Nucleophilicity parameters for strong nucleophiles in dimethyl sulfoxide. A direct alternative to the s(E plus N) equation

A scale of nucleophilicity (N''') for relatively strong nucleophiles (e.g. carbanions and amines), spanning over 5 orders of magnitude was constructed directly from experimental rate constants for reactions of 34 nucleophiles with a benzhydrylium cation, (lil)(2)CH+ (log k=N''' in dimethyl sulfoxide at 20 degrees C). The equation log k = (E''' + s(E)N''' + c), where E''' is an electrophilicity parameter, s(E) is a Swain-Scott type of response parameter (to variation in nucleophilicity) for the electrophile, and c is a residuals term, is used to correlate second order rate constants for reactions of nucleophiles with other benzhydrylium cations, quinone methides (QM) and Michael-acceptor electrophiles. Contrary to published claims (Mayr et al. Angew Chem. Int. Ed. 2002, 41, 92, and later work), s(E) increases as the reactivity of the QM decreases. The N''' scale was extended a further 3 orders of magnitude by an extrapolation involving a QM electrophile. In contrast, published procedures involve over 40 reference electrophiles and over 100 adjustable parameters obtained from the equation log k = s(N)(E + N), where k is the rate constant, and s(N) is a nucleophile parameter. Values of N-N''' in DMSO increase by 8 log units, as the reactivity of the nucleophile increases, because N is a floating scale whereas N''' is a fixed scale. Copyright (C) 2010 John Wiley & Sons, Ltd.