Base-catalysed deuterium exchange reactions of 6,6-dimethyl-2-methylenebicyclo[3.1.1]heptane [pin-2-(10)-ene] and 6,6-dimethylbicyclo[3.1.1]heptan-2-one (nopinone)

Abstract

Deuterium exchange reactions of nopinone with NaOD–D₂O–dioxan and of pin-2(10)-ene with NaH–[²H₆]DMSO during isomerization to pin-2-ene were monitored by mass and ¹H n.m.r. spectral means after separating the products by g.l.c. In both cases, there was relatively rapid uptake of one deuterium atom, followed by the slower incorporation of a second. The first deuterium was shown to be on C-3 and trans to the 6,6-gem-dimethyl bridge, by reducing the exchanged ketone or olefin to the corresponding alcohol and examining the ¹H n.m.r. spectra of mixtures of the purified alcohols with an n.m.r. shift reagent. From analysis of the rate equations for each species, these results were rationalized in terms of base and solvent mainly approaching C-3 from the side trans to the gem-dimethyl group to abstract a hydrogen atom and donate a deuterium atom to the intermediate. Attack on the cis-face is precluded by steric hindrance.