Functionalization of the C−H Bond of N‐Heteroaromatics Assisted by Early Transition‐Metal Complexes

Abstract

This review focuses on direct C−H bond functionalization reactions of N‐heteroaromatic compounds by group 3 and 4 organometallic complexes. Early transition‐metal‐carbon and ‐nitrogen bonds are highly reactive to C−H bond activation via a σ‐bond metathesis pathway to form new organometallic species. By further inserting other unsaturated molecules into the new metal‐carbon bond, C−H bond functionalized products are selectively obtained. To clarify the reaction mechanism, we categorize the reaction by the chelation size of the metallacycle intermediate generated from the organometallic species with N‐heteroaromatic compounds, and include both stoichiometric and catalytic reactions with their reaction mechanism.

Tris(benzhydryl) and Cationic Bis(benzhydryl) Ln(III) Complexes: Exceptional Thermostability and Catalytic Activity in Olefin Hydroarylation and Hydrobenzylation with Substituted Pyridines

“…Addition of aryl C( sp 2 )‐H and benzylic C( sp 3 )‐H bonds of alkylpyridines across multiple C−C bonds is the most straightforward and atom‐economical way for the synthesis of substituted pyridines [21] . To date these catalytic transformations were successfully performed using transition [22] and alkali [23] metals complexes and the achievements are summarized in a series of reviews [2d,25] . Hou demonstrated a powerful potential of half‐sandwich cationic rare‐earth alkyl complexes (mainly of Sc and Y) in o ‐aryl C( sp 2 )‐H and benzylic C( sp 3 )‐H bonds alkylation [24] and established possible pathways and mechanisms of these reactions.…”

Thermally Stable Half-Sandwich Benzhydryl Ln(II) (Ln = Sm, Yb) Complexes Supported by Sterically Demanding Carbazolyl and Fluorenyl Ligands

“…This chemistry started recently in the early 1990s with the discovery of Yb­(II) complexes with bulky silyl-substituted ligands. − The presence of two highly reactive centers (σ-Ln–C bonds and a metal ion in a low oxidation state) provides two different patterns of rich and diverse reactivity of these alkyl species. Due to the Ln–C bonds they can undergo insertions of multiple CC and CE bonds and σ-bond metathesis , and mediate activation of normally inert C–H bonds, while the Ln­(II) ion is responsible for a reductive chemistry. The development of the field was hampered by the extremely high air sensitivity of these complexes as well as by the synthetic problems associated with difficulties in stabilization of the coordination sphere of large electropositive Ln­(II) ions prone to ligand exchange reactions and β-hydrogen elimination or β-hydrogen abstraction of hydrocarbyl ligands .…”

3d Transition Metals for C–H Activation

“…As a direct consequence, earth-abundance is an increasing strong demand for the development of powerful methods based on less toxic 3d-transition metals. − Particularly, environmentally-friendly C–H activation greatly benefits from the use of earth-abundant elements (Figure ) that avoid the use of precious transition metals. To this end, considerable recent progress has been achieved by main group element compounds − as well as frustrated Lewis-pairs .…”

“…Scandium is the first member of 3d transition metal series, which has been extensively investigated in catalysis research during the past decades. Scandium complexes are predominantly known in the oxidation state +3; however, its lower oxidation states +2, +1, and 0 are available in organoscandium compounds as well. − Scandium­(III) salts, such as Sc­(OTf) 3 , are considered as useful Lewis-acids exhibiting unique features, including water-compatibility and reusability, and have attracted significant attention in catalytic organic synthesis. − In the past years, half-sandwich scandium complexes have emerged as efficient catalysts for the polymerization of olefins. − Furthermore, there is extensive precedent of stoichiometric scandium-mediated inter- and intramolecular C–H metalation reactions that provide the basis for new catalytic C–H functionalization reactions. ,,, …”