Ortho C−H Activation of Haloarenes and Anisole by an Electron-Rich Iridium(I) Complex: Mechanism and Origin of Regio- and Chemoselectivity. An Experimental and Theoretical Study

Abstract

Reaction of (PNP)Ir(COE)+PF6- (1) (PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine; COE = cyclooctene) with benzene yields a stable unsaturated square pyramidal Ir(III) hydrido-aryl complex, 2, which undergoes arene exchange upon reaction with other arenes at 50 °C. Upon reaction of 1 with haloarenes (chlorobenzene and bromobenzene) and anisole at 50 °C, selective ortho C−H activation takes place. No C−halogen bond activation was observed, even in the case of the normally reactive bromobenzene and despite the steric hindrance imposed by the halo substituent. The ortho-activated complexes (8a, 9a, and 10a) exhibited a higher barrier to arene exchange; that is, no exchange took place when heating at a temperature as high as 60 °C. These complexes were more stable, both thermodynamically and kinetically, than the corresponding meta- and para-isomers (8b,c, 9b,c, and 10b,c). The observed selectivity is a result of coordination of the heteroatom to the metal center, which kinetically directs the metal to the ortho C−H bond and stabilizes the resulting complex thermodynamically. Upon reaction of complex 1 with fluorobenzene under the same conditions, no such selectivity was observed, due to low coordination ability of the fluorine substituent. Competition experiments showed that the ortho-activated complexes 8a, 9a, and 10a have similar kinetic stability, while thermodynamically the chloro and methoxy complexes 8a and 10a are more stable than the bromo complex 9a. Computational studies, using the mPW1K exchange−correlation functional and a variety of basis sets for PNP-based systems, provide mechanistic insight. The rate-determining step for the overall C−H activation process of benzene is COE dissociation to form a reactive 14e complex. This is followed by formation of a η2C-C intermediate, which is converted into an η2C-H complex, both being important intermediates in the C−H activation process. In the case of chlorobenzene, bromobenzene, and anisole, η1-coordination via the heteroatom to the 14e species followed by formation of the ortho η2C-H complex leads to selective activation. The unobserved C−halide activation process was shown computationally in the case of chlorobenzene to involve the same Cl-coordinated intermediate as in the C−H activation process, but it experiences a higher activation barrier. The ortho C−H activation product is also thermodynamically more stable than the C−Cl oxidative addition complex.