Preferential reduction

Originally, general methods of separation were based on small differences in the solubilities of their salts, for examples the nitrates, and a laborious series of fractional crystallisations had to be carried out to obtain the pure salts. In a few cases, individual lanthanides could be separated because they yielded oxidation states other than three. Thus the commonest lanthanide, cerium, exhibits oxidation states of h-3 and -t-4 hence oxidation of a mixture of lanthanide salts in alkaline solution with chlorine yields the soluble chlorates(I) of all the -1-3 lanthanides (which are not oxidised) but gives a precipitate of cerium(IV) hydroxide, Ce(OH)4, since this is too weak a base to form a chlorate(I). In some cases also, preferential reduction to the metal by sodium amalgam could be used to separate out individual lanthanides. 

Reduction of the sodium salt of equilenin 17-ethylene ketal with lithium, sodium or potassium in ammonia at —70° occurs predominantly in the B-ring, affording, after acid hydrolysis, equilin (29) in up to 76% yield (55% isolated). The preferential reduction of the B-ring reflects the relative, but not absolute, resistance to reduction conferred on the A-ring by the naphthoxide ion. Some A-ring reduction does compete kinetically with B-ring reduction, since the epimeric 3-hydroxyestra-5,7,9-trien-17-ones are the major reaction by-products. Simple phenoxide ions usually reduce slowly... 

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). 

Most catalyst systems, if they show selectivity at all, favor preferential reduction of the less hindered 4-nitro group hydrogenation of 11 over Pt-on-C in acidic alcohol affords 10 in 70% yield (24). Similarly, Lazer et at. (61), selectively prepared 10 from 27 mmol II, 10 ml of 50% H2SO4, 60 ml HOAc, and 350 mg 5% Pt-on-C at 85 C and 30 psig. The hydrogenation is stopped at... 

Competitive reduction tests for cyclohexanone styrene, under transfer conditions, show preferential reduction of cyclohexanone however, under hydrogenation conditions the styrene is reduced exclusively.99 It is worth mentioning that the OsH2(r)2-H2)(CO)(P Pr3)2 precatalyst, formed by addition of NaBH4 to OsHCl (CO)(P Pr3)2, rapidly reduces phenylacetylene to styrene, under transfer conditions, but the reaction rate falls progressively due to the formation of Os(C=CPh)2 (CO)(P Pr3)2.72 As previously mentioned, an alkynyl-dihydrogen intermediate... 

General undernourishment or dietary deficiencies of specific substances can lead to a preferential reduction in myelin formation 649... 

The reductive cleavage (Eq. (11)) is more common. TBHP can also undergo preferential reductive cleavage to the alkoxyl radical ... 

Selective reduction of ketones.1 This reagent can be used to effect selective reduction of the more hindered of two ketones by DIBAH or dibromoalane. Thus treatment of a 1 1 mixture of two ketones with 1-2 equiv. of 1 results in preferential complexation of the less hindered ketone with 1 reduction of this mixture of free and complexed ketones results in preferential reduction of the free, originally more hindered, ketone. An electronic effect of substituents on a phenyl group can also play a role in the complexation. This method is not effective for discrimination between aldehydes and ketones, because MAD-complexes are easily reduced by hydrides. MAD can also serve as a protecting group for the more reactive carbonyl group of a diketone. The selectivity can be enhanced by use of a more bulky aluminum reagent such as methylaluminum bis(2-f-butyl-6-( 1,1-diethylpropyl)-4-methylphenoxide). 

Preferential reduction of a monosubstituted double bond in the presence of an unsym-metrically disubstituted double bond is shown in equation 62154. 

The isolation of benzene and cyclohexane from chlorobenzene and thiophenol, and cyclohexane from fluorobenzene, suggests the preferential reductive cleavage of the substituent prior to hydrogenation of the ring. However, fluorocyclohexane decomposes slowly to cyclohexene, which could give rise to the cyclohexane higher yields of fluorocyclohexane are obtained at lower temperatures. 

Preferential reduction of a nitro group in the presence of a carbonyl group in 4-nitroacetophenone ISD, intramolecular rearrangements of o-nitro-benzanilides 32) intramolecular cyclizations of o-nitro-ferf-anilines to benzimidazol-1-oxides 153,154) cyclizations of acylated 2-nitrodiphenylamines to phenazine-l-oxides i ), intramolecular additions of nitro groups to double bonds 156) remarkably ef-... 

Because of the similar potentials between fully lithiated graphite and lithium metal, it has been suggested that the chemical nature of the SEIs in both cases should be similar. On the other hand, it has also been realized that for carbonaceous anodes this formation process is not expected to start until the potential of this anode is cathodically polarized (the discharge process in Figure 11) to a certain level, because the intrinsic potentials of such anode materials are much higher than the reduction potential for most of the solvents and salts. Indeed, this potential polarization process causes one of the most fundamental differences between the SEI on lithium metal and that on a carbonaceous anode. For lithium metal, the SEI forms instantaneously upon its contact with electrolytes, and the reduction of electrolyte components should be indiscriminate to all species possible,while, on a carbonaceous anode, the formation of the SEI should be stepwise and preferential reduction of certain electrolyte components is possible. 

