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Abstract

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—C and C—C 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.

Item Type:	Thesis - PhD
Authors/Creators:	Heard, GL
Keywords:	Ylides, Rearrangements (Chemistry), Stevens rearrangement, Ammonium compounds
Copyright Holders:	The Author
Copyright Information:	Copyright 1995 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
Additional Information:	Thesis (Ph.D.)--University of Tasmania, 1996. Includes bibliographical references
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