LIGAND-ENABLED PLATINUM--CARBON BOND FUNCTIONALIZATION UTILIZING DIOXYGEN AS THE TERMINAL OXIDANT

Abstract

The use of organotransition metal complexes for selective functionalization of hydrocarbons is of great importance. Dioxygen is the most practical oxidant for large-scale applications in the petroleum industry. The focus of this work is the development of ligand-modulated platinum-based systems that can utilize O2 or air for selective transformation of organoplatinum(II) derivatives into alcohols, diols, aminoalcohols and epoxides in aqueous media. We found that the hemilabile tripod ligand dipyridylmethanesulfonate (dpms) enables facile aerobic functionalization of various PtIIMe complexes and some olefin hydroxo PtII complexes in hydroxylic solvents such as water and alcohols. Complexes LPtII(R)(HX) (L = dpms; R = Me, Ph; HX = H2O, MeOH, PhNH2) are oxidized by O2 to yield virtually quantitatively LPtIV(R)(X)(OH). Some of the derived PtIV alkyls LPtIV(Alk)(X)(OH) (X = OH, OMe) can reductively eliminate methanol in high yield. The mechanism of C-O elimination from LPtIV(Me)(X)(OH) (X = OH, OMe) in acidic aqueous media involves two concurrent pathways: an SN2 attack by water and an SN2 attack by a hydroxo or methoxo ligand of another PtIV species. In the latter case dimethyl ether is produced. The complex (dpms)Pt(ethylene)(OH) is oxidized by O2 in water to give a PtIV hydroxyethyl derivative that reductively eliminates ethylene oxide and ethylene glycol in aqueous solutions. The complexes derived from cyclic alkenes, cis-cyclooctene, norbornene, benzonorbornadiene, (dpms)PtII(cy-alkene)(OH), undergo olefin oxoplatination to give 1,2-oxaplatinacyclobutanes (PtII oxetanes). The derived PtII oxetanes are easily oxidized by O2 to produce PtIV oxetanes. The latter eliminate cleanly the corresponding epoxides by the mechanism of direct C(sp3)-O reductive eliminations, unprecedented in organoplatinum chemistry. The 1,2-azaplatinacyclobutanes (PtII azetidines) LPtII(CH2CH2NHR-&kappaC,&kappaN) (R = t-Bu, Me) are oxidized by O2 in the presence of acids to give PtIV azetidine complexes, [LPt(CH2CH2NHR-&kappaC,&kappaN)(OH)]+. The latter undergo reductive elimination of N-alkyl ethanolammonium salts, HOCH2CH2NH2R+, in acidic aqueous solutions at elevated temperatures. Efficient catalytic systems based on palladium acetate, di(6-pyridyl)ketone and 6-methyldi(2-pyridyl)methanesulfonate ligands, suitable for selective oxidation of ethylene with H2O2 to glycol acetates were developed. Glycol acetates were obtained in high selectivity and high yield on H2O2 under mild reaction conditions.