Cycloaddition donor radical cations

One of the problems associated with thermal cyclodimerization of alkenes is the elevated temperatures required which often cause the strained cyclobutane derivatives formed to undergo ring opening, resulting in the formation of secondary thermolysis products. This deficiency can be overcome by the use of catalysts (metals Lewis or Bronsted acids) which convert less reactive alkenes to reactive intermediates (metalated alkenes, cations, radical cations) which undergo cycloaddilion more efficiently. Nevertheless, a number of these catalysts can also cause the decomposition of the cyclobutanes formed in the initial reaction. Such catalyzed alkene cycloadditions are limited specifically to allyl cations, strained alkenes such as methylenccyclo-propane and donor-acceptor-substituted alkenes. The milder reaction conditions of the catalyzed process permit the extension of the scope of [2 + 2] cycloadditions to include alkene combinations which would not otherwise react. 

Likewise, benzyldihydroisoquinolinium derivatives can be used in a photochemical synthesis of tetrahydroisoquinolines. Thus, 2-(2-trimethyl-silylmethylphenylmethyl)-3,4-dihydroisoquinoliniun perchlorates have been successfully cyclized, as in the synthesis of the protoberbine alkaloids (+)xylopinine and (+)stylopine. The reaction proceeds via SET from the xylyl donor to the iminium moiety, fragmentation of the benzylsilane radical cation and carbon-carbon bond formation in the intermediate diradical. The synthesis is rather general and the yields compare favorably with those obtained from related substrates via a dipolar cycloaddition methodology [298] (Sch. 27). 

Bauld and coworkers, especially, developed the analogous Diels-Alder (4 + 2) cycloaddition reactions. These reactions are conveniently catalyzed by tris(4-bromophenyl)aminium hexachloroantimonate (78) or by photosensitization with aromatic nitriles. The radical cation-catalyzed Diels-Alder reaction is far faster than the uncatalyzed one, and leads to some selectivity for attack at the least substituted double bond for the monoene component (Scheme 18, 79 —> 80), but only modest endo selectivity (e- and x-80) [105]. Cross reactions with two dienes proved to be notably less sensitive to inhibition by steric hindrance of alkyl groups substituted on the double bonds than the uncatalyzed reactions, as cyclohexadiene adds detectably even to the trisubstituted double bond of 2-methylhexadiene (82), producing both 83 and 84. Dienes such as 85 react with donor-substituted olefins (86) to principally give the vinylcyclobutene products 87, but they may be thermally rearranged to the cyclohexene product 88 in good yield [105]. Schmittel and coworkers have studied the cation radical catalyzed Diels-Alder addition of both... 

Radical-anion and radical-cation intermediates, for example, reaet with each other after PET in donor-acceptor systems. After proton reorganization they undergo cyclization to provide a direct synthetic route to macrocycles and A-heterocycles with a variety of ring sizes [230]. Cycloadditions via radical ion pairs [231] and the C -C bond formation between Ceo and A,0-ketene acetals [232] also fit this eategory. 

The formation of CT complexes between alkenes is considered to be the first step of the cycloaddition reactions, and it may also be the first step of some types of olefin polymerization23. The CT complex obtained from strong electron donors and strong electron acceptors may produce a complete charge separation with formation of an ion-radical pair (cation radical and anion radical pair), as illustrated by Scheme 2. 

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