Fun with radicals

Radical reactions offer many of the properties desired by synthetic organic chemists, in terms of variety, mildness of conditions, and a selectivity that is often complementary to that of ionic chemistry, making many protection steps superfluous. There is however one major difficulty, which derives from the propensity of radicals to interact with themselves (dimerisation, disproportionation) with extremely fast rates that are close to diffusion. In order to overcome this complication, it is essential to keep the steady-state concentration of radical species very low. This can be accomplished for example by contriving a chain reaction where the propagating steps are themselves quite fast, as for example in the typical, and now extremely popular, stannane based processes. While various unimolecular cyclisation and fragmentation steps can be efficiently incorporated into the radical sequence, kinetically slower bimolecular transformations, and in particular intermolecular additions to un-activated alkenes, have proven more difficult to implement. In the case of stannanes, the relatively slow addition to the alkene has to compete with premature hydrogen atom abstraction from the organotin hydride, a step that is usually thousands of times faster.

Over the years, we have devised a number of methods that allow the generation of a variety of nitrogen and carbon centred radicals under conditions where the radicals can be captured in both inter- and intra-molecular fashion. By exploiting the specific properties of the thiocarbonyl group, the long-standing problem of intermolecular radical additions to un-activated alkenes could be solved in a simple, practical manner. These explorations have opened numerous synthetic opportunities and allowed an expedient access to complex structures not readily available by other routes. Furthermore, the possibility of iterating the process in one case has resulted in a powerful route to block-polymers, which has now been applied on an industrial scale. In this lecture, the story behind the discovery of some of these reactions will be described and the various mechanistic and synthetic implications will be briefly discussed.

Speaker: Professor Samir Zard,  Ecole Polytechnique, Université Paris-Saclay, France