Mechanistic Studies of Alkyl Chloride Acetoxylation by Pt–Sb Complexes

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 kobs = 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 kobs = 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.