Excited-State ? Succimidyl and Glutarimidyl Radicals: Reversible Ring Opening

Article abstract of DOI:10.1021/ja00389a058

The free-radical isomerization of N-bromosuccinimide to β-bromopropionyl isocyanate has been examined.Of the two varieties of succinimidyl radical (S_? or S_?), only the ? excited state undergoes the ring opening to the β propionyl isocyanatyl radical.The conversion optimally takes place in >95percent yield.The dependence of NBS concentration along with results obtained from deuterium labeling studies indicate that the ring opening of S_? is a reversible process.This explains the failure of N-chlorosuccinimide to produce β-chloropropionyl isocyanate, as well as the increase in ring-opened product for N-bromosuccinimides upon methyl substitution at the 2- and/or 3-position of the succinimidyl ring, since the open-chain radical intermediates are more stable.In the N-bromoglutarimide system, methyl groups on the 2-position are required for the glutarimidyl radicals to undergo the isomerization, ultimately producing isocyanates.The radical-chain nature of these systems is confirmed.