Selective reductions of functional

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. 

The selective reduction of functional groups in the pyridine series over ring hydrogenation is probably due to the effect of the nitrogen atom. It is conceivable that this selectivity could be changed, particularly... 

In a series of papers, Caubere and co-workers have described their continued exploration of the use of complex reducing agents in the selective reduction of functional groups. For example, the readily prepared NaH-Bu ONa-FeCls reduces oct-l-ene to n-octane in 90—95% yield, and shows selectivity towards exocyclic double bonds. Aliphatic and aromatic halides are reduced to hydrocarbons in high yield by the same reagent, but ketones are unaffected. 

Figure 10, Selective reductions of functional groups in benzonitriles.

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 ... 

Very selective c/s-hydrogenations are also achieved by reduction with diiminc (N2H2, S. Hiinig, 1965 C.E. Miller, 1965 D.J. Pasto, 1991). The reagent can be used at low temperatures and has been employed in the selective reduction of C C double bonds, e.g. in the presence of a sensitive peroxidic function (W. Adam, 1978). 

The Al-dtfluoroaimno substituent with no a-hydrogen is resistant to lithium alununum hydnde [5/] (equation 65), and the selective reduction of the ester functions of polychlorofluorocarboxylates to alcohols without loss of chlorme is accomplished with sodium borohydnde [52] (equation 66)... 

There is a complication in choosing a catalyst for selective reductions of bifunctional molecules, For a function to be reduced, it must undergo an activated adsorption on a catalytic site, and to be reduced selectively it must occupy preferentially most of the active catalyst sites. The rate at which a function is reduced is a product of the rate constant and the fraction of active sites occupied by the adsorbed function. Regardless of how easily a function can be reduced, no reduction of that function will occur if all of the sites are occupied by something else (a poison, solvent, or other function). 

A frequent problem is selective reduction of an acetylene to the olefin in the presence of other easily reducible functions. Usually the reaction can be done without difficulty because of the relatively strong and preferential adsorption of the acetylenic function on the catalyst. Functions adjacent to the triple bond may cause special problems if the resulting allylic compound is itself susceptible to facile hydrogenolysis (18,23). The product composition is, as expected, sensitive to steric effects (82). 

Selective reduction of acetylenes containing carbonyl functions seems to present no difficulties if the groups are not conjugated. 

In molecules containing both an acetylenic and a nitro function, either or both may be reduced. Preferential reduction of the acetylenic function is best achieved with palladium (42,44). Ruthenium, on the other hand, favors selective reduction of an aromatic nitro function high yields of (3-aminophenyljacetylene were obtained from the corresponding nitro compound. Catalyst life is prolonged by protection of the acetylenic function (70). Cobalt polysulffde and ruthenium sulffde catalysts have been used similarly, but more vigorous conditions are required (100°C, 25-70 atm) (71). 

For a review of selective reduction of aliphatic nitro compounds without disturbance of other functional groups, see Ioffe, S.L. Tartakovskii, V.A. Novikov, S.S. Russ. Chem. Rev., 1966, 35, 19. 

One or both carbonyls in /3-diketones can be reduced, as well as the carbonyl function in acyl cyanides (210). Similar treatment of a,/3-unsat-urated ketones and aldehydes can lead to the saturated carbonyl products via selective reduction of the olefinic bond (207, 208, 210) see Eq. (51) in Section III,A,4. Some terpenes (a- and /3-ionone, pulegone) were reduced in this way (208). Platinum(II) phosphine complexes have been used for the hydrosilylation of saturated ketones and could be used for the reduction (211). 

Selective reduction of the ester function in products (37), step (2), affords nitro alcohols (38), which are either transformed into halo-containing compounds... 

The possibility of synthesizing unnatural amino acids from the resulting functionalized oxazines was exemplified by the selective reduction of product (521a) to amino acid (527) (473). 

In the other approach, again harmalane (150) was treated with methyl 2-(di-ethylphosphono)acrylate (174), resulting in iminophosphonate 175. By its sodium borohydride reduction and subsequent lactonization, the amidophospho-nate 176 has been obtained, Wittig-Homer reaction of 176 with acetaldehyde followed by selective reduction of the carbonyl group of the enamide function supplied ( )-deplancheine in good yield (116). 

Hydrogenation of double and triple bonds has already been discussed. If the compound contains reducible functional groups, they may also be reduced but selective reduction of double bonds has been possible under suitable conditions. 

Scheme 11.19 Selective formation of functionalized cyclopentanol 63 via electrochemical reduction ofallenic ketone 61 [80],...

The further reactivity of these complexes was then examined, and it was found that treatment of 148.a with sodium borohydride resulted in a quantitative and entirely selective reduction of the uncomplexed organic carbonyl functionality to give complex 149. 

The selective reduction of the C=0 functional group of organic substrates has been reported using hydrogen gas and various iridium complexes as precatalysts. 

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