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Mechanism of cobalt(II) porphyrin-catalyzed C-H amination with organic azides: radical nature and H-atom abstraction ability of the key cobalt(III)-nitrene intermediates.
J Am Chem Soc. 2011 Aug 10; 133(31):12264-73.JA

Abstract

The mechanism of cobalt(II) porphyrin-catalyzed benzylic C-H bond amination of ethylbenzene, toluene, and 1,2,3,4-tetrahydronaphthalene (tetralin) using a series of different organic azides [N(3)C(O)OMe, N(3)SO(2)Ph, N(3)C(O)Ph, and N(3)P(O)(OMe)(2)] as nitrene sources was studied by means of density functional theory (DFT) calculations and electron paramagnetic resonance (EPR) spectroscopy. The DFT computational study revealed a stepwise radical process involving coordination of the azide to the metal center followed by elimination of dinitrogen to produce unusual "nitrene radical" intermediates (por)Co(III)-N(•)Y (4) [Y = -C(O)OMe, -SO(2)Ph, -C(O)Ph, -P(O)(OMe)(2)]. Formation of these nitrene radical ligand complexes is exothermic, predicting that the nitrene radical ligand complexes should be detectable species in the absence of other reacting substrates. In good agreement with the DFT calculations, isotropic solution EPR signals with g values characteristic of ligand-based radicals were detected experimentally from (por)Co complexes in the presence of excess organic azide in benzene. They are best described as nitrene radical anion ligand complexes (por)Co(III)-N(•)Y, which have their unpaired spin density located almost entirely on the nitrogen atom of the nitrene moiety. These key cobalt(III)-nitrene radical intermediates readily abstract a hydrogen atom from a benzylic position of the organic substrate to form the intermediate species 5, which are close-contact pairs of the thus-formed organic radicals R'(•) and the cobalt(III)-amido complexes (por)Co(III)-NHY ({R'(•)···(por)Co(III)-NHY}). These close-contact pairs readily collapse in a virtually barrierless fashion (via transition state TS3) to produce the cobalt(II)-amine complexes (por)Co(II)-NHYR', which dissociate to afford the desired amine products NHYR' (6) with regeneration of the (por)Co catalyst. Alternatively, the close-contact pairs {R'(•)···(por)Co(III)-NHY} 5 may undergo β-hydrogen-atom abstraction from the benzylic radical R'(•) by (por)Co(III)-NHY (via TS4) to form the corresponding olefin and (por)Co(III)-NH(2)Y, which dissociates to give Y-NH(2). This process for the formation of olefin and Y-NH(2) byproducts is also essentially barrierless and should compete with the collapse of 5 via TS3 to form the desired amine product. Alternative processes leading to the formation of side products and the influence of different porphyrin ligands with varying electronic properties on the catalytic activity of the cobalt(II) complexes have also been investigated.

Authors+Show Affiliations

Van 't Hoff Institute for Molecular Sciences (HIMS), Homogeneous and Supramolecular Catalysis, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands.No affiliation info availableNo affiliation info availableNo affiliation info availableNo affiliation info availableNo affiliation info available

Pub Type(s)

