DOCSLIB.ORG

Benzoin Condensation

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Benzoin Condensation GENERAL ARTICLE Benzoin Condensation The Cyanide Connection with Tapioca and Vitamin B1 Gopalpur Nagendrappa Benzoin condensation is an important carbon–carbon bond forming reaction. It is achieved by generating an acyl anion equivalent from one aldehyde molecule which adds to a second aldehyde molecule. The reaction is traditionally catalysed by a cyanide ion. Cyanohydrin anion is the first intermediateandistheprecursortotheacylanionequivalent. G Nagendrappa, retired Cyanohydrins are found in plants as glycosides. A reaction from Bangalore Univer- completely analogous to benzoin condensation occurs in our sity, Bangalore, is body, which however neither involves cyanohydrin interme- presently Professor and diate nor is catalysed by cyanide ion. It is catalysed by the Head of the Department of Medicinal Chemistry, Sri thiazolium moiety of the co-enzyme thiamine pyrophosphate Ramachandra University, (TPP). This article shows the common links and inclusive Porur, Chennai. His main chemistry aspects among cyanohydrin formation, naturally work is in the areas of occurring cyanohydrins, conversion of cyanohydrins to ben- organosilicon chemistry, organic synthesis, reaction zoins/acyloins, the role of vitamin B1 (thiamine) andthe use of mechanism and synthetic thiazolium compounds in benzoin/acyloin condensation. methodologies. Introduction Cyano Group in Natural Products 1. Cyanoglycosides Hydrogen cyanide is a deadly poisonous substance. A variety of plants produce it, though in the hidden form of cyanoglycosides, the sugar derivatives of cyanohydrins. Cyanohydrins are formally the products of HCN addition to ketones or aldehydes, the addi- Keywords tion being reversible. Cyanoglycosides hydrolyse enzymatically Benzoin condensation, acyloin as well as nonenzymatically in the body to sugar and cyanohy- condensation, cyanoglycosides, cyanohydrins, vitamin B cataly- drins, which release hydrogen cyanide, Scheme 1. Ingestion of 1 sis, cyanide catalysis, thiazolium cyanoglycosides through consumption of such plant materials salt catalysis, nitrile-containing may cause cyanide poisoning if the cyanide concentration in the natural products. RESONANCE April 2008 355 GENERAL ARTICLE 1 1 R R R1 hydrolysis Sugar O + sugar + HCN 2 O 2 OH R R 2 CN CN R Linamarin: R1 = R2 = CH3; Sugar = -D-glucose Amygdalin: R1 = Ph, R2 = H; Sugar = -D-gentiobiose Prunarin: R1 = Ph, R2 = H; Sugar = -D-glucose Scheme 1. body goes beyond the tolerance limit, (see Box 1). This works as a defense mechanism in plants, since high cyanoglycoside con- tent makes such plant parts (seeds, roots, leaves, etc.) bitter. However, we consume many food materials that contain cyanoglycosides. Box 1. Toxicity and Detoxification of Cyanide Cyanide’s Murderous Course: Thecyanideinhaled asHCN orconsumedas itssalts such asNaCN, KCNetc, orreleased in the bodyon hydrolysisof cyanogenicglycosides eaten as food,causes poisoning. Orl-hmn LDL0 –1 –1 –1 = 2857gkg for NaCN or KCN. Orl-rat LD50 = 5 mgkg for KCN, and 6.44 mg kg for NaCN. This is How it Poisons: Cyanide ion binds very strongly to Fe3+ ofmethemoglobin in mytochondria and forms cyanomethemoglobin, which cannot carry oxygen to tissues. The cellular respiration is arrested and the death is imminent. _ _ 3+ HbFe + CN HbFe3+CN Methemoglobin Cyanohemoglobin Detoxification: Small amounts of cyanide present as glycosides in foods we consume is detoxified by the enzyme rhodanase present in liver, erythrocytes, and other tissues by facilitating the conversion of cyanide to thiocyanate.MinorquantitiesofCN– areremovedbyoxidationtocyanate(CNO–)orcombiningwithcobalamin to form cyanocobalamin (vitamin B12). _ _ HbFe3+CN HbFe3+ + CN _ _ Rhodana+ se CN SCN a sulphurtransferase – Antidote: Nitrite (NO2 ) functions as antidote for cyanide poisoning. Nitrite is administered either by – inhalation or injection, oxygen being given as adjunct along with or after NO2 . Giving oxygen alone is not effective. 