Rational Design in Homogeneous Catalysis. Ir(I)-Catalyzed Addition of Aniline to Norbornylene via N-H Activation

The first successful demonstration of the amination of an olefin by a transition-metal-catalyzed N-H activation mechanism was accomplished in a stepwise manner with an Ir(I) catalyst and the substrates aniline and norbornylene. The initial N-H oxidative addition step envisioned in such a mechanism was demonstrated for the complexes Ir(PMe₃)₄PF₆, Ir(PMe₃)₃(C₈H₁₄)Cl, and Ir(PEt₃)₃Cl, all of which gave rise to stable cis anilido hydride complexes when reacted with aniline. The second step, olefin insertion, was accomplished from the reaction of Ir(PEt₃)₂(C₂H₄)₂Cl (1) with aniline and norbornylene. The resulting insertion product, Ir(PEt₃)₂(NHPhC₇H₁₀)(H)Cl (6), was characterized by NMR spectroscopy and single-crystal X-ray diffraction and was found to have an azoiridacyclobutane type structure. The relevant crystal data for 6 are a = 10.702 (3) Å,b= 13.176 (21) Å, c = 19.498 (5) Å, β= 98.61 (2); P2₁/c. T = -70 °C, d_c = 1.591 g/cm³, μ = 51.21 cm⁻¹; 3029 reflections, R = 0.039 and R_w= 0.036. Decomposition studies of 6 established two competing reaction pathways. C-N and Ir-C bond breaking, the microscopic reverse of the formation of 6, gave norbornylene and a reactive Ir intermediate. The competing pathway of C-H reductive elimination occurs by prior ligand dissociation and yields the amination product, exo-2-(phenylamino)norbornane (7). Labeling studies indicated an overall cis addition of the N-H group across the exo face of norbornylene. The individual steps for this reaction were elaborated into a catalytic cycle for the addition of aniline to norbornylene and a mechanism based on these individual steps proposed.