C(sp2)–H Activation with Pyridine Dicarbene Iron Dialkyl Complexes: Hydrogen Isotope Exchange of Arenes Using Benzene-d6 as a Deuterium Source

Treatment of pyridine dicarbene iron dialkyl complexes with low (≪1 atm) pressures of H2 in a benzene-d6 solution promoted rapid hydrogen isotope exchange (HIE) of the C(sp2)–H bonds in both electron-poor and -rich aromatic and heteroaromatic rings with benzene-d6 as the deuterium source. The iron-catalyzed reaction proceeded with predictable regioselectivity, engaging sterically accessible C–H bonds including ortho-to-fluorine sites. The site selectivity for the catalytic HIE reaction was studied to identify the kinetic preferences for C–H activation. Structure–activity relationship studies with a series of iron precatalysts established that introduction of substituents at the 3- and 5-positions of pyridine of the pincer significantly accelerated HIE. Mechanistic studies identified N2 as an inhibitor of C–H activation, while H2 served to generate the active catalyst.