356 RESONANCE April 2008 GENERAL ARTICLE Roots of cassava (tapioca), an important food crop in many Even in the case of countries of the world, including India, contain acetone cyanohy- apple seeds, seeds of drin glucoside called linamarin. Tapioca that contains linamarin other fruits, and bitter in excess of 100 mgkg–1 of fresh roots is not recommended for tapioca which have a food use. Even then, to make it suitable for edible purpose it has relatively high to be processed properly to bring down the toxin content to less cyanoglycoside than 50 mgkg–1, a limit that is considered as acceptable. content, one has to A long recognised source of a cyanogenic glucoside is bitter consume huge almond, whosebitterness is due to amygdalin, or D-mandelonitrile- quantities of them -D-gentiobioside, (Scheme 1). Historically, benzaldehyde was before they could first obtained from bitter almonds. pose toxicity problem. Seeds of apple, peaches, plums, apricots, cherries, etc., contain considerable amounts of amygdalin. Cyanoglycosides are present even in common edible plants such as sorghum, soybeans, lima beans, maize, millet, sweet potatoes, spinach, sugar cane, and bamboo shoots. However, the toxin content in these is low, and is handled by liver and eliminated, (see Box 1). Even in the case of apple seeds, seeds of other fruits, and bitter tapioca which have a relatively high cyanoglycoside content, one has to consume huge quantities of them before they could pose toxicity problem. The name cyanide evokes, in laypersons, a spectre of poisoning and instant death, made famous by authors of detective stories. Indeed, cyanide is one of the most toxic substances. Detoxifica- tion of minor amounts of cyanide in the body is brought about by the enzyme rhodanase present in liver, erythrocytes, and other tissues, through its rapid conversion to thiocyanate, (see Box 1). Some insects and mollusks eat plants containing cyanogenic glycosides and accumulate sufficient quantities of these glyco- Detoxification of minor sides which serve as defense against their predators. amounts of cyanide in the body is brought 2. Non-glycosidic Cyano Compounds about by the enzyme Cyano group occurs in natural products not only in cyanohydrin rhodanase present in glycosides, but also in non-cyanohydrin form. Some examples liver, erythrocytes, and are given in Figure 1. other tissues. RESONANCE April 2008 357 GENERAL ARTICLE H Me Me CH2CONH2 H2NOC CH2 H Me CH2CH2CONH2 N CN N Me Co+ H NH N H Me H NOCH C 2 2 Me Me H H2C Me CH2CH2CONH2 CH2 N Me CO HN H2C N Me O O Me OH O P H H -O H Vitamin B12 O CH2OH H H3C NC NC NC O CN Glc-O Glc-O Glc-O OR1 O OR2 N N O 3 CH3 OH OH OR Ricinin (Alkaloid) (name depends on Merisclausin Ehreticide B (from Castor oil seeds) R1, R2, R3 ) Non cyanogenic cyanoglycosides Figure 1. Nitrile-(Cyano Group) Containing Drugs Though cyanohydrins pose as much toxicity problem as the inorganic cyano compounds, (NaCN, KCN, HCN, Cu(CN)2, K3Fe- (CN)6, etc.,), the other types of organic nitriles may not be as toxic 358 RESONANCE April 2008 GENERAL ARTICLE Figure 1. Continued... O OH O O NH O NC NC MeO H N O N H H O H H O Saframycin A OMe Cyanopuupehenone (antibiotic and antitumor activities) (antiviral activitiy against HIV II) OH O MeO O N H NMe2 OH N O H2 O3PO OH H O H O CN OH OMe Calyculin J (antitumour activitiy) Br R2 R1O CN 1 2 3 R = H, R = R = CH2CH = C(CH3)2 1 2 3 R = R = H, R = CH2CH = C(CH3)2 R1 = R2 = R3 = H CN OR1 3 R Epurpurins RESONANCE April 