Work

Catalytic C–H Amination Mediated by Dipyrrin Cobalt Imidos

Year: 2019

Type: article

Abstract: Reduction of ( Ar L)Co II Br ( Ar L = 5-mesityl-1,9-(2,4,6-Ph 3 C 6 H 2 )dipyrrin) with potassium graphite afforded the novel Co I synthon ( Ar L)Co I . Treatment of ( Ar L)Co I with a stoichiometric amount of various alkyl azides (N 3 R) furnished three-coordinate Co III alkyl imidos ( Ar L)Co(NR), as confirmed by single-crystal X-ray diffraction (R: CMe 2 Bu, CMe 2 (CH 2 ) 2 CHMe 2 ). The exclusive formation of four-coordinate cobalt tetrazido complexes ( Ar L)Co(κ 2 -N 4 R 2 ) 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 ( Ar L)Co(NAd) exists in equilibrium in the presence of pyridine with a four-coordinate cobalt imido ( Ar L)Co(NAd)(py) ( K a = 8.04 M –1 ), as determined by 1 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 III imido. Further, ( Ar 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 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 ( Ar 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.

Source: Journal of the American Chemical Society

DOI: 10.1021/jacs.9b01262

Authors: Yunjung Baek, Theodore A. Betley

Institution: Harvard University

Language: English

Topic: Synthesis and Catalytic Reactions

Subfield: Organic Chemistry

Field: Chemistry

Domain: Physical Sciences

Open Access status: green

Funders: Camille and Henry Dreyfus Foundation, Harvard University, Kwanjeong Educational Foundation, National Institute of General Medical Sciences

Awards: Correlation of electronic structure to iron catalyzed C-H bond functionalization, G1787376792