Reactivity Studies on Coenzyme Model Compounds
by Nicholas S. Milutinović
A thesis presented to the University of Guelph
In partial fulfillment of requirements for the degree of Master of Science in Chemistry
Guelph, Ontario, Canada © Nicholas S. Milutinović, December, 2017
ABSTRACT
REACTIVITY STUDIES ON COENZYME MODELS COMPOUNDS
Nicholas S. Milutinović Advisor: University of Guelph, 2017 Professor Michael K. Denk
Hydride transfer is an elementary biochemical step performed by coenzymes like methylenetetrahydrofolate (MTHF) and nicotinamide adenine dinucleotide (NADH). This thesis describes the discovery of a new reactivity of
MTHF and NADH model compounds in the reductive dehalogenation of halocarbons. Formal hydride transfer from the model compounds reduces halocarbons, mimicking the well-known in vivo reducing activity of the biomolecules themselves. The reactions are capable of dehalogenating a variety of halocarbons under biomimetic conditions in the presence of air and water including those with toxicological and environmental relevance. Investigations of the notorious pesticide DDT and the inhalation anesthetic and greenhouse molecule halothane demonstrated rapid conversion to known in vivo metabolites, which suggests the involvement of MTHF and NADH in the still obscure metabolic degradation of both compounds. Conversely, the depletion of MTHF and NADH models by halocarbons suggests a new toxicological activity of these molecules.
Acknowledgments
My deepest gratitude goes to Professor Michael Denk for taking me on as a graduate student. I am incredibly honored to have worked under his guidance and privileged to have partaken in a novel research project. Although a great part of our discussions centered on the exciting results, our frequent discussions of topics aside from chemistry, like politics, philosophy, and history, were equally important for stimulating thought and making our working environment tolerable.
I would like to thank Professor Dmitriy Soldatov and Professor William
Tam for reading my thesis and Professor Mario Monteiro for being an early enthusiast of our work and for establishing our intellectual property connections within the university. The expertise and comments provided by Professor Em.
Gordon Lange and Professor Athanasios Paschos at various points in time were also invaluable.
The helpful and knowledgeable team at the NMR Centre and the expertise at the Catalyst Centre within the University of Guelph are sincerely acknowledged for their instrumental roles in advancing and financing the research. The contributions of the many undergraduate students who worked in the lab over the years and are also recognized.
Finally, I would like to acknowledge my friends and family, especially my grandparents, M. & P. Milutinovich, for all that they have given to me.
iii Table of Contents
Acknowledgments ...... iii List of Abbreviations ...... vi List of Figures ...... viii List of Schemes ...... ix List of Tables ...... xii 1 – Introduction ...... 1 1.1 Biochemical Coenzymes ...... 2 1.2 Folate ...... 3 1.2.1 5,10-Methylenetetrahydrofolate (5,10-MTHF) ...... 6 1.3 Methenyl-H4MPT+ ...... 7 1.4 Pyridine Dinucleotides ...... 9 1.4.1 History ...... 10 1.4.2 Redox Functions ...... 11 1.4.3 Non-Redox Functions ...... 12 1.5 Model Compounds for Biomolecules ...... 14 1.5.1 Model Compounds for 5,10-MTHF ...... 14 1.5.1.1 Carbenes ...... 15 1.5.1.2 Carbenium Salts ...... 16 1.5.1.3 Oxidative Synthesis of Imidazolidinium Salts with Halocarbons ...... 17 1.5.2 Model Compounds for NAD(P)H ...... 20 1.5.2.1 Hantzsch Esters ...... 21 1.5.2.2 1,4-Dihydronicotinamides ...... 23 1.6 Dehalogenation in Synthetic Chemistry ...... 26 1.6.1 Organotin Hydrides ...... 28 1.6.2 Samarium Diiodide ...... 29 1.6.3 Silanes ...... 31 1.7 Research Objectives ...... 32 1.8 References ...... 33
iv 2 – Reduction of Halocarbons by Folate Models ...... 39 2.1 Introduction ...... 40 2.2 Results ...... 41 2.3 Conclusion ...... 48 2.4 References ...... 49 3 – Reductive Dehalogenation of DDT by Folate Models ...... 51 3.1 Introduction ...... 52 3.2 Results ...... 53 3.3 Conclusion ...... 55 3.4 References ...... 56 4 – Reaction of Halocarbons with NAD(P)H Models ...... 58 4.1 Introduction ...... 59 4.2 Results ...... 60 4.3 Conclusion ...... 69 4.4 References ...... 70 5 – Synthesis and Host-Guest Properties of Diaminocarbenium Salts ...... 72 5.1 Introduction ...... 73 5.2 Results ...... 75 5.3 Conclusion ...... 83 5.4 References ...... 84 6 – Experimental Section ...... 85 6.1 General Methods and Instrumentation ...... 86 6.2 Chapter 2 – Reaction and Spectroscopic Data ...... 87 6.3 Chapter 3 – Reaction and Spectroscopic Data ...... 97 6.4 Chapter 4 – Reaction and Spectroscopic Data ...... 103 6.5 Chapter 5 – Reaction and Spectroscopic Data ...... 111 6.6 References ...... 115 Appendix ...... 117
v List of Abbreviations
5-MTHF 5-methyltetrahydrofolate 5,10-MTHF 5,10-methylenetetrahydrofolate ACCN 1,1’-azobis-(cyclohexanecarbonitrile) ADP-ribose adenosine diphosphate ribose AIBN azobisisobutyronitrole Ar aromatic BDE bond dissociation energy Bn benzyl Bu butyl Bz benzoyl CBS complete basis set CCSD(T) coupled cluster single double (triple) CPU central processing unit d day dec. decomposition DBP 4,4’-dichlorobenzophenone DDD 1,1-dichloro-2,2-bis(4-chlorophenyl)ethane DDE 1,1-dichloro-2,2-bis(4-chlorophenyl)ethene DDM bis(4-chlorophenyl)methane DDMU 1-chloro-2,2-bis(4-chlorophenyl)ethene DDT 1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane DFT density functional theory DHF dihydrofolate DHFR dihydrofolate reductase DMSO dimethyl sulfoxide DNA deoxyribonucleic acid dTMP deoxythymidine monophosphate dUMP deoxyuridine monophosphate FAD flavin adenine dinucleotide (quinone form) FADH flavin adenine dinucleotide (semiquinone form) FADH2 flavin adenine dinucleotide (hydroquinone form) e- electron ee enantiomeric excess Eq. equivalent Et2O diethyl ether Et ethyl GC-MS gas chromatography-mass spectrometry h hour HMD 5,10-methylenetetrahydromethanopterin dehydrogenase HMPA hexamethylphosphoramide hv ultraviolet light inst. instantaneous iPr iso-propyl KHMDS potassium bis(trimethylsilyl) amide
vi Me methyl min minute mmol millimole mol mole MTHFR methylenetetrahydrofolate reductase m.p. melting point NAD+ nicotinamide adenine dinucleotide (oxidized form) NADH nicotinamide adenine dinucleotide (reduced form) NADP+ nicotinamide adenine dinucleotide phosphate (oxidized form) NADPH nicotinamide adenine dinucleotide phosphate (reduced form) NaDSS 4,4-dimethyl-4-silapentane-1-sulfonate, sodium salt NOX NADPH oxidase NMR nuclear magnetic resonance Ph phenyl ppt. precipitate pTol para-tolyl RAM random-access memory ROS reactive oxygen species SAM S-adenosylmethionine sec second SHMT serine hydroxymethyltransferase SIRT sirtuin tBu tert-butyl TG thermal gravimetric THF tetrahydrofolate TTMSS tris(trimethylsilyl)silane w week
vii List of Figures
Figure 1.1. Structure of 5,10-methylenetetrahydrofolate 2H. ……………………………...6 Figure 1.2. Structures of imidazolidinium [1]+ and imidazolium [1’]+ cations. …...16 Figure 1.3. Structural relationship between 1,4-dihydronicotinamide 4H/[4]+ and Hantzsch ester 6H/[6]+. ……………………………………………..21 Figure 1.4. Structures of chiral 1,4-dihydronicotinamides 4H. …………………………..24 Figure 1.5. Single-step hydride transfer mechanism. ………………………………………...25 Figure 2.1. Structural relationship between imidazolidines 1H, 5,10- methylenetetrahydrofolate 2H, and methenyl-H4MPT 3H...... 40 Figure 2.2. Oxidized cationic form of 5,10-methylenetetrahydrofolate [2]+ and methenyl-H4MPT+ [3]+. …………………………………………………………..40 Figure 3.1. Structures of DDT and its metabolites. ……………………………………….53 Figure 4.1. Structural relationship between the systems 1,4- dihydronicotinamide 4H/[4]+, Hantzsch ester 6H/[6]+, and NAD(P)H 5H/[5]+. …………………………………………………………………………59 Figure 5.1. Structures of imidazolidinium [1]+ and imidazolium [1’]+ cations. …...73 Figure 5.2. Structures of diphenylethylenediamine and diphenylpiperazine. ……..78 Figure 5.3. Thermal gravimetric curve of [1]+Br- (R: tBu)halothane inclusion compound. …………………………………………………………………………………….81 Figure 5.4. Thermal gravimetric curve of [1’]+Br- (R: tBu)halothane inclusion compound. …………………………………………………………………………………….82
viii List of Schemes
Scheme 1.1. Nicotinamide adenine dinucleotide (NAD+/NADH) and flavin
adenine dinucleotide (FAD/FADH/FADH2) redox coenzyme systems. …………………………………………………………………………………………2 Scheme 1.2. Cellular folate cycle. ……………………………………………………………………….4 Scheme 1.3. Structures and interconversion of cellular dihydrofolate and synthetic folic acid. …………………………………………………………………………5 Scheme 1.4. Oxidation of hydrogen by methenyl-H4MPT+ [3]+ catalyzed by 5,10-methylenetetrahydromethanopterin dehydrogenase (HMD). …...8 Scheme 1.5. Oxidized and reduced forms of NAD+/NADH [5]+/5H and NADP+/NADPH [5]+/5H. ………………………………………………………………...9 Scheme 1.6. Oxidation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate by NAD+ catalyzed by glyceraldehyde-3- phosphate dehydrogenase. …………………………………………………………...11 Scheme 1.7. Protein deacetylation by sirtuins using NAD+ as a cosubstrate. ………12 Scheme 1.8. Synthesis of imidazolidines 1H from substituted ethylenediamines and paraformaldehyde. ……………………………………..14 Scheme 1.9. Wallach’s synthesis of symmetric imidazolium salts. ……………………..16 Scheme 1.10. Arduengo’s and Gridnev’s synthesis of nonsymmetrical imidazolium salts. ……………………………………………………………………….17 Scheme 1.11. Wanzlick’s synthesis of imidazolidinium chloride salts by imidazolidine oxidation with carbon tetrachloride. ……………………..17 Scheme 1.12. Wanzlick’s proposed radical chain mechanism for the oxidation of imidazolidines. ………………………………………………………………………..18 Scheme 1.13. Wanzlick’s proposed ionic transfer mechanism for the oxidation of imidazolidines. …………………………………………………………………….....18 Scheme 1.14. Appel conversion of geraniol to geranyl chloride. ………………………...19 Scheme 1.15. Hantzsch dihydropyridine synthesis. …………………………………………..21 Scheme 1.16. Reduction of α,β-unsaturated carbonyl compounds with Hantzsch ester using catalytic dibenzylammonium trifluoroacetate. …………………………………………………………………………22
ix Scheme 1.17. Asymmetric reduction of α,β-unsaturated carbonyls with Hantzsch ester using catalytic dibenzylammonium trifluoroacetate. ………………………………..………………………………………...22 Scheme 1.18. Synthesis of 1,4-dihydropyridines 4H from N-substituted nicotinamides [4]+ with sodium dithionite in alkaline conditions. …………………………………………………………………………………23 Scheme 1.19. Stereoselective reduction of a pyruvate derivative by a chiral NADH model. ……………………………………………………………………………..25 Scheme 1.20. Reductions of (Z)-ethyl-α-cyano-β-bromoethylcinnamate by Hantzsch ester under thermal and photochemical conditions. ……..26 Scheme 1.21. Organotin hydride radical chain mechanism for the formation of carbon-carbon bonds between alkyl halides and olefins. …………28 Scheme 1.22. Tributyltin hydride radical cyclization in the synthesis of the natural product enantiomers sativene and copacamphene. ………….29 Scheme 1.23. Reductive dehalogenation with TTMSS in water. …………………………31 Scheme 2.1. Oxidation of imidazolidines 1H by halocarbons R’-X. ……………………..41 Scheme 3.1. Degradation pathways of DDT by folate models 1H under aerobic and anaerobic conditions. …………………………………………………55 Scheme 4.1. Reduction of halocarbons R’-X by 1,4-dihydronicotinamides 4H. …...60 Scheme 4.2. Reduction of 1,2-dibromo alkyl halides by 1H and 4H. …………………..64 Scheme 4.3. Reduction of 1,1,2,2-tetrabromoethane by 1H and 4H. ………………….64 Scheme 4.4. Reduction of 1-bromo-2-chloroethane by 4H. ……………………………….64 Scheme 4.5. Reduction of ethyl bromoacetate by 4H. ………………………………………..65 Scheme 4.6. Reductive degradation pathways in the reaction of DDT with NAD(P)H models 4H. ……………………………………………………………………66 Scheme 4.7. Proposed catalytic reduction of carbon tetrabromide with 4H. ………67 Scheme 4.8. Attempted catalytic reduction of carbon tetrabromide with [4]+Br- (R: Et) and sodium dithionite. ……………………………………………67 Scheme 4.9. Control reaction of sodium dithionite and carbon tetrabromide. ……68 Scheme 4.10. Reduction of NAD(P)+ model [4]+Br- with folate model 1H. …………69 Scheme 5.1. Synthesis of imidazolidinium salts [1]+X- from N,N’-ethylenediamines. …………………………………………………………………75
x Scheme 5.2. Optimized synthetic procedure for imidazolidinium halide salts [1]+X- (X: Br, I, R: alkyl, aryl) from ethylenediamine, ammonium halide, and triethyl orthoformate. ……………………………….76 Scheme 5.3. One-pot synthesis of 1-tert-butyl-3-phenylimidazolidinium bromide [1]+Br- (R1: tBu, R2: Ph). …………………………………………………76 Scheme 5.4. Two step synthesis of 1-tert-butyl-3-phenylimidazolidinium bromide [1]+Br- (R1: tBu, R2: Ph). …………………………………………………77 Scheme 5.5. Synthesis and purification of [1]+Br- (R: Ph) and diphenylpiperazine. ……………………………………………………………………...79 Scheme 5.6. Synthesis of 1-phenyl-3-tert-butyl-imidazol-4,5- dihydro-2-ylidene. ………………………………………………………………………..79
xi List of Tables
Table 2.1. Reduction of halomethanes by folate model 1H. ……………………………….42 Table 2.2. Reduction of aryl halides by folate model 1H. …………………………………...43 Table 2.3. Reduction of vicinal halocarbons by folate model 1H. …………………….....44 Table 2.4. Reaction of folate model 1H with alkylating agents. ………………………….46 Table 2.5. Computational free energies ΔG° for reduction of halomethanes by folate model 1H. ………………..……………………………..……………………………..47 Table 2.6. Computational free energies ΔG° for the reduction of halomethanes by 5,10-MTHF 2H. …………………………………………………………………………..48 Table 4.1. Reduction of halomethanes with 1,4-dihydropyridine model 4H. .……..61 Table 4.2. Computational free energies ΔG° for reduction of halomethanes by 1,4-dihydropyridine model 4H. …………………………………………………..62 Table 4.3. Reduction of vicinal halocarbons by 1,4-dihydropyridine model 4H. ……………………………………………………………………………………....63 Table 4.4. Computational free energies ΔG° for hydride transfer between folates 1H, 2H and NAD(P)+ models [4]+. …………………..……………………..69 Table 5.1 Yields of imidazolidinium salts [1]+X- synthesized by the optimized orthoformate procedure. ………..……………………………………………………...... 77
xii
Chapter 1
Introduction
1 1.1 Biochemical Coenzymes
The transformation of energy stored within molecules is essential for the operation of cellular processes. Redox coenzymes, including organic molecules derived from vitamins, shuttle electrons from energy-rich substrates, like carbohydrates, lipids, and proteins, to enzyme-catalyzed reactions involved in the synthesis of adenosine triphosphate – the universal molecular unit of energy. A vast range of redox coenzymes exists in nature, and the reversible oxidation/reduction of these molecules allows for both one and two electron transfers. Nicotinamide adenine dinucleotide (NAD+/NADH) and flavin adenine dinucleotide (FAD/FADH/FADH2) are among the most ubiquitous and versatile of the redox coenzyme systems (Scheme 1.1).1
O H H O
2e- + H+ NH2 NH2
N N
NAD+ NADH
2e- + 2H+
O O O H H N N N NH e- + H+ NH e- + H+ NH
N N O N N O N N O H
FAD FADH FADH2 Scheme 1.1. Nicotinamide adenine dinucleotide (NAD+/NADH) and flavin adenine dinucleotide (FAD/FADH/FADH2) redox coenzyme systems.
In addition to the transfer of electrons by redox coenzymes, numerous other coenzymes exist to allow the transfer of functional groups for metabolic
2 processes. For example, the coenzyme biotin is a carrier of carbon dioxide for carboxylase enzymes,2 coenzyme A transfers acetyl groups in the oxidation of fatty acids,3 and folates methylate DNA and proteins.4
1.2 Folate
Folate, vitamin B9, is an essential nutrient obtained in the diet from green plants.5 The word folate colloquially describes several closely-related derivatives of tetrahydrofolate (THF) present in the folate cycle of cells including tetrahydrofolate (THF), dihydrofolate (DHF), 5,10-methylenetetrahydrofolate
(5,10-MTHF), and 5-methyltetrahydrofolate (5-MTHF) (Scheme 1.2). All folate substrates function as one-carbon donors or acceptors for a number of fundamental biochemical processes including the methylation of DNA and syntheses of amino acids and DNA nucleobases.4,6–10 THF is the central folate form through which all folates are regenerated after demethylation.
3 homocysteine methionine
methionine synthase vitamin B12
O Me O + H NADP NADPH N N N NH N NH DHFR H2N N N H2N N N H H H H O 5-methyltetrahydrofolate tetrahydrofolate N (5-MTHF) (THF) N NH
H2N N N serine H H H SHMT dihydrofolate NADP+ O H N MTHFR glycine (DHF) N thymidylate N NADPH H synthase
H2N N N H H dUMP dTMP 5,10-methylenetetrahydrofolate (5,10-MTHF)
Scheme 1.2. Cellular folate cycle.
Specific enzymes couple the methylation of substrates, such as serine and homocysteine, to the demethylation of folates. These one-carbon transfer pathways have made folate coenzymes the methylation reagents of nature. The methylation of macrobiomolecules, such as DNA and proteins, by folates is crucial at all stages of life, and especially during prolific stages of cell growth like pregnancy and cancer. Deficiencies of folate have been attributed to many chronic diseases including anemia,8 cancer,11 and neurodegenerative illnesses including depression.12–14 As a result, many countries now mandate the fortification of grain products with folic acid to support population health. It is important to note, however, that folic acid, while similar in structure to folates, is not the same as naturally occurring folates. The synthetic forms of folic acid added to grain
4 products incorporate an aromatic pteridine ring within their structures, which is not present in natural folates (Scheme 1.3).15 Synthetic folic acid itself has no coenzyme activity and is reduced to dihydrofolate (DHF) by dihydrofolate reductase (DHFR).
O O O H N N N HN NH DHFR N NH DHFR N NH
H2N N N H2N N N H2N N N H H H H Synthetic folic acid Dihydrofolate (DHF) Tetrahydrofolate (THF) Scheme 1.3. Cellular dihydrofolate and synthetic folic acid. Folic acid must be reduced to dihydrofolate before entering the folate cycle.
This enzyme, which naturally reduces DHF to THF, reduces synthetic folic acid
1300 times more slowly than DHF. Consumption of folic acid exceeding the 0.4 milligrams per day recommendation inhibits the reduction of DHF by DHFR.15
Evidence has suggested that folic acid consumption exceeding one milligram per day may exacerbate existing forms of cancer16 and neurological damage associated
17 with vitamin B12 deficiency. However, there is no evidence to suggest that synthetic folic acid is detrimental to human health at or below currently recommended levels of consumption.15 The case of synthetic folic acid demonstrates, however, that any molecular imbalance or disruption of the folate cycle, such as toxicological disruptors, have the potential to cause adverse physiological health effects.
5 1.2.1 5,10-Methylenetetrahydrofolate (5,10-MTHF)
Within the folate cycle, 5,10-MTHF 2H (Figure 1.1) functions as a one- carbon transfer agent in the biosynthesis of the nucleic acid thymine18 and serves as the immediate precursor to 5-MTHF.8,10
H2N N O H H HN O N N COOH HN HN
2H COOH
Figure 1.1. Structure of 5,10-methylenetetrahydrofolate (5,10-MTHF) 2H.
In this latter role, the imidazolidine methylene ring of 5,10-MTHF 2H is reduced by 5,10-MTHF reductase (MTHFR) to 5-MTHF (Scheme 1.2). This folate substrate methylates the non-protein amino acid homocysteine in a vitamin B12-dependant step to form methionine,9 which, upon subsequent conversion to S- adenosylmethionine (SAM), functions as a universal methylating agent19 for DNA, proteins, lipids, and neurotransmitters, among others.20 Factors that inhibit cellular production of 5-MTHF, and the folic acid cycle in general, lead to an increase of cellular homocysteine, which impedes methylation of DNA and proteins by SAM.19 This physiological condition, known as hyperhomocysteinemia, results from folate deficiency and is a known risk factor for renal failure21 and thrombosis,22 and has also been attributed to several chronic neurodegenerative diseases like dementia,23 schizophrenia,24 and Alzheimer’s disease.25 The recent discovery that 5,10-MTHF 2H also acts as a hydride donor in the light-activated
6 repair enzyme DNA photolyase demonstrates that 5,10-MTHF offers significant surprises even after more than sixty years of extensive research.26,27
In a second biochemical role, 5,10-MTHF 2H reductively methylates deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate
(dTMP) – a precursor to the DNA nucleic acid thymine. This methylation pathway is the only de novo method available for the biosynthesis of dTMP and is crucial for imparting the lipophilic nature to thymine that allows for its structural discernment from the other three DNA nucleic acids by DNA proteins.18
The intimate relationship between folate and the synthesis and functionalization of DNA has made folate inhibition a target for cancer treatment.
