Integration of phase transfer catalysis into aqueous transfer hydrogenation

• Using phase transfer catalysis can alleviate the substrate inhibition problem in the aqueous transfer hydrogenation.
• Reaction conditions were thoroughly screened to give the best catalytic activities.
• It provides a potential route for the large-scale production based on the aqueous transfer hydrogenation.
• HCOONa as hydrogen source bypasses the requirement of high temperature and high pressure in traditional hydrogenation.

Aqueous transfer hydrogenation using Noyori's catalyst (η6-arene)Ru(H)TsDPEN (TsDPEN = N-(p-toluenesulfonyl)-1,2-diphenylethylenediamine) was found to be dependent on the substrate/catalyst ratio. The reaction was suppressed when a large amount of substrate (acetophenone) was used. To relieve this substrate inhibition problem, common phase transfer catalysts were applied to transfer the formate anion from aqueous layer to organic substrate layer. Quaternary ammonium salts with long alkyl chains were shown to enhance the hydrogenation reactivities up to 5 times. The reaction conditions, including formate loading, phase transfer catalyst loading, amount of water, temperature, stirring speed and counterions, were optimized to afford the best catalytic activities.

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