Allenes carbon-centered radicals

In recent years, interest in radical-based transformations of allenes has been renewed for two major reasons. First, a number of useful intramolecular additions of carbon-centered radicals to 1,2-dienes have been reported, which allowed syntheses of complex natural product-derived target molecules to be accomplished in instances where other methods have failed to provide similar selectivities. Further, a large body of kinetic and thermochemical data has become accessible from results of experimental and theoretical investigations in order to predict selectivities in addition reactions to allenes more precisely. Such contributions originated predominantly from (i) studies directed towards an understanding of the incineration process,... 

Most of the recent synthetic developments in the field of radical cyclization have involved the reactions of carbon-centered radicals with alkenes and alkynes. Other useful acceptors include allenes,31 dienes30 and vinyl epoxides.32 The same methods are used for cyclizations to these acceptors as for radical additions, and the preceding chapter should be consulted for specific details on an individual method (the organization of this section parallels that of Section 4.1.6). Selection of a particular method to conduct a proposed cyclization is based on a variety of criteria, including the availability of the requisite pre-... 

Simultaneous introduchon of both sulfur and selenium functions into carbon-carbon unsaturated compounds via a radical mechanism is also demonstrated by selenosulfonahori [149] and selenothiocarboxylatiorl [150] (Scheme 15.70). In these addihon reactions, attack of sulfur-centered radicals at the terminal position of alkenes and the subsequent Sh2 reaction on the selenium lead to the formahon of anti-Markovnikov adducts regioselechvely. The selenosulfonahon can be apphed to a variety of unsaturated compounds, for example alkynes, allenes, and vinylcyclopropanes, and combination with the selenoxide syn-elimination procedure... 

In these reactions, a er-bond is formed at the expense of two re-bonds and, thus, the process leads to a net loss of one chemical bond that is intrinsically unfavorable thermodynamically. Formation of the new er-bond leads to ring closure, whereas the net loss of a bond leads to the formation of two radical centers, which can be either inside (the endo pattern in Scheme 1) or outside of the newly formed cycle (the exo pattern). Note that er-radicals are formed through the endo path, while exo-closures may produce either a er-radical when a triple bond is involved or a conjugated re-radical when the new bond is formed at the central carbon of an allene. The parent version of this process is the transformation of enediyne 1 into p-benzyne diradical2 (the Bergman cyclization), shown in Scheme 2. 

Radical C -C Myers-Saito cyclization of enyne-allene, as well as the reaction of C -C cyclization do not depend on the donor properties of the solvent [427]. For enyne carbodiimides the situation is different, because the nitrogen atom is a potential donor center and is well known for the high electrophilicity of the central carbon atom of the car-bodiimide group. Indeed, the study of the thermolysis of carbodiimide 3.971a showed a strong dependence of the cyclization rate constant on the solvent properties. At 85°C, the reaction was seven times faster in dioxane k = 5.3 x 10 s ) and was nine times faster in acetonitrile K = 6.93 X 10 s ) than in benzene (k = 7.83 x 10 - s ). The rate constant of the thermal C -C cyclization of enyne-carbodiimides correlates better with the donor properties of the solvent rather than its dielectric constant, which is different from the reactions of enyne-allenes. Therefore, any discussion of the mechanism requires the consideration of alternative routes (Scheme 3.147) [424]. 

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