So-called antifolate drugs, such as methotrexate, antagonize the cycle through competitive binding inhibition of dihydroflate reductase (DHFR), which prevents the reduction of DHF to THF. The ultimate effects of this inhibition are cessations of thymine biosynthesis by 5,10-MTHF and methylation of existing DNA by 5-
MTHF, which prevent the synthesis of new DNA in proliferating cells.28
1.3 Methenyl-H4MPT+
An oxidized imidazolidine functional group is also present in the molecule methenyl-H4MPT+ [3]+ found in methanogenic archaea.29 This imidazolidinium coenzyme functions with the enzyme 5,10-methylenetetrahydromethanopterin dehydrogenase (HMD) to reversibly oxidize molecular hydrogen, H2, during the conversion of carbon dioxide to methane by these organisms (Figure 1.5).29,30
7 H R H R O N O H N HMD N CH3 + H N CH3 + HN 2 HN + H H H
H2N N N CH3 H2N N N CH3 H H
[3]+ 3H Scheme 1.4. Oxidation of hydrogen by methenyl-H4MPT+ [3]+ catalyzed by 5,10- methylenetetrahydromethanopterin dehydrogenase (HMD).
The oxidation of hydrogen by HMD, which is slightly exothermic (≈ -5.5 kJ/mol) and essentially a thermoneutral reaction,31 is dependent upon the presence of the methenyl-H4MPT+ [3]+ imidazolidinium coenzyme. The hydrogen-hydrogen bond is one of the strongest single bonds known (BDE = 436 kJ/mol) and is typically only capable of being activated by metals or very strong oxidizing agents.29 The oxidation of hydrogen by HMD at methenyl-H4MPT+ [3]+ is unique among all known hydrogenase enzymes in that no metal is present at this site.29 Before the discovery of HMD by Thauer et al.,32 all hydrogenase enzymes were classified into two phytogenetically unrelated categories: iron-iron ([FeFe]) hydrogenases and iron-nickel ([FeNi]) hydrogenases.33 The active sites of these metalloenzymes contain the dimeric metal center, with separate iron-sulfur clusters distributed throughout the protein structure. The discovery and characterization of HMD necessitated a new class designation of hydrogenase, as the protein contained only an iron center ([Fe]-only) within its active site and no iron-sulfur clusters.33 The involvement of an imidazolidine ring in archaeal hydrogen production demonstrates the ancient and fundamental role of this functional group in the origin of life.
8 1.4 Pyridine Dinucleotides
Pyridine dinucleotides are cellular coenzymes derived from niacin (vitamin
+ + B3). Nicotinamide adenine dinucleotide (NAD /NADH) [5] /5H (R: H) and nicotinamide adenine dinucleotide phosphate (NADP+/NADPH) [5]+/5H (R: O-
3- PO4 ) are the two pyridine dinucleotide forms present in terrestrial life. The structures of the nucleotides differ only by the presence of a phosphate group in the adenine moiety of the latter compound (Scheme 1.5). The oxidized forms,
NAD+/NADP+ [5]+, possess a net charge of -1, while the reduced forms,
NADH/NADPH 5H, have a net charge of -2.34
O NH2 O NH2
N N NH2 N NH2 N
N O O N N N O O N N 2e- + H+ O P O P O O P O P O O O O O O O O O
OH OH OH R OH OH OH R
[5]+ 5H
Scheme 1.5. Oxidized and reduced forms of NAD+/NADH [5]+/5H (R: H) and
NADP+/NADPH [5]+/5H (R: O-PO43-).
NADH and NADPH are structurally very similar and share parallel functionality and reactivity. Although ubiquitously associated as electron carriers for metabolic redox pathways, recent discoveries have also shed light on important non-redox functions of NADH and NADPH, and the significance of these molecules for a wide range of cellular functions and signaling processes is now realized.34 The discovery of these biomolecules and the identification of their central roles in cellular processes have been instrumental in the establishment of modern biochemistry.34
9 1.4.1 History
In 1897 Eduard Buchner demonstrated that sugars could be fermented to carbon dioxide using a cell-free ‘press juice’ obtained from yeasts.35,36 For this remarkable demonstration of an organic transformation in the absence of living cells, Buchner was awarded the Nobel Prize in Chemistry in 1907. In 1904, while extending this study of fermentation, Arthur Harden observed that the activity of a high molecular weight fermenting agent, ‘zymase,’ was dependent upon the presence of a second, low molecular weight agent, ‘cozymase.’36,37 This smaller agent was subsequently isolated by Hans von Euler-Chelpin, and its structure was found to contain sugar, adenine, phosphate, and pyridine subunits.34,37 Together, the nicotinamide mononucleotide and adenosine monophosphate dinucleotides comprising its structure designated this molecule nicotinamide adenine dinucleotide (NAD). For their combined early work in unveiling the structure and function of fermentative enzymes, Harden and von Euler-Chelpin shared the Nobel
Prize in Chemistry in 1929.34 In 1936 Otto Heinrich Warburg clarified the role of
NADH as a carrier of hydridic hydrogen for metabolic processes.38 His ensuing discovery of a second electron carrier, NADPH, which possessed similar reactivity to NADH, demonstrated the functional significance of these coenzyme redox systems.37 During the course of this work, Warburg also identified the pyridine ring of NAD(P)H as the subunit from which electron transfer occurs.37
10 1.4.2 Redox Functions
Thousands of enzymes utilize NAD(P)H as coenzymes to shuttle electrons between chemical bonds in the synthesis and breakdown of molecules. Within cells, NAD exists predominantly in its oxidized NAD+ form, which allows it to function as an oxidizing agent for the capture of electrons in catabolic reactions.34
Specific dehydrogenase enzymes catalyze the oxidation of energy-rich molecules by NAD+, which causes it to be reduced to NADH by the relinquished electrons. For example, glyceraldehyde-3-phosphate, an intermediate substrate of glucose metabolism during glycolysis, is oxidized by NAD+ to 1,3-bisphosphoglycerate through the enzyme glyceraldehyde-3-phosphate dehydrogenase (Scheme 1.6).39
glyceraldehyde- 3-phosphate O O O O dehydrogenase O O O O P P P O O H O O O O P + NAD+ NADH OH i OH glyceraldehyde-3-phosphate 1,3-bisphosphoglycerate
Scheme 1.6. Oxidation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate by NAD+ catalyzed by glyceraldehyde-3-phosphate dehydrogenase.
The oxidation reactions transform complex organic molecules, like sugars and fats, into simpler compounds, and the reduced NADH that is formed is ultimately funneled into the electron transport chain for the synthesis of ATP. Converse to
NAD, NADP exists primarily in its reduced NADPH form and acts as an electron source for the synthesis of molecules in anabolic reactions.34
11 1.4.3 Non-Redox Functions
In addition to the universal roles of pyridine dinucleotides in energy transfer, their functions as non-redox substrates in a number of cellular processes, including aging,40 cell signaling,41 and ion channel regulation,42 have been discovered in recent years. One such role is the fascinating participation of NAD+ in the functionality of the ‘anti-aging’ sirtuin proteins.
Silent information regulator proteins (sirtuins) are a family of universal enzymes that catalyze substrate-specific deacetylation of lysine amino acids within structural proteins.43 Sirtuins function through an NAD+-dependent process, wherein deacetylation is coupled to the hydrolysis of NAD+. In the conversion, the glycosidic bond between nicotinamide and ADP-ribose subunits of
NAD+ is broken, and free nicotinamide and deacetylated protein are released from the reaction along with an acetylated O-acetyl-ADP-ribose unit (Scheme 1.7).
O NH2 O N NH2 N NH N N O O N + O P O P O O O acetylated O O protein OH OH OH R
+ NAD sirtuin H2O [5]+
NH2
N N
O NH3 O O N N NH2 + O P O P O + HO O O O O N deacetylated protein O OH OH R nicotinamide O acetylated O-acetyl-ADP-ribose
Scheme 1.7. Protein deacetylation by sirtuins uses NAD+ as a cosubstrate and generates nicotinamide, acetylated O-acetyl-ADP-ribose, and deacetylated protein.
12 Within the family of sirtuin proteins, SIRT1 has gained specific attention for its
DNA histone-specific deacetylase activity.43,44 Deacetylation of histones causes
DNA to be packaged more tightly around these structural proteins, which limits the copying of DNA and, ultimately, gene expression.45 Overexpression of SIRT1 has been shown to extend the lifespan of lower organisms, such as yeast, worms, and mice, while also delaying the onset of age-related diseases.40,46 Although many histone deacetylase enzymes are known, SIRT1 is the only one that uses NAD+ as a cosubstrate, and the activity of SIRT1 is, therefore, linked to the metabolic state of the cell.40,43
NADH and NADPH are electron sources for cellular reactive oxygen species
- 47,48 (ROS) like superoxide (O2 ) and hydrogen peroxide (H2O2). The production of
ROS is, primarily, the result of the direct reduction of molecular oxygen by NADH in the mitochondrial electron transport chain and NADP oxidase (NOX) enzymes found in the extracellular membranes of cells. Although an overabundance of ROS has been implicated in cancer, aging, and pathological diseases like vascular fibrosis and cardiac hypertrophy,49 the necessary involvement of these compounds in a variety of cellular immune response mechanisms and cellular signaling processes has become clear. The discovery that NOXs catalyze electron transfer from NADPH to molecular oxygen for the direct purpose of generating superoxide demonstrates the general role of ROS in cellular function. Accordingly, the production of ROS has been linked to the mediation of cellular migration, circadian rhythm, and stem cell proliferation.47
13 1.5 Model Compounds for Biomolecules
Simple and readily accessible synthetic model compounds have played a crucial role in our understanding of complex biological systems. Milestones include the study of reversible oxygen uptake through hemoprotein models,50 cytochrome P450 models51 for the oxygenation of organic substrates, and nitrogenase models52 for the reduction of molecular nitrogen. Remarkably, two of the central hydride transfer agents, 5,10-MTHF 2H and NAD(P)H 5H, have not been studied with model systems to a great extent.
1.5.1 Model Compounds for 5,10 MTHF
Imidazolidines 1H share a common structural element with the systems
5,10-MTHF 2H and methenyl-H4MPT 3H described previously, and are investigated as model compounds for these hydride systems in this thesis.
Imidazolidines 1H are conveniently obtained from substituted ethylenediamines and paraformaldehyde (Scheme 1.8).53
R R NH N (CH2O)n Et O NH 2 N R R
1H
Scheme 1.8. Synthesis of imidazolidines 1H from substituted ethylenediamines and paraformaldehyde.
Historically, imidazolidines and imidazolidinium carbenium salts were the focus of investigation as synthetic precursors for N-heterocyclic carbenes54 – neutral molecules of dicoordinate carbon.
14 1.5.1.1 Carbenes
For nearly sixty decades, the chemistry of carbenes has attracted much scientific interest and was even the basis of a Nobel Prize. Additionally, the relevance of these compounds to the field of vitamin chemistry cannot be overlooked. Beginning in the 1950s, Breslow proposed that carbenes derived from thiamine (vitamin B1) existed as short-lived intermediate compounds in a number of biochemical reactions55,56 such as pyruvate decarboxylation and benzoin-type condensations.57 Subsequent synthetic investigations by E. O. Fischer,58 U.
Schöllkopf,59 and, most notably, H.-W. Wanzlick60 were unfruitful in yielding stable carbenes. In 1991 the first persistent carbene, 1,2-di(adamantyl)imidazole-2- ylidene, was reported by Arduengo et al.61 Surprisingly, this compound was thermodynamically stable at room temperature in the absence of air and moisture, and could even be sublimed and melted without decomposition.61 As a result of this groundbreaking synthesis, stable carbenes and their metal complexes have become important synthetic tools for a wide range of chemical applications; from traditional syntheses of natural products to the large-scale industrial manufacturing of polymers. Recently, isolable carbenes have allowed for the examination of Breslow’s early proposal of carbene involvement in biochemical reactions, and his hypothesis, which was instrumental for establishing the field of carbene chemistry, has been proven correct.57
15 1.5.1.2 Carbenium Salts
Imidazolidinium [1]+ and aromatic imidazolium [1’]+ carbenium salts are common precursors to persistent carbenes (Figure 1.2).
H H R R N N N R N R
[1]+ [1’]+ Figure 1.2. Imidazolidinium [1]+ and aromatic imidazolium [1’]+ cations.
A number of synthetic approaches for carbenium salts exist. In 1925
Wallach described the synthesis of a symmetric aromatic imidazolium salt obtained by reaction of a primary amine with formaldehyde, glyoxal, and a mineral acid (Scheme 1.9).62 This method typically uses hydrochloric acid to generate the corresponding chloride carbenium salts. Previous work in our lab has shown that using hydrobromic acid gives a highly impure imidazolium bromide product.
H O R O HX X 2 H2NR + + N O H H N R
Scheme 1.9. Wallach’s synthesis of symmetric imidazolium salts.
A similar synthetic approach for nonsymmetrical imidazolium salts has been developed by Arduengo and Gridnev (Scheme 1.10),63,64 which has proven to be useful for the synthesis of chiral carbenes for use in enantioselective catalytic processes.65
16 H H O O R1 R1 + + + H3PO4 R2X H2NR1 O NH4Cl N N X H H N N R2
Scheme 1.10. Arduengo’s and Gridnev’s synthesis of nonsymmetrical imidazolium salts.
1.5.1.3 Oxidative Synthesis of Imidazolidinium Salts with Halocarbons
In 1973, while investigating the synthesis of carbenium salts, Wanzlick observed that imidazolidines 1H (Ar: Ph, p-MeOPh, p-O2NPh) were oxidized by carbon tetrachloride to the corresponding imidazolidinium chloride salts (Scheme
1.11).66 Chloroform was formed as a reaction byproduct, which amounted to a formal hydride transfer from the imidazolidine to carbon tetrachloride.
Ar Ar
N N Cl CCl4 H - CHCl N 3 N Ar Ar Scheme 1.11. Wanzlick’s synthesis of imidazolidinium chloride salts (Ar: Ph, p-MeOPh, p-
O2NPh) by imidazolidine oxidation with carbon tetrachloride.
Wanzlick observed that radical initiators, such as benzoyl peroxide, led to a rapid oxidation of the imidazolidine by carbon tetrachloride, and a radical chain mechanism was accordingly proposed to account for the reactivity (Scheme 1.12).
17 radical initiator CCl4 CCl3
Ar Ar N N H H + CCl3 + CHCl3 H N N Ar Ar
Ar Ar N N H Cl + CCl4 H + CCl3 N N Ar Ar
Scheme 1.12. Wanzlick’s proposed radical chain mechanism for the oxidation of imidazolidines.
Further experiments demonstrated that, even in the presence of radical inhibitors, like hydroquinone, and exclusion of light, the reactions could not be prevented, and oxidized imidazolidinium salt was still formed, albeit more slowly. To account for this curious observation, Wanzlick proposed the existence of a second competing hydride transfer mechanism (Scheme 1.13).
Ar Ar N Cl N H Cl CCl H + CHCl3 H 3 N N Ar Ar Scheme 1.13. Wanzlick’s proposed ionic transfer mechanism for the oxidation of imidazolidines.
Under the radical inhibition conditions, oxidation of imidazolidines bearing electron-donating aryl substituents, like p-MeO, was accelerated, which lent support to the existence of a competitive ionic mechanism.
Although not commonly regarded for their oxidative properties, carbon tetrahalides, CX4 (X: Cl, Br), have precedence as oxidizing agents in synthetic
18 protocols.67–69 For example, the Appel reaction converts carboxylic acids67 and primary and secondary alcohols69 to acyl and alkyl halides (Cl, Br), respectively, using a phosphine and carbon tetrahalide (Scheme 1.14).
OH PPh3 (1.3 eq.) Cl
CCl4, reflux, 1 h - P(O)Ph3, - CHCl3
Scheme 1.14. Appel conversion of geraniol to geranyl chloride. Chloroform is formed as a reaction byproduct.70
Mechanistically, carbon tetrachloride serves to oxidize the phosphine, which generates a phosphine oxide and chloroform as reduction byproducts. Formation
- of an ion-stabilized phosphonium-trichloromethyl anion (CCl3 ) intermediate and phosphine P=O double bond have been proposed as driving forces for the reaction.69 Considering the oxidation of imidazolidines by carbon tetrachloride, the lattice energy afforded by the carbenium salt byproduct can be viewed as a driving force for this reaction.
Beginning in 2005 our group observed that Wanzlick’s 1973 carbon tetrachloride oxidation of imidazolidines was general for a variety of halomethanes, and the method allowed for a high yielding synthesis of imidazolidinium salts. Notably, the use of brominated and iodinated halomethanes resulted in faster oxidations and produced non-hygroscopic imidazolidinium bromide and iodide salts. Ultimately, the oxidizing power of the investigated compounds was found to follow the order CI4 > CHI3 > CBr4 > CHBr3 ≈ CCl4, with dialkylimidazolidines (R, R’ = Me, Et, iPr) being more readily oxidized than diarylimidazolidines. When performed stoichiometrically, these oxidations
19 afforded the respective imidazolidinium halide salts in quantitative yield.
However, in the presence of excess halogenated hydrocarbon, over-oxidation of the imidazolidinium cation [1]+ to the imidazolium cation [1’]+ occurred as a significant side reaction. Additionally, oxidations of 1H (R: tBu) formed unavoidable mixtures of reaction products, and the desired N,N’-di-tert- butylimidazolidinium salt was only a minor component. However, this second dehydrogenation step was too slow to compete with the oxidation of 1H.
Nonetheless, as over-oxidation could not be prevented, the final N,N’-di-tert- butylimidazolidinium salt product was impure, and separation of the two ionic components proved challenging. In 2014 the discovery that a wide variety of functionalized halocarbons, such as aryl halides and those of toxicological and environmental relevance, was capable of oxidizing imidazolidines initiated a paradigm shift to view imidazolidines as a new class of useful reducing agents for halogenated molecules, which formed the motivation for the model reactions described in this thesis.
1.5.2 Model Compounds for NAD(P)H
1,4-Dihydronicotinamides 4H and Hantzsch esters 6H share a 1,4- dihydropyridine ring with the coenzymes NADH and NADPH. The oxidized forms of 4H and 6H, pyridines [4]+ and [6]+, are also common to NAD+ and NADP+
(Figure 1.3).
20 O O O O O O
NH2 NH2 RO OR RO OR
N N N N H H R R 4H [4]+ 6H [6]+
Figure 1.3. Structural relationship between 1,4-dihydronicotinamide 4H/[4]+ and Hantzsch ester 6H/[6]+.
1.5.2.1 Hantzsch Esters
First synthesized by Arthur Hantzsch in 188271 several decades before the discovery and characterization of NADH from living organisms, Hantzsch esters
6H represent the first model compounds of NAD(P)H (Scheme 1.15).
O O O O NH + 2 3 EtO OEt CO2Et H H N H Scheme 1.15. Hantzsch dihydropyridine synthesis from formaldehyde, ammonia, and ethyl acetoacetate.
Hantzsch esters have found synthetic utility for the reduction of a variety of organic substrates. Organic compounds capable of being reduced include carbonyls,72 imines,73 nitroolefins,74 and α,β-unsaturated carbonyls.75 For the latter case, in particular, Hantzsch esters have provided a metal-free hydrogenation approach for conjugated olefins. These reductions, which are performed catalytically using a variety of organic catalysts, allow for symmetric and asymmetric hydrogenation depending on the catalyst employed.76 For example, α,β-unsaturated aldehydes are reduced to the respective α,β-saturated
21 aldehydes by Hantzsch esters and catalytic dialkylammonium salts (Scheme 1.16).
The reactions generate iminium intermediates, which are reduced in situ to aldehydes by the Hantzsch ester while leaving other double bonds unchanged.
H i), ii) O O THF, rt H
O R O Bn RO OR Bn i) ii) N CF COO N H 3 H H
Scheme 1.16. Reduction of α,β-unsaturated carbonyl compounds with Hantzsch ester using catalytic dibenzylammonium trifluoroacetate.77 i): 4 eq. 3,5-bis(ethoxycarbonyl)- 1,4-dihydro-2,6-dimethylpyridines. ii): 5 mol % dibenzylammonium trifluoroacetate. Yield 82 %.
Asymmetric reductions of α,β-unsaturated carbonyls using Hantzsch esters are achieved through the employment of chiral imidazolidinone salts (Scheme 1.17).
O R O O R O i), ii) R O N i) RO OR ii) CHCl , -40 C N H 3 H N H H 2 CF COO 3 Scheme 1.17. Asymmetric reduction of α,β-unsaturated carbonyls with Hantzsch ester using catalytic dibenzylammonium trifluoroacetate.78 i): 1.1 eq. 3,5-bis(ethoxycarbonyl)- 1,4-dihydro-2,6-dimethylpyridines. ii): 20 mol % imidazolidinone trifluoroacetate. Yield 90 – 96 %, 93 % ee, 24 h.
The asymmetric hydrogenations occur rapidly in high product yield with high enantiomeric excess. The conversions, which are purely organic, have proven to be useful alternatives to expensive metal hydrogenation catalysts.
22 1.5.2.2 1,4-Dihydronicotinamides
1,4-Dihydronicotinamides 4H are structurally very similar to NAD(P)H 5H.
However, compared to Hantzsch esters 6H, these compounds have not been studied to the same extent. A number of syntheses for these compounds exist, but they are typically low yielding and result in product mixtures.79 Direct reduction of pyridines and pyridinium salts with strong reducing agents, such as borohydride or boranes, is complicated by the formation of mixtures of 1,2- and
1,4-dihydronicotinamides. Reduction in lithium-liquid ammonia is selective for the formation of the 1,4-isomer,79 however, the complicated experimental setup has made it an unattractive synthetic method. A facile alternative is the reduction of pyridinium salts with sodium dithionite (Scheme 1.18).80
O O O
R-X Na2S2O4 (4 eq.) NH2 NH2 NH2 MeOH NaHCO3 (6 eq.) H O / CH Cl N N 2 2 2 N X R R
[4]+ 4H Scheme 1.18. Synthesis of 1,4-dihydropyridines 4H from N-substituted nicotinamides [4]+ with sodium dithionite in alkaline conditions.
Water is used as the solvent, and dithionite selectively forms the 1,4- isomer only.
Alkaline conditions are required to slow the decomposition of dithionite in solution. Depending on the polarity of the reduced 1,4-dihydronicotinamides, the reactions can, however, still be low yielding in the case of highly water-soluble
NAD(P)H derivatives. The reduction capabilities of 1,4-dihydronicotinamides closely parallel those of Hantzsch esters. However, in addition to synthetic issues
23 described previously, a number of other factors have rendered these molecules less desirable model compounds. The instability of 1,4-dihydronicotinamides in acidic media and the tendency to give undesirable side reactions can be taken as reasons for this.81 Additionally, 1,4-dihydropyridines are known to be slightly weaker hydride donors compared to Hantzsch esters.82 Interestingly, for both groups of compounds, the rates of substrate reduction are accelerated by the presence of divalent metal ions, (Mg2+, Zn2+, Ni2+, Co2+, Mn2+)83 – a reactivity feature which may have relevance to the biological origin of the molecules.
Despite their general lack of current synthetic use, chiral 1,4- dihydronicotinamides have demonstrated value for stereoselective reductions of carbonyl compounds (Figure 1.4).
O Ph O
N NH2 H N N Pr
Figure 1.4. 1,4-dihydronicotinamides functionalized with chiral and/or bulky substituents allow for stereoselective reduction.84,85
Generally, stereoselective reduction is effected by incorporation of sterically- demanding side chains on the pyridine nitrogen or bulky substituents at the carbon 2- and 5-positions of the pyridine ring.86 A variety of carbonyl compounds have been reduced by chiral 1,4-dihydronicotinamides in moderate yield and high enantiomeric excess (Scheme 1.19). When the reductions are performed with stoichiometric quantities of divalent metal salts, the counter anion of the oxidized
NAD+ salt is acquired from the divalent metal salt.
