Functional groups selective reduction

Functional group selectivity is often easy to achieve in reduction and condensation reactions since several highly selective reagents for reduction and for protection offunctional groups are available. 

Reduction of aldehydes to primary alcohols is very easy and can be accomplished by a legion of reagents. The real challenge is to reduce aldehydes containing other functional groups selectively or to reduce the other function in preference to the aldehyde group. 

Another fact is the quite impressive functional group selectivity of this method. Because of their greater reactivity imines can be reduced in the presence of ketones, although chiral Ru-complex 9 catalyzes the transfer hydrogenation of ketones. Besides this catalytic enantioselective reduction of imines others are known.8... 

It has been of interest to know if the electrochemical method can provide functional group selectivity. As test cases the inter- and intramolecular competition between reduction of benzene rings and terminal alkynes were studied 45). It was found that the amount of water in the solvent has a large effect on the ratio alkyne/benzene that reacted and that it could be used to control the ratio of alkene/dihydrobenzene prod-... 

As illustrated, the major steps in the conversion of 140 to 139 correspond to non-isohypsic transformations of functional groups the reduction of an aldehyde to a primary alcohol, the oxidation of a secondary alcohol to a ketone, and the oxidation of a primary alcohol to a carboxylic acid. The introduction and removal of the isopropylidene protecting groups and the use of the bacterium Aeetobacter suboxydans (a non-typical oxidizing agent) ensures selectivity in the reactions of the polyfunctional intermediate compounds. 

E. R. H. Walker, Chem. Soc. Rev., 1976, 23 lists all aluminum and boron hydrides used for the reduction of organic compounds. A table of functional group selectivity is given. 

Liberation of the 4-ammo function requires selective reduction of the azido group in the presence of the double bond. This has been achieved by first gaining water solubility by removal of the acetate protecting groups with... 

The consonant charge pattern and the presence of a p-hydroxy ketone moiety in the TM suggest a retroaldol transform. Either the hydroxy-bearing carbon or the carbonyl carbon of the TM may serve as an electrophilic site and the corresponding a-carbons as the nucleophilic sites. However, path b is preferable since it does not require a selective functional group interconversion (reduction). 

Reduction of a multisubstituted aromatic compound is functional group selective, where selectivity is based on the reduction potentials. For example, the aldehyde group is reduced in p-cyanobenzaldehyde and p-acetobenzal-dehyde, whereas cyano and aceto groups are not. The nitro group, however, is reduced more readily than the aldehyde group when p-nitrobenzaldehyde is employed as a substrate [23]. The reduction efficiency for an aliphatic aldehyde is much less than that for an aromatic one because of the difference in reduction potentials. 

In the area of preparative chemistry, there are reviews of the functional-group selectivity of complex hydride reducing agents, the reduction of organic compounds with diborane, the formation of C—C bonds using organoboranes," and a report on the preparation, properties, and synthetic applications of catecholborane. ... 

The high functional group selectivity of NaB HgCN-mediated reductions is illustrated by that of the l-a-D-(2,3-di-0-acetyl-4-fonnylarabinofuranosyl)-2-nitroimidazole (222) to the corresponding [ 1 H]alcohol in 50% radiochemical yield without affecting the ancillary nitro and ester functions. In contrast, NaB H4 was found to cleave the acetate protecting... 

Lithium aluminium hydride LiAlH is a useful and conveuient reagent for the selective reduction of the carbonyl group and of various other polar functional groups. It is obtained by treatment of finely powdered lithium hydride with an ethereal solution of anhydrous aluminium chloride ... 

The reduction of a benzenoid ring, except in benzoic acid derivatives, occurs only in the presence of a proton donor having a pKa of 19 or less (pKa of ammonia is about 33). With the exception of the vinyl group, the other functional groups listed above do not require a proton donor of this acidity in order to be reduced, although the course of reduction may then be complex, e.g. as with esters. " Consequently, a variety of functional groups should be capable of selective reduction in the presence of a benzenoid ring if the reaction medium does not contain an acid of pKa <19. A few examples of such selective reductions have been reported in the steroid literature. 

Catalytic hydrogenation has been utilized extensively in steroid research, and the method has been found to be of great value for the selective and stereospecific reduction of various functional groups. A number of empirical correlations concerning selectivity and product stereochemistry compiled for steroid hydrogenations has been listed in a previous review. ... 

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