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Syntheses of Elusive Unsaturated Silacycles Gary Thomas Burns Iowa State University Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1981 Syntheses of elusive unsaturated silacycles Gary Thomas Burns Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Organic Chemistry Commons Recommended Citation Burns, Gary Thomas, "Syntheses of elusive unsaturated silacycles " (1981). Retrospective Theses and Dissertations. 7402. https://lib.dr.iastate.edu/rtd/7402 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 was produced from a copy of a document sent to us for microfilming. While the most advanced technological means to photograph and reproduce this document have been used, the quality is heavily dependent upon the quality of the material submitted. The following explanation of techniques is provided to help you understand markings or notations which may appear on this reproduction. 1. The sign or "target" for pages apparently lacking from the document photographed is "Missing Page(s)". If it was possible to obtain the missing page(s) or section, they are spliced into the film along with adjacent pages. This may have necessitated cutting through an image and duplicating adjacent pages to assure you of complete continuity. 2. When an image on the film is obliterated with a round black mark it is an indication that the film inspector noticed either blurred copy because of movement during exposure, or duplicate copy. 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In all cases we have filmed the best available copy. UniversiN Micnxilms International 300 N /fcfcB RD . ANN ARBUH, Ml 48106 8209102 Bums, Gary Thomas SYNTHESES OF ELUSIVE UNSATURATED SILACYCLES /oMo Slate University PH.D. 1981 University Microfilms IntGrnâtIOnâl W N. zeeb Romt Ann Arbor. MI 48106 Syntheses of elusive unsaturated sllacycles by Gary Thomas Burns 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. In Cha o Signature was redacted for privacy. For the Major Dep r Signature was redacted for privacy. For the Graduate College Iowa State University Ames, Iowa 1981 ii TABLE OF CONTENTS Page DEDICATION 11i I. INTRODUCTION 1 II. NOMENCLATURE 2 III. HISTORICAL 5 A. Slletenes 5 B. C-Unsubst1tuted Slides 13 C. Sllabenzene 17 D. Nonannulated Seven-Membered Rings Containing One Silicon Atom 29 E. Silafulvenes 40 IV. RESULTS AND DISCUSSION 48 A. Syntheses of Siletenes and Attempted Syntheses of a Silacyclobutadiene 48 B. Syntheses of 1-Methy1si1o1e, 1,1-Dimethyl si loie and 2-Methy1-2-si1aindene 63 C. Synthesis of 1-Methyl-1-silabenzene 83 D. Syntheses of Nonannulated Seven-Membered Rings Containing One Silicon 109 E. Syntheses of Silafulvenes 129 V. CONCLUSION 142 VI. EXPERIMENTAL 144 A. Instrumentation 144 B. Procedure and Results 145 VII. BIBLIOGRAPHY 207 VIII. ACKNOWLEDGEMENTS 214 iii DEDICATION To Steven 1 I. INTRODUCTION Since the establishment of the existence of (p-p)m bonded silicon compounds In 1966 by Nametkin and coworkers (1) and shortly thereafter by Gusel'nikov and Flowers (2), considerable Interest has been focused on the attempts to generate, Isolate, and utilize these reactive Inter­ mediates. This endeavor has recently culminated In the Isolation of a stable compound containing a sterlcally hindered (p-p)w silicon-carbon double bond by A. Brook and coworkers (3). This dissertation will describe the synthesis and reactions of 1-methyl-l-s11abenzene and 6,6-d1methyl-6-s11afulvene, two compounds containing a (p-p)n silicon-carbon double bond often postulated to be thermodynamically stable. The retro-ene elimination of propene from cyclic diallylsilanes used to prepare the aforementioned compounds was also utilized in the first synthesis of a C-unsubstituted silole and 2-methyl-2-silaindene. A new method for the generation of a (p-p)ir silicon-carbon double bond was developed based upon the ^-elimination of trimethylroethoxysilane. This method of forming a silicon-carbon (p-p)n double bond was utilized in the synthesis of silacyclobutenes, 6,6-dimethyl-6-s1lafulvene and 1,2-benzo-6,6-d1methyl-6-s11afulvene. In addition, a synthesis of 1,1-dimethylsilole, two silacycloheptadienes and a nonannulated silepin were developed. 