The effect of phenyl substituents on elimination stereochemistry: A mechanistic manifold in alkoxide promoted decomposition of 1-phenyl-1-propyltrimethylammonium ion

Abstract

Reactions of the positionally isomeric 1-phenyl-1-propyl (I) and 1-phenyl-2-propyltrimethylammonium (II) ions with CH3OK-CH3OH, t-C4H9OK-t-C4H9OH and t-C4H9OK-C6H6 systems have been investigated with aid of the deuterated analogues erythro-2-D-I, threo-2-D-I, 1-D-I and threo-1-D-II. A least five mechanistic components (anti-β-elimination, syn-β-elimination, α',β-elimination, Sommelet-Hauser rearrangement and SN2substitution) have been found to participate in the reaction of the quaternary compound I, in proportions varying greatly with base-solvent combination. The corresponding reactions of the isomeric compound _____CHYBI_CISLO_____ proceeded in a more simple manner, without the intervention of ylide pathways in the olefin as well as in the amine formation. The stereochemistry of β-elimination determined for the two phenyl-substituted 'onium compounds has been compared with that reported previously for structurally related aliphatic analogues. The anomalously low propensity for syn-elimination as well as the anomalously high values of trans/cis-olefin ratios in anti-elimination stigmatizing the presence of phenyl substituents are proposed to originate from a lack of base-approach hindrance in the reaction.