Competing Rearrangements of Ammonium Ylides: A Quantum Theoretical Study.

The competition between the Stevens [1,2] and Sommelet-Hauser [2,3] rearrangements for a prototype ylide, N-methyl-3-propenylammonium methylide, has been investigated using ab initio and semiempirical molecular orbital methods. The activation energies for the two processes are remarkably close, separated by only 2 kJ mol(-)(1) at ROMP/6-311+G(d,p). Increasing the size of the basis set leads to a relative stabilization of the Sommelet-Hauser transition geometry, while higher levels of electron correlation (such as CCSD(T)) favor the Stevens rearrangement. Incorporation of solvent effects via the SCRF polarizable continuum model leads to a lowering of the energy barrier of the concerted [2,3] rearrangement, but has little effect on the dissociative [1,2] pathway. The activation energies of both pathways have been calculated for ylides bearing substituents on the ammonium nitrogen and the double bond. Substituents at nitrogen lead to an ylide which is sterically unstable and hence a preference for the dissociative [1,2] rearrangement. Electron-withdrawing substituents on the double bond show a preference for the [2,3] rearrangement, while mildly electron-donating alkyl substituents have very little effect on activation energies.