Catalytic C-H Amination Mediated by Dipyrrin Cobalt Imidos.

Reduction of (L)CoBr (L = 5-mesityl-1,9-(2,4,6-PhCH)dipyrrin) with potassium graphite afforded the novel Co synthon (L)Co. Treatment of (L)Co with a stoichiometric amount of various alkyl azides (NR) furnished three-coordinate Co alkyl imidos (L)Co(NR), as confirmed by single-crystal X-ray diffraction (R: CMeBu, CMe(CH)CHMe). The exclusive formation of four-coordinate cobalt tetrazido complexes (L)Co(κ-NR) 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)Co(NAd) exists in equilibrium in the presence of pyridine with a four-coordinate cobalt imido (L)Co(NAd)(py) ( K = 8.04 M), as determined by H NMR titration experiments. Kinetic studies revealed that pyridine binding slows down the formation of the tetrazido complex by blocking azide coordination to the Co imido. Further, (L)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/ k = 10.2(9)] and initial rate analysis with para-substituted azides, suggesting a two-step radical pathway. Lastly, the enhanced reactivity of (L)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.