Steric Effects in ortho C-H Activation of Aromatic Systems, Z. Novák, B. L. Tóth, A. Stirling, Handbook of C-H functionalization. Chapter: Role of Directing Groups. 2022, Wiley. DOI: 10.1002/9783527834242.chf0080 | [Full Text Link] [Table of Content of The Book]

Transition metal-catalyzed directed aromatic ortho C-H activation offers many possibilities for the functionalization of various substrates having different directing groups. However, the reactivity of the substrate is determined by electronic and steric factors. Herein, we present and discuss a recently developed model which considers only the steric properties of the substrates to assess their reactivity in ortho C-H activation in the presence of ortho substituents. Dihedral angle between the directing group and the aromatic ring influences the coordination and the C-H activation elementary step, thus determine reactivities and the stability of metal-complex intermediates. The presentation of the theoretical model is followed by several literature examples from substrates having borderline reactivities, to explore and describe the most interesting region of reactivity. The most commonly used directing groups are discussed taking into consideration their key structural elements, and reactivity trends are presented with the help of complex formation and reactions of each substrate group. The lack of transformation is also discussed by showing representatives of unreactive substrates.