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• W14105770 abstract "Ab initio and semi-empirical molecular orbital methods have been used tostudy the rearrangement pathways of ammonium ylides. There are two primarycompeting rearrangements of ammonium ylides, a [1,2] migration (Stevensrearrangement) and a [3,2] rearrangement (usually followed by rearomatisation as theSommelet-Hauser rearrangement).The mechanism of the Stevens rearrangement has been determined by aninvestigation of twelve model rearrangements. A dissociative radical mechanism ispredicted to be the true mechanism in all cases of alkyl migration. There is nocompetition from the formally symmetry-forbidden concerted mechanism, or from anion-pair dissociative pathway. The interaction of lithium ions from the bases used togenerate ammonium ylides does not affect the mechanism. The effects of solvationhave been taken into account using polarisable continuum models,supermoleculecalculations (at PM3) and a hybrid polarisable continuum-supermolecule model (in aneffort to take into account both electrostatic and specific solvent-solute interactions).Incorporation of solvent effects does not change the prediction of a radical pairpathway for the Stevens rearrangement. The concerted transition geometry for the [3,2] rearrangement has beencharacterised for fifteen model rearrangements. The important factor in the activationenergy of the [3,2] rearrangement is in aligning the carbanion lone pair to be in afavourable position to interact with the vacant It* orbital of the double bond. Thisrequires rotation about the N—C and C—C bonds.The competition between the [1,2] and [3,2] rearrangements for a prototypeylide, N-methyl-3-propenyl ammonium methylide, has been investigated. Theactivation energies for the two processes are remarkably close, separated by 2 kJ mol -1at ROMP2/6-311+G(d,p). Increasing the size of the basis set leads to a relativestabilisation of the [3,2] transition geometry, while higher levels of electron correlation (such as CCSD(T)) favour the [1,2] rearrangement. Incorporation of solvent effectsvia the SCRF polarisable continuum model leads to a lowering of the energy barrier ofthe concerted [3,2] rearrangement, but have little effect on the radical [1,2]rearrangement.The activation energies of both pathways have been calculated for ylidesbearing substituents on the ammonium nitrogen and the double bond. Substituents atnitrogen lead to an ylide which is sterically unstable, and hence a preference for thedissociative [1,2] rearrangement. Electron-withdrawing substituents on the doublebond show a preference for the [3,2] rearrangement, mildly electron-donating alkylsubstituents 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 Stevensrearrangement, which, in the sulfonium case, has a similar barrier to dissociation as inthe nitrogen case." @default.
• W14105770 created "2016-06-24" @default.
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• W14105770 date "1995-01-01" @default.
• W14105770 modified "2023-09-27" @default.
• W14105770 title "A Theoretical Investigation of Organic Rearrangements" @default.
• W14105770 hasPublicationYear "1995" @default.
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