Aromatic carbonyl compounds reducing conditions

Early work by Papa et al. indicated that reduction of carbonyl compounds with Raney nickel in alkaline solution gave the corresponding hydrocarbon or alcohol products, and formation of the hydrocarbon was only feasible in the case of aromatic carbonyl compounds at 80-90 C. Mitchell et al. reported an improved method under neutral conditions using W-7 Raney nickel in 50% aqueous ethanol, aryl aldehydes, alkyl aryl and diaryl ketones can be reduced to the methylene products in high yields. Aromatic substituents such as nitro, cyano and halogen also suffer reduction under these conditions. 

Manganese. Li and Chan [34] and Rieke [35] have independently reported that manganese reagents can accomplish the pinacol coupling of aromatic carbonyl compounds. In the Li/Chan study, reaction of an array of aromatic aldehydes proceeded in good to excellent yield in the presence of Mn/Ac0H/H20, albeit with poor diastereoselectivity (Eq. 3.15). Under these conditions, aliphatic aldehydes are reduced to the corresponding alcohol, and ketones (aromatic or aliphatic) do not react. 

Saturated aldehydes and ketones are usually easily reduced electrochemically. Alcohols are usually the products, but glycols and pinacols are readily obtained by suitable choice of reaction conditions, especially from aromatic carbonyl compounds. In some cases hydrocarbons may be obtained. ... 

In aromatic carboxylic acids, the carboxylic acid group (CO2H) is an electron-withdrawing moiety (-1, —M effects), hence it reduces the reactivity of the attached aromatic nucleus towards electrophilic substitution. So like aromatic carbonyl compounds (Section 7), aromatic carboxylic acids are more resistant towards sulfonation than the parent aromatic hydrocarbons. The reaction therefore generally requires rather forcing conditions, e.g. the use of a large excess of chlorosulfonic acid at temperatures of more that 120 °C. The reactions of benzoic, cinnamic and phenylacetic acid with chlorosulfonic acid were reported by Suter. ... 

Vardanyan [65,66] discovered the phenomenon of CL in the reaction of peroxyl radicals with the aminyl radical. In the process of liquid-phase oxidation, CL results from the disproportionation reactions of primary and secondary peroxyl radicals, giving rise to trip-let-excited carbonyl compounds (see Chapter 2). The addition of an inhibitor reduces the concentration of peroxyl radicals and, hence, the rate of R02 disproportionation and the intensity of CL. As the inhibitor is consumed in the oxidized hydrocarbon the initial level of CL is recovered. On the other hand, the addition of primary and secondary aromatic amines to chlorobenzene containing some amounts of alcohols, esters, ethers, or water enhances the CL by 1.5 to 7 times [66]. This effect is probably due to the reaction of peroxyl radicals with the aminyl radical, since the addition of phenol to the reaction mixture under these conditions must extinguish CL. Indeed, the fast exchange reaction... 

Reduction of aldehydes and ketones. Earlier work on amine borane reagents was conducted mainly with tertiary amines and led to the conclusion that these borane complexes reduced carbonyl compounds very slowly, at least under neutral conditions, and that the yield of alcohols is low. Actually complexes of borane with primary amines, NHj or (CH3)3CNH2, reduce carbonyl compounds rapidly and with utilization of the three hydride equivalents. BH3 NH3 is less subject to steric effects than traditional complex hydrides. A particular advantage is that NH3 BH3 and (CH3)3CNH2 BH3 reduce aldehyde groups much more rapidly than keto groups, but cyclohexanone can be reduced selectively in the presence of aliphatic and aromatic acyclic ketones. 

Reduction of ArN02.4 Aromatic nitro compounds are reduced by SnCl2 2H20 in ethanol or ethyl acetate at 70° to amines in 90-99% yield. Under these conditions, carbonyl, cyano, halo, and benzyl groups are not reduced. 

Reduction of carbonyl compounds.2 The reagent reduces aromatic and aliphatic ketones, aldehydes, and acid chlorides to alcohols in moderate to excellent yield in chloroform solution. The order of substrate reactivity is RCOCl>RCHO>R2CO. Only three of the eight hydrogens are transferred under the reaction conditions. 

Although aromatic rings can be hydrogenated, as you saw above, neither they nor the aldehyde product are reduced under these conditions and, as with hydride reductions of carbonyl compounds, we can draw up a sequence of reactivity towards hydrogenation. The precise ordering varies with the catalyst, especially with regard to the interpolation of the (less important, because other methods are usually better) carbonyl reductions (in yellow). Some catalysts are particularly selective... 

Catalytic hydrogenation of carbonyl compounds to alkanes is a difficult proposition under normal conditions, although limited success is attainable with aromatic ketones. However, certain enolates derived from ketones have been shown to undergo catalytic reduction to alkanes quite efficiently. For example, enol triflates of ketones are reduced over platinum oxide catalyst to alkanes (equation 56) . Similarly, enol phosphates, conveniently prepared from ketones, can be quantitatively hydrogenated to alkanes (equation 57) . ... 

---