Aldehyde pinacol radical reaction

Probably the most familiar radical reactions leading to 1,2-D systems are the so called acyloin condensation and the different variants of the "pinacol condensation". Both types of condensation involve an electron-transfer from a metal atom to a carbonyl compound (whether an ester or an aldehyde or a ketone) to give a radical anion which either dimerises directly, if the concentration of the species is very high, or more generally it reacts with the starting neutral carbonyl compound and then a second electron is transferred from the metal to the radical dimer species (for an alternative mechanism of the acyloin condensation, see Bloomfield, 1975 [29]). 

One typical radical reaction is a coupling reaction. Oxidative decarboxylation coupling reaction of carboxylic acids by electrolysis (Kolbe electrolysis), intramolecular coupling reaction of diesters with Na (acyloin condensation), formation of pinacols from ketones or aldehydes with Na or Mg are well known classical methods [1,2]. Recently, oxidative... 

Pinacol coupling reactions also involve radical-radical recombinations thus, enan-tiocontrol in these reactions remains a daunting problem. Initially it was found that addition of TMEDA as an additive increased the solubility of TiCl2, thus increasing the conversion of the aldehyde to diol product [19]. It was hypothesized that addition of an optically active amine would not only help to solubilize the Lewis acid, but also offer enantiocontrol in the coupling. When chiral diamines such as 41 were added, modest levels of enantioselectivity were achieved (Eq. 14). 

The same idea allows us to make symmetrical 1,2-diols 33 by the pinacol reaction. Again we avoid cations and anions by making both halves radicals. These are generated by addition of electrons from metals to aldehydes and ketones. So an electron from sodium adds to acetone to give a radical anion 35 that might dimerise to give 33. 

The ketyl radical anion intermediates can be exploited in carbon-carbon bond-forming reactions. Intermolecular and intramolecular pinacol couplings between the carbonyl groups of ketones and aldehydes are well known (Chapter 5, Section 5.1), as are intermolecular and intramolecular carbonyl-alkene couplings (Chapter 5, Section 5.2). 

You ve seen examples of pinacol and McMurry reactions of ketones and aldehydes. What about esters You would expect the ketyl radical anion to form from an ester in the same way, and then to undergo radical dimerization, and this is indeed what happens. 

The relative ease of pinacolization is primarily determined by the reduction potential of the carbonyl group involved. Many reductants are therefore selective for aromatic and other electronically activate systems. Moreover, as a result of this ready reduction, pinacolization of such carbonyls can be effected by either anionic or radical routes. For example, treatment of aromatic aldehydes or ketones with Mg/TMSCl in HMPA promotes pinacolization via formation of an a-silyloxy carbanion - and nucleophilic attack on a second carbonyl group (equation 2). Furthermore, with benzaldehyde the reaction is stereodirecting with a preference for /it-coupling. Whilst an alternate coupling metht using the milder... 

In fact, presenting photoreactions as involving biradical intermediates was natural. The energy of UV photons is of the same order as that of covalent bonds, and in photochemical reactions bonds were cleaved and formed. Thus, with aldehydes and ketones, the light-activated state was not uimaturally drawn as a biradical, with a large or small separation of the radical sites and the reaction pictured as a radical process, e.g., in hydrogen abstraction from alcohols to give pinacols [33, 34]. 

Aqueous TiCU also promotes a chemoselective reduction of a,p-dicar-bonyl compounds to give ketyl radicals that add to the carbonyl carbon of aldehydes affording a,p-dihydroxyketones. The reaction yields are excellent and the diastereoselection depends on the steric hindrance of the groups bonded to the reagents [80e]. When the ketyl radical is generated from methyl phenylglyoxalate and added to p-ketoesters and p-keto acids, the intermediate pinacols rapidly afford lactones by intramolecular cyclization [80d]. 

---