Compounds to Pinacols

Imines can be stereoselectively hy-drodimerized to 1,2-diamines with rac/ meso rahos of 0.9 to 1.1 similar to the cathodic coupling of carbonyl compounds to pinacols in an acidic medium [304]. With 4,6-dimethylpyrimid-2-one only the meso-diamine was obtained [305]. Electroreduction of diimines prepared from... 

The reductive coupling of carbonyl compounds to pinacols (i.e., 1,2-diols) is usually performed in the presence of a low-valent metal such as Li(0), Sm(II) or Ti(III). Under... 

Preparative-scale conversion of aromatic carbonyl compounds to pinacols is best carried out in aqueous alkaline solution. The pinacol is isolated as a mixture of stereoisomers. Reduction in acetonitrile or dimethylformamide can under some circumstances, discussed below, favor formation of the ( ) pinacol. Data for the ) meso ratio for pinacols from acetophenone and propiophenone are collected in Table 6. 

An acyclic v c-diimine was hydrogenated at a mercury cathode to give a mixture of the dj- and me o-diamines in 70% yield [183]. Imines can be stereoselectively hydrodi-merized to 1,2-diamines in a manner similar to the hydrodimerization of carbonyl compounds to pinacols [184,185]. 

Mukaiyama, T., Sato, T., Hanna, J. Reductive coupling of carbonyl compounds to pinacols and olefins using titanium(IV) chloride and zinc. Chem. Lett. 1973, 1041-1044. 

Imamoto, T., Kusumoto, T., Hatanaka, Y., and Yokoyama, M., Reductive coupling of carbonyl compounds to pinacols by using low-valent cerium. Tetrahedron Lett., 23,1353, 1982. 

Aldehydes and ketones can be hy-drodimerized to pinacols (Eq. 2) [34-37]. With aromatic carbonyl compounds, the yields and selectivities are mostly higher than with aliphatic ones. The reaction has been extended to imines (Eq. 2, X = NAr, N-Bn) [38-41] and to heterohydrodimerizations affording, for example, y-lactones (Eq. 3) [42-44]. 

Cathodic reduction of retinal leads to a regioselective coupling in the presence of malonic ester to produce the corresponding pinacol (Fig. 26) [129]. Chromium(III) ions facilitate the reduction and favor the regioselective coupling of conjugated dienones to pinacols. A Cr(III)-carbonyl compound complex is evidenced as the reason for the selectivity [130]. 

Reductions with aluminum are carried out almost exclusively with aluminum amalgam. This is prepared by immersing strips of a thin aluminum foil in a 2% aqueous solution of mercuric chloride for 15-60 seconds, decanting the solution, rinsing the strips with absolute ethanol, then with ether, and cutting them with scissors into pieces of approximately 1 cm [141,142]. In aqueous and non-polar solvents aluminum amalgam reduces cumulative double bonds [143], ketones to pinacols [144], halogen compounds [145], nitro compounds [146, 147], azo compounds [148], azides [149], oximes [150] and quinones [151], and cleaves sulfones [141, 152, 153] and phenylhydrazones [154] (Procedure 30, p. 212). 

Although benzil was the first a-dicarbonyl compound to be investigated,1-4 its photoreactions have not been as thoroughly studied as those of biacetyl. By 1916 it had been established that a precipitate was formed when a solution of benzil in ether,1,3 ethanol,2 aldehydes,3 or alkyl substituted benzenes3 was exposed to sunlight. The thermal and photochemical instability of the precipitate caused some confusion about its structure until Cohen demonstrated that it was pinacol 10 which decomposed to benzil and benzoin on heating.4... 

Treatment of porphyrins with hydrogen peroxide and sulfuric acid, or with osmium tetroxide, results in formation of a dihydroxychlorin (44) under acidic conditions these compounds suffer pinacol rearrangement to give compounds, such as (45), known as gemini-ketones (67TL2185,69JCS(C)564). 

Bolourtchian, M., Zadmard, R. and Saidi, M.R., Microwave promoted reductive coupling of carbonyl compounds to bis(trimethylsilyl) pinacols under solvent-free conditions, Synth. Commun., 1998,28, 2017-2020. 

