LSU Historical Dissertations and Theses

1990

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Frank K. Cartledge

Abstract

A new synthetic route for the preparation of $\alpha$-halobenzylsilanes is reported. The new route is based on carbene generation from diazirines and insertion into the silicon-hydrogen bond. Phenylhalocarbenes were generated by the thermal decomposition of 3-halo-3-phenyldiazirines. The reaction was carried out without solvent and with use of an excess of halodiazirine. This methodology is very versatile in allowing for a variety of different aryl substituents at the benzylic position in the targeted $\alpha$-halobenzylsilanes. The synthesis and rearrangement under Lewis-acid catalyzed conditions of 1-methyl-1-(dichloromethyl)silacyclopentane and 1-ethyl-1-(dichloromethyl)silacyclopentane is reported. The product is formed via two consecutive $\alpha$-halosilane rearrangements, the first of which gives ring expansion producing a silacyclohexane intermediate, and the second gives ring contraction to a cyclopentane. Conformational analysis on the silacyclohexane intermediate using MM2 calculations provides theoretical evidence of an antiperiplanar conformational requirement of both migrating groups. The synthesis and rearrangement of ($\alpha$-halobenzyl)silacyclopentanes provides a new synthetic route for 1-halo-2-phenylsilacyclohexanes. The Lewis acid-catalyzed rearrangement of the 1-($\alpha$-chlorobenzyl)-1-methylsilacyclopentane produces a ring expansion affording a mixture of isomers of 1-methyl-1-chloro-2-phenylsilacyclohexane. 1-($\alpha$-Fluorobenzyl)-1-methylsilacyclopentane rearranges under thermal conditions to give a mixture of the analogous fluoro derivative. In both cases the major isomer was the (E)-isomer. Ab initio results for ethylfluorosilane confirm the preference of the Me group to adopt a gauche conformation. MM2 calculations for 1-methyl-1-chloro-2-phenylsilacyclohexane predict a greater stability for the Z-isomer. The synthesis and separation of cis-and trans-bis(trimethylsilyl)cyclohexane (BTMSC) is reported. Two-dimensional NMR techniques have been utilized to study the conformational preferences of trans-BTMSC. The presence of a long range coupling of equatorial hydrogens, and an upfield displacement in the $\sp{13}$C and $\sp1$H NMR signals comparing the cis to the trans isomer, indicates a rigid conformation. Coupling constants extracted from an ECOSY experiment are in close agreement with those calculated with a Karplus type equation, using dihedral angles calculated for the diaxial chair.

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