Insights into the formation of borabenzene adducts via ligand exchange reactions and TMSCl elimination from boracyclohexadiene precursors

Article abstract of DOI:10.1021/om500524j

The bonding properties of borabenzene with various neutral Lewis bases have been investigated. 1-Chloro-4-isopropyl-2-(trimethylsilyl)-2,4- boracyclohexadiene reacts with a number of Lewis bases-notably pyridine, PMe₃, PCy₃, and PPh₃-to afford 4-isopropylborabenzene-base adducts. These adducts can undergo ligand exchange to afford new borabenzene complexes. The scope and mechanism of the reaction, as well as the steric and electronic properties of different adducts, were studied experimentally and computationally.

Full text of DOI:10.1021/om500524j

Article
pubs.acs.org/Organometallics
Insights into the Formation of Borabenzene Adducts via Ligand
Exchange Reactions and TMSCl Elimination from
Boracyclohexadiene Precursors
Marc-Andre Legare,† Guillaume Belanger-Chabot,† Guillaume De Robillard,† Andre Languerand,†
́
́
́
́
́
́
Laurent Maron,‡ and Frederic-Georges Fontaine*,†
́
́
†Dep
́ ́ ́
artement de Chimie, Centre de Catalyse et de Chimie Verte (C3V), Universite Laval, 1045 Avenue de la Medecine,
Queb
́
ec (Queb
́
ec), Canada G1V 0A6
‡Universite
́
de Toulouse, INSA, UPS, LCPNO, CNRS, UMR 5215 CNRS-UPS-INSA, 135 avenue de Rangueil, Toulouse, France
S
* Supporting Information
ABSTRACT: The bonding properties of borabenzene with
various neutral Lewis bases have been investigated. 1-Chloro-
4-isopropyl-2-(trimethylsilyl)-2,4-boracyclohexadiene reacts
with a number of Lewis basesnotably pyridine, PMe3, PCy3,
and PPh3to afford 4-isopropylborabenzene−base adducts. These
adducts can undergo ligand exchange to afford new borabenzene
complexes. The scope and mechanism of the reaction, as well
as the steric and electronic properties of different adducts,
were studied experimentally and computationally.
Even when it was generated by flash thermolysis and
condensed at 10 K in an argon matrix, the base-free
borabenzene moiety has never been isolated or observed.11
Base-free borabenzene has been calculated to possess a stable
aromatic ring, but the low-lying σ* orbital on boron requires
stabilization by a Lewis base.12 Although the boratabenzene
moiety, the anionic equivalent of borabenzene, was first
reported by Herberich in 1970 (Figure 1C),13 the pyridine−
borabenzene adduct, the first stable neutral borabenzene,
was only reported in 1985 (Figure 1D).14 In 1996, an elegant
general synthetic route for the generation of neutral
borabenzene derivatives was designed by Fu, where the
aromatization of trimethylsilyl-substituted chloroboracyclohexa-
diene derivatives is initiated by the addition of a Lewis base L,
which induces the elimination of SiMe3Cl (Scheme 1).15 The
aromatization, however, relies heavily on the capacity of the
Lewis base to isomerize the 2,5-boracyclohexadiene precursor
INTRODUCTION
■
Both the electron-donating properties and the steric hindrance
of a Lewis base can play a major role in the stability of a Lewis
adduct.1 On one hand, work has been done to minimize
the binding interactions of Lewis adducts in order to exploit the
Lewis character of each individual component. It is notably
the case in the now popular “frustrated Lewis pairs”2 that
can activate small molecules, such as hydrogen3 and carbon
dioxide,4 and in the design of ambiphilic molecules,5 notably
as ligands for transition metals6 and for ion sensing.7 On the
other hand, some research has been aiming at increasing
the strength of these interactions in order to stabilize very
reactive Lewis acid sites from further transformations.
Such a strategy has been recently devised in the work of
Braunschweig to stabilize the borolyl anion (Figure 1A),8 of
Piers to synthesize stable boraanthracene adducts (Figure 1B),9
and of Bertrand to generate the highly reactive borylene
fragment.10
Scheme 1. General Synthesis of Neutral Borabenzene Adducts
Figure 1. Carbene-stabilized borolyl anion (A), boraanthracene
adduct (B), phenylboratabenzene (C), and borabenzene−pyridine
adduct (D).
Received: May 16, 2014
Published: June 18, 2014
© 2014 American Chemical Society
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to the 2,4-boracyclohexadiene and to form a strong enough
bond with boron to stabilize the aromatic product. In such
regard, it was observed that the phospholyl complex CpFe(3,4-
Me2C4H2P) failed to react with the boracyclohexadiene pre-
cursor to generate a borabenzene adduct because of the lack of
nucleophilicity at the phosphorus atom.16
character of the borabenzene adducts. All 13C resonances for
the borabenzene derivatives are in the expected range for this
class of molecules. Interestingly, the 11B chemical shift of
neutral borabenzene adducts is highly dependent on the nature
of the Lewis base ligand. Indeed, whereas 2-Lu and 2-Py both
have resonances at 32.7 ppm, the carbene and phosphine
adducts of borabenzene were found to have 11B resonances in
the 17−22 ppm range. Therefore, little correlation between the
bond strength and the 11B chemical shift was observed. In the
case of the phosphine adducts, the 11B NMR peaks are present
as doublets, whereas in the 31P spectra the resonances appear as
broad quadruplets. In both 11B and 31P NMR spectra, coupling
constants of 97 and 110 Hz were measured for 2-PPh3 and
2-PMe3, respectively, whereas no coupling constant was
observed with 2-PCy3.
In the past few years, there have been a large number of
reports on the properties of boratabenzene derivatives, notably
as ligands for transition metals.17 There are, however, only a
limited number of transition-metal adducts with neutral
borabenzene analogues as ligands,18 and although several
boraaromatic adducts have been synthesized by Piers for their
optoelectronic properties,19 a limited number of studies have
been published on the reactivity of the neutral fragment.14,20 It
was observed that, although the boron atom is formally
electronically saturated in the borabenzene adduct, the Lewis
acidic character remains prevalent in borabenzene complexes.