2008 359 GENERAL ARTICLE H3C CH3 CH3 N CN OCH3 H3CO (S)-Verapamil OCH 2+ 3 H3CO (Ca channel blocker) O CN N N CH3 N O N N Ph + + N (H / K ATPase inhibitor) CN Zaleplan (ultrashort acting sleep inducer) Ph Ph NC N H C H H Ph 3 N N S CH3 CO2C2H5 HN Diphenoxylate N N (antidiarrheal agent) CN Cimetidine (Tagamet) (antiulcer drug) Figure 2. because they do not release the cyanide easily. In fact, a good number of common drug molecules contain cyano group as an important functional group. A few examples are given in Figure 2. Nitrile (Cyano Group):A Versatile Intermediary Func- tional Group Synthetic routes of even larger number of drugs employ nitriles as 360 RESONANCE April 2008 GENERAL ARTICLE 2 OH OR2 OR LDA _ RCHO + HCN R C H R C H R C CN CN CN 2 R = O , R1X R1 X = Variety of electrophiles OR2 1 R R 1 Hydrolysis R C R C O CN intermediates because of their versatility for further useful trans- Scheme 2. formation. The nitrile can be transformed into a variety of other functions such as amine, carboxylic acid, ester, amide, ketone, aldelhyde, heterocycle and others. Consequently nitriles have acquired great importance as intermediates in the manufacture of chemicals, including drugs. Being a strong electron withdrawing group the cyano group facilitates the formation of -carbanion and then ensures its stability. This has been exploited in pole reversal (umpolung) of aldehydic carbon from being electrophilic to nucleophilic, as represented in Scheme 2. Cyanide: A Good Nucleophile Cyano group can be introduced into organic molecules by a variety of methods. Since cyanide is one of the very effective and The cyano group efficient classic nucleophiles, the most common methods of facilitates the cyanation exploit this property, such as in (1) the direct displace- formation of - ment of halides, tosylates or similar leaving groups by cyanide, carbanion and then and (2) the addition of cyanide to aldehydes, ketones and their ensures its stability. ,-unsaturated analogues. This has been Cyanide’s high nucleophilic efficiency is due to its easy exploited in pole polarisability and low steric hindrance to its attack. (It is the reversal (umpolung) smallest carbon nucleophile). It is ranked high in the nuclephilicity of aldehydic carbon. RESONANCE April 2008 361 GENERAL ARTICLE - - - - - - - - - - RS > ArS > I > CN > OH > N3 > Br > ArO > Cl > Pyr > AcO > H2O Figure 3. - order for SN2 reactions (in protic solvents), much above OH , (Figure 3).
Load more

Recommended publications
  • Sep. 5, Hydroacylation by Brandon Reinus
    Hydroacylation and Related Topics Dong Group Seminar Brandon Reinus Wed, Sept. 5th 2012 Why? ¡ Looking at the reaction, it is a highly atom- economical approach to synthesizing ketones ¡ Umpolung (ex: deprotonating dithioacetals) ¡ Using acrylate derivatives generates a 1,4 diketone relationship, a hard relationship to establish using classical organic synthesis. Presentation Overview 1. Hydroformylation (extremely brief) 2. Rh-Catalyzed Hydroacylation ¡ Intramolecular ¡ Intermolecular ¡ Other 3. NHC Catalyzed Hydroacylation ¡ Benzoin reaction ¡ Stetter reaction ¡ other Part 1 : Background Reppe Roelen Science of Synthesis, Stereoselective Synthesis 1, 2011, pg.409 Hydroformylation Metal-Organic Cooperative Catalysis Park et al. Introduction SCHEME 2 Among the many elegant examples of transition metal cat- alyzed activation of C-HandC-Cbonds,1 chelation-as- sisted protocols have recently attracted increasing attention in organometallic chemistry. Directed metalation processes have been demonstrated by valuable applications in organic synthesis, showing remarkable efficiency and chemoselectivity.2 In general, a chelation-assisted proto- col facilitates the formation of either the kinetically or ther- modynamically favored five- or six-membered metallacycle; aprepositionedcoordinatinggroupinducesspatialproxim- ing decarbonylation, their structures are too specific to apply ity between the C-HorC-Cbondsandthetransitionmetal for common aldehydes. center.1,2 Despite the magnificent usefulness in activating otherwise stable C-HandC-Cbonds,amajordrawbackof
