250
A. R. Jupp
including solid-supported intramolecular FLPs, combinations of solid Lewis acids
and bases, metal-organic frameworks, mesoporous silica, zeolites, polyoxometalate
clusters, metal oxide surfaces and graphene. These will be discussed in the remainder
of this chapter.
7.3.1 Solid-Supported Intramolecular FLPs
Semi-immobilised FLP systems were based on either the Lewis acid or the Lewis
base being bound to a solid support. Therefore, arguably the simplest conceptual
approach to bridge the gap between semi-immobilised FLP systems and fully immobilised systems is to attach both the acid and base to the support. Intramolecular
FLP systems contain both the acidic and basic component in the same molecule; a
few examples are shown in Fig. 7.1. These have been explored since the very start
of the FLP movement [2, 60], and it is therefore somewhat surprising that the first
solid-supported intramolecular FLP was first described very recently by Fontaine
et al. in 2019, in a continuation of their work on metal-free C–H activation chemistry [61]. It is worth noting that a polymer-supported Wulff-type amino-boronic
acid was reported ten years prior, featuring a Lewis acidic boron and a Lewis basic
nitrogen centre [62]. However, in this case, the intramolecular N–B interaction was
exploited to lower the pK a of the boronic acid moiety, enabling the efficient binding
of monosaccharides, but no FLP-type reactivity was explored.
To understand the context of the heterogeneous catalyst reported by Fontaine et al.,
it is worth first describing the preceding homogeneous catalyst development. In a
breakthrough discovery in 2015, it was shown that homogeneous FLPs can mediate
the activation of C–H bonds [63]. The intramolecular FLP 14 was able to catalyse
the borylation of heteroarenes such as furans, pyrroles and electron-rich thiophenes
(Fig. 7.10a), giving complementary selectivity to transition metal catalysts. This
system, like many other FLPs, is highly sensitive towards moisture, and specialist
handling using dry solvents and inert atmospheres is required, which reduces their
utility in synthetic applications. In recent years, great efforts have been made to
generate water-tolerant FLP systems [64]. The group of Fontaine broadened the
applicability of the C–H borylation catalyst 14 by synthesising air- and moisturestable precatalysts (15–17) that could be handled on the benchtop (Fig. 7.10b) [65].
It is proposed that after an induction period, these precatalysts are converted to the
active catalyst 14 under the reaction conditions, and they could catalyse the same
borylation reactions as before. This methodology could even be used to carry out the
borylation of 1-methylindole on a kilogram scale [66].
The ease of synthesis and manipulation of these precatalysts prompted
the authors to explore solid-state analogues for heterogeneous catalysis.
Alkylammoniotrifluoroborate-functionalised polystyrene derivatives 18–20
(Fig. 7.10c) were synthesised from the functionalised styrene monomers via a
radical polymerisation process, initiated by azabisisobutyronitrile (AIBN) in hot
cyclohexanol [61]. 18, 19 and 20 could be obtained on gram scales in respectable
A. R. Jupp
including solid-supported intramolecular FLPs, combinations of solid Lewis acids
and bases, metal-organic frameworks, mesoporous silica, zeolites, polyoxometalate
clusters, metal oxide surfaces and graphene. These will be discussed in the remainder
of this chapter.
7.3.1 Solid-Supported Intramolecular FLPs
Semi-immobilised FLP systems were based on either the Lewis acid or the Lewis
base being bound to a solid support. Therefore, arguably the simplest conceptual
approach to bridge the gap between semi-immobilised FLP systems and fully immobilised systems is to attach both the acid and base to the support. Intramolecular
FLP systems contain both the acidic and basic component in the same molecule; a
few examples are shown in Fig. 7.1. These have been explored since the very start
of the FLP movement [2, 60], and it is therefore somewhat surprising that the first
solid-supported intramolecular FLP was first described very recently by Fontaine
et al. in 2019, in a continuation of their work on metal-free C–H activation chemistry [61]. It is worth noting that a polymer-supported Wulff-type amino-boronic
acid was reported ten years prior, featuring a Lewis acidic boron and a Lewis basic
nitrogen centre [62]. However, in this case, the intramolecular N–B interaction was
exploited to lower the pK a of the boronic acid moiety, enabling the efficient binding
of monosaccharides, but no FLP-type reactivity was explored.
To understand the context of the heterogeneous catalyst reported by Fontaine et al.,
it is worth first describing the preceding homogeneous catalyst development. In a
breakthrough discovery in 2015, it was shown that homogeneous FLPs can mediate
the activation of C–H bonds [63]. The intramolecular FLP 14 was able to catalyse
the borylation of heteroarenes such as furans, pyrroles and electron-rich thiophenes
(Fig. 7.10a), giving complementary selectivity to transition metal catalysts. This
system, like many other FLPs, is highly sensitive towards moisture, and specialist
handling using dry solvents and inert atmospheres is required, which reduces their
utility in synthetic applications. In recent years, great efforts have been made to
generate water-tolerant FLP systems [64]. The group of Fontaine broadened the
applicability of the C–H borylation catalyst 14 by synthesising air- and moisturestable precatalysts (15–17) that could be handled on the benchtop (Fig. 7.10b) [65].
It is proposed that after an induction period, these precatalysts are converted to the
active catalyst 14 under the reaction conditions, and they could catalyse the same
borylation reactions as before. This methodology could even be used to carry out the
borylation of 1-methylindole on a kilogram scale [66].
The ease of synthesis and manipulation of these precatalysts prompted
the authors to explore solid-state analogues for heterogeneous catalysis.
Alkylammoniotrifluoroborate-functionalised polystyrene derivatives 18–20
(Fig. 7.10c) were synthesised from the functionalised styrene monomers via a
radical polymerisation process, initiated by azabisisobutyronitrile (AIBN) in hot
cyclohexanol [61]. 18, 19 and 20 could be obtained on gram scales in respectable