The coplanarity between the borabenzene plane and the
conjugated system of the acrolein in (3-(dimethylamino)-
acrolein−borabenzene)Cr(CO)3 complex suggests that the
borabenzene acts as a π-acceptor ligand.18b However, the
π-acidity of the metal-free borabenzene adducts has never been
clearly determined.
It has been demonstrated on several occasions that the
addition of anionic nucleophiles on borabenzene−PMe3 cleanly
generates the boratabenzene analogues by an associative
pathway and that other routes, such as the formation of a
borabenzyne intermediate, were unlikely.20e We reported
that the reverse reaction, the generation of pyridine and
trimethylphosphine borabenzene species from an anionic
chloroboratabenzene, was also possible, presumably through
an associative pathway.21 Although it has been reported that
(CO)3Cr(THF−borabenzene) could undergo substitution
reactions at boron with neutral Lewis bases, to our knowledge
no such reaction has been carried out on the metal-free
borabenzene species. It was even reported by Fu that the
addition of d9-PMe3 to borabenzene−PMe3 in THF at 20 °C
did not yield ligand exchange.20e
Addition of weaker Lewis bases (acetone, tetrahydrofuran,
acetonitrile) to 1 did not afford any of the aromatized products
and left the starting material unreacted. Interestingly, the re-
action of 1 with excess diisopropylamine leads to the consump-
tion of the boracyclohexadiene without the elimination of
TMSCl. Indeed, NMR spectroscopy experiments showed
complete disappearance of the signals associated with 1 and
consumption of amine 12 h after the addition of 2 equiv of
1
HNiPr2. By H NMR spectroscopy, 3a shows the presence of
two doublets at δ 6.87 and 6.51 (J = 12.3 Hz). Also present are
two other signals at δ 6.10 and 2.38 that were found to belong
to adjacent protons according to COSY experiments. The
SiMe3 was still present as a singlet at 0.97 ppm. These results
are consistent with the presence of a 2-(trimethylsilyl)-3,5-
boracyclohexadiene framework as shown in Scheme 2.22 The
Scheme 2. Reactivity of 1 with Secondary Amines Generating
Derivatives 3 (3a, R1 = R2 = iPr; 3b, R1 = H, R2 = tBu)
Following our interest in the generation of novel coordina-
tion modes for bora(ta)benzene derivatives and in the
generation of Z-type ligands for transition metals, we decided
to evaluate the possibility of synthesizing κB-borabenzene
complexes. One possible route for making such a complex is by
a substitution reaction between a neutral borabenzene adduct
and a nucleophilic metal center, to generate the metal−κB-
boratabenzene species and the free Lewis base. In order to
correctly probe the choice of borabenzene adduct to use for
such a reaction, we report herein our study on the formation of
neutral borabenzene adducts from 1-chloro-2-(trimethylsilyl)-
3,5-boracycloexadiene and the possibility to generate novel
borabenzene adducts from ligand exchange reactions on a large
array of borabenzene adducts.
observation of four diastereotopic resonances for the methyl
groups of the two iPr substituents of the amido group, by both
1H and 13C NMR spectroscopy, indicates that no rotation
occurs around the B−N bond, which suggests a double-bond
character. These findings are consistent with the formation of
1-(diisopropylamido)-2-(trimethylsilyl)-3,5-boracyclohexadiene
(3a), which is reminiscent of the previously reported
1-(dimethylamino)-3-methylene-1,2,3,6-tetrahydroborinines.23
This assignment was further supported by 11B NMR spectro-
scopy, which showed a ∼10 ppm upfield shift of the signal from
1, consistent with the substitution of a chlorine atom at boron
with an amido group. Similar reactivity was observed with
H2NtBu (Scheme 2). In the latter case, only one regioisomer
was observed for 3b. Although there is no structural evidence
for it, one would suspect the tBu group to be further away from
the bulky SiMe3 substituent for steric reasons (see the
Experimental Section). As reported by our group recently in
the generation of mesityl boratabenzene species, the first step in
the generation of borabenzene and boratabenzene adducts is
the nucleophilic attack on boron.22 The strong π-donation of
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Borabenzene
Adducts. The synthesis of borabenzene adducts 4-iPr-
C6H4B−L (2-L; L  PMe3, pyridine (Py), lutidine (Lu),
PCy3, PPh3, 1,3-dimesitylimidazol-2-ylidene (IMes)) was
carried out by aromatization of the boracyclohexadiene
precursor (1) induced by the coordination of the correspond-
ing Lewis bases L and driven by the release of TMSCl (see
1
Scheme 1, R = i-Pr). As expected, H NMR spectra of species
2-L exhibit two resonances at low field, confirming the aromatic
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the electron lone pair on nitrogen to the empty p orbital on
boron makes 3 a very stable conjugated base. Therefore, the
elimination of the acidic proton in Int3 to generate the HCl salt
of the corresponding amine is a strong driving force for the
generation of derivatives of 3 rather than the formation of
SiMe3Cl and the expected borabenzene adduct.
Single crystals were obtained for products 2-PMe3, 2-PCy3,
and 2-IMes. The ORTEP representations are shown in
Figures 2−4, respectively, and the important structural data
Figure 3. ORTEP drawing of 2-PCy3, with anisotropic atomic
displacement ellipsoids shown at the 50% probability level. Hydrogen
atoms are omitted for clarity.
In all cases, as expected, the association of Lewis bases to base-
free borabenzene proved to be exothermic and exogenic.
However, the formation of base-free borabenzene via
aromatization of precursor 1 with the elimination of SiMe3Cl
was found to be endothermic by 18.2 kcal mol−1 (step a,
Scheme 3). Therefore, a ligand has to stabilize the borabenzene
by at least 18 kcal mol−1 in order for the overall process to be
thermodynamically favorable. Otherwise, the reverse reaction
the addition of SiMe3Cl on the borabenzeneis expected to be
thermodynamically favored.25
As can be visualized in Figure 5, a direct trend was observed
between the Tolman electronic parameter of the phosphines
modeled and the binding energies for the various phosphine−
borabenzene analogues, suggesting that the σ-donating
capability of the Lewis pair plays an important role in the
overall stability of the borabenzene adduct.26 Indeed, strongly
donating PCy3 and P(i-Pr)3 give respectively ΔG° values of
−30.9 and −28.1 kcal mol−1, but electron-deficient phosphines such
as P(OPh)3 and PF3 have binding energies under −20 kcal mol−1.
