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 Pt<super>II</super>Me complexes and some olefin hydroxo Pt<super>II</super> complexes in hydroxylic solvents such as water and alcohols.

Complexes LPt<super>II</super>(R)(HX) (L = dpms; R = Me, Ph; HX = H₂O, MeOH, PhNH₂) are oxidized by O₂ to yield virtually quantitatively LPt<super>IV</super>(R)(X)(OH). Some of the derived Pt<super>IV</super> alkyls LPt<super>IV</super>(Alk)(X)(OH) (X = OH, OMe) can reductively eliminate methanol in high yield. The mechanism of C-O elimination from LPt<super>IV</super>(Me)(X)(OH) (X = OH, OMe) in acidic aqueous media involves two concurrent pathways: an S_N2 attack by water and an S_N2 attack by a hydroxo or methoxo ligand of another Pt<super>IV</super> species. In the latter case dimethyl ether is produced.

The complex (dpms)Pt(ethylene)(OH) is oxidized by O₂ in water to give a Pt<super>IV</super> hydroxyethyl derivative that reductively eliminates ethylene oxide and ethylene glycol in aqueous solutions.

The complexes derived from cyclic alkenes, cis-cyclooctene, norbornene, benzonorbornadiene, (dpms)Pt<super>II</super>(cy-alkene)(OH), undergo olefin oxoplatination to give 1,2-oxaplatinacyclobutanes (Pt<super>II</super> oxetanes). The derived Pt<super>II</super> oxetanes are easily oxidized by O₂ to produce Pt<super>IV</super> oxetanes. The latter eliminate cleanly the corresponding epoxides by the mechanism of direct C(sp<super>3</super>)-O reductive eliminations, unprecedented in organoplatinum chemistry.

The 1,2-azaplatinacyclobutanes (Pt<super>II</super> azetidines) LPt<super>II</super>(CH₂CH₂NHR-&kappaC,&kappaN) (R = t-Bu, Me) are oxidized by O₂ in the presence of acids to give Pt<super>IV</super> 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.