Journal of the Chemical Society. Perkin transactions II p. 867 - 875 (1980)
Update date:2022-07-30
Topics:
Hegarty, Anthony F.
McCormack, Margaret T.
Brady, Kieran
Ferguson, George
Roberts, Paul J.
The O-acylisourea (16a), which is a model for the proposed intermediate in carbodi-imide condensations, and is formed on reaction of the chloride (15) with benzoate ion, was shown using X-ray crystallography to have the Z-configuration.Crystals are monoclinic, space group P21/c with four molecules in a cell of dimensions a = 15.732(3), b = 16.246(5), c = 7.628(1) Angstroem, β = 108.94(1) deg.The structure was solved by direct methods and refined by full-matrix least-squares calculations to an R value of 0.044 for 1 281 observed reflexions.The benzoyl group is trans to the lone pair of the imine nitrogen and the C=N bond length is 1.286(6) Angstroem.The isourea undergoes uncatalysed (at 25 degC in 1:4 dioxan-water, μ = 1.0) O --> N acyl transfer to give the N-acylamide (17) at pH > 7; the slow step is a Z --> E isomerization.At lower pH (3-6), Z --> E isomerization of the isourea (and thus O --> N acyl transfer) is acid catalysed, but at pH < than the pKa of (16a), intramolecular acyl transfer is inhibited by protonation of the substrate.Competing intermolacular reaction of (16a) with the solvent
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