Work

Direct Manipulation of Metal Imido Geometry: Key Principles to Enhance C–H Amination Efficacy

Year: 2019

Type: article

Abstract: We report the catalytic C–H amination mediated by an isolable Co III imido complex ( Tr L)Co(NR) supported by a sterically demanding dipyrromethene ligand ( Tr L = 5-mesityl-1,9-(trityl)dipyrrin). Metalation of ( Tr L)Li with CoCl 2 in THF afforded a high-spin ( S = 3/2) three-coordinate complex ( Tr L)CoCl. Chemical reduction of ( Tr L)CoCl with potassium graphite yielded the high-spin ( S = 1) Co I synthon ( Tr L)Co which is stabilized through an intramolecular η 6 -arene interaction. Treatment of ( Tr L)Co with a stoichiometric amount of 1-azidoadamantane (AdN 3 ) furnished a three-coordinate, diamagnetic Co III imide ( Tr L)Co(NAd) as confirmed by single-crystal X-ray diffraction, revealing a rare trigonal pyramidal geometry with an acute Co–N imido –C angle 145.0(3)°. Exposure of 1–10 mol % of ( Tr L)Co to linear alkyl azides (RN 3 ) resulted in catalytic formation of substituted N -heterocycles via intramolecular C–H amination of a range of C–H bonds, including primary C–H bonds. The mechanism of the C–N bond formation was probed via initial rate kinetic analysis and kinetic isotope effect experiments [ k H / k D = 38.4(1)], suggesting a stepwise H–atom abstraction followed by radical recombination. In contrast to the previously reported C–H amination mediated by ( Ar L)Co(NR) ( Ar L = 5-mesityl-1,9-(2,4,6-Ph 3 C 6 H 2 )dipyrrin), ( Tr L)Co(NR) displays enhanced yields and rates of C–H amination without the aid of a cocatalyst, and no catalyst degradation to a tetrazene species was observed, as further supported by the pyridine inhibition effect on the rate of C–H amination. Furthermore, ( Tr L)Co(NAd) exhibits an extremely low one-electron reduction potential ( E ° red = −1.98 V vs [Cp 2 Fe] +/0 ) indicating that the highly basic terminal imido unit contributes to the driving force for H–atom abstraction.

Source: Journal of the American Chemical Society

DOI: 10.1021/jacs.9b09015

Authors: Yunjung Baek, Elisabeth T. Hennessy, 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