7 Heterogeneous Catalysis by Frustrated Lewis Pairs
245
by acetophenone in Fig. 7.5b. This reaction was more selective when 4 Å MS were
used as the Lewis base instead of α-CD, presumably because the former could trap
the water by-product to drive the reaction to the styrene product. Finally, applying
the same reaction conditions to diaryl ketones, such as benzophenone, resulted in
reductive deoxygenation to give diphenylmethane. These reactions demonstrate the
wide scope of Lewis bases that can be used in FLP reactions, and highlights the ease
in some cases of transitioning from homogeneous to heterogeneous catalysis.
The examples so far have all targeted the synthesis of heterogeneous FLP systems
as catalysts, but, for completeness, solid-supported bases in combination with Lewis
acids have also been used in polymerisation chemistry, specifically for modifying the
surface of the material. The cooperative action of Lewis acids and bases to promote
the efficient and controlled polymerisation of a range of monomer substrates has
been well-studied, most notably by the group of Chen [40]. This approach was
extended to surface-initiated polymerisation (SIP) by Zhang and Zhang [41]. A selfassembled monolayer containing N-heterocyclic olefin (NHO) moieties supported on
a silica wafer was prepared, and used as a Lewis base in conjunction with Al(C 6 F 5 ) 3
to promote the polymerisation of lactones, giving rise to linear polymer brushes
(Fig. 7.6a). A different surface modification could be achieved by using a grafted
polymer chain bearing NHO functionalities in combination with Al(C 6 F 5 ) 3 , as in
this case bottle-brush brushes were formed (Fig. 7.6b). This novel and conceptually
simple application of solid-supported FLP reactivity highlights the growing utility
of such systems in diverse fields.
Understanding the interactions of Lewis acids and bases at heterogeneous interfaces is crucial for controlling such surface reactions. The “frustration” of these FLP
systems, that is the reason that Lewis adduct formation does not inhibit reactivity, is
often not related to steric hindrance, which is the common cause in typical homogeneous systems. To assess this phenomenon, Dawlaty et al. used a solid Lewis base
consisting of 4-mercaptobenzonitrile bound to gold, and B(C 6 F 5 ) 3 as a Lewis acid,
and used vibrational spectroscopy to measure the stretching frequency of the nitrile
as a probe for the strength of adduct formation [42]. They showed that the surface
adducts were weaker than those in the bulk. Three different origins for this frustration
were proposed: surface steric frustration; solvation electric field differences between
the surface and the bulk; and the alignment of energy levels between the frontier
molecular orbitals of the acid and the base. It was concluded that multiple factors
affect the observed surface interactions, and the relative importance of each effect
will almost certainly be dependent on the system in question.
7.2.2 Solid Lewis Acid with a Soluble Lewis Base
There are also several examples in the literature of solid Lewis acids used in conjunction with soluble Lewis bases for FLP reactivity, where the former can be a solidsupported Lewis acid or a transition metal surface. O’Hare et al. adopted a conceptually similar approach to the aforementioned work of Taoufik [29], in that they adapted
245
by acetophenone in Fig. 7.5b. This reaction was more selective when 4 Å MS were
used as the Lewis base instead of α-CD, presumably because the former could trap
the water by-product to drive the reaction to the styrene product. Finally, applying
the same reaction conditions to diaryl ketones, such as benzophenone, resulted in
reductive deoxygenation to give diphenylmethane. These reactions demonstrate the
wide scope of Lewis bases that can be used in FLP reactions, and highlights the ease
in some cases of transitioning from homogeneous to heterogeneous catalysis.
The examples so far have all targeted the synthesis of heterogeneous FLP systems
as catalysts, but, for completeness, solid-supported bases in combination with Lewis
acids have also been used in polymerisation chemistry, specifically for modifying the
surface of the material. The cooperative action of Lewis acids and bases to promote
the efficient and controlled polymerisation of a range of monomer substrates has
been well-studied, most notably by the group of Chen [40]. This approach was
extended to surface-initiated polymerisation (SIP) by Zhang and Zhang [41]. A selfassembled monolayer containing N-heterocyclic olefin (NHO) moieties supported on
a silica wafer was prepared, and used as a Lewis base in conjunction with Al(C 6 F 5 ) 3
to promote the polymerisation of lactones, giving rise to linear polymer brushes
(Fig. 7.6a). A different surface modification could be achieved by using a grafted
polymer chain bearing NHO functionalities in combination with Al(C 6 F 5 ) 3 , as in
this case bottle-brush brushes were formed (Fig. 7.6b). This novel and conceptually
simple application of solid-supported FLP reactivity highlights the growing utility
of such systems in diverse fields.
Understanding the interactions of Lewis acids and bases at heterogeneous interfaces is crucial for controlling such surface reactions. The “frustration” of these FLP
systems, that is the reason that Lewis adduct formation does not inhibit reactivity, is
often not related to steric hindrance, which is the common cause in typical homogeneous systems. To assess this phenomenon, Dawlaty et al. used a solid Lewis base
consisting of 4-mercaptobenzonitrile bound to gold, and B(C 6 F 5 ) 3 as a Lewis acid,
and used vibrational spectroscopy to measure the stretching frequency of the nitrile
as a probe for the strength of adduct formation [42]. They showed that the surface
adducts were weaker than those in the bulk. Three different origins for this frustration
were proposed: surface steric frustration; solvation electric field differences between
the surface and the bulk; and the alignment of energy levels between the frontier
molecular orbitals of the acid and the base. It was concluded that multiple factors
affect the observed surface interactions, and the relative importance of each effect
will almost certainly be dependent on the system in question.
7.2.2 Solid Lewis Acid with a Soluble Lewis Base
There are also several examples in the literature of solid Lewis acids used in conjunction with soluble Lewis bases for FLP reactivity, where the former can be a solidsupported Lewis acid or a transition metal surface. O’Hare et al. adopted a conceptually similar approach to the aforementioned work of Taoufik [29], in that they adapted