Absolute Rate Constants for Bromine Abstraction from N-Bromoimides and Br2 by Alkyl Radicals

Article abstract of DOI:10.1021/ja00218a035

Imidyl radicals react with cyclopropanes solely via hydrogen abstraction.In the case of methylcyclopropane, the major product (cyclopropylcarbinyl bromide) is derived from abstraction of hydrogen from the methyl group.The resultant cyclopropylcarbinyl radical is partioned between two pathways: (1) abstraction of Br from N-bromoimide and (2) rearrangement to the allylcarbinyl radical (eventually yielding 4-bromo-1-butene).Since the absolute rate of the rearrangement is known, an absolute rate constant for the abstraction of Br from N-bromoimides by alkyl radicals can be derived (CH2Cl2 solvent, 15 deg C), k ca. (1.3-1.6)1xE10 M^-1s^-1.Reactions carried out in the presence of Br2 provide a third pathway for scavenging of the cyclopropylcarbinyl radical, providing k_Br2=2.2x1E10 M^-1s^-1.Thus, trapping of primary R^. by either N-bromoimides or Br2 occurs at rates that are diffusion-controlled.