Redução de compostos carbonílicos catalisada por complexos de rutênio e ósmio contendo ligantes do tipo pinça

Abstract

The reduction of esters to alcohols is a key process for the synthesis of various industrial products. The transition metal complex catalysis (TMCC) emerges as an excellent alternative, given that milder reaction conditions can be employed. In this work, the complexes [RuCl2(PPh2(CH2)2NH2)2] - Ru-1; [RuCl2(PPh3){PyCH2NH(CH2)2PPh2}] - Ru-2; [RuCl2(PPh3){HN(C2H4SEt)2}], Ru-3; [OsHCl(CO){PyCH2NH(CH2)2NHPtBu2}] - Os-1, [OsHCl(CO){PyCH2NH(CH2)2NHPAd2}] Os-2 and [OsH2(CO){PyCH2NH(CH2)2NHPtBu2}] - Os-3 were employed as catalysts in the reduction of esters and aldehydes using both hydrogenation and hydrogen transfer (TH). Various reaction parameters were evaluated to optimize these reactions. The Ru-1, Ru-2 and Ru-3 catalysts were efficient for the hydrogenation of octyl octanoate at 70 °C, 50 bar of hydrogen, using 0.05 mol% of the catalyst and 5 mol% of sodium methoxide, and the order of efficiency was Ru-2 > Ru-1 > Ru-3. It is noteworthy that the reaction can be carried in anisole, an environmentally friendly solvent. Other substrates, methyl 10-undecenoate, cinnamaldehyde and myrtenal were also selectively hydrogenated. In the reduction of methyl 10-undecenoate, the Ru-3 catalyst showed the best chemoselectivity (99%) in reducing the ester group as compared to the C=C group. Furthermore, the Ru-2 and Ru-3 catalysts were able to hydrogenate the ester group at a pressure of 5 bar and a temperature of 40 °C. These catalysts were also efficient in the selective hydrogenation of myrtenal and cinnamaldehyde. In hydrogen transfer (TH), in addition to the mentioned ruthenium catalysts, three osmium catalysts (Os-1, Os-2 and Os-3) were used. Different solvents were evaluated and anisole and cyclopentyl methyl ether (CPME), which are regarded as sustainable, were the best. Among the hydrogen donors evaluated, ethanol was the best. Using Os-3 it was possible to reduce both esters and aldehydes without the use of basic promoter. In the TH of methyl 10-undecenoate, the osmium catalysts showed better chemoselectivity as compared to the ruthenium ones. Finally, in the hydrogen transfer reduction of cinnamaldehyde with ruthenium catalysts, the formation of esters by oxidative esterification as a side reaction, which was controlled by changing the hydrogen source.