Carbocationic n-endo-trig cyclizations

Unsaturated benzyl cations (4-MeOC₆H₄)CH ₊-(CH₂)_n-CH=CH₂ (1) have been generated laser-flash photolytically in acetonitrile in the presence of enol ethers or 2-methylfuran. The reactions of the cations 1 (n = 2 and 4) with these π-nucleophiles follow second-order rate laws with rate constants comparable to those of the analogous saturated species (4MeOC₆H ₄)CH₊-(CH₂)₃CH₃. Product studies show the absence of cyclization products. In contrast, the carbocation (4-MeOC₆H₄)CH⁺-(CH₂) ₃-CH=CH₂ (Id) undergoes a highly reversible 6endo-trig cyclization which is approximately 10⁷ times faster than the corresponding intermolecular reaction of (4MeOC₆H₄)CH ⁺-(CH₂)₃CH₃ with hex1-ene. This cyclization yields a highly electrophilic, partially bridged carbocation, which accounts for the finding that Id is consumed eight times faster in trifluoroethanol solution than all other carbocations of this series. Quantum-chemical calculations (B3LYP/6311G(d,p) and MP2/6-31+G(2d,p)) have been performed to elucidate the structures of the involved carbocations. Consequences of these findings on the role of π-participation in solvolysis reactions are discussed.