The behavior of 3-ferrocenyl-1-methyl-1,2-pentamethyleneallyl and 1,5-diferrocenyl-3-methyl-2,4-tetramethylene-1,4-dienyl carbocations in the cationic dimerization of 1,3-dienes

3-Ferrocenyl-1-methyl-1,2-pentamethyleneallyl carbocation 9 and 1,5-diferro cenyl-3-methyl-2,4-tetramethylene-1,4-dienyl carbocation 10 are transformed into linear 21 and cyclic 28 dimers, respectively, in low yields under con ditions of the cationic dimerization (treatment with N,N-dimethylaniline). The former is preferentially deprotonated into 1-ferrocenylmethylidene-2-me thylidenecycloheptane (19) and 3-ferrocenylmethylidene-2-methylcycloheptene (20), while the latter is reduced into 1,3-bisferrocenylmethylidene-2-meth ylcycloheptane (26). However, a linear dimer was obtained in the reaction o f salt 9 with s-cis-diene 19, while the reaction of salt 10 with 1,3-bisfer rocenylmethylidene-2-methylidenecycloheptane (27) results in a cyclodimer 2 8 together with a linear dimer 29. The results obtained corroborate the ste pwise mechanism of cationic cycloaddition of 1,3-dienes and manifest the ro le of the electronic factor in the intramolecular cyclization of an interme diate linear dimeric allylic cation. (C) 2000 Elsevier Science S.A. All rig hts reserved.