Synthesis of Unsymmetrical N-Heterocyclic Carbene–Nitrogen–Phosphine Chelated Ruthenium(II) Complexes and Their Reactivity in Acceptorless Dehydrogenative Coupling of Alcohols to Esters

Abstract

Two novel ruthenium complexes RuH­(CO)­Cl­(PPh3)­(κ2-CP) (1) and [fac-RuH­(CO)­(PPh3)­(κ3-CNP)]Cl (2) bearing unsymmetrical N-heterocyclic carbene–nitrogen–phosphine (CNP) were synthesized and characterized with 1H NMR, 31P NMR, and HRMS. The structure of complex 2 was further confirmed by single-crystal X-ray diffraction. An anion exchange experiment proved that complex 2 could transform into complex 1 in solution. The two complexes exhibited a highly catalytic performance in acceptorless dehydrogenative coupling of alcohols to esters, and the excellent isolated yields of esters were given in a catalyst loading of 1% for para- and meta-substituted benzyl alcohols and long-chain primary alcohols. Although some ortho-substituted benzyl alcohols displayed a relatively low reactivity due to the steric hindrance and the coordination of electron donor with the ruthenium center, the good product yields were still obtained by prolonging the reaction time. Especially, this system successfully realized the dehydrogenative cross-coupling to esters between two different primary alcohols.

Visible-Light-Driven Dehydrogenative Coupling of Primary Alcohols with Phenols Forming Aryl Carboxylates

“…Among the methods developed, oxidant-free direct reactions from primary alcohols to carboxylic acid derivatives, evolving gaseous dihydrogen, are highly attractive . Such examples include (a) reactions with aliphatic alcohols forming alkyl esters, (b) reactions with amines forming amides, and (c) reactions with aliphatic thiols forming alkyl thioesters . High temperature is generally required to facilitate the elimination of hydrogen and therewith to promote the reaction forward.…”

N–Alkylation of Amines Catalyzed by a Ruthenium–Pincer Complex in the Presence of in situ Generated Sodium Alkoxide

“…A similar 16‐electron NNN–Ru species was shown to be active for methanol dehydrogenation by Kundu and co‐workers . Very recently, Yu and Li have also proposed the release of phosphine and generation of CNP–Ru(II) species as the first step in the dehydrogenation of benzyl alcohol . The β‐hydride elimination from the benzyloxide in complex 9 through TS 10 could lead directly to the formation of 14 along with benzaldehyde ( 2′ ) (purple dotted arrows, Scheme ).…”

Dehydrogenative ester synthesis from enol ethers and water with a ruthenium complex catalyzing two reactions in synergy

“…(For other selected examples of fac isomers of pincer complexes, see ref. 15,[56][57][58][59][60][61]. Density functional theory (DFT) calculations were carried out to gain further insight regarding the reaction mechanism, with ethyl vinyl ether as the model substrate (Fig.…”