Rhodium-catalyzed hydrogenation, reaction

The difficulty of this task became obvious in an attempt to achieve a diastereoselective hydroformylation of a simple methallylic alcohol system. It was expected that in analogy to the known substrate-directed rhodium-catalyzed hydrogenation reaction, substrate direction via the hydroxyl substituent would control diastereoselectivity in the course of the hydro-formylation reaction [9], However, a completely stereorandom hydroformylation product formation was observed (1—>3) [10, 11]. 

The first rhodium-catalyzed reductive cyclization of enynes was reported in I992.61,61a As demonstrated by the cyclization of 1,6-enyne 37a to vinylsilane 37b, the rhodium-catalyzed reaction is a hydrosilylative transformation and, hence, complements its palladium-catalyzed counterpart, which is a formal hydrogenative process mediated by silane. Following this seminal report, improved catalyst systems were developed enabling cyclization at progressively lower temperatures and shorter reaction times. For example, it was found that A-heterocyclic carbene complexes of rhodium catalyze the reaction at 40°C,62 and through the use of immobilized cobalt-rhodium bimetallic nanoparticle catalysts, the hydrosilylative cyclization proceeds at ambient temperature.6... 

Rhodium species in oxidation states I and III are involved in the process. Rhodium-catalyzed hydrogenations generally involve oxidative addition reactions, followed by the reverse process of reductive elimination in the final step. Another common elimination process is the so-called (l-elimination, which accounts for the frequent side reaction of isomerization of alkenes, according to Eq. (1) ... 

Recently, two new P- and C-chiral monodentate phosphines 13 were reported. The ligands were applied in a number of transition metal-catalyzed reactions, though ee-values in the rhodium-catalyzed hydrogenation of N-acyl dehydrophenylalanine were only moderate [37]. 

The hydrogenation in a liquid-liquid system with ionic liquids as the catalyst phase was also applied to the hydrogenation of polymers. The first studies were presented by the group of Rosso et al. [91], who investigated the rhodium-catalyzed hydrogenation of polybutadiene (PBD), nitrile-butadiene rubber (NBR) and styrene-butadiene rubber (SBR) in a [BMIM][BF4]/toluene and a [BMIM][BF4]/tolu-ene/water system. The activity of the catalyst followed the trend PBD>NBR> SBR, which is the same order as the solubility of the polymers in the ionic liquid. The values in percentage total hydrogenation after 4 h reaction time were 94% for PBD and 43% for NBR, and after a reaction time of 3 h was 19% for SBR. 

Becalski et al.98b found that the (E)/(Z) ratio of imines under rhodium-catalyzed hydrogenation conditions stayed essentially constant during the course of the reaction. This observation was explained in terms of two possibilities (1) the rates of hydrogenation of the two isomers were identical or, more likely, (2) the rate of interconversion of the two isomers was faster than the rate of hydrogenation, leaving the (E)/(Z) ratio of the substrate constant. 

Wittig yhdes have been shown to be compatible with hydroformylation conditions, and may thus be used in a domino reaction sequence such as from 16a to 38 (Scheme 5.15) [20]. When an a-unsubstituted ylide is employed, the resulting alkene undergoes in-situ rhodium-catalyzed hydrogenation in a triple tandem reaction to convert 10 a to 39. Several other examples were reported establishing the generality of this domino reaction sequence. 

One of the success stories of transition metal catalysis is the rhodium-complex-catalyzed hydrogenation reaction. Asymmetric hydrogenation with a rhodium catalyst has been commercialized for the production of L-Dopa, and in 2001 the inventor, Knowles, together with Noyori and Sharpless, was awarded the Nobel Prize in chemistry. After the initial invention, (enantioselective) hydrogenation has been subject to intensive investigations (27). In general, hydrogenation reactions proceed... 

Elimination. Two kinds of elimination reaction are important in homogeneous catalysis. A catalytic cycle which has involved an oxidative addition generally ends with the reverse process of reductive elimination (equation 6). Rhodium-catalyzed hydrogenations end with this step. 

Understanding of phase behavior in biphasic systems such as PEG/C02 is critical for the design of an efficient and environmentally friendly reaction and separation process. Jessop et al. developed the first PEG/scC02 scheme in the rhodium catalyzed hydrogenation of styrene to ethyl benzene, in which the reaction is conducted at 40 °C and then swept with scC02 to remove the products, and the catalyst is immobilized in the PEG phase and recycled five times with no loss... 

In the initial rhodium-catalyzed hydrogenation experiments on substituted olefins with MonoPhos (24a) as ligand, it was found that the reaction is strongly solvent dependent. Very good enantioselectivities were obtained in the rhodium-catalyzed hydrogenation of 5 in nonprotic solvents (Table 14.5).28-31... 

Compared to the rhodium-catalyzed stereoselective reactions, studies on the iridium-catalyzed reactions have been limited until recently. Usually lower selectivities have been observed in the Ir(i)-catalyzed reactions.459,460 The asymmetric hydrosilylation of imines affords optically active secondary amines. These are very valuable compounds, but the studies on that reaction are quite limited.461 Close examinations of these reactions revealed that they proceed via a transfer hydrogenation. Other conditions such as the 2-propanol/base system in the presence of an appropriate metal complex have been employed as well, but only low selectivities were obtained.462... 

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