Catalyzed Reductions in Aqueous Media

1 Special features of catalytic hydrogenation in water by organometallic complexes 

Additives such as surfactants or salts in aqueous solvent have marked influence on the solvation properties of reacting species. Selke, Oehme, and coworkers have reported that the use of micelle-forming additives has beneficial effect on the catalyst activity and selectivity. In several cases, micelle acts both as a solubilizing agent for poorly soluble organic substrates and as reaction medium for the catalytic reaction. 

Thermodynamic stability of M—C bond in water has raised questions about lifetimes of intermediates in the aqueous organometallic catalytic cycles. From deuterium-labeling experiments (Eq. 6.3), Sinou and coworkers have concluded that the protonation of the M—C bond can occur readily. When the reduction of (Z)-acetamidocinnamic acid methyl ester was performed under hydrogen in a two-phase system (Et0Ac-D20) in the presence of the catalyst [Rh(cod)Cl]2- - TPPTS [tris-(OT-sulfonato-phenyl)phosphine], regioselective 

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The electroreduction of NO to NH2OH and NH3 in aqueous media is also catalyzed by Mnin-TMPyP complexes.334 In contrast to the behavior found with iron porphyrin complexes, the reaction proceeds through an ECE mechanism, in which reduction of Mn111 to Mn11 is followed by NO coordination and then by electroreduction of [Mnn(NO)(TMPyP)]. 

The 1,4-conjugate addition of triorganoindium to enones is promoted by a catalytic amount of Ni(COD)2 (Scheme 8.71) [100], Allylindium reagents can be used in Pd-catalyzed cross-coupling reaction with aryl halides (Scheme 8.72). The Pd-catalyzed allylic substitution of allyl carbonate produces 1,5-dienes (Scheme 8.73) [101]. The indium-mediated palladium-catalyzed Ullmann-type reductive coupling of aryl halides proceeds in aqueous media under air (Scheme 8.74) [102]. 

Detailed study of the electrochemical reduction of quinoxaline in aqueous media has revealed that 1,2,3,4-tetrahydroquinoxaline is formed via a sequence involving 1,4-dihydroquinoxaline and acid-catalyzed rearrangement of the 1,4-dihydro derivative to 1,2-dihydroquinoxaline (Scheme... 

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