Published: June 28, 2011 r2011 American Chemical Society 12264 dx.doi.org/10.1021/ja204800a | J. Am. Chem. Soc. 2011, 133, 12264–12273 ARTICLE pubs.acs.org/JACS 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 Volodymyr Lyaskovskyy, † Alma I. Olivos Suarez, † Hongjian Lu, ‡ Huiling Jiang, ‡ X. Peter Zhang,* ,‡ and Bas de Bruin* ,† † Van 0 t Hoff Institute for Molecular Sciences (HIMS), Homogeneous and Supramolecular Catalysis, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands ‡ Department of Chemistry, University of South Florida, Tampa, Florida 33620-5250, United States b S Supporting Information ’ INTRODUCTION Direct functionalization of CH bonds is of outstanding importance because of its far-reaching practical applications as atom-, time-, and cost-efficient alternatives to traditional hydro- carbon functionalization approaches, which involve stepwise stoichiometric modifications. 1 This has led to the development of prominent methodologies for hydrocarbon transformations such as palladium-catalyzed cross-coupling of CH bonds 2 and CH insertion of carbenoid and nitrenoid species 3 as well as a number of efficient protocols for selective modification of nonactivated alkanes. 4 Numerous applications of amines have stimulated an intensive exploration of direct CH amination methods. 3,5 These reactions typically employ iminoiodanes (i.e., PhIdNTs), 6 haloamine-T compounds, 6a,7 carbamates, 6a,8 or azides 6a,9 as the nitrene sources. Among several catalysts that have been tested, the most successful ones are probably still those based on expensive Rh 3,10 and Ru 3,11 complexes, although less costly Mn-, 12 Cu-, 7bd,8c,9i and Fe-based 13 catalysts have also been reported. Organic azides are among the most promising and environmentally friendly nitrene sources for these reactions, for which a large substrate scope is available. Similar to carbene formation from diazo reagents, azides can generate nitrene ligands at transition metals by a simple loss of harmless Received: May 25, 2011 ABSTRACT: The mechanism of cobalt(II) porphyrin-cata- lyzed 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 com- putational 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 0 • and the cobalt(III)amido complexes (por)Co III NHY({R 0 • 333 (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 0 , which dissociate to afford the desired amine products NHYR 0 (6) with regeneration of the (por)Co catalyst. Alternatively, the close-contact pairs {R 0 • 333 (por)Co III NHY} 5 may undergo β-hydrogen-atom abstraction from the benzylic radical R 0 • 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. Downloaded by CHINA AGRICULTURAL UNIV at 01:03:40:683 on June 08, 2019 from https://pubs.acs.org/doi/10.1021/ja204800a.