Carbocationic n-endo-trig Cyclizations

Chem. Eur. J., 2009, 15, 8533-8541 published on 23.07.2009
Chem. Eur. J. 2009
Unsaturated benzyl cations have been generated laser-flash photolytically in acetonitrile in the presence of enol ethers or 2-methylfuran. The reactions of the cations with these π-nucleophiles follow second-order rate laws with rate constants comparable to those of the analogous saturated species. Product studies show the absence of cyclization products. In contrast, the carbocation undergoes a highly reversible 6-endo-trig cyclization which is approximately 107 times faster than the corresponding intermolecular reaction of (4-MeOC6H4)CH+(CH2)3CH3 with hex-1-ene. This cyclization yields a highly electrophilic, partially bridged carbocation, which accounts for the finding that 1 d 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 π-participation in solvolysis reactions are discussed
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