2 II. NOMENCLATURE The nomenclature used In this dissertation will, with the exceptions described below, follow the conventions set down by lUPAC, Divalent silicon species will be named as derivatives of the parent sllylene (zSIHg). Simple organoslllcon compounds will be named as derivatives of sllane (SIH^), while more complicated linear and cyclic systems will be named as si la-analogs of the corresponding carbon systems. Examples: MeSICIgH Methyldichlorosliane Me Me Me Me—Si—ll—ll—CI 1 -Chioroheptamethyl trisilane I I I Me Me Me 1 -Chioro-1-methyl-1-silacyclohexa- 2,4-dlene The unsaturated four-membered rings containing one silicon atom will be named as derivatives of slletene. Sllacyclopentadlene ring systems will be named as derivatives of slloles. Exampies : 1,1-Dimethylslletene CSI—Me L 3 ^Me Si^ 1,1-Dimethyl silole D Me All compounds containing silicon-carbon (p-p)m bonded silicon will be named as analogs of the corresponding unsaturated carbon system. When referring to the general class of compounds containing the silicon- carbon double bond, the term sllene shall be used. In the case of sllabenzene, the term shall denote a cyclohexatrlene ring system In which one carbon atom has been replaced by silicon. Substltuents will be numbered In direct analogy with the carbon system. Examples: Me ^Sla-CHg 2-Methyl-2-s11apropene Me _Me S1^ 6,6-D1methyl-6-s11afulvene D- Me I 1-Methyl-1-sllabenzene ^S1^ I Me Unsaturated seven-membered rings containing one silicon atom will be named as analogs of the corresponding carbon systems. Finally, when referring to a cycloheptatrlene ring system containing one silicon atom, the term sllepin shall be used. 4 Examples: 1»1-Dimethyl-l-s11acyclohepta- 3,5-dlene 1,1-01methyl silepIn Me Me 5 III. HISTORICAL A. Siletenes Study of the chemistry of silacyclobutenes or siletenes has long been hampered by their relatively poor accessibility. Multi-step syntheses, low overall yields plus lack of silicon functionality have characterized the approaches to date. The first synthesis of a silacyclobutene was reported in 1964 by Gil man and Atwell (4). Intramolecular cyclization of either 1 or 2 with magnesium gave the diphenylbenzosilacyclobutene 3 although higher yields were obtained from 2. A more convergent synthesis of 3 was accomplished by the magnesium induced coupling of o-bromobenzylbromide and diphenyl- dichlorosilane in ethereal solvents. (ÔÇH (gQi (0E3« ^ 2 3 ^ Ng.Et^O /(yield 30%) 6 The first nonannulated siletene was prepared the following year by Gilman and Atwell (5,6). Their approach revolved around the ability to selectively brominate benzylic protons with NBS. Thus, dibrotnination of (3-phenylpropyl)d1phenylsi lane with NBS gave the dibromide 4. Purification of 4 was hampered by its low thermal stability. However, treatment of crude 4 with magnesium in THF gave the desired 1,1,2- triphenyl-l-silacyclobutane in a 52% yield. Benzylic bromination of 5 with NBS followed by dehydrohalogenation with an excess of phenyl* magnesium bromide gave 1,1,2-triphenylsiletene in an overall yield of 8%, ? O-fCHgig-Si-H Zsq- NBS) g_CH-(CHp),-S1-Br M9,THF, I | L ^ ^ A /L—Si-0 i 0 NBS jnsi-0 ' ml Br-jUji-j 7 Nine years elapsed before another slletene synethels appeared In the literature. In 1974, Weber and coworkers published a synthesis similar to Oilman and AtwelTs for 1,1-dimethyl-2-phenylslletene (7,8). Hydrosilylation of allyl benzene with dimethylchlorosilane followed by benzyllc bromination with NBS gave the analogous dihalosilane 6. Intramolecular cycllzation with magnesium afforded 1,1-dimethyl-2- phenyl-l-s11acyclobutane in a 50% yield from allyl benzene. Desatura- tlon to the slletene was accomplished by benzyl1c bromination with NBS followed by dehydrohalogenation with pyridine in methylene chloride. The overall yield from allyl benzene was 29%. Br Mg,THF 1) NBS Me ^ si -Me 2) Me Me 9 8 The first pyrolytic route to the slletene ring system was reported in 1978 by E. Block and L. Revel le (9). They found that flash vacuum pyrolysis (FVP) of dimethyldlallylsilane at 710°C and 0.1 torr afforded 1,1-dimethylsiletene in a 25% yield (41% based upon reacted starting material). The reaction mechanism favored by them involved the retro- ene elimination of propene as shown in Scheme 1. However, the alternate radical chain process involving abstraction of an allylic hydrogen from dimethyl diallylsi lane by an allyl radical followed by disproportionation to the 1-silabutadiene 7 was not ruled out (Scheme 2).
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