8 Earth-Abundant d-Block Metal Nanocatalysis for Coupling …
261
Heck–Mizoroki-type cross-coupling Since the pioneering and independent works
from Gilman and Lichtenwalter [57], and Kharasch and Fields [70] in the 1930s
and 1940s respectively, concerning the homocoupling reaction of Grignard reagents
promoted by cobalt(II) salts, scarce works were carried out up to the 1990s ([22] and
references therein). Cobalt complexes have proven their efficiency for the formation
of C(sp
2 )–C(sp
2 ) bonds; particularly, Co-based catalysts are interesting for crosscouplings where alkyl halides are involved, because β-hydrogen elimination of alkyl
intermediates is not favored in contrast to the analogous Pd and Ni organometallic species. In the frame of the present contribution, Co(II) anchored to chitosanfunctionalized Fe 3 O 4 NPs has found applications in Heck- and Sonogashira-type
couplings [61]. Fe 3 O 4 NPs containing chitosan were modified by reaction with
methyl salicylate to give amide-phenol groups at their surface, which coordinate
CoCl 2 (Scheme 8.9).
This represents an example of molecular-like catalytic reactivity using functionalized nanoparticles as support. For the Heck-type reaction, the catalyst was efficient
for the coupling of aryl halides (chloro, bromo, iodo) with styrene or methyl acrylate
using PEG as solvent (Scheme 8.10). In contrast to the non-functionalized Fe 3 O 4
NPs, the functionalized ones were more active and could be recycled up to five times
preserving their efficiency, without metal leaching. The Sonogashira-type coupling
between aryl halides (bromo, iodo) and phenylacetylene derivatives gave moderate
yields under harsher conditions than those used for the Heck couplings.
Kumada-type cross-coupling Kumada–Tamao–Corriu reaction, coupling between
a Grignard reagent and an organic halide, was initially reported using Ni-based
catalysts [30, 119]; other efficient systems such as those based on palladium and
iron, have proven their efficacy, the main part of them involving molecular catalysts,
but more recently copper, nickel, and palladium nanoparticles have been used as well
([64, 92] and references therein). Iron, representing the second more abundant metal
in the Earth’s crust, has found interesting applications in C–C couplings [3, 13, 96].
The main part of reported works assumes the contribution of molecular intermediate
species in the catalytic transformation, but also nanoparticles have been identified
for low oxidation states (zero-valent FeNPs should be more stable under catalytic
conditions than organometallic Fe(0) complexes) and it has been postulated that they
act as a reservoir of molecular species exhibiting higher oxidation state [11]. Bedford
and coworkers studied the reaction of alkyl halides with aryl Grignard compounds
catalyzed by in situ generated FeNPs from FeCl 3 in the presence of PEG-14000
(Scheme 8.11 and Fig. 8.6) [12]. Authors proved that the Grignard reagent acted as
reducing agent. Preformed FeNPs/PEG (diameter in the range 7–13 nm, determined
by TEM) afforded the same reactivity as those formed in situ.
261
Heck–Mizoroki-type cross-coupling Since the pioneering and independent works
from Gilman and Lichtenwalter [57], and Kharasch and Fields [70] in the 1930s
and 1940s respectively, concerning the homocoupling reaction of Grignard reagents
promoted by cobalt(II) salts, scarce works were carried out up to the 1990s ([22] and
references therein). Cobalt complexes have proven their efficiency for the formation
of C(sp
2 )–C(sp
2 ) bonds; particularly, Co-based catalysts are interesting for crosscouplings where alkyl halides are involved, because β-hydrogen elimination of alkyl
intermediates is not favored in contrast to the analogous Pd and Ni organometallic species. In the frame of the present contribution, Co(II) anchored to chitosanfunctionalized Fe 3 O 4 NPs has found applications in Heck- and Sonogashira-type
couplings [61]. Fe 3 O 4 NPs containing chitosan were modified by reaction with
methyl salicylate to give amide-phenol groups at their surface, which coordinate
CoCl 2 (Scheme 8.9).
This represents an example of molecular-like catalytic reactivity using functionalized nanoparticles as support. For the Heck-type reaction, the catalyst was efficient
for the coupling of aryl halides (chloro, bromo, iodo) with styrene or methyl acrylate
using PEG as solvent (Scheme 8.10). In contrast to the non-functionalized Fe 3 O 4
NPs, the functionalized ones were more active and could be recycled up to five times
preserving their efficiency, without metal leaching. The Sonogashira-type coupling
between aryl halides (bromo, iodo) and phenylacetylene derivatives gave moderate
yields under harsher conditions than those used for the Heck couplings.
Kumada-type cross-coupling Kumada–Tamao–Corriu reaction, coupling between
a Grignard reagent and an organic halide, was initially reported using Ni-based
catalysts [30, 119]; other efficient systems such as those based on palladium and
iron, have proven their efficacy, the main part of them involving molecular catalysts,
but more recently copper, nickel, and palladium nanoparticles have been used as well
([64, 92] and references therein). Iron, representing the second more abundant metal
in the Earth’s crust, has found interesting applications in C–C couplings [3, 13, 96].
The main part of reported works assumes the contribution of molecular intermediate
species in the catalytic transformation, but also nanoparticles have been identified
for low oxidation states (zero-valent FeNPs should be more stable under catalytic
conditions than organometallic Fe(0) complexes) and it has been postulated that they
act as a reservoir of molecular species exhibiting higher oxidation state [11]. Bedford
and coworkers studied the reaction of alkyl halides with aryl Grignard compounds
catalyzed by in situ generated FeNPs from FeCl 3 in the presence of PEG-14000
(Scheme 8.11 and Fig. 8.6) [12]. Authors proved that the Grignard reagent acted as
reducing agent. Preformed FeNPs/PEG (diameter in the range 7–13 nm, determined
by TEM) afforded the same reactivity as those formed in situ.