Kinetic applications of electron paramagnetic resonance spectroscopy. 40. Intramolecular reactions of some N-alkylcarboxamidyl radicals

An extensive search for an amidyl radical that would undergo a rearrangement at a rate that could be measured by kinetic EPR spectroscopy, and could therefore serve as a free-radical clock, is described. Approximate or limiting rate constants at specific temperatures were obtained for some intramolecular H-atom abstractions and cyclizations onto C=C double bonds involving acyclic amidyl radicals and for some ring-opening reactions of N-cycloalkylamidyl radicals. These kinetic data have been combined with estimated Arrhenius preexponential factors to yield approximate activation energies. Rate constants at 300 K estimated for H abstractions via six-center cyclic transition states are 1 7 105 s-1 and 4 7 104 s-1 for abstraction from a methyl group in the alkyl and acyl moieties, respectively, and 5 7 106 s-1 for abstraction from an allylic position in the acyl group. For exo cyclizations, rate constants at 300 K are estimated to be 651 7 107 s-1 and 1 7 106 s-1 for the formation of five- and six-membered rings involving the acyl moiety but only 5 7 104 s-1 for the formation of a five-membered ring involving the alkyl moiety. Rate constants for the ring openings of N-cyclopropyl-, N-cyclobutyl-, and N-cyclopentylamidyl radicals are estimated to be 6b2 7 108 s-1, >4 7 108 s-1, and