Examples of reductants

Sodium dithionite (Na2S204) is a reductant that is commonly used for the extraction of Fe oxides from kaolin and from soils (both to determine the total amount of Fe oxides and to improve the dispersibility of the day minerals Mehra Jackson, 1960). The overall reaction may be written as  

This reduction reaction is counterbalanced by disproportionation of dithionite, i. e. 

The rate of disproportionation increases with decreasing pH and rising temperature and these conditions also favour oxide reduction. To achieve a reasonable rate of dissolution, one has to compromise on the pH. A pH of 3 is used in kaolin bleaching (Jepson, 1988), whereas in soil analysis the system is usually buffered with citrate and bicarbonate at ca. pH 7 (Mehra Jackson, 1960). Citrate also complexes the dissolved Fe and prevents its precipitation as Fe sulphide. For the dithionite/EDTA system, Rueda et al. (1992) found a maximum efficiency at pH 5-6 and an activation energy for goethite dissolution of 70 kj mol . They stressed the importance of adsorption of S2O4 on the surface to ensure reduction of Fe . 

M ascorbic acid at pH 4 with various amounts of oxalic acid adsorbed by the goethite (figures at the curves). Insert relationship between the initial dissolution rate k and the amount of oxalate adsorbed by the goethite (StummetaL, 1985, with permission). 

Various phenolic reductants have been used to dissolve synthetic goethite and hematite (LaKind Stone, 1989). In hydroquinone, goethite dissolved according to the following reaction  

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Table 15.3 gives some examples of reductive deoxygenation of 2-acyl and 3-acyl indoles. 

Further examples of reduction of simple iminium salts are given in Table 7. 

There have been only a few examples of reduction of the C=N+ function of catalytic hydrogenation since the reductions with complex hydrides are so easy to do in the laboratory. A possible reduction of an iminium salt 45 to 46 with platinum oxide was reported by McKay et al. (91). A report that platinum oxide reduces 2l tio).jgj yjj.Qqyjp Qjj2idijjjujn perchlorate (25) in quantitative yield to 47 indicates that such reduction should be facile (47). 

There are only a few examples of reductions of sulphones in acidic media. One of the earliest was a mention by Bordwell and McKellin63 (see Section III.A.l) that... 

Further examples of reductions with sodium dithionate are given in References 220-222. The stereoselective syntheses of 1,3- and 1,4-dienes and pheromones are described in Reference 223. 

Two important examples of reductive metabolism of xenobiotics are the reductive dehalogenation of organohalogen compounds, and the reduction of nitroaromatic compounds. Examples of each are shown in Figure 2.13. Both types of reaction can take place in hepatic microsomal preparations at low oxygen tensions. Cytochrome P450 can catalyze both types of reduction. If a substrate is bound to P450 in the... 

Examples of reduction of multidimensional problems to chains of onedimensional ones. It is apparent from that discussion that some class of problems for which a solution of problem (6) or problem (11) coincides on the grid with the exact solution of the multidimensional problem (5) plays an important role. 

Development of new reduction systems that reduce sterically hindered compounds The reported examples of reduction of carbonyl compounds are usually for the substrates that can be easily reduced such as methyl ketones. Since the demand for reduction of various types of compounds is increasing, investigation of new biocatalytic reductions is required. Photosynthetic organisms are not investigated yet, and they may have new type of enzymes, which can reduce sterically hindered compounds. 

Scheme 5.7 illustrates these and other applications of the hydride donors. Entries 1 and 2 are examples of reduction of alkyl halides, whereas Entry 3 shows removal of an aromatic halogen. Entries 4 to 6 are sulfonate displacements, with the last example using a copper hydride reagent. Entry 7 is an epoxide ring opening. Entries 8 and 9 illustrate the difference in ease of reduction of alkynes with and without hydroxy participation. 

Scheme 5.13 provides several examples of reductive carbon-carbon bond formation, including formation of diols, alkenes, and acyloins. Entry 1 uses magnesium amalgam in the presence of dichlorodimethylsilane. The role of the silane may be to... 

