Cycloadditions, radical cation dimerizations

The lattice-stabilization effects allow the isolation of [MF6]- salts (M=As, Sb) of [S3N2]2+ in the solid state from the cycloaddition of [SN]+ and [S2N]+ cations in S02.66 The S-S and S-N bond distances in the planar, monomeric dication are shorter than those in the dimeric radical cation dimer, as anticipated for the removal of an electron from a n orbital. 

A variation of the reaction involved the use of the alkene itself as nucleophile. In this case, a radical cation dimer was formed by attack of the alkene radical cation by the neutral alkene, forming a distonic radical cation (Scheme 14.9, left part). With a-methylstyrene (17) as the alkene, a cychzation took place and the neutral radical resulting from the ensuing deprotonation coupled with the radical anion of the acceptor (in this case TCB), leading to the NOCAS adduct 18 as a diastereo-isomeric mixture in overall 90% yield [55]. The irradiation of aromatic nitriles in the presence of aUcenes may lead to different products, particularly when carried out in an apolar medium. As an example, 1,4-dicyanobenzene gave isoquinohnes by a [4-1-2]-cycloaddition with a cyano group through irradiation in the presence of diphenylethylenes in benzene via a polar exciplex [56]. 

The mechanism which could explain the formation of these products is described in Scheme 27. In an EC mechanism, the intermediate radical cation 48a could undergo a follow-up reaction with water as a nucleophile to form radical 48b which could than dimerize through S-N or S-S bond formation or react with 48a to yield 50 and 51 as the fianl one-electron oxidation products. In an ECE mechanism, intermediate 48b is further oxidized to 48c which reacts with acetonitrile as a solvent to give 49 as the final two-electron oxidation product. The cation intermediate 48c can react with the parent molecule 48 through [2 -f 3]-cycloaddition to give the final products 50 and 51. The [2 -f 3]-... 

Scheme 29 describes a plausible mechanism for the formation of the products which fit the observed coulometric (n 0.45 F/mol) and preparative results. The intramolecular cyclization process involves a dimerization between a radical cation 52a and the ketene imine 52 to form the intermediate radical cation 52b which then cyclizes to the radical 52c which can abstract a hydrogen atom leading to 54 or can be further oxidized and transformed through a cyclization and deprotonation reaction to 53 which involves 1 F/mol. However, it seems that the [2 -1- 3]-cycloaddition between the parent compound 52 and the cation 52d giving rise to 55 is the fastest reaction as compared with the intramolecular cyclization of 52d to 53. This can also explain the low consumption of electricity. 

Intramolecular bond formations include (net) [2 + 2] cycloadditions for example, diolefin 52, containing two double bonds in close proximity, forms the cage structure 53. This intramolecular bond formation is a notable reversal of the more general cycloreversion of cyclobutane type olefin dimers (e.g., 15 + to 16 +). The cycloaddition occurs only in polar solvents and has a quantum yield greater than unity. In analogy to several cycloreversions these results were interpreted in terms of a free radical cation chain mechanism. 

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... 

Although hole-catalyzed (cycloaddilions involving radical cation intermediates) and PET (photochemical electron transfer) mixed [2 -I- 2] cycloadditions have been reported from electron-rich alkenes, the only report of a cyclodimerization is that of (E)-4-(prop-l-enyl)anisole which gives stereoisomeric mixtures of the head-to-head dimers 1 and 2.12... 

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. 

TABLE 4.9 Dimerization of 8a SELECTIVITY IN RADICAL CATION CYCLOADDITIONS Ratio 9a/10a in CH3CN Sensitized by 6 ... 

Although cycloadditions have frequently been observed in radical-cation chemistry, this reaction mode is apparently very rare in radical-anion chemistry because of the electron repulsion term. Few examples are known of Diels-Alder dimerizations [355], [2 -I- 2] cycloadditions [356], retro-[2 - - 2] cycloadditions [357], and cyclo-trimerizations [358]. Equally, little is known about electrocyclic reactions, despite their interesting stereochemical course [359]. 

Recent studies [382] have provided rate data for cycloaddition reactions. Accordingly, steric effects at the electrophile site, and the capacity of the added unsaturated component RCH=CH2 to stabilize radicals and cations, play a vital role, the importance of which is reflected in a rate decrease for R = Ph OR > vinyl > alkyl by a factor of 100-300. In principle, the observed trends follow those for addition of car-bocations to alkenes [292]. A study of the [2-1-1] cycloaddition of 4-methoxystyrene also emphasizes the importance of the rapid one-electron reduction of the intermediate dimer radical cation [383]. A direct view of 4-center 3-electron cyclobutane [384] and bisdiazene-oxide [385] radical cations has been obtained with polycyclic, rigid systems. 

