Reduction hydrogen sources

Figure 8.1 NADH recycling using alcohol as a hydrogen source for reduction [2],...

Alcohols such as ethanol, 2-propanol, and so on, have been widely used to recycle the coenzyme for the reduction catalyzed by alcohol dehydrogenase since the enzyme catalyzes both reduction and oxidation. Usually, an excess amount of the hydrogen source is used to push the equilibrium toward formation of product alcohols. 

Formate is one of the most representative hydrogen sources for the biocatalytic reduction because CO2 formed by the oxidation of formate is released easily from the reaction system [4]. For example, for the reduction of aromatic ketones by the... 

Figure 8.2 Reduction of ketone with alcohol dehydrogenase from Thermoanaerobacter brockii using glucose-6-sulfate as a hydrogen source [3],...

Figure 8.4 Reduction of 6-bromotetralone with reductase from Trichosporon capitatum using formate as a hydrogen source [4b]...

Figure 8.6 Reduction of ketone with ruthenium complex and alcohol dehydrogenase using molecular hydrogen as a hydrogen source [5c],...

The first example of an asymmetric reduction of C=N bonds proceeding via DKR was reported in 2005 by Lassaletta et al. In this process, the transfer hydrogenation of 2-substituted bicyclic and monocyclic ketimines could be accomplished via DKR by using a HCO2H/TEA mixture as the hydrogen source and a chiral ruthenium complex including TsDPEN ligand,... 

Reductive elimination with release of the aldol product through the action of the hydrogen source M - H (M = Si, B, etc.)... 

Apart from the Meerwin-Ponndorf-Verley (MPV) reaction,16 18catalytic asymmetric transfer hydrogenation has remained quite primitive,111,112 with successful examples of reduction of activated olefins, using alcohols or formic acid as hydrogen source, being reported only recently.113,114... 

Homogeneous catalysts have now been reported for hydrogenation of carbon monoxide, a combustion product of coal (see Section VI,B). More effective catalysts will undoubtedly be discovered in the near future. Polynuclear or, at least, binuclear sites are favored for reduction of the triple bond in carbon monoxide (see Section VI,B), and this together with the popular parallelism to heterogeneous systems, has renewed interest in metal clusters as catalysts (see Section VI). A nickel cluster is the first catalyst reported for mild (and selective) hydrogenation of the triple bond in isocyanide (see Section VI,A). The use of carbon monoxide and water as an alternative hydrogen source is reattracting interest (see Section VI,C). 

Use of LiAIDi. produced perdeutero-hydrocarbons, confirming reduction of CO with Li AID as the only hydrogen source. Small amounts of partially or non-deuterated hydrocarbons were also observed (< 10% of the total) probably as a result of impurity in the LAD (98% D). 

The efficiency of nitrobenzene photoreduction may be increased remarkably in 2-propanol/hydrochloric acid mixtures. In 50% 2-propanol/water containing 6 moles l i HCl, acetone and a complex mixture of chlorinated reduction products are formed i ). Both HCl and 2-propanol (as hydrogen source) are needed. When sulfuric acid is substituted for HCl, enhanced photoreduction does not occtu . When using mixtures of HCl and LiCl to maintain a constant chloride concentration (6 M) and vary [H+], a constant disappearance quantum yield 366 =0.15 is found within the [H+]-range 0.05—6 moles l i. This strongly suggests that chloride ions play an essential role, probably via electron transfer to 3(n, tt )-nitrobenzene i > [Eq. (1)], but it is also evident from the data presented that the presence of add is probably important in subsequent steps, [Eq. (3)]. 

Cord-Ruwish R, Widdel F. 1986. Corroding iron as a hydrogen source for sulphate-reduction in growing cultures of sulphate-reducing bacteria. Appl Microbiol Biotechnol 25 169-74. 

Complexation of [Cp IrCl2]2 with iV-heterocyclic carbenes has led to complexes such as 25, developed by Peris and coworkers [107, 108], and 133, developed by Crabtree and coworkers [12]. Complex 24 is activated by the addition of silver triflate and is effective for the iV-alkylation of amines with alcohols and for the iV-alkylation of anilines with primary amines. Complex 25 has also been shown to couple benzyl alcohol 15 with a range of alcohols, including ethanol 134, to give ether products such as ether 135 (Scheme 31). Complex 133 was an active hydrogen transfer catalyst for the reduction of ketones and imines, using 2-propanol as the hydrogen source. It was also an effective catalyst for the iV-alkylation of amines... 

A series of chiral N,S-chelates was synthesized as ligands for the iridium(l)-catalyzed reduction of ketones using either HCOOH/NEtj or isopropanol as hydrogen sources. The ligands were obtained by sulfoxidation of an (R)-cysteine-based aminosulfide, providing a diastereomeric ligand family containing a chiral sulfur... 

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