Synthesis and Reactivity of Metal–Ligand Cooperative Bifunctional Ruthenium Hydride Complexes: Active Catalysts for β-Alkylation of Secondary Alcohols with Primary Alcohols

Abstract

Three unsymmetrical NNN ligands with a 2-hydroxypyridyl fragment were used to react with RuHCl­(PPh3)3(CO), affording the three bidentate ruthenium hydride complexes [(R1-C5H3N-CH2-C5H3N-C5H3N-R2)­RuH­(PPh3)2(CO)]­[Cl] (R1 = R2 = OH, 2a; R1 = OH, R2 = H, 2b; R1 = H, R2 = OH, 2c), respectively. When 2a,b were treated with t-BuOK, the two tridentate products [(O-C5H3N-CH2-C5H3N-C5H3N-R2)­RuH­(PPh3)­(CO)] (R2 = OH, 3a; R2 = H, 3b) were obtained, via selective deprotonation of the −OH group of PyCH2PyOH moiety, indicating that this −OH group is more acidic than that of the PyPyOH moiety. The reaction of 2c with t-BuOK generated the bidentate product [(C5H4N-CH2-C5H3N-C5H3N-O)­RuH­(PPh3)2(CO)] (3c) and the tridentate product [(C5H4N-CH2-C5H3N-C5H3N-O)­RuH­(PPh3)­(CO)] (3d). 3d could be further transformed to the diruthenium complex [(C5H4N-CH-C5H3N-C5H3N-O)­Ru­(PPh3)­(CO)]2 (3e) via C–H activation of the −CH2– group in boiling toluene. The catalytic activity for β-alkylation of secondary alcohols with primary alcohols of these eight ruthenium complexes was tested, and the bidentate complexes 2c and 3c exhibit the highest activity. Complex 3c can be regarded as the intermediate of 2c. These results are important for developing more efficient bifunctional catalysts for such reactions.

Ruthenium-Catalyzed β-Alkylation of Secondary Alcohols and α-Alkylation of Ketones via Borrowing Hydrogen: Dramatic Influence of the Pendant N-Heterocycle

“…To explore if the pendant pyridyl group in complex C participates in the catalytic cycle (Figure , C ), we first replaced the uncoordinated pyridyl of its ligand by a phenyl group, and synthesized a ligand precursor [HO–C 5 H 3 N–C 5 H 3 N–CH 2 –C 6 H 5 ] ( L 1 ) (Scheme S1 in Supporting Information). Reaction of L 1 with RuHCl­(PPh 3 ) 3 (CO) in refluxing methanol generated a yellow product, [(O–C 5 H 3 N–C 5 H 3 N–CH 2 –C 6 H 5 )­RuH­(PPh 3 ) 2 (CO)] ( 1 ) in 82% yield (Scheme ).…”

“… a Reaction conditions: catalyst (0.5 mol %), 1-phenylethanol (2.0 mmol), benzyl alcohol (2.0 mmol) and t -BuOK (1 mmol) in reflux condition in toluene for 60 min under N 2 atmosphere. b Determined by GC analysis based on secondary alcohol. c Determined by GC analysis. d Data from ref . …”

“…Recently, our group reported ruthenium hydride complex C (Figure , C ), based on the NNN ligand with a 2-hydroxypyridyl fragment, for β-alkylation of secondary alcohols with primary alcohols . There is a pendant pyridyl group existing in complex C .…”

Alkyl Phosphine Free, Metal‐Ligand Cooperative Complex Catalyzed Alcohol Dehydrogenative Coupling Reactions

“…In 2018, Chen group reported an unsymmetrical bidentate (NN)Ru(II) complex ( 12 a ) which efficiently catalyzed the β‐alkylation secondary alcohols with primary alcohols [39] . Under the reaction conditions, 2‐hydroxypyridine unit was converted to pyridonate intermediate 12a’ which was also catalytically active.…”

Methylation of Amines and Ketones with Methanol Catalyzed by an Iridium Complex Bearing a 2-Hydroxypyridylmethylene Fragment

“…When Cs 2 CO 3 was replaced by a weaker base (K 2 CO 3 ), a relatively low yield (84%) was obtained. 5g 7 Hence, by the introduction of a methylene group into the 6,6′-dihydroxy-2,2′-bipyridine part of complex b, the acidity of the resulting complex will increase, and it might achieve better catalytic activity for N−CH 3 and C−CH 3 coupling in the presence of a weaker base (Figure 1 yield. The 1 H NMR spectrum of 1 in d 6 -DMSO exhibits one singlet at 11.60 ppm for its two −OH groups, three signals between 7.36 and 6.05 ppm for its aromatic protons, one singlet at 3.66 ppm for its methylene protons, and one singlet for the Cp* at 1.63 ppm.…”