The steric hindrance of phosphines also seems to play a key role in
their binding ability with borabenzene. Indeed, the three species
having binding energies stronger than the trend observed are those
with the smallest cone angles (PMe3, P(OMe)3, and PF3), whereas
P(t-Bu)3 and P(o-tolyl)3, the two most encumbered phosphines
with Tolman cone angles superior to 180°, clearly do not show
the trend expected on the basis of the electronic contribution. In
both cases, the binding energies are lower than the expected trend
by 7−8 kcal mol−1.
Figure 2. ORTEP drawing of 2-PMe3, with anisotropic atomic
displacement ellipsoids shown at the 50% probability level. Hydrogen
atoms are omitted for clarity.
are given in Table 1. Species 2-PCy3 exhibits a disordered
borabenzene moiety that was easily resolved but which
nevertheless affects the precision of the atomic coordinates
and of the structural data. The P−B bond lengths in 2-PMe3
and 2-PCy3 are 1.905(3) and 1.917(17) Å, respectively, which
are in the expected range for phosphine−borabenzene species.
The Ccarbene−B distance in 2-IMes of 1.570(5) Å is comparable
to the B−C bond lengths observed by Herberich in 1-(1,3,4,5-
tetramethylimidazol-2-ylidene)−3,5-dimethylborabenzene
(1.596 Å)20f and by Piers in a IMes−boraanthracene adduct
(1.607 Å).9b The torsion angle of 25.5° between the plane of
the borabenzene ring and the imidazolyl moiety of the carbene
does not suggest any electronic communication. The B−C
bond and C−C bond lengths in the structurally characterized
species are unremarkable.
DFT Study on Borabenzene Adducts. In order to better
quantify the stabilizing ability of neutral Lewis pairs, a quantum
chemical study was done on the binding energies of 49 neutral
Lewis bases to base-free borabenzene (step b, Scheme 3) in
the gas phase using DFT methods and the B3LYP hybrid
functional.24 Hess’ law suggests that the thermodynamic values
given in Table 2 will be an indication of the overall trend
observed in borabenzene adduct stability even in the absence of
the base-free borabenzene species in solution. For simplicity,
unsubstituted borabenzene adducts were modeled instead of
the iPr analogues synthesized and will be referred to as 2′-L.
The steric influence of the amines seems to be a much more
important factor in the ΔG° of formation of borabenzene
adducts than in the case of phosphines. Although NMe3, NEt3,
and NiPr3 have similar Lewis basicities,1c there is a very
significant difference in affinity between the various adducts
formed between the borabenzene and the tertiary amines.
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Figure 4. ORTEP drawing of 2-IMes, with anisotropic atomic displacement ellipsoids shown at the 50% probability level. Hydrogen atoms are
omitted for clarity.
orbitals on boron, notably in the generation of highly reactive
fragments such as the borolyl anion and boraanthracene.8,9
It is well documented that boratabenzene species possess
π-accepting capabilities by the presence of an electrophilic pz
orbital on boron, notably evidenced by the planarization of the
nitrogen atom on amidoboratabenzene species.17 η6-Boraben-
zene transition-metal complexes also exhibit some level of
π-acidity, as shown by the coplanarity between the borabenzene
plane and the conjugated system of the boron-coordinated
3-(dimethylamino)acrolein in a chromium(0) borabenzene
complex.18b However, to our knowledge there has been no
report on the π-acidity of metal-free borabenzene adducts. In
that regard, we computationally investigated π-basic ketones
and aldehydes as neutral ligands for borabenzene. We found
that they do not significantly bind borabenzene and are at the
low end of the stability spectrum for borabenzene adducts
with ΔG° values for MeC(O)Me, PhC(O)Ph, and MeC(O)Ph,
respectively, of −19.1, −19.4, and −22.0 kcal mol−1. The
optimized geometry of these species shows coplanarity between
the aromatic borabenzene ring and the carbonyl moiety, suggesting
some level of π-donation from the ketones to borabenzene.
Modeling of an antiplanar structure for 2′-acetone and
2′-formaldehyde showed that free energy gains of respectively
1.1 and 9.3 kcal mol−1 were associated with coplanarization of
the borabenzene and ligand fragments. This energy value is
representative of the amount of π-bond character but will
include, especially in the case of acetone, a steric hindrance
component. Therefore, it appears that the B−O π-bond is
highly dependent on the steric factors. Nevertheless, the
strength of the π-bond is significantly less important than that
observed in amido-borane species, which are in the range of
30 kcal mol−1.27
Table 1. Selected Structural Data for 2-L
2-PCy3
Distances (Å)
2-PMe3
2-IMes
B−L
1.917(17)
1.44(2)
1.905(3)
1.484(4)
1.479(4)
1.383(4)
1.380(5)
1.388(5)
1.394(4)
1.570(5)
1.482(5)
1.486(5)
1.393(4)
1.390(4)
1.400(4)
1.381(4)
B−C1
B−C5
C1−C2
C2−C3
C3−C4
C4−C5
1.537(13)
1.358(13)
1.364(9)
1.409(8)
1.412(10)
Angles (deg)
L−B−C1
L−B−C5
C1−B−C5
124.6(10)
120.2(10)
114.9(12)
122.1(2)
120.8(2)
117.0(3)
122.2(3)
122.2(3)
115.5(3)
Scheme 3. Formation of Borabenzene Adducts As Modeled
using DFT
Indeed, the smallest amine, NMe3, possesses the highest affinity
for borabenzene at −26.9 kcal mol−1, which is about 7 kcal mol−1
more favorable than NEt3 (−19.7 kcal mol−1), which in turn is
more favorable than NiPr3 by 12 kcal mol−1 (−7.7 kcal mol−1).
The same trend can be observed with secondary amines. As for
the primary amines, the formation of adducts, even with tBuNH2,
is highly exergonic with energies close to −30 kcal mol−1.
