Hydride transfer transition metal catalyst

A variety of synthetic methods have been developed for the conjugate reduction of a,/3-unsatnrated carbonyl compounds. This section comprises only those methodologies that involve the nse of palladinm catalysts. More general reviews that cover a broader spectrum of methods, including those involving metal hydrides, different transition metal catalysts, electron transfer reagents, and biomimetic approaches, have already been published. ... 

These transition-metal catalysts contain electronically coupled hydridic and acidic hydrogen atoms that are transferred to a polar unsaturated species under mild conditions. The first such catalyst was Shvo s diruthenium hydride complex reported in the mid 1980s [41 14], Noyori and Ikatiya developed chiral ruthenium catalysts showing excellent enantioselectivity in the hydrogenation of ketones [45,46]. 

Compared with alkynes, alkenes are rather inert to hydroalumination. Most required an external catalyst that aided hydride transfer from aluminum to alkenes [169]. A variety of transition metal catalysts was extensively studied for this among these the Group IV metals Ti and Zr were found generally apphcable. 

A transition metal catalyst has also been used to effect the reductive alkylation of amino groups on proteins [41], This reaction uses [Cp Ir(4-4 -dimethoxybipy)(H20)]S04 31 as a mild transfer hydrogenation catalyst and formate ion as the stoichiometric hydride source, in Fig. 10.3-11 (a). Presumably, this reaction occurs via the reversible formation of imine 33 with free amino groups on the protein surface, followed by reduction of iridium hydride 32. For most proteins, multiple modifications are observed (Fig. 10.3-ll(b)), although the overall level of conversion can be altered through variation of either the reaction temperature or the concentrations of the aldehyde and catalyst. In general, the reaction has shown excellent reliability for protein alkylation between pH 5 and 7.4. 

Many rhodium(II) complexes are excellent catalysts for metal-carbenoid-mediated enantioselective C-H insertion reactions [101]. In 2002, computational studies by Nakamura and co-workers suggested the dirhodium tetracarboxylate catalyzed diazo compounds insertion reaction to alkanes C-H bonds proceed through a three-centered hydride-transfer-like transition state (Fig. 25) [102]. Only one rhodium atom of the catalyst is involved in the formation of rhodium carbene intermediate, while the other rhodium atom served as a mobile ligand, which enhanced the electrophilicity of the first one and facilitate the cleavage of rhodium-carbon bond. In this case, the metal-metal bond constitutes a special example of Lewis acid activation of Lewis acidic transition-metal catalyst. 

Other catalytic reactions involving a transition-metal allenylidene complex, as catalyst precursor or intermediate, include (1) the dehydrogenative dimerization of tributyltin hydride [116], (2) the controlled atom-transfer radical polymerization of vinyl monomers [144], (3) the selective transetherification of linear and cyclic vinyl ethers under non acidic conditions [353], (4) the cycloisomerization of (V2V-dia-llyltosylamide into 3-methyl-4-methylene-(V-tosylpyrrolidine [354, 355], and (5) the reduction of protons from HBF4 into dihydrogen [238]. 

In transition metal chemistry, ligand variation has proven to be the key to obtaining highly active polymerization catalysts. In particular, sterically hindered monocationic alkyl complexes with an empty site seem to be well suited for polymerization. The steric bulk prevents (associative) -hydrogen transfer, while the positive charge destabilizes the free hydride and thus opposes (dissociative) /(-elimination. 

Electrochemical reductions of CO2 at a number of metal electrodes have been reported [12, 65, 66]. CO has been identified as the principal product for Ag and Au electrodes in aqueous bicarbonate solutions at current densities of 5.5 mA cm [67]. Different mechanisms for the formation of CO on metal electrodes have been proposed. It has been demonstrated for Au electrodes that the rate of CO production is proportional to the partial pressure of CO2. This is similar to the results observed for the formation of CO2 adducts of homogeneous catalysts discussed earlier. There are also a number of spectroscopic studies of CO2 bound to metal surfaces [68-70], and the formation of strongly bound CO from CO2 on Pt electrodes [71]. These results are consistent with the mechanism proposed for the reduction of CO2 to CO by homogeneous complexes described earlier and shown in Sch. 2. Alternative mechanistic pathways for the formation of CO on metal electrodes have proposed the formation of M—COOH species by (1) insertion of CO2 into M—H bonds on the surface or (2) by outer-sphere electron transfer to CO2 followed by protonation to form a COOH radical and then adsorption of the neutral radical [12]. Certainly, protonation of adsorbed CO2 by a proton on the surface or in solution would be reasonable. However, insertion of CO2 into a surface hydride would seem unlikely based on precedents in homogeneous catalysis. CO2 insertion into transition metal hydrides complexes invariably leads to formation of formate complexes in which C—H bonds rather than O—H bonds have been formed, as discussed in the next section. 

Catalytic and ionic hydrogenations may be combined to carry out catalytic ionic hydrogenation.216 In these cases the catalyst, either a heterogeneous or a homogeneous transition metal, serves as a hydride ion transfer agent. 

When activated by metallic catalysts, hydrogen may be transferred from the metallic center to unsaturated organic molecules. The nature and reactivity of transition metal hydrides depend on the central metals as well as on the electronic and steric properties of the ligands. Metal hydrides with optically active ligands are chiral and thus, are capable of asymmetric hydrogenation. 

It is known that the polymerization of ethylene by trialkyl aluminum is not a rapid reaction at normal pressures and temperatures. Ziegler, Gellert, Holzkamp, Wilke, Duck and Kroll (72) have found that ethylene was polymerized to higher trialkylaluminums only at elevated temperatures and pressures. Anionic hydride transfer commonly occured under these conditions. However, the addition of a transition metal halide such as titanium tetrachloride, the classical Ziegler catalyst, polymerized ethylene rapidly under mild conditions. 

Recent mechanistic studies on transition metal-catalysed hydrogen transfer reactions have been reviewed. Experimental and theoretical studies showed that hydrogen transfer reactions proceed through different pathways. For transition metals, hydridic routes are the most common. Within the hydridic family there are two main groups the monohydride and dihydride routes. Experimentally, it was found that whereas rhodium and iridium catalysts favour the monohydride route, the mechanism for ruthenium catalysts proceeds by either pathway, depending on the ligands. A direct hydrogen transfer mechanism has been proposed for Meerwein-Ponndorf-Verley (MPV) reductions.352... 

Moreover, MPVO reactions are traditionally performed with stoichiometric amounts of Al(III) alkoxides. Some improvements came from the use of dinuclear AI(III) complexes that can be used in catalytic amount [6, 7]. This is why there has been an ever-increasing interest in catalytic MPVO reactions promoted by lanthanides and transition-metal systems [8]. In these cases, it is believed that reaction proceeds via formation of a metal hydride, in contrast with the mechanism accepted for traditional aluminum alkoxide systems, which involves direct hydrogen transfer by means of a cyclic intermediate [9]. As well as La, Sm, Rh and Ir complexes, Ru complexes have been found to be excellent hydrogen transfer catalysts. The high flexibility of these systems makes them very useful not only for MPVO-type reactions, but also for isomerization processes [10]. 

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