Catalytic C–H Amination Mediated by Dipyrrin Cobalt Imidos
journal contributionposted 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.
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single-crystal X-ray diffractionpotassium graphitefour-coordinate cobalt tetrazido complexesstoichiometric amountfour-coordinate cobalt imidorate analysiscrystal fieldDipyrrin Cobalt Imidos ReductionN 4 R 2pyridine bindingKinetic studiespyridine coordinationCMe 2 BuCo III imidoazide coordinationproduct sequestrationAr L2 CHMe 2CH 2CMe 2isotope effect experiments1 H NMR titration experimentstetrazene speciesturnover numbersspin-state populationaminationgeometry changeN 3 Ralkyl azidesimido moietynovel Cothree-coordinate Co III alkyl imidos