Significant advances have been made in recent years in the transition metal-catalyzed selective functionalization of aryl C-H bonds positioned ortho to �directing� groups. The iridium fragment (tBuPCP)Ir (tBuPCP = {?3-2,6-bis[(di-tert-butylphosphino)methyl]phenyl}) rapidly and reversibly adds the C-H bond of benzene, giving a kinetically labile addition product. With substituted arenes, we find that the products of C-H activation ortho to weakly or non-coordinating substituents (e.g., Cl, Br, CF3) are kinetically more stable than those of the meta- and para-substituted analogs. However, there is no thermodynamic preference for the ortho-substituted complexes. Thus, rates of both C-H addition and elimination are substantially inhibited by bulky ortho substituents. We attribute this effect to the geometry of the addition/elimination transition state; ortho substituents inhibit rotation around the aryl C-Ir bond which is required for elimination.

Effects of aryl substituents on the kinetics and thermodynamics of C-H bond activation by a pincer-ligated iridium complex

243rd National Meeting (2012)

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