Isomerisation of Alkenes Bearing Stannyl and Boryl Groups at the C=C Bond

Abstract

Alkenes bearing organometallic substituents such as boryl and stannyl groups isomerise slowly in the absence of UV irradiation. In addition to cis/trans isomerisation the boryl and stannyl group may also exchange places. This is shown for (Z)-2-chloro(dimethyl)stannyl-3-diethylboryl- pent-2-ene 5b (conversion into (Z)-3-chloro(dimethyl)stannyl-2-diethylboryl-pent- 2-ene 7b) and 3,4-diethyl-1,1,2,5-tetramethyl-1,2-dihydro-1,3-stannaborole8b (conversion into 3,5-diethyl-1,1,2,4-tetramethyl-1,2-dihydro-1,3-stannaborole 9b). Zwitterionic structures, involving a bridging boryl group with π-σ delocalisation, the stabilising effect exerted by a stannyl group in β-position with respect to a positively charged centre, and migration of a stannyl group, are suggested in oder to explain the isomerisation. The heterocycle 9b was prepared independently from the reaction of 7b with lithium diisopropylamide.