Progressing the Frustrated Lewis Pair Abilities of N-Heterocyclic Carbene/GaR3Combinations for Catalytic Hydroboration of Aldehydes and Ketones

Article abstract of DOI:10.1021/acs.inorgchem.1c01276

Exploiting the steric incompatibility of the tris(alkyl)gallium GaR3 (R = CH2SiMe3) and the bulky N-heterocyclic carbene (NHC) 1,3-bis(tert-butyl)imidazol-2-ylidene (ItBu), here we report the B-H bond activation of pinacolborane (HBPin), which has led to

Full text of DOI:10.1021/acs.inorgchem.1c01276

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Progressing the Frustrated Lewis Pair Abilities of N‑Heterocyclic
Carbene/GaR3 Combinations for Catalytic Hydroboration of
Aldehydes and Ketones
Leonie J. Bole, Marina Uzelac, Alberto Hernán-Gómez, Alan R. Kennedy, Charles T. O’Hara,
and Eva Hevia*
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ABSTRACT: Exploiting the steric incompatibility of the tris-
(alkyl)gallium GaR3 (R = CH2SiMe3) and the bulky N-
heterocyclic carbene (NHC) 1,3-bis(tert-butyl)imidazol-2-ylidene
(ItBu), here we report the B−H bond activation of pinacolborane
(HBPin), which has led to the isolation and structural
authentication of a novel ion pair, [{ItBu-BPin}+{GaR3(μ-H)-
GaR3}−] (2). Contrastingly, neither ItBu or GaR3 was able to react
with HBPin under the conditions of this study. Combining an
NHC-stabilized borenium cation, [{ItBu-BPin}+], with an anionic
dinuclear gallate, [{GaR3(μ-H)GaR3}−], 2 proved to be unstable in
solution at room temperature, evolving to the abnormal NHC-Ga
complex [BPinC{{N(tBu)]2CHCGa(R)3}] (3). Interestingly, the
structural isomer of 2, with the borenium cation residing at the C4
position of the carbene, [{aItBu-BPin}+{GaR3(μ-H)GaR3}−] (4), was obtained when the abnormal NHC complex [aItBu·GaR3]
(1) was heated to 70 °C with HBPin, demonstrating that, under these forced conditions, it is possible to induce thermal frustration
of the Lewis base/Lewis acid components of 1, enabling the activation of HBPin. Building on these stoichiometric studies, the
frustrated Lewis pair (FLP) reactivity observed for the GaR3/ItBu combination with HBPin could then be upgraded to catalytic
regimes, allowing the efficient hydroboration of a range of aldehydes and ketones under mild reaction conditions. Mechanistic
insights into the possible reaction pathway involved in this process have been gained by combining kinetic investigations with a
comparative study of the catalytic capabilities of several gallium and borenium species related to 2. Disclosing a new cooperative
partnership, reactions are proposed to occur via the formation of a highly reactive monomeric hydride gallate, [{ItBu-
BPin}+{GaR3(H)}−] (I). Each anionic and cationic component of I plays a key role for success of the hydroboration, with the
nucleophilic monomeric gallate anion favoring the transfer of its hydride to the CO bond of the organic substate, which in turn is
activated by coordination to the borenium cation.
butylimidazol-2-ylidene (ItBu) with tris(alkyl)gallium GaR3 (R
= CH2SiMe3), it is possible to activate unsaturated organic
molecules such as ketones and aldehydes (via CO
insertion)8 as well as the cleavage of X−H acidic bonds of
alcohols, amines, and acetylenes, to name just a few.9 When
structural studies were combined with NMR spectroscopic
monitoring of the reactions and density functional theory
(DFT) calculations, our previous studies shed new light on the
possible mechanisms involved in CO reduction processes,
INTRODUCTION
■
Since the inception of frustrated Lewis pair (FLP) chemistry,1
sterically incompatible Lewis acid/base (LA/LB) pairings have
become a powerful and widespread protocol for the activation
of small molecules,2 which in some cases can be upgraded from
stoichiometric conditions to catalytic regimes.3 Some of the
most popular FLP systems have been modeled from the
classical phosphine/borane combinations, where the borane
often contains strongly electron-withdrawing and sterically
encumbering perfluoroarene (ArF) substitutents.4 However,
deviations from the phosphine/borane systems that explore,
for instance, N-heterocyclic carbenes (NHCs) as the Lewis
basic component5 or aluminum complexes6 as an alternative
Lewis acid choice are possible. Despite the significant use of
Lewis acidic group 13 elements B and Al, Ga has been
significantly less employed.7 Recently, we have shown that, by
combining the particularly bulky and basic NHC 1,3-di-tert-
Special Issue: Advances in Small-Molecule Activation
Received: April 26, 2021
https://doi.org/10.1021/acs.inorgchem.1c01276
© XXXX American Chemical Society
Inorg. Chem. XXXX, XXX, XXX−XXX
A

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Figure 1. Selected examples of main-group catalytic approaches for hydroboration reactions. Ar* = 2,6-(iPr)2-C6H3, HMDS = N(SiMe3)2, and R =
CH2SiMe3.
