Carbonyl groups in hydrogenation

The additional structure C, which cannot be drawn for an unconjugated carbonyl derivative, implies that the carbonyl band in an enone has more single bond character and is therefore weaker. The involvement of a carbonyl group in hydrogen bonding reduces the frequency of the carbonyl stretching vibration by about 10 cm-. This can be rationalised in a manner analogous to that proposed above for free and H-bonded 0-H vibrations. 

Nor is the hydrogen directly attached to the carbonyl group in aldehydes affected. Only the a hydrogen is replaced. 

Diketones 1 can be converted into the salt of an a-hydroxy carboxylic acid upon treatment with alkali hydroxide after acidic workup the free a-hydroxy carboxylic acid 2 is obtained. A well-known example is the rearrangement of benzil (R, R = phenyl) into benzilic acid (2-hydroxy-2,2-diphenyl acetic acid). The substituents should not bear hydrogens a to the carbonyl group, in order to avoid competitive reactions, e.g. the aldol reaction. 

Reduction of unsaturated aldehydes seems more influenced by the catalyst than is that of unsaturated ketones, probably because of the less hindered nature of the aldehydic function. A variety of special catalysts, such as unsupported (96), or supported (SJ) platinum-iron-zinc, plalinum-nickel-iron (47), platinum-cobalt (90), nickel-cobalt-iron (42-44), osmium (<55), rhenium heptoxide (74), or iridium-on-carbon (49), have been developed for selective hydrogenation of the carbonyl group in unsaturated aldehydes. None of these catalysts appears to reduce an a,/3-unsaturated ketonic carbonyl selectively. 

If 4-azidoquinolines with substituents such as a carbonyl group in position 2 or 8 are irradiated in the presence of sodium methoxide the resulting 1//-benzodiazepines 3 are stabilized by hydrogen bonding between the ring NH and a neighboring 2- or 9-acyl carbonyl group and are readily isolated.216,217... 

This comparison suggests that of these two similar reactions, only alkene additions are likely to be a part of an efficient radical chain sequence. Radical additions to carbon-carbon double bonds can be further enhanced by radical stabilizing groups. Addition to a carbonyl group, in contrast, is endothermic. In fact, the reverse fragmentation reaction is commonly observed (see Section 10.3.6) A comparison can also be made between abstraction of hydrogen from carbon as opposed to oxygen. 

Ohkuma, T. and Noyori, R. (2004) Hydrogenation of carbonyl groups, in Comprehensive Asymmetric Catalysis, Supplement I (eds. E.N. Jacobsen, A. Pfaltz and H. Yamamoto), Springer, Berlin, pp. 1-41. 

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