Kinetics and Mechanism of the Aminolysis of Aryl Methyl Phosphorochloridothionates in Acetonitrile

초록

The kinetics and mechanism of the reactions of Z-aryl bis(4-methoxyphenyl) phosphates, (4-MeOC6H4O)2P(=O)OC6H4Z, with pyridines (XC5H4N) are investigated in acetonitrile at 55.0 C. In the case of more basic phenolate leaving groups (Z = 4-Cl, 3-CN), the magnitudes of X (nuc) and Z (lg) indicate that mechanism changes from a concerted process (X = 0.22-0.36, Z = -0.42 to -0.56) for the weakly basic pyridines (X = 3-Cl, 4-CN) to a stepwise process with rate-limiting formation of a trigonal bipyramidal pentacoordinate (TBP-5C) intermediate (X = 0.09-0.14, Z = -0.08 to -0.28) for the more basic pyridines (X = 4-NH2, 3-CH3). This proposal is supported by a large negative cross-interaction constant (XZ = -1.98) for the former and a positive XZ (+0.97) for the latter processes. In the case of less basic phenolate leaving groups (Z = 3-CN, 4-NO2), the unusually small magnitude of Z values is indicative of a direct backside attack TBP-5C TS in which the two apical sites are occupied by the nucleophile and leaving group, ap(NX)-ap(LZ). The instability of the putative TBP-5C intermediate leading to a concerted displacement is considered to result from relatively strong proximate charge transfer interactions between the -lone pairs on the directly bonded equatorial oxygen atoms and the apical bond (nO(eq) - *(ap)).