Grignard reactions single-electron transfer

However, there is evidence that reactions of aluminium hydride produced in situ involve single-electron-transfer (SET) processesThe reactions described by Trost and Ghadiri have most likely not been studied in sufficient detail to permit an adequate description of the reaction mechanism to be given at this stage. It is, however, quite likely that the Grignard reactions catalyzed by copper(II) and nickel(II) complexes , as developed by julia - and by Masaki , do involve SET processes, although, if this is so, the preservation of stereochemistry in some of the examples described by these workers is quite remarkable. (In this context, the reader s attention is drawn to Reference 196, end of this section.)... 

The most reactive Michael acceptors, such as alkylidene malonates, gem-dicyanoalkenes and nitroalkenes, react with a-halozinc esters in a conjugate fashion. Beautiful examples were offered by two stereocontrolled conjugate additions to piperidinone 102 and pyrro-lidinone 104 leading to optically active bicyclic lactams 103147 (equation 60) and 105 (equation 61)148. With these electron-poor alkenes a Grignard two-step protocol is to be adopted in order to avoid the single electron transfer reactions from the metal to the Michael acceptor, which should afford olefin dimers. The best solvent is found to be a... 

However, in a recently published, detailed study on the stereochemistry of the addition reaction of allylic Grignard reagents (mainly crotylmagnesium chloride) with a,/ -ethylenic ketones [15], addition to aryl-substituted enones occurred without any selectivity at all. The authors suggested that this was due to a single-electron transfer, since poor diastereoselectivity is observed when a carbon-carbon bond is formed by a radical- radical combination [64]. No further mechanistic details on this difference in reactivity were given. 

It has been made clear, in Chapter 11, that the nature of the Grignard reagent has a great influence on the mechanism of its reactions. The extremes in the reactivity spectrum of organomagnesium compounds in their interaction with ketones, for example, are (I) reactions in which a single electron transfer occurs as the first step, or (2) reactions in which a concerted bond-breaking-bond-making takes place within a cyclic transition state. 

This chapter will deal with the predictability of the results of reactions of organomagnesium compounds, in general, and of Grignard reagents, in particular, with substrates of different kinds. It is evident that such a predictability requires a clear understanding of the mechanisms of the reactions involved. For the single-electron transfer reactions (see Chapter 11 for the discussion of the mechanism of such reactions), encouraging results have recently been obtained. 

Many reactions of N Mg and C Mg (Grignard) reagents were classified in terms of efficiency of single-electron transfer (SET). From the detailed study of product distribution, minor factors arising from the aggregation of excess reagent molecules and from fx-complexation of reactants were disclosed. 

It has been reported that the MIRC reaction of brominated alkylidenemalonates carrying a tertiary halide with nucleophiles occurs by a single-electron transfer from the initial Michael adduct anion to the nonbonding orbital of the carbon-halide bond. The resulting 1,3-diradial then collapses to the final cyclopropane. The results obtained with Grignard compounds as nucleophilic component are poor (Table 22, entries 1 and 2). 

On the basis of these observations the draft mechanism shown in Scheme 3.160 has been proposed for the catalytic reaction, by analogy with the previous reaction. The reaction of CoCl2(dpph) with M( jSiCH2MgOl gives complex 168, which is electron-rich, because of coordination of the Grignard reagent. Complex 168 effects single-electron transfer to an alkyl halide to yield an anion radical of the halide and cobalt complex 169. Immediate loss of halide from the anion radical affords an alkyl radical intermediate, which adds to styrene to yield a benzyl radical. Cobalt species 169 would then recombine with the carbon-centered radical to form cobalt species 170. Finally, /i-hydride elimination provides... 

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