Complex hydride reduction nitro compounds

With the exception of the nitroso stage, all the intermediate stages of the reduction of nitro compounds can be obtained by controlled catalytic hydrogenation [572], and all reduction intermediates were prepared by reduction with appropriate hydrides or complex hydrides. However, the outcome of many hydride reductions is difficult to predict. Therefore more specific reagents are preferred for partial reductions of nitro compounds. 

Reduction of nitro compounds with complex hydrides. 135... 

Hydroxylamines can be synthesized from various aliphatic and aromatic nitro compounds by reduction with metals and other one-electron donors, with complex hydrides and other two-electron donors, and by hydrogenation. In all cases the reduction proceeds stepwise and, depending on reaction conditions, can provide both amines and hydroxylamines. 

A simultaneous reduction-oxidation sequence of hydroxy carbonyl substrates in the Meerwein-Ponndorf-Verley reduction can be accomplished by use of a catalytic amount of (2,7-dimethyl-l,8-biphenylenedioxy)bis(dimethylaluminum) (8) [33], This is an efficient hydride transfer from the sec-alcohol moiety to the remote carbonyl group and, because of its insensitivity to other functionalities, should find vast potential in the synthesis of complex polyfunctional molecules, including natural and unnatural products. Thus, treatment of hydroxy aldehyde 18 with 8 (5 mol%) in CH2CI2 at 21 °C for 12 h resulted in formation of hydroxy ketone 19 in 78 % yield. As expected, the use of 25 mol% 8 enhanced the rate and the chemical yield was increased to 92 %. A similar tendency was observed with the cyclohexanone derivative. It should be noted that the present reduction-oxidation sequence is highly chemoselective, and can be utilized in the presence of other functionalities such as esters, amides, rert-alco-hols, nitriles and nitro compounds, as depicted in Sch. 10. 

Two novel complex hydrides are likely to find applications in steroid chemistry lithium perhydro-9b-boraphen yl hydride affords unusually high proportions of axial alcohols in model compounds sodium bis(methoxyethoxy)aluminium dihydride, Na (MeOCH2CH20)JAlH2, a very safe and convenient substitute for lithium aluminium hydride, readily reduces not only ketones but also acids, nitro-compounds, oximes, amides, lactones, etc. An improved procedure for Clemmensen reduction of steroid ketones involves saturating an ethereal solution with hydrogen chloride while stirring with zinc. 5a-Cholestane was obtained from the 3-one in 89% yield. ... 

A trinuclear cobalt(I) complex, PhCCo3(CO)9, can also catalyse the reduction of nitro compounds in the presence of hydroxide ion at room temperature under a normal pressure of CO [49]. Satisfactory results were obtained under phase transfer conditions. The catalyst and the aromatic nitro compounds were dissolved in benzene under carbon monoxide and an aqueous solution of sodium hydroxide containing cethyltrimethylammonium bromide was added. At a substrate/cat =10 ratio, ca. 60-80 % of amine was obtained in a 18 h reaction. The reaction also proceeded in a homogeneous phase (methanol-water, methanol, dioxane-water) but with lower conversions (less than 45 %). Cobalt complexes such as MeCCo3(CO)9 and MeCo(CO)4 were also active, but less effective. At the end of the reaction, the catalyst was recovered only in part (ca. 15 %). In the organic phase, an IR absorption at 1891 cm, attributable to [Co(CO)4] anion, was observed. Strangely enough, the preformed [Co(CO)4] anion has not been tested as catalyst. The active species was supposed to be the hydride cluster anion reported in Scheme 6. 

Other complex metal hydrides can only rarely be applied to reduction of C=C bonds. Sodium borohydride, which can be used in aqueous-alcoholic solution, seems not normally to attack ethylenic bonds. A few cases only of partial reduction of cyclic iminum salts96 and of selective reduction of unusually activated ethylenic bonds97 have been reported. However, some polynitro aromatic compounds, e.g., 1,3,5-trinitrobenzene, can be converted in high yield with saturation of the aromatic system into nitrocyclohexanes or nitro-cyclohexenes.98 Sodium hydridotrimethoxyborate has proved valuable as a mild reducing agent for preparation of a series of nitroparaffins from nitro-alkenes."... 

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