Radical cations chemistry

Although cycloaddition reactions have yet to be observed for alkene radical cations generated by the fragmentation method, there is a very substantial literature covering this aspect of alkene radical cation chemistry when obtained by one-electron oxidation of alkenes [2-16,18-26,28-31]. Rate constants have been measured for cycloadditions of alkene and diene radical cations, generated oxidatively, in both the intra- and intermolecular modes and some examples are given in Table 4 [91,92]. 

Due to the fact that the removal of a bonding electron from the HOMO of the substrate RH leads to a radical cation with enhanced reactivity with respect to fragmentation reactions, the pathway often employed in radical cation chemistry results in the separation of charge and spin by dissociative processes, such as deprotonation, desilylation or cleavage of a stable cationic leaving group... 

Radical cation structure types can be classified according to the nature of the donor molecules, viz., it-, n-, or cr-donors, from which they are generated. Radical cations derived from typical it-donors may be closely related to the structure of their precursors, whereas substantial differences may be observed between the structures of radical cation and precursor for cr-donors. The potential surfaces of radical cations and their parents may differ in three features reaction barriers may be reduced, free energy differences between isomers may be reduced or reversed and energy minima on the radical cation surface may have geometries corresponding to transition structures on the parent potential surface. The pursuit of such novel structure types has given new direction to radical cation chemistry. Representative radical cation structures are discussed to document their rich variety and to illustrate the molecular features that determine their structures. 

Compared to the analogous reactions of the parent molecules, many radical cation reactions show a dramatic decrease in activation barriers, one of the most striking aspects of radical cation chemistry. Intuitively, this observation can be ascribed to the fact that the highest occupied molecular orbital (HOMO) of a radical cation is occupied by a single electron. As a result, the bond strength of one or more key bonds must be reduced and the bonds more easily decoupled. However, the barriers to some radical cation rearrangements appear to lie even lower than might be expected on the basis of this simple model. 

Both we and others have established various radical cation structure types, which deviate in important features from the structures of their neutral diamagnetic precursors. The pursuit of these novel structure types has given new direction to radical cation chemistry. We have noted that some of these species resemble plausible transition structures for the thermal rearrangement of the parent molecules, i.e. saddle points on the corresponding potential surfaces. From a different point of view, they can be envisaged as one-electron oxidation products of biradicals or zwitterions. However, this relationship rarely serves as a practical approach to their generation, since the potential bifunctional precursors are often unstable and not readily accessible. These radical cations are usually generated from related hydrocarbons or cyclic azo compounds. 

Interestingly, the radical cation 138 can be generated also by light-induced isomerization of cyclooctatetraene radical cation (140). The conversion of the red non-planar ion 140 (4n - 1 n electrons) upon irradiation with visible light had been observed previously [395], but the blue photo-product had not been recognized as the cyclic conjugated species 138 with 4n + 1 n electrons. This interconversion is one of only a few orbital symmetry allowed processes documented in radical cation chemistry [393]. 

The rich variety of radical cation reactions and the diversity of structure types portrayed in this article may lead the reader to the conclusion that this field has exhausted its growth potential. However, this impression would be premature on the contrary, radical cation chemistry remains in a phase of rapid development and can be expected to retain a high level of attention for the foreseeable future. 

The purpose of this review is to discuss some recent computational studies of radical cations in the context of qualitative concepts of classical physical organic chemistry. In particular, we will demonstrate how such basic, well-understood concepts such as conjugation and electronic state or even more fundamental notions of structure, bonding, and mechanism can lead to new and interesting effects in radical cation chemistry, which are quite different than what is usually expected in the chemistry of neutral compounds. We will also discuss how these effects need to be taken into consideration to understand the chemistry of radical cations. This relatively broad scope means that this review will necessarily be limited to a focused discussion rather than a comprehensive review of the different aspects of radical cation chemistry. Thus, we will concentrate on computational results from our own laboratory, and will discuss experimental data only in the context of the calculational data. A number of recent reviews5 and book chapters6 provide much more detail on aspects that cannot be covered in this limited contribution. 

Enol radical cation intermediates have rerantly been invoked in the ribonucleotide reductase process. According to a hypothesis by Stubbe [127], they are formed through water loss from the 3 -ribonucleotide radical. They are supposed to react subsequently with H (or alternatively via a very unlikely two-electron reduction followed by protonation) to an intermediate 3 -hydroxy radical that is finally transformed to the deoxyribonucleotide. The above mechanistic evidence on simple enol radical cation chemistry, however, argues against this mechanistic model, since deprotonation should be much faster than nucleophilic attack even under physiological conditions. 

A Unifying Picture of Radical-anion and Radical-cation Chemistry... 

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]. 

There is a considerable current interest in synthetic applications of radical cation chemistry. Alkene radical cations have been studied extensively, notable examples include the anti Markovnikov addition of nucleophiles [209], and the photo-NO-CAS reaction [210]. The synthetic utility of radical cation mediated chemistry, and... 

Despite n bonds in benzohomobenzvalene (37) its radical cation chemistry mimics that of the saturated cases and the radical cation of benzonorcarene (38) is observed as an intermediate . It is not at all surprising that the above rapid rearrangements occur for the radical cations. Actually it is the longevity of the neutrals, given their considerable strain energies, that is noteworthy. 

The last aspect of general bicycloalkane radical cation chemistry we discuss relates to the di- and tetradehydro[3.3]paracyclophanes , 39 and 40, that may be said to be derivatives of bicyclo[3.1.0]hexane and tricyclo[3.1.0.0 ]hexane formed by stretching For the radical cations of both the di- and tetradehydro cyclophanes, both the unpaired electron and the charge in the radical cation are delocalized over both rings and all of the benzylic carbons. 

For radical cations this situation is typically observed when deprotonation of the dimer dication is slow and for radical anions under conditions that are free from electrophiles, for example, acids, that otherwise would react with the dimer dianion. Most often, this type of process has been observed for radical anions derived from aromatic hydrocarbons carrying a substituent that is strongly electron withdrawing, most notably and well documented for 9-substituted anthracenes [112,113] (see also Chapter 21). Examples from the radical cation chemistry include the dimerization of the 1,5-dithiacyclooctane radical cations [114] and of the radical cations derived from a number of conjugated polyenes [115,116]. 

When the primary species (products of catalysis) must be deduced from knowledge of the radical cation chemistry, interpretation becomes more... 

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 following three sections discuss recent time-resolved experiments on inter- and intramolecular cycloadditions of aryl-alkene radical cations. These studies address some of the mechanistic issues raised by the earlier studies and also provide kinetic data for the cycloadditions of a number of aryl and diaryl-alkene radical cations. Such kinetic data are essential for the development of this chemistry as a useful synthetic strategy and as a mechanistic probe for radical cation chemistry. 

The combined data in Tables 7-9 for the additions of styrene radical cations to their neutral precursors (dimerizations) and to other alkenes lead to a potentially important conclusion with respect to the design of cross-addition reactions. These data indicate that dimerization rate constants are frequently several orders of magnitude greater than the rate constants for cross addition. The absolute rate constants for the two reactions can be used to adjust the concentrations of the neutral styrene that leads to the radical cation and the alkene in order to maximize the yield of the cross-addition product. The kinetic and mechanistic data obtained for these reactions thus provides the basis for the development of synthetic strategies that utilize radical cation chemistry. 

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. 

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