N-heterocyclic carbenes (NHCs) prove to be the strongest
neutral donors, giving binding energies of −50.4 kcal mol−1 in
the case of IMes and of −55.9 kcal mol−1 with the less hindered
N-methyl-containing carbene (IMe). The latter result is not
surprising, since NHCs are known to stabilize Lewis acid
We also modeled adducts of borabenzene and π-accepting
ligands. These ligands were found to bind borabenzene more
strongly than their Lewis basicity would suggest. The ΔG°
values for the CO,28 CNMe, CNtBu, CNPh, and CN(C5H11)
adducts were found to be respectively −29.1, −38.1, −38.6,
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Table 2. Gas-Phase Binding Energies of Selected Lewis Bases (Ligand) with Borabenzene at the B3LYP Level of Theory
at 298 K using TZVP as a Basis Set for All Atoms
ligand
NMe3
ΔH°, kcal/mol
ΔG°, kcal/mol
ligand
ΔH°, kcal/mol
ΔG°, kcal/mol
−40.1
−34.0
−22.6
−44.4
−36.5
−43.0
−42.1
−39.1
−45.8
−41.6
−43.7
−43.0
−46.3
−44.8
−33.0
−43.9
−44.8
−41.5
−37.1
−43.7
−36.1
−41.2
−36.4
−31.0
−23.0
−34.2
−34.3
−26.9
−19.7
−7.7
P(p-tolyl)3
P(p-OMe)3
P(p-C6H4CF3)3
P(p-C6H4F)3
THF
−38.1
−39.3
−34.9
−36.4
−30.0
−35.4
−29.0
−24.4
−20.0
−34.7
−38.0
−30.8
−30.6
−30.1
−12.8
−34.5
−31.7
−67.2
−65.3
−18.7
−39.8
−48.4
−49.2
−50.3
−49.0
−29.9
−24.9
−26.0
−20.6
−23.5
−18.3
−24.1
−17.2
−13.4
−10.0
−24.3
−26.3
−19.1
−18.0
−18.3
−9.0
−22.0
−19.4
−55.9
−50.4
−8.2
−29.1
−38.1
−38.6
−38.8
−38.4
−17.4
NEt3
NiPr3
NEt2H
−31.2
−22.7
−30.8
−29.6
−26.7
−32.9
−27.8
−30.5
−29.7
−33.5
−31.1
−21.6
−29.5
−32.2
−30.2
−24.0
−30.9
−22.5
−28.1
−24.1
−17.9
−11.4
−21.5
−20.2
NiPr2H
NiPrH2
NtBuH2
NH3
DMSO
SMe2
Et2O
pyridine
2,6-lutidine
2,4-lutidine
2-picoline
3-picoline
4-picoline
aniline
Ph2O
NCMe
NCPh
acetone
a
acetone (anti)
HC(O)H
HC(O)H (anti)
acetophenone
Ph2CO
IMe
a
quinuclidine
piperidine
PMe3
PPh3
IMes
PCy3
N2
P(tBu)3
P(iPr)3
P(OMe)3
P(OPh)3
PF3
CO
CNMe
CNtBu
CNPh
CN(C5H11)
C2H4
PPh2Cl
P(o-tolyl)3
a
Energy of the transition state corresponding to the rotation of the carbonyl ligand bound to borabenzene.
Figure 5. Diagram of the bonding energy of phosphine−borabenzene adducts (kcal mol−1) as determined by DFT methods as a function of the
Tolman electronic parameters calculated from Ni(CO)3L complexes.1a
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Figure 6. Representations of the highest occupied molecular orbitals of 2′-CO (A, MO 27) and 2′-CNMe (B, MO 31) corresponding to the back-
donation of the π-system of the borabenzene ring to the π* orbitals of the ligand.
−38.8, and −38.4 kcal mol−1. These results reveal that the
borabenzene ring behaves more as a π-donor than a π-acceptor.
This latter property is further shown by the geometry of the
HOMO of the borabenzene adducts of π-acceptor ligands
(Figure 6). This molecular orbital is strongly delocalized
between the aromatic cycle and the antibonding π orbital of
CO and CNMe. Therefore, these numeric data suggest that the
borabenzene ring is a better π-donor than a π-acceptor. How-
ever, one should note that the values given in Table 2 do not
take into account the possible rearrangements for borabenzene
adducts, which are numerous, as demonstrated by the
formation of 3 in the presence of secondary amines or the
lack of experimental evidence for the generation of the CO
adduct.
Table 3. Outcome of the Addition of Various Lewis Bases to
Species 2-L
a
IMes
Py
X
PCy3
lutidine
PPh3
2-IMes
2-Py
X
X
X
X
X
X
X
X
X
X
O
O
X
eq
eq
2-PMe3
2-PCy3
2-Lu
eq
eq
X
X
X
2-PPh3
O
O
O
X
a
Conditions: in benzene-d6 at 80 °C for 3 days. Legend: X, reaction
does not take place; O, complete substitution occurs; eq, equilibrium
has been reached.
Experimental Validation of the Thermodynamic
Stability of Borabenzene Adducts. It was previously
demonstrated that the addition of anionic nucleophiles on
borabenzene−PMe3 adducts could lead to a large array of
boratabenzene species.20e However, no report of substitution of
a neutral Lewis base on a metal-free borabenzene adduct was
ever reported. In order to test the possibility of such a reaction
to occur, series of reactions were carried out where neutral
Lewis bases (IMes, pyridine (Py), PCy3, 2,6-lutidine (Lu), and
PPh3) were added to species 2-L (L  IMes, Py, PCy3, Lu,
PMe3, PPh3) and heated in benzene-d6 at 80 °C for 3 days or
until equilibrium was reached (Scheme 4), with the results
displayed in Table 3.
do not undergo an exchange reaction with IMes. The latter
result suggests that the steric environment around boron plays
an important factor in the substitution reactions. This is
supported by the evidence that 2-Lu does not undergo a
substitution reaction with any of the ligands that were used or
that the addition of 2,6-lutidine to any borabenzene adduct
does not form new adducts. As can be seen on the DFT model
of 2-Lu (Figure 7B), the boron pz orbital is shielded on both
sides by the methyl groups of the 2,6-lutidine, which should
prevent an associative substitution at the boron atom, especially
in comparison with 2-PMe3, where the pz orbital on boron is
much more exposed (Figure 7).
