KINETICS OF PROTON-TRANSFER FROM PHOSPHONIUM IONS TO ELECTROGENERATEDBASES - POLAR, STERIC AND STRUCTURAL INFLUENCES ON KINETIC ACIDITY AND BASICITY

Derivative cyclic voltammetry (DCV) and linear sweep voltammetry (LSV) have been used to measure rates of proton transfer in DMSO solution b etween different types of electrogenerated base (EGB) and a series of phosphonium ions of relevance to ylide formation for synthetic reactio ns, Although the electrochemical methods are convenient for the measur ement of rates of proton transfer in these systems a major conclusion of the study is that considerable care must be exercised in the applic ation of the methods and in drawing general conclusions from the resul ts, In particular, comparison of kinetic acidity with thermodynamic pK values in DMSO shows that a single Bronsted relationship does not hol d for the series of phosphonium ions, The kinetic acidities are profou ndly affected by: whether the EGB is a carbon or nitrogen base; the pr opensity of some of the: phosphonium ions to enolise; and steric facto rs, Other measures of electron-demand at the acidic methylene groups ( C-13 and H-1 chemical shifts, reduction potentials) are consistent wit h the pK(DMSO) values, The kinetic results confirm that the Ph3P+ grou p is, for non-enolisable phosphonium salts, more activating than Bu3P.