Elimination–addition. Part VIII. Structures of acetylene–amine adducts

Abstract

Addition of primary and secondary amines to acetylenes activated by the groups PhCO–, PhCH₂N(Me)·CO–, MeO·CO–, PhCH₂·SO–, and PhCH₂·SO₂– has been investigated. In all cases, 1 : 1-adducts are formed and these have the enamine structure.

Secondary amines give the thermodynamically more stable trans-enamines only; the exceptional behaviour of ethyleneimine, reported by other workers, is discussed.

Primary amines give equilibrium mixtures of cis- and trans-enamines whose compositions have been determined by proton magnetic resonance spectroscopy. The proportions of cis- and trans-isomers depend on the solvent used for p.m.r. measurements, and upon the structures both of the activating group and the amine.

Change of solvent from chloroform to dimethyl sulphoxide shifts the equilibrium in favour of the trans-isomer; this observation is accounted for in terms of the balance between inter- and intra-molecular hydrogen bonding.

For adducts with benzylamine, the proportion of cis-isomer falls along the series of activating groups as follows: Ph·CO– < PhCH₂·N(Me)CO– < MeO·CO– and PhCH₂·SO– < PhCH₂·SO₂–. The adducts obtained from the acetylenic ester and ketone with t-butylamine contain a greater proportion of cis-isomer than those obtained with benzylamine. By contrast, the cis–trans ratios depend little upon either the basicity of the amine or the polarity of the N-alkyl group. These observations are discussed.