Cycloadditions, radical cation alkene

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. 

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]. 

Table 4 Rate constants for cycloadditions with alkene radical cations a ...

The gas-phase reactions of the fulvene radical cation with neutral 1,3-butadiene, alkenes and 2-propyl iodide have been investigated by Russell and Gross131a using ICR mass spectrometry. Unlike ionized benzene, ionized fulvene undergoes no C—C coupling with 2-propyl iodide. On the basis of deuterium and 13C labelling, the reaction of ionized fulvene with 1,3-butadiene was suggested to occur by [6 + 4] cycloaddition to yield tetrahydroazulene radical cations. Cycloadditions of neutral fulvene were also studied in this work. 

Electron transfer from the alkene leads to a radical cation that can undergo coupling (Scheme la). The radical cation can also react with the nucleophilic heteroatom of a reagent to afford addition or substitution products (Scheme lb). Adducts can be likewise obtained by oxidation of the nucleophile to a radical that undergoes radical addition. Reactions between alkenes and nucleophiles can be realized too with chemical oxidants that are regenerated at the anode (mediators) (see Chapter 15). Finally, cycloadditions between alkenes can be initiated by a catalytic anodic electron transfer. These principal reaction modes are subsequently illustrated by selected conversions. 

Cycloaddition Reactions with Alkenes Olefins can react with electrogenerated radicals, cationic species or dienophiles. 

The same group studied the radical cation cycloaddition of 2-vinylbenzofti-rans with various alkene and diene compounds initiated by photoinduced electron transfer. Depending on the unsaturated compound used, yields up to 60% were feasible. In contrast to 2-vinylindoles, 2-vinylbenzofurans prefer to react as die-nophiles, very similar to styrenes [82]. 

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... 

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. 

Another mode for catalyzed cycloaddition involves the generation of radical cations from electron-rich alkenes with single-electron oxidants such as tris(4-bromophenyl)amminium hexachloroantimonate (TBAH). An equivalent reaction involves the photosensitized electron transfer (PET) process (see Section 1.3.2.3.). These processes have been recently reviewed,9 and are limited to electron-rich alkenes capable of producing radical cations. Furthermore, some of the cyclobutanes themselves undergo secondary isomerization under the oxidative conditions, e.g. formation of 31-35.10-12... 

There are a number of other mechanisms by which alkenes can undergo photochemical f2 + 2) cycloaddition, one of which works well for electron-rich alkenes and electron-acceptor sensitizers. The pathway is through the radical cation of the alkene, which attacks a second, ground-state alkene molecule and then cydizes and accepts an electron to give the product cyclobutane. Typical of this group of reactions is the formation of 1,2-dialkoxycydobutanes from alkoxy-ethylenes with drcyanonaphthalene as sensitizer 12.78). 

A number of electrocyclic reactions under PET conditions have been reported. In this way, A-benzyl-2.3-diphcnylaziridinc (40) underwent a 3 + 2-cycloaddition with alkene and alkyne dipolarophiles to afford substituted pyrrole cycloadducts (41) via the radical cation intermediate (42) see Scheme 7.80 Elsewhere, novel arylallenes have been used as dienophiles in a radical cation-catalysed Diels-Alder cycloaddition reaction with 1,2,3,4,5-pentafluromethylcyclopentadiene, which often occurred with peri-, chemo-, facial- and stereo-selectivity.81... 

Recently Geiger and coworkers found that the electrochemically generated CpRe(CO)3 cation catalyzes the [2+2] cycloaddition of unactivated alkenes [304]. The reaction proceeds via SET oxidation of the olefin to the corresponding radical cation, addition, cyclization, and back electron transfer (BET), which can occur either from the reduced catalyst or the alkene. 

Arene-substituted alkenes can undergo radical-cation [4+2] cycloadditions with 1,3-dienes when they are irradiated in the presence of an electron acceptor (Sch. 13) [49]. Good levels of regio and stereoselectivity... 

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. 

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. 

Electron Transfer Processes - A single electron-transfer mechanism is involved in the cycloaddition of alkenes, such as 2-methylpropene, to 1,2-dicyanonaphthalene. Reaction of the alkene radical cation with the radical anion of the sensitiser results in the products shown in the Scheme 1. Incorporation of solvent to give (55) occurs as one of the main products in addition to what are essentially photo-NOCAS products (56). 

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