Hydrodimerization of Carbonyl Compounds

The hydrodimerization of aromatic or aliphatic aldehydes and ketones yields pinacols (.100) (Eq. (184) ). 

The reduction mechanism of carbonyl compounds and its dependence on pH has been outlined in section 8.2. Pinacol formation occurs either by dimerization of the hydroxymethyl radical 101 (Eq. (185) ) or by mixed coupling of 101 with the ketyl 102. Dimerization of 102 seems less probable due to electrostatic repulsion of the two negative charges. Besides coupling, 101 or 102 may be further reduced to the alcohol 103. With active cathodes (e.g., Hg, Sn) 101 forms organo- 

Pinacol yields are normally better with aromatic carbonyl compounds. Some examples are given in Table 14. A more extensive coverage - especially of the older literature - has been given by Allen 6 Popp and Schultz 9 and Fichter 4). 

Pinacol (30-40%) 2,2 -Dihydroxy-3,3 -dimethoxy-hydrobenzoin (70%) 4,4 -Bis(acetamido)hydrobenzoin (80%) 

Available data on pinacol formation are not consistent enough to allow firm prescriptions on conditions for optimal yields. Pinacols are obtained at low and high pH, though a low pH may be more favorable. The dependence of yields on the cathode metal and the SSE are not yet fully understood. Too low and too high a pH are to be avoided to suppress nonelectrolytic pinacol rearrangements or aldol condensations as side reactions. 

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Scheme 167 Samarium-mediated cathodic hydrodimerization of carbonyl compounds.

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]. 

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. 

An oxidation using a nickel hydroxide electrode is shown in 15.8.433 Electrochemistry is also a way to produce radicals and anions. The hydrodimerization of acrylonitrile to adiponitrile just mentioned may involve the coupling of free radicals. The coupling of carbonyl compounds, such as p lolualdehyde, to form pinacols with up to 100% selectivity, by way of free radicals, can be done electrically.434 Anions can also be formed electrochemically and used in situ, as in example (15.9).435... 

Steric factors are also important in hydrodimerizations carried out in acidic media. Excessive steric hindrance about the 0-carbon in an a, 0-unsaturated carbonyl compound can retard tail-to-tail coupling, e.g., 2 130 - 131, and lead to products of head-to-head (and occasionally head-to-tail) coupling. Thus in the reduction of mesityl oxide at pH lg.4 there is also formed a small amount of ketone 140, apparently formed via head-to-head coupling of 130 and subsequent pinacol rearrangement of 139 134) ... 

Intermolecular coupling Many papers on hydrodimerization of aromatic carbonyl compounds have appeared indicating the importance of this reaction. The rac/meso ratio for the pinacolization of acetophenone in aqueous ethanol ranges between 0.9 and 1.4 in acidic medium and between 2.5 and 3.2 in basic medium. The diastereoselectivity is independent of the cathode material mercury, tin, or copper. Electrolysis conditions such as current density, potential, or current-controlled electrolysis also do not influence the diastereoselectivity. The same holds for propiophenone. For benzaldehyde, the rac/meso ratio is 1.1 to 1.2 in acidic as well as in basic media [283]. In the presence... 

There is insufficient information on the stereochemistry of the experimentally less simple hydrodimerization of aliphatic carbonyl compounds in a protic... 

Cathodic hydrodimerization of active olefins and carbonyl compounds. 

Schiff bases of substituted benzaldehydes can be hydrodimerized at the mercury cathode in ethanol-ethyl acetate - water - Bu4N+Br in 25% to 66% yield to the diastereomeric 1,2-diamino compounds 407 Coelectrolysis of a Schiff base with a carbonyl compound forms aminoalcohols. 

Some interesting results have been reported on this subject. Although most stereochemical investigations of hydrodimerization have been performed using aromatic carbonyl compounds with unsubstituted and substituted phenyl groups, some examples dealing with other aromatic groups have appeared [141-144,154-156]. The hydrodimerization of... 

The stereochemistry of hydrodimerization of aliphatic carbonyl compounds has very rarely been studied. It was reported that an aliphatic ketone gave an almost equal quantity of the isomeric hydrodimers at a mercury cathode [160]. 

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