Cycloaddition reactions radical anions

A particular case of a [3C+2S] cycloaddition is that described by Sierra et al. related to the tail-to-tail dimerisation of alkynylcarbenes by reaction of these complexes with C8K (potassium graphite) at low temperature and further acid hydrolysis [69] (Scheme 24). In fact, this process should be considered as a [3C+2C] cycloaddition as two molecules of the carbene complex are involved in the reaction. Remarkable features of this reaction are (i) the formation of radical anion complexes by one-electron transfer from the potassium to the carbene complex, (ii) the tail-to-tail dimerisation to form a biscarbene anion intermediate and finally (iii) the protonation with a strong acid to produce the... 

Recently it has been shown that radical anionic cyclization of olefinic enones effectively compete with intramolecular [2 -I- 2]-cycloaddition to form spirocy-clic compounds [205, 206], 3-Alkenyloxy- and 3-alkenyl-2-cyclohexenones 235 are irradiated in the presence of triethylamine. As depicted in Scheme 46 two reaction pathways may operate. Both involve electron transfer steps, either to the starting material (resulting in a direct cyclization) or to the preformed cyclobutane derivative 239, which undergoes reductive cleavage. The second... 

CgH (n = 6, 7, 8). A novel collision-induced isomerization of CgH7 (10a), which has a sttained allenic bond, to (lOyS) has been reported to occur upon SIFT injection of (10a) at elevated kinetic energies (KE) and collision with helium. In contrast, radical anions (9) and (11) undergo electron detachment upon collisional excitation with helium. Bimolecular reactions of the ions with NO, NO2, SO2, COS, CS2, and O2 have been examined. The remarkable formation of CN on reaction of (11) with NO has been attributed to cycloaddition of NO to the triple bond followed by eliminative rearrangement. 

This section is devoted to cyclizations and cycloadditions of ion-radicals. It is common knowledge that cyclization is an intramolecular reaction in which one new bond is generated. Cycloaddition consists of the generation of two new bonds and can proceed either intra- or intermolecularly. For instance, the transformation of 1,5-hexadiene cation-radical into 1,4-cyclohexadienyl cation-radical (Guo et al. 1988) is a cyclization reaction, whereas Diels-Alder reaction is a cycloaddition reaction. In line with the consideration within this book, ring closure reactions are divided according to their cation- or anion-radical mechanisms. 

Nitrogen heterocycles continue to be valuable reagents and provide new synthetic approaches such as NITRONES FOR INTRAMOLECULAR -1,3 - DIPOLAR CYCLOADDITIONS HEXAHYDRO-1,3,3,6-TETRAMETHYL-2,l-BENZISOX AZOLINE. Substituting on a pyrrolidine can be accomplished by using NUCLEOPHILIC a - sec - AM IN O ALKYL ATION 2-(DI-PHENYLHYDROXYMETHYL)PYRROLIDINE. Arene oxides have considerable importance for cancer studies, and the example ARENE OXIDE SYNTHESIS PHENANTHRENE 9,10-OXIDE has been included. An aromatic reaction illustrates RADICAL ANION ARYLATION DIETHYL PHENYLPHOSPHONATE. 

The formation of CT complexes between alkenes is considered to be the first step of the cycloaddition reactions, and it may also be the first step of some types of olefin polymerization23. The CT complex obtained from strong electron donors and strong electron acceptors may produce a complete charge separation with formation of an ion-radical pair (cation radical and anion radical pair), as illustrated by Scheme 2. 

In all examples discussed up to now the radical cation of Qo is involved in the reaction mechanism. However, due to the electronic features reduction of the fullerenes leading to radical anions should be much easier performed. For example, a useful method to synthesize 1-substituted l,2-dihydro-[60]fullerenes is the irradiation of Q0 with ketene silyl acetals (KAs) first reported by Nakamura et al. [216], Interestingly, when unstrained KAs are used, this reaction did not yield the expected [2 + 2]-cycloaddition product either by the thermal, as observed by the use of highly strained ketene silyl acetals [217], or by the photochemical pathway. In a typical reaction Q0 was irradiated for 10 h at 5°C with a high pressure mercury lamp (Pyrex filter) in a degassed toluene solution with an excess amount of the KA in the presence of water (Scheme 11). Some examples of the addition of KAs are summarized in Table 11. 

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 the case of metal ion-promoted hydride transfer and cycloaddition reactions described above, binding of two metal ions to radical anions of electron acceptors can accelerate ET from electron donors to acceptors, leading to more efficient... 

This review article deals with addition and cycloaddition reactions of organic compounds via photoinduced electron transfer. Various reactive species such as exdplex, triplex, radical ion pair and free radical ions are generated via photoinduced electron transfer reactions. These reactive species have their characteristic reactivities and discrimination among these species provides selective photoreactions. The solvent and salt effects and also the effects of electron transfer sensitizers on photoinduced electron transfer reactions can be applied to the selective generation of the reactive species. Examples and mechanistic features of photoaddition and photocycloaddition reactions that proceed via the following steps are given reactions of radical cations with nucleophiles reactions of radical anions with electrophiles reactions of radical cations and radical anions with neutral radicals radical-radical coupling reactions addition and cycloaddition reactions via triplexes three-component addition reactions. 

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]. 

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. 

Two major side reactions compete with the coupling reaction protonation of the radical anion followed by further reduction and protonation leading to the saturated dihydro product, and polymerization induced by the basic dianion formed by coupling of two radical anions. Other, less typical reaction pathways include reaction between a radical anion and a molecule of substrate. Scheme 2, dimerization of two radicals formed by protonation of the initial radical anion. Scheme 3, or, infrequently, cleavage of the radical anion followed by coupling. However, for radical anions derived from monoactivated alkenes, the pathway in Scheme 2 has only been unequivocally established as a major pathway in a few cases in which the final zero-electron product is a cyclobutane, that is, a cycloaddition product. 

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