Synthetically, the question of which product is formed from a given carbonyl compound, the pinacol or the alcohol, is obviously of great interest. We have already noted the critical importance of the proton-donating ability of the medium in this respect. Attention has turned to the effect of other variables on the pinacol/alcohol ratio. Formaldehyde can be reduced cleanly to ethylene glycol at an electrode composed of a special type of carbon, in a process of potential commercial significance50. Nonaka and coworkers have found that reduction of aldehydes proceeds more selectively to aldehydes at nickel-Teflon-silica gel composite electrodes51. Nonaka and coworkers have also shown that for aromatic... 

The reduction of carbonyl compounds to form pinacol dimers can be accomplished photochemically, electrochemically or with chemical reducing agents. When conducted under acidic conditions or in protic solvents, pinacols are likely produced by coupling of two neutral ketyl radicals (vs radical anions). The electrochemical reduction is especially complicated in terms of the role of the electrode surface, counterion and solvent, and an excellent review has appeared on the subject32. 

This reaction involves the reductive homo-coupling of a carbonyl compound to produce a symmetrically substituted 1,2-diol. TheJQrst step is single electron transfer of the carbonyl bond, which generates radical ion intermediates that couple via carbon-carbon bond formation to give a 1,2-diol. The example depicted above shows the preparation of pinacol itself. 

Since the introduction of the titanocene chloride dimer 67a to radical chemistry, much attention has been paid to render these reactions catalytic. This field was reviewed especially thoroughly for epoxides as substrates [123, 124, 142-145] so only catalyzed reactions using non-epoxide precursors and a few very recent examples of titanium-catalyzed epoxide-based cyclization reactions, which illustrate the principle, will be discussed here. A very useful feature of these reactions is that their rate constants were determined very recently [146], The reductive catalytic radical generation using 67a is not limited to epoxides. Oxetanes can also act as suitable precursors as demonstrated by pinacol couplings and reductive dimerizations [147]. Moreover, 5 mol% of 67a can serve as a catalyst for the 1,4-reduction of a, p-un saturated carbonyl compounds to ketones using zinc in the presence of triethylamine hydrochloride to regenerate the catalyst [148]. 

Electrolytic formation of carbon-carbon bonds occurs in the reduction of ketones to pinacols, in the Kolbe synthesis, and in the hydrodimerization of activated double bonds. Of these only the last reaction has been used in the preparation of heterocyclic compounds. 

Cyclic immonium salts such as 205 are reduced in a nonaqueous solvent such as acetonitrile through a radical to a dimer (206).207 The reaction is analogous to pinacol formation from carbonyl compounds. 

Electrolysis of carbonyl compounds provides pinacols, alcohols or hydrocarbons, depending on the conditions, such as pH, the nature of the electrode, and its potential. Fundamental studies have been carried out on the mechanisms of hydrocarbon formation using acetone as a substrate. Although several electrodes, such as Cd, Pt, Pb or Zn, are recommended, carbonyl compounds, including aryl and alkyl derivatives, require strong aqueous acidic media for reduction to the hydrocarbons. The mechanism of the electrolytic reduction is probably similar to that of Clemmensen reduction, which starts from anion radical formation by one-electron transfer, as indicated in Scheme 3. The difference is that electrolytic reduction takes place in an electric double layer, rather than on the surface of the zinc metal. 

Many common rearrangement reactions are related to the rearrangement of 1,2-dihydrojqr compounds to carbonyl compounds. Often these reactions are called pinacol rearrangements, because one of the first examples was the transformation of pinacol to pinacolone ... 

Meyer (17) in the following year then expressed the view that the products of the action of nitrous acid on the ketones were not true nitroso compounds (4) but isomeric oximes (5), and he considered their reduction to be analogous to the conversion of a nitro to amino group, rather than that of ketone to pinacol. Thereafter the Treadwell formulation of pyrazines was abandoned. 

The stereochemistry of the cathodic reaction of carbonyl compounds has been extensively studied from synthetic and mechanistic aspects. In this section the reaction is discussed in three parts hydrogenation to alcohols, hydrodimerization to pinacols, and cross-dimerization with unsaturated compounds. 

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