Mechanism of the metal-mediated carbalkoxylation of vinyl electrophiles. 1. Preparation, molecular structure, and alcoholysis of vinylic acyl platinum(II) complexes

Article abstract of DOI:10.1021/om00039a005

The two key steps of the nickel triad metal-mediated carbalkoxylation of vinyl electrophiles were investigated using platinum complexes: (a) carbonyl insertion of σ-vinyl Pt(II) halides and (b) alcoholysis of vinylic acyl Pt(II) complexes. NMR spectral studies suggest that the carbonyl insertion into the alkenyl C - Pt bond proceeds via the formation of a σ-vinyl Pt(II) carbonyl species, followed by phosphine dissociation and migratory insertion. While the resulting vinylic acyl Pt(II) halides are unreactive toward alcohols, the corresponding cationic triflate complexes readily undergo alcoholysis in the absence of base. Kinetic and NMR spectral studies establish that the alcoholysis of vinylic acyl Pt(II) triflate complexes occurs via a preequilibration to form a reactive, trigonal-planar intermediate, followed by the rate-determining trans-cis isomerization and nucleophilic attack of alcohols prior to the facile reductive elimination to the α,β-unsaturated carboxylic esters. Both the covalent and cationic vinylic acyl Pt(II) complexes were isolated and fully characterized. A single-crystal molecular structure determination is reported for trans-(CH₃)₂C=CCOPt(PPh₃)₂I (9b). Complex 9b crystallizes in the monoclinic space group P2₁/n with a = 11.924 (5) A?, b = 20.354 (10) A?, c = 15.606 (7) A?, β = 105.93 (3)°, and Z = 4. The structure was solved and refined to R = 0.036 and R_w = 0.045 by using 3245 observed independent reflections. Comparison with the vinyl precursor clearly indicates the stronger trans influence for the acyl moiety than for the vinyl ligand.