Acyl anions samarium

In the absence of the aldehyde, homocoupled pinacol and a ctxnpound resulting from a double carbon-ylation are isolated in low yields. Both products are consistent with initial formation of a samarium acyl anion. 

In addition to the carbon monoxide insertion route, samarium acyl anions can also be prepared under reductive conditions by reaction of SmI with acyl halides. " In the absence of any other electrophiles, the acyl halides provide moderate yields of a-diketones (equation 77). The main by-product generated in these reactions is the a-ketol. 

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 . ... 

Samarium acyl anions can be trapped by electrophiles other than acid halides. For example, addition of a mixture of a carboxylic acid chloride and an aldehyde or ketone to a solution of Sml2 in THF results in the synthesis of a-hydroxy ketones (equations 78 and 79). Intramolecular versions of the reaction have also been performed, although the scope of the reaction is limited owing to the difficulty in obtaining suitable substrates for the reaction (equation 80). ... 

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). 

Souppe et al. (1984) have also elucidated mechanistic aspects of diiodosamaiium/acid chloride reaction systems that produce symmetrical a-dike-tones via acyl anions. Nucleophilic acylation occurs through in situ trapping when an acid chloride is reduced by samarium(II) iodide in the presence of aldehydes or ketones. 

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). 

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]. 

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). 

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