Stereoselectivity of proton loss for ''E1-like'' 1,3-eliminations in tertiary, benzylic systems

The objective of this study was to learn whether γ-proton loss from a carbocation to form a cyclopropane ring prefers the semi-W or semi-U pathway. Two rigid tricyclic systems in which the intermediate carbocation in the overall 1,3-elimination is tertiary and carries a p-anisyl group were studied. One substrate, 2-chloro-2-p-anisylnoradamantane, was synthesized from 2-noradamantanone by action of p-anisyllithium, to give the tertiary alcohol, followed by treatment with dry HCl in pentane. The other substrate, 2-chloro-2-p-anisylbrendane, was obtained by a similar sequence from 2-brendanone. When heated in hexane, these tertiary chlorides underwent net 1,3-elimination to produce their respective cyclopropyl products, 2-p-anisyltriaxane and 2-p-anisyldeltacyclane. To elucidate stereochemistry of the proton loss, the corresponding 4-axial-d- and 4-equatorial-d analogues in the noradamantyl series and the corresponding 9-exo-d analogues in the brendyl series were synthesized. The deuterium losses in the 1,3-eliminations were determined mass spectrally. After correction for an isotope effect (K/sub H/k/sub D/) of 2.04 in the noradamantyl series, the stereoselectivity for γ-proton loss was 7.4/1 in favor of equatorial over axial (i.e., semi-W over semi-U). In the brendyl skeleton loss of the exo proton (i.e., semi-W) was favored over loss of the endo proton (i.e., semi-U) by a factor of ca. 27/1. Findings with these two tertiary skeletons are contrasted with those reported for secondary norbornyl systems