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.
Naphthyl-derived
orthometalated RuII-NHC complexes have
been developed for catalytic applications considering the stereoelectronic
profiles of the NHC ligands, which can be modified easily via alteration
of the NHC donors (ImNHC/1,2,4-tzNHC) as well as their substitution
pattern at the naphthyl ring. The azolium salts [(2a–c)-H]Br, precursors for the NHC ligands, react with [Ru(p-cymene)Cl2]2 in the presence of
a base to deliver the ortho-metalated RuII-NHC complexes 3a–c with the general formula [(NHC)Ru(p-cymene)Br]. Orthometalation in these complexes can be
exploited for further functionalization of the NHC ligands. This is
depicted by the generation of the diphenylethylene-inserted isolable
intermediate complex 4, from the reaction of 3a with diphenylacetylene in a 1:1 ratio, which eventually provides
an annulated salt 5a-Br via reductive elimination. Gratifyingly,
this process can also be made catalytic, which directly provides several
mono-/bis-annulated cationic N-heterocyclic compounds starting from
the imidazolium salts [(2a,b)-H]Br using
[Ru(p-cymene)Cl2]2 as a precatalyst.
Furthermore, subtle variations in the electronic profiles of the complexes 3a–c in combination with steric alterations
are observed to influence their activity in the transfer hydrogenation
of acetophenone, a model for the hydride transfer process. Among all
the complexes studied here, complex 3b with an ImNHC
donor at the second position of the naphthyl ring was identified as
a superior catalyst in comparison to 3a,c featuring either a different NHC donor or substitution pattern with
a low loading of 0.1 mol%.
A general method for catalyst- and solvent-free room temperature reductive amination has been developed and it efficiently delivers a wide range of sterically and electronically diverse secondary amines in one-pot.
An unprecedented base-catalyzed hydroarylation of isocyanates with azolium salts was developed, which follows a simple reaction pathway and provided facile access to diverse C2-amidated azolium salts under mild conditions. Importantly, this methodology can also be applied for the [a
Effective access to azolium salts with varying stereoelectronic properties has always fascinated the organometallic chemists. Herein, we described a Ru(II)‐catalyzed one‐step access to diverse meta‐alkylated N‐aryl (benz)imidazolium salts (>40 examples) via σ‐bond activation strategy starting from the readily available azolium salts using alkyl bromide. Notably, the present method is compatible with a wide range of azolium salts and secondary/tertiary alkyl bromides, including the biologically relevant motifs. Various control experiments along with the DFT calculation established the reaction pathway of orthometalated Ru(II)‐NHC complex (6) formation followed by the generation of a Ru(III)‐intermediate, which controls the observed meta‐selectivity, via single electron transfer. Crucially, detailed insight of the reaction mechanism helped to comprehend why some substrates are challenging for this methodology.
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