Structural determination of a carbocation-derived rearrangement product observed during Thiele cage opening: insight into the mechanism of an alkyl-extended pinacol reaction

Abstract

A substituted bishomocubane structure colloquially known as a Thiele cage was previously observed to undergo an alkyl-extended pinacol-type rearrangement, wherein 1,2-aryl migration and ketone formation occurred together with (or at least in close succession to) opening of a strained cyclobutane bond. While there was some indication that the rearrangement reaction might proceed via a stepwise process involving a sequence of carbocation intermediates, previous studies did not uncover any direct evidence for the formation of carbocations, and did not fully explain the regio- and stereospecificity of the reaction. Here we describe the isolation and detailed characterization of a second rearrangement product formed under the pinacol reaction conditions, the existence of which implicates the formation of discrete carbocation intermediates along the reaction pathway. Observation of this new product also finally explains the fate of any Thiele cage material that is converted to a carbocation, but which is not geometrically predisposed to react through a facile 1,2-aryl migration. As such, our findings resolve previous questions surrounding the origin of regio- and stereospecificity in the alkyl-extended pinacol rearrangement.