Effect of substituents on reactivity and reaction mechanism observed in Sn reaction of some organophosphorus compounds: based on Physical organic methodologies

Abstract

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 S_N2 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 S_N2 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