A design to prevent floating within the N scale of nucleophilicity

摘要

Two nucleophilicity scales (NT and N") including 96 nucleophiles, spanning 10~(17) in rate constant, have been constructed directly from experimental rate constants for reactions of nucleophiles with dianisyl (log k - N') or dimethylaminobenzhydrylium (log k - N") cations in dichloromethane at 20 °C. The two scales are linked/unified by the formula: N' - N" - 6.6. In contrast, published procedures (H. Mayr et al J. Am. Chem. Soc. 2001,123,9500, and later work) involve 23 benzhydrylium cations and over 200 adjustable parameters obtained from the equation log k - s(E + N), where k is the rate constant, E is an electrophilicity parameter and s is a nucleophile-specific sensitivity parameter. The definitions of N' and N" avoid a floating N scale, i.e. having less than the usual attachment to fixed reference points. New equations, log k-(E + sN') or log k-(E + sN"), where s is now an electrophile-specific parameter, are then used to correlate over 350 second-order rate constants for reactions of other benzhydrylium cations with nucleophiles. For weaker, less polar, nucleophiles, s～1 and the new E and N' parameters are in satisfactory agreement with published data for E and N. For more polar nucleophiles, s≠1 and values of N - N" vary by five log units.