Mechanistic Studies of Alkyl Chloride Acetoxylation by Pt–Sb Complexes

Abstract

The bis-acetate complexes (SbQ₃)Pt(OAc)₂ (1) and (SbQ₂Ph)Pt(OAc)₂ (2) (Q = 8-quinolinyl) were used to study C–Cl acetoxylation of 1,2-dichloroethane (DCE) to generate 2-chloroethyl acetate and the complexes (SbQ₃)PtCl₂ (1b) and (SbQ₂Ph)PtCl₂ (2b), respectively. The first acetoxylation step produced the intermediates (SbQ₃)Pt(Cl)(OAc) (1a) and (SbQ₂Ph)Pt(Cl)(OAc) (2a). The reaction was studied using pseudo first order kinetics (excess DCE) in order to compare the rates of reaction of 1 and 2, which revealed that k_obs = 2.44(6) × 10^–4 s^–1 for 1 and 0.51(2) × 10^–4 s^–1 for 2. The intermediate 1a was synthesized independently, and the solid-state structure was determined using single crystal X-ray diffraction. A non-Sb containing control complex, (^tbpy)Pt(OAc)₂ (3) (^tbpy = 4,4′-di-tert-butyl-2,2′bipyridine), was studied for the acetoxylation of DCE to form (^tbpy)Pt(Cl)(OAc) with k_obs = 0.46(1) × 10^–4 s^–1. Density Functional Theory (DFT) calculations were used to examine possible Pt-mediated mechanisms for the reactions of 1, 2, or 3 with DCE. The lowest energy calculated substitution mechanism occurs with nucleophilic attack by the Pt center on the C−Cl bond followed acetate reaction with the Pt−C bond. However, close in energy and potentially also a viable mechanism is a direct substitution mechanism where the coordinated acetate anion directly reacts with the C−Cl bond of DCE. In addition, the rate of acetoxylation for complex 1 in heated dichloromethane-d₂ and chloroform-d was determined (0.43(1) × 10^–4 s^–1 for dichloromethane-d₂ and 0.37(1) × 10^–4 s^–1 for chloroform-d) and compared to the rate of acetoxylation of DCE.