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

2002; American Chemical Society; Volume: 67; Issue: 7 Linguagem: Inglês

10.1021/jo0162742

1520-6904

Hai Whang Lee, Arun Kanti Guha, Chang Kon Kim, Ikchoon Lee,

Structural and Chemical Analysis of Organic and Inorganic Compounds

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 βX (βnuc) and βZ (βlg) indicate that mechanism changes from a concerted process (βX = 0.22−0.36, βZ = −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 (βX = 0.09−0.14, βZ = −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 (ρXZ = −1.98) for the former and a positive ρXZ (+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 βZ 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 π-lone pairs on the directly bonded equatorial oxygen atoms and the apical bond (nO(eq) − σ*(ap)). These are supported by the results of natural bond orbital (NBO) analyses at the NBO-HF/6-311+G**//B3LYP/6-311+G** level of theory.