272
A. R. Jupp
Pd as the Lewis acid to heterolytically cleave the H–H bond, resulting in a FLP-type
mechanism. The heterogeneous catalyst could easily be recycled and reused without
loss of activity over multiple cycles. Related Pd/CN materials, albeit prepared in
a different manner, were also shown to be effective catalysts for the deoxygenation of aryl aldehydes and ketones compounds under mild conditions (25 °C, 1 bar
H 2 ) [130]. A similar mechanism was proposed by Rossi et al. for a gold-containing
analogue [131]. Pyrolysis of Au(OAc) 3 in the presence of 1,10-phenanthroline over
TiO 2 afforded a highly active and selective Au-NP catalyst embedded in a nitrogendoped carbon support, labelled as Au@N-doped carbon/TiO 2 . The catalyst was able
to promote the selective hydrogenation of alkynes to Z-alkenes in high yields under
mild conditions, by analogy with the gold surfaces and nanoparticles discussed earlier
in the chapter. The mechanism was proposed to involve the heterolytic cleavage of H 2
by the Au and N sites, which was supported by a Hammett plot of various substituted
alkynes: a strong substituent effect was observed for the Au@N-doped carbon/TiO 2
catalyst, with a rate enhancement for the substrates containing electron-withdrawing
groups, suggesting a charge build-up on the substrate during the catalytic cycle. This
result is in contrast to the established Lindlar catalyst, which had a ρ-value of close
to 0 for the same experiments, and is consistent with a homolytic H 2 dissociation
pathway. Once again, the catalyst exhibited excellent stability and recyclability, and
was able to catalyse the alkyne reduction selectively even in the presence of other
reducible organic functional groups.
There are other surfaces that have been invoked in FLP chemistry, either directly or
used as supports for additional elements. Hexagonal boron nitride (h-BN) is isoelectronic with the graphite/graphene carbon surfaces discussed above, and has high
chemical and thermal stability [132]. Blair et al. studied defect-laden h-BN as a
heterogeneous catalyst for the hydrogenation of a range of alkenes, and demonstrated
rates that are significantly faster than other metal-free FLP systems and graphene catalysts [133]. Mechanochemistry, and specifically a reactor based on a ball mill, was
used to ensure a large number of defects on the h-BN surface, and to prevent cluster
formation. DFT was used to computationally study the different types of defects
and their significance in the reaction mechanism of the hydrogenation of the simple
alkene, propene. The defects that were studied were boron vacancies (V B ), nitrogen
vacancies (V N ), Stone-Wales defects and boron substitution for nitrogen (B N ), and
it was shown that the hydrogenation most likely occurs at V N sites, as supported by
solid-state NMR spectroscopic measurements. In this case, the authors noted that
although there are similarities between the hydrogenation mechanism on the h-BN
surface and that for FLPs, ultimately their mechanism is closer to the Horiuti–Polanyi
mechanism normally seen for transition metals, which involves hydrogen transfer
from the surface to the β carbon of the alkene, and subsequent reductive elimination
of the free alkane from the surface [134].
Phosphorene, a two-dimensional allotrope of phosphorus, is a single layer of the
stable black phosphorus, and was isolated for the first time in 2014 by multiple
independent research groups [135–137]. Chen et al. subsequently computationally
designed a catalyst based on phosphorene doped with B or Al, where the P and Al/B
sites can function as an FLP [138]. Both materials were predicted to heterolytically
A. R. Jupp
Pd as the Lewis acid to heterolytically cleave the H–H bond, resulting in a FLP-type
mechanism. The heterogeneous catalyst could easily be recycled and reused without
loss of activity over multiple cycles. Related Pd/CN materials, albeit prepared in
a different manner, were also shown to be effective catalysts for the deoxygenation of aryl aldehydes and ketones compounds under mild conditions (25 °C, 1 bar
H 2 ) [130]. A similar mechanism was proposed by Rossi et al. for a gold-containing
analogue [131]. Pyrolysis of Au(OAc) 3 in the presence of 1,10-phenanthroline over
TiO 2 afforded a highly active and selective Au-NP catalyst embedded in a nitrogendoped carbon support, labelled as Au@N-doped carbon/TiO 2 . The catalyst was able
to promote the selective hydrogenation of alkynes to Z-alkenes in high yields under
mild conditions, by analogy with the gold surfaces and nanoparticles discussed earlier
in the chapter. The mechanism was proposed to involve the heterolytic cleavage of H 2
by the Au and N sites, which was supported by a Hammett plot of various substituted
alkynes: a strong substituent effect was observed for the Au@N-doped carbon/TiO 2
catalyst, with a rate enhancement for the substrates containing electron-withdrawing
groups, suggesting a charge build-up on the substrate during the catalytic cycle. This
result is in contrast to the established Lindlar catalyst, which had a ρ-value of close
to 0 for the same experiments, and is consistent with a homolytic H 2 dissociation
pathway. Once again, the catalyst exhibited excellent stability and recyclability, and
was able to catalyse the alkyne reduction selectively even in the presence of other
reducible organic functional groups.
There are other surfaces that have been invoked in FLP chemistry, either directly or
used as supports for additional elements. Hexagonal boron nitride (h-BN) is isoelectronic with the graphite/graphene carbon surfaces discussed above, and has high
chemical and thermal stability [132]. Blair et al. studied defect-laden h-BN as a
heterogeneous catalyst for the hydrogenation of a range of alkenes, and demonstrated
rates that are significantly faster than other metal-free FLP systems and graphene catalysts [133]. Mechanochemistry, and specifically a reactor based on a ball mill, was
used to ensure a large number of defects on the h-BN surface, and to prevent cluster
formation. DFT was used to computationally study the different types of defects
and their significance in the reaction mechanism of the hydrogenation of the simple
alkene, propene. The defects that were studied were boron vacancies (V B ), nitrogen
vacancies (V N ), Stone-Wales defects and boron substitution for nitrogen (B N ), and
it was shown that the hydrogenation most likely occurs at V N sites, as supported by
solid-state NMR spectroscopic measurements. In this case, the authors noted that
although there are similarities between the hydrogenation mechanism on the h-BN
surface and that for FLPs, ultimately their mechanism is closer to the Horiuti–Polanyi
mechanism normally seen for transition metals, which involves hydrogen transfer
from the surface to the β carbon of the alkene, and subsequent reductive elimination
of the free alkane from the surface [134].
Phosphorene, a two-dimensional allotrope of phosphorus, is a single layer of the
stable black phosphorus, and was isolated for the first time in 2014 by multiple
independent research groups [135–137]. Chen et al. subsequently computationally
designed a catalyst based on phosphorene doped with B or Al, where the P and Al/B
sites can function as an FLP [138]. Both materials were predicted to heterolytically