When a radical is alpha to a three-membered ring, it is common for the ring to open via β-scission (to relieve ring strain):
When the ring contains a heteroatom there are a couple of different possibilities:
Ab initio studies of these reactions1 showed that for both X = O and X = NH, the C–C cleavage was thermodynamically favoured (presumably due to stabilisation of the subsequent radical by the adjacent lone pair). For X = O, the C–C cleavage was also found to be kinetically favoured, but for X = NH, the C–N cleavage is kinetically favoured!
This regioselectivity is also backed up by experiment2 and also has potential for use in synthesis.3
Is there an explanation for why the kinetically favoured products are different for X = O and NH?
Pasto, D. J. Ab Initio Theoretical Studies on the Ring-Opening Modes of the Oxiranyl-, Aziridinyl-, Oxaziridinyl-, and Thiaranylmethyl Radical Systems. J. Org. Chem. 1996, 61 (1), 252–256. DOI: 10.1021/jo9505942.
Marples, B. A.; Toon, R. C. An investigation into the β-cleavage of aziridinylcarbinyl radicals. Tetrahedron Lett. 1999, 40 (26), 4873–4876. DOI: 10.1016/S0040-4039(99)00900-4.
Prévost, N.; Shipman, M. Intramolecular Radical Rearrangement Reactions of 2-Methyleneaziridines: Application to the Synthesis of Substituted Piperidines, Decahydroquinolines, and Octahydroindolizines. Org. Lett. 2001, 3 (15), 2383–2385. DOI: 10.1021/ol016194k.