SRN1 reactions transfer

The mechanistic aspects of the SRN1 reaction were discussed in Section 11.6 of Part A. The distinctive feature of the SRN1 mechanism is an electron transfer between the nucleophile and the aryl halide.181 The overall reaction is normally a chain process. 

Since the publication of the review on Single Electron Transfer and Nucleophilic Substitution in this same series,1 reviews or research accounts have appeared concerning several particular points among those addressed here, namely, dynamics of dissociative electron transfer,2-6 single electron transfer and Sn2 reactions,2,7 9 and SRN1 reactions.10,11... 

It is a pleasure to acknowledge the essential contribution of Dr. C. P. Andrieux to most of the work reported above as well as that of Dr. D. Lexa in the field of porphyrins, Professor Moiroux and Dr. A. Anne to cation radical reactivity, Dr. M. Robert to photoinduced dissociative electron transfer and to the stepwise/concerted competition and Drs. P. Hapiot and Medebielle to recent work on thermal SRn1 reactions. Many students from our group have also contributed effectively to the work, namely, C. Costentin, G. Delgado, V. Grass, A. Le Gorande, C. Tardy and D. L. Wang. Fruitful and pleasant... 

There are not many successful examples of arylation of carbanions by nucleophilic aromatic substitution. A major limitation is the fact that aromatic nitro compounds often react with carbanions by electron-transfer processes.111 However, such substitution can be carried out under the conditions of the SRN1 reaction (see Section 11.4). 

The most common electron transfer process in organic chemistry, the SRN1 reaction, takes place somewhat differently. This particular reaction [e.g. (90)]... 

Another instance is the benzylation of AW-dimethylanilines and derivatives upon irradiation of benzyl cyanides, which cleave via the radical anion formed by electron transfer [212]. Yet another path for the benzylation of (hetero)aromatics is the SRN1 reaction, as shown by the smooth photoassisted reaction of the enolate from ethyl phenylacetate with 2-bromopyridine (77% yield) [213]. 

The fragmentation of radical anions and the reverse reaction, the addition of anions to radicals, are the critical steps of SRN1 reactions [110] which constitute perhaps the largest class of fragmentation reactions initiated by photoinduced electron transfer. These reactions are chain processes and photoinduced ET is involved only in the initiation step, which is usually poorly defined. The reactions may also be initiated by other means, not involving absorption of a photon. The SRN1 reactions and related redox-activation processes have been recently extensively reviewed [72a, 110,127] and will not be discussed here. 

Photoreactions employing an electron transfer are discussed. Among these are recent examples of photochemical SRN1 reactions, photoalkylations of carbanions and photoreductions initiated by oxyanions and radical anions. Anions used in their ground state as electron donating quenchers are also considered. Intra ion pair electron transfers as well as the use of anion-like precursors in charge transfer complexes or charge transfer excited states are presented. 

The situation may however be more complicated if charge transfer complexes (CTC) between the substrate and the anion are formed. This was suggested [104] in the case of the SRN1 reaction between ethyl-, phenyl- and diethyl phosphite anion in DMF where the UV spectrum of the mixture shows an enhanced absorption attributed to a CTC, the last one being the possible mediator for the photochemical activation of the reaction. 

The possibility of a thermally activated electron transfer from an anion to an acceptor is not always excluded. As explained by Bordwell [106], this will of course depend on the oxidation potential of the anion or on its related basicity and this author has shown that the electron transfer reactivity of carbanions rapidly decreases with a decrease of basicity. It is thus possible that among the dark SRN1 reactions some of them are activated by an initial ground state electron transfer from the anion to the accepting substrate. 

Perhaps the most common use of carbanions in organic photochemistry is in the synthetically useful SRN1 reaction. The reaction proceeds via a radical chain mechanism, which requires the transfer of an electron in an initiation step. Photoinduced electron transfer from a carbanion, which also serves as the nucleophile, is a convenient and mild method of initiation. A generalized mechanism is shown in Scheme 9. The excited state anion, with its enhanced... 

It is clear from this review that the topic of photoinduced electron transfer from carbanions is well-developed, with important synthetic applications, as exemplified by the SRN1 reactions. There is sufficient data available to indicate that electron transfer from photoexcited carbanions is a reasonably general process. It is now possible to predict with some certainty which systems will undergo PET. This area will see continued development especially with respect to the details of reaction. Much less is known with respect to PET to carbocations. However, it is clear that this is a developing area and the examples presented provide us with new opportunities for exploratory studies. Whereas neutral molecules have been traditional substrates for PET studies in the past, it is clear that both carbanions and carbocation can also serve as substrates for such investigations, which may lead to interesting results. 

The SRN1 reaction thus appears as a reaction in which single electron transfer plays a pre-eminent role but is by no means a single elementary step. A different problem is that of the possible involvement of single electron transfer in reactions that are not catalysed by electron injection (or removal). A typical example of such processes is another substitution reaction, namely,... 

In other classes of organic halides, for example perfluoroalkyl and vinyl halides, the distinction between stepwise and concerted electron-transfer-bond-breaking upon reduction by outer sphere heterogeneous and/or homogeneous electron donors is less unambiguous than in the case of aryl and alkyl halides. As discussed in Section 3, they also present the interest of being active substrates in SRN1 reactions. 

A different mechanism for the SRN1 reaction has been proposed in an attempt to unify conceptually the SN2, SRN1 and SN1 mechanisms (Shaik, 1985). As sketched in Scheme 14, one electron is transferred, thermally or... 

The question we address now is that of the possible role of single electron transfer in substitution reactions that, unlike SRN1 reactions, are not catalysed by electron injection. The problem is twofold. One side of it consists in answering the questions do bond breaking and bond formation belong to two different and successive processes, i.e. (135) followed by (136), or, more... 

Light can often be used to promote SRN1 reactions [2]. Indeed, the photo-chemically induced, cobalt-catalyzed carbonylation of haloarenes, PhCl included, readily occurs under phase-transfer conditions. This interesting methodology was first developed by Brunet, Sidot, and Caubere [23,69] and subsequently used for the carbonylation of various chloroarenes in the presence of catalytic amounts of cobalt compounds (Sect. 3.3). 

Substitution reactions by anions at carbon are also known to occur by initial electron transfer. The mechanism of such transformations was first characterized by Russell and Danen (14) and Kornblum et al. (15), and Bunnett (16) significantly developed its applications and named it the SRN1 reaction an... 

Nitroarenes, on the other hand, are strong electron acceptors and easily undergo one-electron reduction (12, 13). Thus, nitrobenzene, to cite one example, has been customarily used as an effective quencher in chain reactions involving radical anion intermediates, such as in SRN1 reactions (3). Under different conditions, nitroarene radical anions are reactive species. In particular, Zinin (14) reported that treatment of nitroarenes with hot alkaline alcoholic solutions results in products of reduction, mainly the azoxy derivative (equation 2). These complex multistep processes involve nitroarene radical anion intermediates and are quite effectively inhibited by oxygen (10, 15, 16). In 1964, Russell et al. (17) wrote that apparently much of the chemistry of aromatic nitro, nitroso and azo compounds in basic solution involves electron-transfer processes . 

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