Stereoselectivity of 7-methyl-7-norborn(en)yl cations

Article abstract of DOI:10.1002/jlac.199719970211

7-Methyl-7-norbornyl cations were generated, starting from 1-methylbicyclo[3.2.0]hept-2-yl substrates. While the exo diazonium ion (26) reacted exclusively with migration of the C-1-C-7 bond, competitive shifts of C-7 and C-5 were observed with the endo diazonium ion (25) and the endo brosylate (24). The stereorandom capture of 7-methyl-7-norbornyl cations was demonstrated by means of deuterium labels, thus supporting the classical structure 12 (C_2v). The inductive and hyperconjugative effects of the 7-Me group in 12 override the σ delocalization that is characteristic of the analogous secondary cation (7, C₁). 1,2-Dimethylbicyclo[3.2.0]hept-2-yl cations (34), labeled with deuterium, were shown to give 1,7-dimethyl-7-norbornanol (35) without undergoing a degenerate shift of C-5. A variety of 1-methylbicyclo[3.2.0]hept-6-en-2-yl substrates were found to display a similar pattern of rearrangements as their saturated analogs. However, the 7-methyl-7-norbornenyl cation (48) thus generated accepts nucleophiles stereoselectively, i.e., the delocalization of π electrons to C-7 is not affected by methyl substitution. VCH Verlagsgesellschaft mbH, 1997.