24 (S)-chiral NADH model O OH Mg(ClO4)2 81 % CH CN 98 % ee CO nBu 3 CO nBu 2 rt, 4d 2 (R) 16 other examples Scheme 1.19. Stereoselective reduction of a pyruvate derivative by a chiral NADH model.85
Several mechanisms have been described to account for the reactivity of 1,4- dihydropyridines. As the reductions amount to a formal hydride transfer, a polar mechanism in which partial negative charge is distributed between NAD, hydride, and substrate, akin to an SN2 mechanism, has been proposed to explain the reactivity (Figure 1.5).87
!+ … !- … !+ + … … + NADH + A [NAD H A] NAD + HA Figure 1.5. Single-step hydride transfer mechanism.
Studies involving deuterium labeling under anhydrous and anaerobic conditions seemed to support the presence of an ionic mechanism.88 Additionally, investigations of hydride transfer between various NAD(P)H and NAD(P)+ model compounds have also pointed to a one-step reduction process.89 However, the relevance of this mechanism to the biological systems from which the models have been inspired does not seem to have been considered to a great extent. Although an ionic process may be a contributing mechanism under anhydrous conditions, the instability of hydride anions in the presence of water suggests its unlikely existence in biological systems. Reduction mechanisms involving electron transfer and the formation of transient neutral and ionic radical species have also been proposed.87 Such processes involve multistep mechanisms in which initial electron transfer is the rate-determining step, and intermediate transient radicals and ions
25 generally have fleeting lifetimes.87 Mechanistic work by Zhu et al. investigating the reductions of cis-allylic bromides by Hantzsch esters under thermal and photochemical conditions supported the presence of ionic and radical reduction mechanisms, respectively (Scheme 1.20).88
Thermal conditions
EtOOC COOEt
NC COOEt NC COOEt Ph N NC H
Br in the dark Br Ph Ph EtOOC H
Photochemical conditions
EtOOC COOEt
NC COOEt NC COOEt NC COOEt EtOOC CN N H
Br hv, 420 nm Br Br Br Ph Ph Br Ph Ph
NC COOEt EtOOC CN +
Ph Ph (E) 70 % (Z) 30 % Scheme 1.20. Reductions of (Z)-ethyl-α-cyano-β-bromoethylcinnamate by Hantzsch ester under thermal and photochemical conditions demonstrate the existence of ionic and radical reduction pathways.
Photochemical reactions generated a mixture of cis and trans olefin products while thermal conditions formed only the cyclopropane product. Similar to Wanzlick’s proposal of separate ionic and radical mechanisms to explain the reduction of carbon tetrachloride by imidazolidines, mechanistic studies of 1,4- dihydronicotinamides show the same level of complexity.
1.6 Dehalogenation in Synthetic Chemistry
A variety of reagents exist in the synthetic toolbox of organic chemistry for
26 reductive conversions of functional groups. Sodium borohydride and lithium aluminum hydride are among the most commonly used reducing agents for their extensive reduction capabilities and their general ease of handling. At room temperature, both compounds rapidly reduce aldehydes, ketones, and acid chlorides to alcohols. Lithium aluminum hydride is further capable of reducing esters, carboxylic acids, and amides, which, comparably, makes it a less selective reducing agent.90 Other inorganic and organometallic reagents, such as boranes, metal alloys and amalgams, and Schwartz’s reagent, are used for similar reductions of carbon-oxygen and carbon-nitrogen bonds, often with enhanced functional group specificity. However, the strong reducing nature of these materials and their reactivity towards air and/or moisture are features that often render their use problematic.
Organohalides are useful substrates in organic synthesis for nucleophilic substitution and radical coupling reactions. However, few reagents exist to selectively reduce carbon-halogen bonds to carbon-hydrogen bonds. Organotin hydrides, samarium diiodide, and silanes are the most prominent reagents employed for this synthetic transformation. In the years since the discoveries of their respective dehalogenation activity, the radical mechanistic nature through which these reagents operate has become clear, and new synthetic reactions using organohalide substrates have been developed. However, as with the inorganic hydride and organometallic reducing agents commonly employed in organic synthesis, the often-indiscriminate reducing ability of these reagents towards other functional groups limits their utility for halogen reduction.
27 1.6.1 Organotin Hydrides
Beginning with their synthesis in the 1940s,91 the synthetic utility of organotin hydrides for functional group transformation quickly became clear, and their preeminent status as reagents for reductive dehalogenation has remained to this day. The strong reducing power of organotin hydrides is the result of the weak tin-hydrogen bond, which facilitates homolytic cleavage to generate hydrogen radicals. The reductive pathway follows a radical chain mechanism instigated by radical initiators, such as AIBN or ultraviolet light, and is driven forward by the strong tin-halide bond formed in the reaction.92 The reaction generates an alkyl radical intermediate, which, after abstracting a hydrogen radical from the organotin hydride, generates the reduced product. The dehalogenation reactions hold additional synthetic utility by mediating carbon-carbon bond formation through the coupling of the alkyl radical intermediates with olefins (Scheme
1.21).93
R
Y Y Bu3SnH R
R
RX Bu Sn 3 Y Scheme 1.21. Organotin hydride radical chain mechanism for the formation of carbon- carbon bonds between alkyl halides and olefins.
Halide reductions with organotin hydrides allow for the coupling of primary, secondary, and tertiary bromides and iodides to electron-withdrawing alkenes.94
This coupling method has been used extensively in the synthesis of cyclic natural
28 products via intramolecular radical cyclizations of alkyl halides (Scheme 1.22).95
Br
Bu3SnH 62 % AIBN
O O Scheme 1.22. Tributyltin hydride radical cyclization in the synthesis of the natural product enantiomers sativene and copacamphene.96
However, the high reactivity of organotin hydrides prevents selective halide reduction in the presence of other reducible functional groups such as alcohols, selenides, and nitro compounds. Tin hydrides also have a strong affinity for carbon-sulfur bonds, which has been exploited in decarboxylation reactions.97
Despite their attractive synthetic utility, tin hydrides are highly toxic and lipophilic, and their use in synthesis is now usually avoided. A related issue with their use has been the challenge of removing tin-containing impurities from product mixtures. Recently, new organotin hydrides have been designed and synthesized to facilitate removal of tin species from the product mixture.98
Additionally, new radical reagents based on indium(III)99 and silylated cyclohexadienes100 have been developed that share similar reactivity to organotin hydrides toward alkyl and aryl halides.
1.6.2 Samarium Diiodide
In the 1980s Henri Kagan demonstrated the utility of samarium diiodide as a mild reducing agent for a variety of functional groups. Solutions of samarium diiodide in THF allowed for time efficient and high yielding deoxygenation of
29 epoxides and sulfoxides, selective reduction of aldehydes in the presence of ketones, reduction of conjugated double bonds, formation of tertiary alcohols from ketones and alkyl halides, and reduction of tosylate and alkyl halides to alkanes. In the case of the reductions of allylic and benzylic halides, coupling could be obtained at room temperature in moderate yields. The reducibility of the halides was shown to be in the order of I > Br >> Cl, and reaction yields followed a similar trend.101 Subsequent work by Inanaga et al. modified Kagan’s original samarium diiodide-THF method to include 5% HMPA as a cosolvent, which resulted in nearly quantitative yields for the reduction of alkyl iodides and bromides while shortening reaction times from hours or days to only minutes, and under milder reaction temperatures.102 Compared to the organotin hydrides, samarium diiodide offered the advantages of easier handling and a better compatibility with functional groups, such as alcohols and esters, that would otherwise be reduced by organotin hydrides. The samarium diiodide protocols also provided access to aryl radicals from aryl halides; an improvement from the organotin hydride approach.103 The synthetic utility of samarium diiodide for halide reduction and carbon-carbon bond formation has garnered much interest in the mechanism of the reactions. Deuterium labeling experiments demonstrated the apparent involvement of both radical and anionic mechanisms depending on the substrates investigated,102 and further mechanistic studies demonstrated the involvement of an intermolecular samarium Barbier reaction in the case of ketone coupling with alkyl halides and a samarium Grignard reaction in the case of both reductions and couplings of aryl and alkyl halides.104 Experimental evidence has also pointed to
30 the involvement of an intermediate samarium diiodide-HMPA complex
105 SmI2(HMPA)4 in an outer-sphere electron transfer mechanism. Recently, methodological developments of samarium diiodide chemistry have demonstrated its reduction capabilities in the presence of water, which has further expanded the synthetic utility of this versatile reagent.106
1.6.3 Silanes
Tris(trimethylsilyl)silane (TTMSS) was introduced by Chatgilialoglu et al. as a useful substitute for tin hydrides in dehalogenation reactions.107 The reagent allowed for the reduction of halides (Cl, Br, I), sulfides, selenides, primary and secondary alcohols, acyl derivatives, and nitroxides. Compared to the organotin hydride dehalogenation method, reductions with silanes give fewer side products and are compatible with water108,109 for both soluble and insoluble substrates
(Scheme 1.23). However, TTMSS is readily degraded by oxygen under ambient conditions, which necessitates inert gas handling techniques.
water-insoluble water-soluble product substrate product
(TMS)3SiH (1.2 - 2 eq.) (TMS)3SiH (1.2 eq.) ACCN (0.3 eq.) HOCH2CH2SH (0.3 eq.) H2O, 100 °C, 4 h ACCN (0.3 eq.) H O, 100 °C, 4 h 2 Scheme 1.23. Reductive dehalogenation (X: Br, I) with TTMSS in water. For both water- soluble and –insoluble substrates 1,1’-azobis-(cyclohexanecarbonitrile) (ACCN) serves as the radical initiator. Water-soluble substrates require the presence of catalytic quantitites of 2-mercaptoethanol.108
In the case of water-soluble substrates, the addition of an amphiphilic thiol is required for reduction to occur in heterogenous media.110
31 Reductions of alkyl halides with TTMSS in the presence of olefins formed carbon-carbon bonds, which demonstrated the radical nature of the reactions.111
The bond dissociation energy of TTMSS was determined through photolysis experiments to be 79.0 kcal/mol, which is similar to the 74 kcal/mol bond dissociation energy of tributyltin hydride.112 However, compared to tributyl tin hydride, this greater bond dissociation energy nonetheless necessitates higher reaction temperatures107 (100 °C) to initiate formation of the silyl radical. The low bond dissociation energy of TTMSS has been attributed to stabilization of the silyl radical of the central silicon atom through bonding interactions with the beta- silicon atoms.113 Attempted reductions of organohalide substrates using triethylsilane were unsuccessful in producing hydrocarbons,114 and the experimentally determined silicon-hydrogen bond dissociation energy of 90.1 kcal/mol112 for this silane justified the absence of reduction.
1.7 Research Objectives
Biological model compounds have played an important role in our understanding of biochemical processes. This project intends to investigate the reactivity of folate and NAD(P)H model compounds towards halogenated organic molecules. The synthetic applications of these models to organic chemistry have been investigated extensively for a variety of reducible substrates. However, the literature shows that reductions of halocarbons have not been investigated to any significant extent. Given the known toxicity of halocarbons, the results of the
32 studies are likely to provide insight into the rather obscure toxicology and environmental degradation of these compounds.
1.8 References
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33 18. Avendano, C.; Menendez, J. C. In Medicinal Chemistry of Anticancer Drugs; Elsevier Academic Press, 2015, p 20. 19. Selhub, J. Annu. Rev. Nutr. 1999, 19, 217. 20. Botto, L. D.; Yang, Q. Am. J. Epidemiol. 2000, 151, 862. 21. van Guldener, C. Nephrol. Dial. Transplant. 2006, 21, 1161. 22. Cattaneo, M. J. Thromb. Haemost. 1999, 81, 165. 23. Stanger, O.; Fowler, B.; Petrzik, K.; Huemer, M.; Haschke-Becher, E.; Semmler, A.; Lorenzl, S.; Linnebank, M. Expert Rev. Neurother. 2009, 9, 1393. 24. Dietrich-Muszalska, A.; Malinowska, J.; Olas, B.; Glowacki, R.; Bald, E.; Wachowicz, B.; Rabe-Jablonska, J. Neurochem. Res. 2012, 37, 1057. 25. Morris, M. S.; Lancet Neurol. 2003, 2, 425. 26. Sancar, A. Chem. Rev. 2003, 103, 2203. 27. Pauszek III, R. F.; Kodali, G.; Stanley, R. J. J. Phys. Chem. A. 2014, 118, 8320. 28. Rajagopalan, P. T. R.; Zhang, Z.; McCourt, L.; Dwyer, M.; Benkovic, S. J.; Hammes, G. G. Biochem. 2002, 99, 13481. 29. Shima, S.; Thauer, R. K. Chem. Rec. 2007, 7, 37. 30. Salomone-Stagni, M.; Stellato, F.; Whaley, C. M.; Vogt, S.; Morante, S.; Shima, S.; Rauchfuss, T. B.; Meyer-Klaucke, W. Dalton Trans. 2010, 39, 3057. 31. Geierstanger, B. H.; Prasch, T.; Griesinger, C.; Hartmann, G.; Buurman, G.; Thauer, R. K. Angew. Chem. Int. Ed. Engl. 1998, 37, 3300. 32. Zirngibl, C.; Hedderich, R.; Thauer, R. K. FEBS Lett. 1990, 261, 112. 33. Fontecilla-Camps, J.C.; Volbeda, A.; Cavazza, C.; Nicolet Y. Chem. Rev. 2007, 107, 4273. 34. Nakamura, M.; Bhatnagar, A.; Sadoshima, J. Circ. Res. 2012, 111, 604. 35. Buchner, E. Ber. Dtsch. Chem. Ges. 1897, 30, 117. 36. Manchester, K. L. Trends Biochem. Sci. 2000, 25, 89. 37. Berger, F.; Ramírez-Hernández, M. H.; Ziegler, M. Trends Biochem. Sci. 2004, 29, 111. 38. Warburg, O.; Christian, W. Biochem. Z. 1936, 287, 291. 39. Arnon, D. I.; Rosenberg, L. L.; Whatley, F. R. Nature. 1954, 173, 1132. 40. Imai, S.; Guarente, L. Trends Cell Biol. 2014, 24, 464.
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36 86. Wang, N.-X.; Zhao, J. Adv. Synth. Catal. 2009, 351, 3045. 87. Gębicki, J.; Marcinek, A.; Zielonka, J. Acc. Chem. Res. 2004, 37, 379. 88. Zhu, X.-Q.; Liu, Y.-C.; Cheng; J.-P. J. Org. Chem. 1999, 64, 8980. 89. Bunting, J.; Bioorg. Chem. 1991, 19, 456. 90. Chaikin, S. W.; Brown, W. G. J. Am. Chem. Soc. 1949, 71, 122. 91. Finholt, A. E.; Bond Jr., A. C.; Wilzbach, K. E.; Schlesinger, H. I. J. Am. Chem. Soc. 1947, 69, 2692. 92. Neumann, W. P.; Heymann, E.; Kleiner, F.; Lind, H.; Niermann, H.; Pedain, J.; Rübsaman, K; Schneider, B.; Sommer, R. Angew. Chem. 1964, 76, 849. 93. Giese, B. In Radicals in Organic Synthesis: Formation of Carbon-Carbon Bonds; Baldwin, J. E. Ed.; Pergamon Press, 1986, p 7. 94. Giese, B.; González-Gómez, J. A.; Witzel, T. Angew. Chem. Int. Ed. Engl. 1984, 23, 69. 95. Giese, B. In Radicals in Organic Synthesis: Formation of Carbon-Carbon Bonds; Baldwin, J. E. Ed.; Pergamon Press, 1986, p 155. 96. Bazukis, P.; Campos, O. O. S.; Bazukis, M. L. F. J. Org. Chem. 1976, 41, 3261. 97. Barton, D. H. R.; Crich, D.; Motherwell, W. B. J. Chem. Soc. Chem. Commun. 1983, 939. 98. Clive, D. L. J.; Wang, J. J. Org. Chem. 2002, 67, 1192. 99. Inoue, K.; Sawada, A.; Shibata, I.; Baba, A. J. Am. Chem. Soc. 2002, 124, 906. 100. Amrein, S.; Studer, A., Angew. Chem. Int. Ed. Engl. 2000, 39, 3080. 101. Girard, P.; Namy, J. L.; Kagan, H. B. J. Am. Chem. Soc. 1980, 102, 2693. 102. Inanaga, J.; Ishikawa, M.; Yamaguchi, M. Chem. Lett. 1987, 16, 1485. 103. Beckwith, A. L. J.; Gara, W. B. J. Chem. Soc. Perkin Trans. 2 1975, 795. 104. Curran, D. P.; Fevig, T. L.; Jasperse, C. P.; Totleben, M. J. Synlett. 1992, 12, 943. 105. Shabangi, M.; Kuhlman, M. L.; Flowers II, R. A. Org. Lett. 1999, 1, 2133. 106. Szostak, M.; Spain, M.; Parmar, D.; Procter, D. J. Chem. Commun. 2012, 48, 330. 107. Chatgilialoglu, C.; Griller, D.; Lesage, M. J. Org. Chem. 1988, 53, 3641. 108. Postigo, A.; Kopsov, S.; Ferreri, C.; Chatgilialoglu, C. Org. Lett. 2007, 9, 5159. 109. Barata-Vallejo, S.; Postigo, A. J. Org. Chem. 2010, 75, 6141.
37 110. Ferreri, C.; Samadi, A.; Sassatelli, F.; Landi, L.; Chatgilialoglu, C. J. Am. Chem. Soc. 2004, 126, 1063. 111. Ballestri, M.; Chatgilialoglu, C.; Clark, K. B.; Griller, D.; Giese, B.; Kopping, B. J. Org. Chem. 1991, 56, 678. 112. Kanabus-Kaminska, J. M.; Hawari, J. A.; Griller, D.; Chatgilialoglu, C. J. Am. Chem. Soc. 1987, 109, 5267. 113. Chatgilialoglu, C. Acc. Chem. Res. 1992, 25, 188. 114. Giese, B.; Kopping, B. Tetrahedron Lett. 1989, 30, 681.
38
Chapter 2
Reduction of Halocarbons by Folate Models
39 2.1 Introduction
The structural similarity between 5,10-methylenetetrahydrofolate (MTHF)
2H1–6 and methenyl-H4MPT 3H7–9 led us to investigate imidazolidines 1H as model compounds for the reactivity of these biomolecules (Figure 2.1).
H R H N
N R 1H
H H H H O N O N
N N CH3 HN HN H H
H2N N N H2N N N CH H H 3 2H 3H
Figure 2.1. Structural relationship between imidazolidines 1H, 5,10- methylenetetrahydrofolate 2H, and methenyl-H4MPT 3H.
A type of reactivity shared by 2H and 3H is the formal transfer of negatively charged “hydridic” hydrogen leading to the very stable cationic species [2]+ and
[3]+ (Figure 2.2).
H H O N O N
N N CH3 HN HN H H
H2N N N H2N N N CH H H 3 [2]+ [3]+
Figure 2.2. Oxidized cationic forms of 5,10-methylenetetrahydrofolate [2]+ and methenyl- H4MPT [3]+.
40 2.2 Results
Previous studies by our group10,11 demonstrated that imidazolidines 1H (R: methyl, tert-butyl)12 are unstable in chloroform and resemble many organometallic compounds and hydridocomplexes in this respect. A closer examination revealed that the reason for this instability was the reduction of chloroform to dichloromethane by 1H acting as a hydride donor. The byproduct of the reduction is the salt [1]+Cl-. An excess of the halogenated hydrocarbon and prolonged reaction times lead to further oxidation of [1]+X- to [1’]+X- (Scheme
2.1).
X X H R R R H H H N N N R'-X R'-X N - R'-H N - R'-H N R R R + - 1H [1] X [1’]+X-
Scheme 2.1. Oxidation of imidazolidines 1H by halocarbons R’-X.
This type of reduction by 1H is remarkably general and allows for the dehalogenation of a wide variety of halogenated hydrocarbons including those of toxicological, environmental, and pharmacological significance.
To further explore the scope of the reductive dehalogenation the reactivity of 1H towards a series of chloromethanes and bromomethanes was investigated first (Table 2.1). With the exceptions of dichloromethane, chloromethane, and bromomethane, all chlorine- and bromine-substituted methanes are reduced by
1H. The reductions occur at room temperature, and the time required for a complete conversion ranges from weeks for chloroform to only minutes for
41 bromoform and carbon tetrabromide.
Table 2.1. Reactivity of halomethanes (A → B) with 1.1 eq. of 1H (R: tert-butyl).
A B Temperature (°C) Time 1st ppt.
CI4 CHI3 25 inst.
CBr4 CHBr3 25 inst.
CCl4 CHCl3 25 10 sec
CHI3 CH2I2 25 inst.
CHBr3 CH2Br2 25 3 sec
CHCl3 CH2Cl2 25 3 d 25 no ppt. CH2Cl2 CH3Cl 200 no ppt.
CFCl3 CFHCl2 25 30 min
The reactions showed the rate of halide reduction followed the order I > Br >>> Cl for all cases. CBr4, CCl4, CHBr3, CHCl3, and CH2Br2 are completely reduced, even at room temperature, while CH2Cl2 requires extensive heating to obtain reduction.
Variation of solvent polarity with diethyl ether, benzene, and 1,4-dioxane, an excess of 1H, or an excess of the halogenated methane showed no significant effect on the rates of the reductions.
The successful reductions of halomethanes encouraged us to study a range of other halocarbons. Initially, 1,3,5-tribromobenzene was investigated, and its rapid conversion to 1,3-dibromobenzene by 1H at 50 °C demonstrated the potential synthetic utility of the reactions. Other aryl halides were subsequently examined, and the rates of halide reduction were observed to follow the same order as the halomethanes, I > Br >>> Cl (Table 2.2).
42 Table 2.2. Reactivity of aryl halides (A → B) with 1.1 eq. of 1H (R: tert-butyl).
A B Temperature (°C) Time 1st ppt.
Br Br Br 50 5 min Br Br
Br Br Br
Br Br 60 20 min
Br Br Br 100 3 h
Br 150 1 w
I I 110 10 min I
I I 110 10 min I
I
150 2 d
Cl Cl Cl none 120 no ppt. Cl Cl
Cl none 200 no ppt.
Reaction of 1H with 1,2,4-tribromobenzene was of interest due to the three possible reduction products that could form. 13C NMR revealed that 1,3- dibromobenzene is formed as the major component (~90 %) with 1,4- dibromobenzene as the minor product. No 1,2-dibromobenzene was generated, and the reduction of 1,2,4-tribromobenzene is clearly regioselective. As an extension of this unique reactivity, 1-bromoadamantane was investigated as a substrate to determine the potential of reducing tertiary bridgehead halides.
