Iridium-Catalyzed Borylation of Benzene with Diboron. Theoretical Elucidation of Catalytic Cycle Including Unusual Iridium(V) Intermediate

Iridium-catalyzed borylation of benzene with diboron was theoretically investigated with the DFT method, where an iridium(I) boryl complex, Ir(Beg)(NN) 1, and an iridium(III) tris(boryl) complex, Ir(Beg)(3)(NN) 14, (eg (ethyleneglycolato) = -OCH(2)CH(2)O-, NN = HN=CHCH=NH (diim) or 2,2'-bipyridine (bpy)) were adopted as models of active species and B(2)(eg)(2) was adopted as a model of bis(pinacolato)diboron (pinacolato = -OCMe(2)CMe(2)O-). Oxidative addition of a benzene C-H sigma-bond to 1 takes place with an activation barrier (E(a)) of 11.2 kcal/mol, followed by reductive elimination of phenylborane, Ph-Beg, from Ir(Beg)(H)(Ph)(diim) with an activation barrier of 15.6 kcal/mol. Though the oxidative addition and the reductive elimination occur with moderate activation barriers, B(2)(eg)(2) much more easily reacts with 1 to afford 14 than does benzene, of which the activation barrier is very small (2.9 kcal/mol). Oxidative addition of the benzene C-H sigma-bond to 14 occurs with a moderate activation barrier of 24.2 kcal/mol to afford an unusual seven-coordinate iridium(V) complex, Ir(H)(Ph)(Beg)(3)(bpy) 16. From this complex, phenylborane Ph-Beg is produced through the reductive elimination with concomitant formation of IrH(Beg)(2)(bpy) 17, where the activation barrier is 4.9 kcal/mol. Complex 17 further reacts with diboron to form Ir(H)(Beg)(4)(bpy) (E(a) = 8.0 kcal/mol), followed by the reductive elimination of borane H-Beg (E(a) = 2.6 kcal/mol) to regenerate Ir(Beg)(3)(bpy), when diboron exists in excess in the reaction solution. After consumption of diboron, IrH(Beg)(2)(bpy) reacts with borane, H-Beg, to form Ir(H)(2)(Beg)(3) (E(a) = 21.3 kcal/mol) followed by the reductive elimination of H(2), to regenerate Ir(Beg)(3)(bpy) with concomitant formation of H(2). Formation of the iridium(III) tris(boryl) complex 14 from IrCl(diim) and diboron was also theoretically investigated; IrCl(diim) undergoes two steps of oxidative addition of diboron to afford a seven-coordinate iridium(V) complex, IrCl(Beg)(4)(NN), from which the reductive elimination of Cl-Beg takes place easily to afford 14. From these results, it should be clearly concluded that the iridium(III) tris(boryl) complex is an active species and an unusual iridium(V) species is involved as a key intermediate in the reaction. Detailed discussion is presented on the full catalytic cycle and the importance of a seven-coordinate iridium(V) intermediate.