Selectivity in reductions

Bases function in a more complex way than simply by acting as a halogen acceptor, for they cannot always be used interchangeably, and the product may depend on the base used (58,68,69). Diamines, such as ethylene-, propylene-, or hexamethylene diamines may function differently than monoamines. Diamines were thought to function through chelate formation (32). Loven and Speckamp (40) concluded that selectivity in reduction of a... 

The inertness of ordinary double bonds toward metallie hydrides is quite useful, since it permits reduction of, say, a carbonyl or nitro group, without disturbing a double bond in the same molecule (see Chapter 19 for a discussion of selectivity in reduction reactions). Sodium in liquid ammonia also does not reduce ordinary double bonds, although it does reduce alkynes, allenes, conjugated dienes, and aromatic rings (15-14). 

Iridium nanopartides also catalyze the hydrogenation of benzyhnethylketone, with high selectivity in reduction of the aromatic ring (92% selectivity in saturated ketone, 8% in saturated alcohol at 97% benzylmethylketone conversion). This preferential coordination of the aromatic ring can be attributed to steric effects that make carbonyl coordination difficult. Therefore, metallic iridium nanoparticles prepared in ILs may serve as active catalysts for the hydrogenation of carbonyl compounds in both solventless and biphasic conditions. 

Chromium complexes,163 cationic homogeneous complexes of rhodium164 and ruthenium,165 166 and supported palladium complexes141167 have been found to be extremely selective in reduction of alkynes to cis alkenes ... 

An excellent, broad review of the last 60 years of hydride reductions has been published,235 and the use of selectrides, Li and K tri-.v-butylborohydridcs or trisiamylborohydrides, has also been reviewed.236 A review of sodium borohydride-carboxylic acid as a reagent with novel selectivity in reductions has been written in particular, this reagent is useful for the A -alkylation of primary and secondary amines, through a sequence that is believed to involve sequential carboxylic acid to aldehyde reduction followed by reductive animation.237... 

A rhodium bis(oxazoline) catalyst gives high ee and anti-selectivity in reductive... 

There are many more examples of selectivity in reduction. These few are simply meant to illustrate that the simple principles discussed in this section, and the models used, can be applied to more complex systems with reasonable success. 

Regioselective reduction of aryl-substituted epoxides with NaBHj is catalyzed by PdCl2 in the presence of moist alumina (H2O content, 19 wt%) in hexane. The selectivity in reduction of aryl-substituted epoxides was the same as with NaBILj and diborane. In addition, the alumina can be easily recovered and reused without further treatment. 

Nickel boride (P — 2) generated from nickel acetate and NaBH4 in ethanol exhibits notable selectivity in reduction depending on the substitution pattern of the alkene " ". Sodium borohydride with Co(II) has also proved to be of general utility for the reduction of various alkenes and alkynes (equation... 

A catalytic system involving supported palladium and a phenantroline in equimolar ratio, has been shown to be active and selective in reductive carbonyl ation of aromatic nitro compounds to the corresponding urethanes when the reaction is conducted in anhydrous ethanol and in the presence of a catalytic amount of a weak, non ester ifiable Broensted acid [252. ... 

Selectivity in reduction with the hydrated electron and oxidation with the hydroxyl radical have been shown to occur in redox reactions of dicopper(II) complexes with different ligand fields. The stronger field tetraazacomplexes exhibit and Cu / " couples at significantly more negative potentials... 

In the last steps, all that remained was to reduce the azide and methylate the resulting amine. Reduction of azide was cleanly effected by hydrogen over Pd/C. The problem of selectivity in reduction of carbamate to Me group was circumvented, however, by using LiAlH(OMe3) in refluxing THE. 

Solved Exercise 13-13 Working with the Concepts Selectivity in Reduction... 

Difference in the reduction rates of maleic and fumarlc acids occurs not only at the mercury electrode but also at lead [12, 13], zinc [11, 12], and silver [12] electrodes. It is interesting to note that the differences in reduction rate are not the same on different faces of a lead single crystal. This effect is not noticeable with silver, although in both cases selectivity in reduction of the acids from their mixtures depends on the index of the crystal face. 

Syntheses of 3-0-methyl-7,8,15,16-tetrahydrobatrachotoxinin A (Scheme VIII) and of 3-0-methyl-7,8-dihydrobatrachotoxinin A (Scheme IX) were now accomplished by the Zurich scientists 115,117). Both C-20-epimers of 7,8-dihydrobatrachotoxinin A were ultimately obtained (Scheme IX) 118, 119). Certain improvements in the earlier routes were introduced, resulting in more quantitative conversions and better selectivity in reductions to desired 20S and 20R alcohols. The 7,8-dihydro-derivative with the unnatural 20- 3-hydroxy configuration (20S) was analyzed by x-ray crystallography and its structural assignment confirmed 152). Proton magnetic resonance spectra of the 20-epimers of 7,8-dihydro compounds were depicted and analyzed in detail 118, 119). Mass spectra were also reported. 

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