Acyl radicals samarium

Mechanistic studies strongly suggest the intermediacy of a samarium acyl anion. For example, the phe-nylacetyl radical (PhCHiCO ) is known to rapidly decarbonylate k = 5.2 X 10 s" ), providing a benzyl radical which dimerizes to bibenzyl. However, addition of phenylacetyl chloride to a solution of Sml2 in THF leads to a 75% yield of the expected diketone, and neither toluene nor bibenzyl is detected. Apparently, intermolecular reduction of this acyl radical to the corresponding anion proceeds at a rate which is greater than 5.2 x 10 s . ... 

Intramolecular trapping studies have verified the intermediacy of acyl radicals in the conversion of carboxylic acid chlorides to samarium acyl anions by Smli. Treatment of 2-allyloxybenzoyl chlorides with Sml2 resulted in a very rapid reaction, frtrni which cyclopropanol products could be isolated in yields of up to 60% (equation 8iy Apparently, initial formation of the acyl radical was followed by rapid radical cyclization. The 3-keto radical generated by this process undergoes cyclization by a radical or anionic process, affording the observed cyclopropanols (Section 1.9.2.3.1). 

For example, a solution of acrylamide and thiol ester (1.5 equiv) in THF was added dropwise to a 0.1 M solution of Sml2 cooled to -78 °C. After the mixture was stirred for 1 h, followed by oxidation of the excess Sml2 with oxygen, y-keto-amide was formed in 49% yield. That decarbonylation (as shown in Eq. 29) is not observed implies that the reacting species is not a free acyl radical. It is proposed that electron transfer into the carbonyl group could generate a ketyl radical anion equivalent to the structure 44 coordinating to samarium(III) (Scheme 10). 

A very fast reaction occurs between acid chlorides RCOCl and two equivalents of diiodosamarium in THF solution at room temperature (Girard et al., 1981). The a-diketone RCOCOR is obtained in good yields (table 11). The mechanism of this unusual coupling should start from an acyl radical, RCO , which either dimerizes or is further reduced into a samarium acyl anion (fig. 7). 

Preparation of chiral nitroxide radicals is important because of their potential as molecules with unique optoelectronic properties. The radical enolate 151 was generated from 150 on treatment with samarium(II) diiodide at low temperatures [95JOC6820]. The intermediate enolate was acylated to provide the stable free radical 152 in good yield. 

The secondary reduction of the terminal radical by Sml2 generates samarium alkyl species which are suitable for classical organometallic reactions, e.g. protonation, acylation, reactions with carbon dioxide, disulfides, diselenides, or the Eschenmoser salt. A broad variety of products is available (hydroxy-substituted alkanes, esters, carboxylic acids, thioethers, selenoethers, tertiary amines) by use of the double-redox four-step (reduction-radical reaction-reduction-anion reaction) route (Scheme 20) [73]. 

Scheme 5.10 Samarium(ll) iodide-induced domino radical cyclization/acylation reaction.

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