Bimolecular reduction

Acetone when treated in ethanol with sodium undergoes reduction mainly to isopropanol. By modifying the conditions, however, acetone may be induced to undergo a bimolecular reduction to pinacol. 

Tetraphenylethylene Glycol. This prcp iration illustnites the mild conditions under which aryl ketones may undergo bimolecular reduction to com-... 

Solvent plays a key role in the course of reductions by active metals Changing the solvent of the reaction of hexafluoroacetone with magnesium from tetrahydro turan to dimethylformamide induces bimolecular reduction to the pinacol [5] (equation 2)... 

High-boiling products found in this procedure and in similar experiments involving cyclohex-2-enone derivatives5 probably result from bimolecular reduction processes.15 3-Methylcyclohexanone, which arises by protonation rather than alkylation of the enolate (and which made up ca. 12% of the volatile products), is probably the result of reaction of allyl bromide with liquid ammonia to form the acidic species allyl ammonium bromide.5 10... 

Direct Electron Transfer. We have already met some reactions in which the reduction is a direct gain of electrons or the oxidation a direct loss of them. An example is the Birch reduction (15-14), where sodium directly transfers an electron to an aromatic ring. An example from this chapter is found in the bimolecular reduction of ketones (19-55), where again it is a metal that supplies the electrons. This kind of mechanism is found largely in three types of reaction, (a) the oxidation or reduction of a free radical (oxidation to a positive or reduction to a negative ion), (b) the oxidation of a negative ion or the reduction of a positive ion to a comparatively stable free radical, and (c) electrolytic oxidations or reductions (an example is the Kolbe reaction, 14-36). An important example of (b) is oxidation of amines and phenolate ions ... 

Addition of a masked Grignard reagent to an aldehyde or ketone From aromatic aldehydes and carbanions Bimolecular reduction of aldehydes or ketones... 

Gardfeldt K, Jonsson M. 2003. Is bimolecular reduction of Hg (II) complexes possible in aqueous systems of environmental importance J Phys Chem A(107) 4478 482. 

Acetone is reduced by amalgamated magnesium largely to a bimolecular reduction product, tetramethylethylene glycol or plnacol (CHj) jC(OH)C(OH)(CH3), some isopropyl alcohol is also formed ... 

Diaryl imines 109 > and immonium salts no) have been found to undergo bimolecular reduction to v/c-diamines under the appropriate experimental... 

The bimolecular reduction of ketones to pinacols by magnesium may involve ketyls as intermediates, the bivalence of magnesium favoring the bimolecular reduction product. Ketyl formation with sodium in liquid ammonia eventually leads to the reduction of the ketone to the alcohol. The corresponding pinacol is also cleaved to the alcohol under the same conditions.128... 

Bimolecular reductive elimination of metal (or metal complex) hydrides n2 2HCo(CO)4 <-> Co2(CO)8 + H2 12... 

Symmetrical 1,2-glycols, known as pinacols, are prepared by bimolecular reduction of aldehydes or ketones. 

The bimolecular reduction of nitro compounds is believed to involve reduction of some of the starting material to a nitroso compound and another portion to either a substituted hydroxylamine or an amine. These intermediates, in turn, condense to form the azo compound. The exact mechanism of the reaction requires critical study. On the one hand, reducing conditions are always on the alkaline side to prevent the benzidine rearrangement of an intermediate hydrazo compound under acidic conditions, yet it is difficult to visualize the formation of hydrazo compounds by the indicated condensation. As a practical matter, this method is of value only if symmetrically substituted azo compounds are desired. 

The preparation of ethyl a-azoxyisopropyl ketone is a typical example of the bimolecular reduction with stannous chloride [37]. 

The bimolecular reduction of aromatic nitro compounds, depending on reaction conditions, may proceed by way of azoxy and azo compounds to 1,2-diarylhydrazines (also referred to as hydrazo compounds). This may be... 

The nature of the azo bond is such that only a very limited number of possible functional groups can be considered to have the necessary features to serve as starting materials for reductive methods of preparation. In a sense, the Bogo-slovskii reaction [17, 18] may be considered a reduction of a diazonium salt by copper(I) ions. However, because the reaction resembles the other condensations of diazonium salts, its classification among the condensation reactions seems appropriate. The direct reduction of azoxy compounds as such is of minor preparative importance except as a method of identification of an azoxy compound. However, in the various bimolecular reduction procedures of aromatic nitro compounds, it has been postulated that an azoxy intermediate forms in the course of the reaction. This intermediate azoxy compound is ultimately reduced to an azo compound. 

