The organic chemistry of pericyclic reactions

Abstract

In the previous chapters it should have become abundantly clear that concerted pericyclic reactions are promoted either by thermal activation or by photochemical activation, and are almost always highly stereospecific. It is fair to say that in a large number of cases the photochemical reactions have not been investigated with sufficient thoroughness to be sure they are in fact concerted. Stereospecificity in a process is not an unambiguous criterion for concertedness. Implicit to a concerted reaction is the intervention between reactants and products of a single transition state; this ‘species’ has a lifetime of the order of the period of a vibration (c. 10⁻¹³ s). It is possible that in a stepwise reaction the intermediates may also, in certain circumstances, have exceedingly short kinetic lifetimes. If such a process is also stereospecific, as may well be the case, then the distinction between concertedness and non-concertedness becomes blurred, and may not be open to simple experimental tests.