Radical reactions carbonyl derivatives

Radical reaction products, derived from the reagents, which are often found in Barbier reactions, may result from interaction of the radical XMgs with the carbonyl group of the substrate. 

Radical anion EGBs derived from aromatic carbonyl compounds are expected to be relatively weak bases but since the radical anions undergo dimerization, the more basic dimer dianions may be active as EGBs for substrates with pK values in the range 20 to 23. Aromatic carbonyl compounds have primarily been used as PBs in catalytic reactions in which the PB also functions as an electrophile (cf. Sect. 14.9.2). 

It is worth mentioning that in a few cases the (3-elimination of the silyl radical from the a-silyl alkoxyl radical (47) with the formation of corresponding carbonyl derivative was observed [63,64]. Evidently the fate of a-silyl alkoxyl radical depends on the method of radical generation and/or the nature of the substrate. Two examples that delineate the potentialities of this rearrangements are reported in Reactions (5.33) and (5.34). In the former, the 5-exo cyclization of secondary alkyl radical on the carbonyl moiety followed by the radical Brook rearrangement afforded the cyclopentyl silyl ether [65], whereas Reaction (5.34) shows the treatment of an a-silyl alcohol with lead tetracetate to afford the mixed acetyl silyl acetal under mild conditions [63]. 

The dissociation of carbonyl compounds. Many carbonyl derivatives undergo a homolytic dissociation between the GO group and a saturated hydrocarbon chain e.g. Figure 4.33). The stability of the free radical R plays a very important role in the efficiency of this reaction. In a series of aromatic carbonyl compounds dissociation is not observed with benzophenone (R = C6H5), benzaldehyde (R = H) or acetophenone (R = CH3), but is observed with the longer R chains. 

In the elaboration of /Mactam derivatives, an allyl group has been introduced alpha to the carbonyl group by reaction of an a-bromo-/Mactam with an allyl trialkylstannane under radical conditions (equation 113)704. The allyl group is introduced in a stereospecific fashion, thus indicating how useful radical reactions can be in the synthesis of complex molecules ... 

A tandem carbonylation-cyclization radical process in heteroaromatic systems bearing electron-attracting substituents such as l-(2-iodoethyl)indoles and pyrroles 970 result in the formation of 2,3-dihydto-l//-pyrrolo[l,2- ]indol-1-ones and 2,3-dihydro-l//-pyrrolizin-l-ones 974 (Scheme 188). The AIBN-induced radical reaction of compounds 970 with Bu3SnH under pressure of CO suggests that the acyl radical 972, derived from radical 971 and CO, would undergo intramolecular addition to C-2 of heteroaromatic system, and the benzylic radical 973 so obtained, upon in situ oxidation would produce final product 974 <1999TL7153>. 

Miyazoe et al. introduced silyltellurides as partners for group-transfer three-component couplings with isocyanides (Scheme 60) [ 170]. The reaction probably started through initial addition of silyl radicals to a carbonyl derivative, yielding ketyl radical 185, which underwent addition to phenylisocyanide (a Cl radical synthon). Final phenyltelluride group transfer to intermediate... 

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