Catalytic C-H Amination Mediated by Dipyrrin Cobalt Imidos

Abstract

Reduction of (L-Ar)(CoBr)-Br-II (L-Ar = 5-mesityl-1,9-(2,4,6-Ph3C6H2)dipyrrin) with potassium graphite afforded the novel Co-I synthon (L-Ar)Co-I. Treatment of (L-Ar)Co-I with a stoichiometric amount of various alkyl azides (N3R) furnished three-coordinate Co-III alkyl imidos (L-Ar)Co(NR), as confirmed by single-crystal X-ray diffraction (R: CMe2Bu, CMe2(CH2)(2)CHMe2). The exclusive formation of four-coordinate cobalt tetrazido complexes (L-Ar)Co(kappa(2)-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 (L-Ar)Co(NAd) exists in equilibrium in the presence of pyridine with a four-coordinate cobalt imido (L-Ar)Co(NAd)(py) (K-a = 8.04 M-1), as determined by H-1 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, (L-Ar)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 (L-Ar)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.