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Nitrogen-centred radicals hold much promise as useful synthetic intermediates. Even though their popularity is still extremely limited and very far from matching that of carbon radicals, the recent development of various routes allowing their generation under mild conditions and a better appreciation of their reactivity thanks to the increased availability of absolute rate constants should encourage their use. It is hoped that this tutorial review will help increase the awareness of synthetic chemists and help revive the interest in these forgotten species.

The capability of three chain‐transfer agents, O‐alkyl‐S‐(1‐ethoxycarbonyl)ethyl xanthates (CH3CHCO2C2H5)S(CS)OZ′, to control the free‐radical polymerization of styrene and ethyl acrylate by the MADIX process was examined. The reactivity of the xanthates varied according to the following trend: Z′  CH2CH3 < CH2CF3 < CH[P(O)(OEt)2]CF3. This change in reactivity allowed a lowering of the polydispersity index from 2.0 for Z′  CH2CH3 to 1.15 for Z′  CH[P(O)(OEt)2]CF3 in the case of the polymerization of styrene. Evolution of Mw/Mn with conversion during the polymerization of ethyl acrylate in the presence of xanthates X1, X2 and X3. Reaction conditions: [EA]0 = 4.6 M, [X]0 = 5.75 × 10−2 M, [AIBN]0 = 1.72 × 10−3 M ; T = 80 °C ; solvent: toluene.magnified imageEvolution of Mw/Mn with conversion during the polymerization of ethyl acrylate in the presence of xanthates X1, X2 and X3. Reaction conditions: [EA]0 = 4.6 M, [X]0 = 5.75 × 10−2 M, [AIBN]0 = 1.72 × 10−3 M ; T = 80 °C ; solvent: toluene.

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, eact 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‐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.

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Control of the free‐radical polymerization of styrene was achieved using a series of eight xanthates of the general structure RS(CS)OEt as reversible addition fragmentation chain‐transfer agents. The influence of the nature of the R leaving group was explored. It was found that the transfer ability of the xanthate is markedly improved with increasing stability of R and its steric hindrance. Group R strongly influences the Mn evolution profile during polymerization, but only influences the polydispersities to a small extent. The cyanoisopropyl group was shown to be the best leaving group, leading to an increase in the molecular weight during polymerization that was close to linearity. The living character of the polymerization and the high purity of the chain structures were supported by matrix‐assisted laser desorption ionization time‐of‐flight (MALDI‐TOF) mass spectrometry and 13C NMR spectroscopy. A zero‐order dependence of the kinetics of polymerization on the xanthate concentration was observed.

S-Alkyl and S-acyl xanthates are valuable sources of alkyl and acyl radicals which can be trapped inter-or intra-molecularly b y various alkenes in a radical chain process.