Allylic radicals reductive elimination

The reductive elimination of a variety of )3-substituted sulfones for the preparation of di-and tri-substituted olefins (e.g. 75 to 76) and the use of allyl sulfones as synthetic equivalents of the allyl dianion CH=CH—CHj , has prompted considerable interest in the [1,3]rearrangements of allylic sulfones ". Kocienski has thus reported that while epoxidation of allylic sulfone 74 with MCPBA in CH2CI2 at room temperature afforded the expected product 75, epoxidation in the presence of two equivalents of NaHCOj afforded the isomeric j ,y-epoxysulfone 77. Similar results were obtained with other a-mono- or di-substituted sulfones. On the other hand, the reaction of y-substituted allylic sulfones results in the isomerization of the double bond, only. The following addition-elimination free radical chain mechanism has been suggested (equations 45, 46). In a closely related and simultaneously published investigation, Whitham and coworkers reported the 1,3-rearrangement of a number of acyclic and cyclic allylic p-tolyl sulfones on treatment with either benzoyl peroxide in CCI4 under reflux or with... 

The mechanism of the Kharasch-Sosnovsky reaction remains unclear. The generally accepted version, as proposed by Kochi and co-workers (94-96) and later improved by Beckwith and Zavitsos (97), is illustrated in Scheme 8. Cuprous ion reduces the perbenzoate to Cu(II)OBz (Bz = benzoyl) and free /-BuO radical. The radical abstracts an allylic hydrogen atom generating an allyl radical that combines with the cupric salt to form an allylcopper(III) species. Reductive elimination with... 

A tandem radical 5-exo cyclization/radical addition/allylic substitution reaction was subsequently described [292]. Allylic ot-bromo acetal 242b cyclized cobalt-catalyzed. Addition to diene 245 and subsequent coupling with coformed organocobalt(I) species generates an allylcobalt complex, which undergoes reductive elimination to cyclic product 246 in 93% yield (cf. Fig. 56). 

Alternative methods for the deprotection of the allyl group at the anomeric position include Pd(PPh3)4/AcOH [402], in which the reaction proceeds by the formation of a 7r-allyl complex, or PdCl2/CuCl/02 followed by photolysis in the presence of triethylamine [403]. Per-fluoroalkylation with perfluoroalkyl iodide under sodium dithionite and sodium bicarbonate followed by elimination in the presence of zinc powder and ammonium chloride has also been disclosed as an efficient procedure for deprotection of the anomeric allyl group of carbohydrates (O Scheme 73) [404]. The reaction goes through the intermediacy of a radical addition of a perfluoroalkyl iodide to the double bond followed by Zn-mediated reductive -elimination. 

Of course the opening of the lower homologs of 449 is a very fast process. For instance, the opening of the oxiranylmethyl radical to allyloxy radical is a fast process >10 sec (Reference 127). By using a-iodoepoxides as radical probe this permitted the conclusion to be reached that the reductive elimination of a-iodoepoxides to allylic alcohols with sodium naphthalides, as well as the... 

When 1,3-diene is the radical acceptor, a three-component coupling reaction of alkyl halide, 1,3-diene, and trimethylsilylmethyl-magnesium chloride takes place to afford homoallylsilane The final step of the three-component coupling is reductive elimination from allyl(trimethylsilylmethyl)cobalt that predominates over conceivable /3-hydride elimination. 

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