Bromine, free radical transfer reactions

Bromination in the absence of free radical catalysts, for example, gives high yields of 1-bromoadamantane (Eq. (53)) 18s The Koch 189> and Ritter 188> 19°) reactions, which involve the initial generation of the 1-ada.-mantyl cation either by means of hydride transfer to the t-butyl cation or... 

Catalysis of these reactions by potassium bromide has been observed.232,236 Aralkyl bromides are formed as intermediates that undergo acetolysis to the corresponding acetates under the reaction conditions. Results236 were consistent with a free radical mechanism, even with reactive toluenes, in contrast to the electron transfer mechanism observed in the absence of potassium bromide. Relative reactivities236 corresponded closely to those observed in photochemical brominations, suggesting that bromine atoms, formed by electron transfer oxida-... 

Since the pyridinyl radical is soluble in n-hexane, and BrCHjBr has a dipole moment of l.OD, the initial state is not very polar and the transition state can thus not be very polar. A free-energy versus reaction coordinate diagram (Fig. 24a) illustrates the point. The rate-limiting step must be the transfer of a bromine atom from the halocarbon to the pyridinyl radical, yielding a bromomethyl radical and one or two bromodihydropyridines. The latter dissociate to the pyridinium bromide, while the former combines with a second pyridinyl radical to form two bromo-methyldihydropyridines. The mechanism is shown in Fig. 25 for the reaction with BrCH Cl. 

Other atoms and groups apart from hydrogen are susceptible to abstraction by free radicals. The most important from a synthetic point of view are bromine, iodine, sulfur, and selenium substituents. Group transfer reactions can occur inter- or intramolecularly. Indeed, we have already encountered one example in the addition of polyhalogenated methanes to alkenes. The chain is propagated by a bromine atom transfer. 

End functionalization can be achieved by the addition of allyl bromide to form the corresponding vinyl-functionalized polymer. Allyl bromide acts as a chain transfer agent (CTA) in free-radical polymerizations and can be used to prepare chain-end-functionalized polymer via controlled free-radical polymerizations. The functionalization reaction proceeds via addition followed by fragmentation (i.e., elimination of a bromine radical). When ATRP is quenched with allyl bromide, a bromine radical is eliminated resulting in the formation of a Cu (II) species. The Cu(II) drives the equilibrium to the deactivated species, reducing the propensity of the chain end to add to allyl bromide. In order to overcome this, Cu(0) needs to be added to... 

Silane radical atom transfer (SRAA) was demonstrated as an efficient, metal-free method to generate polystyrene of controllable molecular weight and low polydispersity index values. (TMSlsSi radicals were generated in situ by reaction of (TMSlsSiH with thermally generated f-BuO radicals as depicted in Scheme 14. (TMSlsSi radicals in the presence of polystyrene bromide (PS -Br), effectively abstract the bromine from the chain terminus and generate macroradicals that undergo coupling reactions (Reaction 70). 

In non-polar solvents a decrease in yield was observed in the reactions with N-bromosuccinimide, and a loss of stereoselectivity in the reaction of the exo-isomer with bromine. The inversion of configuration in the reaction with N-bromosuccinimide was explained by direct electrophilic displacement (Scheme 5), but the reaction with bromine was considered to proceed by an initial electron-transfer step, followed by nucleophilic attack of bromide ion on the resulting organopentafluorosilicate radical-ion (Scheme 6). Steric constraints or a reduction in the polarity of the solvent would allow dissociation of the radical ion to a free alkyl radical, and loss of stereoselectivity, as observed (Scheme 7). 

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