Hydrogenation of the Carbonyl Group

In this contribution, we first review the hydrogenation of carbohydrates to polyols. Section 8.4.3 will focus on the reductive amination of carbohydrates and, finally (Section 8.4.4), the facile dehydrogenation of carbohydrates under alkaline conditions will be reported. The hydrogenolysis of carbohydrates is beyond the scope of this chapter. Readers interested in this subject should consult Ref. 3 and references cited therein. 

Industrially the most important carbohydrate hydrogenation product is sorbitol (o-glucitol), which is obtained by reduction of the carbonyl group of o-glucose [4-7]. Worldwide production is estimated to be 650 000 tons p. a. Sorbitol is used in numerous cosmetic, food and drinks formulations, and as a starting material e. g. in the manufacture of ascorbic acid (vitamin C). 

In batch processes a 45-50% (w/v) aqueous solution of o-glucose is hydrogenated in the presence of Raney nickel (3-6% w/w relative to o-glucose) at 

A mixture of high polyols has been prepared by the simultaneous action of a- or yff-amylase and nickel catalyst on starch (maize) under hydrogen pressure [14]. A syrup with DE 11 and 82% solubles was obtained. Higher DE-level syrups were not obtained by this combined hydrolysis-hydrogenation process because the enzyme was found to be inhibited by leached nickel from the catalyst. 

Typical reaction conditions for the conversion of starch are batch autoclave, 180 °C, 55 bar H2, starch concentration 30% wlw), Ru/starch wlw ratio 0.002. Under these conditions essentially quantitative conversion is reached within 1 h. Sorbitol selectivity is 95 % and the catalyst can be re-used many times. 

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Hydrogenation of the carbonyl group yields benzyl alcohol. 

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. 

Hydrogenation of the carbonyl group of a mono- or disaccharide gives the corresponding sugar alcohol. On paper, this reaction is simple, and it can schematically be written as O... 

A 95% yield of cinnamic alcohol is obtained by selective hydrogenation of the carbonyl group in cinnamaldehyde with, for example, an osmium-carbon catalyst [145]. 

The hydrogenation of the carbonyl group can, in principle, proceed in two ways (Scheme 9) by the addition of adsorbed hydrogen to the C=0 bond (ketonic mechanism), or by the addition of adsorbed hydrogen to the C=C bond of the enol form (enolic mechanism). Deuteration appears to be a good method to distinguish the two mechanisms the ketonic mechanism would give rise to a C(l)—D bond, whereas the enolic mechanism... 

In general, Pt/C catalysts show little selectivity in the hydrogenation of the carbonyl group of a,P-unsaturated aldehydes but a Pt/graphite catalyst promoted the hydrogenation of cinnamaldehyde at 60°C and 40 atmospheres in an aqueous iso-propyl alcohol solution containing some sodium acetate. Cinnamyl alcohol was obtained in 83% selectivity at 50% conversion. Comparable selectivities... 

Although selective hydrogenation of the carbonyl group of a,P-unsaturated ketones does not appear possible at this time, the keto group can be selectively hydrogenated with unconjugated, unsaturated ketones provided the... 

Although ruthenium is the most active known catalyst for hydrogenation of the carbonyl group, it is possible to reduce a,/3-unsaturated aldehydes to saturated aldehydes quantitatively. The Engelhard workers, however, regard palladium as the catalyst of choice for this conversion. For hydrogenation to the saturated alcohol, they prefer a two-step process Pd-reduction of the double bond, followed by Ru-reduction of the aldehyde group. 

Reduction of either cis- or tra i-2-benzoylcyclopropyl isocyanate (3) with lithium aluminum hydride gave 4-methylamino-l-phenylbutan-l-ol (4) by cleavage of the activated cyclopropyl bond and hydrogenation of the carbonyl groups. ... 

As it is visible from Fig. 1, the selectivity towards COL increased with conversion. This was most clearly observed in non-reduced catalysts. Previously [23], it was demonstrated that in the liquid-phase hydrogenation of a similar molecule (e.g. crotonaldehyde), there was a clear increase of the selectivity as a function of conversion. This kinetic pattern did not obey the general behavior characteristic for parallel-consecutive reactions, thus calling for the introduction [23] of the reaction-induced formation of active sites. One can thus speculate that, also in cinnamaldehyde hydrogenation, during the reaction there is in situ formation of sites responsible for the hydrogenation of the carbonyl group (formation of unsaturated alcohol). 

Catalytic hydrogenation of the carbonyl group of benzoylformic acid and pyruvic acid bound to an optically active alcohol [(— )-menthol, (+ )-bomeol] or an optically active amine or amino isobutylester yields chiral mandelic acid and lactic acid °. The... 

In contrast to Raney nickel catalysts ( 3.4.1), heterogeneous hydrogenation catalysts based on Pt, Rh or Pd do not induce asymmetry in the presence of tartaric acid [113, 578], Platinum catalysts modified by cinchona alkaloids 3.1 and 3.2 cause asymmetric hydrogenation of the carbonyl group of a-ketoesters with a high enantiomeric excess (> 90%). From other types of ketones, the enantioselectivities are lower. 

Catalysts constituting a C2-symmetric 1,2-diamine have been used to hydrogenate a-aryl aldehydes to yield chiral alcohols, under dynamic kinetic resolution conditions. Hydrogenation of the carbonyl group of acylsilanes with 3 (presence of f-AmOK or NaBHr as activator) is apphcable to acquisition of a-silyl aUylc alcohols from conjugated acylsilanes. ... 

Ketones carrying a sulfone substituent in the -position were subjected to enantioselective hydrogenation of the carbonyl group. With the heterogeneous Raney nickel catalyst, modified with tartaric acid and sodium bromide (see Section 2.3.1.1.), l-methylsulfonyl-2-butanone was reduced to (R)-l-methylsulfonyl-2-butanol, 1-methylsulfonyl-2-heptanone to (R)-l-methylsul-fonyl-2-heptanol, and l-methylsulfonyl-2-deeanone to (/ )-l-methylsulfonyl-2-decanol in 100% yield 67-71% ee51. 

Here we wish to report that chemoselective hydrogenation of the carbonyl group can be achieved in the presence of copper catalysts by using 2-propanol as hydrogen donor or as a solvent. 8% Cu/Silica catalysts prepared by a non-conventional technique and reduced at 270°C can be used. Molecular hydrogen addition is also possible, but for open chain substrates messy reactions are observed. 

EN Bakhanova, AS Astakhova, KhA Brikenshtein, VG Dorokhov, VI Savchenko, ML Khidekel, Selective hydrogenation of the carbonyl group of a,P-unsaturated aldehydes in the presence of an iridium catalyst. Izvestiya Akademii Nauk SSSR, Seria Khimicheskaya 1993-1998, 1972. 

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