Hydrogen transfer carbonyl compounds

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...

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). 

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. ...

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. 

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. 

Other chiral diamine-( -arene)ruthenium catalysts were developed by Noyori where the chirality was centred at the metal (see Figure 3.18). These complexes were effective catalysts for asymmetric transfer hydrogenation of carbonyl compounds and a mechanism involving a metal-ligand bifunctional process was proposed. 

Baniwati, B., Polshettiwar, V., Varma, R. S. (2009). Magnetically recoverable supported ruthenium catalyst for hydrogenation of alkynes and transfer hydrogenation of carbonyl compounds. Tetrahedron Letters, 50, 1215—1218. http //dx.doi.org/10.1016/ j.tetiet.2009.01.014. 

Cp Ir (NHC) complexes are known to be efficient catalysts in the transfer hydrogenation of carbonyl compounds. One of these catalysts has been used by Corberan and Peris in the one-pot enzymatic DKR of a jS-branched aldehyde. Thus, the treatment of this aldehyde by Amano lipase PS-D I and this catalyst at 80°C in the presence ofp-chlorophenyl acetate as the acyl donor provided the corresponding acetate in good yield, albeit with moderate enantioselectivity of 61% ee, as shown in Scheme 4.50. 

HRu(PCy3)2(CO)(CH3CN]BF4 for the transfer hydrogenation of carbonyl compounds. ... 

The reduction reaction occurs by the transfer of hydrogen atoms bound to the surface of the metal catalyst to the carbonyl oxygen and carbon atoms. We recall that the same types of catalysts are used for the hydrogenation of alkenes, a much faster reaction. Alkenes can be reduced at room temperature under 1 atm. pressure of hydrogen gas. Carbonyl compounds require higher temperatures and pressures as high as 100 atm. Therefore, transition metal-catalyzed reduction of carbonyl compounds that also have a carbon—carbon double bond results in reduction of both functional groups. 

Shvo s catalyst 1 is a cyclopentadienone-ligated dimthenium complex, [Ru2(CO)4 (/t-H)(C4Ph4COHOCC4Ph4)]. It was first synthesized in 1984 by Shvo et al. [1, 2], Since then it has been widely applied in various hydrogen transfer reactions, including hydrogenation of carbonyl compounds [2, 3], transfer hydrogenation of ketones and imines [4,5], disproportion of aldehydes to esters [6], and Oppenauer-type oxidations of alcohols [7-9] and amines [10-12]. Shvo s complex 1 has also been found to be effective as a racemization catalyst for secondary alcohols and amines, and complex 1 has therefore been used together with enzymes in several dynamic kinetic resolution (DKR) protocols [13-18]. 

Gracia et al. (2009) investigated the transfer hydrogenation of carbonyl compounds to their corresponding alcohols using supported Pt and Pd nanoparticles on Al-SBA-15 materials under microwave irradiation with short times of reaction (15-30 min). 

L = P(CH3)3 or CO, oxidatively add arene and alkane carbon—hydrogen bonds (181,182). Catalytic dehydrogenation of alkanes (183) and carbonylation of bensene (184) has also been observed. Iridium compounds have also been shown to catalyse hydrogenation (185) and isomerisation of unsaturated alkanes (186), hydrogen-transfer reactions, and enantioselective hydrogenation of ketones (187) and imines (188). 

The reaction between the photoexcited carbonyl compound and an amine occurs with substantially greater facility than that with most other hydrogen donors. The rate constants for triplet quenching by amines show little dependence on the amine a-C-H bond strength. However, the ability of the amine to release an electron is important.- - This is in keeping with a mechanism of radical generation which involves initial electron (or charge) transfer from the amine to the photoexcited carbonyl compound. Loss of a proton from the resultant complex (exciplex) results in an a-aminoalkyl radical which initiates polymerization. The... 

The catalytic hydrosi(ly)lations of other C=X functional groups (X = O, NR) constitute alternative routes to the reduction of aldehydes, ketones, imines and other carbonyl compounds (Scheme 2.9), circumventing the use of molecular hydrogen or occasionally harsh transfer hydrogenation conditions. 

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