2-PPh3, which according to the DFT results is the weakest of
the synthesized borabenzene adducts with a ligand−borabenzene
bond strength of 24 kcal mol−1, undergoes complete substitu-
tion with PMe3, pyridine, and IMes. 2-PPh3 proved to be an
excellent precursor for substitution reactions. Indeed, although
borabenzene−NEt3 and borabenzene−PMe3 have both been
used as starting reagents for accessing novel bora- and
boratabenzene species and are useful because NEt3 and PMe3
are volatile side products, borabenzene−PPh3 proves to be
easier to isolate and more stable in solution than borabenzene−
NEt3. Furthermore, PPh3 is significantly less expensive and
easier to handle than PMe3. Interestingly, the addition of the
Lewis bases L (L = PMe3, Py, PCy3) to species 2-L′ (L′ ≠ L =
Py, PMe3, PCy3) leads to an equilibrium between 2-L and 2-L′.
A series of experiments was conducted in which various com-
binations of L and 2-L′ in different ratios and concentrations
were combined in benzene-d6 with hexamethylbenzene as an
Scheme 4. Exchange Reaction of Borabenzene Adducts
As can be expected from the DFT results, the species 2-IMes
with a very strong carbene−boron interaction does not undergo
an exchange reaction with any other ligand. On the other hand,
the addition of IMes to 2-PMe3, 2-Py, and 2-PPh3 leads to the
formation of 2-IMes, which is again explained by the stability of
the N-heterocyclic carbene adduct. However, 2-PCy3 and 2-Lu
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Figure 7. Representations of the molecular orbitals of (A) 2′-PMe3 (LUMO; MO 42) and (B) 2′-Lu (LUMO+5; MO 55), showing the pz orbital on
boron.
internal standard and allowed to reach equilibrium. The
equilibrium constants were then calculated as the average of
three equilibrium reactions and were used to calculate the
difference in Gibbs free energies between the studied systems
(Table 4). The experimental results are in agreement with
degrade slowly over time in solution, as we observed with the
NEt3 adduct. Once the bond energy gets closer to 30 kcal mol−1,
as is observed with the PCy3, pyridine, and PMe3 adducts, the
compounds become stable in solution for a prolonged period of
time.
The absence of any exchange reaction when lutidine is added
to a Lewis base−borabenzene adduct, or when a Lewis base is
added to a borabenzene−lutidine adduct, suggests that an
associative process is taking place at the boron atom. However,
a DFT model of the transition state for a direct exchange
reaction between borabenzene−pyridine and PMe3 goes against
such a proposal, since the activation barriers of 28.9 (enthalpy)
and 40.4 kcal mol−1 (Gibbs free energy), respectively, are too
important for such a process to occur, even at 70 °C. Fu did
mention in a previous report that the exchange of
borabenzene−PMe3 with borabenzene−PMe3-d9 did not
occur in THF at room temperature.20e Our attempts to study
the kinetics of this reaction are in accordance with the
observations of Fu, since the rate of the reaction was highly
dependent on the purity of the sample studied and the rates
were irreproducible. In the case of the exchange reaction using
2-PMe3 and pyridine to generate 2-Py at 70 °C, the reaction
with freshly sublimed starting material was very slow, whereas a
sample that had been left in a freezer in a glovebox for 2 weeks
underwent exchange reactions much more rapidly.
Therefore, it is highly likely that this exchange can be cata-
lyzed by the presence of a trace amount of some electrophile
(notably some H+- or B-containing species) in solution that
could arise from thermal degradation of the borabenzene
adducts on heating to 70 °C. Nevertheless, the observations
made above suggest that it is still highly dependent on the steric
hindrance around the boron atom. The most likely process
for such an exchange reaction would be the generation of
borenium species that are generated readily in the presence of
Table 4. Determination of ΔG° between the Various
Borabenzene Adducts According to the Equilibrium
Constants and the DFT Calculations (B3LYP-TZVP)
a
ΔG°, kcal mol−1
ligand
DFT
exptl
L = PMe3, L′ = pyridine
L = PMe3, L′ = PCy3
L = pyridine, L′ = PCy3
−2.7
−0.7
−2.0
0.5 0.1
−0.6 0.1
−1.0 0.2
a
L and L′ are represented in Scheme 4.
computations that indicated a very small energy gap between
those three adducts. PCy3 is unsurprisingly found to form the
strongest bond with borabenzene as the strongest donor of the
studied ligands. The positive ΔG° value of 0.5 kcal mol−1 found
in the case of the substitution of trimethylphosphine by pyridine is
opposed to the calculated energy gap of −2.9 kcal mol−1 but
remains in the error margin of the computational method.
DFT Study on the Ligand Exchange Reaction. Our
observations indicate that the exchange of the Lewis bases at
boron is possible with borabenzene adducts. As expected, the
NHC−boratabenzene adducts were the most thermodynami-
cally favored borabenzene species that can be obtained. Toward
the lower end of thermodynamic stability, other than the NEt3
which has been made on several occasions and has proved to be
quite labile, we can include the triphenylphosphine analogue,
which undergoes full conversion to the PCy3, pyridine, PMe3,
and IMes borabenzene adducts. However, these species tend to
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Figure 9. Reaction pathway for the ligand exchange between 2-Py and
PMe3 to generate 2-PMe3 and pyridine.
In order to experimentally verify this mechanistic hypothesis,
1-triphenylphosphine-4H-borabenzene was reacted with 1 equiv
of PCy3 in deuterated benzene in the presence of 10 mol % of
triphenylphosphonium bromide. A control experiment without
the acidic phosphonium was conducted at the same time under
similar conditions. Within 15 min, we observed complete con-
version of borabenzene triphenylphosphine in the acid-catalyzed
reaction by 1H and 31P NMR (see the Supporting Information).
The control experiment did not show any conversion under
these conditions. These findings give strong support to the idea
that ligand exchange at borabenzene is an acid-catalyzed process
involving protonation of the aromatic cycle and formation of a
borenium intermediate.
Figure 8. Possible pathways for the ligand substitution reaction on
borabenzene.
H+, as previously reported by Piers, which would make the
boron atom much more electrophilic and would reduce the
energy of the transition state (Figure 8).20c
Calculations were performed at the DFT level using the
B3PW91 hybrid functional to investigate the possibility of the
borenium ligand exchange pathway. The borenium 2+-Py was
modeled and used as a starting point for the exchange reaction.
