Kinetics of proton transfer from phosphonium ions to electrogenerated bases: polar, steric and structural influences on kinetic acidity and basicity

Abstract

Derivative cyclic voltammetry (DCV) and linear sweep voltammetry (LSV) have been used to measure rates of proton transfer in DMSO solution between different types of electrogenerated base (EGB) and a series of phosphonium ions of relevance to ylide formation for synthetic reactions. Although the electrochemical methods are convenient for the measurement of rates of proton transfer in these systems a major conclusion of the study is that considerable care must be exercised in the application of the methods and in drawing general conclusions from the results. In particular, comparison of kinetic acidity with thermodynamic pK values in DMSO shows that a single Brønsted relationship does not hold for the series of phosphonium ions. The kinetic acidities are profoundly affected by: whether the EGB is a carbon or nitrogen base; the propensity of some of the phosphonium ions to enolise; and steric factors. Other measures of electron-demand at the acidic methylene groups (¹³C and ¹H chemical shifts, reduction potentials) are consistent with the pK (DMSO) values. The kinetic results confirm that the Ph₃P⁺ group is, for non-enolisable phosphonium salts, more activating than Bu₃P⁺.

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