Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1972 Synthetic routes to heterocycloheptatrienes Albin James Nelson Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Organic Chemistry Commons Recommended Citation Nelson, Albin James, "Synthetic routes to heterocycloheptatrienes " (1972). Retrospective Theses and Dissertations. 5945. https://lib.dr.iastate.edu/rtd/5945 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. INFORMATION TO USERS This dissertation was produced from a microfilm copy of the original document. 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University Microfilms 300 North Zeeb Road Ann Arbor, Michigan 48106 A Xerox Education Company 73-3918 NELSON, Albin James, 1946- SYNTHETIC ROUTES TO HETEROCYCLOHEPTATRIENES. lowa State University, Ph.D., 1972 Chemistry, organic University Microfilms, A XEROX Company, Ann Arbor. Michigan THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RECEIVED. Synthetic routes to heterocycloheptatrienes by Albin James Nelson A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment of The Requirements for the Degree of DOCTOR OF PHILOSOPHY Department: Chemistry Major: Organic Chemistry Approved Signature was redacted for privacy. irï charge of Work Signature was redacted for privacy. Fp^the Major Department Signature was redacted for privacy. For the Graduate College Iowa State University Ames, Iowa 1972 PLEASE NOTE: Some pages may have indistinct print. Filmed as received. University Microfilms, A Xerox Education Company 11 TABLE OP CONTENTS Page DEDICATION ill A NOTE ABOUT NOMENCLATURE Iv THEORETICAL ASPECTS 1 APPROACHES TOWARD SYNTHESIS, LITERATURE METHODS 20 PHOTOCHEMISTRY OP HETEROCYCLOPENTADIENES THE RELATION OF STRUCTURE TO REACTIVITY 33 Photodimerlzatlon and aromaticlty 33 Cycloaddition to silole 1 60 Synthesis of a sllepln from initial photocycloaddition 65 Photoadditions to a silacyclopentene (^) 103 DIELS-ALDER REACTIONS OF HETEROCYCLOPENTADIENES 108 RING CLOSURE USING SULFUR DICHLORIDE 138 EXPERIMENTAL 171 General Information 171 REFERENCES CITED 222 ACKNOWLEDGEMENTS 234 APPENDIX 2 35 ill DEDICATION This thesis is dedicated to the memory of my grandfather Albin James Nelson who told me never to fish in the middle of the lake. iv A NOTE ABOUT NOMENCLATURE Two systems of nomenclature are prevalent in the literature of heterocyclic organic chemistry. The first treats all heterocycles as a perturbation of the all-carbon system. The number of the position and the name of the heteroatom is then added to the carbo-name in the same place a substituent would be put. Thus the molecule drawn below is 1-methyl-l-azacycloheptane. The other system has special names for unsaturated heterocycles and treats the saturated compounds as perturba­ tions of the unsaturated ones. The ending on the root name indicates the degree of unsaturation. Further, the root name changes depending on the presence of nitrogen. Some common roots are ole. for an unsaturated five membered ring, epin, for an unsaturated seven membered ring, enon. for an unsatu­ rated nine membered ring. Ete and ajn are the roots for four and six membered rings respectively. 1,1-dimethylsilepin is shown below. In the interest of space economy, the shortest name will always be employed in this work. In addition, a general name such as heterocyclopentadienes will be understood to mean a five membered ring with four carbons and one heteroatom. Possession of more than one heteroatom by a ring will be explicitly stated (there are only three in this thesis). V CH, Ai 1,1-dimethyIsllepin 1-methyl-l-azacycloheptane The interested reader may refer to any of the current books about general organic chemistry for more nomenclature rules. A good one is J. D. Roberts and Marjorle C. Caserlo, "Basic Principles of Organic Chemistry", W. A. Benjamin, New York, 1905. Look on page 969. vi LIST OF FIGURES Figure Title Page 1 Orbital symmetry