Untersuchungen über Allylgruppenwanderungen in aliphatischen Carbenium‐Ionen

Abstract

Investigations on the Migratory Aptitude of Allyl Groups in Aliphatic Carbenium‐IonsThe acetolysis (80°) of the 4‐bromobenzenesulfonates given in Scheme 6 were investigated in regard to determine type allyl/methyl migratory aptitudes in the secondary carbenium ion a (Scheme 24). In all cases olefins (about 80%) and acetates (about 20%) were formed which can be derived from the rearranged tertiary carbenium ions b (being formed by allyl group migration) and c (being formed by methyl group migration).Olefin A and acetate H, originated in carbenium ion a, occurred in the acetolysis mixture only in minor amounts (<2%). By acetolysis of [l4C]‐20, isolation of [14C]‐4,5‐dimethyl‐l, 3‐hexadiene ([14C]‐45), and degradation of this diene (Scheme 16) it could be shown (4 Scheme 15) that the ions b and c (Scheme 24, R1R4H) are not interconverted by a [1,2]‐hydride shift (extent < 1%). Since olefin D arises by proton loss from ion b as well as from ion c, [14C]‐4,5‐dimethyl‐l,4‐hexadiene ([14C]44 D, R1R4 H) was also degraded (cf. Scheme 15 and Scheme 17). It was found that [14C]‐44 contained 48% of the label in the methyl group at C(4) and 52% in the methyl groups at C(5), i.e. 48% of 44 is formed via the allyl migration path and 52% via the methyl migration path. In addition, acetolysis of d3‐20 and product analysis showed, that the d3‐ally1 moiety migrates as expected only in a [1,2]‐fashion. Product analysis of the acetolysis mixtures of erythro‐ and threo‐24 (cf. Scheme 19 and Tables 4 and 5) revealed that carbenium ion a must exist as an intimate ion pair (with the 4‐bromobenzenesulfonyloxy‐ion) which has lost its configuration at C(1) only partially. This is indicated by reversed ratios (1: 11 and 10: 1, resp.) in the formation of erythro‐ and threo‐2,3,4‐trimethyl‐l, 5‐hexadiene (erythro‐ and threo‐77) arising from ion b (Scheme 24, R1R3  H, R4 CH,). The acetolysis of 1,2,2,4‐tetramethyl‐4‐pentenyl4‐bromobenzenesulfonate (23) was not studied in detail, but the appearance of a seventh product in the olefin part cannot be explained by the genesis paths in Scheme 24. Thus, it may be concluded that in this case a third tertiary carbenium ion d3 (Scheme 21) is produced by cyclization of a3. Cyclizations of this type are known to occur in carbenium ions bearing β‐substituted allyl groups (see Scheme 22). The kinetic data of the acetolysis of all 4‐bromobenzenesulfonates (Table 6) are in accord with a rate determining ionization step leading to a since all activation enthalpies resp. entropies are within 25.5 L± 0.6 kcal/mol resp. −0.2 ± 1.7 e.u. The migratory aptitudes given in Table 7 show, that allyl groups migrate only slightly easier than methyl groups in ion a. This is in strong contrast to allyl substituted methylcyclohexadienyl cations (generated in the acid catalyzed dienone/phenol and dienol/benzene rearrange‐ment) which undergo exclusively [1,2]‐ally1 migrations (Schemes 3‐5).