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):

Ring opening of cyclopropylmethyl radical

When the ring contains a heteroatom there are a couple of different possibilities:

Epoxide/aziridine analogues

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

C–N bond cleavage, taken from ref 3

Is there an explanation for why the kinetically favoured products are different for X = O and NH?


  1. 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.

  2. 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.

  3. 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.


The 2-methyl-3-phenyloxiranylcarbinyl radical has been shown EXPERIMENTALLY to cleave the C-O bond kinetically while the C-C bond is cleaved thermodynamically. This means that, in this example, The oxygen-centered radical forms faster than the highly stable benzyl radical. The formation of the oxygen radical is necessarily reversible. See ref. 11 in your ref. 2 (Marples paper)

Ziegler, F. E.; Petersen, A. K. Allyloxy Radicals are Formed Reversibly from Oxiranylcarbinyl Radicals: A Kinetic Study, J. Org. Chem. , 1995, 60, 2666.

  • $\begingroup$ Fair enough, but there's the followup question: why is C–X cleaved kinetically despite forming a less stable radical? Thanks for pointing this out, nonetheless. I'll edit the question when I get a chance to. $\endgroup$ – orthocresol Nov 19 '17 at 21:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.