Catalytic Deoxygenation of 1,2-Propanediol to Give n-Propanol

Deoxygenation of 1,2-propanediol (1.0M in sulfolane) catalyzed by bis(dicarbonyl)(mu-hydrido) (pentamethylcyclopentadiene)ruthenium trifluoromethanesulfonate ({[Cp*Ru(CO)(2)](2)(mu-H)}+OTf-) (0.5 mol%) at 110 degrees C under hydrogen (750 psi) in the presence of trifluoromethanesulfonic acid (HOTf) (60 mM) gives n-propanol as the major product, indicating high selectivity for deoxygenation of the internal hydroxy group over the terminal hydroxy group of the diol. The deoxygenation of 1,2-propanediol is strongly influenced by the concentration of acid, giving faster rates and proceeding to higher conversions as the concentration of HOTf is increased. Propionaldehyde was observed as an intermediate, being formed through acid-catalyzed dehydration of 1,2-propanediol. This aldehyde is hydrogenated to n-propanol through an Ionic pathway involving protonation of the aldehyde, followed by hydride transfer from the neutral hydride, dicarbonyl(pentamethylcyclopentadiene)ruthenium hydride [Cp*Ru(CO)(2)H]. The proposed mechanism for the deoxygenation/hydrogenation reaction involves formation of a highly acidic dihydrogen complex [Cp*Ru(CO)(2)(eta(2)-H-2)]+OTf-.