STEREOSELECTIVITY OF 7-METHYL-7-NORBORN(EN)YL CATIONS

7-Methyl-7-norbornyl cations were generated, starting from 1-methylbic yclo[3.2.0]hept-2-yl substrates. While the exo diazonium ion(26) react ed 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 sigma delocalization that is characteristic of the analogous secondary cation (7, C-1). 1,2-Dim ethylbicyclo[3.2.0]hept-2-yl cations (34), labeled with deuterium, wer e shown to give 1,7-dimethyl-7-norbornanol (35) without undergoing a d egenerate 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 rearrangemen ts as their saturated analogs. However, the 7-methyl-7-norbornenyl cat ion (48) thus generated accepts nucleophiles stereoselectively, i.e., the delocalization of pi electrons to C-7 is not affected by methyl su bstitution.