43 Heating at 150 °C for one week formed adamantane, although approximately half of the starting substrate remained.
To explore the potential biochemical and toxicological implications of the reductions with the MTHF model compounds the reaction of the anesthetic and greenhouse molecule halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) with 1H was investigated. Reaction occurred within minutes at room temperature, and analysis with 19F NMR showed the selective reduction of the carbon-bromine bond to form 2-chloro-1,1,1-trifluoroethane. Reactions of 1H and other vicinal alkyl dihalides were also investigated (Table 2.3).
Table 2.3. Reactivity of alkyl vicinal halides (A → B) with 1.1 eq. of 1H (R: tert-butyl).
A B Temperature (°C) Time 1st ppt.
Br H H H F F Cl Cl 25 5 min F F F F I 25 15 sec I Br 25 30 min Br Br Cl Cl Cl 100 1 w Br Br Br Br 100 2 h
Br 60 30 min Br Br Cl Br Br Cl Br Cl Cl 100 3 h
Br Br Br Br Br 25 5 min Br Br
44 1,2-Diiodoethane was rapidly reduced at room temperature, and monitoring of the reaction with NMR revealed its complete conversion to ethene within five minutes at room temperature. As expected, the reaction of 1,2-dibromoethane occurred slower, and 1,2-dichloroethane required weeks of extended heating before reaction was observed. Interestingly, vinyl halides were the reduction products obtained from these compounds. In the case of 1,2-dibromopropane, selective
3 formation of cis-1-bromopropene was inferred from the 6.7 Hz JHH coupling
3 constant characteristic of cis alkenes. JHH coupling constants for trans isomers are typically larger than 12 Hz. To examine an application of this vicinal alkyl dihalide reactivity, the flame-retardant 1,2,5,6,9,10-hexabromocyclododecane, purchased as a mixture of stereoisomers, was reacted with 1H (R: tert-butyl). NMR showed the formation of [1]+Br- after several minutes at 75 °C and GC-MS and NMR revealed a complex product spectrum.
Reactions of 1H with alkylating agents, such as bromomethane and allyl bromide, did not generate reduced hydrocarbons, and instead formed mixtures of
N-alkylated ethylenediamine compounds (Table 2.4). Although multiple substitution products were obtained for some substrates, the compounds listed in
Table 2.4 represent the most abundant product identified by GC-MS in each case.
[1]+X- was formed in approximately equal amount as the ring-opened product
(NMR), which indicated that reduction of a halogenated intermediate occurred.
45 Table 2.4. Reaction temperatures and time until the first appearance of precipitate to form ring-opened products of 1H (R: tert-butyl) with alkylating agents.
A B Temperature (°C) Time 1st ppt.
Me tBu N Bromomethane 25 5 sec Me N tBu Me tBu N Bromohexane 60 5 min nHex N tBu Me tBu N Allyl bromide 90 10 min N tBu Me tBu N Benzyl bromide 60 10 min N Ph tBu Me tBu N Ethyl-2-bromoacetate 60 5 min O N tBu O Me tBu N O Benzoyl chloride 25 3 min N Ph tBu
To circumvent N-alkylation a second MTHF model 1H (R: para-tolyl) with reduced nucleophilicity on nitrogen was investigated. In each case, carbenium salt was formed; however, the product spectra were very complex, and no reduction products could be detected.
To investigate if the lack of reaction observed for some of the halogenated
46 substrates was due to thermodynamic reasons or was the result of kinetic factors the reaction energies ΔG° for the halide–hydride exchange were determined by quantum chemical calculations (Table 2.5).13 A computational solvation model for water was used in order to simulate biological conditions.14
Table 2.5. Computational free energies ΔG° (CBS-QB3 level,15 H2O-SMD solvent model,14 kcal/mol) for the reductions of halomethanes (A → B) by 1H (R: tert-butyl).
ΔG° A B X: Cl X: Br
CX4 CHX3 -48.3 -37.9
CHX3 CH2X2 -44.3 -39.1
CH2X2 CH3X -41.0 -36.5
CH3X CH4 -39.8 -29.3 PhX PhH -35.9 -31.3
The calculated energies show that all reactions are thermodynamically favorable.
The lack of reduction to hydrocarbons observed for bromomethane, chloromethane, and dichloromethane is accordingly the result of low reaction rates. The observed reaction rates show no correspondence to the calculated free energies of reaction ΔG°, but closely match the order of decreasing carbon– halogen bond energies (CBS-QB3 level, kcal/mol): CH3Br (77.1), CHCl3 (77.0),
CH2Br2 (71.7), CCl4 (71.9), CHBr3 (65.3), CBr4 (62.9). It must, therefore, be concluded that the breaking of the carbon-halogen bond is the rate-determining step of the reductions.
Identical calculations to determine the reaction energies for the reductions of chloromethanes by 2H were also performed (Table 2.6). As with MTHF models
1H, the free energies of reduction are all thermodynamically favorable.
47 Table 2.6. Computational free energies ΔG° (CBS-QB3 level,15 H2O-SMD solvent model,14 kcal/mol) for the reductions of halomethanes (A → B) by MTHF 2H.
A B ΔG°
CCl4 CHCl3 -31.7
CHCl3 CH2Cl2 -27.8
CH2Cl2 CH3Cl -24.4
CH3Cl CH4 -23.2
2.3 Conclusion
While in vivo studies will inevitably be required to further study the biochemical implications of these findings, the reactivity of the folate model 1H does suggest that folate-dependent biochemical pathways are likely to be disrupted by halogenated hydrocarbons. The reductive dehalogenation reactions described in this thesis are highly unusual for a simple, neutral, and stable organic molecule like 1H, but have precedence in the fascinating reactivity of dehalohydrogenases.16,17 Dehalohydrogenases have been isolated from a variety of microorganisms16–18 and have attracted great interest for their ability to reduce a number of highly toxic, halogenated hydrocarbons like the notorious 2,3,5,6- tetrachlorodibenzodioxin.19 A notable difference between the model system 1H and dehalohydrogenases reported to date is that the latter require metal- containing cofactors like iron-sulfur proteins or cobalamines.20,21 The fact that dehalohydrogenase-type activity is possible with the metal-free folate models 1H suggests that metal-free dehalohydrogenases may exist as well.
The ready synthetic accessibility of the model compounds 1H as well as their dehalogenation activity in the presence of air and moisture recommend their use for the remediation of toxic halogenated hydrocarbons. The reductions
48 constitute a new synthetic approach for dehalogenation. The high reactivity and selectivity demonstrated by 1H suggest that they may become a new substitute for organotin hydrides and silanes previously used for selective debrominations. A significant advantage would be that the conversions to hydrocarbons proceed in the presence of air and moisture.
2.4 References
1. Kallen, R. G.; Jencks, W. P. J. Biol. Chem. 1966, 241, 5851. 2. Jaenicke, L. Angew. Chem. 1961, 73, 449. 3. Jaenicke, L. Annu. Rev. Biochem. 1964, 33, 287. 4. Fox, J. T.; Stover, P. J. In Folic Acid and Folate; Litwack, G. Ed.; Elsevier Academic Press Inc., San Diego, 2008, p 1-44. 5. Quinlivan, E. P.; Hanson, A. D.; Gregory, J. F. Anal. Biochem. 2006, 348, 163. 6. Sancar, A. Chem. Rev. 2003, 103, 2203. 7. Pauszek III, R. F.; Kodali, G.; Stanley, R. J. J. Phys. Chem. A. 2014, 118, 8320. 8. Shima, S.; Thauer, R. K. Chem. Rec. 2007, 7, 37. 9. Geierstanger, B. H.; Prasch, T.; Griesinger, C.; Hartmann, G.; Buurman, G.; Thauer, R. K. Angew. Chem. Int. Ed. Engl. 1998, 37, 3300. 10. Denk, M. K.; Gupta, S.; Brownie, J.; Tajammul, S.; Lough, A. J. Chem.-Eur. J. 2001, 7, 4477. 11. Denk, M. K.; Rodezno, J. M.; Gupta, S.; Lough, A. J. J. Organomet. Chem. 2001, 617-618, 242. 12. Rabe, E.; Wanzlick, H.-W. Liebigs Ann. Chem. 1973, 0, 40. 13. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery Jr, J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.;
49 Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. GAUSSIAN 09 (Revision D.01), Gaussian Inc., Wallingford, CT, 2013. 14. Marenich, A.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B, 2009, 113, 6378. 15. Montgomery, J. A.; Frisch, M. J.; Ochterski, J. W.; Petersson, G. A. J. Chem. Phys. 2000, 112, 6532. 16. Adriaens, P.; Grbić-Galić, D. Chemosphere. 1994, 29, 2253. 17. Adrian, L.; Szewzyk, U.; Wecke, J.; Görisch, H. Nature. 2000, 408, 580. 18. Dolfing, J.; van den Wijngaard, A. J.; Janssen, D. B. Biodegradation. 1993, 4, 261. 19. Smidt, H.; de Vos, W. M. Annu. Rev. Microbiol. 2004, 58, 43. 20. Bommer, M.; Kunze, C.; Fesseler, J.; Schubert, T.; Diekert, G.; Dobbek, H. Science. 2014, 346, 455. 21. Payne, K. A.; Quezada, C. P.; Fisher, K.; Dunstan, M. S.; Collins, F. A.; Sjuts, H.; Levy, C.; Hay, S.; Rigby, S. E.; Leys, D. Nature. 2015, 517, 513.
50
Chapter 3
Reductive Dehalogenation of DDT by Folate Models
51 3.1 Introduction
First described in 1874,1 DDT only gained widespread use after Paul
Herrmann Müller identified it as a highly potent insecticide.2 The use of DDT as an effective suppressant for insect-borne diseases, such as typhus and malaria, was recognized by the Nobel Prize in Physiology or Medicine in 1948. The ability of
DDT3 and its metabolites to act as endocrinological disruptors towards estrogen receptors3,4 and androgen receptors5 resulted in the ban of DDT in the United
States and most other countries. Despite legal restrictions on its production and use, the great persistence of DDT and its metabolites in the environment represents an on-going concern,6–8 which has made DDT one of the most studied insecticides from both environmental and toxicological perspectives.
While details of the metabolic degradation of DDT depend on the organism, the reduction of the carbon-chlorine bonds is common for all organisms studied to date.7–10 The existence of this reductive pathway was demonstrated by Kallman &
Andrews through in vivo studies with yeasts as early as 1963.10 In 1964, Datta,
Laug, and Klein confirmed the operation of the dehalogenation metabolism for
DDT in mammals.11 Subsequent studies demonstrated the dominance of the dehalogenation pathway for a wide range of chlorinated halocarbons, in addition to DDT.12 Remarkably, the agent involved in the reductive dehalogenation of DDT has remained obscure.
Reduced cytochromes were postulated as reducing factors by Wedemeyer in 1966,13 and the subsequent finding that simple iron porphyrin models are capable of reducing DDT in vitro14–16 can be taken as supporting evidence.
52 Nicotinamide adenine dinucleotide phosphate (NADPH) was implied as the metabolic reducing agent by K. A. Hassel in 1972,17 while subsequent studies favored the involvement of flavin adenine dinucleotide (FADH).18–20 Remarkably, the ubiquitous hydride donor methylenetetrahydrofolate (MTHF) 2H does not seem to have been implicated in the reductive metabolism of DDT.
3.2 Results
The observation that MTHF models are capable of reducing a wide range of halocarbons suggested that DDT might likewise be reduced. Our experiments demonstrated that MTHF models 1H not only reduce DDT under biomimetic conditions but also produce the five most common DDT metabolites. Using two standard protocols, one aerobic and one anaerobic, the reduction of DDT by the
MTHF models 1H (R: tert-butyl, para-tolyl) formed the metabolites 3.4–3.8 identified by previous in vivo studies (Figure 3.1).
Cl Cl Cl Cl Cl Cl Cl
Cl Cl Cl Cl Cl Cl DDT DDD DDE 3.3 3.4 3.5
H Cl O
Cl Cl Cl Cl Cl Cl DDMU DDM DBP 3.6 3.7 3.8
Figure 3.1. DDT (1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane 3.3, and its metabolites DDD (1,1-dichloro-2,2-bis(4-chlorophenyl)ethane) 3.4, DDE (1,1-dichloro-2,2-bis(4- chlorophenyl)ethene) 3.5, DDMU (1-chloro-2,2-bis(4-chlorophenyl)ethene) 3.6, DDM (bis(4-chlorophenyl)methane) 3.7, and DBP (4,4’-dichlorobenzophenone) 3.8.
53 Comparison of aerobic and anaerobic conditions revealed that reaction of
DDT and 1H gave 3.4, 3.5, 3.6, and 3.7 under anaerobic conditions. Under aerobic conditions,22,23 metabolite 3.8 has been observed as well23 (Scheme 3.1). The formation of DBP 3.8 is of particular interest as it has previously been attributed to the oxidation of DDT by cytochrome P450.24 Our observations show that 3.8 can be obtained from MTHF models 1H under aerobic conditions without the involvement of cytochrome P450.
To map the degradation pathways in detail the reactions of the individual
DDT metabolites 3.4–3.8 with 1H (R: tert-butyl) were examined. The observed degradation pathways are summarized in Scheme 3.1. DDMU 3.6 forms from both
DDD 3.4 and DDE 3.5 and is the final stage of the degradation processes under both aerobic and anaerobic conditions.
Surprisingly, DDM 3.7 does not form from any of the metabolites 3.4, 3.5,
3.6, 3.8, and must accordingly form directly from DDT 3.3. DBP 3.8 is obtained under aerobic conditions from DDT 3.3 and from the pure metabolites 3.4–3.6.
However, as its formation from DDT 3.3 is much faster than from the pure metabolites 3.4–3.6, its direct formation from 3.3 must be assumed as the dominant pathway.
54 DDD DDD
DDT DDMU DBP + DDMU Folate Models
DDE DDE
Aerobic Conditions DDM Anaerobic Conditions
Aerobic & Anaerobic Conditions
Scheme 3.1. Degradation pathways of DDT by folate models 1H. Aerobic and anaerobic conditions degrade DDT to DDMU via DDD and DDE. In the presence of oxygen, DBP forms rapidly from DDT. Prolonged reaction times show that DBP forms slowly from DDE, DDE, and DDMU. DDM is formed directly from DDT under aerobic and anaerobic conditions.
To confirm the generality of our findings a second MTHF model 1H with an aryl substituent on nitrogen (R: para-tolyl) was investigated. The spectrum of DDT metabolites obtained was identical but the formation of all metabolites was slower. It is noteworthy that the in vitro degradation rates observed for both 1H models are in good qualitative agreement with the metabolic half-life of DDT25 observed by in vivo studies.
3.3 Conclusion
The reduction of DDT by the MTHF models 1H to a series of well-known metabolites is strong evidence that MTHF is the biological reducing agent involved in the metabolism of DDT. The findings further suggest the capability of DDT and its metabolites to act as disruptors towards folate-dependent pathways.
The ready accessibility of the MTHF models 1H and their capability to degrade DDT in the presence of air and water suggest their use for the remediation27 of DDT waste sites.
55 3.5 References
1. Zeidler, O. Ber. Dtsch. Chem. Ges. 1874, 7, 1180. 2. Läuger, P.; Martin, H.; Müller, P. H. Helv. Chim. Acta. 1944, 27, 892. 3. Bitman, J.; Cecil, H. C.; Harkis, S. J.; Fries, G. F. Science. 1968, 162, 371. 4. Gellert, R. J.; Heinrichs, W. L.; Swerdloff, R. S. Endocrinology. 1972, 91, 1095. 5. Kelce, W. R.; Stone, C. R.; Laws, S. C.; Gray, L. E.; Kemppainen, J. A.; Wilson, E. M. Nature. 1995, 375, 581. 6. Longnecker, M. P.; Rogan, W. J.; Lucier, G. Annu. Rev. Public Health. 1997, 18, 211. 7. Beard, J. Sci. Total Environ. 2006, 355, 78. 8. Tiemann, U. Reprod. Toxicol. 2008, 25, 316. 9. Smith, J. N. Annu. Rev. Entomol. 1962, 7, 465. 10. Kallman, B. J.; Andrews, A. K. Science. 1963, 141, 1050. 11. Datta, P. R. Laug, E. P.; Klein, A. K. Science. 1964, 145, 1052. 12. Mohn, W. W. Tiedje, J. M. Microbiol. Rev. 1992, 56, 482. 13. Wedemeyer, G. Science. 1966, 152, 647. 14. Miskus, P.; Blair, D. P.; Casida, J. E. J. Agr. Food Chem. 1965, 13, 481. 15. Castro, C. E. J. Am. Chem. Soc. 1964, 86, 2310. 16. Mansuy, D.; Lange, M.; Chottard, J. C. J. Am. Chem. Soc. 1978, 100, 3213. 17. Hassel, K. A. Pestic. Biochem. Physiol. 1972, 1, 259. 18. Esaac, E. G.; Matsumura, F. Pestic. Biochem. Physiol. 1980, 13, 81. 19. Sugihara, K.; Kitamura, S.; Ohta, S. Biochem. Mol. Biol. Int. 1998, 45, 85. 20. Wedemeyer, G. Appl. Microbiol. 1967, 15, 569. 21. Denk, M. K.; Milutinović, N. S.; Marczenko, K. M.; Sadowski, N. M.; Paschos, A. Chem. Sci. 2017, 8, 1883. 22. Subba-Rao; R. V.; Alexander, M. Appl. Environ. Microbiol. 1985, 49, 509. 23. Langlois, B. E.; Collins, J. A.; Sides, K. G. J. Dairy Sci. 1970, 53, 1671. 24. Xiao, P. F.; Mori, T.; Kamei, I.; Kondo, R. Biodegradation. 2011, 22, 859.
56 25. DDT and Its Derivatives: Environmental Aspects (EHC 83, 1989). World Health Organization, Geneva.
57
Chapter 4
Reaction of Halocarbons with NAD(P)H Models
58 4.1 Introduction
The reduction of halocarbons by folate model compounds 1H poses the question whether other biochemical hydride transfer systems might likewise be capable of reducing halocarbons. Nature’s ubiquitous reducing agent, NAD(P)H, was chosen as the second system for investigation for this thesis. Throughout a long and venerable history beginning with Hantzsch esters 6H/[6]+ (Figure 4.1),
NAD(P)H models have been shown to reduce a variety of functional groups including imines,1 nitro compounds,2 and carbonyl compounds.3,4 However, to the best of our knowledge, reductions of carbon-halogen bonds have not been reported.
O O O O O O
NH2 RO OR NH2 RO OR
N N N N R R R R 4H 6H [4]+ [6]+
O NH2 O NH2
N N NH2 N NH2 N
N O O N N N O O N N O P O P O O P O P O O O O O O O O O
OH OH OH R OH OH OH R 5H [5]+
Figure 4.1. Structural relationship between the 1,4-dihydronicotinamide system 4H/[4]+, Hantzsch ester system 6H/[6]+, and NAD(P)H/NAD(P)+ 5H/[5]+. For NADH/NAD+, R: H; for NADPH/NAD(P)+, R: PO43-.
The functions of NAD(P)H/NAD(P)+ in metabolic processes are well understood,5 but evidence for the roles of these systems in the reductive metabolism of anthropogenic compounds, including halocarbons, is lacking. In order to examine the interaction of halocarbons with NAD(P)H/NAD(P)+ 5H/[5]+, the 1,4-
59 dihydronicotinamide system 4H/[4]+ was chosen as a convenient in vitro model system (Figure 4.1).
4.2 Results
The halocarbons investigated in our previous studies with folate models
1H6 (chapter 3) served as the starting point for the investigation of NAD(P)H models 4H. Like the folate models, the NAD(P)H models rapidly reduce halocarbons to hydrocarbons by formally acting as hydride donors. Byproducts of the reactions are the pyridinium salts [4]+X- (Scheme 4.1).
O O
NH2 R’-X NH2
N - R’-H N X R R + 4H [4]
Scheme 4.1. Reduction of halocarbons R’-X by 1,4-dihydronicotinamides 4H.
Several 1,4-dihydronicotinamides 4H (R: Et, Me, Bn, nBu) were initially synthesized7 to be tested as NAD(P)H models. For R: Me, Bn the models 4H were found to have very low solubility in non-polar as well as polar solvents, respectively. For R: Me, 4H could not be isolated in sufficient quantity for further reactivity studies. For R: nBu, the synthesis of 4H8 led to a mixture of products that proved difficult to separate. Only the synthesis of 4H (R: Et) from [4]+Br- by reduction with dithionite gave a pure product in acceptable yield (15 %).
Formation of [4]+Br- from nicotinamide and ethyl bromide was quantitative.
Subsequent attempts to improve the yield in the reduction step by variation of the
60 reaction conditions were unsuccessful. The simple 1H NMR spectrum and its solubility in both polar and non-polar solvents made 4H (R: Et) the most suitable model. An investigation to determine the stability of 4H (R: Et) in air demonstrated its air sensitive nature. After several days of exposure, the 1H NMR spectrum showed formation of the oxidized nicotinamide cation [4]+. Heating 4H in a flame-sealed NMR tube in C6D6 under inert atmosphere resulted in a slow degradation of 4H at approximately 110 °C. The limited thermal stability of 4H accordingly requires lower reaction temperatures than the folate models 1H.
To explore the scope of the reductive dehalogenation the reactivity of 4H
(R: Et) towards a series of chloromethanes and bromomethanes was investigated
(Table 4.1).
Table 4.1. Reaction temperatures and time until the first appearance of precipitate for the reductions of halomethanes (A → B) with 1.1 eq. of 4H (R: Et) in benzene.
A B Temperature (°C) Time 1st ppt.
CI4 CHI3 25 inst.
CBr4 CHBr3 25 inst.
CHBr3 CH2Br2 25 30 min
CCl4 CHCl3 25 2 min 25 1 w CHCl3 CH2Cl2 90 3 d
The reactions showed the rate of halide reduction followed the order I ≈ Br >> Cl for all cases. CBr4, CCl4, CHBr3, and CHCl3 were reduced, even at room temperature, while CH2Br2 and CH2Cl2 could not be reduced below the decomposition temperature of 4H, even with extended heating. Variation of solvent polarity with benzene and 1,4-dioxane, an excess of 4H, or an excess of the
61 halogenated methane showed no significant effect on the rates of the reductions.
To investigate if the lack of reduction observed for some of the halomethanes was due to thermodynamic reasons or was the result of kinetic factors the reaction energies ΔG° for the halide–hydride exchange were determined9 by high precision quantum chemical calculations (Table 4.2).10 The thermochemically accurate SMD solvent model of Trular and Marenich11 was used to predict the thermochemistry.
Table 4.2. Computational free energies ΔG° (CBS-QB3 level,12 H2O-SMD solvent model,11 kcal/mol) for the reductions of halomethanes (A → B) by 4H (R: Me).
ΔG° A B X: Cl X: Br
CX4 CHX3 -40.0 -32.2
CHX3 CH2X2 -36.1 -32.8
CH2X2 CH3X -32.8 -30.5
CH3X CH4 -31.5 -22.8 PhX PhH -27.7 -23.1
The computational free energies showed that all reductions are thermodynamically favorable. The lack of reactivity observed for bromomethane, chloromethane, dibromomethane, and dichloromethane is, accordingly, the result of low reaction rates.
