Kinetics of deprotonation of triphenyl(2‐substituted‐9‐fluorenyl)phosphonium ions in 50% dimethyl sulfoxide–50% water. Intrinsic rate constants, imbalances and evidence for changes in transition state structure

Rate constants ( k B 1 and k BH −1 ) for the reversible deprotonation of triphenyl(2‐Z‐9‐fluorenyl)phosphonium ions (Z = H, Br, NO 2 ) by piperidine, morpholine, n ‐butylamine, 2‐methoxyethylamine, glycine ethyl ester, cyanomethylamine, OH − and water were determined in 50% Me 2 SO–50% (v/v) water at 20°C. Brønsted α CH (variation of carbon acid) and β values (variation of amine), and intrinsic rate constants [log k 0 = log k B 1 / q when p K BH a ‐ p K CH a + log( p/q ) = 0] were obtained. α CH decreases with increasing basicity of the amine whereas β 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 = ∂β B /∂p K CH a = ∂α CH / ‐ ∂p K BH a = 0.03 (primary amines) and 0.01 (piperidine/morpholine). α CH is smaller than β 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 Ph 3 P + group on the intrinsic rate constant is analyzed in terms of possible contributions by inductive/field (I), 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 Ph 3 P + group relative to the COOMe group enhances log k 0 substantially; the fact that the R effect of Ph 3 P + is weaker than that of COOMe also contributes to an increase in k 0 , 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 Ph 3 P + group. A similar analysis for the Me 2 S + 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 k 0 for the Me 2 S + derivative is more than ten times higher than for the Ph 3 P + derivative; another potential reason is a difference in the steric effect.