Hypophosphorous acid, reduction

There are also reports of improved yields in the hypophosphorous acid reduction of diazonium salts, particularly in the presence of a trace of cuprous oxide (Korzeniowski et al., 1977a see also Sec. 10.2), and in bromo-, and cyano-de-diazoniations if acetate ions are present (Korzeniowski and Gokel, 1977 see also Eustathopoulos et al., 1985). The positive effect of crown ethers on the yield in the... 

Hydrochloric and sulfuric acids have been employed with about equal frequency in hypophosphorous acid reductions. Recent work has shown that, in general, the reaction goes more smoothly with the diazonium chlorides,82 so that, with certain exceptions discussed below, diazotiza-tion with hydrochloric acid is advisable. The striking results obtained with the toluidines are recorded in Table I. The isomeric nitroanilines... 

As has already been noted in the section on ethanol deaminations, upon diazotizing some nitroamines in hydrochloric acid, a diazonium salt is obtained in which the nitro group has been replaced by chlorine (pp. 272-273). Consequently, hypophosphorous acid reduction gives a deaminated product Containing chlorine in place of the nitro group. For example, when 5-amino-8-nitroisoquinoline (XVIII) is diazotized with hydrochloric acid and then treated with hypophosphorous acid, 8-chloro-isoquinohne (XIX) is obtained in 60-70% yield instead of 8-nitro-isoquinoline.86... 

When the preceding compound is heated on the water-bath with hypophosphorous acid, reduction rapidly takes place and the arseno-compound is obtained by diluting the reaction mixture with w ater. It is a yellow, amorphous substance, insoluble in water, alkalis, alcohol, acetone, chloroform, and benzene, but soluble in hot acetic acid. In amyl alcohol it gives a colourless solution and is readily soluble in pyridine. Dilute sulphuric acid does not dissolve it, but it is readily soluble in the concentrated acid. Concentrated hydrochloric acid forms the hydrochloride, which is precipitated by excess of acid. 

The replacement of 2-amino group by a hydrogen can be achieved by diazotization, followed by reduction with hypophosphorous acid (1-8, 13). Another method starting from 2-aminothiazole is to prepare the 2-halo-thiazole by the Sandmeyer reaction (prepared also from the 2-hydroxy-thiazole), which is then dehalogenated chemically or catalytically (1, 9, 10). 

Reductive deamination of primary arylamines The ammo substituent of an arylamine can be replaced by hydrogen by treatment of its de rived diazonium salt with ethanol or with hypophosphorous acid... 

Gravimetric methods more suitable for general use involve the precipitation of metallic gold from tetrachloraurate solutions by reduction with oxaUc acid, SO2, or hydroquinone. Formaldehyde, hydrazine, ferrous sulfate, and hypophosphorous acid also have been used but are considered less efficient (40). 

A widely used procedure for determining trace amounts of selenium involves separating selenium from solution by reduction to elemental selenium using tellurium (as a carrier) and hypophosphorous acid as reductant. The precipitated selenium, together with the carrier, are collected by filtration and the filtered soflds examined directly in the wavelength-dispersive x-ray fluorescence spectrometer (70). Numerous spectrophotometric and other methods have been pubHshed for the deterruination of trace amounts of selenium (71—88). 

Diarsines and Diarsenes. Under certain conditions, the reduction of compounds with two organic groups attached to arsenic may give rise to tetraalkyl-or tetraaryldiarsines. Thus a number of diarsines have been obtained by the reduction of arsinic acids with phosphorous or hypophosphorous acid (100). Diarsines can also be prepared by the treatment of a metal dialkyl- or diarylarsenide with iodine (101) or a 1,2-dihaloethane (102). 

A large number of polymeric substances, (RAs) or (ArAs), are also known (113). They are usually prepared by the reduction of arsonic acids with hypophosphorous acid (100,114) or sodium dithionite (115). Most of these polymers have not been well characterized. An insoluble, purple material, poly(methylarsinidene) [26403-94-1], (CH As), prepared by the interaction of methylarsine and a dihalomethylarsine, however, has been shown by an x-ray investigation to have a ladderlike polymeric stmcture in which the inter-mng distances correspond to one-electron bonds (116) ... 

In general, however, reduction by ethanol is recommended only in cases where one has a reliable analytical method for distinguishing between products resulting from hydrogen- or ethoxy-substitution. For all other cases we recommend Kornblum s dediazoniation in an aqueous solution of hypophosphorous acid, in some cases in the presence of a catalyst, e.g., 0.05-0.10 mol% CuS04. The procedure is notable for its simplicity of operation. In Organic Syntheses the diazotization and hydro-de-diazoniation of 3,3 -dimethyl- and 3,3 -dimethoxybenzidine are described by Kornblum (1955) and the formation of 2,4,6-tribromobenzoic acid by Robison and Robison (1963). 

Reduction by hydrogen atom donors involves free radical intermediates and usually proceeds by chain mechanisms. Tri-n-butylstannane is the most prominent example of this type of reducing agent. Other synthetically useful hydrogen atom donors include hypophosphorous acid, dialkyl phosphites, and tris-(trimethylsilyl)silane. The processes that have found most synthetic application are reductive replacement of halogen and various types of thiono esters. 

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. 

Hydrosulphurous acid is also formed as an intermediate product in the reduction of sulphurous acid by hypophosphorous acid.8... 

Aqueous solutions of the salts, or of tellurium dioxide in acids, easily undergo reduction.to elementary tellurium. Phosphorus, phosphorous acid,-7 hypophosphorous acid,8 sulphurous acid,9 thiosulphuric acid,10 hyposulphurous acid,11 hydriodic acid,12 hydrogen sulphide,13 ferrous salts, stannous salts,14 hydrazine 15 and phenylhydrazine, as well as various metals,18 e.g. zinc, iron, tin, cadmium, antimony and copper, are able to effect this reduction. 

Cyclization of an allyl A-acyl-1 -phenylglycinate (398) with triphenylphosphine, hexa-chloroethane and triethylamine affords a 2-allyl-3-oxazolin-4-one (400) (81AG(E)395, 77AG(E)394). Reductive cleavage of the oxazolinone with chromium(II) acetate and aqueous hypophosphorous acid furnishes a j3,y-unsaturated ketone (Scheme 88). 

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