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 pi-nucleophiles follow second-order rate laws with rate constants comparable to those of the analogous saturated species (4-MeOC6H4)CH+-(CH2)(3)CH3. Product studies show the absence of cyclization products. In contrast, the carbocation (4-MeOC6H4)CH+-(CH2)(3)-CH=CH2 (1d) undergoes a highly reversible 6-endo-trig cyclization which is approximately 10(7) times faster than the corresponding intermolecular reaction of (4MeOC(6)H(4))CH+-(CH2)(3)CH3 with hex-1-ene. This cyclization yields a highly electrophilic, partially bridged carbocation, which accounts for the finding that 1d is consumed eight times faster in trifluoroethanol solution than all other carbocations of this series. Quantum-chemical calculations (B3LYP/6-311G(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 pi-participation in solvolysis reactions are discussed.