Formation and thermodynamic stability of novel methyl–vinyl derivatives of PtIV as possible intermediates for Csp3–Csp2 cross-coupling

Abstract

Methyl–vinyl derivatives of PtIV were obtained in a sequence of the reactions of methyl iodide oxidative addition to PtII iodo complexes followed by iodoplatination of C–C triple bond of R1CCR2 (R1 = Ph, R2 = C(O)OC2H5; R1 = R2 = C(O)OCH3) in acetone solution under excess of I−. In the reaction of the ethyl phenylpropiolate equilibrium formation of PtIV methyl–vinyl derivative ▪ occurs. The reaction is highly exothermic, ΔH = –(81 ± 7) kJ/mol, and this provides a favorable thermodynamics even though a significant decrease in entropy, ΔS = −(249 ± 23) J/(K mol). In the reaction of dimethyl acetylenedicarboxylate the chelate metallocycle ▪ is quantitatively formed. The quantitative yield of the organometallic product is explained by more favorable thermodynamics of the reaction: chelation of the metal center by the oxygen atom of the carboxymethyl group in the σ-vinyl ligand forming metallocycle provides an additional stabilization of the product. In the formed organoplatinum compounds vinyl and methyl ligands are located in the mutual cis-positions, which is essential for further accomplishment of Csp3–Csp2 coupling reaction.