6 Lewis Acidic Boranes in Frustrated Lewis Pair Chemistry
211
the Erker group developed a flexible intramolecular ethylidene-bridged phosphinoborane (Fig. 6.1b). Again, this species was capable of activating H 2 at room temperature
[2].
It became obvious that the combination of steric hindrance of the Lewis pair,
coupled with high Lewis acidity and basicity of the FLP components, was responsible
for the activation of H 2 . Based on these observations, intermolecular FLPs were
reported in 2007 together with their catalytic applications in H 2 activation [3]. These
major breakthroughs have laid the foundation for a field that has garnered much
attention over recent years and has inspired chemists to search for new applications
[4, 5]. The high importance of this in advancing synthetic main group chemistry is
reflected by a large number of reviews and even books published on FLP chemistry
[6–15].
This chapter will give an overview of the synthesis and application of Lewis acidic
boranes, which have been utilized in FLP-based chemistry. Indeed, the Lewis acidity
of the borane used in the FLP is central to the reactivity of the FLP in terms of small
molecule activation and catalytic activity. At this point, it is poignant to recap how
the Lewis acidity of boranes can be measured. Lewis acidity determination can be
classified into three main categories: effective, global and intrinsic metrics (Fig. 6.2)
[16]. Effective metrics typically consist of spectroscopic approaches, measuring the
effect of the Lewis acid on a probe molecule [17, 18]. Gutmann–Beckett and the
Childs methods both fall into this category [19–21]. The Gutmann–Beckett method
involves the coordination of triethylphosphine oxide (Et 3 PO) to a Lewis acid and
recording the change in
31 P NMR resonance. The Lewis basic oxygen in Et 3 PO forms
an adduct with the borane, thus causing a deshielding of the adjacent phosphorus
atom. This perturbation can be measured to ascertain the Lewis acidity by attributing
an acceptor number (AN) to the borane. The Childs method correlates the Lewis
acidity by measuring the change in the
1 H NMR resonance of the H
3 proton in
crotonaldehyde upon its complexation to a Lewis acid. From this method, it is possible
to measure the relative acidity of the borane compared with boron tribromide (set to
1.0).
Global metrics, on the other hand, are computationally derived and consider the
whole process of adduct formation. These techniques enable extrapolation of thermodynamic outputs such as intramolecular coordination in the initial Lewis acid (E intra ),
deformation energies and preorganization (E prep ) in addition to the immediate interaction energy (E inter ). The common examples of this technique are the fluoride ion
affinities (FIA) method and the hydride ion affinity (HIA) [22, 23]. FIA calculates
the change in enthalpy upon coordination of a fluoride anion (generated from a fluorophosgene precursor) to a free gaseous Lewis acid. HIA, on the other hand, utilizes
the isodesmic reaction between superhydride and the borane in question.
The final class is intrinsic metrics; these enable the electronic structure of the free
Lewis acid to be probed through a combination of quantum-theoretical numbers or
spectroscopy in a non-invasive fashion. The main approach utilized for this technique
is the Global Electrophilicity Index (GEI) [24], which is based on the propensity of
a molecule to take up electron density and as such the ranking of Lewis acidities is
not defined with respect to a specific base.
211
the Erker group developed a flexible intramolecular ethylidene-bridged phosphinoborane (Fig. 6.1b). Again, this species was capable of activating H 2 at room temperature
[2].
It became obvious that the combination of steric hindrance of the Lewis pair,
coupled with high Lewis acidity and basicity of the FLP components, was responsible
for the activation of H 2 . Based on these observations, intermolecular FLPs were
reported in 2007 together with their catalytic applications in H 2 activation [3]. These
major breakthroughs have laid the foundation for a field that has garnered much
attention over recent years and has inspired chemists to search for new applications
[4, 5]. The high importance of this in advancing synthetic main group chemistry is
reflected by a large number of reviews and even books published on FLP chemistry
[6–15].
This chapter will give an overview of the synthesis and application of Lewis acidic
boranes, which have been utilized in FLP-based chemistry. Indeed, the Lewis acidity
of the borane used in the FLP is central to the reactivity of the FLP in terms of small
molecule activation and catalytic activity. At this point, it is poignant to recap how
the Lewis acidity of boranes can be measured. Lewis acidity determination can be
classified into three main categories: effective, global and intrinsic metrics (Fig. 6.2)
[16]. Effective metrics typically consist of spectroscopic approaches, measuring the
effect of the Lewis acid on a probe molecule [17, 18]. Gutmann–Beckett and the
Childs methods both fall into this category [19–21]. The Gutmann–Beckett method
involves the coordination of triethylphosphine oxide (Et 3 PO) to a Lewis acid and
recording the change in
31 P NMR resonance. The Lewis basic oxygen in Et 3 PO forms
an adduct with the borane, thus causing a deshielding of the adjacent phosphorus
atom. This perturbation can be measured to ascertain the Lewis acidity by attributing
an acceptor number (AN) to the borane. The Childs method correlates the Lewis
acidity by measuring the change in the
1 H NMR resonance of the H
3 proton in
crotonaldehyde upon its complexation to a Lewis acid. From this method, it is possible
to measure the relative acidity of the borane compared with boron tribromide (set to
1.0).
Global metrics, on the other hand, are computationally derived and consider the
whole process of adduct formation. These techniques enable extrapolation of thermodynamic outputs such as intramolecular coordination in the initial Lewis acid (E intra ),
deformation energies and preorganization (E prep ) in addition to the immediate interaction energy (E inter ). The common examples of this technique are the fluoride ion
affinities (FIA) method and the hydride ion affinity (HIA) [22, 23]. FIA calculates
the change in enthalpy upon coordination of a fluoride anion (generated from a fluorophosgene precursor) to a free gaseous Lewis acid. HIA, on the other hand, utilizes
the isodesmic reaction between superhydride and the borane in question.
The final class is intrinsic metrics; these enable the electronic structure of the free
Lewis acid to be probed through a combination of quantum-theoretical numbers or
spectroscopy in a non-invasive fashion. The main approach utilized for this technique
is the Global Electrophilicity Index (GEI) [24], which is based on the propensity of
a molecule to take up electron density and as such the ranking of Lewis acidities is
not defined with respect to a specific base.