The reduction of aromatic nitro compounds is believed to proceed to an intermediate mixture of nitroso compounds and substituted hydroxylamines which are not isolated but condense to form an azoxy compound which, in turn, is reduced to an azo compound. Contributing evidence to substantiate this mechanism is that the reduction of a mixture of two aromatic nitro compounds leads to a mixture of azo compounds consistent with that predicted if each of the nitro compounds were reduced to a nitroso compound and a hydroxylamine and these, in turn, reacted with each other in all possible combinations. This observation also implies that the bimolecular reduction of nitro compounds is practical only from the preparative standpoint for the production of symmetrically substituted azo compounds. Spectrophotometric studies of the reaction kinetics of the reduction of variously substituted nitro compounds may, however, uncover reasonable procedures for the synthesis of unsymmetrical azo compounds. 

Among the reductive methods of preparing azoxy compounds is the reduction of aliphatic nitroso compounds with stannous chloride. Triethyl phosphite has been used for the bimolecular reduction of fully fluorinated aromatic nitroso compounds. 

The bimolecular reduction of aromatic nitro compounds, depending on reaction conditions, may produce azoxy compounds, azo compounds, hydrazo compounds (1,2-diarylhydrazines), benzidines, or amines. Whereas the reduction with zinc and sodium hydroxide leads to azo compounds, zinc and acetic acid/acetic anhydride produces azoxy compounds. Other reducing agents suggested are stannous chloride, magnesium with anhydrous methanol, a sodium-lead alloy in ethanol, thallium in ethanol, and sodium arsenite. 

The bimolecular reduction of aliphatic nitroso compounds is complex and somewhat unreliable. With careful control of reaction conditions, a-nitroso ketones (in dimeric form) may be reduced with stannous chloride in an acidic medium at room temperature to the azoxy compounds, while dimeric a-nitroso acid derivatives may be reduced at about 50°C [10, 35, 36]. Nitrosoalkanes, on the other hand, are decomposed at room temperature to alcohols and nitrogen, and are reduced to amines at 50°-60°C. It has been postulated that only the dimeric nitroso compounds can be reduced to azoxy compounds and, in fact, that the dimer has a covalent nitrogen-nitrogen bond. Equations (31)—(34) summarize these data [10]. 

The details of the mechanism for the conversion of nitrobenzene into azoxy-benzene need further amplification. It also should be pointed out that, in the preparation of azo compounds by bimolecular reduction, hydrazo compounds seem to form invariably since the directions invariably call for the reoxidation of the hydrazo product with air (see Chapter 14, Azo Compounds and Meisenheimer and Witte [39]). 

Direct electron transfer We have already met some reactions in which the reduction is a direct gain of electrons or the oxidation a direct loss of them. An example is the Birch reduction (5-10), where sodium directly transfers an electron to an aromatic ring. An example from this chapter is found in the bimolecular reduction of ketones (9-62), where again it is... 

Bimolecular Reduction of Aldehydes and Ketones to 1,2-Diols 21O-Hydrogen-coupling... 

Bimolecular Reduction of Aldehydes or Ketones to Alkenes De-oxygen-coupling... 

Thiophenecarbaldehydes add smoothly to a,f3-unsaturated ketones and nitriles under cyanide ion catalysis to form y-diketones (366) and y-ketonitriles (367) respectively (76CB534). The 2,5-dicarbaldehyde gives the bis-adduct (368). The aldehydes undergo normal reduction to the hydroxymethylthiophenes by sodium borohydride. However, electrochemical reduction of the 2,5-dialdehyde on a mercury electrode at pH 1-3 gives the bimolecular reduction product (369) as a mixture of meso- and ( )-forms in the ratio 7 3. Reduction with zinc and acetic acid gives only the meso -form of (369) (75CR(C)(280)165>. 

Bimolecular reduction of ketones has also been recorded. Thus electrochemical reduction of the ketones (378) gave the pinacols (379) <74CR(C)(279)779,74CR(C)(279)25>. 

Reduction of aryl nitro compounds with less-powerful reducing agents, especially in alkaline media, gives what may appear to be a mysterious conglomerate of bimolecular reduction products. For example, with nitrobenzene,... 

All of these substances can be reduced to benzenamine with tin and hydrochloric acid. As a result, each could be, but not necessarily is, an intermediate in the reduction of nitro compounds to amines. Formation of the bimolecular reduction products is the result of base-induced reactions between nitroso compounds and azanols or amines and possibly further reduction of the initially produced substances (see Exercise 24-18). 

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