    [Show full text]
  • Benzoin and Stobbe Reactions
    ____________________________________________________________________________________________________ Subject Chemistry Paper No and Title 9; Organic Chemistry-III (Reaction Mechanism-2) Module No and Title 21; Named Reactions: Benzoin Condensation and Stobbe Condensation Module Tag CHE_P9_M21 CHEMISTRY Paper 9: Organic Chemistry-III (Reaction Mechanism-2) Module NO. 21: Benzoin Condensation and Stobbe Condensation ____________________________________________________________________________________________________ TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. Benzoin Condensation 3.1 Mechanism of Benzoin Condensation 3.2 Characteristics of Benzoin Condensation 3.3 Reactions of Benzoin 4. Stobbe Condensation 4.1 Mechanism of Stobbe Condensation 4.2 Characteristics of Stobbe Condensation 4.3 Few Examples of Stobbe Condensation 5. Summary CHEMISTRY Paper 9: Organic Chemistry-III (Reaction Mechanism-2) Module NO. 21: Benzoin Condensation and Stobbe Condensation ____________________________________________________________________________________________________ 1. Learning Outcomes After studying this module, you shall be able to: Know what are Benzoin condensation and Stobbe condensation reactions Learn mechanism of Benzoin and Stobbe condensation reactions Know about the role of CN- ion in Benzoin condensation Identify the products of reduction and oxidation of Benzoin condensation Understand the product formation in Benzoin and Stobbe condensation. 2. Introduction A condensation reaction, also commonly referred to as dehydration
    [Show full text]
  • CHEM 33 Unless You Have Completed the Laboratory and Turned in a Notebook, So Be Sure to Turn in Your Notebook on Time to Your Laboratory Instructor
    SANTA CLARA UNIVERSITY SUMMER TERM 2019 ORGANIC CHEMISTRY III CHEMISTRY 33L LABORATORY SYLLABUS INSTRUCTORS Raymond Gipson, Ph.D.; Beatrice Ruhland, Ph.D. TIMES • Monday/Wednesday: 8:00-12:30 p.m. • Monday/Wednesday: 4:30-9:00 p.m. • Tuesday/Thursday: 8:00-12:30 p.m. • Tuesday/Thursday: 4:30-9:00 p.m. Laboratories begin on Monday, July 29th and are held in DS 126 MATERIALS 1. Course syllabus (available on Canvas, Chem33 lab course page or on google sites https://sites.google.com/a/scu.edu/organic-chemistry-laboratory/, and from Copy Craft Printing, 341 Lafayette St.) 2. Laboratory Techniques in Organic Chemistry by Mohrig, Alberg, Hofmeister, Schatz and Hammond, 4th Edition 3. Bound laboratory notebook embossed with "Santa Clara University", Scientific Notebook Company (available from the bookstore); you may use the same notebook as you purchasEd for Chem32L 4. Safety splash goggles 5. Laboratory coat (available from the bookstore) LABORATORY GUIDELINES 1. The laboratory in Daly Science 100 were modernized and redesigned in 1994, with safety as the primary concern. The main improvement in these laboratories was increasing the number of hoods so that each student has a workstation in a hood, two students per hood. This significant enhancement in safety protects everyone because many organic compounds are volatile and using a hood minimizes your exposure to fumes. Therefore, virtually 100% of your chemical work should be performed in a fume hood. 2. To do organic chemistry safely, you should treat all chemicals with respect; glovEs should be worn unless otherwise directed by the instructor. 3.
