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Mesylate Derivatives of α-Hydroxy Phosphonates. Formation of Carbocations Adjacent to the Diethyl Phosphonate Group
Creary, Xavier,Geiger, Cristina C.,Hilton, Kathryn
, p. 2851 - 2858 (2007/10/02)
Mesylates 3-6 have been prepared and reacted in a variety of solvents.Product, rate, and solvent effect studies implicate carbocationic intermediates in these solvolyses despite the electron-withdrawing PO(OEt)2 group.Mesylate 3 gave exclusive substitution products.Optically active 3 gave racemic products on trifluoroacetolysis. α-Deuterium isotope effects were also in line with a cationic intermediate.Mesylate 4 gave some elimination product, 27, along with the substitution product 28, also via a cationic intermediate.Mesylates 5 and 6 gave exclusive elimination products.A β-deuterium isotope effect study gave a kH6/kD6 value of 2.8.This isotope effect, along with a small m value (0.45), suggested the intermediacy of a reversibly formed ion pair which subsequently loses a proton.This mechanism represents the merging of the classical E1 mechanism and the E2C(+) mechanism, the latter representing the cationic counterpart of the E1cb mechanism.Analysis of the solvolysis rates of 3 and 4 led to the conclusion that cationic intermediates are formed quite easily.Reasons are suggested for this unexpectedly facile generation of cations adjacent to the potent electron-withdrawing PO(OEt)2 substituent.
