2-Propanol. as hydrogen source

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)]. 

Ketones are reduced by asymmetric hydrogen transfer from either HCO2H or 2-propanol as hydrogen sources, catalysed by chiral Ru complexes [66]. HCO2H is used... 

In both cases dicarbonylation of the alkyne triple bond occurs with reduction of one of the resulting carbonyl groups (2-propanol as hydrogen source). General surveys of carbocarbony-lation reactions of this type (also leading to bifuran diones) are available -2. Similar reaction types are observed in multicomponent carbonylative cyclizations (see Section 1.5.8.3.7.). 

Henbest and Mitchell [78] have shown that water can be used as hydrogen source with chloroiridic acid (6) as the catalyst through oxidation of phosphorous acid (59) to phosphoric acid (60) in aqueous 2-propanol. Under these conditions, no hydrogen transfer occurs from 2-propanol. However, iridium complexes with sulfoxide or phosphine ligands show the usual transfer from 2-pro-panol [79-81]. 

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... 

The transfer hydrogenation of a-keto- S -unsaturated esters, catalyzed by Ru(p-cymene)(TsDPEN) (TsDPEN monotosylated l,2-diphenylethylene-l,2-dia-mine) with 2-propanol as the hydrogen source, has been developed as an efficient method for the preparation of a-hydroxy-)S, y-unsaturated esters or acids. 

The use of 2-propanol for these substrates, besides its role as hydrogen source, allows suppression of cydization reactions promoted by the acidity of the catalyst. Nevertheless, when cydization leads to valuable products, as in the case of 3,3,5-trimethylpyran derived from sulcatol, it is possible to steer the reaction in this direction by using molecular hydrogen and a hydrocarbon solvent [27]. 

Finally, Noyori and coworkers have shown that chiral Ru"-diamine complexes are efficient catalysts for the enantioselective transfer hydrogenation of several prochiral ketones, using 2-propanol as the hydrogen source, to give the corresponding alcohols in high ee [I6aj. The reaction... 

With ferrocenyl diamines such as 32, the transfer hydrogenation of I -acetonaphthone reached 90 % ee at -30 °C with 2-propanol as the hydrogen source [63]. Even amino acids have been used as ligands for ruthenium [64, but, more than 90 % ee results only when tetralone is the substrate. 

Ir(COD)Cl]2 and 2-propanol, as an in situ source of hydrogen, are used for the carbon-carbon double bond hydrogenation of an a,(3-unsaturated ketone ... 

Several of the most common hydrogen donors, such as formic acid and formates, ascorbic acid, EDTA or 2-propanol are well or at least sufficiently soluble in water. In addition, H20 itself can serve as a source of hydrogen. Frequently, hydrogenation of unsaturated substrates is achieved by using C0/H20 mixtures such reactions are discussed in 3.8. As written in eq. (3.11) the hydrogen transfer reaction is often reversible, an obvious example being the reduction of ketones using 2-propanol as donor. 

The same HCOOH-EtjN system as hydrogen source, in conjunction with Ru(II) catalyst "8. is also quite superior to 2-propanol for the asymmetric reduction of aryl ketones.It allows for a much higher substrate concentration (2-10 M vs. < 0.1 M in 2-propanol). 

Before considering the mechanism of this important variant of asymmetric hydrogen transfer, let us first look at earlier methods used in the field. Most of them were based on 2-propanol as the favourable organic source of hydrogen, and represent catalytic variants of the Meerwein-Verley-Ponndorf name reaction which uses large quantities of Al-isopropoxide at elevated temperatures (Scheme 11.7) [27, 28]. 

In a similar way, the same authors also reported new half-sandwich t -Cp -rhodium(ra) and r -Cp -ruthenium(ii) complexes 31-34 (Scheme 8), from the corresponding bis(phosphino)amine ligands, thiophene-2-(A ,Af-bis(diphenylphosphino)methylamine) 31 and 32 or furfutyl-2-(N,N-bis(diphenylphosphino)amine) 33 and 34. The structures of the new complexes were elucidated by spectroscopic techniques and successfully applied to transfer hydrogenation of various ketones in the presence of 2-propanol as the hydrogen source. 

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. 

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