Cycloadditions, radical cation importance

It is important to note that the reactions are fundamentally different from similar radical cation Diels-Alder reactions initiated with the use of a photochemical electron-transfer reaction [35, 36]. In photochemical reactions, a one-electron oxidation of the substrate leads to a cycloaddition that is then terminated by a back electron transfer . No net change is made in the oxidation state of the substrate. However, the reaction outlined in Scheme 13 involves a net two-electron oxidation of the substrate. Hence, the two pathways are complementary. 

Several factors that are less important in the case of neutral cycloadditions have to be considered because they can potentially be exploited for greater selectivity. As a result of the charged nature of the radical cations, solvents and the solvation state of the ions as contact ion pairs, solvent separated ions pairs, or free ion pairs involved in the reaction now play a much greater role than for the cycloadditions of a neutral substrate, which for the most part show only a weak solvent dependence. The... 

Enol ether radical cations undergo a variety of reactions, e.g. carbon-carbon bond formation [206,230-235], isomerization [236,237], oxygenation [238-241], cycloaddition [242-245], and they play an important role in the damage to DNA by ionizing radiation [246,247]. According to ESR studies most of the spin density resides at the -carbon (89% for EtO" = CH-CH2 [248]) [234,249,250]. The focus of the present brief section, however, will be conremed with their role as potential intermediates in a formal a-Umpolung reaction of ketones. 

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. 

Another important application of DCN-sensitized photodissociation of strained ring compoimds has been demonstrated by Muller and Mattay [127] for synthesizing iV-substimted imidazoles (147) by the [3-l-2]-cycloaddition of the 2-azaallenyl radical cation (144), produced by the cleavage of corresponding radical cation from azirine (143), with imines. This strategy is further extended [128] for the synthesis of pyrrolophane 3,4-dimethyl ester (152) by the ring opening cycloaddition reaction of (148) with dimethyl acetylene dicarb-oxylate (Scheme 32). 

A number of alkene radical cations have been generated in matrices at low temperature and have also been studied by ESR, CIDNP, and electrochemical methods. However, until recently very little absolute kinetic data have been available for the reactions of these important reactive intermediates in solution under conditions comparable to those used in mechanistic or synthetic studies. In a few cases, competitive kinetic techniques have been used to estimate rates for nucleophilic additions or radical cation/alkene cycloaddition reactions. In addition, pulse radiolysis has been used to provide rate constants for some of the radical cation chemistry relevant to the pho-topolymerization of styrenes. More recently, wc and others have used laser flash photolysis to generate and characterize a variety of alkene radical cations. This method has been extensively applied to the study of other reactive intermediates such as radicals, carbenes, and carbenium ions and is particularly well-suited for kinetic measurements of species that have lifetimes in the tens of nanoseconds range and up and that have at least moderate extinction coeffleients in the UV-visible region. 

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 reactivity trends observed for both the nucleophilic addition and cycloaddition of alkene radical cations parallel trends observed for the reactions of carbocations with nucleophiles and alkenes. However, the observed variations in reactivity towards oxygen - and substituent effects on the competition between addition of methanol and the neutral monomer for diphenylethene radical cations indicate that variations in both spin and charge density are important in determining the overall reactivity patterns. It is clear that further experimental and theoretical studies are required to provide a detailed model for understanding and ultimately predicting the reactivity of radical cations. 

A tris(4-bromophenyl)ammonium hexachloroantimonate catalyst has been utilized to promote a cation radical mechanism in the Diels-Alder cycloaddition polymerization of a bis(diene) with an ionizable bis(dienophile) (Scheme 2). The polymers were obtained with molecular weights up to ca 10 000 and a polydispersity index of ca 2. The electron-transfer reactions of phenols and its derivatives are also important to the polymer industry for the stabilization of polymers, fats and oils. Pulse radiolysis of naphthols and hydroxybiphenyls in n-butyl chloride at room temperature forms two species-phenol-type radical cations and phenoxyl-type radicals. Two different electron-transfer channels are proposed. The naphthol and hydroxybiphenyl radical cations show increased stability compared with phenol radical cations, presumably due to extensive delocalization over the whole aromatic system. 

It is important to note that the efficiency of the various cycloaddition reactions presented above arises from a rapid cleavage of the resulting cation radical intermediates, which renders the back electron transfer process ineffective. 

In this account, we will focus on the transient analysis of these systems, which has strongly contributed to a deeper understanding of the diverse reaction modes (Patemo-Buchi, proton abstraction, cycloaddition). In general, aromatic ketones were selected as electron acceptors for reasons of suitable excitation and long wavelength absorption of the radical anion intermediates. Among them, fluorenone 3 is particularly well suited since the concentration, solvent, temperature, and cation radius dependence of the absorption spectra of pairs formed with metal cations are already known [29]. Hogen-Esch and Smid [30, 10] pointed out that a differentiation between CIP and SSIP is possible for fluorenone systems. On the other hand, FRI s and SSIP s cannot be differentiated simply by their UV/Vis absorption spectra, whereas for instance conductance measurements may be successful. However, the portion of free radical ions in fluorenyl salt solutions was shown to be less important [9, 31]... 

In practical terms, particularly important are the cycloaddition reactions catalyzed through the formation of cation-radicals. A theoretical analysis of the PES of these reactions [94] has helped to evolve some general notions concerning the mechanism of ionradical pericyclic transformations. 

During the past 15 years since Qo became available in macroscopic quantities in 1990 [1], a wide variety of its derivatives have been synthesized as part of the explosive development of the study of its chemistry [2). Various organic reactions have been reported, most of which are cycloadditions, nucleophilic additions, and radical additions. Fullerenes, as represented by Qo, are now commonly accepted to behave as electron-deficient olefins, hence there have been numerous studies on their anions. This has led to a situation where the other equally important species, the fullerene cations, have been left unexplored for nearly a decade in spite of their significance in both fundamental and application studies. Clearly, a systematic study of this class of species is needed. 

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