Quantum Chemical Characterization of Cycloaddition Reactions between the Hydroxyallyl Cation and Dienes of Varying Nucleophilicity

Hydroxyallyl cation prefers to undergo cycloaddition reactions with dienes in a stepwise fashion. Analysis of positive charge distribution in the intermediate formed after initial C−C bond formation provides a simple unifying method for rationalizing changes in reactivity as a function of diene nucleophilicity. Predicted product distributions are consistent with results from experimental studies. Interestingly, for the initial intermediate generated from 1,3-butadiene, [3 + 2] cycloaddition reactions are favored, but closure to form a C−O bond as the second step results in an intermediate that can undergo a low-energy Claisen rearrangement to form a seven-membered-ring product identical to that expected from compact [4 + 3] cycloaddition.