Its formation energy from 2-Py could not be calculated, since
the electrophilic impurity reacting with 2-L under our reaction
conditions is unknown. However, this borenium cation has
been reported by Piers as the product of the reaction of 2-L
with HCl and thallium tetraperfluorophenylborate.20c
A stable tetracoordinated boronium structure (IM1) was
found as the product of the association of PMe3 on 2+-Py. This
stable intermediate is reminiscent of [C5H6B(Py)2]+[B(C6F5)4]− ,
also reported by Piers.25 Unsurprisingly, IM1 is thermodynamically
favored, as the boronium fragment is then stabilized by two Lewis
bases. Either pyridine or PMe3 can easily dissociate from IM1
to give the corresponding monocoordinated borenium com-
plex. The highest energy barrier found for the ligand exchange
reaction of 2+-Py with PMe3 to give 2+-PMe3 and free pyridine
is 15.1 kcal mol−1, which is consistent with a reaction that
occurs rapidly at room temperature. The reaction pathway is
illustrated in Figure 9.
These results indicate that ligand exchange on borenium
derivatives of borabenzene should be easy and proceed through
an associative pathway. The rate of reaction is thus expected to
be highly dependent on the steric properties of the ligands
involved and their ability to form a tetracoordinated inter-
mediate. Borenium complexes of bulky ligands, such as 2,6-
lutidine, that completely protect the boron atom of the
borenium are expected to be inert with regard to ligand
substitution. Unfortunately, the thermodynamics of the relation
between borenium derivatives 2+-L and borabenzene com-
plexes 2-L remain unknown. The main factors governing the
rate of the overall ligand substitution on neutral borabenzene
adducts through a borenium pathway have to include the
nature and concentration of the catalytic electrophile.
CONCLUSION
■
The borabenzene fragment has found applications in a large
array of fields, from catalysis to material sciences. In that regard,
the nature of the substituent on boron can play a large role in
the activity and properties of the molecules of interest. While it
is widely known that the borabenzene is kinetically reactive, we
have quantified in this report, by computational and experimental
results, the thermodynamic stability of the borabenzene adducts
according to the nature of the substituents and demonstrated
that trace impurities can greatly enhance exchange reactions. We
have also demonstrated that the neutral borabenzene fragment
mostly acts as a π-donor rather than a π-acceptor, because of the
availability of electron density from the aromatic ring. These
results should be helpful in the design of more stable and durable
boron heterocycles.
EXPERIMENTAL SECTION
■
General Procedures. All manipulations were conducted under a
nitrogen atmosphere using standard Schlenk and glovebox techniques.
Reactions were carried out either in a sealed J. Young NMR tube, in
which case NMR conversions are indicated, or in standard flame-dried
Schlenk glassware. Dry deoxygenated solvents were employed for all
manipulations. All solvents were distilled from Na/benzophenone.
Benzene-d6 and toluene-d8 were purified by vacuum distillation from
Na/K alloy. 1-chloro-2-(trimethylsilyl)-4-isopropyl-2−5-boracyclohexa-
diene19 (1), 1-pyridine-4-isopropylborabenzene (2-Py),20c and
1-tricyclohexylphosphine−4-isopropylborabenzene (2-PCy3)17o were
prepared and characterized according to literature procedures. NMR
spectra were recorded on a Varian Inova NMR AS400 spectrometer at
400.0 MHz (1H), 100.580 MHz (13C), and 161.923 MHz (31P), on a
Bruker Advance NMR 400 MHz spectrometer at 128.336 MHz (11B),
or on a Bruker NMR AC-300 at 300 MHz (1H), 75.435 MHz (13C),
and 121.442 MHz (31P). 1H NMR and 13C{1H} NMR chemical shifts
are referenced to residual solvent signals in deuterated solvent. Multi-
plicities are reported as singlet (s), doublet (d), triplet (t), quartet (q),
multiplet (m), overlapping (ov) or broad (br). Chemical shifts are
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reported in ppm. Peak assignment was confirmed using COSY (1H)
and gHSQC (13C) 2D NMR experiments. Coupling constants are
reported in Hz. HRMS characterization was performed with an Agilent
Technologies 6210 LC time of flight mass spectrometer. Products in
toluene solutions were introduced to the nebulizer by direct injection.
Neutral borabenzene adducts were characterized using APPI ionization
in positive mode.
NMR (C6D6): δ 146.1 (s, Mes(ipso)), 139.3 (s, Mes(p-C)), 135.5
(s, C3), 135.0 (s, Mes(o-C)), 131.9 (s, C2), 129.8 (s, Mes(m-C)), 121.3
(s, Im), 35.9 (s, C4), 25.6 (s, C5), 21.1 (s, Mes(p-CH3)), 17.9 (s,
Mes(o-CH3)), C3 and carbene not located. 11B NMR (C6D6): δ 21.0
(s). DI-MSTOF (APPI, m/e): (M+) 422.2892 (calcd 422.2893).
1-(Diisopropylamido)-2-(trimethylsilyl)-4-isopropyl-3,5-boracy-
clohexadiene (3a).
Synthesis of Borabenzene Adducts.
1-Trimethylphosphine−4-Isopropylborabenzene (2-PMe3). Trime-
thylphosphine (0.23 mL, 168 mg, 2.21 mmol) was added dropwise at
room temperature to 1 (500.3 mg, 2.21 mmol) in hexane (25 mL).
