An Immobilized Ortho-Palladated Dimethylbenzylamine Complex as an Efficient Catalyst for the Methanolysis of Phosphorothionate Pesticides

Abstract

The methanolysis of a series of P=S phosphorothionate pesticides (fenitrothion, coumaphos, diazinon, and dichlofenthion) catalyzed by an ortho-palladated complex covalently attached to two different solid supports, macroporous polystyrene and amorphous silica gel, was studied. Both the polystyrene and the silica-based catalysts showed excellent activity in methanol near neutral pH (neutral s(s)pH = 8.38) at ambient temperature. These heterogeneous catalysts can be readily recovered and reused without significant loss of activity. Fifty milligrams of the silica-supported catalyst SiPd1 offered an acceleration of up to 8.6 x 10(9)-fold for the methanolysis of fenitrothion (2) over the methoxide-promoted background reaction at s(s)pH = 8.8. For the same reaction, 50 mg of polystyrene-supported complex PSPd2 provided a 3.7 x 10(9)-fold acceleration at s(s)pH = 8.8. When accounting for the amount of palladium in the solid, the slight superiority of silica over polystyrene as a solid support is believed to be a result of several possible factors including a higher concentration of active sites accessible to the reaction solvent and a more hydrophilic surface environment that allows better interaction of the methanol solvent with the attached palladacycle. Unlike the behavior in homogeneous solution, the rate of methanolysis of the substrates catalyzed by the solid catalysts was relatively insensitive to the nature of the substrate, probably indicating that a mass transport process is rate limiting. The solid-supported materials effectively decompose malathion at roughly stoichiometric ratios, but they are strongly inhibited by the thiol product resulting from the cleavage of the P=S(SR) linkage.