    [Show full text]
  • Mechanisms and Uses of Aldol Condensations
    Mechanisms and Uses of Aldol Condensations Tom Crowley Missouri Western State University CHE 445 Advanced Topics in Chemistry Advanced Organic Dr. Steven P. Lorimor 4-5-2007 Aldol condensations form a very important class of reactions in organic synthesis. The reaction was discovered independently by Charles-Adolph Wurtz and Alexander Porfyrevich Borodin in 1872. The name aldol was chosen because the product of an aldol condensation often contains an aldehyde and an alcohol group. Aldol condensations are extremely important in pharmaceuticals, used in the production of Lipitor, the immunosuppressant FK506, tetracycline antibiotics, and the antifungal agent Amphotericin B. Aldol condensations are also very important in biological processes, the breakdown of glucose in cells through glycolysis uses enzyme catalyzed aldol reactions.1 In general, an aldol condensation is the attack of a nucleophile on a carbonyl to make a β-hydroxy ketone or aldehyde. Usually the nucleophile is an enolate of an aldehyde or ketone that attacks another molecule of the aldehyde or ketone. The aldol condensation can be catalyzed by either an acidic or basic solution. The mechanism for the aldol condensation is as follows:2 Acid catalyzed aldol condensation Base catalyzed aldol condensation. Ketones are harder to use in aldol condensations, they usually produce much smaller yields than aldehydes. 2 Aldol condensations are reversible, forming equilibria. To drive an aldol reaction to completion, dehydration is used to remove the aldol product from the reaction. The dehydration can also be carried out by acidic or basic solutions. Prior to the development of the Wittig reaction, an aldol condensation followed by dehydration was the best way to link two molecules by a carbon-carbon double bond.
    [Show full text]
  • Cyanohydrin Reaction
    Cyanohydrin reaction A cyanohydrin reaction is an organic chemical reaction by an Cyanohydrin reaction aldehyde or ketone with a cyanide anion or a nitrile to form a Named after Friedrich Urech cyanohydrin. This nucleophilic addition is a reversible reaction but with aliphatic carbonyl compounds equilibrium is in favor of the Reaction type Addition reaction reaction products. The cyanide source can be potassium cyanide, sodium cyanide or trimethylsilyl cyanide. With aromatic aldehydes such as benzaldehyde, the benzoin condensation is a competing reaction. The reaction is used in carbohydrate chemistry as a chain extension method for example that of D-xylose. Contents Examples Reaction mechanism Asymmetric synthesis References External links Examples Reaction of acetone with sodium cyanide to hydroxyacetonitrile Reaction of benzoquinone with trimethylsilylcyanide, catalyst KCN is introduced as a 1:1 complex with the Crown ether 18-crown-6 Reaction mechanism Asymmetric synthesis The asymmetric cyanohydrin reaction of benzaldehyde with trimethylsilylcyanide is made possible by employment of (R)-Binol[1] at 1–10% catalyst loading. This ligand firsts reacts with a lithium alkoxy compound to form a lithium binaphtholate Complex. Asymmetric reaction of benzaldehyde with (R)–Binol–lithium(i-propyloxy) gives (S)-acetonitrile with 98% ee The chemist Urech in 1872 was the first to synthesize cyanohydrins from ketones with alkali cyanides and acetic acid[2] and therefore this reaction also goes by the name of Urech cyanohydrin method. With HCN in acidic conditions – i.e. the cyanohydrin is the functional group CN–C–OH. References 1. Hatano, Manabu; Ikeno, Takumi; Miyamoto, Takashi; Ishihara, Kazuaki (2005). "Chiral Lithium Binaphtholate Aqua Complex as a Highly Effective Asymmetric Catalyst for Cyanohydrin Synthesis".
    [Show full text]
  • Enzyme Catalyzed Cross Acyloin Reactions Via C-C Bond Cleavage
    ENZYME CATALYZED CROSS ACYLOIN REACTIONS VIA C-C BOND CLEAVAGE A THESIS SUBMITTED TO THE GRADUATE SCHOOL OF NATURAL AND APPLIED SCIENCES OF MIDDLE EAST TECHNICAL UNIVERSITY BY GÖKÇİL BİLİR IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN CHEMISTRY DECEMBER 2015 Approval of the thesis ENZYME CATALYZED CROSS ACYLOIN REACTIONS VIA C-C BOND CLEAVAGE submitted by GÖKÇİL BİLİR in partial fulfillment of the requirements for the degree of Master of Science in Chemistry Department, Middle East Technical University by, Prof. Dr. Gülbin Dural Ünver Dean, Graduate School of Natural and Applied Sciences Prof. Dr. Cihangir Tanyeli Head of Department, Chemistry Assist. Prof. Salih Özçubukçu Supervisor, Chemistry Dept., METU Examining Committee Members: Prof. Dr. Cihangir Tanyeli Dept. of Chemistry, METU Assist. Prof. Dr. Salih Özçubukçu Dept. of Chemistry, METU Prof. Dr. Özdemir Doğan Dept. of Chemistry METU Assist. Prof. Dr. Yunus Emre Türkmen Dept. of Chemistry, Bilkent University Assist. Prof. Dr. Ferdi Karadaş Dept. of Chemistry, Bilkent University Date: I hereby declare that all information in this document has been obtained and presented in accordance with academic rules and ethical conduct. I also declare that, as required by these rules and conduct, I have fully cited and referenced all material and results that are not original to this work. Name, Last name: Gökçil Bilir Signature iv ABSTRACT ENZYME CATALYZED CROSS ACYLOIN REACTIONS VIA C-C BOND CLEAVAGE Bilir, Gökçil M.Sc., Department of Chemistry Supervisor: Assist. Prof. Dr. Salih Özçubukçu December 2015, 67 pages Enzyme-catalysis is an effective tool for asymmetric synthesis under environmentally-friendly conditions, with an increasing number of examples at industrial scale.