Journal Article

Language

eng

PubMed ID

21711027

Citation

Lyaskovskyy, Volodymyr, et al. "Mechanism of cobalt(II) Porphyrin-catalyzed C-H Amination With Organic Azides: Radical Nature and H-atom Abstraction Ability of the Key cobalt(III)-nitrene Intermediates." Journal of the American Chemical Society, vol. 133, no. 31, 2011, pp. 12264-73.
Lyaskovskyy V, Suarez AI, Lu H, et al. Mechanism of cobalt(II) porphyrin-catalyzed C-H amination with organic azides: radical nature and H-atom abstraction ability of the key cobalt(III)-nitrene intermediates. J Am Chem Soc. 2011;133(31):12264-73.
Lyaskovskyy, V., Suarez, A. I., Lu, H., Jiang, H., Zhang, X. P., & de Bruin, B. (2011). Mechanism of cobalt(II) porphyrin-catalyzed C-H amination with organic azides: radical nature and H-atom abstraction ability of the key cobalt(III)-nitrene intermediates. Journal of the American Chemical Society, 133(31), 12264-73. https://doi.org/10.1021/ja204800a
Lyaskovskyy V, et al. Mechanism of cobalt(II) Porphyrin-catalyzed C-H Amination With Organic Azides: Radical Nature and H-atom Abstraction Ability of the Key cobalt(III)-nitrene Intermediates. J Am Chem Soc. 2011 Aug 10;133(31):12264-73. PubMed PMID: 21711027.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Mechanism of cobalt(II) porphyrin-catalyzed C-H amination with organic azides: radical nature and H-atom abstraction ability of the key cobalt(III)-nitrene intermediates. AU - Lyaskovskyy,Volodymyr, AU - Suarez,Alma I Olivos, AU - Lu,Hongjian, AU - Jiang,Huiling, AU - Zhang,X Peter, AU - de Bruin,Bas, Y1 - 2011/07/18/ PY - 2011/6/30/entrez PY - 2011/6/30/pubmed PY - 2011/6/30/medline SP - 12264 EP - 73 JF - Journal of the American Chemical Society JO - J Am Chem Soc VL - 133 IS - 31 N2 - The mechanism of cobalt(II) porphyrin-catalyzed benzylic C-H bond amination of ethylbenzene, toluene, and 1,2,3,4-tetrahydronaphthalene (tetralin) using a series of different organic azides [N(3)C(O)OMe, N(3)SO(2)Ph, N(3)C(O)Ph, and N(3)P(O)(OMe)(2)] as nitrene sources was studied by means of density functional theory (DFT) calculations and electron paramagnetic resonance (EPR) spectroscopy. The DFT computational study revealed a stepwise radical process involving coordination of the azide to the metal center followed by elimination of dinitrogen to produce unusual "nitrene radical" intermediates (por)Co(III)-N(•)Y (4) [Y = -C(O)OMe, -SO(2)Ph, -C(O)Ph, -P(O)(OMe)(2)]. Formation of these nitrene radical ligand complexes is exothermic, predicting that the nitrene radical ligand complexes should be detectable species in the absence of other reacting substrates. In good agreement with the DFT calculations, isotropic solution EPR signals with g values characteristic of ligand-based radicals were detected experimentally from (por)Co complexes in the presence of excess organic azide in benzene. They are best described as nitrene radical anion ligand complexes (por)Co(III)-N(•)Y, which have their unpaired spin density located almost entirely on the nitrogen atom of the nitrene moiety. These key cobalt(III)-nitrene radical intermediates readily abstract a hydrogen atom from a benzylic position of the organic substrate to form the intermediate species 5, which are close-contact pairs of the thus-formed organic radicals R'(•) and the cobalt(III)-amido complexes (por)Co(III)-NHY ({R'(•)···(por)Co(III)-NHY}). These close-contact pairs readily collapse in a virtually barrierless fashion (via transition state TS3) to produce the cobalt(II)-amine complexes (por)Co(II)-NHYR', which dissociate to afford the desired amine products NHYR' (6) with regeneration of the (por)Co catalyst. Alternatively, the close-contact pairs {R'(•)···(por)Co(III)-NHY} 5 may undergo β-hydrogen-atom abstraction from the benzylic radical R'(•) by (por)Co(III)-NHY (via TS4) to form the corresponding olefin and (por)Co(III)-NH(2)Y, which dissociates to give Y-NH(2). This process for the formation of olefin and Y-NH(2) byproducts is also essentially barrierless and should compete with the collapse of 5 via TS3 to form the desired amine product. Alternative processes leading to the formation of side products and the influence of different porphyrin ligands with varying electronic properties on the catalytic activity of the cobalt(II) complexes have also been investigated. SN - 1520-5126 UR - https://www.unboundmedicine.com/medline/citation/21711027/Mechanism_of_cobalt_II__porphyrin_catalyzed_C_H_amination_with_organic_azides:_radical_nature_and_H_atom_abstraction_ability_of_the_key_cobalt_III__nitrene_intermediates_ L2 - https://doi.org/10.1021/ja204800a DB - PRIME DP - Unbound Medicine ER -
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