The novel four-center, two-electron delocalized a-bishomoaromatic species 182,183,188,190a, and 192 are representatives of a new class of 27i-aromatic pericyclic systems. These may be considered as the transition state of the Woodward-Hoffmann allowed cycloaddition of ethylene to ethylene dication or dimerization of two ethylene radical cations (Fig. 5.11,193). Delocalization takes place among the orbitals in the plane of the conjugated system, which is in sharp contrast to cyclobutadiene dication 194 having a conventional p-type delocalized electron structure (Fig. 5.11). 

Cycloadditions only proceeding after electron transfer activation via the radical cation of one partner are illustrated by the final examples. According to K. Mizono various bis-enolethers tethered by long chains (polyether or alkyl) can be cyclisized to bicyclic cyclobutanes using electron transfer sensitizer like dicyanonaphthalene or dicyano-anthracene. Note that this type of dimerization starting from enol ethers are not possible under triplet sensitization or by direct irradiation. Only the intramolecular cyclization ci the silane-bridged 2>. s-styrene can be carried out under direct photolysis. E. Steckhan made use of this procedure to perform an intermolecular [4+2] cycloaddition of indole to a chiral 1,3-cyclohexadiene. He has used successfully the sensitizer triphenylpyrylium salt in many examples. Here, the reaction follows a general course which has been developed Bauld and which may be called "hole catalyzed Diels-Alder reaction". 

The styrylpyrilium salt derivatives (130) undergo (2 + 2)-cycloaddition to afford the corresponding dimers. The magnetic properties of the radical cations formed from these dimers were compared with those formed from (130). Styryl dyes of the type shown as (131) undergo E-Z isomerization on irradiation at 436 nm. The dyes align themselves in the pattern shown in (132, where the filled blob represents the crown ether complex). These undergo dimerization on irradiation to afford compounds (133), from which the magnesium ions can be removed. 

Cycloaddition reactions of alkene radical cations have been the subject of a number of mechanistic studies and are potentially useful synthetic reactions. - - - Most of the initial work on radical cation mediated cycloadditions focused on the dimerization of arylalkenes. with one of the first examples being Ledwith s report of the chloranil-sensitized dimerization of M-vinylcarba-zole to generate a diarylcyclobutane. This work led to the development of the mechanism outlined in Scheme 2, in which addition of the radical cation to neutral alkene generates an acyclic 1,4-radical cation as the primary intermediate. This intermediate cyclizes to a cyclobutane radical cation that is then reduced by the neutral alkene and regenerates a second radical cation to carry the chain. 

Despite the demonstrated utility of alkene radical cation cycloadditions, little kinetic data for these reactions are currently available. However, two recent studies have provided rate constants for the initial step in the cyclobutanation or Diels-Alder reactions of a number of styrene radical cations.Previous work by Bauld had shown that the rrradical cation reacts with a variety of alkenes to generate either cyclobutane or Diels—Alder adducts (Eqs. 23, 24) 110 j, g [jnetic data for the styrene radical cation cycloadditions, in combination with the dimerization results discussed above, provide a detailed assessment of the effects of radical cation and alkene structure on dimerization and cross addition reactions. 

The data on cycloadditions of alkene radical cations indicate that dimerization will usually compete efficiently with cross additions and demonstrate the necessity for obtaining detailed kinetic data in order to design appropriate synthetic methods based on radical cation chemistry. The mechanistic data obtained from both time-resolved and steady-state experiments demonstrate the complexity of cycloaddition chemistry. This may be a particular limitation in the use of cycloaddition reactions in the design of mechanistic probes for assessing whether a particular reaction involves radical cation intermediates. The results also highlight the importance of using both product studies and the kinetic and mechanistic data obtained from time-resolved methods to develop a detailed understanding of the reactions of radical cations. 

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). ... 

The cycloaddition of two ethylenes or the cycloreversion of cyclobutane is one of the textbook examples used in the illustration of the Woodward-Hoffman rules [20] of orbital symmetry. Studies on the cyclobutane radical cation [21,22] showed a low activation energy for the cycloaddition of an ethylene radical cation to ethylene, in remarkable contrast with the high activation energy for the corresponding neutral reaction [23]. The dissociation reaction of cyclobutane radical cation is endothermic. Although there is a cyclobutane ring in the pyrimidine dimer, its electronic structure is likely to be different from cyclobutane itself, because of the presence of the two pyrimidine rings. 

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