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Reactions Involving Free Radicals
Free radical reactions involve one electron species, frequently generated by homolysis (shown below)
Radical reactions can be divided into three steps:1) Initiation2) Propagation3) Termination
Weaker bonds are more readily cleaved by homolysis
Some bonds can be cleaved by heating
Radical Bromination
Allylic (and benzylic) bromination with NBS (N-Bromosuccinimide)
NBS is regarded as a source of trace amounts of Br2 via the mechanism shown below.
Reductions utilizing tributyltin hydride as a hydrogen atom donor
The Sn-H bond is relatively weak (82 kcal/mole), relative to the C-H bond (99 kcal/mole)By contrast, tin forms stronger bonds to bromine, iodine, and sulfur than does carbon.
Sn-S 111 kcalSn-Br 132 KcalSn-I 56 kcal
C-S 65 kcalC-Br 68 kcalC-I 51 kcal
The Barton-McCombie Deoxygenation
Mechanism
Radical Reaction with Allyltributylstannane
The Barton Decarboxylation
