Hydrogenation of carbonyl compounds

Ru/tppms and Ru/pta are also active catalysts for the hydrogenation of a series of substituted benzaldehydes to the corresponding benzyl alcohols with sodium formate as the source of hydrogen.247,261,491 

Hernandez and Kalck495,496 synthesized various Ru/tppts complexes and used them as catalysts in the hydrogenation of various cc,J3-unsaturated aldehydes and ketones in a two phase system. Using Ru(H)2(tppts)4 high conversions of cinnamaldehyde (97%) and high selectivity to cinnamylalcohol (95%) were observed under mild reaction conditions (40°C, 20 bar, 3h) 495 More forcing conditions (80°C, 35 bar, 16h) were needed for the hydrogenation of unsaturated a, 3-ketones such as 4-hexen-3-one or benzylideneacetone and only low chemo-selectivities were observed.495 

Hydrogenation of saturated ketones such as cyclohexanone or 2-butanone catalysed by Ru/tppts complexes gave saturated alcohols in 100% selectivity at 5-99% conversion. 

5 to 2.9 and at low pH Ru/tppts-NEt3/HCOOH catalytic systems exhibited very low catalytic activity.494 

Ru/tppms catalysts exhibited excellent yields (98%) in the transfer hydrogenation of unsaturated aldehydes such as cinnamaldehyde or crotonaldehyde, to the corresponding unsaturated alcohols under mild reaction conditions (30-80°C), with HCOONa in an aqueous/organic two phase system. Similarly, ruthenium modified with the water soluble, air stable phosphine 100 (pta Table 5) is an effective catalyst for the chemoselective transfer hydrogenation of a,P-unsaturated aldehydes to unsaturated alcohols using formate in an aqueous/ organic two phase system. In contrast, Rh/pta afforded the cor- 

Hernandez and synthesized various Ru/tppts complexes and used 

Basset et showed that Ru/tppts catalysts exhibit extremely high 

When the earbonyl of ketones or aldehydes is eonjugated to aromatie systems, hydrogenolysis produets are often the major isolated products. Catalytic hydrogenation of 2-naphthaldehyde (395) with palladium on harium sulfate, gave only 2-methylnaphthalene (397).When the Adam s catalyst (Pt02) was used, with ferric chloride (FeCly) as a promoter, an 80% yield of alcohol 396 was obtained although addition of excess promoter induced further hydrogenolysis to 397. 

Some acid derivatives are reduced by catalytic hydrogenation, but not all. There is a clear difference in reactivity, apart from specificity for the metal catalyst. The ability of hydrogen to reduce acid derivatives follows the expected order and acid chlorides will be reduced faster than esters, for example. Aldehydes and ketones are reduced significantly faster than all acid derivatives except acid chlorides. OO xhe general order of reactivity for hydrogenation is 

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The transfer hydrogenations of carbonyl compounds to alcohols catalysed by a variety of NHC complexes have been intensively studied. The strong bond... 

Scheme 2.4 Experimental evidence in support of the mechanism for the base-free transfer hydrogenation of carbonyl compounds catalysed by complex 43...

Table 3.3 Representative conditions for the hydrogenation of carbonyl compounds including enantiomeric reactions. 

Transfer hydrogenations of carbonyl compounds are often conducted using 2-propanol as the hydrogen donor. One advantage of this compound is that it can be used simultaneously as a solvent. A large excess of the hydrogen donor shifts the redox equilibrium towards the desired product (see also Section 20.3.1). 

Transition Metal-Complex Catalyzed Hydrogenation of Carbonyl Compounds... 

Figure 9.1 Catalytic cycle for hydrogenation of carbonyl compounds.

The selectivity in favor of the desired monobenzylated product was found to be >99% and the immobilized Pt02 was found to be 4-5 times more active than the commercial Adams catalysts. In solution or in immobilized form, the PtOz colloid is effective in the hydrogenation of carbonyl compounds or of olefins. Recently, the heterogeneous catalytic amination of aryl bromides by immobilized Pd(0) particles has been reported [163], Secondary amines such as piperidine and diethyl amine are used in the amination of aryl bromides and the reaction proceeds with good turnover numbers and regio-control. The catalysts can be reused repeatedly without loss of activity or selectivity after filtration from the reaction mixture. 

There have been many reports of the use of iridium-catalyzed transfer hydrogenation of carbonyl compounds, and this section focuses on more recent examples where the control of enantioselectivity is not considered. In particular, recent interest has been in the use of iridium A -heterocyclic carbene complexes as active catalysts for transfer hydrogenation. However, alternative iridium complexes are effective catalysts [1, 2] and the air-stable complex 1 has been shown to be exceptionally active for the transfer hydrogenation of ketones [3]. For example, acetophenone 2 was converted into the corresponding alcohol 3 using only 0.001 mol% of this... 

