Other methods of reduction

Although as already stated the use of metal amalgams, and in particular use of the Jones reductor or of the related silver reductor, is the best method of reducing solutions in preparation for titration with an oxidant, it may happen that for occasional use there is no Jones reductor available, and a simpler procedure will commend itself. In practical terms, the need is most likely to arise in connection with the determination of iron, for which the reduction of iron(III) to iron(II) may be necessary. 

Tin(II) chloride solution. Many iron ores are brought into solution with concentrated hydrochloric acid and the resulting solution may be readily reduced with tin(II) chloride  

The small amount of mercury(I) chloride in suspension has no appreciable effect upon the oxidising agent used in the subsequent titration, but if a heavy precipitate forms, or a grey or black precipitate is obtained, too much tin(II) solution has been used the results are inaccurate and the reduction must be repeated. Finely divided mercury reduces permanganate or dichromate ions and also slowly reduces Fe3+ ions in the presence of chloride ion. 

After the addition of the mercury(II) chloride solution, the whole is allowed to stand for five minutes, then diluted to about 400 mL and titrated with standard 

Blank runs on the reagents should be carried through all the operations, and corrections made, if necessary. 

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The stereoselectivity of different catalysts in catalytic hydrogenations is discussed in the chapter on catalysts (pp. 4, 50). In addition to catalytic hydrogenation, a few other methods of reduction can be used for saturation of carbon-carbon double bonds. However, their practical applications are no match for catalytic hydrogenation. 

Although lithium aluminum hydride and boranes are very useful reagents, they are expensive and impractical to employ on a large scale. Other methods of reduction then may be necessary. Of these, the most important are reduction of esters with sodium and ethanol (acids do not react readily),... 

Other methods of reduction include the use of metal-acid systems, catalytic hydrogenation over Raney nickel or palladium-on-charcoal, and modified metal hydride reducing agents such as Red-Al .209... 

A recent report by Evans14 of azide reduction by SnCl2 in aqueous dioxane and in situ amine protection by addition of B0C2O has demonstrated that other methods of reduction of azides may be feasible if catalytic reduction of the azide is precluded because of the presence of protective groups sensitive to hydrogenolysis. 

Several related reactions involve reduction of cyclic carboxylic acid derivatives to masked aldehydes which resist further reduction but can be converted into the required aldehydes by acid hydrolysis. In a series of papers, it was established that carboxylic acids could be converted into dihydro-1,3-thiazines or dihydro-1,3-oxazines which could be reduced by NaBH4 in weakly acidic ethanol. Thus, as shown in Scheme 20, dihydro-1,3-thiazines (41) were reduced to tetrahydro-1,3-thiazines (42) in yields of 66-84%. The resulting tetrahydro compounds could be hydrolyzed to aldehydes by aqueous acid. - In a later publication, these workers showed that there was little evidence for ring opening during reduction and that other methods of reduction e.g. hydrogenation over Pt, Pd or Rh or use of dissolving metals such as Zn, Sn or Na) were totally unsuccessful. In closely similar work, reduction of 5,6-dihydro-4W-... 

Carbon-carbon double bonds oc9 / to the carbonyl group are not attacked, which contrasts with the results of other methods of reduction. Nitro groups, ester groups, and even activated halogen are unaffected. And a further advantage of the method is the generally good yields obtained. 

Other methods of reduction are said to fail. The above method leads to partial reduction of 2,4-dinitrophenol, to 2-amino-4-nitrophenol, in good yield.69... 

Catalytic reductions can be carried out in batches or in continuous processes, in the liquid phase or in the vapor phase. This method has many advantages over other methods of reduction, particularly for lai e-volume production. With low-cost hydrogen, as is the case when by-product hydrogen is available from other installations or when large hydrocarbon-steam units are installed, this process cannot be matched by other methods of reduction in so far as economics and quality of product are concerned. 

The key intermediate benzothiazine 25 was accessed via displacement of fluoride in the dinitrobenzene 23 with the lithium salt of methyl thioglycolate to provide thioether 24, which after tin(II)-mediated reduction of the nitro groups and cyclization furnished benzothiazine 25 in modest yield (Scheme 6). It was noted by the authors that other methods of reduction such as hetero- or homogenous hydrogenation did not improve the yields for this step. It is also noteworthy that subsequent chemoselective reduction of the ester group to give the requisite alcohol proved problematic due to the ease of reduction of the benzothiazinone amide functionality. 

Other methods of reduction 7.4.3 Reductions with diimide... 

Reduction of the sugar acid lactones to aldose sugars also may be accomplished by catalytic hydrogenation (I40) or, very conveniently, with sodium borohydride in aqueous solution ) Other methods of reduction include the catalytic hydrogenation of the acetylated aldonyl chlorides (14 ) or thio esters (I4S). 

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