Scheme 1. (a) Synthesis of 1 and (b) Synthesis of 2 via FLP Splitting of HBPin
where the NHC can act as a Lewis base not only via its normal
C2 position (kinetic product) but also via its C4 (abnormal)
site (thermodynamic product). This isomerization seems to be
driven by the relief of the steric hindrance around the new C−
O group.10 Evidencing the synergistic behavior of these NHC/
GaR3 pairings, when separated, none of the components are
able to activate these ketones or aldehydes on their own.
Building on these studies, here we assess the capabilities of the
ItBu/GaR3 combination to induce the activation of hydridic
B−H bonds using HBPin as a model substrate. Furthermore,
we also investigate its ability to operate under catalytic
conditions to facilitate hydroboration of carbonyl compounds.
Involving the direct addition of a B−H bond across
unsaturated organic compounds (i.e., alkenes, alkynes, or
carbonyl derivatives), hydroboration catalysis has gained
increasing importance in synthesis, becoming a fundamental
tool to access organoborane compounds.11 While a large
number of these studies used transition-metal complexes as
catalysts, the past decade has witnessed several examples of s-
and p-block metal hydrides (Figure 1a) that have emerged as
competent hydroboration catalysts toward a variety of
unsaturated substrates, with the vast majority of these studies
using HBPin as the borane of choice.12−17 For carbonyl-based
substrates, these reactions are generally accepted to proceed
through an initial insertion of the substrate across the metal−
hydride bond, leading to an alkoxide intermediate. A σ-bond
metathesis step can then ensue, promoted by the stoichio-
metric excess of HBPin present, leading to the final product
with regeneration of the active hydride catalyst. Metal-free
examples have also been established in recent years, with
examples from Stephan et al.18 and Oestreich et al.19 that have
shown that electrophilic boranes such as Piers’ borane
[HB(C6F5)2] and B(3,5-(CF3)2-C6H3)3 are able to catalyze
the hydroboration of alkenes and alkynes with HBPin (Figure
1b). This approach has also been used for hydroboration of
imines, ketones, aldehydes, and alkynes, where a series of
system-specific mechanisms have been proposed to be in
operation.20 Not limited to single-component systems, highly
Lewis acidic boranes, such as the tris(pentafluorophenyl)-
borane B(C6F5)3, in the presence of a Lewis base (Figure 1c)
can perform activation of the B−H bond of HBPin, leading to
ion-pair intermediates,21,22 often described as [HB(C6F5)3]−
balanced by the borenium cation [LB·BPin]+. The latter has
been proposed to actually be the active catalyst in studies by
Crudden et al. when using 1,4-diazabicyclo[2.2.2]octane
(DABCO) as a LB for the hydroboration of imines (Figure
1c).21
Herein we investigate the catalytic ability of a novel gallium
hydride species (Figure 1d) obtained by the activation of
HBPin using ItBu/GaR3 (vide infra) to facilitate the
hydroboration of ketones, aldehydes, and imines. Through
the isolation of key reaction intermediates and kinetic studies,
we have provided mechanistic insights into how these
processes may take place.
RESULTS AND DISCUSSION
■
Taking heed from our previous studies,23 combining equimolar
amounts of GaR3 and ItBu in a nonpolar solvent at room
temperature instantly leads to the formation of a stable,
abnormal NHC/Ga complex, aItBu·GaR3 (1; Scheme 1a). The
B
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Figure 2. Molecular structures of 1 and 2 with displacement ellipsoids at 50% probability and all H atoms omitted for clarity, with the exception of
the bridging hydride of 2. SiMe3 groups have been drawn at 70% transparency for clarity. Selected bond angles (deg) and distances (Å) of 1: C12−
Ga1−C16 108.96(7), C12−Ga1−C20 112.18(8), C12−Ga1−C4 103.09(7), C16−Ga1−C2 110.70(7), C16−Ga1−C4 108.29(7), C20−Ga1−C4
113.27(7); Ga1−C12 2.0255(17), Ga1−C16 2.0231(18), Ga1−C20 2.0154(18), Ga1−C4 2.0771(17). Selected bond angles (deg) and distances
(Å) of 2: O1−B1−C25 120.46(19), O2−B1−C25 123.79(19), O2−B1−O2 115.73(19), N1−C25−B1 126.47(18), N1−C25−N2 107.35(17),
B1−C25−N2 125.92(19); Ga1−C1 2.004(2), Ga1−C5 2.010(2), Ga1−C9 2.012(2), B1−C25 1.588(3).
