The cleavage of aromatic CH bonds promoted by a metal and an intramolecular base has been described over 50 years ago. Herein, discussion of selected mechanistic studies of this transformation will be presented. The basic ligand on the metal was proven to play a pivotal role in the CH bond cleavage step and evidence of a single operative concerted metallation deprotonation mechanism unifies the different mechanistic studies.

The concerted metalation-deprotonation mechanism predicts relative reactivity and regioselectivity for a diverse set of arenes spanning the entire spectrum of known palladium-catalyzed direct arylation coupling partners. An analysis following an active strain model provides a more complete portrayal of the important arene/catalyst parameters leading to a successful coupling. The breadth of arenes whose reactivity can be predicted by the CMD mechanism indicates that it may be far more widespread than previously imagined.

A comprehensive understanding of the C-H bond cleavage step by the concerted metalation-deprotonation (CMD) pathway is important in further development of cross-coupling reactions using different catalysts. Distortion-interaction analysis of the C-H bond cleavage over a wide range of (hetero)aromatics has been performed in an attempt to quantify the various contributions to the CMD transition state (TS). The (hetero)aromatics evaluated were divided in different categories to allow an easier understanding of their reactivity and to quantify activation characteristics of different arene substituents. The CMD pathway to the C-H bond cleavage for different classes of arenes is also presented, including the formation of pre-CMD intermediates and the analysis of bonding interactions in TS structures. The effects of remote C2 substituents on the reactivity of thiophenes were evaluated computationally and were corroborated experimentally with competition studies. We show that nucleophilicity of thiophenes, evaluated by Hammett σ(p) parameters, correlates with each of the distortion-interaction parameters. In the final part of this manuscript, we set the initial equations that can assist in the development of predictive guidelines for the functionalization of C-H bonds catalyzed by transition metal catalysts.

Conditions for the palladium-catalyzed direct arylation of a wide range of heterocycles with aryl bromides are reported. Those conditions employ a stoichiometric ratio of both coupling partners, as well as a substoichiometric quantity of pivalic acid, which results in significantly faster reactions. An evaluation of the influence of the nature of the aryl halide has also been carried out.

The challenge of achieving selective and predictable functionalizations at C-H bonds with complex poly(hetero)aromatic substrates was addressed by two different approaches. Site-selectivity can be obtained by applying various reaction conditions that are (hetero)arene specific to substrates that contain indoles, pyridine N-oxide, and polyfluorinated benzenes. An experimental classification of electron-rich heteroarenes based on their reactivity toward palladium-catalyzed C-H functionalization was established, the result of which correlated well with the order of reactivity predicted by the DFT-calculated concerted metalation-deprotonation (CMD) pathway. Model substrates containing two reactive heteroarenes were then reacted under general reaction conditions to demonstrate the applicability this reactivity chart in predicting the regioselectivity of the palladium-catalyzed direct arylation and benzylation reactions.

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Broadly applicable palladium-catalyzed heteroarene benzylation reactions are described with a focus on the most challenging heterocyclic classes under traditional benzylation techniques such as sulfur-containing heterocycles and those bearing functional groups that would be incompatible with reactions requiring Lewis acids and/or strong bases.

A versatile core structure has been prepared that should provide a foundation for the syntheses of the hetidine and hetisine type of diterpenoid alkaloids. The synthesis of the caged polycyclic core structure, which features nine contiguous stereocenters, utilizes a Ga(III)-catalyzed cycloisomerization of alkynyl indenes as well as a Michael/aldol sequence to build the bicyclo-[2.2.2] framework.