Nucleophilic aliphatic radical processes

The reactivities of the substrate and the nucleophilic reagent change vyhen fluorine atoms are introduced into their structures This perturbation becomes more impor tant when the number of atoms of this element increases A striking example is the reactivity of alkyl halides S l and mechanisms operate when few fluorine atoms are incorporated in the aliphatic chain, but perfluoroalkyl halides are usually resistant to these classical processes However, formal substitution at carbon can arise from other mecharasms For example nucleophilic attack at chlorine, bromine, or iodine (halogenophilic reaction, occurring either by a direct electron-pair transfer or by two successive one-electron transfers) gives carbanions These intermediates can then decompose to carbenes or olefins, which react further (see equations 15 and 47) Single-electron transfer (SET) from the nucleophile to the halide can produce intermediate radicals that react by an SrnI process (see equation 57) When these chain mechanisms can occur, they allow reactions that were previously unknown Perfluoroalkylation, which used to be very rare, can now be accomplished by new methods (see for example equations 48-56, 65-70, 79, 107-108, 110, 113-135, 138-141, and 145-146)... 

Cyclo-coupling between arylalkenes and an aliphatic ester function is achieved by electrolysis in tetrahydrofuran using cathode and anode both of magnesium in an undivided cell. The first electron addition is to the arylalkene. The bond forming steps involves nucleophilic attack by radical-anions or dianions derived from the alkene. Magnesium ions generated at the anode are essential to the process. The... 

That is, the act of shifting the single electron from Y to X may occur either with or without free-radical formation. Usually, the concerted non-radicaloid process is energetically favoured. For a more detailed discussion of the various mechanisms of nucleophilic substitution reactions in aliphatic compounds and their solvent dependence, see references [14, 483, 782-785]. 

Even though fragmentation of radical anions represents a key step in SrnI reactions (Scheme 76) and in aliphatic nucleophilic substitution reactions (Sn2) proceeding via single electron transfer (Scheme 77), such processes and their mechanistic implications will not be discussed in this section (several reviews are available [271-277]). 

Photoamination of la with ammonia and aliphatic primary amines (RNH ) in the presence of m-DCB in MeCN-HjO (9 1) was kinetically analyzed by Yasuda and coworkers. The fluorescence of la was quenched by m-DCB at a diffusion-controlled limit, but not at all by RNHj, and photoamination of la with RNH did not occur in the absence of m-DCB. Therefore, an initiation process for photoamination is the photoinduced electron transfer from la to m-DCB. For kinetic analysis, Scheme 6.37, involving the nucleophilic addition of RNH to the cationic intermediate, was postulated. As the cationic species, the cation radical of la (D ) and the ion pair [D /... 

Thiols may be used as transfer agents in a wide variety of free radical polymerization processes. Scheme 1.12 shows the general reaction mechanism for this class of transfer agents. Nucleophilic radicals react more readily with thiols than electrophilic radicals, so transfer coefficients are higher for vinyl esters and styrene than for acrylates and methacrylates. Aromatic thiols react more readily than aliphatic ones, i.e., the chain transfer constant is higher, but they also show a stronger retardation effect as the resulting S-centered radicals are less prone for monomer addition due to their increased stability. The product of the transfer reaction is a thiyl radical, which is electrophilic and will react preferably with the more electron rich monomer in copolymerizations. 

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