structure of 1 was established by X-ray crystallographic studies
(Figure 2), and it can be considered to be a deactivation
product because, at room temperature, it is a highly robust
species that shows no dissociation or further FLP reactivity.
However, in our previous work, we discovered that its
formation can be minimized by lowering the reaction
temperature to 0 °C. Introducing a suitable substrate under
these conditions could then promote its subsequent activation
(via X−H activation or CO insertion).8,9 Thus, we started
our stoichiometric studies by reacting HBPin with equimolar
amounts of GaR3 and ItBu at 0 °C for 1 h in hexane. Cooling
the solution to −30 °C led to isolation of the borenium gallate
complex [{ItBu-BPin}+{GaR3(μ-H)GaR3}−] (2), which was
characterized by X-ray crystallographic analysis and can be
envisaged to form as a result of the FLP splitting of the B−H
bond of borane. The formation of 2 is clearly synergic because
no apparent reaction of the individual components is observed
with HBPin under those conditions. For ItBu, on its own, not
even coordination to HBPin is observed, whereas GaR3 slowly
generates RBPin over long periods of time at room
temperature (64% conversion after 2 days).
achieved by a [{B(3,5-Cl2-C6H3)4}−] anion.26 The authors
comment that the side-on arrangement of the borenium to the
NHC allows for electrostatic interactions between Me groups
of the NHC’s tBu substituents and the cationic B atom, with a
mean distance of 2.846 Å. Within 2, it is therefore likely that
the orthogonal binding of [{BPin}+] to the NHC maximizes
the opportunity for stabilization of the borocation via
interaction of the Me groups to the empty p orbital of B
with detected distances of 3.081(4) and 3.028(3) Å from the
crystallographic data.
While discussions thus far have labeled [{ItBu-BPin}+] as a
borenium cation, it could also be formally considered to be a
borylimidazolium cation (Scheme 2). Previous theoretical
Scheme 2. Borenium and Imidazolium Representations of
[{ItBu-BPin}+] of 2
Compound 2 consists of a cationic NHC-BPin fragment,
with the B residing on the C2 position of the carbene (C25 in
Figure 2). Both C and B atoms exhibit trigonal-planar
environments (the sum of angles around C25 and B1 and
359.75° and 359.98°, respectively). The length of the B1−C25
bond [1.588(3) Å] is consistent with those reported in the
literature for related NHC-stabilized bis(aryl)borenium
cations.24 Unsurprisingly, this B−C bond distance is noticeably
shorter than that recently reported by Mandal et al. for a
coordination adduct between a tetrasubstituted abnormal
NHC and HBPin [1.668(2) Å], which can be rationalized
considering the higher coordination number of B in the latter
and its neutral character in comparison with the borenium
cation.25 Considering the near-perpendicular binding of BPin
to the NHC, similarities can be drawn to an example reported
studies by Stephan et al. have focused on a constitutionally
analogous species to 2, reached by FLP activation of
catecholborane by tBu3P and B(C6F5)3, leading to the ion
pair [{tBu3P-BCat}+{HB(C6F5)3}−].4a Natural bond order
calculations then predicted that the charge of the cation
formally lies on the P atom, rendering the species as a
borylphosphonium rather than a phosphine-stabilized bore-
nium cation. Although a similar understanding could be
applied to the cation of 2, onward reactivity of our system
suggests that under catalytic conditions the [{ItBu-BPin}+]
fragment can react as a borenium cation (vide infra).
The counterion for this structure is a dinuclear gallate
comprising two GaR3 units connected by a bridging hydride.
