Chemistry of methylnorbornyl cations. I. Introduction and general survey

Formation of synthetically useful camphor derivatives: mechanistic aspects

“…Authentic examples of 3,2-endo-methyl shifts are exceedingly rare (7,8), and indeed, in the absence of special structural features, molecules will tend to rearrange by a circuitous route involving 3,2-exo-methyl shifts rather than by a more direct route involving endo-methyl shifts (cf. transformation of longifolene to isolongifolene (9) and albene structure and synthesis (10)). In spite of these reservations, we considered (6) the possibility of reversing the normal preference (9-17) for 3,2-exomethyl over 3,2-endo-methyl shifts in bicyclo[2.2.l]heptyl systems by having a bulky group (i.e., a bromine atom) in the 7-syn-position of intermediate 15 (Scheme 3).…”

“…Subsequent brornination ( l c ) then provided the corresponding 3,3-dibromo derivatives. The almost exclusive occurrence of 3,2-exo rather than 3,2-endo methyl shifts in rearrangements involving bornyl or norbornyl carbocations has been thoroughly documented by several research groups (9)(10)(11)(12)(13)(14)(15)(16)(17), and a stereoelectronic effect (8,20,21) has been proposed to account for this stereoselectivity. The stereoelectronic effect can be illustrated in the case of camphene (47) racemization (cf.…”

Predicting the Most Favored Path and the Overall Rate of Multistep: 2-Norbornyl Cation Rearrangements. The Assignment of a Trial Activation Energy for Each Unique Step

“…In the analysis of the observed line broadening, one cannot distinguish between an exo-3,2-hydride shift and an endo equivalent or a mixture of both but, in the corresponding TT process, the exo shift is favored over the corresponding endo shift by at least 6.9 kcal/mol (see below). There is also much solvolysis evidence showing that exo shifts are highly preferred (21) in the secondary system. FIG.…”

“…Several results (11,21) suggest that an exo-3,2-hydride shift will be much faster than the endo shift. The process 5HSS accomplishes the same overall permutations (unless the C2 and C3 protons are labelled) as the apparently much easier 2HSS process and the former need not be considered further (i.e.…”

Carbonium Ion Rearrangement with Retention at the Migration Terminus

“…Bicyclo[2.2.l]hept-2-yl cations are captured with a strong exo preference, even if exo approach is hindered by a 7-methyl substituent (3)(4)(5). Rearrangement of such cations, by 2,3-hydride transfer (6) or alkyl migration (7,8), is also highly exo selective. Similarly, all 7-norbornenyl cations studied to date react faster with nucleophiles at the anti-C, face than at the syn face.…”