cis-hydroxyplatination of dimethyl maleate: Modeling the intermediates in a catalytic alkene-hydration cycle with organoplatinum(II)-hydroxo complexes

Article abstract of DOI:10.1021/ja00137a006

Dimethyl maleate adds to the Pt-OH bonds of cis-Pt(OH)(Me)(PPh₃)₂ (2), Pt(OH)(Me)(dppe) (3), and Pt(OH)(Me)(pdmp) (5) [dppe = l,2-bis(diphenylphosphino)ethane, Ph₂PCH₂CH₂PPh₂; pdmp = o-phenylenebis-(dimethylphosphine), o-C₆H₄(PMe₂)₂] to give Cis-PtMe{CH(CO₂Me)CH(OH)(CO₂Me)}L₂ [L2 = 2PPh₃ (6), dppe (7), pdmp (8)]. Crystals of 6 are monoclinic, space group P2₁/n, with a = 12.658(2) ?, b = 21.688(4) ?, c = 14.340(2) ?, β= 97.25(1)°, and Z = 4. The structure was solved by the heavy atom method and refined by least-squares analysis to R = 0.027 and R_w = 0.032 based on 4508 unique observed reflections. The platinum atom is planar-coordinated by two cis-PPh₃ ligands, a methyl group [r(Pt-C) = 2.111(6) ?], and a CH(CO₂Me)CH(OH)-(CO₂Me) group [r(Pt-C = 2.152(7) ?]. The threo-(R*,R*)-configuration of the PtCH(CO₂Me)CH(OH)(CO₂Me) moiety indicates that dimethyl maleate undergoes syn addition to the Pt-OH bond of 2, probably by cis-migratory insertion into the Pt-OH bond of a five-coordinate intermediate Pt(OH)(Me)(PPh₃)₂(η²-(Z)-MeO ₂CCH=CHCO₂-Me). Dimethyl maleate does not insert into the Pt-OH bonds of trans-Pt(OH)(Ph)(PEt₃)₂ (1) or Pt(OH)(CF₃)(dppe) (4), and dimethyl fumarate is inert toward 1, 2, and 4, but it reacts with 5 to give the monomethyl fumarato complex PtMe{(E)-OC(O)CH=CHCO₂Me}(pdmp) (9). Treatment of 6 or 7 with aqueous acids cleaves their Pt-CH(CO₂-Me)CH(OH)(CO₂Me) bonds forming [PtMe(H₂O)L₂]⁺ and dimethyl malate, MeO₂CCH₂CH(OH)CO₂Me, thus demonstrating two steps necessary for catalysis of addition of water to dimethyl maleate.