Experimental and DFT studies show the selectivity of C–H bond activation at [MCl2Cp*]2 (M = Ir, Rh) species can be controlled by the choice of metal catalyst, reflecting kinetic control at M = Ir and thermodynamic control at M = Rh.

IntroductionModern computational chemistry is a key tool by which insight into organometallic reaction mechanisms can be gained. The ability to characterize short-lived intermediates and transition states provides an ideal complement to experiment, where such information is often extremely difficult, if not impossible, to obtain. Recent years have seen great advances in understanding the mechanisms of C-H bond activation, and this area was the subject of several major reviews toward the end of the last decade [1,2]. More recently, the focus has shifted to how C-H activation can be integrated into catalytic cycles for useful organic transformations. The C-H bond activation event is itself mechanistically diverse, with oxidative addition (OA), σ-bond metathesis (SBM), and electrophilic activation (EA) all potentially available at late transition metal centers, depending on the metal, its oxidation state, and the coordination environment. C-H activation assisted by a heteroatom base (typically a carboxylate or carbonate) falls into the last of these categories and forms the focus of this chapter. More recently, computational studies of catalytic cycles for C-H activation and functionalization have become more common. These reveal the complexity of what are usually multiple-step processes, and calculations are particularly well placed to test different mechanistic possibilities. Such studies are most effectively pursued through a close interaction between experiment and computation, and increasingly this is allowing for a more quantitative assessment of computed reaction mechanisms.As well as progress in mechanistic understanding, the last 10 years have seen important developments in the computational methodologies available to model transition metal reactivity. While density functional theory (DFT) remains the core method of choice, the ability to model larger systems that more closely reflect experiment has highlighted the known shortcomings of DFT in describing dispersion interactions. These long-range, stabilizing interactions are individually weak, but their cumulative effect in large systems can be significant. Methods to incorporate this component include its separate calculation (e.g., Transition Metal-Catalyzed Heterocycle Synthesis via C-H Activation, First Edition. Edited by Xiao-Feng Wu.

Absolute rate coefficients for the reactions of the hydroxyl radical with ethane (kl, 297-800 K) and propane (kz, 297-690 K) were measured using the flash photolysis-resonance fluorescence technique. The rate coefficient data were fit by the following temperature-dependent expressions, in units of cm3/molecule-s: kl(T) = 1.43 X 10-14T1.05exp (-9ll/T) and k z ( T ) = 1.59 X 10-15T1.40 exp (-428/T). Semiquantitative separation of OH-propane reactivity into primary and secondary H-atom abstraction channels was obtained.

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This chapter surveys computational studies on heteroatom-assisted C-H activation at group 8 and 9 metal centres and will cover the literature since 2009. The chapter first considers work where the mechanism of the C-H activation step is the primary concern and categorizes these into intramolecular (with directing groups) and intermolecular processes. Studies on C-H activation and functionalization will be presented, classified in terms of the nature of the functionalization process (oxidative coupling to form heterocycles, alkenylation and amination).