Bulky NHC–Cobalt Complex-Catalyzed Highly Markovnikov-Selective Hydrosilylation of Alkynes

The hydrosilylation of alkynes is one of the most attractive and, at the same time, most challenging catalytic transformations, usually demanding the use of noble transition metals. We describe a catalytic system, based on cobalt(0) complex and bulky N-heterocyclic carbene (NHC) ligands, permitting the highly effective hydrosilylation of a broad scope of alkynes and silanes. The application of bulky NHC ligands allowed a decrease in the amount of cobalt necessary for an effective reaction run to 2.5 mol% and provided excellent selectivity towards challenging α-vinylsilanes. The developed method tolerates a number of substituted aryl, alkyl, and silyl acetylenes. Moreover, it is suitable for both tertiary and secondary silanes. Our findings confirm that steric hindrance around the metal center can effectively increase the activity of a catalyst and ensure better selectivity than those of analogous complexes bearing smaller ligands.

Preparation of cost-effective Ni-Ag bimetallic catalyst for hydrodehalogenation of aryl halides under mild conditions

Cost-effective Ni-Ag bimetallic nanoparticles were prepared using an aqueous-phase surfactant-aided co-reduction process and used as catalyst for the hydrodehalogenation of aryl halides CAr-X (X=F, Cl, Br). The CAr-X could be...

Metal-free thioesterification of α,β-unsaturated aldehydes with thiols

For the first time, the synthesis of thioesters starting from enals and thiols has been performed in the presence of a bulky N-heterocyclic carbene (NHC) as a catalyst.

Solvent-free hydroboration of alkynes catalyzed by an NHC–cobalt complex

A new cobalt complex bearing a bulky N-heterocyclic carbene (NHC) ligand is described as a pre-catalyst for alkyne hydroboration. The proposed catalytic system, synthesized using easily accessible reagents, allowed obtaining a series of mono- and dialkenylboranes in solvent-free conditions with excellent efficiency and selectivity. The results have been compared to those obtained in the presence of the same cobalt complex containing smaller NHC ligands and those achieved for the catalytic system based on a CoCl₂ – NHC precursor.

Cobalt complex bearing bulky NHC ligand enable selective hydroboration of alkynes in solvent-free conditions.

Accelerating the insertion reactions of (NHC)Cu-H via remote ligand functionalization

Most ligand designs for reactions catalyzed by (NHC)Cu-H (NHC = N-heterocyclic carbene ligand) have focused on introducing steric bulk near the Cu center. Here, we evaluate the effect of remote ligand modification in a series of [(NHC)CuH]2 in which the para substituent (R) on the N-aryl groups of the NHC is Me, Et, t Bu, OMe or Cl. Although the R group is distant (6 bonds away) from the reactive Cu center, the complexes have different spectroscopic signatures. Kinetics studies of the insertion of ketone, aldimine, alkyne, and unactivated α-olefin substrates reveal that Cu-H complexes with bulky or electron-rich R groups undergo faster substrate insertion. The predominant cause of this phenomenon is destabilization of the [(NHC)CuH]2 dimer relative to the (NHC)Cu-H monomer, resulting in faster formation of Cu-H monomer. These findings indicate that remote functionalization of NHCs is a compelling strategy for accelerating the rate of substrate insertion with Cu-H species.

Evidence for “cocktail”-type catalysis in Buchwald–Hartwig reaction. A mechanistic study

The mechanism of the C–N cross-coupling reaction, catalyzed by Pd/NHC, was evaluated at the molecular and nanoscale levels. The first evidence for the involvement of a “cocktail”-type system in the Buchwald–Hartwig reaction is provided.