Vinylic Cations from Solvolysis. 42. Cyclization on Methyl, Capture by Solvent, and Degenerate Rearrangement of the Trimesitylvinyl Cation

Trimesitylvinyl tosylate, prepared in situ from trimesitylethenol (2) and tosyl chloride, cyclizes in benzene to 2,3-dimesityl-4,6-dimethylindene (5). AgBF₄-assisted reaction of trimesitylvinyl chloride (6) in the alcohols ROH, R = Me, Et, and t-Pr, gives mixtures of 5 and the corresponding alkyl trimesitylvinyl ether 7, with 7/5 ratios of 8.1, 4.6, and 0.32 for R = Me, Et, and t-Pr, respectively. Only 5 was formed when R = t-Bu. (Z)-1,2-Dimesityl-2-phenylvinyl chloride (8) gives no indene in t-BuOH under similar conditions. A plot of log ([7}/{51) is linear with log k for the reaction of trianisylvinyl cation with ROH. Reactions of Me-d₉-a- or R-mesityl-labeled 2 with SOCl₂ in CH₂C1₂ gave 6 with statistically scrambled mesityl groups and small amount of 5. Cyclization of the isotopomeric mixture of labeled 6 gave a mixture of three isotopomeric indenes from which a k_H/k_D isotope effect of 1.6 ± 0.4 for the cyclization was calculated. It is suggested that the rearrangement, the capture by the ROH, and the cyclization proceed via the intermediacy of the trimesitylvinyl cation 3. Two alternative routes for the unusual cyclization on the saturated 0-o-Me group are discussed. β-Mesityl rearrangement in 3 is faster than capture by Cl^- which is faster than the cyclization. The capture by ROH is subjected to steric hindrance and is faster than cyclization for R = Me and Et, whereas it is slower than cyclization for the bulkier alcohols, R = i-Pr and t-Bu.