The stereochemistry of the SN2' reaction between (S)-(+)-3-chloro-1-butene-Z-1-d and methylamine

Allylic compounds are known to undergo substitution reactions and some are thought to proceed by an S_N2' mechanism, that is, with rearrangement of the allylic system. While the general reaction mechanism itself has enjoyed much conjecture, the stereochemical considerations of the mechanism have received far less attention. The reaction between (S)-(+)-3-chloro-1-buten-Z-1-d and monomethylamine was shown to proceed with syn stereoselectivity to produce a mixture of optically active amines. In order to have meaningful comparisons with analogous, saturated, optically active, reference compounds, it was necessary to reduce this initial mixture with diimide. A reference compound of known optical purity and absolute configuration was then synthesized and its optical rotations were corrected by comparison with the rotations of the alcohol from which it was made in order to obtain its [α]_max. After correcting for the deuterium content, the relative amounts of E and Z isomer in the initial S_N2' mixture, and the optical activity of the allylic substrate, the rotations of the reduced S_N2' mixture were compared with those of the reference compound. These comparisons led to the conclusion that the mechanism was proceeding with syn stereoselectivity by 75:25; that is, the nucleophile was approaching the allylic system from the same side as the leaving group was departing.