Abstract
others in the field, including those put forth in numerous excellent reviews [1]. The diversity of these and other reviews reflects the remarkably interdisciplinary range of approaches and perspectives that have been brought to bear on the inspiring challenge of functionalizing C-H bonds. In the modern historiography and taxonomy of C-H bond activation, electrophilic chemistry is often considered the earliest class of mechanisms discovered, whereas concerted metalation deprotonation (CMD) is perhaps the most recent important example. The distinction between these two classes of mechanisms, however, is less clear upon careful consideration. In that context we note that this lack of a clear boundary between various classes of C-H activation extends well beyond this particular example; indeed, blurry lines are more the rule than the exception [2] . Although CMD was first described independently in 2005-2006 by Davies and Macgregor [3], Daugulis [4], Maseras and Echavarren [5], its importance was first particularly recognized and exploited by Fagnou [6]. However, despite being recognized relatively recently, CMD is perhaps the operative pathway for the first selective C-H bond functionalizations ever identified. The term is used to refer to a mechanism in which a C-H bond is associated with a vacant site on an electrophilic metal center (typically an electrophilic, late transition metal) through an initial sigma complex (3-center-2-electron bond). This leads to an acidification of the C-H bond, which enables a basic ligand, most classically a carboxylate, to abstract the hydrogen as proton and often dissociate synchronously with carbon-metal bond formation. Selective functionalization of "unactivated" C-H bonds by a transition metal can arguably be dated back to 1891 [7] . BASF used fuming sulfuric acid to oxidize naphthalene to phthalic anhydride, a key intermediate for production of synthetic indigo dye [8] . During one batch, Eugene Sapper, the technician on duty, decided to stir the hot mixture of acid and naphthalene with the nearest object available -a mercury thermometer. The thermometer broke and the mercury entered the reactor, where it was quickly taken into solution. However unplanned, this procedural deviation sharply increased the yield of phthalic anhydride and was quickly commercialized by BASF. It also appears to have initiated academic research into reactions of mercury with aromatic compounds. Thus in 1892, Jacob Volhard, then at Friedrichs-Universität Halle, discovered that aqueous mixtures of mercuric chloride and sodium acetate could mercurate thiophene alpha to sulfur [9] . With a bit of heating, both alpha positions could be substituted. This reaction, however, only afforded modest conversions, and it led to complex mixtures.
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