Table 5.7 Transfer hydrogenation of carbonyl compounds with [Cp lr(H20)3f (24) and HCOONa in water at pH 3.2. ...

Iridium nanopartides also catalyze the hydrogenation of benzyhnethylketone, with high selectivity in reduction of the aromatic ring (92% selectivity in saturated ketone, 8% in saturated alcohol at 97% benzylmethylketone conversion). This preferential coordination of the aromatic ring can be attributed to steric effects that make carbonyl coordination difficult. Therefore, metallic iridium nanoparticles prepared in ILs may serve as active catalysts for the hydrogenation of carbonyl compounds in both solventless and biphasic conditions. 

Meerwein-Pondorf-Verley reduction, discovered in the 1920s, is the transfer hydrogenation of carbonyl compounds by alcohols, catalyzed by basic metal compounds (e.g., alkoxides) [56-58]. The same reaction viewed as oxidation of alcohols [59] is called Oppenauer oxidation. Suitable catalysts include homogeneous as well as heterogeneous systems, containing a wide variety of metals like Li, Mg, Ca, Al, Ti, 2r and lanthanides. The subject has been reviewed recently [22]. In this review we will concentrate on homogeneous catalysis by aluminium. Most aluminium alkoxides will catalyze MPV reduction. 

Kumada and co-workers have introduced the ferrocene-based ligand BPPFOH (62) for the rhodium-catalyzed asymmetric hydrogenation of carbonyl compounds. 

The heterogeneous catalytic hydrogenation of carbonyl compounds using chirally modified metal catalysts has been reviewed in recent years315,455-459. The conclusions can be summarized as follows ... 

Transfer hydrogenation of carbonyl compounds.4 Carbonyl compounds are reduced to alcohols by formic acid with this ruthenium catalyst in high yield without a solvent. 

Delepine and Horeau also compared the activating effects of the six platinum group metals on Raney Ni in the hydrogenation of carbonyl compounds. Osmium, iridium, and platinum were the most effective, ruthenium and rhodium followed them, and palladium was the least effective.66... 

FTIR model experiments were performed to reveal the nature of catalyst deactivation in C02. The spectrum taken at 15 bar in a C02/H2 mixture is shown in Fig. 1. The bands at 2060 and 1870 cm 1 indicate considerable coverage of Pt by linearly and bridge-bonded CO [12], formed by the reduction of C02 on Pt (reverse water gas shift reaction). The three characteristic bands at 1660, 1440 and 1235 cm 1 are attributed to C02 adsorption on A1203, likely as carbonate species [13, 14], It is well known [15] that CO is a strong poison for the hydrogenation of carbonyl compounds on Pt, but can improve the selectivity of the acetylene — olefin type transformations. Based on the above FTIR experiments it cannot be excluded that there are other strongly adsorbed species on Pt formed in small amounts. It is possible that the reduction of C02 provides also -COOH and triply bonded COH, as proposed earlier [16]. 

Systematic studies on the enantioselective heterogeneous catalytic hydrogenation of carbonyl compounds were carried out by Izumi using Raney nickel modified with various chiral reagents. Hydroxy acids or amino acids were used for the modification of the nickel catalyst, and (-i-)-tartaric acid (2R,3R)... 

On the other hand, rhodium complexes with fully alkylated phosphine ligands were used for the hydrogenation of carbonyl compounds, a-ketoamides " and ketopantolactone (66% ee). When a rhodium complex with CyDlOP (23) was used, a slightly higher asymmetric yield (71% ee) was observed in the hydrogenation of a-ketoamides. Hydrogenation of A(-(a-ketoacyl)amino acid esters was... 

Several studies on the hydrogenation of carbonyl compounds using cobalt complexes have been reported, including the hydrogenation of benzil in the presence of bis(dimethylglyoximato)co-balt(II)-chiral amine complexes. The highest ee (78%) was obtained when quinine was used as the chiral base (equation 17). Cyano cobalt complexes have also been reported to be effective for the hydrogenations of ketones. ... 

Ruthenium complexes have been used for the hydrogenation of alkenes but they had not been used for hydrogenations of carbonyl compounds until recently, when several highly selective catalytic hydrogenations of carbonyl compounds (P-keto esters, a-diketones, a-amino ketones etc., ee > 95%) were reported using BINAP-Ru" complexes (equation 18). ° ... 

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