On the Trail of Xanthates: Some New Chemistry from an Old Functional Group

The use of free radical reactions in organic synthesis has witnessed an extraordinary development in recent years. When properly conceived, radical processes often exhibit many of the properties desired by synthetic organic chemists, such as flexibility, mildness, and selectivity. Unfortunately, the number of synthetically useful radical-generating systems is still limited, and most applications have relied on tin hydride chemistry. Secondary O-alkyl-S-methyl xanthates, for example, react with tributyltin hydride to give the corresponding alkane (the Barton-McCombie reaction). It was, however, found that the isomeric O-methyl-S-alkyl xanthates undergo cleavage of the weaker carbon sulfur bond and that a chain reaction can be sustained without tin or other heavy metals. A variety of synthetically interesting free radicals can thus be produced and captured, the last propagating step being a reversible transfer of the xanthate group. S-Propargyl xanthates represent a special class. Their radical chemistry can be easily overshadowed by hitherto unknown but equally interesting nonradical behavior. Upon heating, a thermal [3,3] sigmatropic rearrangement occurs to give the allenyl isomer, which is in equilibrium with a novel betaine structure. This species is at the heart of a number of new transformations, including formal [3 + 2] cycloadditions, formation of esters with inversion in the case of secondary alcohols, conversion into 1,3-dithiol-2-ones, generation of cisoid dienes, carbon-carbon bond formation through reaction with acid chlorides etc. This account provides a brief description of this original radical and nonradical chemistry of xanthates, an old family of compounds that still harbors many mysteries.