While developing novel catalysts for carbon-carbon or carbon-heteroatom coupling (N, O, or F), we were able to introduce tridentate NHC-amidate-alkoxide palladium(II) complexes. In aqueous solution, these NHC-Pd(II) complexes showed high ability for C-H activation of various hydrocarbons (cyclohexane, cyclopentane, dimethyl ether, THF, acetone, and toluene) under mild conditions.
Keywords
NHC ligand; C-H activation; H/D exchange; Palladium complex; Intermolecular reactionCarbon-carbon or carbon-heteroatom cross-coupling reactions catalyzed by transition metal catalysts have been widely investigated and established because of their significance in organic synthesis. In particular, homogeneous Pd catalysis has been well studied in the most powerful and versatile synthetic process such as Heck [1] , Suzuki [2] , Stille [3] , Sonogashira [4] , Negishi [5] , and Buchwald-Hartwig reactions [6] . However, these reactions have required alkenyl/aryl halide substrates. Alternatively, the more desirable direct functionalization of hydrocarbons via C-H bond activation has still remained a challenge in cross-coupling reactions.In recent years, a substantial number of metal complexes that are able to selectively activate C-H bonds under mild conditions have been discovered. [7] In spite of these advances, practical catalysts for the C-H bond functionalization remain elusive, due to the requirement of high energy to break C-H bond and subsequent vulnerability of the metal-carbon bond. Additionally, the C-H bond activation is often inhibited by water or by the product eliminated from the metal complex during the reaction. [8] Recently, N-Heterocyclic carbene (NHC) ligands, which are known to enhance σ-donor coordination, have shown improved behaviour toward C-H bond activation during the past several years. [9] A number of organometallic species at various oxidation states (Pd, Pt, Ru, and Ir) are stabilized by the use of NHC ligands, increasing the capability to obtain efficient catalysts for C-H activation. [10] However, most examples refer to intramolecular processes, while intermolecular or catalytic examples being rare. With these challenges in mind, we report herein the efficient C-H bond activation of hydrocarbons in aqueous solution (1)Recently, we have succeeded in preparing air-stable tridentate NHC-amidate-alkoxide ligand/palladium complex 1 from an amino alcohol. [12] We found that H/D exchange on benzene using D 2 O as both the solvent and deuterium source, occurred via C-H bond activation in the presence of Pd complex 1 and silver tetrafluoroborate. As shown in equation (1), the H/D exchange reaction was demonstrated with benzene (20 μL), palladium complex 1 (5 mol %) and AgBF 4 in deuterium oxide (700 μL) for 22 hours at 55 °C and 100°C . The efficiency of the H/D exchange giving deuterated isotopomers showed significant enhancement, when temperature was increased from 55 °C to 100 °C. While evaluating H/D exchange with palladium complex 1 and AgBF 4 , we found that a dimeric structure 2 was present ...