    [Show full text]
  • (Thdp)-Dependent Enzymes for Carboligation
    Dörte Gocke New and optimised thiamine diphosphate (ThDP)-dependent enzymes for carboligation Creation of a toolbox for chiral 2-hydroxy ketones Thesis for doctoral degree (Dr. rer. nat.) New and optimised thiamine diphosphate (ThDP)- dependent enzymes for carboligation - Creation of a toolbox for chiral 2-hydroxy ketones Inaugural-Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultät der Heinrich-Heine-Universität Düsseldorf vorgelegt von Dörte Gocke aus Dorsten Köln, November 2007 Aus dem Institut für Molekulare Enzymtechnologie der Heinrich-Heine-Universität Düsseldorf Gedruckt mit der Genehmigung der Mathematisch-Naturwissenschaftlichen Fakultät der Heinrich-Heine-Universität Düsseldorf Referent: PD Dr. Martina Pohl Korreferent: Prof. Dr. Dieter Willbold Tag der mündlichen Prüfung: 14. April 2008 "Das Schönste, was wir erleben können, ist das Geheimnisvolle. Es ist das Grundgefühl, das an der Wiege von wahrer Kunst und Wissenschaft steht. Wer es nicht kennt und sich nicht mehr wundern, nicht mehr staunen kann, der ist sozusagen tot und sein Auge erloschen." Albert Einstein, aus ‚Mein Weltbild’ Für Jörn, David und Valentin Content I ABSTRACT Various thiamine diphosphate (ThDP)dependent enzymes catalyse the carboligation of aldehydes to chiral 2hydroxy ketones with high stereoselectivity. Since 2hydroxy ketones are versatile building blocks for organic syntheses, the aim of the thesis was the creation of a ThDPdependent enzyme toolbox in order to access a diversely substituted and enantio complementary 2hydroxy ketone platform. Several goals could be achieved: 1. Development of a quick and reliable high-throughput assay to screen large toolboxes of purified enzymes or whole cell catalysts for the product formation of diversely substituted 2hydroxy ketones. 2. Enlargement of the enzyme toolbox by six wild type enzymes and four variants showing carboligase activity.
    [Show full text]
  • Umpolung Strategy: Advances in Catalytic C-C Bond Formations
    Turkish Journal of Chemistry Turk J Chem (2013) 37: 586 { 609 http://journals.tubitak.gov.tr/chem/ ⃝c TUB¨ ITAK_ Research Article doi:10.3906/kim-1303-85 Umpolung strategy: advances in catalytic C-C bond formations Serkan EYMUR,1;2 Mehmet GOLL¨ U,¨ 1 Cihangir TANYELI_1∗ 1Department of Chemistry, Middle East Technical University, Ankara, Turkey 2Department of Chemistry, Sel¸cukUniversity, Konya, Turkey Received: 27.03.2013 • Accepted: 28.04.2013 • Published Online: 12.07.2013 • Printed: 05.08.2013 Abstract: This mini-review examines the recent advances in umpolung strategy, devised originally by Corey and Seebach. Although numerous stoichiometric variants have been published to date, this article covers just the catalytic C-C bond forming reactions due to their major benefits such as atom economy, less pollution, and workable simplicity. In the context of umpolung, the studies are evaluated under 3 main titles: enzyme, N -heterocyclic carbene, and cyanide ion catalyzed reactions. In particular, enzyme and NHC catalyzed reactions mainly involve asymmetric applications. Key words: Umpolung, thiamine pyrophosphate (TPP), N -heterocyclic carbene, asymmetric synthesis 1. Introduction to umpolung As an outcome of extensive efforts dedicated to the development of new approaches for the construction of either complex synthetic targets or small synthetic building blocks, numerous synthetic organic methods are now available for a wide range of bond-forming reactions. 1 In particular, carbon-carbon (C-C) bond-forming reactions have been the key point among all the organic reactions in the history of organic synthesis. 2 However, the investigation of catalytic methods for C-C bond formation reactions, while generating functionality, remains a formidable challenge in the ongoing development of effective and reliable chemical processes.
