Ketone dianions, ketone-ketyl coupling, and possible reaction-during-mixing effects in alkali metal/ammonia reductions of saturated ketones

✍ Scribed by Valentin Rautenstrauch

• Publisher: Elsevier Science
• Year: 1988
• Tongue: French
• Weight: 397 KB
• Volume: 44
• Category: Article
• ISSN: 0040-4020
• DOI: 10.1016/s0040-4020(01)86722-3

✦ Synopsis

The hypothesis of Pradhan that alkali metal/NH3 reductions of saturated ketones lead to ketone &anions which then deprotonate unreacted ketone can be excluded because the dianions would have been protonated in experiments in which 50-110 equiv of tert.-Bush, Et&L, and H20 were present in the reaction medium: the phenomenon interpreted in terms of the above deprotonation persisted in the presence of these proton sources and is probably instead due to a radical disproportionation of the monoanions (ketyls). Another hypothesis of Pradhan, that pinacol coupling of saturated ketones by Li dissolving in NH3 involves ketone-ketyl coupling, is implausible because this coupling would be thermodynamically highly unfavorable. Possible alternative interpretations of Pradhan's findings are proposed. Alkali metal/NH3 reductions of enolizable ketones R-CH2-CO-R, R = alkyl or alkenyl with a remote double bond, for example 1 in Scheme I, give 1:l mixtures of the corresponding enolate 6 and alcoholate 7' _ . We believe' that the underlying mechanism is a radical disproportionation2 of the M+ ketyls 2, thus of the monoanions of 1, cf. Scheme II. We think that these disproportionations are likely to have all the characteristics of normal radical disproportionations2, for example, neardiffusion-controlled rates. This opinion is to some extent justified by the fact that the selfreactions (disproportionation and combination) of the corresponding ketyl radicals (which would result if the ketyls 1 and 1 were protonated) do have the same characteristics as those of alkyl radicalsebp3. Pradhan has instead hypothesized' that the 1:l enolate 6 alcoholate 7 mixture is formed through deprotonation of the ketone 1 by its dianions 9 or 5, cf. Scheme I.