Transformation of Carbinols by RuCl2(PPh3)3 and by Some Other Transition-Metal Catalysts

Several platinoid metal catalysts have been shown to promote reductive coupling, dehydration, disproportionation, and dehydrogenation of diarylcarbinols. Mechanistic studies were performed at 180-210 °C with benzhydrol as substrate and RuCl2(PPh3)3 as catalyst. In aromatic hydrocarbon solvents the main process is reductive coupling. In this medium solvated RuCl2(PPh3)2 is suggested tobe the active catalyst. In dimethyl sulfoxide the starting complex is transformed initially into RuCl2(PPti3) (MegSOlg and causes chiefly carbinol dehydrogenation. Ruthenium alkoxides are implied ascommon reaction intermediates in all four catalyses. Ruthenium hydrides are suggested to take part in the reductive coupling, disproportionation, and dehydrogenation processes. Some aliphatic and primary aromatic alcohols that do not react by themselves in the presence of RuCl2(PPh3)3 can both serve as active hydrogen donors and form crossover products in the presence of secondary and tertiaryaromatic carbinols.