A theoretical investigation of organic rearrangements

Ab initio and semi-empirical molecular orbital methods have been used to study the rearrangement pathways of ammonium ylides. There are two primary competing rearrangements of ammonium ylides, a [1,2] migration (Stevens rearrangement) and a [3,2] rearrangement (usually followed by rearomatisation as the Sommelet-Hauser rearrangement). The mechanism of the Stevens rearrangement has been determined by an investigation of twelve model rearrangements. A dissociative radical mechanism is predicted to be the true mechanism in all cases of alkyl migration. There is no competition from the formally symmetry-forbidden concerted mechanism, or from an ion-pair dissociative pathway. The interaction of lithium ions from the bases used to generate ammonium ylides does not affect the mechanism. The effects of solvation have been taken into account using polarisable continuum models,supermolecule calculations (at PM3) and a hybrid polarisable continuum-supermolecule model (in an effort to take into account both electrostatic and specific solvent-solute interactions). Incorporation of solvent effects does not change the prediction of a radical pair pathway for the Stevens rearrangement. The concerted transition geometry for the [3,2] rearrangement has been characterised for fifteen model rearrangements. The important factor in the activation energy of the [3,2] rearrangement is in aligning the carbanion lone pair to be in a favourable position to interact with the vacant It* orbital of the double bond. This requires rotation about the N‚ÄövÑvÆC and C‚ÄövÑvÆC bonds. The competition between the [1,2] and [3,2] rearrangements for a prototype ylide, N-methyl-3-propenyl ammonium methylide, has been investigated. The activation energies for the two processes are remarkably close, separated by 2 kJ mol -1 at ROMP2/6-311+G(d,p). Increasing the size of the basis set leads to a relative stabilisation of the [3,2] transition geometry, while higher levels of electron correlation (such as CCSD(T)) favour the [1,2] rearrangement. Incorporation of solvent effects via the SCRF polarisable continuum model leads to a lowering of the energy barrier of the concerted [3,2] rearrangement, but have little effect on the radical [1,2] rearrangement. 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 [3,2] rearrangement, mildly electron-donating alkyl substituents have very little effect on activation energies. The sulfonium ylide is shown to have a much smaller barrier to the [3,2] rearrangement than its nitrogen analogue, and there is no competition from the Stevens rearrangement, which, in the sulfonium case, has a similar barrier to dissociation as in the nitrogen case.