Bromine atom transfer

In Reaction (7.20) is reported the cyclization of a thermally unstable propar-gyl bromide cobalt complex mediated by Ph2SiH2 at room temperature and Et3B/02 as the radical initiator. However, a mixture of reduced and bromine atom-transfer products (1 1.8 ratio) are isolated due to the low hydrogen donation of the employed silane [31]. 

Iodocarbonyls are excellent substrates for atom transfer cyclization, as shown by examples from our recent work in Scheme 29.19-129 When two carbonyl (or cyano) groups are present, bromides can also serve as radical precursors. Photolysis with 10% ditin usually provides excellent yields of kinetic products at high concentration, and alkene substituents often dictate the regioselectivity. The y-iodo ester products are particularly versatile for subsequent transformations, which can often be conducted in situ. Although tertiary iodine products sometimes go on to give lactones or alkenes, primary and secondary iodides can often be isolated if desired. The last example is particularly noteworthy the kinetic product from the cyclization presented in Scheme 27 is trapped, because bromine atom transfer is much more rapid that reverse cyclization. 

Bromo-enamides have been reported to give radical cyclization in excellent yields (82-99%) to p-lactams using catalytic amounts (30%) of tripyridylamine (TPA) copper halide complex [184]. The p-lactam developed under mild conditions via 4-exo bromine atom transfer and subsequent elimination of the tertiary bromide that could be readily achieved by reaction with DBU (Scheme 83). 

The negligible solvent effect of this radical reaction with dibromomethane demonstrates that the activated complex for bromine atom-transfer has the same charge separation as the initial reactants. The dipole moment expected for a molecule like the pyridinyl radical is probably (0... 10) 10 ° Cm (0...3 D). Dibromomethane has a modest dipole moment of 5 10 Cm (1.5 D). Consequently, in view of the negligible solvent effect upon rate, the activated complex must also have a dipole moment between (0... 10) 10-3 Cm [214, 570]. 

Table 2. Rate and Equilibrium Constants for the Reversible Bromine Atom Transfer Reaction of Polymeric and Low Molecular Model Compounds with Cu—Complexes. For Ligand Structures, See Scheme 33...

Scheme 6.6 Bromine atom transfer reactions in organic and alternative media.

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. 

No product derived from the transannular hydrogen abstraction is observed in the addition of bromotrichloromethane because bromine atom transfer is sufficiently fast to prevent effective competition from the intramolecular hydrogen abstraction. 

Metathetical reactions of bromine atoms transfer of hydrogen... 

We showed that radical relay in the presence of phenyliodine dichloride but with an excess of CBr4 afforded the bromosteroid, with bromine at the point that is normally chlorinated [55]. When (SCN)2 was the additive, rather than CBr4, an SCN group was formed [55]. The mechanism shown (Scheme 6-8) involves a bromine atom transfer to... 

If both the radical source and the radical trap are in the same molecule, then an intramolecular atom-transfer radical cyclisation reaction occurs. In 2001, Yang et al. reported the oxazoline 39d-magnesium-catalysed bromine atom-transfer radical cyclisation reaction of unsaturated p-keto ester 49 for... 

Scheme 3.17 Bromine atom-transfer radical cyclisation reaction.

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