Magnesium amalgam reaction with ketones

These dimerizations are analogous to those of the radical anions R2C 0 which are intermediates in the reduction of ketones to pinacols. Indeed, in the presence of magnesium amalgam, pyridine condenses with acetophenone to give alcohol 396 by oxidation of the intermediate dihydropyridine. In a similar reaction type, pyridine with zinc and acetic anhydride or ethyl chloroformate yields (397 R = Me or OEt, respectively). 

This might be achieved by reacting a ketone with magnesium amalgam in dry ether, under anaerobic conditions, i.e. without oxygen present. Accordingly, write down the complete reaction pathway for the synthesis of pinacol itself, (CH3)2C(OH)C(OH)(CH3)2, starting with Mg and propanone. 

Emmert reaction. Formation of 2-pyridyldial-kylcarbinols by condensation of ketones with pyridine or its homologs in the presence of aluminum or magnesium amalgam. 

Pinucolic coupling of ketones and aldehydes. The reaction of TiCU and 70 -80 mesh magnesium amalgamated with HgCIa leads to a Ti(H) species that is effective for intermolecular coupling of aldehydes and ketones as shown in the examples. ... 

A convenient procedure for the reduction of small amounts of ketones involves the periodic addition of small pieces of sodium to a slowly stirred mixture of an ethereal or benzene solution of the ketone and water or a concentrated solution of sodium carbonate. Sodium and alcohol are used for the conversion of methyl n-amyl ketone to 2-heptanol (65%). These reagents are used to prepare secondary alcohols from olefinic ketones obtained by the aldol condensation. Benzophenone and related compounds are reduced by zinc dust and sodium hydroxide, magnesium and methanol, and sodium amalgam. With the last reagent the reaction has been shown to take place through the intermediate sodium ketyl, (C,H,)jCONa. 

By use of weaker reductants, the C-C coupling can be stopped at the stage of the pinacols [48]. The best metals for achieving chemical pinacolization from ketones are magnesium in the form of its amalgam and mixtures of Zn-Hg with titanium tetrachloride (Scheme 11). In the latter reaction the Ti(II) species presumably initiates electron transfer [49]. 

Another interesting example of the Barbier reaction is that of 3-butylpyridine with aliphatic ketones in the presence of amalgamated magnesium [40]. With ethyl methyl ketone, 5-butyl-a-ethyl-a-methyl-2-pyridine methanol is isolated, in 70% yield. The corresponding a,a-dimethyl derivative is isolated when acetone is used as the ketone source. The ketyl-coupling products, R,R C(OH)-C(OH)R,R, are also produced in both reactions. 

Although reduction of ketones with alkali metals, for example, sodium (Na°) in ethanol (CH3CH2OH), to the corresponding alcohol, as already noted, has been replaced with complex hydrides, a minor product in the original reaction, the result of coupling two ketones at their respective carbonyl carbons, was occasionally found on workup of the reaction mixture. The diol (a pinacol) thus formed could be made the major product if the reaction was carried out with magnesium (Mg°) or amalgamated aluminum (Al-Hg) instead of sodium (Na°). The process, which doubtlessly involves a series of one-electron transfer reactions, is shown in Equation 9.21. 

Emmert and Asendorf showed that when ketones were heated with pyridine and amalgamated magnesium or aluminium, 2- and 4-hydroxy-alkylpyridines resulted. The yields from the reaction are moderate to good, and it has been used preparatively (the literature is summarized ). 

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