Anilinolysis of Diethyl Phosphinic Chloride in Acetonitrile

Table 1. The second-order rate constants (kH(D) × 10 /M‒1 s‒1), selectivity parameters (ρX and βX), and deuterium kinetic isotope effects (kH/kD) for the reactions of diethyl phosphinic chloride (2) with XC6H4NH2(D2) in MeCN at 50.0 C X kH × 10 /M s kD × 10 /M s kH/kD 4-MeO 44.4 ± 0.2 45.6 ± 0.6 0.974 ± 0.001 4-Me 29.8 ± 0.9 31.2 ± 0.9 0.955 ± 0.001 3-Me 23.0 ± 0.6 25.1 ± 0.7 0.931 ± 0.001 H 16.2 ± 0.6 17.7 ± 0.3 0.915 ± 0.001 3-MeO 11.8 ± 0.3 13.2 ± 0.4 0.894 ± 0.003 4-Cl 6.89 ± 0.04 8.08 ± 0.20 0.853 ± 0.003 3-Cl 4.24 ± 0.06 5.12 ± 0.06 0.828 ± 0.003 –ρX 1.59 ± 0.05 1.48 ± 0.05 βX 0.56 ± 0.03 0.52 ± 0.03 The σ values were taken from Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165. The pKa values of X-anilines in water were taken from Streitwieser, A. Jr.; Heathcock, C. H.; Kosower, E. M. Introduction to Organic Chemistry, 4th ed.; Macmillan: New York, 1992; p 735. Standard deviation. Calculated from kH. Correlation coefficient, r = 0.998. r = 0.998. Calculated from kD. r = 0.993. r = 0.993. Standard error {= 1/kD [(∆kH) + (kH/kD) × (∆kD)]} from Crumpler, T. B.; Yoh, J. H. Chemical Computations and Errors; John Wiley: New York, 1940; p 178. Nucleophilic substitution at a phosphoryl (P=O) or thiophosphoryl (P=S) center generally proceeds either through stepwise mechanism with a trigonal bipyramidal pentacoordinate (TBP-5C) intermediate or a concerted SN2 mechanism with a TBP-5C transition state (TS). Continuing the studies on phosphoryl transfer reactions, the reactions of diethyl phosphinic chloride [Et2P(=O)Cl; 2] with substituted anilines (XC6H4NH2) are investigated kinetically in acetonitrile at 50.0 C (eq 1). The deuterium kinetic isotope effects (DKIEs; kH/kD) involving deuterated aniline (XC6H4ND2) nucleophiles are also investigated, since the DKIEs are one of the strong tools to clarify the reaction mechanism. The DKIEs have provided a useful means to determine the TS structures in nucleophilic substitution reactions, and how the reactants, especially through changes in substituents, alter the TS structures. Incorporation of deuterium in the nucleophile has an advantage in that the α-DKIEs reflect only the degree of bond formation. When partial deprotonation of the aniline occurs in a rate-limiting step by hydrogen bonding, the kH/kD values are greater than unity, primary normal (kH/kD > 1.0). In contrast, the DKIEs can only be secondary inverse (kH/kD < 1.0) in a normal SN2 reaction, since the N-H(D) vibrational frequencies invariably increase upon going to the TS because of an increase in steric congestion in the bond-making process. This work aims to gain further information into the phosphoryl transfer reactions, as well as to compare the mechanism and DKIEs of the anilinolyses of dimethyl [Me2P(=O)Cl; 1], methyl phenyl [MePhP(=O)Cl; 3], and diphenyl [Ph2P (=O)Cl; 4] phosphinic chlorides in MeCN.