    [Show full text]
  • Aerobic Oxidative N-Heterocyclic Carbene Catalysis Via Multistep Electron Transfer
    THESIS FOR THE DEGREE OF LICENTIATE OF PHILOSOPHY Aerobic Oxidative N-Heterocyclic Carbene Catalysis via Multistep Electron Transfer Method Development and Applications Anton Axelsson Department of Chemistry and Chemical Engineering CHALMERS UNIVERSITY OF TECHNOLOGY Gothenburg, Sweden 2017 Aerobic Oxidative N-Heterocyclic Carbene Catalysis via Multistep Electron Transfer Method Development and Applications Anton Axelsson ©Anton Axelsson, 2017. Licentiatuppsatser vid Institutionen för kemi och kemiteknik Chalmers tekniska högskola. Nr 2017:22 Department of Chemistry and Chemical Engineering Chalmers University of Technology SE-412 96 Gothenburg Sweden Telephone + 46 (0)31-772 1000 Cover: Graphical representation of how the combination of electron transfer mediators and N-heterocyclic carbenes enables the use of oxygen as terminal oxidant in chemical reactions, see section 2.3 Catalytic Aerobic Oxidations. Printed by Chalmers Reproservice Gothenburg, Sweden 2017 I Aerobic Oxidative N-Heterocyclic Carbene Catalysis via Multistep Electron Transfer Method Development and Applications Anton Axelsson Department of Chemistry and Chemical Engineering Chalmers University of Technology Abstract Oxidation reactions are ubiquitous in synthetic chemistry, but generally suffer from formation of large amounts of potentially toxic byproducts. Aerial oxygen represents an ideal oxidant since it is inexpensive, non-toxic and only forms water as a byproduct. However, aerobic oxidations are characterized by high activation barriers leading to formation of kinetic side products. A common way to circumvent this is by introducing electron transfer mediators (ETMs) to achieve a kinetically useful reaction. N-heterocyclic carbenes (NHCs) are an important group of organocatalysts that have been used in a wide range of both redox neutral and oxidative transformations. In this thesis, an ETM strategy is used to enable aerobic NHC catalysis.
    [Show full text]
  • ENANTIOSELECTIVE, NUCLEOPHILIC CARBENE-CATALYZED REACTIONS of ACYL ANION EQUIVALENTS Reported by Michelle M
    ENANTIOSELECTIVE, NUCLEOPHILIC CARBENE-CATALYZED REACTIONS OF ACYL ANION EQUIVALENTS Reported by Michelle M. Adams October 4, 2004 INTRODUCTION Nontraditional bond disconnections become available by reversing the alternating donor- acceptor reactivity pattern imposed by heteroatoms. This inversion of a normal reactivity pattern is described as “umpolung.”1 Change of polarity is achieved by a temporary heteroatom modification that imparts opposite electronic character on an adjacent carbon. The reversal of the normally electrophilic reactivity of aldehydes is commonly effected though intermediates such as cyanohydrin or dithiane, which function as a nucleophilic acyl anion equivalents after deprotonation at the modified carbonyl carbon. Asymmetric carbon-carbon bond forming reactions HO S using umpolung chemistry are found in nature. Vitamin B (1) 1 N N H3C functions as a cofactor for thiamin diphosphate-dependent CH3 1 N enzymes, including transketolase, pyruvate decarboxylase, and H2N acetolactate synthase, to catalyze the formation of stabilized Figure 1. Thiamin, Vitamin B1 acyl anion intermediates.2 Umpolung chemistry of the carbonyl carbon is also featured in several familiar organic reactions that create new carbon-carbon bonds.3,4 One of the oldest known reactions in organic chemistry, the cyanide-catalyzed benzoin condensation, employs this mode of reactivity. Ugai5 and Breslow4 have shown that under basic conditions, the benzoin condensation can also be catalyzed by thiazol-2-ylidenes by a mechanism analogous to that of thiamin diphosphate in nature. Unlike classic cyanide catalysis, nucleophilic carbene catalysis can be used with enolizable aldehyde substrates because of the mildly basic reaction conditions. In addition, high enantioselectivities and yields can be obtained using an asymmetric variant.
