Transition metal catalysis hydrogenation

Key words Bioconjugation, Directed evolution, Enantioselectivity, Hybrid catalysts, Hydrogenation, Transition metal catalysis... 

Selected Examples of the Application of Ionic Liquids in Transition Metal Catalysis 5.2.4.1 Hydrogenation... 

The mechanism for hydrosilylation in Figs. 6 and 7 clearly has much in common with suggestions regarding homogeneous transition metal catalysis for other processes involving olefins, such as hydrogenation, isomerization, the oxo reaction, and oligo- and polymerization. 

Not all C-H activation chemistry is mediated by transition metal catalysts. Many of the research groups involved in transition metal catalysis for C-H activation have opted for alternative means of catalysis. The activation of methane and ethane in water by the hexaoxo-/i-peroxodisulfate(2—) ion (S2O82) was studied and proceeds by hydrogen abstraction via an oxo radical. Methane gave rise to acetic acid in the absence of external carbon monoxide, suggesting a reaction of a methyl radical with CO formed in situ. Moreover, the addition of (external) CO to the reaction mixture led to an increase in yield of the acid product (Equation (ll)).20... 

As shown in Figure 1.26, a chiral Sm(III) complex catalyzes asymmetric reduction of aromatic ketones in 2-propanol with high enantioselectivity. Unlike other late-transition-metal catalysis, the hydrogen at C2 of 2-propanol directly migrates onto the carbonyl carbon of substrate via a six-membered transition state 26A, as seen in the Meerwein-Ponndorf-Verley reduction. ... 

By far the most generally useful synthetic application of allyltributyltin is in the complementary set of transition metal- and radical-mediated substitution reactions. When the halide substrates are benzylic, allylic, aromatic or acyl, transition metal catalysis is usually the method of choice for allyl transfer from tin to carbon. When the halide (or halide equivalent) substrate is aliphatic or alicyclic, radical chain conditions are appropriate, as g-hydrogen elimination is generally not a problem in these cases. 

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... 

The entries into transition metal catalysis discussed so far, required the presence of a specific bond (a polar carbon-heteroatom bond for oxidative addition or a carbon-carbon multiple bond for coordination-addition processes) that was sacrificed during the process. If we were able to use selected carbon-hydrogen bonds as sacrificial bonds, then we could not only save a lot of trouble in the preparation of starting materials but we would also provide environmentally benign alternatives to several existing processes. In spite of the progress made in this field the number of such transformations is still scarce compared to the aforementioned reactions. 

For a more complete description of the principles of transition metal catalysis, a number of the excellent reviews published recently should be consulted. These include, in addition to those already cited, several reviews of rhodium (89) and palladium (90-93) chemistry, addition (94, 95) and insertion (96) reactions, homogeneous catalytic hydrogenation (97), hydride complex chemistry (98), nitrogen fixation (99), and organometallic complexes as potential synthetic reagents (100). 

Enantioselective isomerization of olefins for preparation of optically active olefins has great synthetic potential with a long tradition [1] and the non-stereoselective version is probably the most intensively investigated reaction in transition metal catalysis [2]. In particular, stereoselective hydrogen migration in a-functionalized olefins, for example allyl alcohols and allylamines [3], affording optically active aldehydes, ketones and amines, is part of the standard repertoire of enantioselec-... 

The reaction is catalysed by many transition-metal complexes, and a mechanism for the hydrosilylation of an alkene under transition-metal catalysis is depicted in Figure Si5.7. Initial coordination of the alkene to the metal is followed by cis addition of the silicon-hydrogen bond. A hydride migratory insertion and elimination of the product silane complete the cycle. 

The first transition metal catalysis using BINAP-ruthenium complex in homogeneous phase for enantioselective hydrogenation of P-ketoesters was developed by Noyori and co-workers [31]. Genet and co-workers described a general synthesis of chiral diphosphine ruthenium(II) catalysts from commercially available (COD)Ru(2-methylallyl)2 [32]. These complexes preformed or prepared in situ have been found to be very efficient homogeneous catalysts for asymmetric hydrogenation of various substrates such as P-ketoesters at atmospheric pressure and at room temperature [33]. 

A few examples of the use of transition metal catalysis in conjunction with hydrogen peroxide for aldehyde oxidations have also been reported.228,229... 

The crossover product, propionaldehyde-l,3-d-3- C 12, clearly demonstrated that the isomerization occurred via intermolecular 1,3-hydrogen shift. These results are consistent with a modified metal hydride addition-elimination mechanism which involves exclusive 1,3-hydrogen shift through oxygen-directed Markovnikov addition of the metal hydride to the carbon-carbon double bond (Scheme 12.2). The directing effect of functional groups on the selectivity of transition metal catalysis is well presented [9], and an analogous process appears to be operative in the isomerization of allylamines to enamines [10]. 

In 1972, Parshall used an ionic liquid for the first time for the immobilization of a transition metal catalyst in a biphasic reaction set-up [7]. He described the hydrogenation of CC-double bonds with PtCl2 dissolved in tetraethylammonium chloride associated with tin dichloride ([Et4N][SnCl3], m.p. 78 °C) at temperatures between 60 and 100 °C. A substantial advantage of the molten salt medium [over conventional organic solvents]. .. is that the product may be separated by decantation or simple distillation . The use of ionic liquids as novel media for transition metal catalysis started to receive increasing attention when in 1992 Wilkes reported on the synthesis of... 

R. Noyori shared the Nobel Prize in Chemistry in 2001 with W. S. Knowles, who pioneered the use of Rh-catalyzed asymmetric hydrogenation, and K. B. Sharpless, who is known for fundamental work on asymmetric epoxidation and dihydroxylation of alkenes involving transition metal catalysis. 

Cyclopropanation reactions with disubstituted diazo esters are given in Table 13. The reactions with ethyl 2-diazopropanoate (entry 1) shows that 1,2-hydrogen shift at the carbenoid stage, a process that occurs readily in free carbenes and can also be effected by transition metal catalysis, is not competitive under these conditions. 

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