Comparison of the Reaction Mechanism of N- and S- ylide rearrangement : A Quantum Theoretical Study

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When a reactant has two different active points (allyl group and benzyl group) in the molecule in [2,3] sigmatropic rearrangement of N- or S- ylides, the experimental results showed that selectivity of the N-ylide was poor and the S-ylide gave only one product. Difference of the chemoselectivity was investigated by molecular orbital theory.Geometries of the ylides and the transition state of the [2,3] sigmatropic rearrangement were optimized by various calculation levels and energy barriers for the reaction were estimated. Semi-empirical calculations failed to analyze the reason of the difference. Ab initio calculations with Density Functional Method (B3L YP/6-31GィイD1*ィエD1 level show that the transition states of the N-ylide rearrangement are early transition structures and those of the S-ylide are late transition structures. These transition structures are much affected by reactivity of the ylides. Lone pair electron of S atom stabilize the S-ylide to give the late transition structure. Result of this study verified the Evans-Polanyi Proposal.In the process of this research program, it was found that interaction of lone pair electron on S atom with π system of phenyl ring influence conformation of the sulfonium salt. The influence was quantitatively analyzed by B3LYP/6-31GィイD1*ィエD1.