Unexpectedly fast catalytic transfer hydrogenation of aldehydes by formate in 2-propanol–water mixtures under mild conditions

• Fast transfer hydrogenation of aldehydes from aqueous formate at low temperature.
• Unsaturated aldehydes reduced with complete selectivity to unsaturated alcohols.
• Mixtures of water–2-propanol as solvent, high water proportion is required.

Unsaturated aldehydes were efficiently reduced by transfer hydrogenation from sodium formate in water–2-propanol mixtures using a water-soluble Ru(II)-tertiary phosphine catalyst. The reaction yielded unsaturated alcohols with complete selectivity and without hydrogenation or isomerization of CC bonds of the substrates. Very high reaction rate was observed in the transfer hydrogenation of cinnamaldehyde already at 30 °C with turnover frequency of 160 h−1 and this increased to 3800 h−1 at 70 °C. Consequently, the method is applicable to the synthesis of unsaturated alcohols in case of heat sensitive or highly volatile aldehydes, too. Based on multinuclear NMR investigations, trans-[RuH2(H2O)(mtppms)3] is suggested as the key catalytic species.

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