An Air/Water‐Stable Tridentate N‐Heterocyclic Carbene‐ Palladium(II) Complex: Catalytic CH Activation of Hydrocarbons via Hydrogen/Deuterium Exchange Process in Deuterium Oxide

Abstract

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 ...

Conversion of saccharides into formic acid using hydrogen peroxide and a recyclable palladium(ii) catalyst in aqueous alkaline media at ambient temperatures

“…In effect, this study is a continuation of our previously reported work on the direct conversion of glycerol into formic acid, 11 which also utilized our NHC-amidate Pd(II) complex 12 using hydrogen peroxide in aqueous media. Hydrogen peroxide continues to be the oxidant used in this procedure since it is convenient, accessible, and breaks down into environment-friendly side products unlike other oxidants.…”

Potentially Hemilabile N-Heterocyclic Carbene Palladium Complexes: Synthesis and Catalytic Applications

“…The same group also described the synthesis and characterization of palladium (II) complexes 78 and 79 of amidate-alkoxide-NHC ligands (Chart 21). The water soluble Pd (II) complexes showed efficient H/D exchange of various organic substrates in D 2 O under mild conditions (55 o C to 100 o C), and the catalytic activity was not inhibited by the coordination of water to the palladium compound [64]. Similar pincer Pd complex 80 containing a CNO ligand has been employed to one-pot multicomponent Strecker reactions [65].…”

Mechanistic considerations on catalytic H/D exchange mediated by organometallic transition metal complexes

“…and complexes used as catalysts. In particular, several organometallic species operating with this mechanism and containing metal such as Nb [18], Ta [18], Cu [19], Ru [20], Pt [11a,21], Os [20g,h,22], Sc [23], Pd [24], Re [25], Mo [26], Ni [27], Rh [20a,28], Zr [18b], have been used and above all Ir represents the most prevalent metal for the catalytic H/D exchange reactions [20a,29]. The analysis of some of these systems lets us understand the mechanistic issues in catalytic H/D exchange.…”