Subsequent investigations of the reactivity of 4H towards aryl halides, which were successfully reduced by folate models 1H, showed no signs of reduction. Compared to our folate models 1H, the results show a generally lower reactivity of the NAD(P)H models 4H towards halocarbons.
In order to explore the specifics of halocarbon reductions with 4H, a series
62 of vicinal alkyl dihalides was investigated (Table 4.3).
Table 4.3. Reactivity and selectivity of selected vicinal halocarbons (A → B) with 1.1 eq. of 4H (R: Et) per reducible halogen atom in benzene.
A B Temperature (°C) Time 1st ppt.
Br H H H F F Cl Cl 25 5 min F F F F H
I H 25 1 min I H H H Br H Br H 90 3 d H Cl Cl Cl 90 2 w Br Br 90 3 d
Br Br Br Br Br Br Br 25 1 min Br Br Br Br Br Br Cl 90 3 d Cl Br Cl Cl
Halothane and 1,2-diiodoethane were rapidly reduced to 2-chloro-1,1,1- trifluoroethane and ethene, respectively, while 1,2-dichloroethane was slowly converted to vinyl chloride. This product spectrum is identical to that obtained from the folate reductions. By way of contrast, reactions of 1,2-dibromoalkanes with 4H gave only propene while reductions with 1H also resulted in the formation of vinyl bromides (Scheme 4.2).
63 Br 4H
Br 1H Br + + Br
Scheme 4.2. Reduction of 1,2-dibromo alkyl halides by 1H and 4H.
Additional insight into the qualitative differences in the reductions with 1H versus
4H was obtained from reduction of 1,1,2,2,-tetrabromoethane. 4H generated
1,1,2-tribromoethene, cis-1,2-dibromoethene, and trans-1,2-dibromoethene in a ratio of approximately 20 %, 60 %, and 20 % (1H NMR) respectively, while 1H formed only 1,1,2-tribromoethene (Scheme 4.3).
Br Br Br
4H Br Br H + + H H H Br Br H Br Br Br Br Br 1H Br H Br
Scheme 4.3. Reduction of 1,1,2,2-tetrabromoethane by 1H and 4H.
A particularly surprising result with potentially valuable mechanistic implications was obtained in the reduction of 1-bromo-2-chloroethane. The substrate was converted to 1,2-dibromoethane and 1,2-dichloroethane with trace amounts of vinyl chloride (Scheme 4.4).
Cl Br Cl Cl 4H + +
Br Br Cl
Scheme 4.4. Reduction of 1-bromo-2-chloroethane by 4H.
64 This halogen scrambling, which was previously discovered in our group for the folate models 1H is consistent with a radical mechanism for the reductions.
Judging by the reactions of 4H with 1,2-dibromoethane and 1,2-dichloroethane, formation of ethene and vinyl chloride was the expected outcome. To the best of our knowledge, halogen scrambling has rarely been reported in the literature and only for aryl halides according to a polar mechanism involving carbanions.13
Halocarbons that were previously found to alkylate folate models 1H were also tested with the NAD(P)H models 4H. 4H (R: Bn), the first model investigated, reacted slowly with ethyl bromoacetate and benzyl bromide at room temperature to form the salt [4]+Br-. Heating the reactions generated trace amounts of the reduced products, and led to a complex reaction mixture dominated by aromatic signals in the NMR spectra. However, the reaction of ethyl bromoacetate with 4H
(R: Et) proceeded cleanly and led to the exclusive formation of the reduction product ethyl acetate and [4]+Br- (Scheme 4.5).
O 4H O
Br + - O - [4] Br O
Scheme 4.5. Reduction of ethyl bromoacetate by 4H.
Surprisingly, allyl bromide was inert toward 4H (R: Et) at room temperature, as well as after extended heating to 80 °C.
To follow up on our earlier study investigating the reactivity of folate models 1H towards DDT 3.3, the reaction of NAD(P)H models 4H was carried out.
Under anaerobic conditions, DDT 3.3 was cleanly converted to the metabolites
65 DDD 3.4, DDE 3.5, and DDMU 3.6, which are also formed from the folate models
1H and DDT (Scheme 4.6).
DDD DDD
DDT DDMU + DDMU NAD(P)H Models
DDE DDE
Aerobic Conditions Anaerobic Conditions
Scheme 4.6. Reductive degradation pathways in the reaction of DDT with NAD(P)H models 4H.
An important difference between 4H and 1H is the fact that the former did not produce the known metabolites DDM (bis(4-chlorophenyl)methane) 3.7 or DBP
(4,4’-dichlorobenzophenone) 3.8 that are formed in the reaction of DDT with the folate models 1H. These results suggest that, like 5,10-MTHF 2H, NAD(P)H 5H can, in principle, also be involved in the still obscure reductive metabolism of DDT. The involvement of NADPH was in fact postulated by Hassel as early as 1972.14
During the course of the investigations with the models 4H, it was proposed that the halocarbon reductions might be carried out in a catalytic fashion by in situ regeneration of 4H by sodium dithionite – the reducing agent also employed for the synthesis of 4H from [4]+Br- (Scheme 4.7).
66 Na2S2O4
[4]+ 4H (7.5 mol %)
CHBr3 CBr4
Scheme 4.7. Proposed catalytic reduction of carbon tetrabromide with 4H.
To test the idea, carbon tetrabromide, sodium dithionite, and a catalytic quantity
(7.5 mol %) of [4]+Br- (R: Et) were reacted in a biphasic system consisting of alkaline water/benzene. After 18 hours at room temperature, carbon tetrabromide was completely consumed. GC-MS and 13C NMR showed the exclusive formation of bromoform and 1,1,2,2-tetrabromoethene in approximately equal proportion (Scheme 4.8).
Br i) CBr CHBr + Br 4 3 Br Br
Scheme 4.8. Attempted catalytic reduction of carbon tetrabromide with [4]+Br- (R: Et). i)
Na2S2O4 (1.1 eq.), [4]+Br- (R: Et) (7.5 mol %), NaHCO3 (4 eq.), benzene (12 mL), deionized water (25 mL).
To test whether dithionite alone without [4]+/4H might be responsible for the observed debromination the reaction of carbon tetrabromide with sodium dithionite was repeated without [4]+ but with otherwise identical reaction conditions (Scheme 4.9).
67 Br i) CBr CHBr + Br 4 3 Br Br
Scheme 4.9. Control reaction of sodium dithionite and carbon tetrabromide. i) Na2S2O4
(1.1 eq.), NaHCO3 (4 eq.), benzene (12 mL), deionized water (25 mL).
The spectrum of reduced products was identical to that obtained with [4]+/4H, which serves to demonstrate that sodium dithionite alone was the agent responsible for the reduction of carbon tetrabromide. The result is in agreement with previous applications of dithionite for direct reduction of organochloride compounds.15,16 Although catalytic reduction of halocarbons with 1H and 4H was not achieved, the carbon-carbon bond formation in the course of the reduction of halocarbons with dithionite has not been reported previously and warrants further investigation.
During the course of investigations with NAD(P)H/NAD(P)+ models
4H/[4]+ and folate models 1H/[1]+, we were also interested to test the possibility of a direct hydride transfer between the two systems. Apart from mechanistic implications, such a transfer would also demonstrate the possibility of an as yet undocumented biochemical pathway directly linking NAD(P)H/NAD(P)+ 5H/[5]+ and 5,10-MTHF 2H/[2]+. Initially, the free energies of reaction ΔG° for the hydride exchange were determined9 by quantum chemical calculations (Table 4.4).10 A computational solvation model for water was used to simulate biological conditions.11
68 Table 4.4. Computational free energies ΔG° (CBS-QB3 level,12 H2O-SMD solvent model,11 kcal/mol) for hydride transfer between folates H and NAD(P)+ models [4]+.
Folate H NAD(P)+ model [4]+ ΔG° 1H (R: tBu) R: Me -8.22 1H (R: Ph) R: Me +5.47 2H R: Me -8.36
The data suggest that 5,10-MTHF 2H should be a stronger hydride donor and that
1H should be capable of reducing NAD(P)+ models [4]+. However, equilibrium also depends on nitrogen substituents of 1H. For 1H (R: tBu) the reaction is predicted to reduce [4]+, while the reverse is predicted for 1H (R: Ph). For the combination of 1H (R: tBu) and [4]+ (R: Et), the predicted hydride transfer was, indeed, confirmed experimentally (Scheme 4.10).
tBu O tBu O N N NH2 air NH2 + H + D2O N N N N Br Br tBu Et tBu Et
1H [4]+ [1]+ 4H
Scheme 4.10. Reduction of NAD(P)+ model [4]+Br- with folate model 1H.
As the reaction was performed in water, and hydride anions (H-) are unstable in protic environments, a ‘hydride’ transfer mechanism involving free hydride ions is unlikely. However, a transition state with a bridging hydride cannot be ruled out and requires further studies.
4.3 Conclusion
The reduction of halocarbons by 1,4-dihydronicotinamide model 4H constitutes a previously unknown type of reactivity with obvious relevance to
69 biochemistry, toxicology, and possibly environmental remediation. Despite the moderate air sensitivity of the model 4H, the reactions can be carried out in the presence of air and water and also mimic the biological activity of NAD(P)H 5H.
The results of the NAD(P)H model reactions are evidence that NAD(P)H 5H can be involved in the in vivo reductive metabolism of halocarbons, and that halocarbons can in turn function as disruptors of NAD(P)H. The halogen scrambling observed in the in vitro experiments with NAD(P)H models 4H is consistent with the postulated radical mechanism of the in vivo reductive dehalogenation.17,18
4.4 References
1. Pandit, U. K.; Gase, R. A.; Mas Cabré, F. R.; de Nie-Sarink, M. J. J. Chem. Soc. Chem. Comm. 1975, 211. 2. Vasse, J. L.; Charpentier, P.; Levacher, V.; Dupas, G.; Quéguiner, G.; Bourguignon, J. Synlett. 1998, 10, 1144. 3. Ohnishi, Y.; Kagami, M. Chem. Lett. 1975, 4, 125. 4. Lelais, G.; MacMillan, D. W. C. Aldrichimica Acta. 2006, 39, 79. 5. Nakamura, M.; Bhatnagar, A.; Sadoshima, J. Circ. Res. 2012, 111, 604. 6. Denk, M. K.; Milutinović, N. S.; Marczenko, K. M.; Sadowski, N. S.; Paschos, A. Chem. Sci. 2016, 8, 1883. 7. Eisner, U.; Kuthan, J. Chem. Rev. 1972, 72, 1. 8. Paul, C. E.; Gargiulo, S.; Opperman, D. J.; Lavandera, I; Gotor-Fernández, V.; Gotor, V; Taglieber, A.; Arends, I. W. C. E.; Hollmann, F. Org. Lett. 2013, 15, 180. 9. Denk, M. K. performed quantum chemical calculations. 10. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
70 Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery Jr, J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. GAUSSIAN 09 (Revision D.01), Gaussian Inc., Wallingford, CT, 2013. 11. Marenich, A.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B, 2009, 113, 6378. 12. Montgomery, J. A.; Frisch, M. J.; Ochterski, J. W.; Petersson, G. A. J. Chem. Phys. 2000, 112, 6532. 13. Schnürch, M.; Spina, M.; Khan, A. F.; Mihovilovic, M. D.; Stanetty, P. Chem. Soc. Rev. 2007, 36, 1046. 14. Hassel, K. A. Pestic. Biochem. Physiol. 1972, 1, 259. 15. Nzengung, V. A.; Castillo, R. M.; Cates, W. P.; Mills, G. L. Environ. Sci. Technol. 2001, 35, 2244. 16. Garcia, A. N.; Boparai, H. K.; O’Carroll, D. M. Environ. Sci. Technol. 2016, 50, 5243. 17. Janzen, E. G.; Stronks, H. J.; Dubose, C. M.; Lee Poyer, J.; McCay, P. B. Environ. Health Perspect. 1985, 64, 151. 18. Sang, H.; Janzen, E. G.; Poyer, J. L.; McCay, P. B. Free Radic. Biol. Med. 1997, 22, 843.
71
Chapter 5
Synthesis and Host-Guest Properties of Diaminocarbenium Salts
72 5.1 Introduction
The reductions of halocarbons with folate models 1H generates imidazolidinium salts [1]+X- as the oxidative byproduct (Figure 5.1). These salts, which can be recovered in high yield, are useful precursors to carbenes, and reduction of halocarbons by 1H accordingly represents a useful synthesis for imidazolidinium salts [1]+X-.
H H R R N N N R N R
[1]+ [1’]+
Figure 5.1. Imidazolidinium cation [1]+ and aromatic imidazolium cation [1’]+.
+ - In 2005, following Wanzlick’s 1973 synthesis of [1] Cl (R: Ph, p-OMe, p-NO2) from
1H using carbon tetrachloride,1 our group extended the synthetic applicability of the reaction to carbon tetrabromide. The resulting imidazolidinium bromide salts
[1]+Br- were obtained in shorter reactions times and, unlike the chloride salts, were not hygroscopic. Both are significant advantages to the carbon tetrachloride oxidation. A variety of folate models 1H (R: Me, Et, iPr, tBu) were subsequently oxidized by carbon tetrabromide; however, it was observed that complete conversion to [1]+Br- required a large excess of carbon tetrabromide. For R: tBu,
[1’]+Br- was formed as an impurity that was hard to separate. For all of the salts
[1]+Br- (R: Me, Et, iPr, tBu), X-ray crystal structure analysis revealed that the excess carbon tetrabromide was incorporated as a guest molecule into the host lattice of [1]+Br-. Heating the inclusion compounds to 130 °C under vacuum led to
73 the release of carbon tetrabromide and recovery of the salts [1]+Br- from the residue. Our group subsequently investigated the deliberate formation of inclusion compounds as a potential purification technique for the related imidazolium salt
[1’]+Br- (R: tBu), which, as discussed in chapter 1, was synthesized from glyoxal, hydrobromic acid, and tert-butylamine in low purity. When the crude brown product was dissolved in chloroform, large (~ 1 cm3) colorless crystals of the inclusion [1’]+Br- (R: tBu)chloroform formed at room temperature after a few days. Filtration and simple heating allowed us to obtain [1’]+Br- (R: tBu) for the first time in 100 g quantities.
In this chapter, the host properties of [1’]+Br- (R: tBu) for the inhalation anesthetic halothane are reported. To allow for a stability comparison, the halothane inclusion properties of [1]+Br- were also investigated. To obtain sufficient host material [1]+Br- (R: tBu) for the study, [1]+Br- (R: tBu) was prepared directly from the respective ethylenediamine following literature procedures.2–5 In the course of synthesizing the compound, the protocol was further optimized to allow the synthesis of aryl-functionalized imidazolidinium salts [1]+X-, as well as bromide and iodide salts, which could previously be obtained only in low yield. The utility of the optimized method was demonstrated by the syntheses of several alkyl- and aryl-functionalized salts [1]+X-. One of the compounds, 1-tert-butyl-3-phenylimidazolidinium bromide, was subsequently used as starting material for the synthesis of a new stable carbene.
74 5.2 Results
Halocarbon oxidation of 1H leading to [1]+X- is formally a two-step synthesis beginning from ethylenediamine. An elegant one-step conversion of alkyl-functionalized ethylenediamines to [1]+X- described in the literature2–5 served as the starting point for synthesis of [1]+Br- (R: tBu) (Scheme 5.1).
Two-step halocarbon synthesis of [1]+
R R R NH N N (CH2O)n CX4 H N N NH X R R R [1]+
One-step orthoformate synthesis of [1]+
R R NH N NH4X H HC(OR) 3 N NH X R R [1]+
Scheme 5.1. Synthesis of imidazolidinium salts [1]+X- from N,N’-ethylenediamines.
In the one-step synthesis, orthoformate functions as a C1-building block for ring closure, and ammonium halide as the source of the anion X-. The one-pot procedure required boiling reflux and was found to give excellent yields for [1]+X-
- - - (X: BF4 , PF6 , and Cl ), but only modest yields for the respective bromides and iodides. Alkyl-functionalized salts are obtained in nearly quantitative yield, but yields for aryl-functionalized products range between 30 – 75 %.6 An optimized, one-pot synthetic protocol for these products was developed in which ethylenediamine and ammonium halide salt are first reacted separately until the
75 cessation of ammonia evolution, at which time triethyl orthoformate is added to complete the synthesis of [1]+X- by boiling (Scheme 5.2).
R R R
NH NH4X (1.1 eq.) NH2 HC(OEt)3 (2.2 eq.) N 110 C 110 C H
- NH3 N NH NH X R R R [1]+
Scheme 5.2. Optimized synthetic procedure for imidazolidinium halide salts [1]+X- (X: Br, I, R: alkyl, aryl) from ethylenediamine, ammonium halide, and triethyl orthoformate.
The new optimized method affords yields greater than 90 % for alkyl- and aryl- functionalized products [1]+X- (X: Br, I). A high yielding synthesis of the bromide and iodide salts is valuable, as these compounds were found to be non- hygroscopic, which is advantageous for their use as precursors to carbenes and inclusion compounds. Naturally, the formation of inclusion compounds is disrupted by the competing formation of hydrates. In order to directly compare the value of the optimized orthoformate procedure to the halocarbon oxidation of
1H, the salt [1]+Br- (R1: Ph, R2: tBu) was obtained using both methods. The one- step orthoformate protocol generated the product from the respective ethylenediamine with a pre-crystallization yield of 99 % in a total reaction time of
16 hours (Scheme 5.3).
tBu tBu
NH 1.) NH4Br (1.1 eq.) N 110 C H 99 % 2.) HC(OEt)3 (2.2 eq.) N NH 110 C Br Ph Ph
Scheme 5.3. One-pot synthesis of 1-phenyl-3-tert-butylimidazolidinium bromide [1]+Br- (R1: Ph, R2: tBu).
76 Carbon tetrabromide oxidation of 1H (R1: Ph, R2: tBu), which first required synthesis of 1H from ethylenediamine (R1: Ph, R2: tBu) with paraformaldehyde, generated the product in two steps with an overall yield of 64 % in a total reaction time of 24 hours (Scheme 5.4). The same oxidation procedure using carbon tetrachloride formed many side products and resulted in a total yield of 43 %.
tBu tBu tBu NH N N X yield (CH2O)n CX4 H Cl 43 % Et2O N Et2O N NH X Br 64 % Ph Ph Ph
Scheme 5.4. Two step synthesis of 1-phenyl-3-tert-butylimidazolidinium bromide [1]+Br- (R1: Ph, R2: tBu).
The one-step optimized orthoformate method was thus found to be better suited for the synthesis of alkyl- and aryl-functionalized carbeninum salts [1]+X- (X: Br, I).
The utility and generality of the optimized method were also demonstrated by the syntheses of several bromide and iodide salts [1]+X- (Table 5.1).
Table 5.1 Yields of imidazolidinium salts [1]+X- synthesized by the optimized orthoformate procedure.
tBu Ph tBu pTolyl iPr N N N N N H H H H H N N N N N Br Br Br I I tBu Ph Ph pTolyl iPr
Yield (%) 93 92 99 91 93
[1]+Br- (R: tBu), the host material required for inclusion compound studies, was obtained as a crystalline solid in 93 % yield from the corresponding
77 ethylenediamine starting material. Comparison of the salts showed that salts with large size differences between [1]+ and X- were hygroscopic. For example, [1]+Br- was found to be hygroscopic for R: iPr and non-hygroscopic for [1]+I-. Applying the orthoformate method to the synthesis of diaryl-functionalized imidazolidinium salts [1]+X- (X: Br, I, R: Ph, pTol) was successful for obtaining the products in yields greater than 90 % for all cases. A problem related to the synthesis of [1]+X- (X: Br,
I, R: Ph) was the presence of piperazine impurity (~ 20 %) in the ethylenediamine starting material synthesized by a previous group member (Figure 5.2).
Ph Ph
NH N
NH N Ph Ph
Figure 5.2. Diphenylethylenediamine and diphenylpiperazine.
Utilizing differences in solubility between the desired ionic product [1]+Br- (R: Ph) and piperazine impurity allowed a facile synthesis of [1]+Br- (R: Ph) without the need for a laborious purification of the impure ethylenediamine. The impure ethylenediamine mixture was reacted directly with ammonium bromide and triethyl orthoformate, and crystallization of the resulting product mixture from methanol followed by water separately crystallized piperazine and [1]+Br- (R: Ph), respectively (Scheme 5.5). The method provided pure imidazolidinium salt in 92
% yield along with pure piperazine.
78 Ph Ph Ph Ph
NH N 1.) NH4Br (1.1 eq.) N N 110 C + H + 2.) HC(OEt)3 (2.2 eq.) N NH N 110 C Br N Ph Ph Ph Ph
~ 80 % ~ 20 % Ph crystallization from crystallization from N MeOH H2O H Ph N Br N Ph - N Ph
Scheme 5.5. Synthesis and purification of [1]+Br- (R: Ph) and diphenylpiperazine.
For the subsequent deprotonation of imidazolidinium salts to the corresponding Wanzlick carbenes, 1-phenyl-3-tert-butyl-imidazolidinium bromide salt was reacted with potassium bis(trimethylsilyl)amide. As carbenes bearing two phenyl substituents are known to dimerize rapidly, while carbenes with only tert- butyl substituents or a combination of tert-butyl and alkyl substituents are monomeric,7 the behavior of a Wanzlick carbene bearing one tert-butyl group and one phenyl group was of interest. The carbene was synthesized following literature procedures using the non-nucleophilic base potassium bis(trimethylsilyl) amide (KHMDS) in benzene (Scheme 5.6).8
tBu tBu N N H KHMDS
N C H N Br 6 6 Ph Ph
Scheme 5.6. Synthesis of 1-phenyl-3-tert-butyl-imidazol-4,5-dihydro-2-ylidene.
79 The resulting product was shown to be the monomeric carbene 1-phenyl-3-tert- butyl-imidazol-4,5-dihydro-2-ylidine as evidenced by its characteristically deshielded 13C NMR signal of 237.7 ppm. The flame-sealed NMR sample also showed no evidence of dimerization, even after several months at room temperature. To clarify if the surprising lack of dimerization of 1-phenyl-3-tert- butyl-imidazol-4,5-dihydro-2-ylidine has kinetic or thermodynamic reasons, the dimerization energy ΔG° was calculated at the high precision CBS-QB3 level.9 The
CBS-QB3 level was deemed essential for a thermochemically accurate prediction, as previous benchmark calculations in our group demonstrated surprisingly large errors of even the best DFT methods for the dimerization problem.9 The selection of the CBS-QB3 method was not undertaken lightly, as it was clear from the many related calculations in our group that the CBS-QB3 calculation of the dimer would require enormous computational resources exceeding the capabilities of Sharcnet.
The calculation was successfully carried out on a modified 64bit MacPro workstation with four striped raid0 hard disks, 128 GB of physical RAM and 24 cores. The CCSD(T) step of the CBS-QB3 calculation did require 619 days of CPU time reduced by the number of cores to 33 days of wall time. To accurately reflect the observed behavior of the carbene in C6D6 solution, the thermochemically accurate SMD solvent model of Trular and Marenich was added to the calculations at all five steps of the CBS-QB3 protocol. The obtained dimerization energy ΔG° of -
10.96 kcalmol-1, in combination with the known error margin of the CBS-QB3 method of ± kcalmol-1, shows that dimerization should occur. The lack of dimerization observed for 1-phenyl-3-tert-butyl-imidazol-4,5-dihydro-2-ylidine
80 must accordingly have kinetic reasons only. A precise calculation of the transition state with the CBS-QB3 method and the SMD solvent model is in progress.