The presence of a white precipitate was observed immediately, and the
mixture was stirred for 2 h. Removal of the volatiles in vacuo and
subsequent washing with hexane gave 212.4 mg of a white solid (yield
To a solution of 1 (8.6 mg, 0.04 mmol) in 0.5 mL of C6D6 was added
diisopropylamine (55 μL, 0.2 mmol) by syringe. A white precipitate
was immediately formed. The reaction mixture was kept at room
temperature for 16 h and analyzed by NMR spectroscopy. The
assignation was confirmed on the basis of COSY experiments and with
comparison to similar compounds.16 1H NMR (C6D6): δ 6.87 (d,
3JH−H = 12.3 Hz, 1H, H5), 6.51 (d, 3JH−H = 12.3 Hz, 1H, H6), 6.10 (s,
1
50%). H NMR (benzene-d6): δ 7.91 (br, 2H, H2), 7.23 (dd, J = 8.7,
3
10.3 Hz, 2H, H1), 3.20 (sept, JH−H = 6.9 Hz, 1H, CH(CH3)2), 1.53
1H, H3), 3.74 (br s., 1H, N(CHMe2)2), 3.21 (br s, 1H, N(CHMe2)2),
2.38 (br. ov. s., 1H, CHMe2), 2.38 (br ov s, 1H, H2), 1.31 (br s, 3H,
N(CHMe2)2), 1.28 (br s, 3H, N(CHMe2)2), 1.10 (br, 6H, CHMe2),
1.01 (br, 3H, N(CHMe2)2), 0.91 (br, 3H, N(CHMe2)2), 0.10 (s, 9H,
TMS); signals for excess diisopropylamine are present at δ 2.78 and
0.94. 13C{1H} NMR (C6D6): δ 143.3 (s, C5), 138.8 (s, C4), 131.6 (br,
C6), 129.8 (s, C3), 48.8 (s, N(CHMe2)2), 44.9 (s, N(CHMe2)2), 34.5
(s, CHMe2), 25.5 (s, CHMe2 or N(CHMe2)2), 24.8 (s, CHMe2 or
N(CHMe2)2), 23.0 (s, CHMe2 or N(CHMe2)2), 22.8 (s, CHMe2 or
N(CHMe2)2), 22.7 (s, CHMe2 or N(CHMe2)2), 21.2 (s, CHMe2 or
N(CHMe2)2), 0.0 (s, TMS); signals for excess diisopropylamine are
present at δ 45.3 and 23.7. 11B NMR (C6D6): δ 41.2 (s).
3
2
(d, JH−H = 6.9 Hz, 6H, CH(CH3)2), 0.65 (d, JH−P = 10.8 Hz, 9H,
PMe3). 13C{1H} NMR (C6D6): δ 140.0 (s, C3), 132.0 (d, 18.1 Hz, C2),
129.3 (br. d, C1), 36.2 (s, C4), 25.8 (s, C5), 10.9 (d, JC−P = 41.8 Hz,
2
PMe3). 31P{1H} NMR (C6D6): δ −23.2 (q, 1JP−B = 110 Hz). 11B NMR
1
(C6D6): δ 19.8 (d, JB−P = 110 Hz). DI-MSTOF (APPI, m/e):
[M + H]+ 195.1714 (calcd 195.0691).
1-Triphenylphosphine−4-Isopropylborabenzene (2-PPh3). A sat-
urated solution of triphenylphosphine (526.6 mg, 2.02 mmol) in
hexane was added dropwise at room temperature to 1 (455 mg,
2.02 mmol). The presence of a white precipitate was observed immediately,
and the mixture was stirred for 1 h. Removal of the volatiles in vacuo
and subsequent washing with hexane gave 347.2 mg of a white solid
1
3
4
1-(tert-Butylamido)-2-(trimethylsilyl)-4-isopropyl-3,5-boracyclo-
hexadiene (3b).
(yield 45%). H NMR (C6D6): δ 7.98 (dd, JH−H = 10.3, JP−H = 4.7,
2H, H2), 7.58 (m, 6H, PPh3), 7.46 (dd, 3JH−H = 10.3, 3JP−H = 7.8, 2H,
H1), 6.98 (m, 3H, PPh3), 6.90 (m, 6H, PPh3), 3.21 (sept., 3JH−H = 6.9,
1H, CH(CH3)2), 1.53 (d, JH−H = 6.9, 6H, CH(CH3)2). 13C{1H}
3
NMR (C6D6): δ 140.4 (s, C3), 134.6 (d, JC−P = 10.3 Hz, PPh3), 132.5
(d, 3JC−P = 17.3, C2), 131.6 (d, JC−P = 2.4 Hz, PPh3), 131.5 (d, JC−P
=
3.0 Hz, PPh3), 128.5 (br, C1), 36.2 (s, C4), 25.7 (s, C5). 31P{1H} NMR
(C6D6): δ 8.2 (br). 11B NMR (C6D6): δ 18.8 (d, JB−P = 96.7). DI-
1
MSTOF (APPI, m/e): [M + H − Pr]+ 339.2670 (calcd 339.1468).
i
1-(2,6-Lutidine)−4-Isopropylborabenzene (2-Lu). 2,6-Lutidine
(0.30 mL, 272 mg, 2.54 mmol) was added dropwise at room tem-
perature to a saturated hexane solution of 1 (575.8 mg, 2.54 mmol).
The reaction mixture was stirred for 3 h, and all volatiles were removed
in vacuo. The resulting bright yellow solid was washed with hexane to
give 342.7 mg of a white solid (yield 60%).1H NMR (C6D6): δ 8.00
To a solution of 1 (8.7 mg, 0.04 mmol) in 0.5 mL of C6D6 was added
tert-butylamine (20 μL, 0.2 mmol) by syringe. A white precipitate
was immediately formed. The reaction mixture was kept at room
3
(d, JH−H = 10.2 Hz, 2H, H2), 6.47−6.52 (m, 1H, lut(p-H)), 6.49 (d,
1
3
3JH−H = 10.2 Hz, 2H, H1), 6.11 (d, JH−H = 7.7 Hz, 2H, lut(m-H)),
temperature for 24 h and analyzed by NMR spectroscopy. H NMR
(C6D6): δ 7.00 (d, 3JH−H = 12.3 Hz, 1H, H5), 6.48 (d, 3JH−H = 12.3 Hz,
3
3.29 (sept, JH−H = 7 Hz, 1H, CH(CH3)2), 2.22 (s, 6H, lut(CH3)),
3
1H, H6), 6.07 (d, JH−H = 4.9 Hz, 1H, H3), 3.44 (br, 1H, NH), 2.40
3
1.62 (d, JH−H = 7 Hz, 6H, CH(CH3)2). 13C{1H} NMR (C6D6): δ
(sept, 3JH−H = 6.9 Hz, 1H, CHMe2), 1.80 (d, 3JH−H = 5.4 Hz, 1H, H2),
1.09 (m, 6H, CHMe2), 0.66 (s, 9H, NCMe3), 0.04 (s, 9H, TMS);
signals for excess tert-butylamine are present at δ 1.20 (s) and 0.98 (s).
