Homogeneous catalytic hydrogenation of long-chain esters by an osmium pincer complex and its potential application in the direct conversion of triglycerides into fatty alcohols

The osmium hydride complexes OsH2(CO)[NH(CH2PiPr2)2] (1) and OsHCl(CO)[NH(CH2PiPr2)2] (2) were evaluated in the catalytic hydrogenation of hexyl octanoate and cis-3-hexenyl hexanoate to alcohols as model substrates for triglycerides. Both complexes achieve full conversion of the saturated ester at 220 °C and 800 psi pressure of hydrogen gas. In the presence of unsaturated substrates, the complexes hydrogenate CC bonds, but are subsequently ineffective in the reduction of the ester moiety. However complex 1 is capable of hydrogenating fully saturated triglycerides (i.e., hardened fats as obtained by separate initial hydrogenation of seed oils using either 1 or 2 or a standard heterogeneous hydrogenation catalyst) giving cetyl and stearyl alcohols as the main products.