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The rates of exchange of 2-hydrogens in 3,4-dimethyloxazolium ion, 3,4-dimethylthiazolium ion, and 1,3,4-trimethylimidazolium ion have been measured at a variety of pH's and buffer concentrations. In each case the rate constant for exchange catalyzed by OD~has been evaluated. The relative second-order rate constants are respectively 105•1 2345:103•5:1. The 18C-H coupling constants for ring hydrogens have also been measured. These data for rates are compared with knowledge of the ground states. The similarity of 18C-H coupling constants in homologous imidazolium and thiazolium ions at the 2 position indicates that these C-H bonds are similar in the ground state. The exchange rate 3000 times greater in 3,4-dimethylthiazolium ion than in 1,3,4-trimethylimidazolium ion suggests the transition state for ylide formation must be considerably stabilized in the case of the sulfur heterocycle. This indicates a special role for sulfur and provides partial understanding of the importance of a thiazolium ion in the structure of thiamine.

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The rates of hydrolysis of p-nitrophenyl diphenylphosphinate have been studied in acidic dioxane-water (40:60, v/v). Oxygen-18 studies show that hydrolysis occurs by cleavage of the P-O bond. The dependence of rate on acid concentration passes through a maximum at ~1.5 M HC104. The observed data give a w value (Bunnett) of 12 although AS* is only -27 eu.

The chemical dynamics of the hydrolytic cleavage of the P-N bond in phosphinamides, R2P(0)NR2' (R' = H, CHS), have been studied. The results reflect on the fundamental nature of the P-N bond and therefore are relevant to understanding phosphorus amides as solvents, synthetic intermediates, and phosphate transfer agents in biochemistry. Phosphinamides undergo alkaline hydrolysis with first-order dependence on [HO-]. The rates are comparable to the alkaline hydrolysis of analogous carboxylic amides. Oxygen-18 studies of the basic hydrolysis reveal a small amount of lsO exchange into unhydrolyzed amide: k(hydrolysis)/k(exchange) = 70. This may provide direct evidence for a pentacoordinate intermediate in the mechanism of alkaline hydrolysis.Although phosphinamides are no more reactive toward hydroxide than carboxylic amides, these amides hydrolyze in acidic solutions about 106 times more rapidly than carboxylic amides. In order to elucidate the source of this rate enhancement, a study was made of the hydrolysis of N-substituted diphenylphosphinamides (1), N,N-dimethyl-2,2,3,4,4-pentamethyltrimethylenephosphinamide (2), and A,A-dimethyldiisopropylphosphinamide (3). In acidic media, specific acid catalysis was observed for 1 in the pH range 1.5-3.5. The low solvent isotope effect, ko/kn = 1.3, and the highly negative activation entropy, AS* = -35 eu, indicate an A2 mechanism. Partial hydrolysis in acidic H2lsO introduced 1 equiv of solvent oxygen into diphenylphosphinic acid. The variation in rate with nitrogen substituents, p* = -1.0, is consistent with a mechanism involving the N-protonated species, and it is the tendency to protonate on nitrogen that appears to be the source of the lability of phosphorus amides. Introduction of angle strain at phosphorus in 2 produced a marked inhibition of hydrolysis: 2 reacts 103 times more slowly than its open-chain analog, 3. This unusual retarding effect of small ring geometry in displacement at phosphorus together with the evidence for N-protonation indicates a process involving direct displacement rather than a pentacoordinate intermediate with significant stability.