Sulfur analogs, 2-tetrahydrofuranyl and 2-tetrahydropyranyl thioethers, were reduced by alone to alkyl 4- or alkyl 5-hydroxyalkyl thioethers resulting from the preferential reductive cleavage of the carbon-oxygen (rather than the carbon-sulfur) bond. Thus refluxing for 2 hours with alane in ether converted 2-alkylthiotetrahydrofurans to alkyl 4-hydroxybutyl thioethers in 63-72% yields, and 2-alkylthiotetrahydropyrans to alkyl 5-hydroxypentyl thioethers in 58-82% yields [794]. 

List later reported the asymmetric reductive amination of a wide spectrum of aromatic and aliphatic a-branched aldehydes via dynamic kinetic resolution (Scheme 5.27) [49]. The initial imine condensation product is believed to undergo fast racemization in the presence of the acid catalyst Ih through an imine/enamine tautomerization pathway. Preferential reductive amination of one of the imine enantiomers furnishes the optically pure P-branched amine. 

Another possible reason that ethylene glycol is not produced by this system could be that the hydroxymethyl complex of (51) and (52) may undergo preferential reductive elimination to methanol, (52), rather than CO insertion, (51). However, CO insertion appears to take place in the formation of methyl formate, (53), where a similar insertion-reductive elimination branch appears to be involved. Insertion of CO should be much more favorable for the hydroxymethyl complex than for the methoxy complex (67, 83). Further, ruthenium carbonyl complexes are known to hydro-formylate olefins under conditions similar to those used in these CO hydrogenation reactions (183, 184). Based on the studies of equilibrium (46) previously described, a mononuclear catalyst and ruthenium hydride alkyl intermediate analogous to the hydroxymethyl complex of (51) seem probable. In such reactions, hydroformylation is achieved by CO insertion, and olefin hydrogenation is the result of competitive reductive elimination. The results reported for these reactions show that olefin hydroformylation predominates over hydrogenation, indicating that the CO insertion process of (51) should be quite competitive with the reductive elimination reaction of (52). 

In the synthesis of the head-to-head [4 + 4] dimer of cyclopentadiene, the preferential reductive fission of the C3-C4 bond in bishomocubane again plays a significant role.43 Thus, catalytic hydrogenation of bishomocubane-6,10-diol (14) over 10% palladium on charcoal in ethyl acetate at 2.7-3.4 atmospheres for two days furnished the bishomosecocubanediol tetracy-clo[5.2.1.02,< .04 8]decane-5,10-diol (15).43... 

Actinide and lanthanoid complexes have been employed for hydrogenation reactions, for which they often generate dramatic rate increases and high numbers of turnovers42. Many of these complexes exhibit good selectivity for preferential reduction of the less hindered alkene in situations where more than one is present in a substrate (Scheme l)43. 

Model reactions showed that 2-dodecyl-5-methyl thiophene, benzo[b]thiophene, 2-n-butyl benzo[b]thiophene and dibenzothiophene were unaffected by these reaction conditions after a 4-hour reflux. For the LiAlH4 reduction step the solvent dioxane (b.p. 100°C) is preferred to either diethyl ether or tetrahydrofuran since preferential reduction of certain sulfides in the latter solvents has been observed (2). The results of the sulfide analyses for several petroleums from Alberta are summarized in Table I. 

Meerwein-Ponndorf-Verley reductions. Zirconocene or hafnocene can catalyze reduction of carbonyl compounds with isopropanol. This method is useful for preferential reduction of keto aldehydes to hydroxy ketones and of a,(i-enones or -enals to allylic alcohols.1... 

Although nickel catalysts have served as examples, articles dealing with other metals show that the same concepts apply. This is the case for Co deposition on silica or alumina [82], Cu/y-AhOj [124], and Ag/ /-AhOj and Ag/Ti02 [125], for example. The preparation of bimetallic catalysts is more complicated because of possible preferential reduction of one metal before the other, a phenomenon well known with the bulk oxides [3]. A few studies suggest that approaches similar to those mentioned above can also be used in these cases [122],... 

H NMR spectroscopy has found use in establishing the position of A-substitution upon alkylation [319, 699-703], benzylation [699, 704], acylation [700], and ribo-sylation [705-707] of nitrobenzimidazoles. On the basis of proton spectra the ratio of the N-l and N-2 products of alkylation of 5(6)- and 4(7)-nitrobenzimidazoles was determined [701, 702], The alkylation of 2-nitrobenzimidazole has been studied in [708], The structure of nitroaminobenzimidazoles prepared by preferential reduction of corresponding dinitro compounds has been confirmed [709],... 

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