KINETICS OF DEPROTONATION OF TRIPHENYL(2-SUBSTITUTED-9-FLUORENYL)PHOSPHONIUM IONS IN 50-PERCENT DIMETHYL SULFOXIDE-50-PERCENT WATER - INTRINSIC RATE CONSTANTS, IMBALANCES AND EVIDENCE FOR CHANGES IN TRANSITION-STATE STRUCTURE

Rate constants (k1B and k-1BH) for the reversible deprotonation of triphenyl(2-Z-9-fluorenyl)phosphonium ions (Z = H, Br, NO2) by piperidine, morpholine, n-butylamine, 2-methoxyethylamine, glycine ethyl ester, cyanomethylamine, OH- and water were determined in 50% Me2SO-50% (v/v) water at 20-degrees-C. Bronsted-alpha(CH) (variation of carbon acid) and beta(B) values (variation of amine), and intrinsic rate constants [ log k0 = log k1(B)/q when pK(a)BH - pK(a)CH + log(p/q) = 0] were obtained. alpha(CH) decreases with increasing basicity of the amine whereas beta(B) decreases with increasing acidity of the carbon acid. These trends, which imply changes in the transition-state structure with reactivity, can be described by the interaction coefficient p(XY) = partial derivative beta(B)/partial derivative pK(a)CH = partial derivative-alpha(CH)/ - partial derivative pK(a)BH = 0.03 (primary amines) and 0.01 (piperidine/morPholine). alpha(CH) is smaller than beta(B), indicating an imbalance due to a lag in the delocalization of the negative charge into the fluorenyl moiety at the transition state. The influence of the Ph3P+ group on the intrinsic rate constant is analyzed in terms of possible contributions by inductive/field (1), resonance (R), polarizability (P) and steric (S) effects. Using 9-carbomethoxyfluorene as a reference, it is shown that the stronger electron-withdrawing I effect of the Ph3P+ group relative to the COOMe group enhances log k0 substantially; the fact that the R effect of Ph3P+ is weaker than that of COOMe also contributes to an increase in k0, and so does the P effect of the phosphorus. All these increases are virtually completely offset by the rate-retarding S effect of the bulky Ph3P+ group. A similar analysis for the Me2S+ derivative studied by Murray and Jencks [J. Am. Chem. Soc. 112, 1880 (1990) ] leads to similar conclusions except that the still smaller R effect is probably one of the main reasons why k0 for the Me2S+ derivative is more than ten times higher than for the Ph3P+ derivative; another potential reason is a difference in the steric effect.