diagram for silepin 10 2a 60 MHz NMR spectrum of a mixture of silole 1 and dimer 2 40 2b 60 MHz NMR spectrum of pure dimer £ 40 3 Crystal structure of silole dimer £ 42 i|a 60 MHz NMR spectrum of phosphole dimer 9 46 i|b 60 MHz NMR spectrum of germole dimer 6 46 5 Crystal structure of pyrrole ]_ 53 6 Crystal structure of phosphole ^ 55 7 Crystal structure of silole 1 monomer 59 8a 60 MHz NMR spectrum of phosphole- silole dimer ll^ 64 8b 60 MHz NMR spectrum of silole-maleic anhydride Diels-Alder adduct ^ 64 9 Plot of 1/[1] vs. time 68 10a 60 MHz NMR spectrum of expanded middle region 73 10b 60 MHz NMR spectrum of adduct 13 73 11a 60 MHz NMR spectrum of isolated mesylate isomer l8-l 84 lib 60 MHz NMR spectrum of the mixture of benzoates 1_7 84 12a 60 MHz NMR spectrum of l,l-dimethyl-2,7- diphenylsilepin 19. 90 12b 100 MHz NMR spectrum of 1^ 90 13a 60 MHz NMR spectrum of adduct 2^ and the decomposition intermediate ll6 vil 13b 60 MHz NMR spectrum of 28, the sllole- dlchlorovlnylene carbonate adduct II6 14 Crystal structure of siloxapinone 21 123 15 60 MHz NMR spectrum of a mixture of 29 and 1 at ça. -50°C 132 16 60 MHz NMR spectrum of C22H13SCI 143 17 Crystal structure of adduct ^ 14? 18 Crystal structure of §1^ 156 viil LIST OP TABLES Table Page 1 UV Spectra of Some Heterocyclo- pentadlenes 50 2 UV Spectra of ^ and 77 151 3 Bond Distances of 63_ 157 4 Bond Angles of ^ 158 5 Extent of the Sllole Photolysis Reaction 183 1 THEORETICAL ASPECTS The cycloheptatrlene model Cycloheptatrlene exists as two rapidly interconvertlng boat conformers. P. A. L. Anet^ found the rate inversion to be 180 seccalculated from low temperature NMR studies at -150°C. The energy of activation Is 6.3±0.5 kcal/mole. Substitution around the cycloheptatrlene ring raises the barrier of p Inversion quickly. Conrow found the coalescence tempera­ ture of 1,1,3-trimethyl-2-t-butyltropylidene to be -86°C. Phenyl substitution raises the coalescence temperature even more. Barton" found 2,3>^35-tetraphenyItropylidene coalesced at 38°C. An X-ray structure of g-bromobenzyl-1,1-dimethyl- 4-carboxy-2,4,6-cycloheptatriene ester^ revealed the ring puckering to be 23-7° and 49-7°. This cycloheptatrlene has a typical carbon-carbon double bond distance of 1.33 A. Me Me 49.7® o / 23.7 2 These studies and others like them put to rest any controversy in describing cycloheptatriene either as a planar system or as a homoaromatic one. PLANAR HOMOAROMATIC Removal of a hydride ion from cycloheptatriene creates a planar system having seven atomic "p" orbitals available for an aromatic molecular orbital system with six electrons. Various authors have adequately described the molecular orbital scheme including Streltwieser^ and Dewar^. Dewar has calculated the aromatic energy, for the tropylium cation^ to be 0.93 eV. or 21.4 kcal/mole as compared to the open chain ion. For comparison cyclopropenium cation has an effective pi-bonding energy (E^^) of 1.84 eV. or 42.4 kcal/mole by Dewar's method. Experimental fact corroborates the stability of the tropylium cation. Pyrolysis of the 1,6 bromine addition product of cycloheptatriene gave tropylium bromide 7. The synthesis of tropylium iodide is such a sure-fire procedure 3 8 that it is included in undergraduate laboratory preparations . Q Spectroscopic investigations of the cation demonstrate the correlation between the calculated energy levels for a planar aromatic molecule and the ultraviolet absorptions. The cycloheptatrienyl anion may be contrasted with the cation. It is a 4n pi electron system having a relative aromatic energyof 10.25 eV. or -5.78 kcal/mole as compared to the heptatrienyl anion. This destabilization makes the anion at least an unobserved system. Thus, the cation and anion of cycloheptatriene fit the classic Hiickel 4n+2 rule where the cation is predicted to be aromatic with 6 pi electrons and the anion antiaromatic with 8 pi electrons.