Stabilization Demands of Diethyl Phosphonate Substituted Carbocations as Revealed by Substituent Effects

Article abstract of DOI:10.1021/jo00212a033

Trifluoroethanolyses of a series of mesylate derivatives of diethyl (1-aryl-1-hydroxymethyl)phosphonates, 9, gave a Hammett ρ value of -10.1 in the electron donor substituent region.This value was slightly less than the value of -11.6 seen in the corresponding benzyl mesylates, ArCH2OMs, 12, in hexafluoroisopropyl alcohol.These data suggest that the demand for aryl group stabilization in the intermediate phosphoryl-substituted cation 10 does not surpass that of the α-H analogues, the benzyl cations.Some other factor must therefore account for the relative ease of formation of cations 10, which have the electronegative diethyl phosphonate group attached directly to the cationic center.The likely factor is an offsetting cation stabilizing feature associated with the diethyl phosphonate group.The Hammett plots for both mesylates 9 and 12 show a break, with decreased ρ values (-6.1 and -5.1, respectively) being observed in the electron-withdrawing region of the plot.Solvent effect studies on 9-m-F suggested that a change to "borderline behavior" is the origin of the break in the Hammett plot.A mechanistic change to the kδ process could be ruled out.The triflate derivative of diethyl (1-hydroxyethyl)phosphonate, 14, gave mixtures of substitution and elimination products on solvolysis.Solvent effect studies indicated a largely nucleophilic mechanism, while isotope effect studies were in line with some cationic character in the transition state in the highly ionizing, nonnucleophilic hexafluoroisopropyl alcohol solvent.Ion pair formation or the SN2 (intermediate) mechanism could rationalize the behavior of 14 in more highly ionizing solvents.