Aromatic substitution chain transfer

It was known that polystyrene and poly- >-methoxystyrene initiated by tin tetrachloride have a branched structure, due to aromatic substitution in the course of the polymerization (186). Haas, Kamath and Schuler (93, 124) studied the ionic chain transfer reaction between a polystyrene carbonium chain and poly-/>-methoxystyrene. They were able to separate the homopolymers from the graft copolymers by extraction with methylcyclohexane. 

Equation 2.1). Three propagation steps then follow, including dissociation of the radical anion to an aryl radical and X- (Equation 2.2). In contrast, the corresponding alternative SN1 reaction would lead to the much less stable aryl cation (the empty p-orbital is part of the a-framework, and so cannot be stabilised by the n -electrons). The aryl radical then reacts rapidly with another nucleophile (Y in general or NH2- in this case) to give another radical anion (Equation 2.3) then electron transfer from one radical anion to another reactant molecule (Equation 2.4) initiates another chain. The overall consequence of the three propagation steps is nucleophilic aromatic substitution (Equation 2.5). 

Several chain transfer to polymer reactions are possible in cationic polymerization. Transfer to cationic propagating center can occur either by electrophilic aromatic substitution (as in the polymerization of styrene as well as other aromatic monomers) or hydride transfer. Short chain branching found in the polymerizations of 1-alkenes such as propylene may be attributed to intermolecular hydride transfer to polynier. The propagating carbocations are reactive secondary carbocations that can abstract tertiary hydrogens from the polymer ... 

But before 1980, the foundations for essentially all modern synthetic radical reactions had been laid, sometimes by synthetic organic chemists but more often by physical organic chemists. Kharasch reactions (now often called atom transfer reactions) were known since the 1930s and 1940s, and tributyltin hydride was introduced in the 1960s. In the 1970s, SnAc reactions and redox chain aromatic substitutions (Minisci reactions) were already topical, and allylations with allyl-tributylstannane were first described. In short, there were a number of ways to generate and trap radicals on the one hand, and a number of fundamental transformations of radicals such as addition and cyclization to multiple bonds on the... 

Recent mechanistic investigations have defined a group of nucleophilic aromatic substitutions which proceed by a pathway described as SrnI. The key is an electron transfer from the nucleophile to an aryl halide. The process is a chain... 

Almost quantitative radical nucleophilic aromatic substitution reactions were reported by Corsico and Rossi from the reaction of nucleophile trimethylstannyl ions with mono-, di- and trichloro-substituted aromatic substrates in Hquid ammonia.The chain process shown in Scheme 9 requires an initiation step, and it can be either Hght induced or be a spontaneous electron transfer from the nucleophile to the aromatic substrate. 

Amatore, C., Pinson, J., Saveant, J. M., and Thiebault, A., Electron-transfer-induced reactions, termination steps and efficiency of the chain process in Sgjjl aromatic substitution, /. Am. Chem. Soc., 103, 6930, 1981. 

A useful application in the manufacture of ion-exchange resins may well be possible which avoids the use of carcinogenic chloromethyl ether. Here, a polymer of p-methyl styrene is chlorinated on the side chain with aqueous NaOCl and a phase-transfer catalyst. Sasson et al. (1986) have shown how stubborn . substituted aromatics like nitro/chlorotoluenes can be oxidized to the corresponding acids by using aqueous NaOCl containing Ru based catalyst. 

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