Aldehydes hydrogen donors

One aldehyde molecule has transferred its aldehyde hydrogen during course of the reaction onto another aldehyde molecule, which is why the reactants are called donor and acceptor (see below). 

Aldehydes, both aliphatic and aromatic, can be decarbonylated by heating with chlorotris(triphenylphosphine)rhodium or other catalysts such as palladium. The compound RhCl(Ph3P)3 is often called Wilkinson s catalyst.In an older reaction, aliphatic (but not aromatic) aldehydes are decarbonylated by heating with di-tert-peroxide or other peroxides, usually in a solution containing a hydrogen donor, such as a thiol. The reaction has also been initiated with light, and thermally (without an initiator) by heating at 500°C. 

The dimerization of ketones to 1,2-diols can also be accomplished photochemi-cally indeed, this is one of the most common photochemical reactions. The substrate, which is usually a diaryl or aryl alkyl ketone (though a few aromatic aldehydes and dialkyl ketones have been dimerized), is irradiated with UV light in the presence of a hydrogen donor such as isopropyl alcohol, toluene, or an amine. In the case of benzophenone, irradiated in the presence of 2-propanol, the ketone molecule initially undergoes n — k excitation, and the singlet species thus formed crosses to the T, state with a very high efficiency. 

Prochiral derivatives of propenoic acid were reduced by hydrogen transfer from aqueous solutions of M[HCOO] (M = K+, Na+ and [NH4]+) catalyzed by Rh1 complexes of (117) or the tetrasulfonated cyclobutanediop (132) 345 Aldehydes were reduced in a phase transfer catalytic system having [RuCl2(PPh3)3] as the catalyst in the organic phase (for example chlorobenzene) and the hydrogen donor (Na-methanoate) in the aqueous phase.346... 

Iridium-catalyzed transfer hydrogenation of aldehyde 73 in the presence of 1,1-dimethylallene promotes tert-prenylation [64] to form the secondary neopentyl alcohol 74. In this process, isopropanol serves as the hydrogen donor, and the isolated iridium complex prepared from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and (S)-SEGPHOS is used as catalyst. Complete levels of catalyst-directed diastereoselectivity are observed. Exposure of neopentyl alcohol 74 to acetic anhydride followed by ozonolysis provides p-acetoxy aldehyde 75. Reductive coupling of aldehyde 75 with allyl acetate under transfer hydrogenation conditions results in the formation of homoallylic alcohol 76. As the stereochemistry of this addition is irrelevant, an achiral iridium complex derived from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and BIPHEP was employed as catalyst (Scheme 5.9). 

Gordon used a household microwave oven for the transfer hydrogenation of benz-aldehyde with (carbonyl)-chlorohydridotris-(triphenylphosphine)ruthenium(II) as catalyst and formic acid as hydrogen donor (Eq. 11.43) [61]. An improvement in the average catalytic activity from 280 to 6700 turnovers h-1 was achieved when the traditional reflux conditions were replaced by microwave heating. 

Fig. 15.5 Transfer hydrogenation of aldehydes using [Rh(COD)CI]2/TPPTS at 80°C using i-PrOH as hydrogen donor. Values shown are yields (TOF, h 1).

Nitrogen-containing heterocyclic compounds, including 1,2,3,4-tetrahydroqui-noline, piperidine, pyrrolidine and indoline, are also popular hydrogen donors for the reduction of aldehydes, alkenes, and alkynes [75, 76]. With piperidine as hydrogen donor, the highly reactive 1-piperidene intermediate undergoes trimer-ization or, in the presence of amines, an addition reaction [77]. Pyridine was not observed as a reaction product. 

In summary, the most popular hydrogen donors for the reduction of ketones, aldehydes and imines are alcohols and amines, while cyclic ethers or hydroaromatic compounds are the best choice for the reduction of alkenes and alkynes. 

The reductive alkylation of amines is called the Leuckart-Wallach reaction [112-115]. The primary or secondary amine reacts with the ketone or aldehyde. The formed imine is then reduced with formic acid as hydrogen donor (Scheme 20.27). When amines are reductively methylated with formaldehyde and formic acid, the process is termed the Eschweiler-Clarke procedure [116, 117]. 

Other alcohols such as methanol and ethanol will also react, but are typically less effective as the aldehyde byproducts can interfere in the reaction. Isobutanol is an effective hydrogen donor, and others such as glucose will also react but cannot be used in such high concentrations. Isopropanol can be mixed with an inert solvent, including water, but the rates of reaction fall linearly, as expected. 

Photolysis of 2-naphthyl thioesters in the presence of a hydrogen donor such as 1,4-cyclohexadiene yields aldehydes (80-100%) no photo-Fries reaction products have been reported [60],... 

At the end of reaction the initial hydrocarbon concentration is still important, but there are also many other hydrogen donors. Thus, in the absence of oxygen, reactions of alkoxy radicals will lead to aldehydes by pyrolysis and to alcohols by abstraction. Let us examine these two possible reactions for each alkoxy radical. 

Comparing Tables I and II reveals that amine concentration is the second important variable in the aldehyde hydrogenation reaction. For a given amine, selectivity to alcohol passes through a maximum with increasing concentration of N donor. Thus with DMBA, the alcohol present at 80% olefin cvonversion was 5, 29, and 28% at 1, 2, and —3M (resin), respectively, suggesting an optimum DMBA concentration of about 2M. 

Iridium-monotosylated ethylenediamine [Ts(en)] and Ir-CF3Ts(en) are highly active and chemoselective catalysts for the aqueous-phase transfer hydrogenation of aldehydes using sodium formate as the hydrogen donor.376... 

Halides react with carbon monoxide, usually with palladium complex catalysts, in the presence of hydrogen donors, to give aldehydes (equation 147). 

The Cannizzaro reactionof non-enolizable aldehydes is another example of a hydride transfer reaction. It is carried out under alkaline conditions and involves not only the addition of a hydroxide ion to one aldehyde but the stabilization of the resultant acid as the anion (Scheme 3.41). Methanal (formaldehyde), which gives methanoic acid (formic acid), a relatively strong carboxylic acid, makes a good hydrogen donor in a cross-Cannizzaro reaction. 

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