On rearrangements by cyclialkylations of arylpentanols to 2,3-dihydro-1H-indene derivatives.: Part 2.: An unexpected rearrangement by the acid-catalyzed cyclialkylation of 2,4-dimethyl-2-phenylpentan-3-ol under formation of trans-2,3-dihydro-1,1,2,3-tetramethyl-1H-indene

The acid catalyzed-cyclialkylation of 4-(2-chloro-phenyl)-2,4-dimethylpentan-2-ol (1) gave two products: 4-chloro-2,3-dihydro-1,1,3,3-tetramethyl-1H-indene (2) and also trans-4-chloro-2,3-dihydro-1,1,2,3-tetramethyl-1H-indene (3). A mechanism was proposed in Part 1 (cf, Scheme 1) for this unexpected rearrangement. This mechanism would mainly tic supported by the result of the cyclialkylation of 2,4-dimethyl-2-phenylpentan-3-ol (4), which, with respect to the similarity of ion II in Scheme 1 and ion V in Scheme 2, should give only product 5. This was indeed the experimental result of this cyclialkylation. But the result of the cyclialkylation of 1,1,1,2',2',2'-hexadeuterated isomer [H-2(6)]-4 of 4 (cf Scheme 3) requires a different mechanism as for the cyclialkylation of 1. Such a mechanism is proposed in Schemes 5 and 6. It gives a satisfactory explanation of the experimental results and is supported by the result of the cyclialkylation of 2,4-dimethyl-1-3-phenylpentan-3-ol (9 Scheme 7). The alternative migration of a Ph or of an i-Pr group (cf. Scheme 6) is under further investigation.