Reduction of Functional Groups

J.M. Lalancette et al., Reduction of Functional Groups with Sulfurated Borohydrides, Synthesis 1972, 526. J. Malek u. M. Cerny, Reduction of Organic Compounds by Alkoxyaluminohydrides, Synthesis 1972, 217. S.-C. Chen, Molecular Rearrangements in Lithium Aluminium Hydride Reduction, Synthesis 1974, 691. 

Another example of the reduction of functional groups is that of aromatic ketones. At the mercury electrode, a radical-type intermediate is formed as the primary product ... 

Many enantioselective catalysts have been developed for reduction of functional groups, particularly ketones. BINAP complexes of Ru(II)C12 or Ru(II)Br2 give good enantioselectivity in reduction of (3-ketoesters.49 This catalyst system has been shown to be subject to acid catalysis.50 Thus in the presence of 0.1 mol % HC1, reduction proceeds smoothly at 40 psi of H2 at 40° C. 

The use of free-radical reactions in organic synthesis started with the reduction of functional groups. The purpose of this chapter is to give an overview of the relevance of silanes as efficient and effective sources for facile hydrogen atom transfer by radical chain processes. A number of reviews [1-7] have described some specific areas in detail. Reaction (4.1) represents the reduction of a functional group by silicon hydride which, in order to be a radical chain process, has to be associated with initiation, propagation and termination steps of the radical species. Scheme 4.1 illustrates the insertion of Reaction (4.1) in a radical chain process. 

Glyoxylic Acid Oxalic acid (Japan) Not available Reduction of functional group... 

Electrochemical studies are usually performed with compounds which are reactive at potentials within the potential window of the chosen medium i.e. a system is selected so that the compound can be reduced at potentials where the electrolyte, solvent and electrode are inert. The reactions described here are distinctive in that they occur at very negative potentials at the limit of the cathodic potential window . We have focused here on preparative reductions at mercury cathodes in media containing tetraalkylammonium (TAA+) electrolytes. Using these conditions the cathodic reduction of functional groups which are electroinactive within the accessible potential window has been achieved and several simple, but selective organic syntheses were performed. Quite a number of functional groups are reduced at this limit of the cathodic potential window . They include a variety of benzenoid aromatic compounds, heteroaromatics, alkynes, 1,3-dienes, certain alkyl halides, and aliphatic ketones. It seems likely that the list will be increased to include examples of other aliphatic functional groups. 

For such a modular configuration, it is claimed that a large number of heterogeneously catalyzed reactions can be performed, e.g. Heck and Suzuki coupling, acid-catalyzed esterifications and reductions of functional groups [83], However, for the mentioned process, the catalyst and reactants are not named and no reaction conditions or experimental results, e.g. yield, selectivity or nature of side products, are given. 

Selective reduction of functional groups can be achieved by chemical modification of the LiALH4 for example, lithium tri(t-butoxy)aluminium hydride [LiAIH(t-OBu)3] is a more selective reagent, and reduces aldehydes and ketones, but slowly reduces esters and epoxides. Nitriles and nitro groups are not reduced by this reagent. Carboxylic acids can be converted into the aldehyde via acid chloride with lithium tri(tert-butoxy) aluminium hydride at a low temperature (—78°C). The nitro compounds are not reduced under this condition. Thus, selective reduction of 3,5-dinitrobenzoic acid (6.45) to 3,5-dinitrobenzaldehyde (6.47) can be achieved in two steps. First, 6.45 is converted into 3,5-dinitrobenzoyl chloride (6.46) and then LiAlH(t-OBu)3 reduction of 6.46 gives 6.47. 

Sulfur-containing functions are especially prone to undergo reductive removal. Hydrogenolysis of the thio group proceeds easily in the presence of Raney nickel and this procedure is ideally suited for the reduction of functional groups like thiols, sulfides, or thioacetals. The sulfonyl group can be reductively removed under the action of agents like Na in liquid ammonia ... 

The reduction of functional groups with sulfurated horohydrides has been reviewed by Lalancette et al. The reagent is particularly useful for selective reductions. Thus selective reduction of a nitro, an oxime, or a nitrile group in the presence of an ester group is possible. In the case of steroidal ketones, the carbonyl group at C is the most reactive site selective reduction of a C -carbonyl group in the presence of other carbonyls at C, I, C,2, C,7, or Cjo is possible. The alcohol obtained is the equatorial isomer. Aldehydes can be reduced selectively in the presence of a carhonyl group if the molar ratio of NaBHjS, is kept at a suitable value. 

Stereogenic centers can also be prepared by carbon-carbon bond-forming reactions or reductions of functional groups other than alkenes. Some reactions are also summarized in Table 4 (144) for a comprehensive work on asymmetric catalysts, see Reference 145. In some cases, two stereogenic centers can be created. This result can be achieved either in a single step as... 

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