Studies of 9-fluorenyl carbocations. Intramolecular hydride migration in asubstituted 9-fluorenyl carbocation

The substituted fluorenyl cation, 9-(diphenylmethyl)fluoren-9-yl cation (4) , is formed under stable ion conditions (low temperature/strong acid) from its corresponding alcohol 3. This ion is transformed to a substituted diphe nyl methyl cation 8 at ambient temperature via an apparent 1,2-hydrogen shi ft. Irradiation of 9-(diphenylmethyl)fluoren-9-ol in methanol gives product s derived from the corresponding cation along with radical-derived products from C-C and C-O homolysis processes. The laser flash photolysis of this a lcohol gave a transient corresponding to cation 4. All of the photoproducts are derived from cation 4 or radical pathways. High level MO calculations point to a high barrier (23.8 kcal mol(-1)) for the 1,2-hydride shift. This barrier is the consequence of the minimum energy conformation of this fluo renyl cation which is less than ideal for the periplanar geometry necessary for this process.