252
M. Camats et al.
In the present contribution, we focus on the recent advances in 3d metal nanocatalysts (for a recent review, see: [127]), involving polyols acting as stabilizer and/or
reaction medium for coupling reactions, Carbon–Carbon and Carbon–Heteroatom
bond formation processes, including multicomponent syntheses.
8.2 Carbon–Carbon Bond Formation
C–C cross-coupling reactions have been largely dominated by palladium-based catalyzed processes due to their efficiency and versatility. In particular, the ability of
this metal to stabilize different kinds of species (complexes, nanoparticles, extended
surfaces) and its relatively high robustness under many different reaction conditions
has permitted the elucidation of the corresponding mechanisms [18, 120, 124]. From
a sustainable chemistry point of view, the use of first-row transition metals is obviously preferred and a huge research has been developed in the last years (for instance,
see the contributions published in the Accounts of Chemical Research special issue
“Earth Abundant Metals in Homogeneous Catalysis,” [27]).
8.2.1 Lewis Acid-Catalyzed Coupling Reactions
Heterocyclic motifs are present in a large variety of naturally occurring products
along with industrial compounds [37, 59, 66, 102]. Multicomponent reactions represent an environmentally friendly approach to prepare polyfunctional compounds,
in particular heterocycle derivatives, via one-pot processes involving three or more
reactants, with high atom economy and easy implementation [19, 130, 138]. These
transformations are often promoted by Lewis acids, which favor the kinetics directing the reaction pathway and in consequence improving the selectivity [51]. In this
frame, Khurana and coworkers reported nickel nanoparticles (NiNPs) stabilized by
polyethylene glycol (PEG-4000) and prepared by polyol-based methodology using
ethylene glycol (EG) in the presence of NaBH 4 , which were applied in the synthesis
of spiropyrans [73], interesting materials particularly due to their unique molecular
switch properties that can trigger structural isomerization under the effect of different
external stimuli (light, mechanical stress, temperature…) [75, 83, 131]. They were
synthesized by a multicomponent reaction, constituted of a tandem Knoevenagelcyclo-condensation involving ninhydrin (or related cyclic dicarbonyl compounds),
malonitrile, and dimedone (or related 1,3-dicarbonyl derivatives) (Scheme 8.1). The
role of the nanocatalyst (mean size: ca. 7 nm determined by TEM) was evidenced
by different control tests; in the absence of nickel, the reaction was much slower
(some hours vs. some minutes) and the use of Ni powder (particle size <150 μm) led
to moderate yields after 8 h of reaction. Ethylene glycol was a convenient solvent
permitting a straightforward extraction of products by a biphasic system (using ethyl
acetate as immiscible solvent with ethylene glycol), preserving the catalyst dispersed
M. Camats et al.
In the present contribution, we focus on the recent advances in 3d metal nanocatalysts (for a recent review, see: [127]), involving polyols acting as stabilizer and/or
reaction medium for coupling reactions, Carbon–Carbon and Carbon–Heteroatom
bond formation processes, including multicomponent syntheses.
8.2 Carbon–Carbon Bond Formation
C–C cross-coupling reactions have been largely dominated by palladium-based catalyzed processes due to their efficiency and versatility. In particular, the ability of
this metal to stabilize different kinds of species (complexes, nanoparticles, extended
surfaces) and its relatively high robustness under many different reaction conditions
has permitted the elucidation of the corresponding mechanisms [18, 120, 124]. From
a sustainable chemistry point of view, the use of first-row transition metals is obviously preferred and a huge research has been developed in the last years (for instance,
see the contributions published in the Accounts of Chemical Research special issue
“Earth Abundant Metals in Homogeneous Catalysis,” [27]).
8.2.1 Lewis Acid-Catalyzed Coupling Reactions
Heterocyclic motifs are present in a large variety of naturally occurring products
along with industrial compounds [37, 59, 66, 102]. Multicomponent reactions represent an environmentally friendly approach to prepare polyfunctional compounds,
in particular heterocycle derivatives, via one-pot processes involving three or more
reactants, with high atom economy and easy implementation [19, 130, 138]. These
transformations are often promoted by Lewis acids, which favor the kinetics directing the reaction pathway and in consequence improving the selectivity [51]. In this
frame, Khurana and coworkers reported nickel nanoparticles (NiNPs) stabilized by
polyethylene glycol (PEG-4000) and prepared by polyol-based methodology using
ethylene glycol (EG) in the presence of NaBH 4 , which were applied in the synthesis
of spiropyrans [73], interesting materials particularly due to their unique molecular
switch properties that can trigger structural isomerization under the effect of different
external stimuli (light, mechanical stress, temperature…) [75, 83, 131]. They were
synthesized by a multicomponent reaction, constituted of a tandem Knoevenagelcyclo-condensation involving ninhydrin (or related cyclic dicarbonyl compounds),
malonitrile, and dimedone (or related 1,3-dicarbonyl derivatives) (Scheme 8.1). The
role of the nanocatalyst (mean size: ca. 7 nm determined by TEM) was evidenced
by different control tests; in the absence of nickel, the reaction was much slower
(some hours vs. some minutes) and the use of Ni powder (particle size <150 μm) led
to moderate yields after 8 h of reaction. Ethylene glycol was a convenient solvent
permitting a straightforward extraction of products by a biphasic system (using ethyl
acetate as immiscible solvent with ethylene glycol), preserving the catalyst dispersed