Radical Alliances. Solutions and Opportunities for Organic Synthesis

School of Chemistry Seminar Professor Sam Zard (Ecole Polytechnique, Paris) "Radical Alliances. Solutions and Opportunities for Organic Synthesis" William Bragg LT (2.37)

Dear colleagues,
Please join us for a seminar from Professor Sam Zard, a giant in the field of radical chemistry.  Sam’s research encompasses synthetic and mechanistic aspects of radical chemistry; in particular his pioneering use of xanthates as radical precursors has not only found widespread application in organic synthesis, but led Sam to be one of the co-discoverers of the RAFT/MADIX polymerisation method.

Biography: Samir Z. Zard was born in 1955 in Ife, Nigeria. His training as a chemist started at the American University of Beirut, then at Imperial College, London, and finally at the Université Paris-Sud, Orsay, France, where he received his doctorate in 1983 under the supervision of Professor Sir Derek Barton. His main research concerns the study and development of new reactions and processes, with a special interest in radicals, organosulfur derivatives, alkynes, and nitro compounds. In addition to a number of academic awards, he received in 2007 the Croix de Chevalier de la Légion d’Honneur.

Abstract: 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 shown that xanthates and related thiocarbonylthio derivatives allow the generation of radicals under conditions where the radicals possess a considerably increased effective lifetime even in a concentrated medium. Intermolecular additions to un-activated alkenes, as well as a variety of reputedly difficult radical transformations can now be easily accomplished. No metals, heavy or otherwise, are required, and the starting materials and reagents are cheap and readily available. Complex, densely functionalized structures can be constructed in a convergent, modular fashion. In the course of our study of the scope and limitations of this chemistry, we have uncovered a few surprising transformations. Recent results and some mechanistic aspects will be presented and discussed briefly.