Reductions using homogeneous catalysis

SC899>. Reduction under homogeneous catalysis conditions using [Ru30(Ac0)6(H20)3]Ac0 as a catalyst led to 67% yield of the thionine 86. 

Sequential hydroformylation/reductive amination of dendritic perallylated polyglycerols with various amines in a one-pot procedure to give dendritic polyamines in high yields (73-99%). Furthermore, the use of protected amines provides reactive core-shell-type architectures after deprotection. These soluble but membrane filterable multifunctional dendritic polyamines are of high interest as reagents in synthesis or as supports in homogeneous catalysis as well as nonviral vectors for DNA-transfection (Scheme 18) [65]. 

A recent paper by Kwiatek and co-workers1891 discloses the use of a homogeneous catalysis system for the hydrogenation of epoxides. Reduction of cyclohexene oxide to cydohexanol, and of styrene oxide to 2-phenylethanol, was carried at atmospherio pressure, using Ks[Co(CN) ] to catalyze the reaction. 

The second alternative to bypass a difficult RX reduction consists of using redox catalysis [29], Thus, the reduction of RX can be performed at the much less negative one-electron reversible reduction potential (Equation 12.18) of an adequate redox mediator M, which delivers the electron to RX through an homogeneous electron transfer when RX does exist (Equation 12.19), or for a concerted bondbreaking RX reduction (Equation 12.20) ... 

Rh and Ir complexes stabilized by tertiary (chiral) phosphorus ligands are the most active and the most versatile catalysts. Although standard hydrogenations of olefins, ketones and reductive aminations are best performed using heterogeneous catalysts (see above), homogeneous catalysis becomes the method of choice once selectivity is called for. An example is the chemoselective hydrogenation of a,/ -unsaturated aldehydes which is a severe test for the selectivity of catalysts. 

Short of going that far, the tools that can be used for reduction of complexity are much the same in heterogeneous as in homogeneous catalysis (see Chapter 4 and Section 8.5). Specifically ... 

Hydrogenation with homogeneous catalysis has received increasing attention over the last few decades. Hexarhodium hexadecacarbonyl (181 Scheme 35) under water gas shift conditions forms 1,2,3,4-tetrahydroquinoline (128) or N-formyl-1,2,3,4-tetrahydroisoquinoline (182) with the parent hete-roaromatic. When hydrogen is substituted for carbon monoxide, 4-methylquinoline is reduced to 4-methyl-5,6,7,8-tetrahydroquinoline (183) exclusively.Isoquinoline behaves similarly generating A/-formyl-l,2,3,4-tetrahydroisoquinoline (184). Similar reductions under water gas shift or synthesis gas conditions using transition metal carbonyls derived from Mn, Co and Fe, have been recorded.Promotion by phase transfer agents is observed in some cases. 

These examples show that the use of solid catalysts offers not only the possibility of avoiding the formation of waste with respect to homogeneous catalysis but also offers new possibilities to establish new sustainable processes. In fact, realizing, in one step, complex multistep reactions means a reduction in costs, amount of waste and risks of operations. Therefore, it is a primary objective for a sustainable chemistry. 

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