Aldehydes hydrogen transfer

Transfer hydrogenation of aldehydes with isopropanol without addition of external base has been achieved using the electronically and coordinatively unsaturated Os complex 43 as catalyst. High turnover frequencies have been observed with aldehyde substrates, however the catalyst was very poor for the hydrogenation of ketones. The stoichiometric conversion of 43 to the spectroscopically identifiable in solution ketone complex 45, via the non-isolable complex 44 (Scheme 2.4), provides evidence for two steps of the operating mechanism (alkoxide exchange, p-hydride elimination to form ketone hydride complex) of the transfer hydrogenation reaction [43]. 

Cyclohexanones undergo type I cleavage to produce a mixture of ketenes and aldehydes by hydrogen transfer,<1-3)... 

Two of the three general types of secondary reactions resulting from photochemical a-cleavage of carbonyls, namely molecular rearrangement and hydrogen transfer to yield aldehydes or ketenes, have been discussed. The third type of reaction observed, decarbonylation, will be discussed in this section. The discussion will begin with the decarbonylation of small ring carbonyls. By way of example of this type of reaction, diphenylcyclopropenone decarbonylates upon photolysis to yield diphenylacetylene(57) ... 

An important aspect of hydrogen transfer equilibrium reactions is their application to a variety of oxidative transformations of alcohols to aldehydes and ketones using ruthenium catalysts.72 An extension of these studies is the aerobic oxidation of alcohols performed with a catalytic amount of hydrogen acceptor under 02 atmosphere by a multistep electron-transfer process.132-134... 

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). 

Considerable interest remains in catalyzed hydrogen-transfer reactions using as donor solvents alcohols, glycols, aldehydes, amides, acids, ethers, cyclic amines, and even aromatic hydrocarbons such as alkylben-... 

Mechanisms, exemplified by alcohol as donor (493, 496), usually invoke coordination of the substrate (olefins, saturated and unsaturated ketones, and aldehydes), then coordination of the alcohol and formation of a metal alkoxide, followed by /8-hydrogen transfer from the alkoxide and release of product via protonolysis ... 

Molybdenum and tungsten carbonyl hydride complexes were shown (Eqs. (16), (17), (22), (23), (24) see Schemes 7.5 and 7.7) to function as hydride donors in the presence of acids. Tungsten dihydrides are capable of carrying out stoichiometric ionic hydrogenation of aldehydes and ketones (Eq. (28)). These stoichiometric reactions provided evidence that the proton and hydride transfer steps necessary for a catalytic cycle were viable, but closing of the cycle requires that the metal hydride bonds be regenerated from reaction with H2. 

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).

D-xylose was converted into 2-furaldehyde in acidified, tritiated water, no carbon-bound isotope was detected. This suggested that the 1,2-enediol (2) reacted immediately, as otherwise, tritium would have been detected at the aldehydic carbon atom of 2-furaldehyde, as a result of aldose-ketose interconversion.An acidic dehydration performed with d-[2- H]xylose showed that an intramolecular C-2-C-1 hydrogen transfer had actually occurred. Thus, these data indicated that an intramolecular hydride shift is more probable than the previously accepted step involving a 1,2-enediol intermediate. 

In a water/chlorobenzene biphasic system, reduction of aromatic aldehydes by hydrogen transfer from aqueous sodium formate catalyzed by [ RuCl2(TPPMS)2 2] provided unsaturated alcohols exclusively (Scheme 10.7). Addition of 3-CD shghtly inhibited the reaction [13]. It was speculated that this inhibition was probably due to complexation of the catalyst by inclusion of one of the non-sulfonated phenyl rings of the TPPMS ligand, however, no evidence was offered. 

The highly efficient catalytic system for the chemoselective transfer hydrogenation of aldehydes was reported by Xiao et al. [52]. This system consisted of [Cp IrCl2]2 (1), a diamine and HCOONa, and worked on water and in air. A wide range of aromatic aldehydes were reduced to the corresponding primary alcohols in a highly chemoselective manner some representative examples are summarized in Table 5.9. 

Carbon-Nitrogen Bond Formation Based on Hydrogen Transfer 123 Table 5.9 Transfer hydrogenation of aromatic aldehydes with HCOONa in water. ... 

Hydrogen transfer oxidation of an alcohol to give an aldehyde and an iridium hydride. 

A proposed mechanism for the Cp lr-catalyzed Tishchenko reaction is illustrated in Scheme 5.35. In this reaction, hydrogen transfer from the hemiacetal to aldehyde catalyzed by the Cp lr complex would be crucial. 

Primary alcohols 121 undergo an efficient oxidative dimerization by [IrCl(coe)2]2 under air, without any solvent, to form esters 122 in fair to good yields (Equation 10.30) [54]. The reaction is initiated by the in situ generation of an Ir-hydride complex via hydrogen transfer from alcohols to afford aldehydes, followed by the dehydrogenation of hemiacetals derived from alcohols and aldehydes by action of the Ir-complex to afford esters. 

Some cation-radicals can appear as hydrogen acceptors. Thus, fullerene Cgg is oxidized to the cation-radical at a preparative scale by means of photoinduced electron transfer. As in the case of anion-radical, the fullerene Cgo cation-radical bears the highly delocalized positive charge and shows low electrophilicity. This cation-radical reacts with various donors of atomic hydrogen (alcohols, aldehydes, and ethers) yielding the fullerene 1,2-dihydroderivatives (Siedschlag et al. 2000). 

Similar reductions were effected by heating of triethylaluminum (triethyl-alane) in ether with aldehydes and ketones. The alcohols are formed by hydrogen transfer from the alkyl groups which are transformed to alkenes. 

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