Many recent studies have used KOtBu in organic reactions that involve single electron transfer; in the literature, the electron transfer is proposed to occur either directly from the metal alkoxide or indirectly, following reaction of the alkoxide with a solvent or additive. These reaction classes include coupling reactions of halobenzenes and arenes, reductive cleavages of dithianes, and SRN1 reactions. Direct electron transfer would imply that alkali metal alkoxides are willing partners in these electron transfer reactions, but the literature reports provide little or no experimental evidence for this. This paper examines each of these classes of reaction in turn, and contests the roles proposed for KOtBu; instead, it provides new mechanistic information that in each case supports the in situ formation of organic electron donors. We go on to show that direct electron transfer from KOtBu can however occur in appropriate cases, where the electron acceptor has a reduction potential near the oxidation potential of KOtBu, and the example that we use is CBr4. In this case, computational results support electrochemical data in backing a direct electron transfer reaction.

Recent papers report transition metal-free couplings of haloarenes to arenes to form biaryls, triggered by alkali metal tert-butoxides in the presence of various additives. These reactions proceed through radical intermediates, but understanding the origin of the radicals has been problematic. Electron transfer from a complex formed from potassium tert-butoxide with additives, such as phenanthroline, has been suggested to initiate the radical process. However, our computational results encouraged us to search for alternatives. We report that heterocycle-derived organic electron donors achieve the coupling reactions and these donors can form in situ in the above cases. We show that an electron transfer route can operate either with phenanthrolines as additives or using pyridine as solvent, and we propose new heterocyclic structures for the respective electron donors involved in these cases. In the absence of additives, the coupling reactions are still successful, although more sluggish, and in those cases benzynes are proposed to play crucial roles in the initiation process. Results An explosion of interest has arisen in dehalogenative couplings between haloarenes and arenes to form biphenyls under transition metal-free conditions 1-28 since the original report of coupling of iodobenzene with pyridine and pyrazine by Itami in 2008. 1 The excitement is easily understood, since these reactions avoid the use of costly complexes of palladium, which are the normal catalysts. A common feature of the recent reactions is the use of sodium or potassium tert-butoxide as base. The absence of any signicant contamination with transition metals like palladium, 1,2 together with the properties of the new coupling reactions, has led to a general consensus that the reactions proceed by aryl radical intermediates. 1,6 The corollary coupling between haloarenes and styrenes, also triggered by alkali metal butoxides, 3,7,12 likely follows the same route. The general case for a radical mechanism was well summarised in an essay by Studer and Curran 9 and an attractive proposal from that source is shown in Fig. 1A. Here, the aryl radical 2 adds to benzene to form the arylcyclohexadienyl radical 3. Deprotonation by butoxide affords the radical anion 4,

Coupling of haloarenes to arenes has been facilitated by a diverse range of organic additives in the presence of KO(t)Bu or NaO(t)Bu since the first report in 2008. Very recently, we showed that the reactivity of some of these additives (e.g., compounds 6 and 7) could be explained by the formation of organic electron donors in situ, but the role of other additives was not addressed. The simplest of these, alcohols, including 1,2-diols, 1,2-diamines, and amino acids are the most intriguing, and we now report experiments that support their roles as precursors of organic electron donors, underlining the importance of this mode of initiation in these coupling reactions.

A new ground-state organic electron donor has been prepared that features four strongly π-donating iminophosphorano substituents on a bispyridinylidene skeleton. Cyclic voltammetry reveals a record redox potential of −1.70 V vs. saturated calomel electrode (SCE) for the couple involving the neutral organic donor and its dication. This highly reducing organic compound can be isolated (44 %) or more conveniently generated in situ by a deprotonation reaction involving its readily prepared pyridinium ion precursor. This donor is able to reduce a variety of aryl halides, and, owing to its redox potential, was found to be the first organic donor to be effective in the thermally induced reductive S–N bond cleavage of N,N-dialkylsulfonamides, and reductive hydrodecyanation of malonitriles.

In recent times, metal-free chemistry has received significant attention due to its inherent qualities and its potential savings in the costs of (i) reagents and (ii) environmental treatments of residues. In this context, recently developed neutral organic electron-donors have shown an ability to perform challenging reductions that are traditionally the preserve of reactive metals and metal-based complexes, under mild reaction conditions. Hence, this feature article is aimed at describing the evolution of neutral organic super-electron-donors and their rapidly developing applications in electron-transfer reactions.