addition of radical inhibitors; TEMPO, BHT and BQ indicating a radical aryl species
are involved in the catalytic cycle. With these mechanistic studies in mind, the
authors concluded that a heterogeneous species is formed either by leaching and
re-aggregation or by breaking off from the heterogeneous source.
An important observation to note is that the Pd/C source (i.e. the manufacturer)
had a significant bearing on reactivity, indicating variability in activity of the catalyst
samples. This case study demonstrates how a set of reaction conditions can be tuned
and adapted and then applied to a variety of different aromatic substituents. It also
gives indication of the ability for the widely accessible catalyst, Pd/C, to be
employed in an operationally simple manner to carry out cross-coupling reactions
for the synthesis of valuable chemicals, thus bypassing the need to prepare complex
Pd catalyst scaffolds.
6 Future Perspectives and Conclusions
In this review we have highlighted the importance of heterogeneous/quasiheterogeneous–homogeneous metal-catalysed cross-coupling reactions, with many
important breakthroughs being made in synthetic chemistry. Considerably more
mechanistic work has been completed on classical cross-coupling reactions, such
as Suzuki–Miyaura cross-coupling and Heck alkenylation reactions. We decided to
set out this groundwork for classical cross-coupling reactions to assist researchers in
building and assessing more fully the potential of metal surfaces and aggregated
metal species in functionalising C–H bonds, in suitable substrates. Numerous promising synthetic methodologies have been reported over the last few years, and some
preliminary mechanistic work are completed. However, for the field to move to the
next step, it is necessary to conduct more detailed and comprehensive studies using
techniques such as XAS (XANES/EXAFS) on reactions conducted in operando,
particularly if higher oxidation state Pd
IV species are proposed. Such studies arguably require access to synchrotron facilities and specific expertise, which is difficult
for most research groups working in this field to conduct. In this context collaboration with surface scientists can be highly beneficial.
From our point view we have benefited as a synthetic organic and organometallic
chemistry group by collaborating with physical chemists working in surface catalysis. This has helped gained valuable mechanistic insight in cross-coupling catalysis,
which can also inform future directions in terms of catalyst design. Critically, it is
necessary for the catalysis community at large to seriously consider the role of
aggregated metal catalyst species in frontier-leading C–H bond functionalisation
reactions. Typically, many groups will assess this with a few tests, but the closer one
looks, evidence will usually emerge showing that aggregated metal species form
from homogeneous metal catalyst species. Whether such aggregated species are
active in their own right, a reservoir of catalytically active species or a moribund
form needs to be assessed fully. The increasing evidence showing a role for metal
catalyst surfaces and aggregated species (i.e. nanoparticles) in catalytic crossPd Nanoparticles in C–H Activation and Cross-coupling Catalysis
199
are involved in the catalytic cycle. With these mechanistic studies in mind, the
authors concluded that a heterogeneous species is formed either by leaching and
re-aggregation or by breaking off from the heterogeneous source.
An important observation to note is that the Pd/C source (i.e. the manufacturer)
had a significant bearing on reactivity, indicating variability in activity of the catalyst
samples. This case study demonstrates how a set of reaction conditions can be tuned
and adapted and then applied to a variety of different aromatic substituents. It also
gives indication of the ability for the widely accessible catalyst, Pd/C, to be
employed in an operationally simple manner to carry out cross-coupling reactions
for the synthesis of valuable chemicals, thus bypassing the need to prepare complex
Pd catalyst scaffolds.
6 Future Perspectives and Conclusions
In this review we have highlighted the importance of heterogeneous/quasiheterogeneous–homogeneous metal-catalysed cross-coupling reactions, with many
important breakthroughs being made in synthetic chemistry. Considerably more
mechanistic work has been completed on classical cross-coupling reactions, such
as Suzuki–Miyaura cross-coupling and Heck alkenylation reactions. We decided to
set out this groundwork for classical cross-coupling reactions to assist researchers in
building and assessing more fully the potential of metal surfaces and aggregated
metal species in functionalising C–H bonds, in suitable substrates. Numerous promising synthetic methodologies have been reported over the last few years, and some
preliminary mechanistic work are completed. However, for the field to move to the
next step, it is necessary to conduct more detailed and comprehensive studies using
techniques such as XAS (XANES/EXAFS) on reactions conducted in operando,
particularly if higher oxidation state Pd
IV species are proposed. Such studies arguably require access to synchrotron facilities and specific expertise, which is difficult
for most research groups working in this field to conduct. In this context collaboration with surface scientists can be highly beneficial.
From our point view we have benefited as a synthetic organic and organometallic
chemistry group by collaborating with physical chemists working in surface catalysis. This has helped gained valuable mechanistic insight in cross-coupling catalysis,
which can also inform future directions in terms of catalyst design. Critically, it is
necessary for the catalysis community at large to seriously consider the role of
aggregated metal catalyst species in frontier-leading C–H bond functionalisation
reactions. Typically, many groups will assess this with a few tests, but the closer one
looks, evidence will usually emerge showing that aggregated metal species form
from homogeneous metal catalyst species. Whether such aggregated species are
active in their own right, a reservoir of catalytically active species or a moribund
form needs to be assessed fully. The increasing evidence showing a role for metal
catalyst surfaces and aggregated species (i.e. nanoparticles) in catalytic crossPd Nanoparticles in C–H Activation and Cross-coupling Catalysis
199