13C{1H} NMR (C6D6): δ 146.4 (s, C5), 139.5 (s, C4), 130.2 (s, C3),
129.2 (br, C6), 50.0 (s, N(CMe3)), 35.5 (br, C2), 34.7 (s, CHMe2),
22.9 (s, CHMe2 or N(CMe3)), 23.0 (s, CHMe2 or N(CMe3)), −1.3 (s,
TMS). 11B NMR (C6D6): δ 40.9 (s).
156.9 (s, lut(o-C)), 139.0 (s, lut(p-C)), 133.9 (s, C2), 123.5 (s, lut(m-
C)), 116.9 (br, C1), 35.9 (s, C4), 26.3 (s, C5), 26.0 (s, lut(CH3)), not
located (C3). 11B NMR (C6D6): δ 32.7 (s). DI-MSTOF (APPI, m/e):
(M+) 225.1435 (calcd 225.1689).
1-(1,3-Dimesitylimidazolin-2-ylidene)−4-Isopropylborabenzene
(2-IMes). A saturated solution of 1,3-dimesitylimidazoline-2-ylidene
(310.2.9 mg, 1.01 mmol) in hexane was added dropwise at room
temperature to 1 (231.3 mg, 1.01 mmol). The yellow reaction mixture
was stirred for 2 h. Removal of the volatiles in vacuo and several
washings in hexane gave 212.4 mg of a pale solid. 1H NMR (C6D6): δ
7.51 (d, 3JH−H = 10.5 Hz, 2H, H2), 6.68 (s, 4 H, Mes(m-H)), 6.53 (d,
General Procedure for Ligand Exchange. Qualitative ligand
exchange reactions were performed at the NMR scale with 2-L (L =
2,6-lutidine, pyridine, PMe3, PCy3, PPh3, IMes). In these experiments,
1 molar equiv of L′ (L′ = 2,6-lutidine, pyridine, PMe3, PCy3, PPh3,
IMes) was added to a C6D6 solution of 2-L. The reaction mixtures
were heated to 60 °C overnight. NMR analysis was then used to verify
if the exchange reaction occurred. Reaction mixtures that did not
3JH−H = 10.5 Hz, 2H, H1), 5.92 (s, 2 H, Im(CH)), 2.91 (sept, 3JH−H
=
6.9 Hz, 1H, CH(CH3)2), 2.08 (s, 6H, Mes(p-CH3)), 1.96 (s, 12H,
3
Mes(o-CH3)), 1.26 (d, JH−H = 6.9 Hz, 6H, CH(CH3)2). 13C{1H}
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undergo reaction were heated to 80 °C for up to 5 days more to make
sure no exchange occurred.
(4) (a) Momming, C. M.; Otten, E.; Kehr, G.; Frohlich, R.; Grimme,
̈ ̈
S.; Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed. 2009, 48, 6643−
6646. (b) Berkefeld, A.; Piers, W. E.; Parvez, M. J. Am. Chem. Soc.
2010, 132, 10660−10661. (c) Ashley, A. E.; Thompson, A. L.; O’Hare,
Quantitative analysis of the exchange reaction was done for
L = pyridine, PMe3, PCy3 and L′ = pyridine, PMe3, PCy3. These
reactions were done on the NMR scale in C6D6 with hexamethylben-
zene (HMB) as an internal standard. Solutions of 2-L + HMB and L′
were prepared (4.2 mol L−1). These solutions were mixed in airtight
NMR tubes in 1:2, 1:1, and 2:1 ratios. The reaction mixtures were
heated to 80 °C for 72 h, and the equilibrium constants were measured
by NMR spectroscopy. The Gibbs free energy variation for these
systems was calculated as the average of the measurements taken for
the 1:2, 1:1, and 2:1 mixtures.
Acid-Catalyzed Ligand Exchange Reaction. Two C6D6 (ca. 0.4
mL) solutions of 2-PPh3 (4.4 mg, 0.012 mmol) and tricyclohex-
ylphosphine (3.3 mg, 0.012 mmol) were prepared. To one of them
was added 0.4 mg (0.001 mmol) of triphenylphosphonium bromide.
The solutions thus prepared were introduced into J Young NMR tubes
and analyzed by NMR spectroscopy after 15 min.
́
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Stephan, D. W. J. Am. Chem. Soc. 2010, 132, 1796−1797. (e) Appelt,
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Computational Details. The density functional theory calcu-
lations were carried out with the B3LYP/B3PW91 hybrid functional as
implemented in the G03 program.29 B3LYP is Becke’s three-parameter
functional (B3)30 with the nonlocal correlation provided by the LYP
expression31 and VWN functional III for local correlation.32
The TZVP33 basis set was used for all atoms. The tight geometry
optimizations were performed without symmetry constraints and with
the use of the modified GDIIS algorithm.34 Vibrational analyses were
performed to confirm the optimized stationary points as true minima
on the potential energy surface or as transition states and to obtain the
zero-point energy, thermodynamic data, and orbital analysis. The free
Gibbs energies, G, were calculated for T = 298.15 K. For every
transition state, the reaction path in both directions was followed using
the intrinsic reaction coordinate (IRC).35
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Inorg. Chem. 2010, 49, 4060−4072. (i) Anderson, J. S.; Moret, M. E.;
Peters, J. C. J. Am. Chem. Soc. 2013, 135, 534−5375. (j) Sircoglou, M.;
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ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures, a table, and CIF and xyz files giving spectroscopic
data for all compounds, Cartesian coordinates for numerical
models, and crystallographic data for 2-PCy3, 2-IMes, and 2-
PMe3. This material is available free of charge via the Internet
at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail for F.-G.F.: frederic.fontaine@chm.ulaval.ca.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the NSERC (Canada), CFI (Canada),
CCVC (Queb
́
ec), and C3V (Universite
́
Laval) for financial
support; M.-A.L. and G.B.-C. are grateful to NSERC and
FQRNT for scholarships. Prof. T. Woo and S. Goreslky are
acknowledged for the training of M.-A.L. and G.B.-C. and their
help with DFT calculations. We also acknowledge Prof. A. Hill
for his insightful input.
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