With the optimized high yielding synthesis of [1]+Br- (R: tBu) in hand, the inclusion properties of this salt could be examined in greater detail. As an initial experiment, the formation of an inclusion compound between [1]+Br- (R: tBu) and the inhalation anesthetic halothane, CF3C(Br)HCl, was investigated. Halothane, which was previously reduced by our folate models 1H and our NAD(P)H models
4H, was chosen in relation to the clathrate hypothesis of anesthesia put forth by
Linus Pauling and Stanley Miller.10,11 [1]+Br- (R: tBu) was dissolved in halothane, and the resulting crystals that formed were analyzed by thermal gravimetric (TG) analysis. This technique, which measures mass loss as a function of temperature, was employed to quantify the formation of the inclusion compound (Figure 5.3).
N Br F H H Cl N Br F F
Figure 5.3. Thermal gravimetric curve of [1]+Br- (R: tBu)halothane inclusion compound.
81 TG analysis revealed the loss of halothane from the host system between 70 °C and
97 °C. The observed mass loss of 41 % within this temperature range translated into a molar 1:1 [1]+Br- (R: tBu)halothane ratio. The intermolecular host-guest interactions were strong enough to contain the volatile guest within the host matrix above its normal boiling point of 50.2 °C.12 In order to directly compare the stability of this inclusion compound with the known inclusion properties of
[1’]+Br- (R: tBu) described previously by our group, [1’]+Br- (R: tBu) was dissolved in halothane, and the crystals resulting from slow evaporation were analyzed by TG analysis (Figure 5.4).
N Br F H H Cl N Br F F
Figure 5.4. Thermal gravimetric curve of [1’]+Br- (R: tBu)halothane inclusion compound.
Loss of halothane from the host system now occurred between the even higher temperature range of 94 °C and 116 °C. The observed mass loss of 43 % translated
82 into a molar 1:1 [1’]+Br- (R: tBu)halothane ratio. The intermolecular host-guest interactions for this inclusion compound are significantly stronger than those of the [1]+Br- (R: tBu)halothane inclusion compound synthesized earlier. It is of interest to note in this context that the [1’]+Br- (R: tBu)chloroform inclusion compound loses chloroform (boiling point 61.2 °C) between 73 °C and 104 °C. The results show that [1’]+Br- (R: tBu) may be a useful host material for halocarbons.
The stability of [1’]+Br- (R: tBu)chloroform was obviously crucial for the purification of [1’]+Br- (R: tBu).
5.3 Conclusion
An optimized orthoformate synthesis of imidazolidinium salts [1]+X- (X: Br,
I) was developed for alkyl- and aryl-functionalized salts that were previously only obtained in low yield. The new approach affords yields greater than 90 % in one step from ethylenediamines, which is a valuable improvement considering the high cost or lack of commercial availability of these diamines. The utility of this method for the synthesis of a new carbene was demonstrated, and the stability of the carbene was clarified to be kinetic only by high precision quantum chemical calculations. Formation of a highly stable halothane inclusion compound with
[1’]+Br- (R: tBu) was of significance in relation to the reactivity of the previous reactivity of halothane with folate and NAD(P)H model compounds described in this thesis. Interestingly, 2-chloro-1,1,1-trifluoroethane, the product obtained from the reduction of halothane by the model compounds 1H and 4H is a component in the exhaled breath of anesthetized individuals,13 which may suggest
83 a relationship between the two phenomena. Given the medicinal relevance of halothane, the result also suggests that the strong host-guest interactions may be applicable as a slow-release method for the delivery of anesthetics in pharmaceutical applications. Ultimately, this anesthetic inclusion compound bridges work by Pauling and Miller with our current research on the biochemistry and toxicology of halocarbons.
5.4 References
1. Rabe, E.; Wanzlick, H.-W. Liebigs Ann. Chem. 1973, 40. 2. Saba, S.; Brescia, A.; Kaloustian, M. K. Tetrahedron Lett. 1991, 32, 5031. 3. Aidouni, A.; Bendahou, S.; Demonceau, A.; Delaude, L. J. Comb. Chem. 2008, 10, 886. 4. Benhamou, L.; Chardon, E.; Lavigne, G.; Bellemin-Laponnaz, S. Chem. Rev. 2011, 111, 2705. 5. Arduengo III, A. J.; Krafczyk, R.; Schmutzler, R.; Craig, H. A.; Goerlich, J. R.; Marshall, W. J.; Unverzagt, M. Tetrahedron, 1999, 55, 14523. 6. Leuthäusser, S.; Schwarz, D.; Plenio, H. Chem. Eur. J. 2007, 13, 7195. 7. Denk, M. K.; Hezarkhani, A.; Zheng, F.-L. Eur. J. Inorg. Chem. 2007, 3527. 8. Lloyd-Jones, G. C.; Alder, R. W.; Owen-Smith, G. J. J. Chem. Eur. J. 2006, 12, 5361. 9. Denk, M. K. performed quantum chemical calculations. 10. Pauling, L. Science. 1961, 134, 15. 11. Miller, S. Proc. Natl. Acd. Sci. U. S. A. 1961, 47, 1515. 12. Raventós, J. Br. J. Pharmacol. Chemother. 1956, 11, 394. 13. Chapman, J.; Hill, R.; Muir, J.; Suckling, C. W.; Viney, D. J. J. Pharm. Pharmacol. 1967, 19, 231.
84
Chapter 6
Experimental Section
85 6.1 General Methods and Instrumentation
To retain volatile reaction components reductions of halocarbons were performed in flame-sealed NMR tubes. To mimic biological conditions solvents were not pre-dried unless otherwise noted. Folate models 1H were prepared from the respective N,N’-ethylenediamines1,2 and NAD(P)H models 4H were prepared from the respective N-substituted nicotinamides3,4 as described elsewhere.
Halocarbons were purchased from Sigma-Aldrich Inc. DDT and its metabolites
DDD, DDE, DDMU, DDM, and DPB were obtained from Sigma-Aldrich Inc. as analytical standards and used as received.
NMR data were recorded on Bruker AvanceTM 400 MHz and 600 MHz
UltraShieldTM spectrometers at 298 K. Chemical shifts were referenced versus tetramethylsilane as internal standard unless otherwise noted. 1H NMR spectra are reported as δ [number of protons, multiplicity, coupling constant J, proton identification]. 1H NMR data are reported using standard abbreviations: singlet (s), doublet (d), triplet (t), doublet of doublet (dd), doublet of triplet (dt), quartet (q), multiplet (m). GC-MS data were obtained with an Agilent 7890A GC – 5975C VL
MSD instrument, DB-5ms column (30 m, 250 μm ID, 0.25 μm film), 40 °C – 250 °C, heating rate 20 °C / min. Melting points were determined using a MEL-TEMP melting point apparatus and are uncorrected. Thermal gravimetric (TG) analysis was performed on a Q5000-IR TGA instrument equipped with an infrared furnace, a thermostatted microbalance, and an autosampler in the Soldatov group at the
University of Guelph. A sample of 7 – 8 mg was loaded in a 100 μL platinum pan and heated from ambient temperature to 300 °C at a heating rate of 5 °C/min in a
86 25 mL/min flow of nitrogen gas. Quantum chemical calculations were carried5 out with Gaussian 09, Revision D.01.6 The energies were obtained from full optimizations with the CBS-QB3 method7 and the SMD solvent model8 for water.
Minima were verified by the absence of virtual frequencies.
6.2 Chapter 2 – Reaction and Spectroscopic Data
1,3-Di-tert-butylimidazolidinium chloride [1]+Cl- (R: tBu).9 Scheme 2.1. M.p.
1 202-203 °C (from chloroform); H NMR (400 MHz, CDCl3) δ 1.55
+ 13 1 N Cl [18H, s, C(CH3)3], 4.05 [4H, s, CH2CH2], 8.85 [1H, s, N2CH ]; C{ H} H N NMR (100 MHz, CDCl3) δ 28.2 [C(CH3)3], 45.3 [CH2CH2], 57.2
+ [C(CH3)3], 154.1 [N2CH ].
1,3-Di-tert-butylimidazolidinium bromide [1]+Br- (R: tBu).9 Scheme 2.1. M.p.
1 230-231 °C (dec.) (from chloroform); H NMR (400 MHz, CDCl3) δ
+ 1.56 [18H, s, C(CH3)3], 4.10 [4H, s, CH2CH2], 8.46 [1H, s, N2CH ]; N Br H 13 1 N C{ H} NMR (100 MHz, CDCl3) δ 28.3 [C(CH3)3], 45.5 [CH2CH2],
+ 57.2 [C(CH3)3], 153.1 [N2CH ].
1,3-Di-tert-butylimidazolidinium iodide [1]+I- (R: tBu).9 Scheme 2.1. M.p. 280
1 °C (dec.) (from iso-propanol); H NMR (400 MHz, CDCl3) δ 1.21
+ [18H, s, C(CH3)3], 4.11 [4H, s, CH2CH2], 8.16 [1H, s, N2CH ]; N I 13 1 H C{ H} NMR (100 MHz, CDCl3) δ 28.3 [C(CH3)3], 45.5 [CH2CH2], N + 57.2 [C(CH3)3], 152.2 [N2CH ].
87 Reduction of 2-bromo-2-chloro-1,1,1-trifluoroethane (halothane) by 1H (R: tBu). Table 2.3.
1H (0.02 g, 0.11 mmol) was added to 1.5 mL of benzene-D6 layered over halothane
(0.01 g, 0.05 mmol) in an NMR tube. The contents were frozen with liquid nitrogen and flame-sealed under vacuum. After warming to room temperature the contents of the tube were mixed and tiny colorless crystals formed after 5 min. NMR analysis after 1 d shows formation 2-chloro-1,1,1-trifluoroethane and [1]+Br-.
2-Bromo-2-chloro-1,1,1-trifluoroethane (halothane).9 1H NMR (600 MHz,
3 13 1 2 C6D6) δ 4.58 [q, JHF = 5.4 Hz]; C{ H} NMR (150 MHz, C6D6) δ 50.2 [q, JCF = 40.9
1 19 Hz, -CHClBr], 121.6 [q, JCF = 278.0 Hz, -CF3]; F NMR (565 MHz, C6D6 vs. CFCl3) δ -
3 76.1 [d, JHF = 5.3 Hz].
9 1 3 2-Chloro-1,1,1-trifluoroethane. H NMR (600 MHz, C6D6) δ 2.78 [q, JHF = 8.5
13 1 2 Hz]; C{ H} NMR (150, C6D6) δ 40.1 [q, JCF = 37.7 Hz, - H 1 19 H CH2Cl], 123.1 [q, JCF = 275.7 Hz, -CF3]; F NMR (565 MHz, F Cl 3 F C6D6 vs. CFCl3) δ -71.6 [t, JFH = 8.5 Hz]. F
Reduction of 1,2-dichloroethane by 1H (R: tBu). Table 2.3.
1H (0.02 g, 0.11 mmol) was added to 1.5 mL of benzene-D6 layered over 1,2- dibromoethane (0.005 g, 0.05 mmol) in an NMR tube. The contents were frozen with liquid nitrogen and flame-sealed under vacuum. After warming to room temperature the contents of the tube were mixed and heated at 80 °C in an
88 aluminum block. NMR analysis after 6 w shows formation of vinyl chloride and
[1]+Cl-.
10 1 13 1 1,2-Dichloroethane. H NMR (400 MHz, C6D6) δ 2.95 [s]; C{ H} NMR (100
MHz, C6D6) δ 43.5.
1 10 3 Vinyl chloride. H NMR (400 MHz, C6D6) δ 4.84 [1H, dd, JHH = -
H3 3 1.66, 7.27 Hz, H1], 5.16 [1H, dd, JHH = -1.66, H H1 Cl 3 H 14.6 Hz, H2], 5.80 [1H, dd, JHH = 7.27, 14.6 Hz, Cl H2 13 1 H3]; C{ H} NMR (100 MHz, C6D6) δ 117.4, 125.9. H
Reduction of 1,2-dibromoethane by 1H (R: tBu). Table 2.3.
1H (0.02 g, 0.11 mmol) was added to 1.5 mL of benzene-D6 layered over 1,2- dibromoethane (0.009 g, 0.05 mmol) in an NMR tube. The contents were frozen with liquid nitrogen and flame-sealed under vacuum. After warming to room temperature the contents of the tube were mixed and heated at 80 °C in an aluminum block. A colorless precipitate in a light brown supernatant formed after
3 h. NMR analysis after 3 d showed formation of vinyl bromide and [1]+Br-. 1,2-
Dibromoethane was consumed after 1 w.
10 1 13 1 1,2-Dibromoethane. H NMR (400 MHz, C6D6) δ 2.86 [s]; C{ H} NMR (100
MHz, C6D6) δ 29.6.
1 10 H3 Vinyl bromide. H NMR (400 MHz, C6D6) δ 5.42 [1H, dd, 3JHH =
H1 3 Br -1.86, 7.12 Hz, H1], 5.51 [1H, dd, JHH = -1.86, H H2 H Br
H
89 3 13 1 14.9 Hz, H2], 6.01 [1H, dd, JHH = 7.12, 14.9 Hz, H3]; C{ H} NMR (100 MHz, C6D6) δ
114.0, 121.7.
Reduction of trans-1,2-dibromocyclohexane by 1H (R: tBu). Table 2.3.
1H (0.02 g, 0.11 mmol) was added to 5 mL of benzene and trans-1,2- dibromocyclohexane (0.01 g, 0.05mmol) in a Schlenk tube under argon. The mixture was stirred magnetically and heated at 100 °C. A white precipitate in a yellow supernatant formed after 10 min. NMR analysis after 48 h showed complete reduction of trans-1,2-dibromocyclohexane to cyclohexene and formation of [1]+Br-.
10 1 13 1 Cyclohexene. H NMR (400 MHz, CDCl3) δ 1.61 [m], 1.99 [m], 5.67 [m]; C{ H}
NMR (100 MHz, CDCl3) δ 22.6, 25.1, 127.2.
Reduction of 1,1,2,2-tetrabromoethane by 1H (R: tBu). Table 2.3.
1H (0.02 g, 0.11 mmol) was added to 1.5 mL of benzene-D6 and 1,1,2,2- tetrabromoethane (0.009 g, 0.02 mmol) in an NMR tube. The contents were frozen with liquid nitrogen and flame-sealed under vacuum. After warming to room temperature the contents of the tube were mixed, and tiny colorless crystals formed after 30 sec. Heating at 80 °C for 24 h in an aluminum block formed colorless crystalline solid and a light yellow supernatant. NMR analysis showed
90 complete consumption of 1,1,2,2-tetrabromoethane and formation of 1,1,2- tribromoethene and [1]+Br-.
10 1 1 1,1,2,2-Tetrabromoethane. H NMR (400 MHz, CDCl3) δ 6.07 [s]; H NMR (400
13 1 13 1 MHz, C6D6) δ 5.07 [s]; C{ H} NMR (100 MHz, CDCl3) δ 47.0; C{ H} NMR (100
MHz, C6D6) δ 47.4; GC-MS 9.23 min, m/z (rel. int. %): 350(2), 348(7),
346(10)[M+ ], 344(7), 342(2), 269(31), 267(96), 265(100), 263(34), 188(11),
186(23), 184(12), 107(14), 105(15), 81(11), 79(11).
1 1,1,2-Tribromoethene. H NMR (400 MHz, CDCl3) δ 7.16 [s]; 1H NMR (400 MHz,
13 1 C6D6) δ 6.41 [s]; C{ H} NMR (100 MHz, CDCl3) δ 92.7, H 110.4; 13C{1H} NMR (100 MHz, C D ) δ 92.9, 110.9; GC-MS Br 6 6 Br 6.64 min, m/z (rel. int. %) 268(31), 266(95), 264(100)[M+ ], Br 262(35), 187(44), 185(92), 183(48), 106(31), 104(31),
81(14), 79(14).
Reduction of trichlorofluoromethane (Freon-11) by 1H (R: tBu). Table 2.3.
1H (0.02 g, 0.11 mmol) was added to 1.5 mL of benzene-D6 and trichlorofluoromethane (0.005 g, 0.03 mmol) in an NMR tube. The contents were frozen with liquid nitrogen and flame-sealed under vacuum. After warming to room temperature the contents of the tube were mixed, and tiny colorless crystals formed after 30 min. Heating at 80 °C for 14 h in an aluminum block formed colorless crystalline solid and a light yellow supernatant. NMR analysis showed formation of dichlorofluoromethane and [1]+Cl-.
91 13 1 Trichlorofluoromethane. C{ H} NMR (150 MHz, C6D6) δ 117.7 [d 336.1 Hz];
19 F NMR (565 MHz, C6D6) δ 0.00.
1 2 Dichlorofluoromethane. H NMR (600 MHz, C6D6) δ 6.63 [d, JHF = 53.2 Hz];
13 1 19 C{ H} NMR (150 MHz, C6D6) δ XX; F NMR (565 MHz, C6D6 vs. CFCl3) -80.15
2 [d, JFH = 53.2 Hz].
Reduction of 1,2-diiodoethane by 1H (R: tBu). Table 2.3.
1H (0.02 g, 0.11 mmol) was added to 1.5 mL of benzene-D6 layered over 1,2- diiodoethane (0.014 g, 0.05 mmol) in an NMR tube. The contents were frozen with liquid nitrogen and flame-sealed under vacuum. After warming to room temperature the contents of the tube were mixed, and a dark brown precipitate formed after 20 sec as the supernatant turned yellow. NMR analysis after 2 h showed complete conversion of 1,2-diiodoethane to ethene and formation of
[1]+I-.
10 1 13 1 1,2-Diiodoethane. H NMR (400 MHz, C6D6) δ 2.81 [s]; C{ H} NMR (100 MHz,
C6D6) δ 2.41.
11 1 13 1 Ethene. H NMR (400 MHz, C6D6) δ 5.25 [s, H2C=CH2]; C{ H} NMR (100 MHz,
C6D6) δ 122.9 [H2C=CH2]. H
H H
H
92 Reduction of 1-bromo-2-chloroethane by 1H (R: tBu). Table 2.3.
1H (0.02 g, 0.11 mmol) was added to 1.5 mL of benzene-D6 layered over 1-bromo-
2-chloroethane (0.07 g, 0.005 mmol) in an NMR tube. The contents were frozen with liquid nitrogen and flame-sealed under vacuum. After warming to room temperature the contents of the tube were mixed and heated at 70 °C in an aluminum block, forming a small amount of colorless solid after 1 h. NMR analysis after 3 d showed formation of vinyl bromide, vinyl chloride, 1,2-dibromoethane, and 1,2-dichloroethane.
1 1-Bromo-2-chloroethane. H NMR (400 MHz, C6D6) δ 2.79 [2H, m], 3.02 [2H, m];
13 1 C{ H} NMR (100 MHz, C6D6) δ 30.4, 43.0.
H H Br Cl Br Cl H H Cl Br
H H
Reduction of 1,2-dibromopropane 1H (R: tBu). Table 2.3.
1H (0.02 g, 0.11 mmol) was added to 1.5 mL of benzene-D6 layered over 1,2- dibromopropane (0.01 g, 0.05 mmol) in an NMR tube. The contents were frozen with liquid nitrogen and flame-sealed under vacuum. After warming to room temperature the contents of the tube were mixed and heated at 80 °C in an aluminum block. A small amount of colorless solid formed after 3 h. NMR analysis after 3 d showed formation of propene, 2-bromopropene, and cis-1-bromo-1- propene. After 1 w 1,2-dibromopropane was consumed, and propene, 2-bromo-1-
93 propene, and cis-1-bromo-1-propene were present in approximately equal quantity.
10 1 3 1,2-Dibromopropane. H NMR (400 MHz, C6D6) δ 1.34 [3H, d, JHH = 6.57 Hz],
3 13 1 2.97 [1H, t, JHH = 9.99 Hz], 3.26 [1H, m], 3.64 [1H, m]; C{ H} NMR (100 MHz,
C6D6) δ 23.9, 37.7, 46.0.
11 1 3 Propene. H NMR (600 MHz, C6D6) δ 1.55 [3H, dt, JHH = 1.6 Hz, 3.3 Hz], 4.94 [1H,
13 1 dm], 5.00 [1H, dm], 5.71 [1H, m]; C{ H} NMR (150 MHz, C6D6,
150 MHz) δ 19.4, 115.9, 133.7.
10 1 2-Bromo-1-propene. H NMR (600 MHz, C6D6) δ 1.89 [3H, s], 5.11 [1H, m], 5.17
13 1 Br [1H, m]; C{ H} NMR (150 MHz, C6D6) δ 28.6, 117.3, 129.4.
10 1 3 cis-1-Bromo-1-propene. H NMR (600 MHz, C6D6) δ 1.46 [3H, dd, JHH = 1.7 Hz,
3 13 1 4.9 Hz], 5.63 [1H, q, JHH = 6.7 Hz], 5.83 [1H, m]; C{ H} NMR
Br (150 MHz, C6D6) δ 15.9, 109.1, 129.1.
Reduction of 1,3,5-tribromobenzene 1H (R: tBu). Table 2.2.
1H (0.71 g, 3.9 mmol) and 1,3,5-tribromobenzene (0.37 g, 1.2 mmol) were dissolved in 5 mL of benzene in a Schlenk tube and heated under argon at 90 °C for
20 h in an aluminum block. A colorless precipitate of [1]+Br- in a light brown supernatant formed. NMR and GC-MS showed formation of 1,3-dibromobenzene and trace bromobenzene.
94 9 1 13 1,3,5-Tribromobenzene. H NMR (400 MHz, CDCl3) δ 7.61 [s]; C{1H} NMR
(100 MHz, CDCl3) δ 123.4, 133.0; GC-MS 10.28 min, m/z (rel. int. %)
314(100)[M+], 235(30), 156(15), 74(32).
9 1 1,3-Dibromobenzene. H NMR (400 MHz, CDCl3) δ 7.67 [m], 7.43 [m], 7.11 [m];
13 Br Br C{1H} NMR (400 MHz, CDCl3) δ 123.1, 130.3, 131.2,
134.2; GC-MS 8.57 min, m/z (rel. int. %) 236(100)[M+],
155(41), 75(32).
9 1 13 1 Bromobenzene. H NMR (400 MHz, CDCl3) δ 7.30 [m]; C{ H} NMR (400 MHz,
Br CDCl3) δ 122.5, 126.7, 130.0, 131.4; GC-MS 6.25 min, m/z
(rel. int. %): 156(50)[M+], 77 (100).
Reaction of allyl bromide and 1H (R: tBu). Table 2.4.
