Cyclisation of ω-(isocyanatocarbonyl)alkyl radicals: acyclic precursors of imidyl radicals

Abstract

Imidyl radicals, generated by photolysis of, or halogen-atom abstraction from, N-halogenoimides, at efficiently trapped by Bu₂^tCCH₂ to give relatively persistent adducts which have been studied by e.s. spectroscopy. Bromine-atom abstraction from BrCH₂CH₂C(O)NCO (2) yields H₂ĊCH₂C(O)NCO (1) which undergoes rapid 1,5-endo cyclisation to give the succinimidyl radical. This cyclisation has been investigated using e.s.r. spectroscopy in conjunction with spin-trapping by Bu₂^tCCH₂ and Bu^tNO. The rate coefficient for cyclisation of (1) has been estimated to be 3.7 × 10⁶ s^–1 at 328 K in cyclohexane from analysis of the products from the radical-chain reaction between (2) an triethylgermane. E.s.r. and product-analysis studies show that H₂ĊCMe₂C(O)NCO (14) cyclises, mor rapidly than (1), to give the 2,2-dimethylsuccinimidyl radical which subsequently undergoes rin opening to yield Me₂ĊCH₂C(O)NCO (15). The overall rearrangement of (14) to (15) represents 1,2-shift of the –C(O)NCO group via an intermediate imidyl radical. The glutarimidyl radical is forme by 1,6-endo cyclisation of H₂ĊCH₂CH₂C(O)NCO. It is proposed that the rapid cyclisation of ω-isocyanatoalkyl radicals provides strong evidence that the unpaired electron occupies a σ-orbital in th product imidyl radicals.