Synthesis of Coordinatively Unsaturated Iron Complexes and C-H Bond Activation

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The activation of C-H bonds is a key elementary step of organometallic reactions, and has great utility in organic synthesis. As a requirement for successful C-H bond activation under mild conditions would be preparation of coordinatively unsaturated complexes, we have synthesized coordinatively unsaturated half-sandwich iron complexes of N-heterocyclic carbenes. The sequential C-H bond activation and borylation reactions were found to occur upon treatment of the iron complexes with thiophene, furan, benzothiophene, benzofuran, or pyridine, followed by addition of hydroboranes. Based on these findings on stoichiometric reactions, we have developed the catalytic C-H bond activation/borylation of furans and thiophenes mediated by a half-sandwich iron complex of the methyl-substituted N-heterocyclic carbene. We have also conducted a series of reactions of the new iron complexes with nearly stoichiometric amount of substrate, to gain mechanistic insights.