Reactions of a graded set of radicals with N-bromosuccinimide; two transition states

Recent controversy on imidyl radical chemistry

“…(i0)) has been established to be reversible in competition with the So reactions as shown by deuterium labelling experiments [55]. Therefore, low concentrations of NBS yielded lower BPI, and NCS, being a poor chlorine donor, failed to give 8-chloropropionyl isocyanate CTable II).…”

“…Since 1978, Skell and coworkers have published a series of papers [55][56][57] describing two reactive radical species, in addition to halogen atoms, during decompositions of NBS and other haloimides on the basis of extensive selectivity and product pattern analyses. Two radical species were generated by the propagation step in which NBS reacts with a series of radicals with graded reactivities (Eqs.…”

“…Some relative rate constants for hydrogen abstraction by the two radicals are distinctly different (Table III) and their relative reactivities toward 1 °, 2 ° and 3 ° hydrogens is a key indicator for the generation of So. Extensive studies of various halo imide decompositions showed that the rearrangement is facilitated by the formation of stabilized carbon radicals; e.g., the formation of the 3 ° alkyl radical from the 2,2'-dimethylsuccinimidyl radical was particularly favored [55,56].…”

The chain carriers of intermolecular hydrogen abstraction in the photobromination with N-bromosuccinimide; modifications of the chain carriers

“…Mechanistic studies of the photodecomposition of N-bromosuccinimide (NBS) under various conditions have caused many controversies over the number and nature of the chain propagating species (1)(2)(3)(4)(5)(6)(7)(8). The claim (4-7) of "the generation and identification of excited state and ground state X-II succinimidyl radicals" appears to arise from experimental artefacts and has been withdrawn (3).…”

“…The claim (4-7) of "the generation and identification of excited state and ground state X-II succinimidyl radicals" appears to arise from experimental artefacts and has been withdrawn (3). Recently it was shown (9, 10) that the photoexcitation of NBS in the presence of 1,l -dichloroethene (DCE) causes the, enhanced ring-opening reaction to form extra amounts of ~-'bromopropionyl isocyanate (BPI) from the excited state *NBS directly or via a vibrationally "hot" succinimidyl radical (tS') (Scheme 1); this observation has led to the discussion of probable sources of previous controversies (1)(2)(3)(4)(5)(6)(7)(8). Reviews of the NBS photodecomposition and details of the controversies are described in recent publications (1-3) and will not be repeated here.…”