Scheme 11.6 gives some examples of the various substitution reactions of aryl diazonium ions. Entries 1 to 6 are examples of reductive dediazonization. Entry 1 is an older procedure that uses hydrogen abstraction from ethanol for reduction. Entry 2 involves reduction by hypophosphorous acid. Entry 3 illustrates use of copper catalysis in conjunction with hypophosphorous acid. Entries 4 and 5 are DMF-mediated reductions, with ferrous catalysis in the latter case. Entry 6 involves reduction by NaBH4. 

Taking into account the electron density relocation (Table 2.4) two routes of NO adsorption can be distinguished. Thus, the nitric oxide coordinates to the monovalent Cr, Ni, and Cu ions in an oxidative way (A<2M > 0), whereas for the rest of the TMIs in a reductive way (AgM < 0). Although this classification is based on the rather simplified criteria, it is well substantiated by experimental observations. Examples of reductive adsorption are provided by interaction of NO with NinSi02 and NinZSM-5, leading at T > 200 K to a Ni -NOs+ adduct identified by the characteristic EPR signal [71]. At elevated temperatures, similar reduction takes place for ConZSM-5 [63], whereas in the case of Cu ZSM-5 part of the monovalent copper is oxidized by NO to Cu2+, as it can readily be inferred from IR and EPR spectra [72,73], This point is discussed in more detail elsewhere [4,57],... 

The final example of reduction that I discuss is also the most extreme - gene selectionism in evolutionary biology. All of the senses of reduction that I... 

Apart from the Meerwin-Ponndorf-Verley (MPV) reaction,16 18catalytic asymmetric transfer hydrogenation has remained quite primitive,111,112 with successful examples of reduction of activated olefins, using alcohols or formic acid as hydrogen source, being reported only recently.113,114... 

The stereoselective reduction may be applied to a variety of ketones. Some examples of reductions, as a function both of ketone substrate and amino acid catalyst are provided in Table 11.9. The full scope of this procedure 26-281 has... 

DR. TAUBE There are numerous examples of reduction of CIO at the surfaces of solids, including one from work in my... 

Scheme 3.9 Example of reduction in solid-phase for the synthesis ofbicyclic pyrimidine (135) derivatives [235],...

Further examples of reductive detelluration of tellurium compounds have been shown in connection with tellurolactonization and telluroetherification reactions (see Section 4.5). 

This book encompasses indiscriminately all the types of reductions and superimposes them over a matrix of types of compounds to be reduced. The manner of arrangement of the compounds is a somewhat modified Beilstein system and is explained in the introduction. Numerous tables summarize reducing agents and correlate them with the starting compounds and products of the reductions. Reaction conditions and yields of reductions are mentioned briefly in the text and demonstrated in 175 examples of reductions of simple types of compounds and in 50 experimental procedures. 

Examples of reductive cluster-opening and oxidative cluster-closing reactions are common in the chemistry of metal-hydrocarbon tt complexes. For example, bases convert nido- (hexa-hapto)arene-manganese tricarbonyl complexes into aracAno(pentahapto)-7T-cyclohexadienyl complexes 129,130, 217) ... 

The enolate reactivity associated with this approach to C-glycoside synthesis was, however, first developed with octulosonic acid derivatives, such as 283 [114], 284 [115], and 285 [116], and a series of examples involving aldehyde and halide-based electrophiles are shown in Scheme 74. Related studies involving stereoselective protonation of this class of exocyclic enolate have also been described [13] and Scheme 75 illustrates this with an example of reductive samariation (using ethane-1,2-diol, but no HMPA) of an anomeric acetate 286 [117]. 

Metal Hydrides and Metal-Carbon Compounds. Numerous examples of reductions and additions of metal hydrides or alkyls to unsaturated carbon-nitrogen compounds are known. I shall mention only two examples pertinent to this discussion. 

Applying similar conditions to 8,8-dichlorobicyclo[4.2.0]oct-2-en-7-one, yielded analogous products with approximately the same endojexo ratio.89 Further examples of reduction of di-or monohalobicyclo[3.2.0]heptanes to their monohalo or unsubstituted derivatives are collected in Table 6. 

Selected examples of reductions of eye]obutanones are given in Table 12. 

EXAMPLES OF REDUCTION-BASED, ELECTROCHEMICALLY CONTROLLED H-BONDING SYSTEMS WITH NEUTRAL GUESTS... 

Table 8 Examples of Reductions of Organic Compounds by Vanadium(II) Ions...

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