Radical Aliphatic Substitutions

Thiols react more rapidly with nucleophilic radicals than with electrophilic radicals. They have very large Ctr with S and VAc, but near ideal transfer constants (C - 1.0) with acrylic monomers (Table 6.2). Aromatic thiols have higher C,r than aliphatic thiols but also give more retardation. This is a consequence of the poor reinitiation efficiency shown by the phenylthiyl radical. The substitution pattern of the alkanethiol appears to have only a small (<2-fokl) effect on the transfer constant. Studies on the reactions of small alkyl radicals with thiols indicate that the rate of the transfer reaction is accelerated in polar solvents and, in particular, water.5 Similar trends arc observed for transfer to 1 in S polymerization with Clr = 1.4 in benzene 3.6 in CUT and 6.1 in 5% aqueous CifiCN.1 In copolymerizations, the thiyl radicals react preferentially with electron-rich monomers (Section 3.4.3.2). 

By far the most generally useful synthetic application of allyltributyltin is in the complementary set of transition metal- and radical-mediated substitution reactions. When the halide substrates are benzylic, allylic, aromatic or acyl, transition metal catalysis is usually the method of choice for allyl transfer from tin to carbon. When the halide (or halide equivalent) substrate is aliphatic or alicyclic, radical chain conditions are appropriate, as g-hydrogen elimination is generally not a problem in these cases. 

Radical ions from arenes Birch reduction and arene oxidation Electron transfer in aliphatic substitution 38 Electron transfer in aromatic substitution 38 Electrochemical electron transfer 39... 

A new and general method of substitution at aromatic carbon has been uncovered. Reactions involve electron transfer steps and the formation of radical anion and radical intermediates. Many of the transformations are unprecedented in aromatic chemistry they represent major, new sequences which will have considerable value in syntheses. The radical anion method of substitution at aromatic carbon was first reported by Kim and Bunnett in 1970.96,97) The method is related to radical-anion substitution at aliphatic carbon.98, ")... 

Replacement of aliphatic hydrogens with bromine can be done under free radical substitution conditions, but reaction at aromatic carbons is unfavorable because of the very high energy of the aryl radical. Benzylic substitution is usually the only product observed. 

Alkanes are only poorly directly iodinated with iodine radicals. Aliphatic compounds with a double bond will be preferentially subject to an addition rather than to a substitution. Also, for aromatic compounds, labeling occurs mainly by the substitution of a halogen atom, and rarely by substitution of a hydrogen atom, with ortho and para positions favored, the ortho position being even more reactive if there is no steric hindrance (Argentini, 1982). 

This chain reaction is analogous to radical chain mechanisms for nucleophilic aliphatic nucleophilic substitution that had been suggested independently by Russell and by Komblum and their co-workers. The descriptive title SrnI (substitution radical-nucleophilic unimolecular) was suggested for this reaction by analogy to the SnI mechanism for aliphatic substitution. The lUPAC notation for the SrkjI reaction is (T -t- Dm -t- An), in which the symbol T refers to an electron transfer. When the reaction was carried out in Ihe presence of solvated electrons formed by adding potassium metal to the ammonia solution, virtually no aryne (rearranged) products were observed. Instead, reaction of 95c produced only 98 (40%) and 94 (40%) but no 99, and reaction of 96c produced 99 (54%) and 94 (30%) with only a trace of 98. ... 

If the characteristic group occurs only in a chain that carries a cyclic substituent, the compound is named as an aliphatic compound into which the cyclic component is substituted a radical prefix is used to denote the cyclic component. This chain need not be the longest chain. 

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)... 

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