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.
Both imidazol-2-ylidene
(ImNHC) and 1,2,3-triazol-5-ylidene (tzNHC)
have evolved to be elite groups of N-heterocyclic carbene (NHC) ligands
for homogeneous catalysis. To develop efficient ruthenium(II)-based
catalysts incorporating these ligands for C–N bond-forming
reactions via hydrogen-borrowing methodology, we utilized chelating
ligands integrated with ImNHC and mesoionic tzNHC donors connected
via a CH2 spacer with a diverse triazole backbone. The
synthesized ruthenium(II) complexes 3 are found to be
highly efficient for C–N bond formation across a wide range
of primary amine and alcohol substrates under solvent-free conditions,
and among all of the complexes studied here, catalyst 3a with a mesityl substituent displayed maximum activity. To our delight,
catalyst 3a is also effective for the selective mono-N-methylation
of various anilines utilizing methanol as a coupling partner, known
to be relatively more difficult than other alcohols. Furthermore,
complex 3a also delivers various substituted quinolines
successfully via the reaction of 2-aminobenzyl alcohol with several
secondary alcohols. Importantly, catalyst 3a exhibited
the highest activity among the reported ruthenium(II) complexes for
both the N-benzylation of aniline [achieving a turnover number (TON)
of 50000] and the realization of quinoline 8a by reacting
2-aminobenzyl alcohol with 2-phenylethanol (attaining a TON of 30000).
Imidazol-2-ylidene (ImNHC) and 1,2,3-traizol-5-ylidene (tzNHC) have been established as important classes of carbene ligands in homogeneous catalysis. To develop Ru(II)/Ir(III) complexes based on these ligand systems considering their electronic as well as steric profiles for hydride transfer reactions, we employed chelating ligands featuring combinations of ImNHC and triazole-N or mesoionic tzNHC donors bridged by a CH 2 spacer with possible modifications at triazole backbone. In general, synthesized Ru(II) complexes were found to perform significantly better than analogous Ir(III) complexes in ketone and aldimine reduction. Among the Ru(II) complexes, electron-rich complexes 8/9 of the general formula [(p-cymene)(ImNHC−CH 2 −tzNHC)-Ru II (Cl)]BF 4 with two different carbene donors (ImNHC and tzNHC) were found to perform appreciably better in ketone reduction than analogous complexes with a combination of ImNHC and triazole-N-donor ([(p-cymene)(ImNHC−CH 2 −tz−N)Ru II (Cl)]BF 4 ; 4) explaining the electronic fine-tuning of the catalytic systems. No appreciable variation in activity was observed between complexes 8 and 9 having almost similar electronic profiles. However, less bulky Ru(II) complex 9 with a triazole N-phenyl substituent is more suitable for aldimine reduction than is complex 8, having a triazole N-3,5-dimethylphenyl substituent that explains the steric influence in addition to electronic effect on the reduction process.
A series of simple and electronically tuneable cyclometalated RuII–NHC complexes have been explored as efficient catalysts for various C–C/N bond forming reactions via a BH methodology.
Dialkylation of the 1,3-bis(1,2,4-triazol-1-yl)benzene with ethyl bromide resulted in the formation of [L-H2]Br2 which upon salt metathesis with NH4PF6 readily yielded the bis(triazolium) salt [L-H2](PF6)2 with non-coordinating counterions. [L-H2](PF6)2 and...
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