    [Show full text]
  • (NHC)-Catalysed Benzoin Reactions
    Recent advances in N-heterocyclic carbene (NHC)-catalysed benzoin reactions Rajeev S. Menon1, Akkattu T. Biju2 and Vijay Nair*3 Review Open Access Address: Beilstein J. Org. Chem. 2016, 12, 444–461. 1Department of Chemistry, Central University of Haryana, doi:10.3762/bjoc.12.47 Mahendergarh, Haryana-123 029, India, 2Organic Chemistry Division, National Chemical Laboratory (CSIR), Dr. Homi Bhabha Road, Pune Received: 15 December 2015 411 008, India and 3Organic Chemistry Section, CSIR-National Accepted: 24 February 2016 Institute for Interdisciplinary Science and Technology,Trivandrum Published: 09 March 2016 695 019, India.; Fax: +91 471 2491712; Tel: +91 471 2490406 This article is part of the Thematic Series "N-Heterocyclic carbenes" and Email: is dedicated to Professor Ron Breslow on the occasion of his 85th Vijay Nair* - [email protected] birthday. * Corresponding author Guest Editor: S. P. Nolan Keywords: © 2016 Menon et al; licensee Beilstein-Institut. acyloin reaction; benzoin reaction; N-heterocyclic carbenes; License and terms: see end of document. organocatalysis; umpolung Abstract N-Heterocyclic carbenes (NHCs) have emerged as a powerful class of organocatalysts that mediate a variety of organic transfor- mations. The Benzoin reaction constitutes one of the earliest known carbon–carbon bond-forming reactions catalysed by NHCs. The rapid growth of NHC catalysis in general has resulted in the development of a variety of benzoin and benzoin-type reactions. An overview of such NHC-catalysed benzoin reactions is presented. Introduction The benzoin reaction (or condensation) is named after the prod- (umpolung) of one aldehyde to generate an acyl anion equiva- uct it furnishes via a catalytic assembly of two molecules of lent and this event is mediated by the catalyst.
    [Show full text]
  • Aldol Condensation
    CHAPTER‐12 MECHANISM OF REACTION ALDOL CONDENSATION: An aldol condensation is an organic reaction in which an enol or an enolate ion reacts with a carbonyl compound to form a β‐ hydroxyaldehyde or β‐hydroxyketone, followed by a dehydration to give a conjugated enone. Aldol condensations are important in organic synthesis, providing a good way to form carbon–carbon bonds. The Robinson annulation reaction sequence features an aldol condensation; theWieland‐Miescher ketone product is an important starting material for many organic syntheses. Aldol condensations are also commonly discussed in university level organic chemistry classes as a good bond‐forming reaction that demonstrates important reaction mechanisms. In its usual form, it involves the nucleophilic addition of a ketone enolate to an aldehyde to form a β‐hydroxy ketone, or "aldol" (aldehyde + alcohol), a structural unit found in many naturally occurring molecules and pharmaceuticals. The name aldol condensation is also commonly used, especially in biochemistry, to refer to the aldol reaction itself, as catalyzed by aldolases. However, the aldol reaction is not formally a condensation reaction because it does not involve the loss of a small molecule. The reactions between a ketone and a carbonyl compound lacking an alpha‐Hydrogen(Cross Aldol condensation) is called Claisen‐Schmidt condensation. These reactions are named after two of its pioneering investigators Rainer Ludwig Claisen and J. G. Schmidt, who independently published on this topic in 1880 and 1881. An example is the synthesis of dibenzylideneacetone. MECHANISM: The first part of this reaction is an aldol reaction, the second part a dehydration—an elimination reaction. Dehydration may be accompanied by decarboxylation when an activated carboxyl group is present.
    [Show full text]