Transition-State Variation in the Nucleophilic Substitution of Aryl Di-(4-methoxyphenyl) Phosphates with Pyridines in Acetonitrile

초록

The kinetics and mechanism of the reactions of Z-aryl di-(4-methoxyphenyl) phosphates with pyridines are investigated in acetonitrile at 55.0 C. The magnitudes of βx ( βnuc ) and βz ( βIg ) indicate that mechanism changes from a concerted process ( βx = 0.23 ~ 0.42, βz = -0.56 ~ -0.42) for the weakly basic pyridines ( X = 3-Cl ~ 4-CN ) to a stepwise one with rate-limiting formation of a trigonal bipyramidal pentacoordinate ( TBP-5C ) intermediate ( βx = 0.09 ~ 0.14, βz = -0.08 ~ -0.28 ) for the more basic pyridines ( X = 4-NH2 ~ 3-CH3 ). This proposal is supported by a latge negative cross-interaction constant ( -1.92 ) for the former and a positive ( +1.00 ) for the latter processes.