Electron transfer radical coupling sequence

FIGURE 27. The polar addition mechanism and electron transfer-radical coupling sequence in the addition of lithium pinacolone enolate to benzaldehyde. Reprinted with permission from Reference 28. Copyright 1997 American Chemical Society... 

Two possible reaction pathways have been proposed421 for the Barbler-type carbonyl addition (equation 220) the polar (PL) route and the electron transfer-radical coupling (ET-RC) sequences with rate-determining ET or rate-determining RC (equation 221). 

Therefore, it has been considered that the formation of the dimer involves a mechanism different to the simple head-to-head radical coupling of the parent monomer. As suggested by the authors, it is likely that the overall mechanistic sequence is initiated by the radical-anion 472 of compound 469 formed by a single electron transfer (SET) process, which is the first stage of the bromine-lithium exchange (Scheme 68) [128],... 

All these observations point to the occurrence of a 8 2 rather than an outer sphere, dissociative electron-transfer mechanism in cases where steric constraints at the carbon or metal reacting centres are not too severe. It is, however, worth examining two other mechanistic possibilities. One of these is an electrocatalytic process of the Sg -type that would involve the following reaction sequence. If, in the reaction of the electron donor (nucleophile), the bonded interactions in the transition state are vanishingly small, the alkyl radical is formed together with the oxidized form of the electron donor, D . Cage coupling (144) may then occur, if their mutual affinity is... 

The addition of RLi and other nucleophiles to carbonyl functions in general proceeds via one of the two possible reaction pathways, polar addition (PL) and electron transfer (ET)-radical coupling (RC) sequence (equation 5). Current reaction design for the synthetic purpose of additions of common nucleophiles to aldehydes and ketones is mostly based on the polar mechanism, but apparently the ET process is involved in some reactions of, for example, Grignard reagents Mechanistically there are three possible variations the PL pathway, the ET rate-determining ET-RC route and the RC rate-determining ET-RC route. 

Radicals add to unsaturated bonds to form new radicals, which then undergo addition to other unsaturated bonds to generate further radicals. This reaction sequence, when it occurs iteratively, ultimately leads to the production of polymers. Yet the typical radical polymerization sequence also features the essence of radical-induced multicomponent assembling reactions, assuming, of course, that the individual steps occur in a controlled manner with respect to the sequence and the number of components. The key question then becomes how does one control radical addition reactions such that they can be useful multicomponent reactions Among the possibilities are kinetics, radical polar effects, quenching of the radicals by a one-electron transfer and an efficient radical chain system based on the judicious choice of a radical mediator. This chapter presents a variety of different answers to the question. Each example supports the view that a multicomponent coupling reaction is preferable to uncontrolled radical polymerization reactions, which can decrease the overall efficiency of the process. 

In several photochemical electron transfer reactions, addition products are observed between the donor and acceptor molecules. However, the formation of these products does not necessarily involve direct coupling of the radical ion pair. Instead, many of these reactions proceed via proton transfer from the radical cation to the radical anion, followed by coupling of the donor derived radical with an acceptor derived intermediate. For example, 1,4-dicyanobenzene and various other cyanoaromatic acceptors react with 2,3-dimethylbutene to give aromatic substitution products, most likely formed via an addition-elimination sequence [140]. 

The reactions leading to the formation of these polymers—except polyphenylene—have one feature in common, although they otherwise differ greatly in mechanism the crucial step in the reaction sequence is a one-electron transfer from the monomer to a transition metal ion serving as an electron acceptor. In addition to being an electron acceptor the transition metal ion is probably also involved in the coupling reaction by complexation of radical-like intermediates produced. 

Intramolecular coupling of carbonyl groups with vinylbromides after an addition-elimination sequence involves two electron-transfer steps. In the first step the ketone is reduced to the ketyl radical anion. After 5-exo-trig radical addition of the vinylbromide group, a primary radical is generated which is subsequently reduced to give an alkyl samarium species which eliminates bromide. This reaction... 

The foregoing equations suggest that either electron transfer (eq 14b) or detachment (eq 14c) is the rate-determining step. The oxidized reactant Ox is often a radical that may undergo further non-rate-determining reactions with the oxidant. Equations 14a and 14b may be coupled. The reaction sequence accounts for the observation (24-27) that the reaction rate, R, is proportional to the density of the surface concentration of the surface species, =FeM,R (mol/m2), provided that the concentration of the oxidized reactant Ox is at steady state or is negligible. The reaction rate is given by... 

Products of addition to styrene double bonds can arise as a result of light induced electron transfer reactions. Lewis has studied the intramolecular reaction of l-phenyl-w-amino alkenes (422) 289,290 products arise from electron transfer from the amine nitrogen to the excited state of the styryl group followed by intramolecular proton transfer in the radical ion pair produced. The resultant biradical then couples to yield the isolated products (423) and (424). Sensitisation of the intermolecular analogue of this reaction by 1,4-dicyanobenzene has been reported and is proposed to occur by electron transfer from the styrene to the excited state of the sensitiser followed by attack of an amine on the styrene radical cation. This ultimately leads to the product of anti-Markovnikov addition of the amine across the double bond of the styrene. This is similar to the sequence long since established by... 

Reductive alkylation of N-methylacridinium (87) occurs when it is irradiated with carboxylic acid salts. The reaction is thought to proceed by electron transfer from the carboxylate to the excited acrldinium ring followed by decarboxylation of RCOO coupling of the alkyl radical produced with the acridinyl radical then gives (88). A very similar sequence probably occurs in a reaction proposed as a synthetic procedure for decarboxylation of carboxylic acids.In this case an aza-aromatic compound such as acridine is irradiated with a carboxylic acid in benzene in the presence of tert-butyl thiol. The authors propose that a hydrogen bonded acridine-acid complex is excited and that adiabatic proton transfer is followed by electron transfer. This produces RCOO which decarboxylates, and reduction of the alkyl radical then ensues. The major fate of the acridine is coupling to (89) if the reaction is perfonned in the absence of oxygen. 

---