Catalytic conversions in water: Part 19. Smooth hydroformylation of N-allylacetamide in mono- and biphasic aqueous media

The Rh/tppts (tppts=P(C6H4-m-SO3Na)3) catalysed hydroformylation of N-allylacetamide in water proceeds at a much faster rate and in a much higher selectivity (>99%) towards the aldehydes, 4-acetamidobutanal and 2-methyl-3-acetamidopropanal, than the Rh/PPh3 catalysed reaction in organic solvents, such as THF, toluene and methanol. In water, at 90°C and 50 bar H2/CO, turnover frequencies (TOF)>10.700 h−1 were observed. Unfortunately, both catalysts exhibited a rather low regioselectivity (linear/branched (l/b) ratio=1.1–1.5) which for Rh/tppts was found not to depend on the temperature, pressure, or ligand concentration. By using phosphate buffers the optimum pH of the aqueous reaction mixture was found to be pH 7.0. Under basic conditions (pH 11.0), the l/b ratio increased to l/b=6.5, while the overall selectivity towards the aldehydes decreased to 41%. In a toluene/water biphasic system, due to the presence of water, the selectivity towards the aldehydes remained >99%. Although Rh/PPh3 (operating in the organic phase) was less active compared to Rh/tppts (operating in the aqueous phase), Rh/PPh3 could easily be separated from the aqueous product layer. The hydrophobic Rh/Xantphos catalyst (Xantphos=4,5-bis(diphenylphosphino)-9,9-dimethylxanthene) afforded a product mixture with l/b ratios up to 20 and could be recycled in five consecutive runs without loss in activity.