Non-perfect synchronisation of β-scission with product stabilisation in radical ring-opening reactions

Reduction of cyclobut-2-enylmethyl bromide with tri-n-butyltin hydride gave 3-methylcyclobutene together with trans- and cis-penta-1,3-diene and penta-1,4-diene. The diene products are formed by β-scission of the intermediate cyclobut-2-enylmethyl radicals to give pentadienyl radicals which accept hydrogen at the terminal and central carbon atoms. The rate constants and activation energies of β-scission in the series of radicals cyclobutylmethyl, 3-methylenecyclobutylmethyl, cyclobut-2-enylmethyl were shown to change little with the large increase in product radical stabilisation. This was accounted for in terms of the principle of non-perfect synchronisation, i.e. bond scission occurs before the development of resonance delocalisation. Semi-empirical MNDO calculations for the same series of radicals were in full agreement with this interpretation.