Cycloadditions, radical cation concerted

The mechanism of the cycloaddition appears to be concerted for various reagents however, for several cases, radical cation cycloaddition-cycloreversions have a stepwise component. For example, CIDNP effects observed during the PET induced dimerization of spiro[2.4]heptadiene (97) identify a dimer radical cation with spin density only on two carbons of the dienophile fragment this intermediate must be a doubly linked radical cation ( 99 + 282,283 pulsed laser experiment at high concentrations of 97 supports a second dimer radical cation at high... 

Unlike thermal [2 + 2] cycloadditions which normally do not proceed readily unless certain structural features are present (see Section 1.3.1.1.), metal-catalyzed [2 + 2] cycloadditions should be allowed according to orbital symmetry conservation rules. There is now evidence that most metal-catalyzed [2 + 2] cycloadditions proceed stepwise via metallacycloalkanes as intermediates and both their formation and transformation are believed to occur by concerted processes. In many instances such reactions occur with high regioselectivity. Another mode for [2 + 2] cyclodimerization and cycloadditions involves radical cation intermediates (hole-catalyzed) obtained from oxidation of alkcnes by strong electron acceptors such as triarylammini-um radical cation salts.1 These reactions are similar to photochemical electron transfer (PET) initiated [2 + 2] cyclodimerization and cycloadditions in which an electron acceptor is used in the irradiation process.2 Because of the reversibility of these processes there is very little stereoselectivity observed in the cyclobutanes formed. 

However, only limited experimental evidence is available concerning the key step of the dimerization, i,e. the addition of the radical cation to the parent olefin. Does this addition occur stepwise or in concerted fashion Does the radical cation serve as a the diene component ([3 + 2]cycloaddition) or as dienophile ([4+ l]cy-cloaddition) The observed retention of dienophile stereochemistry and orbital symmetry arguments (Fig. 7) favor the [4 + l]cycloaddition type. Although it is difficult to distinguish the [3 + 2] from the [4 + l]addition type, a stepwise component for the cycloaddition and the complementary cycloreversion has been established in at least one system, viz., spiro[2.4]heptadiene. 

In an earlier report Mazzocchi and his coworkers reported that the photo-reaction of A) methylnaphthalimide (325) with phenyIcyclopropane involved the production of a radical cation/radical anion pair. The product from the reaction was the cyclic ether (326). - A study of the mechanism of this reaction using suitably deuteriated compounds has demonstrated that the reaction is not concerted and takes place via the biradical (327). - Other systems related to these have been studied. In the present paper the photoreactivity of the naphthalimide (328) with alkenes in methanol was examined. Thus, with 1-methylstyrene cycloaddition occurs to the naphthalene moiety to afford the adducts (329) and (330). The mechanism proposed for the addition involves an electron transfer process whereby the radical cation of the styrene is trapped by methanol as the radical (331). This adds to the radical anion (332) ultimately to afford the observed products. Several examples of the reaction were descr ibed. 

A study of the stereoselectivity of the radical-cation Diels-Alder reaction of indole with the diene 139 gave a mixture of both cis and trans adducts. This lack of stereospecificity is consistent with other evidence that the radical-cation cycloaddition is non-concerted. <93TL6391>... 

The kinetic data discussed above demonstrate the effects of varying the structure of both the styrene radical cation and the alkene on the initial step in the cycloaddition reaction. However, the transient experiments do not provide any evidence that would permit one to distinguish between a concerted or stepwise mechanism. The kinetic data obtained for additions to a range of alkenes do show considerable similarities to those reported for the addition of carbenium ions to the same substrates. For example, rate constants for the addition of the bis(4-methyl-phenyl)methyi cation to a series of ring-substituted styrenes also correlate with the Hammett a and a parameters with p and p values of-5.2 and -5.0, respectively." The latter reactions are thought to proceed via a partially bridged transition slate and might, therefore, be expected to show similarities to concerted... 

The concerted mechanism, in which the two new bonds form synchronously (Fig. 7), is probably less common than generally assumed. A concerted non-synchronous mechanism can involve diradicals or zwitterions, which means more or less dissymmetry, geometrical and/or electronic, in the bond formation, which can be increased by the presence of catalysts, such as Lewis acids, especially lithium salts,26 or solvent effects.27 Ionization of one of the reactants (Fig. 8), frequently the dienophile, is efficient in promoting cycloadditions with unreactive reagents, e.g., the [4+2] dimerization of dienes, by a selective transformation to the reactive radical cations ("hole" catalysis). ... 

For some time it has been known that many pericyclic reactions can be greatly accelerated if they are run under single electron transfer (SET) conditions (also known as electron transfer catalysis, ETC). Examples include Diels-Alder reactions, electrocyclic openings of cyclobutenes, and retro [2-1-2] cycloadditions. From the beginning it has been debated whether these SET reactions really are concerted processes with aromatic transition states, or whether they are better thought of as stepwise processes involving radical cation intermediates. 

In view of the demonstrated stereospecificity of at least some cation radical Diels-Alder reactions, it is at least possible that these reactions, like the neutral Diels-Alder, are true pericyclic reactions, i.e., they may occur via a concerted cycloaddition. The results of a variety of calculations, however, make clear that the cydoadditions must at least be highly non-synchronous, so that the extent of the formation of the second bond, which completes the cyclic transition state, is no more than slight [55, 56]. If the cation radical Diels-Alder reaction is nevertheless interpreted as pericyclic and the concept of orbital correlation diagrams is applied to them, it emerges that the cycloaddition is symmetry allowed if the ionized (cation radical) component is the dienophile, but forbidden if it is the diene [39, 55], The former mode of reaction has been referred to as the [4-1-1] mode, and the latter as the [3 -t- 2] mode. Interestingly, the great majority of cation radical Diels-Alder reactions thus far observed seem to represent the formally allowed [4-1-1] mode. An interesting case in point is the reaction of l,l -dicyclohexenyl with 2,3-dimethylbutadiene (Scheme 24) [57]. 

The cation-radical Diels-Alder reactions of cis- and fran5 -l,2-(diaryloxy)ethenes with butadienes are stereospecific, in agreement with a concerted cycloaddition mechanism. " Tris(4-bromophenyl)aminium hexachloroantimonate catalyses the two-step, non-stereospecific cation-radical Diels-Alder reaction of cis- and traui-prop-l-enyl aryl ethers with cyclopenta-1,3-diene in CH2CI2 solution. 

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