1H (1.2 g, 6.5 mmol) and allyl bromide (0.70 g, 5.9 mmol)
were dissolved in 5 mL of benzene in a Schlenk tube and
N heated under argon at 90 °C for 72 h in an aluminum block. A
N colorless precipitate in a light brown supernatant formed, and
NMR analysis indicated it was [1]+Br-. The precipitate was
removed by filtration and the supernatant evaporated under reduced pressure to afford a viscous yellow oil that was purified with flash chromatography (50 % hexanes : 50 % ethyl acetate) to afford a yellow oil of N1- allyl-N1,N2-di-tert-butyl-N2-methylethane-1,2-diamine (0.82 g, 63 %); 1H NMR
95 (400 MHz, C6D6) δ 1.02 [9H, s], 1.04 [9H, s,], 2.21 [3H, s], 2.48 [2H, t,], 2.63 [2H, t],
3.12 [2H, dt], 5.00 [1H, dm], 5.16 [1H, dm], 5.96 [1H, m]; 13C{1H} NMR (400 MHz,
C6D6, 400 MHz) δ 26.2, 27.4, 36.3, 46.8, 50.9, 52.3, 53.5, 54.6, 114.6, 140.5; GC-MS
9.74 min, m/z (rel. int. %) 212(0.1)[M+], 197(9), 126(49), 113(19), 100(38),
84(1), 70(100), 57(24).
Reaction of benzoyl chloride and 1H (R: tBu). Table 2.4.
1H (0.59 g, 3.2 mmol) and benzoyl chloride (0.41 g, 2.9 mmol)
were dissolved in 5 mL of diethyl ether in a Schlenk tube and
N stirred magnetically. A white precipitate formed after 3 min, O and NMR analysis after 24 h indicated it was [1]+Cl-. The N Ph precipitate was removed by filtration and the supernatant
evaporated under reduced pressure to afford a sticky yellow solid that was washed with methanol to afford a sweet smelling off-white solid of spectroscopically pure N-tert-butyl-N-(2-(tert- butyl(methyl)amino)ethyl)benzamide (0.74 g, 87 %); m.p. 66 – 68 °C; 1H NMR
(400 MHz, CDCl3) δ 0.87 [9H, s, C(CH3)3], 1.55 [9H, s, C(CH3)3], 1.79 [3H, s, N-
3 CH3], 2.32 [2H, m, CH2CH2], 3.33 [2H, m, CH2CH2], 7.53 [2H, t, JHH = 7.4 Hz,
3 3 meta-CH], 7.68 [1H, t, JHH = 7.4 Hz, para-CH], 8.17 [2H, d, JHH = 7.2 Hz,
13 1 ortho-CH]; C{ H} NMR (100, MHz, CDCl3) δ 25.6, 29.1, 35.2 46.4, 51.7, 54.9,
57.0, 126.1, 128.4, 128.6, 139.4, 173.4; GC-MS 13.64 min, m/z (rel. int. %)
290(2)[M+], 281(0.8), 162(3), 149(3), 134(10), 105(17), 100(100), 77(5).
96 6.3 Chapter 3 – Reaction and Spectroscopic Data
Reaction of DDT 3.3 and 1H (R: tBu) (aerobic). Scheme 3.1.
DDT (51 mg, 0.14 mmol) and 1H (140 mg, 0.77 mmol) dissolved in 5 mL of benzene was added to a Schlenk flask filled with oxygen. Heating at 80 °C for 15 min yielded a yellow solution and colorless precipitate of [1]+Cl- and [1’]+Cl-. GC-
MS analysis of the solution after 20 h showed formation of DDD, DDE, DDMU,
DDM, and DDB. After 40 h, DDT and DDD were consumed, and DDE, DDMU, DBP, and DDM were the only detectable products.
Reaction of DDT 3.3 and 1H (R: pTol) (aerobic). Scheme 3.1.
DDT (28 mg, 0.079 mmol) and 1H (110 mg, 0.44 mmol) dissolved in 3 mL of benzene was added to a Schlenk flask filled with oxygen. Heating at 80 °C for 15 min yielded a colorless solution and colorless precipitate of [1]+Cl-. GC-MS analysis of the light brown solution after 40 h showed formation of DDD, DDE, DDMU,
DDM, and DBP, while DDT remained the major component. After 2 w, the amount of the metabolites increased, but DDT remained the major component.
Reaction of DDD 3.4 and 1H (R: tBu) (aerobic). Scheme 3.1.
DDD (68 mg, 0.21 mmol) and 1H (170 mg, 0.94 mmol) dissolved in 5 mL of benzene was added to a Schlenk flask filled with oxygen. Heating at 80 °C for 3 h yielded a colorless solution and colorless precipitate of [1]+Cl- and [1’]+Cl-. GC-MS analysis of the solution after 20 h showed formation of DDMU. After 70 h, all DDD was consumed and DDMU was the only component. After 3 w, trace amounts of
DBP could be detected.
97 Reaction of DDE 3.5 and 1H (R: tBu) (aerobic). Scheme 3.1.
DDE (31 mg, 0.097 mmol) and 1H (78 mg, 0.42 mmol) dissolved in 5 mL of benzene was added to a Schlenk flask filled with oxygen. Heating at 80 °C for 1 w yielded a colorless solution and a small amount of colorless precipitate of [1]+Cl- and [1’]+Cl-. GC-MS analysis of the solution showed formation of DDMU, but DDE remained the major component. After 3 w, DDMU remained the major component, and trace amounts of DBP could be detected.
Reaction of DDMU 3.6 and 1H (R: tBu) (aerobic). Scheme 3.1.
DDMU (86 mg, 0.30 mmol) and 1H (190 mg, 1.0 mmol) dissolved in 4 mL of benzene was added to a Schlenk flask filled with oxygen. GC-MS analysis after heating at 80 °C for 1 w showed trace amounts of DBP.
Reaction of DDT 3.3 and 1H (R: tBu) (anaerobic). Scheme 3.1.
DDT (60 mg, 0.17 mmol) and 1H (170 mg, 0.92 mmol) dissolved in 4 mL of dry benzene was added to a flame dried Schlenk flask and the solution degassed with three freeze-pump-thaw cycles. Heating at 80 °C for 15 min yielded a yellow solution and colorless precipitate of [1]+Cl- and [1’]+Cl-. GC-MS analysis of the solution after 20 h showed formation of DDD, DDE, DDMU, and DDM. After 48 h,
DDT was consumed, and DDD, DDE, DDMU, and DDM were the only detectable products. After 1 w, all DDD was consumed.
Reaction of DDT 3.3 and 1H (R: pTol) (anaerobic). Scheme 3.1.
DDT (16 mg, 0.045 mmol) and 1H (68 mg, 0.27 mmol) dissolved in 3 mL of benzene was added to a flame dried Schlenk flask and the solution was degassed
98 with three freeze-pump-thaw cycles. Heating at 80 °C for 15 min yielded a colorless solution and colorless precipitate of [1]+Cl-. GC-MS analysis of the light brown solution after 40 h showed formation of DDD, DDE, DDMU, and trace amounts of DDM, but DDT remained the major component.
Reaction of DDD 3.4 and 1H (R: tBu) (anaerobic). Scheme 3.1.
DDD (85 mg, 0.27 mmol) and 1H (230 mg, 1.25 mmol) dissolved in 4 mL of dry benzene was added to a flame dried Schlenk flask and the solution was degassed with three freeze-pump-thaw cycles. Heating at 80 °C for 3 h yielded a colorless solution and colorless precipitate of [1]+Cl- and [1’]+Cl-. GC-MS analysis of the solution after 48 h showed formation of DDMU. After 1 w, the amount of DDMU increased, but DDD remained the major component.
Reaction of DDE 3.5 and 1H (R: tBu) (anaerobic). Scheme 3.1.
DDE (74 mg, 0.23 mmol) and 1H (200 mg, 1.1 mmol) dissolved in 4 mL of dry benzene was added to a flame dried Schlenk flask and the solution was degassed with three freeze-pump-thaw cycles. Heating at 80 °C for 1 w yielded a colorless solution and a small amount of colorless precipitate of [1]+Cl- and [1’]+Cl-. GC-MS analysis of the solution showed formation of DDMU, but DDE remained the major component.
Reaction of DDMU 3.6 and 1H (R: tBu) (anaerobic). Scheme 3.1.
DDMU (86 mg, 0.30 mmol) and 1H (190 mg, 1.0 mmol) dissolved in 4 mL of dry benzene was added to a flame dried Schlenk flask and the solution was degassed with three freeze-pump-thaw cycles. GC-MS analysis after heating at 80 °C for 1 w
99 showed no reaction.
DDT (1,1,1-Trichloro-2,2-bis(4-chlorophenyl)ethane) 3.3.12 1H NMR
(600 MHz, CDCl3) δ 5.02 [1H, s], 7.32 Cl Cl Cl [4H, m], 7.51 [4H, m]; 1H NMR (600
MHz, C6D6) δ 4.58 [1H, s], 7.03 [8H, m];
13 1 C{ H} NMR (150 MHz, CDCl3) δ 69.6, Cl Cl 100.8, 128.6, 131.3, 134.2, 136.2;
13 1 C{ H} NMR (150 MHz, C6D6) δ 69.6, 101.3, 128.7, 131.6, 134.4, 136.5; GC-
MS 16.65 min, m/z (rel. int. %) 354(2)[M+], 318(2), 282(3), 235(100),
212(10), 199(13), 176(10), 165(40), 136(9), 123(8), 99(3), 75(7).
DDD (1,1-Dichloro-2,2-bis(4-chlorophenyl)ethane) 3.4.12 1H NMR (600
3 MHz, CDCl3) δ 4.54 [1H, d, J = 8.0 Hz], Cl Cl 6.29 [1H, d, 3J = 8.0 Hz], 7.24 [4H, m],
1 7.35 [4H, m]; H NMR (600 MHz, C6D6) δ
Cl Cl 4.03 [1H, d, 3J = 8.6 Hz], 5.67 [1H, d, 3J =
13 1 8.6 Hz], 6.61 [4H, m], 7.15 [4H, m]; C{ H} NMR (150 MHz, CDCl3) δ 61.0,
13 1 73.4, 128.9, 129.8, 133.7, 137.6; C{ H} NMR (150 MHz, C6D6) δ 61.2, 74.3,
129.0, 129.9, 133.7, 138.2; GC-MS 15.77 min, m/z (rel. int. %) 320(3)[M+],
282(1), 235(100), 212(7), 199(13), 176(8), 165(43), 136(5), 101(5), 88(6),
75(7).
100 DDE (1,1-Dichloro-2,2-bis(4-chlorophenyl)ethene) 3.5.12 1H NMR (600
MHz, CDCl3) δ 7.20 [4H, m], 7.32 [4H, Cl Cl
1 m]; H NMR (600 MHz, C6D6) δ 6.70 [4H,
m], 6.98 [4H, m]; 13C{1H} NMR (150
Cl Cl MHz, CDCl3) δ 120.5, 128.6, 130.7,
13 1 134.3, 137.4, 138.2; C{ H} NMR (150 MHz, C6D6) δ 120.6, 128.8, 131.0,
134.5, 137.6, 138.8; GC-MS 14.95 min, m/z (rel. int. %) 318(73)[M+],
281(6), 246(100), 210(15), 176(36), 105(15), 75(9).
DDMU (1-Chloro-2,2-bis(4-chlorophenyl)ethene) 3.6.12 1H NMR (600
MHz, CDCl3) δ 6.58 [1H, s], 7.11 [2H, m], Cl H 7.24 [2H, m], 7.28 [2H, m], 7.38 [2H, m];
1 H NMR (600 MHz, C6D6) δ 6.07 [1H, s],
Cl Cl 6.57 [2H, m], 6.86 [2H, m], 6.96 [2H, m],
7.07 [2H, m]; 13C{1H} NMR (150 MHz,
CDCl3) δ 116.8, 128.6, 128.7, 128.9, 131.2, 134.2, 134.3, 135.5, 138.1, 141.8;
13 1 C{ H} NMR (150 MHz, C6D6) δ 117.1, 128.8, 128.8 (accidental overlap),
129.1, 131.5, 134.3, 134.4, 135.6, 138.3, 141.9; GC-MS 14.36 min, m/z (rel. int. %) 282(68)[M+], 247(18), 212(100), 176(49), 88(18), 75(15).
12 1 DDM (1,1-Bis(4-chlorophenyl)methane) 3.7. H NMR (600 MHz, CDCl3)
δ 3.90 [2H, s], 7.08 [4H, m], 7.25 [4H,
1 m]; H NMR (600 MHz, C6D6) δ 3.35 [2H,
Cl Cl s], 6.59 [4H, m], 7.05 [4H, m]; 13C{1H}
13 1 NMR (150 MHz, CDCl3) δ 40.5, 128.7, 130.2, 132.1, 139.0; C{ H} NMR (150
101 MHz, C6D6) δ 40.4, 128.8, 130.4, 132.4, 139.2; GC-MS 12.81 min, m/z (rel. int.
%) 236(46)[M+], 201(100), 165(75), 125(11), 89(13), 82(19).
12 1 DBP (4,4’-Dichlorobenzophenone) 3.8. H NMR (600 MHz, CDCl3) δ 7.47
[4H, m], 7.73 [4H, m]; 1H NMR (600 O
MHz, C6D6) δ 6.98 [4H, m], 7.29 [4H, m];
13 1 C{ H} NMR (150 MHz, CDCl3) δ 128.8, Cl Cl 131.3, 135.5, 139.2, 194.2; 13C{1H} NMR
(150 MHz, C6D6) δ 128.7, 131.4, 135.9, 138.8, 193.1; GC-MS 13.61 min, m/z
(rel. int. %) 250(30)[M+], 215(8), 139(100), 111(40), 75(27).
1,3-Di-para-tolyl-imidazolidinium chloride [1]+Cl- (R: pTol).12 Colorless
crystals; m.p. 255-257 °C (dec.) (from iso-propanol); 1H
NMR (400 MHz, DMSO-D6) δ 2.26 [6H, s, C-CH3], 4.55 [4H,
3 s, CH2CH2], 7.36 [4H, d, J = 8.3 Hz, meta-CH], 7.54 [4H, d,
3 + 13 1 N Cl J = 8.3 Hz, ortho-CH], 9.94 [1H, s, N2CH ]; C{ H} NMR H N (100 MHz, DMSO-D6) δ 20.3 [C-CH3], 48.1 [CH2CH2],
+ 118.1, 129.9, 133.6, 136.4, 150.9 [N2CH ].
102 6.4 Chapter 4 – Reaction and Spectroscopic Data
Reduction of 2-bromo-2-chloro-1,1,1-trifluoroethane (halothane) by 4H (R: Et). Table 4.3.
4H (0.02 g, 0.13 mmol) was added to 1.5 mL of benzene-D6 layered over halothane
(0.01 g, 0.03 mmol) in an NMR tube. The contents were frozen with liquid nitrogen and flame-sealed under vacuum. After warming to room temperature the contents of the tube were mixed and tiny opaque crystals formed after 15 min. NMR analysis after 12 h showed formation of 2-chloro-1,1,1-trifluoroethane.9 After 48 h, halothane was completely consumed.
Reduction of 1,2-diiodoethane by 4H (R: Et). Table 4.3.
4H (0.02 g, 0.13 mmol) was added to 1.5 mL of benzene-D6 layered over 1,2- diiodoethane (0.01 g, 0.03 mmol) in an NMR tube. The contents were frozen with liquid nitrogen and flame-sealed under vacuum. After warming to room temperature the contents of the tube were mixed and tiny opaque crystals formed after 1 min. NMR analysis showed formation of ethene11 and complete consumption of 1,2-diiodoethane.
Reduction of 1,2-dibromoethane by 4H (R: Et). Table 4.3.
4H (0.02 g, 0.13 mmol) was added to 1.5 mL of benzene-D6 layered over 1,2- dibromoethane (0.01 g, 0.03 mmol) in an NMR tube. The contents were frozen with liquid nitrogen and flame-sealed under vacuum. After warming to room temperature the contents of the tube were mixed and heated at 90 °C for 3 d,
103 forming tiny opaque crystals. NMR analysis showed formation of ethene.11 Heating at 90 °C for 2 w resulted in complete consumption of 1,2-dibromoethane.
Reduction of 1,2-dichloroethane by 4H (R: Et). Table 4.3.
4H (0.02 g, 0.13 mmol) was added to 1.5 mL of benzene-D6 layered over 1,2- dichloroethane (0.01 g, 0.03 mmol) in an NMR tube. The contents were frozen with liquid nitrogen and flame-sealed under vacuum. After warming to room temperature the contents of the tube were mixed and heated at 90 °C for 2 w, forming tiny opaque crystals. NMR analysis showed formation of vinyl chloride.10
Extended heating at 90 °C for 4 w did lead to significant consumption of 1,2- dichloroethane.
Reduction of 1,1,2,2-tetrabromoethane by 4H (R: Et). Table 4.3.
4H (0.02 g, 0.13 mmol) was added to 1.5 mL of benzene-D6 layered over 1,1,2,2- tetrabromoethane (0.01 g, 0.03 mmol) in an NMR tube. The contents were frozen with liquid nitrogen and flame-sealed under vacuum. After warming to room temperature the contents of the tube were mixed and tiny opaque crystals formed after 1 min. NMR analysis after 18 h showed formation of 1,1,2-tribromoethene, cis-1,2-dibromoethene, and trans-1,2-dibromoethene. Heating at 80 °C for 1 w resulted in complete consumption of 1,1,2,2-tetrabromoethane.
1 13 1 cis-1,2-Dibromoethene. H NMR (400 MHz, C6D6) δ 6.15 [s]; C{ H} NMR (100
MHz, C6D6) δ 113.2. Br
Br
104 1 13 1 trans-1,2-Dibromoethene. H NMR (400 MHz, C6D6) δ 5.94 [s]; C{ H} NMR (100
MHz, C6D6) δ 107.2. Br Br
Reduction of 1,2-dibromopropane by 4H (R: Et). Table 4.3.
4H (0.02 g, 0.13 mmol) was added to 1.5 mL of benzene-D6 layered over 1,2- dibromoethane (0.01 g, 0.03 mmol) in an NMR tube. The contents were frozen with liquid nitrogen and flame-sealed under vacuum. After warming to room temperature the contents of the tube were mixed and heated at 90 °C for 3 d, forming tiny opaque crystals. NMR analysis showed formation of propene.11
Heating at 90 °C for 2 w resulted in complete consumption of 1,2- dibromopropane.
Reaction of DDT with 4H (R: Et) (anaerobic conditions). Scheme 4.6.
DDT (18 mg, 0.048 mmol) and 4H (56 mg, 0.30 mmol) dissolved in 3 mL of 1,4- dioxane was added to a flame dried Schlenk flask and the solution was degassed with three freeze-pump-thaw cycles. Heating at 80 °C for 15 min yielded a colorless solution and colorless precipitate of [4]+Cl-. GC-MS analysis of the light brown solution after 40 h showed formation of DDD, DDE, and DDMU, but DDT remained the major component. After 1 w, DDT was consumed, and DDE was the major component.12
Reaction of DDT with 4H (R: Et) (aerobic conditions). Scheme 4.6.
DDT (20 mg, 0.054 mmol) and 4H (54 mg, 0.27 mmol) dissolved in 3 mL of 1,4-
105 dioxane was added to a Schlenk flask filled with oxygen. Heating at 80 °C for 15 min yielded a colorless solution and colorless precipitate of [1]+Cl-. GC-MS analysis of the light brown solution after 40 h showed formation of DDD, DDE, and DDMU, while DDT remained the major component. After 2 w, DDT was consumed, and
DDE was the major component.12
Attempted catalytic reduction of carbon tetrabromide with 4H (R: Bn). Scheme 4.8.
Carbon tetrabromide (0.72 g, 2.2 mmol) in benzene (12 mL) was added to a solution of sodium bicarbonate (0.82 g, 9.8 mmol) and sodium dithionite (0.44 g,
2.5 mmol) in deionized water (25 mL) and stirred magnetically at room temperature. 4H (0.03 g, 0.16 mmol) was added to the mixture. After 18 h, GC-MS and NMR analysis of the organic layer showed complete consumption of carbon tetrabromide, and formation of bromoform and 1,1,2,2-tetrabromoethene in approximately equal proportion.
13 1 1,1,2,2-Tetrabromoethene. C{ H} NMR (150 MHz, CDCl3) δ 92.3; GC-MS 8.75
min, m/z (rel. int. %) 343.6(100)[M+], 262(75), 183(56), Br Br 103(19), 90(8), 80(17). Br
Br
Reduction of carbon tetrabromide with sodium dithionite. Scheme 4.9.
Carbon tetrabromide (0.72 g, 2.2 mmol) in benzene (15 mL) was added to a solution of sodium bicarbonate (0.82 g, 9.8 mmol) and sodium dithionite (0.44 g,
106 2.5 mmol) in deionized water (25 mL) and stirred magnetically at room temperature. After 20 h, GC-MS and NMR analysis of the organic layer showed complete consumption of carbon tetrabromide, and formation of bromoform and
1,1,2,2-tetrabromoethene in approximately equal proportion.
Reduction of NAD(P)+ model [4]+Br- (R: Et) with folate model 1H (R: tBu). Scheme 4.10.
+ - [4] Br (R: Et) (0.32 g, 1.5 mmol) dissolved in D2O (4 mL) was added to a solution of 1H (R: tBu) (0.3 g, 1.6 mmol) dissolved in D2O (7 mL) in a screw cap vial and stirred magnetically at 50 °C. A light yellow solution formed after 10 min. After 24 h, 1H NMR showed formation of 4H (R: Et) and [1]+Br- (R: tBu). After 5 d, [4]+Br- was completely converted to 4H.
1-Ethyl-3-carbamoylpyridinium bromide [4]+Br- (R: Et). Scheme 4.1.
Nicotinamide (12.0 g, 98.3 mmol), methanol (110 mL), O and ethyl bromide (13.9 g, 128 mmol) were heated at 50
NH2 °C in a 250 mL stainless steel autoclave for 24 h. The N Br colorless crystals that formed upon cooling were
recrystallized from methanol to afford pure product as
1 colorless prisms (19 g, 84 %); m.p. 212 – 214 °C; H NMR (600 MHz, DMSO-D6) δ
1.59 [3H, t, J = 7.3 Hz], 4.73 [2H, q, J = 7.3 Hz], 8.18 [1H, br, -NH2], 8.29 [1H, m],
1 8.63 [1H, br, -NH2], 8.97 [1H, m], 9.29 [1H, m], 9.59 [1H, s]; H NMR (400 MHz, D2O vs. NaDSS) δ 1.76 [3H, t, J = 7.4 Hz], 4.84 [2H, q, J = 7.4 Hz], 8.29 [1H, m], 8.97 [1H, dt, J = 5.1, 1.3 Hz], 9.16 [1H, dt, J = 3.6, 1.2 Hz], 9.43 [1H, s]; 13C{1H} NMR (150 MHz,
107 13 1 DMSO-D6,) δ 16.1, 56.6, 127.7, 133.6, 143.2, 144.5, 146.1, 162.7; C{ H} NMR (100
MHz, D2O vs. NaDSS) δ 18.3, 60.6, 131.0, 136.3, 146.4, 146.6, 148.9, 168.3.
