Hydrogen molecular, reaction with ruthenium complexes

The photochemical studies of transition metal hydride complexes that have appeared in the chemical literature are reviewed, with primary emphasis on studies of iridium and ruthenium that were conducted by our research group. The photochemistry of the molybdenum hydride complexes Mo(tj5-C5H5)2M2] and [MoH4(dppe)2] (dppe = Ph2PCH2CH2PPh2), which eliminate H2 upon photolysis, is discussed in detail. The photoinduced elimination of molecular hydrogen from di-and polyhydride complexes of the transition elements is proposed to be a general reaction pathway. 

In a one-pot reaction, a series of ketones were converted to chiral acetates with the help of an achiral ruthenium complex and CALB at 1 atm of hydrogen gas in ethyl acetate. Molecular hydrogen was equally effective in the transformation of enol acetates to chiral acetates in the same catalyst system without addition of additional acyl donors (Jung, 2000b). 

The present hydrogen transfer reaction is extended to the aerobic oxidation of alcohols. Thus, the oxidation of alcohols can be carried out with a catalytic amount of hydrogen acceptor under an O2 atmosphere by a multistep electron-transfer process. As shown in Scheme 3.4, the ruthenium dihydrides formed during the hydrogen transfer can be regenerated by a multistep electron-transfer process including hydroquinone, ruthenium complex, and molecular oxygen. 

On comparing H NMR spectra obtained for the reaction of para-H2 with X at different reaction times (fig. 10), it can be seen that the H2 signal starts as an enhanced negative value (due to the initial transfer from para-H2), and then reaches its thermal equilibrium value by a complex mechanism basically determined by the relaxation rates of the protons in the ruthenium complex. Clearly, the exchange rate of H2 over the triruthenium cluster should be faster than the relaxation rate of the hydride ligands, otherwise no memory of the intermediate state could have been revealed in the molecular hydrogen resonance. [56]... 

Certain neutral complexes of bivalent ruthenium (1) and osmium (1,2), and tervalent and univalent iridium (3,4) have been discovered to activate molecular hydrogen in solution at normal conditions. These reactions are of considerable interest because they permit comparison of hydrogen-activating characteristics of metal complexes with different coordination numbers and formal oxidation states, two properties which are believed to represent important factors in determining catalytic activation of hydrogen. 

Polborn and Severin [23] recently reported ruthenium- and rhodium-based TSAs for the transfer hydrogenation reaction. These complexes were used as catalyst precursors in combination with molecular imprinting techniques. Phosphinato complexes were prepared as analogs for the ketone-associated complex. They demonstrated that the results obtained in catalysis were better in terms of selectivity and activity when these TSAs were imprinted in the polymer. This shows that organometallic complexes can indeed serve as stable TSAs (Figure 4.9). 

Hydrogenation of Carbon-Carbon Multiple Bonds. There are a number of ruthenium complexes that can catalyze hydrogenation of various substrates, either through reduction with molecular hydrogen, or transfer reactions from a hydrogen donor. Generally, the available substrates for hydrogenation include the double bonds present in nitro compounds, alkenes, aldehydes, ketones, and other carboxylic acid derivatives (4). 

Two reviews deal with the relation between homogeneous and heterogeneous catalysis. One of these reviews is of a general physical and theoretical nature, while the other is concerned with specific classes of compounds — alkenes, alkynes, and fats — and concentrates on their hydrogenation and isomerization. Homogeneous catalysis by ruthenium complexes has been reviewed. The application of molecular orbital symmetry rules, to organic as well as to organometallic reaction mechanisms, has been discussed, and a set of rules similar to, but simpler to apply than, the Woodward-Hoffmann rules has been described. ... 

The direct conversimi of esters into amides is a synthetically useful transformation. However, most of the methodologies developed till now usually require harsh reaction conditions, are poorly compatible with sensitive substrates, and present a low atom economy [136, and references cited therein]. Very recently, MUstein and co-workers demonstrated that esters can be selectively converted into amides generating molecular hydrogen as the only by-product (Scheme 31) [137]. The catalytic reactions were carried out with 2 equiv. of amine per ester in toluene or benzene at reflux in the presence of 0.1 mol% of the dearomatized PNN-pincer ruthenium complex [RuH(CO)(PNN)] (28) (see Scheme 21). Strikingly, both the acyl and the aUcoxo units of the starting ester are involved in the amide production. Hence, to avoid mixtures of products, the process was only applied to symmetrical esters. The catalytic protocol was effective for both primary and secondary cyclic amines. In addition, the coupling of piperazine and butylbutyrate provided compound 46, which results from the bis-acylation of the diamine (Scheme 31). 

In the past, this field has been dominated by ruthenium, rhodium and iridium catalysts with extraordinary activities and furthermore superior enantioselectivities however, some investigations were carried out with iron catalysts. Early efforts were reported on the successful use of hydridocarbonyliron complexes HFcm(CO) as reducing reagent for a, P-unsaturated carbonyl compounds, dienes and C=N double bonds, albeit complexes were used in stoichiometric amounts [7]. The first catalytic approach was presented by Marko et al. on the reduction of acetone in the presence of Fe3(CO)12 or Fe(CO)5 [8]. In this reaction, the hydrogen is delivered by water under more drastic reaction conditions (100 bar, 100 °C). Addition of NEt3 as co-catalyst was necessary to obtain reasonable yields. The authors assumed a reaction of Fe(CO)5 with hydroxide ions to yield H Fe(CO)4 with liberation of carbon dioxide since basic conditions are present and exclude the formation of molecular hydrogen via the water gas shift reaction. H Fe(CO)4 is believed to be the active catalyst, which transfers the hydride to the acceptor. The catalyst presented displayed activity in the reduction of several ketones and aldehydes (Scheme 4.1) [9]. 

It is interesting to note that C-H activation on ruthenium NHC complexes is not limited to intramolecular protons located in the N-sidechain of the carbene, but occurs inter-molecularly as well. Leimer et al. reacted [MesIRuH PCyj] with toluene-dg at ambient temperature and observed a rapid H/D exchange reaction involving the four hydride hydrogen atoms on ruthenium, the methyl protons of the mesityl substituents of the carbene ligand and the deuterium atoms on the meta positions of toluene-dg. The ortho-, para- and methyl-deuterium atoms of the solvent did not participate [145]. 

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