Radicals a-acyl

Arylation of activated double bonds with diazonium salts in the presence of copper catalysts is known as the Meerwin reaction. The reaction is postulated to either proceed through an organocopper intermediate or through a chlorine atom transfer from chiral CuCl complex to the a-acyl radical intermediate. Brunner and Doyle carried out the addition of mesityldiazonium tetrafluoroborate with methyl acrylate using catalytic amounts of a Cu(I)-bisoxazoline ligand complex and were able to obtain 19.5% ee for the product (data not shown) [79]. Since the mechanism of the Meerwin reaction is unclear, it is difficult to rationalize the low ee s obtained and to plan for further modifications. 

For the route A, acyl radicals donors like iS are readily generated from acyl selenides (ISa) or acyl cobalt derivatives (iSb) and radicals acceptors 2S are usually multiple bonds as in methyl vinyl ketone (2Sa) -although some homolytic substitutions are possible. On the other hand, nitriles GSal are useful acceptors (3S) in radical cyclisations and 4Sa is an obvious synthon equivalent of radical donor 4S (See Table 7.2). 

The described imidoylation of a-acyl radicals deserves great interest since a-acyl compounds are not only versatile building blocks but also exhibit several biological activities. ... 

Houck and coworkers postulate that the origin of the regioselectivity is at the biradicalforming step and directly affected by the polarity of the alkene. The /J-carbon, considered as nucleophilic, adds rapidly to the less substituted side of the electron-deficient alkene, whereas a position considered as an a-acyl radical (more electrophilic than an alkyl radical) adds rapidly to the less substituted side of electron-rich alkenes. The calculated relative energies for the addition of jtjt triplet acrolein to different substituted alkenes at the first bond-forming step (Table 3) are found to be in good agreement with experimental values determined in the photoaddition of cyclohexenone to the related alkene. 

Nucleophilic radical 79, derived from bromide 78, is expected to add intra-molecularly to the eleetrophilic double bond to yield a-acyl radical 80, but it can also add to the radicophilic alkene group to yield captodative species 81 (Scheme... 

The formation of eight-membered rings by radical cyclization has been found to occur using a-acyl radical species. The reaction occurs at a rate that is faster remarkably even than 5-exo-trig cyclization. Hence, the eight-membered lactone 92 was formed, rather than a five-, six- or seven-membered lactone, on treatment of the bromide 91 with tributyltin hydride (4.72). 

Generation of a-Acyl Radicals. As a one-electron oxidant, Ce can promote the formation of radicals from carbonyl compounds. In the presence of interceptors such as butadiene and alkenyl acetates, the a-acyl radicals undergo addition. The carbonyl compounds may be introduced as enol silyl ethers, and the oxidative coupling of two such ethers may be accomplished. Some differences in the efficiency for oxidative cyclization of, s-, and ,f-unsaturated enol silyl ethers using CAN and other oxidants have been noted (eq 14). ... 

In this case, a facile reaction occurred. On the basis of theoretical as well as kinetic and product studies, the reactions were interpreted as proceeding through attack of one IV at the carbonyl carbon, forming an a-acyl radical intermediate. Then the intermediate radical reacts with another IV at C ... 

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