Some Selective Reductions

The difference in selectivity may be explained. The ease with which the pyridylidene compound was reduced with Raney nickel at low pressure is an indication of its non-aromaticity. The selectivity then is a preferential one of two nonaromatic bonds over one highly conjugated double bond. The selectivity was likely aided by the basic unshielded nitrogen which inhibited the activity of the catalyst and prevented further hydrogen uptake. In the presence of hydrochloric acid the inhibiting effect was removed and uptake proceeded quickly with platinum oxide to l-benzyl-2-phenacylpiperidine. 

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 

Howton and Golding (115) reduced methyl 2-phenacylpyridinium bromide in methanol with platinum oxide and obtained an 88% yield of l-methyl-2-phenacylpiperidine hydrobromide. In another reduction they allowed uptake to proceed further and claimed that 20% of 1-methyl-2-(hexahydrophenacyl)piperidine was formed. In the hydrogenation of the methobromide and metho-p-toluene sulfonate of 2-phenacylpyridine l-methyl-2-phenacylpiperidine was obtained (116). When the methiodide compound was reduced about 8% of l-methyl-2-(2-hydroxy-2-phenethyl)piperidine was also obtained. 

A study of these reductions shows that three reaction routes can be followed, leading to the pyridyl carbinol, piperidyl carbinol, and pipe-ridyl ketone  

Route (A) is the expected reaction route in neutral solvent. In the preparation of the piperidyl carbinol, in the presence of hydrochloric acid, reaction follows route (B), which may involve concurrent reduction of the ring and carbonyl group or may actually follow (A) or (C) with 

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The known property of diorganyl tellurides of reducing metallic salts, applied to tita-nium(IV) chloride, generates a titanium(III) species which is a useful reagent for some selective reductions. By this method benzaldehyde is reduced to dihydrobenzoin, and benzyl to benzoin, and successively to desoxybenzoin. ... 

Reduction of ketones. Reduction of one carbonyl group requires 2 moles of this complex because B2HJ is also released. Aliphatic ketones are reduced readily, but aromatic ketones are reduced much more slowly, and a,(i-cnones even more slowly. Consequently some selective reductions are possible. 

Aromatic or aliphatic nitriles are unreactive towards Sml2- Nitroaro-matics are rapidly reduced to amines. Some selective reductions are then... 

It is convenient to include under Aromatic Amines the preparation of m-nitroaniline as an example of the selective reduction of one group in a polynitro compound. When wt-dinitrobenzene is allowed to react with sodium polysulphide (or ammonium sulphide) solution, only one of the nitro groups is reduced and m-nitroanUine results. Some sulphur separates, but the main reaction is represented by ... 

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

The washed slime is dried and melted to produce slag and metal. The slag is usually purified by selective reduction and smelted to produce antimonial lead. The metal is treated ia the molten state by selective oxidation for the removal of arsenic, antimony, and some of the lead. It is then transferred to a cupel furnace, where the oxidation is continued until only the silver—gold alloy (dorn) remains. The bismuth-rich cupel slags are cmshed, mixed with a small amount of sulfur, and reduced with carbon to a copper matte and impure bismuth metal the latter is transferred to the bismuth refining plant. 

In the early work on the synthesis of prostaglandins, zinc borohydride was used for the reduction of the 15-ketone function and a 1 1 mixture of epimeric 15(S)- and 15(/ )-alcohols was generally obtained. Subsequent studies led to reaction conditions for highly selective reduction to the desired 15(S)-alcohol. Some of the results are summarized in the following table. The most practical method is E which utilizes borane as the stoichiometric reductant and a chiral, enzyme-like catalyst which is shown. 

Figure 18.5 Plausible sequence of steps responsible for rapid and selective reduction of O2 to H2O by mixed-valence CcO. The square frames signify the catalytic site (Fig. 18.4c) imidazole ligation of Cub is omitted for clarity in some or aU intermediates, Cub may additionally be ligated by an exogenous ligand, such as H2O (in Cu ) or OH (in Cu ) such ligation is not established, and hence is omitted in all but compound Pm and the putative hydroperoxo intermediate. The dashed frames signify the noncatalytic redox cofactors. Typically used phenomenological names of the spectroscopically observed intermediates (compounds A, E, H, etc.) are also indicated.

In synthesis, the principal factors that affect the choice of a reducing agent are selectivity among functional groups (chemoselectivity) and stereoselectivity. Chemo-selectivity can involve two issues. One may wish to effect a partial reduction of a particular functional group or it may be necessary to reduce one group in preference to another,78 In the sections that follow, we consider some synthetically useful partial and selective reductions. 

Electrocatalysis employing Co complexes as catalysts may have the complex in solution, adsorbed onto the electrode surface, or covalently bound to the electrode surface. This is exemplified with some selected examples. Cobalt(I) coordinatively unsaturated complexes of 2,2 -dipyridine promote the electrochemical oxidation of organic halides, the apparent rate constant showing a first order dependence on substrate concentration.1398,1399 Catalytic reduction of dioxygen has been observed on a glassy carbon electrode to which a cobalt(III) macrocycle tetraamine complex has been adsorbed.1400,1401... 

In the following, some selected classes of reactions and applications of Ni11 polyazamacrocyclic complexes are discussed. Other applications include, inter alia, the electrochemical reduction of allyl ethers1613 and the epoxidation of alkenes.1614... 

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). 

In some cases, silylation of AN and their derivatives produces nitroso acetals containing the N -siloxy fragment or cyclic ethers of oximes (predominantly substituted 5.6-dihydro-4f/-oxazines). To use these products in strategies for synthesis, it is worthwhile to develop convenient procedures for selective reductions of the above derivatives to the corresponding amines. 

New selected possibilities created by reduction of new types of oxazines (603) in the synthesis of polyfunctional products (473, 554) are presented in Scheme 3.285. Coupling the reduction of the C=N bond with NaBH CN followed by hydrogenation of the N-0 bond or a one-step catalytic hydrogenation, and the double-bond transfer from the C(3) to the C(4) position, enables the synthesis and detection of 14 types of reduction products. In some cases, reduction is stereoselective. 

Reduction of dienes and polyenes has attracted much attention since it is important from both practical and theoretical aspects. In these reactions the major interest is the selective reduction of a double bond in the presence of another. In general, saturation of all the multiple double bonds of nonaromatic compounds can be carried out with any of the catalysts which are suitable for low-pressure reductions or with some reducing chemicals. 

Most species of angiosperms produce hermaphroditic flowers. The unisexuality in some plants most often results from developmentally programmed abortion or selective reduction in sex organ primordia. In dicots, higher levels of auxins, cytokinins, and ethylene usually correlate with female sex expression and in most of them the femaleness is mainly promoted by ethylene. ... 

Some selected reactions of 21b were investigated [32]. In the reaction with [(Ph 0)4W=0j the dinuclear compound 24 is formed (Eq. 16) containing an almost planar W2OP four-membered ring system. The structure of 24 reveals that after the formal cycloaddition reaction a reductive W-W bond formation occurs under loss of OPh moieties. 

Meunier, FC Ukropec, R Stapleton, C Ross, JRH. Effect of the silver loading and some other experimental parameters on the selective reduction of NO with CsH over AI2O3 and Zr02-based catalysts. Appl Catal, B Environmental, 2001, Volume 30, Issues 1-2, 163-172. 

Dissolving metal reductions were among the first reductions of organic compounds discovered some 130 years ago. Although overshadowed by more universal catalytic hydrogenation and metal hydride reductions, metals are still used for reductions of polar compounds and selective reductions of specific types of bonds and functions. Almost the same results are obtained by electrolytic reduction. 

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