The aminolyses of tetracoordinated organophosphorus compounds were investigated by varying substituents around phosphorus center or in nucleophile. The reactivity is expressed in terms of second-order rate constant, k2 and measured conductometrically. Physical organic chemistry tools; Hammett (?), Brönsted (?) LFER, CICs and heavy atom KIE have been used in quest for the mechanistic information. The pyridinolysis of O-aryl phenyl phosphonochloridothioates [PhP(=S)(OPh-Y)Cl, 1, and O,O-diphenyl Z-S-phenyl phosphorothiolates [(PhO)2P(=O)(S-Ph-Z)], 2, in acetonitrile at 35.0°C, were observed by varying substituents around phosphorus centre (Y in 1, Z in 2) or in nucleophile (X) and extended to pyridinolysis of 4-Chlorophenyl phenyl Chlorophosphate [4-ClPhOP(=O)(OPh)Cl], 3, in acetonitrile at 5.0°C (present study). The variation in X and Y in system 1 shows LFER with negative value of the Hammett coefficients, ?X, -(4.35 ~ 4.75), CICs, ?XY = ?0.46, which is in favour of concerted SN2 mechanism. The LFER plots obtained for 2 with the variation in X, with negative value of the ?X, -(4.43 ~ 4.76) indicating same mechanism as the system 1, substituent (Z) variations (log k2 vs. Z) are biphasic concave downwards with breaks at Z = H, ?XZ = ?0.70 for Z = electron donating group, ?XZ = +0.76 for Z = electron withdrawing group interpreting as the change in mechanism at Z = H from concerted to stepwise. In the light of the above reported results the LFER obtained for 3 with negative value of the ?X, -5.66 can be interpreted as SN2 process, with greater extent of bond formation in TS than that of 1, 2. DOI: http://dx.doi.org/10.3329/jasbs.v38i1.15317 J. Asiat. Soc. Bangladesh, Sci. 38(1): 17-27, June 2012
Homolytic substitution at phosphorus for C–P bond formation in organic synthesis
