employing Pd
II /MWCNT, markedly increased yields were observed in halogenations employing N-bromo- and N-chlorosuccinimide as the X
+ source.
For arylations of a range of N-directing substrates, the Pd
II /MWCNT catalyst
performed to an equivalent level or was poorer than Pd(OAc) 2 . On a model substrate,
2-phenyl-3-methylpyridine and para-substituted electron-withdrawing and donating
as well as neutral aromatic substituents were well tolerated, with good yields (aryl
group being transferred from the diaryliodonium salt derivative).
Catalyst recycling studies in a specific reaction, methoxylation of
8-methylquinoline showed a minimal loss of activity, with a remarkable drop of
only 8% conversion after 16 recycles. ICP-MS showed extremely low levels
of leached Pd (<250 ppb) into the reaction solution post-reaction. No further
conversion was observed after hot-filtration tests were administered. Taken together,
these experiments constitute some evidence of a heterogeneous catalytic manifold
being responsible for this transformation.
Recycling studies were additionally carried out with a model arylation reaction
where reactivity was conserved for the first three cycles but found to drop significantly after the third. ICP-MS analysis of the reaction mixture showed 38.4 ppm of
leached Pd, significantly higher than that with the model methoxylation reaction. It is
worthy to note that the catalytic recyclability and the amount of leaching of catalytic
metal vary significantly depending on the type of reaction carried out, thus
underlining that the supported catalysts can behave differently, depending on the
reaction conditions deployed.
This work constitutes a supported heterogeneous analogue of synthetically useful
C–H functionalisation reactions, previously carried out with Pd(OAc) 2 . While generally not performing better than the homogeneous catalyst precursor per reaction,
this system does exhibit the benefit of being able to be recycled and reused multiple
times.
5.3 Case Study III) Pd/C-Catalysed C–H Alkenylations
in Continuous Flow, Using γ-Valerolactone as a Green
Reaction Medium
Vaccaro and Ackermann et al. have focused on boosting the sustainability of a direct
C–H alkenylation process, namely, in the Fujiwara–Moritani reaction [74]. The
simple, widely available heterogeneous supported catalyst Pd/C was deployed.
Acetanilides were functionalised in a site-selective manner at the ortho-C–H position, with activated alkenes in the presence of an acid and oxidant additives. In
optimised reaction conditions, substrates were deployed alongside a polymersupported tosic acid, which allowed for more efficient recycling. γ-Valerolactone
(GVL), a biomass-derived reaction medium, was used as the solvent (Scheme 9).
All reactions reported showed good to excellent yields. Meta- and parasubstituted acetanilides were well tolerated in addition to a variety of different
electron-withdrawing substituents on the alkene coupling partner.
Pd Nanoparticles in C–H Activation and Cross-coupling Catalysis
191
II /MWCNT, markedly increased yields were observed in halogenations employing N-bromo- and N-chlorosuccinimide as the X
+ source.
For arylations of a range of N-directing substrates, the Pd
II /MWCNT catalyst
performed to an equivalent level or was poorer than Pd(OAc) 2 . On a model substrate,
2-phenyl-3-methylpyridine and para-substituted electron-withdrawing and donating
as well as neutral aromatic substituents were well tolerated, with good yields (aryl
group being transferred from the diaryliodonium salt derivative).
Catalyst recycling studies in a specific reaction, methoxylation of
8-methylquinoline showed a minimal loss of activity, with a remarkable drop of
only 8% conversion after 16 recycles. ICP-MS showed extremely low levels
of leached Pd (<250 ppb) into the reaction solution post-reaction. No further
conversion was observed after hot-filtration tests were administered. Taken together,
these experiments constitute some evidence of a heterogeneous catalytic manifold
being responsible for this transformation.
Recycling studies were additionally carried out with a model arylation reaction
where reactivity was conserved for the first three cycles but found to drop significantly after the third. ICP-MS analysis of the reaction mixture showed 38.4 ppm of
leached Pd, significantly higher than that with the model methoxylation reaction. It is
worthy to note that the catalytic recyclability and the amount of leaching of catalytic
metal vary significantly depending on the type of reaction carried out, thus
underlining that the supported catalysts can behave differently, depending on the
reaction conditions deployed.
This work constitutes a supported heterogeneous analogue of synthetically useful
C–H functionalisation reactions, previously carried out with Pd(OAc) 2 . While generally not performing better than the homogeneous catalyst precursor per reaction,
this system does exhibit the benefit of being able to be recycled and reused multiple
times.
5.3 Case Study III) Pd/C-Catalysed C–H Alkenylations
in Continuous Flow, Using γ-Valerolactone as a Green
Reaction Medium
Vaccaro and Ackermann et al. have focused on boosting the sustainability of a direct
C–H alkenylation process, namely, in the Fujiwara–Moritani reaction [74]. The
simple, widely available heterogeneous supported catalyst Pd/C was deployed.
Acetanilides were functionalised in a site-selective manner at the ortho-C–H position, with activated alkenes in the presence of an acid and oxidant additives. In
optimised reaction conditions, substrates were deployed alongside a polymersupported tosic acid, which allowed for more efficient recycling. γ-Valerolactone
(GVL), a biomass-derived reaction medium, was used as the solvent (Scheme 9).
All reactions reported showed good to excellent yields. Meta- and parasubstituted acetanilides were well tolerated in addition to a variety of different
electron-withdrawing substituents on the alkene coupling partner.
Pd Nanoparticles in C–H Activation and Cross-coupling Catalysis
191