1-Ethyl-1,4-dihydronicotinamide 4H (R: Et). Scheme 4.1. To a stirred, ice-
cooled solution of 1-ethyl-3-carbamoylpyridinium O bromide (8.07 g, 34.9 mmol) and NH2 1H NMR sodium bicarbonate (17.9 g, 213 O N C mmol) in deionized water (300 mL) F NH2 G D layered with methylene chloride (200 E N
B mL), small portions of sodium dithionite (24.6 g, 141 mmol) A were added over a period of 10 min. After stirring for 2 h in the
13C NMR dark, the organic phase was separated and washed with cold O C water (2 x 10 mL). Sodium chloride (10 g) and methylene G F H NH2 chloride (2 x 10 mL) were added to the aqueous phase to D E N
B extract additional product. The combined organic layers were A dried over MgSO4, and the filtered solution evaporated under reduced pressure to afford pure product as an orange powder (0.79 g, 15 %); m.p. 74 – 76 °C (dec.); 1H
NMR (600 MHz, DMSO-D6) δ 1.06 [3H, t, J = 7.1 Hz, A-H], 2.95 [2H, m, C-H], 3.12
[2H, q, J = 7.1 Hz, B-H], 4.59 [1H, dt, J = 3.4, 1.1 Hz, F-H], 5.87 [1H, dq, J = 3.2, 1.6 Hz,
1 E-H], 6.51 [2H, br, G-H], 6.87 [1H, d, J = 1.5 Hz, D-H]; H NMR (600 MHz, C6D6) δ
0.58 [3H, t, J = 7.2 Hz, A-H], 2.37 [2H, q, J = 7.2 Hz, B-H], 2.89 [2H, m, C-H], 4.45 [1H, dt, 3.4, 1.2 Hz, F-H], 5.24 [1H, dq, 3.0, 1.7 Hz, E-H], 5.59 [2H, br, G-H], 7.18 [1H, d,
1 1.7 Hz, D-H]; H NMR (600 MHz, CDCl3) δ 1.17 [3H, t, J = 7.2 Hz, A-H], 3.15 [2H, q, J
= 7.2 Hz, B-H], 3.16 [2H, m, C-H], 4.74 [1H, dt, J = 3.5, 0.9 Hz, F-H], 5.28 [2H, br, G-
108 H], 5.73 [1H, dq, J = 3.2, 1.6 Hz, E-H], 7.06 [1H, d, 1.5 Hz, D-H]; 1H NMR (600 MHz,
D2O vs. NaDSS) δ 1.13 [3H, t, J = 7.2 Hz, A-H], 3.06 [2H, m, C-H], 3.20 [2H, q, J = 7.2
Hz, B-H], 4.90 [1H, m, F-H], 5.92 [1H, m, E-H], 7.04 [1H, s, D-H]; 13C{1H} NMR (150
MHz DMSO-D6) δ 15.0 [A-C], 22.3 [C-C], 47.3 [B-C], 99.7 [G-C], 101.5 [F-C], 129.1
13 1 [E-C], 137.3 [D-C], 169.1 [H-C]; C{ H} NMR (150 MHz, C6D6) δ 15.0 [A-C], 23.2 [C-
C], 48.2 [B-C], 99.9 [G-C], 102.5 [F-C], 128.9 [E-C], 138.9 [D-C], 170.0 [H-C]; 13C{1H}
NMR (150 MHz, CDCl3) δ 15.2 [A-C], 22.9 [C-C], 48.6 [B-C], 98.0 [G-C], 102.9 [F-C],
13 1 128.6 [E-C], 139.4 [D-C], 170.3 [H-C]; C{ H} NMR (150 MHz, D2O vs. NaDSS) δ
17.0 [A-C], 24.4 [C-C], 51.2 [B-C], 98.9 [G-C], 106.9 [F-C], 131.3 [E-C], 143.8 [D-C],
176.6 [H-C].
Synthesis of 1-ethyl-3-carbamoylpyridinium iodide [4]+I- (R: Et). Scheme 4.1.
Nicotinamide (3.02 g, 24.7 mmol), methanol (40 mL), O and ethyl iodide (4.68 g, 30 mmol) were refluxed at 60 °C NH 2 in a 250 mL round bottom flask for 24 h. The colorless
N I crystals that formed upon cooling were filtered, washed
with hexanes, and recrystallized from methanol to afford pure product as colorless prisms (5.61 g, 82 %); m.p. 202 – 203 °C; 1H NMR (400
MHz, DMSO-D6) δ 1.58 [3H, t, J = 7.3 Hz], 4.70 [2H, q, J = 7.3 Hz], 8.16 [1H, br], 8.28
[1H, m], 8.53 [1H, br], 8.92 [1H, dt, J = 5.1, 1.6 Hz], 9.23 [1H, dt, J = 3.7, 1.2 Hz], 9.48
1 [1H, s]; H NMR (400 MHz, D2O vs. NaDSS) δ 1.67 [3H, t, J = 7.4 Hz], 4.74 [2H, q, J =
7.4 Hz], 8.20 [1H, m], 8.89 [1H, m], 9.05 [1H, m], 9.34 [1H, s]; 13C{1H} NMR (100
13 1 MHz, DMSO-D6) δ 16.1, 56.7, 127.7, 133.7, 143.1, 144.5, 146.0, 162.7; C{ H} NMR
(100 MHz, D2O vs. NaDSS) δ 18.3, 60.7, 131.0, 136.6, 146.5, 146.8, 149.0, 168.6.
109 1-Ethyl-3-carbamoylpyridinium chloride [4]+Cl- (R: Et). Scheme 4.1. 1-Ethyl-
1,4-dihydronicotinamide (1.03 g, 6.77 mmol), carbon O tetrachloride (1.60 g, 10.4 mmol), and 1,4-dioxane (10 NH 2 mL) were stirred in a Schlenk flask under argon at room
N Cl temperature for 48 h. The thick beige precipitate that
formed was filtered under argon and washed with toluene to afford the product as a beige hygroscopic solid (0.81 g, 64 %); 1H NMR
3 3 (600 MHz, DMSO-D6) δ 1.56 [3H, t, JHH = 7.3 Hz], 4.71 [2H, q, JHH = 7.3 Hz], 8.17
[1H, br], 8.26 [1H, m], 8.87 [1H, br], 9.01 [1H, m], 9.27 [1H, m], 9.70 [1H, s]; 1H
NMR (600 MHz, D2O vs. NaDSS) δ 1.67 (3H, t, J = 7.4 Hz), 4.74 (2H, q, J = 7.4 Hz),
8.20 (1H, m), 8.89 (1H, m), 9.05 (1H, m), 9.34 (1H, s); 13C{1H} NMR 150 MHz,
13 1 DMSO-D6) δ 14.2, 56.7, 127.7, 133.6, 143.4, 144.6, 146.1, 162.7; C{ H} NMR (150
MHz, D2O vs. NaDSS) δ 18.3, 60.7, 131.1, 136.6, 146.5, 146.8, 148.9, 168.6.
1-Methyl-3-carbamoylpyridinium iodide [4]+I- (R: Me). Nicotinamide (0.52 g,
40 mmol), methanol (5 mL), and freshly distilled methyl O iodide (7.1 g, 50 mmol) were stirred magnetically in a NH2 Schlenk tube and heated at 80 °C for 2 h. The yellow N I solution was evaporated under reduced pressure (0.1
Torr) and the resulting off-white solid crystallized from
1 iso-propanol as colorless needles (92 %); m.p. 182 °C; H NMR (400 MHz, D2O vs.
NaDSS) δ 4.35 [3H, s, CH3], 8.07 [1H, t, J], 8.78 [1H, d], 8.86 [1H, d], 9.18 [1H, s];
13 1 C{ H} NMR (100 MHz, D2O vs. NaDSS) δ 45.8, 133.8, 143.7, 145.4, 146.2, 165.6.
110 6.5 Chapter 5 – Reaction and Spectroscopic Data
1-Phenyl-3-tert-butylimidazolidinium bromide [1]+Br- (R1: Ph, R2: tBu).
Scheme 5.3. Ammonium bromide (0.67 g, 6.84 mmol) was
added to a 4 mL triethyl orthoformate solution of N-phenyl- N H N’-tert-butylethylenediamine (1.20 g, 6.24 mmol) in a 20 mL N Br round bottom flask. The mixture began to effervesce as the
ammonium bromide slowly disappeared. The mixture was
refluxed in open air for 12 h, forming a beige precipitate. The solvent was removed under reduced pressure (0.1 Torr) and the residue washed with hexanes (3 x 5 mL) to afford an off-white solid (1.76 g, 99 %) that was recrystallized from iso-propanol as white needles (1.52 g, 86 %); m.p. 240 – 242
1 °C; H NMR (400 MHz, CDCl3,) δ 1.62 [9H, s, C(CH3)3], 4.27 [2H, m, CH2CH2], 4.50
3 [2H, m, CH2CH2], 7.16 [1H, t, JHH = 7.4 Hz, para-CH], 7.36 [2H, m, meta-CH], 7.56
3 + 13 1 [2H, d, JHH = 8.0 Hz, ortho-CH], 9.31 [1H, s, N2CH ]; C{ H} NMR (100 MHz, CDCl3)
δ 28.4 [C(CH3)3], 46.0 [CH2CH2], 48.6 [CH2CH2], 58.5 [C(CH3)3], 118.4, 126.9,
+ 129.8, 135.7, 152.4 [N2CH ].
1-Phenyl-3-tert-butylimidazolidine 1H (R1: Ph, R2: tBu). Scheme 5.4. Diethyl
ether (10 mL) was added to a solid mixture of N-phenyl-N’-tert-
butylethylenediamine (1.44 g, 7.49 mmol) and N paraformaldehyde (0.47 g, 15.7 mmol) in a Schlenk flask. N Heating at 50 °C for 2 d generated a suspension of fine white
solid in a light yellow supernatant. The solvent was removed
111 under reduced pressure (0.1 Torr), and the residue was redissolved in hexanes and filtered to remove unreacted paraformaldehyde. The filtrate was evaporated under reduced pressure to afford an off-white solid (1.43 g, 94 %); m.p. 69 – 71 °C;
1 3 H NMR (400 MHz, CDCl3) δ 1.18 [9H, s, C(CH3)3], 3.04 [2H, t, JHH = 6.2 Hz,
3 3 CH2CH2], 3.43 [2H, t, JHH = 6.2 Hz, CH2CH2], 6.51 [2H, d, JHH = 7.8 Hz, ortho-CH],
3 13 1 6.69 [1H, t, JHH = 7.3 Hz, para-CH], 7.23 [2H, m, meta-CH]; C{ H} NMR (100 MHz
CDCl3) δ 26.0 [C(CH3)3], 45.0 [CH2CH2], 46.6 [CH2CH2], 52.1 [C(CH3)3], 64.0
[N2CH2], 111.5 [meta-C], 116.1 [para-C], 129.1 [ortho-C], 146.6 [ipso-C]; GC-MS
12.04 min m/z (rel. int. %) 204(88)[M+], 203(76), 189(71), 147(99),
106(100), 91(26), 84(34), 77(26).
1-Phenyl-3-tert-butylimidazolidinium chloride [1]+Cl- (R1: Ph, R2: tBu).
Scheme 5.4. 1-Phenyl-3-tert-butylimidazolidine (0.20 g, 0.98
mmol) and carbon tetrachloride (0.18 g, 1.8 mmol) were N H heated in a Schlenk tube under argon at 100 °C for 3 d. The N Cl off-white precipitate was washed with diethyl ether (3 x 5
mL) and crystallized from iso-propanol as white needles
1 (0.07 g, 30 %); H NMR (400 MHz, CDCl3) δ 1.63 [9H, s,
3 C(CH3)3], 4.25 [2H, m, CH2CH2], 4.47 [2H, m, CH2CH2], 7.24 [1H, t, JHH = 7.5 Hz,
3 para-CH], 7.41 [2H, m, meta-CH], 7.73 [2H, d, JHH = 8.0 Hz, ortho-CH], 10.0 [1H, s,
+ 13 1 N2CH ]; C{ H} NMR (100 MHz, CDCl3) δ 28.4 [C(CH3)3], 45.7 [CH2CH2], 48.1
+ [CH2CH2], 58.6 [C(CH3)3], 118.3, 126.8, 129.9, 136.0, 153.6 [N2CH ].
112 1-Phenyl-3-tert-butyl-imidazol-4,5-dihydro-2-ylidene. Scheme 5.5. 1-Phenyl-
3-tert-butylimidazolidinium bromide (50 mg, 0.14 mmol) was
added to an NMR tube and dried under reduced pressure (0.1
N Torr) with heating in an aluminum block at 130 °C for 25 min.
N In a separate Schlenk tube, KHMDS solution (0.25 mL, 0.5 M in
toluene) was evaporated under reduced pressure with heating
in an aluminum block at 90 °C to constant mass and
redissolved in pre-dried (CaH2) benzene-D6 (1.7 mL). The solution was added to the carbenium salt and the NMR tube was flame-sealed.
After 36 h at room temperature, a fine precipitate formed in a yellow solution. 1H
NMR C6D6, 600 MHz: δ 1.32 [9H, s, C(CH3)3], 2.92 [2H, m, CH2CH2], 3.09 [2H, m,
3 CH2CH2], 6.95 [1H, t, JHH = 7.3 Hz, para-CH], 7.25 [2H, m, meta-CH], 7.69 [2H, d,
3 13 1 JHH = 8.6 Hz, ortho-CH]; C{ H} NMR C6D6, 150 MHz: δ 29.8, [C(CH3)3], 44.8
[CH2CH2], 46.6 [CH2CH2], 54.9 [C(CH3)3], 116.5 [meta-C], 121.6 [para-C], 129.0
[ortho-C], 145.9 [ipso-C], 237.7 [N2C:].
1,3-Di-iso-propylimidazolidinium iodide [1]+I- (R: iPr). Scheme 5.2. 1,3-Di-iso-
propylethylenediamine (1.71 g, 12.0 mmol) and ammonium
iodide (1.76 g, 12.0 mmol) were heated in a Schelnk tube at I N 120 °C until the evolution of ammonia ceased. Triethyl H N orthoformate (3.57 g, 24.0 mmol) was added and refluxed
for 12 h. The solvent was removed under reduced pressure
and the resulting solid was washed with benzene (2 x 5 mL)
113 1 to afford a white solid (3.10 g, 93 %); m.p. 152 – 154 °C; H NMR (400 HMz, CDCl3)
3 3 δ 1.42 [2H, sep, JHH = 7.1 Hz, C-H], 4.01 [s, 4H, CH2CH2], 4.24 [12H, d, JHH =
+ 13 7.1 Hz, C(CH3)2], 9.21 [1H, s, N2CH ]; C NMR (400 HMz, CDCl3) δ 20.9, [C-
+ (CH3)2], 45.3 [CH2CH2], 50.6 [C-(CH3)2] 155.1 [N2CH ].
1,3-Di-para-tolylimidazolidinium iodide [1]+I- (R: pTol). Scheme 5.2. 1,3-Di-
para-tolylethylenediamine (0.78 g, 3.0 mmol) and
ammonium iodide (0.47 g, 3.0 mmol) were heated in a
Schelnk tube at 120 °C until the evolution of ammonia
I N ceased. Triethyl orthoformate (1.1 g, 7.0 mmol) was added H N and refluxed for 12 h. The solvent was removed under reduced pressure and the resulting solid was washed with
benzene (2 x 5 mL) to afford a white solid (0.85 g, 89 %);
1 m.p. 199 – 201 °C (dec.); H NMR (400 HMz, CDCl3) δ 2.33
3 [6H, s, C-CH3], 4.64 [4H, s, CH2CH2], 7.12 [4H, d, JHH = 8.4 Hz, meta-CH], 7.62
3 + 13 [4H, d, JHH = 8.4 Hz, ortho-CH], 9.92 [1H, s, N2CH ]; C NMR (400 HMz, CDCl3)
+ δ 21.0, [C-CH3], 49.2 [CH2CH2], 113.2, 119.1, 130.6, 132.5, 138.3 [N2CH ].
1,3-Diphenylimidazolidinium bromide [1]+Br- (R: Ph). Scheme 5.5. Impure
1,3-diphenylethylenediamine/1,3-diphenylpiperazine mixture (80 % : 20 %) (18.6 g), ammonium bromide (7.82 g, 79.8 mmol), and triethyl orthoformate (35.5 g,
240 mmol) were refluxed in a 250 mL round bottom flask for 12 h. The solvent was removed under reduced pressure (0.1 Torr) and the residue was washed with hexanes (3 x 10 mL). Crystallization of the residue from methanol formed
114 colorless plates of pure N,N’-diphenylpiperazine. The
mother liquor was evaporated under reduced pressure, and
the resulting brown residue was washed with methylene N Br chloride and crystallized from water to afford a white solid H N (19.7 g, 90 %); m.p. 252 – 254 °C (two times); 1H NMR (600
3 MHz, DMSO-D6) δ 4.61 [4H, s, CH2CH2], 7.40 [2H, t, JHH =
3 7.4 Hz, para-CH], 7.57 [4H, m, meta-CH], 7.67 [4H, d, JHH
+ 13 1 = 7.7 Hz, ortho-CH], 9.99 [1H, s, N2CH ]; C{ H} NMR (150 MHz, DMSO-D6) δ 48.2
[CH2CH2], 118.4 [meta-C], 126.9 [para-C], 129.6 [ortho-C], 136.0 [ipso-C], 151.7
+ [N2CH ].
1 N,N’-Diphenylpiperazine. M.p. 160 – 162 °C; H NMR (600 MHz, CDCl3) δ 3.35
3 [8H, s, CH2CH2], 6.89 [2H, t, JHH = 7.4 Hz, para-CH], 6.99 [4H, m, meta-CH],
13 1 7.29 [4H, m, ortho-CH]; C{ H} NMR CDCl3, 150 MHz: δ 49.4 [CH2CH2], 116.3,
120.1, 129.2, 151.2 [ipso-C]; GC-MS 15.44 min m/z (rel. int. %) 238(100)[M+],
223(9), 132(51), 105(95), 104(49), 91(15), 77(37).
6.6 References
1. Denk, M. K.; Gupta, S.; Brownie, J.; Tajammul, S.; Lough, A. J. Chem.-Eur. J. 2001, 7, 4477. 2. Denk, M. K.; Rodezno, J. M.; Gupta, S.; Lough, A. J. J. Organomet. Chem. 2001, 617-618, 242. 3. Eisner, U.; Kuthan, J. Chem. Rev. 1972, 72, 1. 4. Paul, C. E.; Gargiulo, S.; Opperman, D. J.; Lavandera, I; Gotor-Fernández, V.; Gotor, V; Taglieber, A.; Arends, I. W. C. E.; Hollmann, F. Org. Lett. 2013, 15, 180.
115 5. Denk, M. K. performed quantum chemical calculations. 6. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery Jr, J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. GAUSSIAN 09 (Revision D.01), Gaussian Inc., Wallingford, CT, 2013. 7. Montgomery, J. A.; Frisch, M. J.; Ochterski, J. W.; Petersson, G. A. J. Chem. Phys. 2000, 112, 6532. 8. Marenich, A.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B, 2009, 113, 6378. 9. Denk, M. K.; Milutinović, N. S.; Marczenko, K. M.; Sadowski, N. M.; Paschos, A. Chem. Sci. 2017, 8, 1883. 10. SDBSWeb: http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology 09/2015–09/2017) 11. Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.; Nudelam, A.; Stolz, B. M.; Bercaw, J. E.; Goldberg, K. I. Organometallics, 2010, 29, 2176. 12. Denk, M. K.; Milutinović, N. S. Chemosphere, 2018, 191, 408.
116
Appendix
Representative Compound Spectra
117 Cl 4,4’-DDT Cl Cl PESTANALTM, analytical standard Sigma-Aldrich Product Number: 31041 Cl Cl Batch Number: BCBS5338V Purity (GC Area %) SPECIFICATION RESULT ≥ 98.0 % 99.3 %
1 H NMR (600 MHz, CDCl3)
118 1 H NMR (600 MHz, C6D6)
119 13 C JMOD NMR (150 MHz, CDCl3)
120 13 C JMOD NMR (150 MHz, C6D6)
121 4,4’-DDD Cl Cl PESTANALTM, analytical standard Sigma-Aldrich Product Number: 35486 Cl Cl Batch Number: BCBS3969V Purity (GC Area %) SPECIFICATION RESULT ≥ 98.0 % 99.9 %
1 H NMR (600 MHz, CDCl3)
122 1 H NMR (600 MHz, C6D6)
123 13 C JMOD NMR (150 MHz, CDCl3)
124 13 C JMOD NMR (150 MHz, C6D6)
125 4,4’-DDE Cl Cl PESTANALTM, analytical standard Sigma-Aldrich Product Number: 35487 Cl Cl Batch Number: BCBS5816V Purity (GC Area %) SPECIFICATION RESULT ≥ 98.0 % 99.8 %
1 H NMR (600 MHz, CDCl3)
126 1 H NMR (600 MHz, C6D6)
127 13 C JMOD NMR (150 MHz, CDCl3)
128 13 C JMOD NMR (150 MHz, C6D6)
129 4,4’-DDMU H Cl PESTANALTM, analytical standard Sigma-Aldrich Product Number: 35489 Cl Cl Batch Number: SZBC313XV Purity (GC Area %) SPECIFICATION RESULT ≥ 98.0 % 99.8 %
1 H NMR (600 MHz, CDCl3)
130 1 H NMR (600 MHz, C6D6)
131 13 C JMOD NMR (150 MHz, CDCl3)
132 13 C JMOD NMR (150 MHz, C6D6)
133 4,4’-DDM PESTANALTM, analytical standard Cl Cl Sigma-Aldrich Product Number: 35488 Batch Number: SZBD302XV Purity (GC Area %) SPECIFICATION RESULT ≥ 98.0 % 99.0 %
1 H NMR (600 MHz, CDCl3)
134 1 H NMR (600 MHz, C6D6)
135 13 C JMOD NMR (150 MHz, CDCl3)
136 13 C JMOD NMR (150 MHz, C6D6)
137 4,4’-Dichlorobenzophenone O Sigma-Aldrich Product Number: 113700 Cl Cl Lot Number: MKBN8261V Assay 99 %
1 H NMR (600 MHz, CDCl3)
138 1 H NMR (600 MHz, C6D6)
139 13 C JMOD NMR (150 MHz, CDCl3)
140 13 C JMOD NMR (150 MHz, C6D6)
141 1-Ethyl-1,4-dihydronicotinamide O
NH2
N
1 H NMR (600 MHz, CDCl3)
142 1 H NMR (600 MHz, D2O)
143 13 C JMOD NMR (150 MHz, CDCl3)
144 13 C NMR (150 MHz, D2O)
145 1-Ethyl-3-carbamoylpyridinium bromide O
NH2
N Br 1 H NMR (600 MHz, D2O)
146 1 H NMR (600 MHz, DMSO-D6)
147 13 C NMR (150 MHz, D2O)
148 13 C NMR (150 MHz, DMSO-D6)
149 1-Ethyl-3-carbamoylpyridinium iodide O
NH2
N I 1 H NMR (400 MHz, D2O)
150 13 C NMR (100 MHz, D2O)
151