Comparison between the epoxide ring-opening of 1-trimethylsilylcyclohexene oxide and some electrophilic additions to 1-trimethylsilylcyclohexene

Abstract

The reactions of 1,2-epoxy-1-trimethylsilylcyclohexane (1) with several nucleophilic reagents under acidic conditions have been studied and the ring-opened products identified. In all cases ring-opening occurs in a trans fashion with attack by the nucleophile on the carbon α to silicon, the only exception being the reaction with hydrogen chloride in which the regioisomeric trans-chlorohydrin, 2-chloro-1-trimethylsilylcyclohexanol is also formed as a minor product in amounts that depend on the solvent. These reactions were compared with those of 1-trimethylsilylcyclohexene with chlorine, bromine, and mercury-based electrophiles in water and in methanol. Whereas the reactions with N-bromoacetamide in water and methoxymercuriation yielded exclusively anti-Markovnikov adducts, reactions with hypochlorous acid in water and in methanol and with N-bromosuccinimide in methanol were less specific, yielding mixtures of regioisomers. The mechanism of these reactions is discussed and it is proposed that the main factor in determining the regioselectivity is the stability of the intermediate mercurinium, halonium, or epoxonium ion and that assistance by a silicon empty d orbital on the attacking nucleophile plays a role only in the reactions involving less-stable three-membered ring intermediates (epoxonium, chloronium, and to a lesser extent bromonium). ¹H N.m.r. data for the prepared adducts point to the existence of those in which a hydrogen bond can form between the two substituents prevalently in chair conformations with an axial trimethylsilyl substituent, in chloroform solutions.