Date of Award

1986

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Abstract

A number of (alpha)-halosilanes, some which are chiral at carbon, have been prepared. Lewis acid-catalyzed rearrangements and thermal rearrangements were performed on ((alpha)-chloroethyl)diphenylmethylsilane. Both the rearrangements proceed cleanly; the major product is formed by the migration of the phenyl group from silicon to carbon. The rearranged product mixture was subjected to a Kumada-type oxidation in which the silicon-carbon is cleaved stereospecifically. Thus the principle has been established that a clean rearrangement can be carried out in a manner which will allow determination of the stereochemistry at the carbon center. Preliminary kinetic studies of the thermolysis of ((alpha)-chlorobenzyl)dimethylphenylsilanes containing p-substituents in the benzyl group were done. The relative ease of rearrangement was found to be p-MeOC(,6)H(,4) > p-t-BuC(,6)H(,4) > C(,6)H(,5), indicating that electron donating groups at carbon assist the rearrangement. This observation is consistent with several mechanistic possibilities which involve positive charge generation at the carbon (alpha) to silicon and concurrent negative charge on silicon forming an 'inverse ylide'. The rotational potential function and structures of 1,4 disilabutane (DSB), propylsilane (PS), ethylmethylsilane (EMS) allylsilane and 1,2 disilacyclobutane have been examined by means of ab initio (3-21G((,*))) and molecular mechanics technique. The two methods yield virtually identical relative conformational energies and barrier heights. For three silanes, DSB, PS, EMS, there exists a butane-like rotational potential exists with gauche and anti energy minima. In the case of EMS equi-energetic gauche and anti conformers are suggested. Very good agreement is found between the electron diffraction and MM2 structures of allylsilane. The minimal energy conformer occurs at a torsion angle of 103(DEGREES). Hyperconjugation may explain this result. In the case of 1,2 bistrimethylsilylcyclohexane conformational energies of various conformers was calculated by means of the molecular mechanics method, and the results were compared with the conformational energies of 1,2 di-t-butylcyclohexane. The 3-21G((,*)) basis set is superior to other similar or smaller basis sets for evaluating structures and conformational properties of silanes. The silane force field developed by Frierson and Allinger represents a valid and useful tool for silane conformational/structural studies.

Pages

185

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