Catalytic C-H Amination Mediated by Dipyrrin Cobalt Imidos

Abstract

Reduction of (^ArL)Co^IIBr (^ArL = 5-mesityl-1,9-(2,4,6-Ph₃C₆H₂)dipyrrin) with potassium graphite afforded the novel Co^I synthon (^ArL)Co^I. Treatment of (^ArL)Co^I with a stoichiometric amount of various alkyl azides (N₃R) furnished three-coordinate Co^III alkyl imidos (^ArL)Co(NR), as confirmed by single-crystal X-ray diffraction (R: CMe₂Bu, CMe₂(CH₂)₂CHMe₂). The exclusive formation of four-coordinate cobalt tetrazido complexes (^ArL)Co(κ²-N₄R₂) was observed upon addition of excess azide, inhibiting any subsequent C-H amination. However, when a weak C-H bond is appended to the imido moiety, as in the case of (4-azido-4-methylpentyl)benzene, intramolecular C-H amination kinetically outcompetes formation of the corresponding tetrazene species to generate 2,2-dimethyl-5-phenylpyrrolidine in a catalytic fashion without requiring product sequestration. The imido (^ArL)Co(NAd) exists in equilibrium in the presence of pyridine with a four-coordinate cobalt imido (^ArL)Co(NAd)(py) ( K_a = 8.04 M^-1), as determined by ¹H NMR titration experiments. Kinetic studies revealed that pyridine binding slows down the formation of the tetrazido complex by blocking azide coordination to the Co^III imido. Further, (^ArL)Co(NR)(py) displays enhanced C-H amination reactivity compared to that of the pyridine-free complex, enabling higher catalytic turnover numbers under milder conditions. The mechanism of C-H amination was probed via kinetic isotope effect experiments [ k_H/ k_D = 10.2(9)] and initial rate analysis with para-substituted azides, suggesting a two-step radical pathway. Lastly, the enhanced reactivity of (^ArL)Co(NR)(py) can be correlated to a higher spin-state population, resulting in a decreased crystal field due to a geometry change upon pyridine coordination.