Xanthates and Related Derivatives as Radical Precursors Synthetic Strategies and Applications
Résumé
This chapter deals with the various radical reactions involving dithiocarbonates (xanthates) and related structures. It will revolve around two main topics: (i) cleavage of the CO bond and (ii) cleavage of the CS bond. The first mode corresponds to the Barton-McCombie reaction and variants thereof. The mechanism of this important reaction is discussed in detail and then illustrated with a few varied examples taken from the recent literature (in order not to overlap too much with existing reviews). Modifications not using organotin hydrides are also described, with emphasis on the practical aspects. The incorporation of the Barton-McCombie process in various CC bond-forming radical sequences is also be presented. Included in this part is an O → S radical chain rearrangement that is sometimes observed. The second mode encompasses the degenerative fragmentation of xanthates. Radicals generated this way have a relatively long lifetime in the medium, allowing various transformations otherwise difficult to perform. Intermolecular additions to unactivated olefins are one such case. The possibility of using the adduct, itself a xanthate, for another radical sequence can also be exploited. Intra- and intermolecular additions to aromatic and heteroaromatic rings, as well as radical-based modifications of the xanthate group, are other possibilities that are described and illustrated with examples