The structure of this anion is closely related to that reported by
́
by Vidovic et al., where ItBu is used to stabilize a
catecholatoborenium cation, [{BCat+}], with charge balance
C
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Scheme 3. (a) Decomposition Pathway of 2 into 3 by Deprotonation of the Imidazole Backbone and (b) Decomposition of 4
into 5 by Nucleophilic Addition across the C2 Position of the Imidazole Fragment
Stephan et al. for [{Ga(C6F5)3(μ-H)Ga(C6F5)3}−], which is
part of a phosphonium gallate resulting from the FLP cleavage
of H2 by a 2:1 combination of Ga(C6F5)3 and PtBu3.27
Interestingly, despite employment of a 1:1:1 stoichiometry, the
anionic fragment of 2 consistently forms with 2 equiv of GaR3,
which we propose to be a crystallization feature for improved
stability of the metal hydride species.28
While the synthesis and solid-state determination of 2 by X-
ray crystallography is reproducible, its isolation at room
temperature as a crystalline solid and subsequent solution-state
characterization proved particularly challenging, with multiple
attempts resulting in decomposition products. Deeper analysis
of the products of decomposition revealed the formation of an
unexpected abnormal NHC-Ga complex, [BPinC{{N-
(tBu)]2CHCGa(R)3}] (3), which can be recrystallized in
17% yield (Scheme 3). Its most salient NMR spectroscopic
Figure 3. Molecular structure of 3 with displacement ellipsoids at 50%
probability and H atoms omitted for clarity. SiMe3 groups have been
drawn at 70% transparency for clarity. Selected bond angles (deg) and
distances (Å): N2−C2−N1 106.79(12), N2−C2−B1 123.65(12),
N1−C2−B1 128.54(13), O2−B1−C2 119.73(15), O2−B1−O1
115.27(15), C2−B1−O1 125.00(15); C2−B1 1.591(2), Ga1−C4
2.0852(2).
1
feature is a singlet at 7.32 ppm in the H NMR spectrum in
C6D6 for the remaining H on the imidazole ring, alongside the
inequivalence of the tBu groups (at 1.64 and 1.26 ppm). In
addition, a resonance at 160.3 ppm is observed in the 13C
NMR spectrum for its carbenic C, a chemical shift similar to
that observed for 1 (159.5 ppm), whereas its 11B NMR shows a
broad resonance at 26.8 ppm, which compares well with
neutral BPin-Csp2 environments.20b The structure of 3 could
also be established by X-ray crystallographic studies (Figure 3).
The key coordination parameters around Ga are similar to
those found in 1, with similar Ccarbenic−Ga distances
[2.0771(17) and 2.0852(15) Å, respectively], whereas the
B−C2 distance [1.591(2) Å] is almost identical with that
found in 2. While the decomposition of 2 into 3 could not be
monitored by NMR spectroscopy, during analysis of isolated
previously noted low kinetic basicity when coordinated to
Ga.9,29
As mentioned above, our previous studies have found that 1
is deactivated toward exhibiting further FLP reactivity. A
similar behavior has been reported by Tamm et al. for the
related aItBu·B(C6F5)3.30 Interestingly, while 1 is inert towards
HBPin at room temperature, we found that increasing the
reaction temperature to 70 °C for 4 h allowed for formation of
the mixed borenium gallate [{aItBu-BPin}+{GaR3(μ-H)-
GaR3}−] (4; Figure 4). Compound 4 was subjected to
Hirshfeld atom refinement (HAR),32,33 using the NoS-
pherA2.34 As shown previously,35 HAR allows for the position
of the bridging hydride to be defined, as well as discussion of
its associated bond lengths and angles. Further details can be
found in the SI.36
The dinuclear gallate anionic fragment in 4 is identical with
that found in 2, but because HAR could be applied to this
structural determination, discussion of its most salient
structural parameters is included here. It comprises two
GaR3 units connected by a bridging hydride, where each Ga
center adopts a distorted tetrahedral geometry (the mean bond
angles around Ga1 and Ga2 are 109.16° and 109.29°,
respectively). The Ga1−H−Ga2 bond angle [134.6(9)°] is
1
samples of 3 by H NMR spectroscopy, variable amounts of
GaR3 could also be detected (see the Supporting Information,
SI). A possible explanation for the formation of 3 could be
deprotonation of the imidazole backbone of 2 by the [{GaR3-
(μ-H)-GaR3}−] fragment, producing a carbenic center at the
C4 position to which GaR3 can then bind with subsequent H2
elimination (Scheme 3a). This is somewhat surprising because
gallium hydrides are expected to be poor bases, but perhaps its
anionic constitution enhances its basicity. No evidence could
be found to support deprotonation of the imidazole backbone
by one of the alkyl groups on Ga, which is consistent with their
D
https://doi.org/10.1021/acs.inorgchem.1c01276
Inorg. Chem. XXXX, XXX, XXX−XXX