Asymmetric hydroformylation in aqueous media

There is very little information available on asymmetric hydroformylation in aqueous solutions or biphasic mixtures despite that asymmetric hydroformylation in organic solvents has long been studied very actively. This is even more surprising since enantioselective hydrogenation in aqueous media has been traditionally a focal point of aqueous organometallic catalysis and several water soluble phosphine ligands have been synthetized in enantiomerically pure form. 

The earliest study is from 1995, when the rhodium complex of a menthyl-substituted phosphine (22) was used for the hydroformylation of styrene [99], Although the catalytic activity was quite good (TOF up to 245 h-1), regioselectivity was low (b/1 = 1.0 - 2.5) and no optical induction was observed in 2-phenylpropanal. 

The very limited set of data in Table 4.4 does not allow extensive generalizations. The most obvious conclusion is that with analogous pairs of ligands (NAPHOS/44, CBD/37, BDPP/36) lower enantioselectivities are obtained in water than in organic solvents. Conversion to aldehydes can be higher in aqueous systems, although in several reactions increased hydrogenation of the product aldehydes to alcohols was also observed [102], 

The pH of the aqueous phase may significantly influence both the rate and the enantioselectivity of the reaction. 

The maximum enantioselectivity of 18 % achieved so far in aqueous hydroformylations may not seem very promising. However, the history of asymmetric hydrogenation of prochiral olefins and ketones demonstrates that such a situation may change fast if there is a strong drive behind the case. 

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