18926-24-4Relevant articles and documents
Cyclisation of ω-(Isocyanatocarbonyl)alkyl Radicals: Acyclic Precursors of Imidyl Radicals
Kaushal, Parveen,Roberts, Brian P.
, p. 1559 - 1568 (2007/10/02)
Imidyl radicals, generated by photolysis of, or halogen-atom abstraction from, N-halogenoimides, are efficiently trapped by But2C=CH2 to give relatively persistent adducts which have been studied by e.s.r. spectroscopy.Bromine-atom abstraction from BrCH2CH2C(O)NCO (2) yields (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 But2C=CH2 and ButN=O.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) and triethylgermane.E.s.r. and product-analysis studies show that (14) cyclises, more rapidly than (1), to give the 2,2-dimethylsuccinimidyl radical which subsequently undergoes ring opening to yield (15).The overall rearrangement of (14) to (15) represents a 1,2-shift of the -C(O)NCO group via an intermediate imidyl radical.The glutarimidyl radical is formed by 1,6-endo cyclisation of .It is proposed that the rapid cyclisation of ω-isocyanatoalkyl radicals provides strong evidence that the unpaired electron occupies a ?-orbital in the product imidyl radicals.
On the Mechanism of N-Bromosuccinimide Brominations. Bromination of Cyclohexane and Cyclopentane with N-Bromosuccinimide
Tanner, Dennis D.,Ruo, Tomoki C.-S.,Takiguchi, Hideki,Guillaume, Andre,Reed, Darwin W.,et al.
, p. 2743 - 2747 (2007/10/02)
The competitive N-bromosuccinimide (NBS) bromination of cyclopentane vs. cyclohexane was shown to proceed by a mechanism dominated by either a bromine atom chain, a succinimidyl chain, or a mixed chain.The dominance of each of the major chain-carrying processes depends upon the solvents used, to some degree upon the reactivity of the substrate, and upon the additives (molecular bromine or ethylene) used to moderate or enhance one or the other of the chain processes.No evidence was obtained, from studies of the NBS halogenation of the two substrates used, which required the intermediacy of an excited-state succinimidyl radical to explain the reactivities obtained.The observation that β-bromopropionyl isocyanate is produced under all of the reaction conditions precludes the requirement that brominations using the NBS-Br2 reagent proceed exclusively by a radical species whose reactions do not correlate with a ring-opening process.
Excited-State ? Succimidyl and Glutarimidyl Radicals: Reversible Ring Opening
Tlumak, Robert L.,Day, James C.,Slanga, Joseph P.,Skell, Philip S.
, p. 7257 - 7267 (2007/10/02)
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.
