The complete chemoselective functionalization of aromatic C(sp2)-H bonds of benzene and alkyl-benzenes by carbene insertion from ethyl diazoacetate was unknown until the recent discovery of an iron-based catalytic system toward such transformation. A Fe(II) complex bearing the pytacn ligand (pytacn=L1=1-(2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane) transferred the CHCO2Et unit exclusively to the C(sp2)-H bond. The cycloheptatriene compound commonly observed through Buchner reaction or, when employing alkyl-benzenes, the corresponding derivatives from C(sp3)-H functionalization are not formed. We herein present a combined experimental and computational mechanistic study to explain this exceptional selectivity. Our computational study reveals that the key step is the formation of an enol-like substrate, which is the precursor of the final insertion products. Experi-mental evidences based on substrate probes and isotopic labelling experiments in favor of this mechanistic interpretation are provided.

Support for this work was provided by the MINECO (CTQ2014-62234-EXP, CTQ2014-52769-C3-R-1, CTQ2014-54306-P, CTQ2014-52525-P, and grant No. BES-2012-052801 to V.P.), the Junta de Andalucía (P10-FQM-06292) and the Generalitat de Catalunya (Project 2014SGR931, Xarxa de Referència en Química Teòrica i Computacional, ICREA Academia prizes 2013 to M.C. and 2014 to M.S.). The EU under the FEDER grant UNGI10-4E-801 (European Fund for Regional Development) has also funded this research