Effect of Ancillary Ligand in Cyclometalated Ru(II)–NHC-Catalyzed Transfer Hydrogenation of Unsaturated Compounds

Abstract

In an effort to develop efficient Ru­(II)–NHC-based catalyst considering their stereoelectronic effect for hydride-transfer reaction, we found that the ancillary NHC ligand can play a significant role in its catalytic performance. This effect is demonstrated by comparing the activity of two different types of orthometalated precatalysts of general formula [(p-cymene)­(NHC)­RuII(X)] (NHC = an imidazolylidene-based ImNHC, compound 2a–c, or a mesoionic triazolylidene-based tzNHC, compound 4) in transfer hydrogenation of carbonyl substrates. The electron-rich precatalyst, 2c, containing p-OMe-substituted NHC ligand performed significantly better than both unsubstituted complex 2a and p-CF3 substituted electron-poor complex 2b in ketone reduction. Whereas bulky mesoionic triazolylidene ligand containing complex 4 was found to be superior catalyst for aldehyde reduction and the precatalyst 2a is more suitable for the selective transfer hydrogenation of a wide range of aromatic aldimines to amines. To the best of our knowledge, this is the first systematic study on the effect of stereoelectronic tuning of ancillary orthometalated NHC ligand in Ru­(II)-catalyzed transfer hydrogenations of various types of unsaturated compounds with broad substrate scope.

(para‐Cymene)Ru(II) Complexes with Chelating Benzotriazole Ligands: Application in Oxidation and Reduction Catalysis

“…Interestingly, the other functional groups such as Cl, Br, I, OMe and SMe on the aromatic backbone were found to be tolerated in this catalysis. The efficiencies of these catalysts were found to be comparable to the other reported ( para ‐cymene)Ru II ‐catalysts supported with chelating bipyridine‐based, NHC‐alkene and NHC‐pyridine, and chelating diamine ligands, but relatively lower than the similar Ru catalysts with NHC ligands (see Table S4, Supporting Information) …”

Ruthenacycles and Iridacycles as Transfer Hydrogenation Catalysts

“…Monometallic complexes bearing closely related CC -cycloruthenated NHC-phenyl ligands were then successively described by the groups of Ramesh [ 16 ] and Rit [ 17 ], and studied for the TH of acetophenone from isopropanol at reflux. While Ramesh et al studied the influence of the nature of the wingtip substituents in their series of three complexes 9a – c , Rit et al studied the influence of the nature of a substituent in meta -position to the C–Ru bond on the cyclometalated phenyl ring of complexes 10a – c ( Scheme 4 ).…”

“…Thus, among complexes 9 , the 9a derivative, bearing an n -butyl substituent, proved to be more active than 9b and 9c , bearing bulkier isopropyl and benzyl wingtips, respectively [ 16 ]. Interestingly, the catalyst loading of 9a could even be decreased to 0.2 mol.%, allowing a TON of 460, which is from far the highest value observed for the CC -cyclometalated complexes 7 – 10 , along with a much lower amount of KOH than in the cases of complexes 7 , 8 , and 10a – c [ 15 , 16 , 17 ]. Among complexes 10 , the m -OMe derivative 10c (0.5 mol.%) displayed the best activity, with 95% conversion to 1-phenylathanol after 1 h reaction (TOF = 190 h −1 ), and the m -CF 3 derivative 10b displayed the lowest, with only 53% conversion after 1 h (TOF = 106 h −1 ).…”

Efficient methylation of anilines with methanol catalysed by cyclometalated ruthenium complexes

“…35,36 Mainly in the transfer hydrogenation of ketones, ruthenium half-sandwich complexes bearing bidentate ligands with a carbon and a classical donor atom have been established. [37][38][39][40][41] Moreover, cyclometalated ruthenium complexes were used in the dehydrogenation of alcohols, 42 the direct hydrogenation of olefins [43][44][45] and the α-alkylation of amides using hydrogen autotransfer. 46 In addition to that, the implementation of a C-donor atom into pincer ligands was accomplished by the group of Baratta and others for efficient transfer hydrogenation, direct hydrogenation or acceptorless dehydrogenation reactions.…”