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Catalytic C–H Amination Mediated by Dipyrrin Cobalt Imidos

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journal contribution
posted on 23.04.2019, 00:00 by Yunjung Baek, Theodore A. Betley
Reduction of (ArL)­CoIIBr (ArL = 5-mesityl-1,9-(2,4,6-Ph3C6H2)­dipyrrin) with potassium graphite afforded the novel CoI synthon (ArL)­CoI. Treatment of (ArL)­CoI with a stoichiometric amount of various alkyl azides (N3R) furnished three-coordinate CoIII 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) (Ka = 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 CoIII 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 [kH/kD = 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.