Enamine chemistry. Part VI. Structure and proton magnetic resonance spectra of dienamines

Abstract

Δ^1,8a-2-Octalones may be converted into mixtures of dienamines consisting of the exocyclic (s-trans) diene (I) and the endocyclic (s-cis) diene (II). Similarly the pyrrolidine dienamine of isophorone (3,5,5-trimethylcyclohex-2-enone) is a mixture of the exocyclic diene (VIII) and the endocyclic diene (VI). The morpholine and piperidine dienamines also contained small amounts of the cross-conjugated isomer (VII). The in-chain olefinic proton of the s-cis-dienes gave signals in the ¹H n.m.r. spectrum at 0·5–0·65 p.p.m. to higher field than those of the corresponding proton of the s-trans-dienes. This difference in chemical shift may be rationalised in terms of the reduced coplanarity and orbital overlap of the double bond system in the s-cis-dienes, resulting in increased electron density at the α-position and increased shielding of the in-chain proton relative to that in the s-trans-dienes.