Mechanistic aspects of the abstraction of an allylic hydrogen in the chlorine atom reaction with 2-methyl-1,3-butadiene (isoprene)

The different channels for the abstraction of an allylic hydrogen in the ch lorine atom reaction with isoprene were explored using ab initio methodolog y. It is shown that the metathesis reaction proceeds through an association -elimination mechanism in which a weakly bound intermediate (HCl . . .C5H7. ) is formed first (formal addition). Further evolution by HCl elimination l eads to the final C5H7. radical. QCISD(T)/aug-cc-pVDZ//MP2/6-31G(d,p) calcu lations show that for two of the possible pathways the barrier heights invo lved are moderate and the formation of the intermediates are exergonic (Del taG< 0). Therefore, the mechanism proposed is both kinetically and thermody namically feasible. The pressure dependence experimentally observed for the Cl + isoprene reaction can be rationalized in terms of the association-eli mination mechanism proposed.