Solvent deuterium kinetic isotope effects for the methanolyses of neutral CO, PO and PS esters catalyzed by a triazacyclododecane : Zn2+-methoxide complex

Abstract

The methanolyses of several organophosphate/phosphonate/phosphorothioate esters (O,O-diethyl O-(4-nitrophenyl) phosphate, paraoxon, 3; O,O-diethyl S-(3,5-dichlorophenyl) phosphorothioate, 4; O-ethyl O-(2-nitro-4-chlorophenyl) methylphosphonate, 5; O,O-dimethyl O-(3-methyl-4-nitrophenyl) phosphorothioate, fenitrothion, 6; O-ethyl S-(3,5-dichlorophenyl) methylphosphonothioate 7) and a carboxylate ester (p-nitrophenyl acetate, 2) catalyzed by methoxide and the Zn²⁺(⁻OCH₃) complex of 1,5,9-triazacyclododecane (1 : Zn²⁺(⁻OCH₃)) were studied in methanol and d₁-methanol at 25 °C. In the case of the methoxide reactions inverse skie's were observed for the series with values ranging from 2 to 1.1, except for 7 where the k_D/k_H = 0.90 ± 0.02. The inverse k_D/k_H values are consistent with a direct nucleophilic methoxide attack involving desolvation of the nucleophile with varying extents of resolvation of the TS. With the 1 : Zn²⁺(⁻OCH₃) complex all the skie values are k_D/k_H = 1.0 ± 0.1 except for 7 where the value is 0.79 ± 0.06. Arguments are presented that the fractionation factors associated with complex 1 : Zn²⁺(⁻OCH₃) are indistinguishable from unity. The skie's for all the complex-catalyzed methanolyses are interpreted as being consistent with an intramolecular nucleophilic attack of the Zn²⁺-coordinated methoxide within a pre-equilibrium metal : substrate complex.