The great impact of the nanoscale organization of reactive species on their performance in chemical transformations creates the possibility of fine-tuning of reaction parameters by modulating the nano-level properties. This methodology is extensively applied for the catalysts development whereas nanostructured reactants represent the practically unexplored area. Here we report the palladium- and copper-catalyzed cross-coupling reaction involving nano-structured nickel thiolate particles as reagents. On the basis of experimental findings we propose the cooperative effect of nano-level and molecular-level properties on their reactivity. The high degree of ordering, small particles size, and electron donating properties of the substituents favor the product formation. Reactant particles evolution in the reaction is visualized directly by dynamic liquid-phase electron microscopy including recording of video movies. Mechanism of the reaction in liquid phase is established using on-line mass spectrometry measurements. Together the findings provide new opportunities for organic chemical transformations design and for mechanistic studies.
Metal complexes with N-heterocyclic
carbene ligands (NHC) are ubiquitously
used in catalysis, where the stability of the metal–ligand
framework is a key issue. Our study shows that Ni-NHC complexes may
undergo facile decomposition due to the presence of water in organic
solvents (hydrolysis). The ability to hydrolyze Ni(NHC)2X2 complexes decreases in the order of NHC = 1,2,4-triazolium
> benzimidazolium ≈ imidazolium. Depending on the ligand
and
substituents, the half reaction time of the complex decomposition
may change from several minutes to hours. The nature of the halogen
is also an important factor, and the ability for decomposition of
the studied complexes decreases in the order of Cl > Br > I.
NMR and
MS monitoring revealed that Ni-NHC complexes in the presence of water
undergo hydrolysis with Ni–Ccarbene bond cleavage,
affording the corresponding N,N′-dialkylated
azolium salts and nickel(II) hydroxide. These findings are of great
importance for designing efficient and recyclable catalytic systems,
because trace water is a common contaminant in routine synthetic applications.
Copper-oxide-catalyzed
cross-coupling reaction is a well-known
strategy in heterogeneous catalysis. A large number of applications
have been developed, and catalytic cycles have been proposed based
on the involvement of the copper oxide surface. In the present work,
we have demonstrated that copper(I) and copper(II) oxides served as
precursors in the coupling reaction between thiols and aryl halides,
while catalytically active species were formed upon unusual leaching
from the oxide surface. A powerful cryo-SEM technique has been utilized
to characterize the solution-state catalytic system by electron microscopy.
A series of different experimental methods were used to reveal the
key role of copper thiolate intermediates in the studied catalytic
reaction. The present study shows an example of leaching from a metal
oxide surface, where the leaching process involved the formation of
a metal thiolate and the release of water. A new synthetic approach
was developed, and many functionalized sulfides were synthesized with
yields of up to 96%, using the copper thiolate catalyst. The study
suggests that metal oxides may not act as an innocent material under
reaction conditions; rather, they may represent a source of reactive
species for solution-state homogeneous catalysis.
Vinyl sulfides represent an important
class of compounds in organic
chemistry and materials science. Atom-economic addition of thiols
to the triple bond of alkynes provides an excellent opportunity for
environmentally friendly processes. We have found that well-known
and readily available Pd-NHC complex (IMes)Pd(acac)Cl is an efficient
catalyst for alkyne hydrothiolation. The reported technique provides
a general one-pot approach for the selective preparation of Markovnikov-type
vinyl sulfides starting from tertiary, secondary, or primary aliphatic
thiols, as well as benzylic and aromatic thiols. In all the studied
cases, the products were formed in excellent selectivity and good
yields.
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