Probing the ruthenium-catalyzed higher polyol hydrogenolysis reaction through the use of stereoisomers

Nine polyol stereoisomers ranging from three to six carbons in length were reacted under hydrogenolysis conditions (205-240 °C, 100 bar H 2 ) over a Ru-C catalyst to better understand the reaction mechanism. Previous reports have postulated the retro-aldol mechanism as the main pathway leading to C-C scission. However, the retro-aldol mechanism was insufficient in explaining the product distribution of tetritols from pentitols, while the decarbonylation mechanism could explain the selectivity results of terminal C-C scission. Retro-aldol scission of internal C-C bonds was confirmed to occur by the tetritol product distribution from hexitols. Therefore, the presence or role of 3-keto and 4-keto intermediates had a negligible effect on the C-C hydrogenolysis of polyols when compared to aldehyde intermediates. The reaction rates of the polyols depended on the configuration of the polyol stereoisomers. The reactivity of the stereoisomers was correlated to the presence of erythro sequences of hydroxyl groups and was independent of the carbon chain length. Hydrogenolysis of higher polyols over Ru-C under basic conditions appears to initiate preferentially via formation of an aldehyde intermediate rather than a ketone intermediate. C-C bond scission in this aldehyde then proceeds competitively through both decarbonylation and retro-aldol reactions.