Synthesis and Reactivity of Fluorinated Triaryl Aluminum Complexes

Article abstract of DOI:10.1021/acs.inorgchem.0c01076

The addition of the Grignard 3,4,5-ArFMgBr to aluminum(III) chloride in ether generates the novel triarylalane Al(3,4,5-ArF)3·OEt2. Attempts to synthesize this alane via transmetalation from the parent borane with trimethylaluminum gave a dimeric structure with bridging methyl groups, a product of partial transmetalation. On the other hand, the novel alane Al(2,3,4-ArF)3 was synthesized from the parent borane and trimethylaluminum. Interestingly, the solid-state structure of Al(2,3,4-ArF)3 shows an extended chain structure resulting from neighboring Al···F contacts. Al(3,4,5-ArF)3·OEt2 was then found to be an effective catalyst for the hydroboration of carbonyls, imines, and alkynes with pinacolborane.

Full text of DOI:10.1021/acs.inorgchem.0c01076

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Synthesis and Reactivity of Fluorinated Triaryl Aluminum
Complexes
Darren M. C. Ould,‡ Jamie L. Carden,‡ Rowan Page, and Rebecca L. Melen*
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Supporting Information
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ABSTRACT: The addition of the Grignard 3,4,5-ArFMgBr to aluminum(III) chloride
in ether generates the novel triarylalane Al(3,4,5-ArF)3·OEt2. Attempts to synthesize this
alane via transmetalation from the parent borane with trimethylaluminum gave a dimeric
structure with bridging methyl groups, a product of partial transmetalation. On the other
hand, the novel alane Al(2,3,4-ArF)3 was synthesized from the parent borane and
trimethylaluminum. Interestingly, the solid-state structure of Al(2,3,4-ArF)3 shows an
extended chain structure resulting from neighboring Al···F contacts. Al(3,4,5-ArF)3·OEt2
was then found to be an effective catalyst for the hydroboration of carbonyls, imines, and
alkynes with pinacolborane.
Grignard C6F5MgBr in ether led to an explosion, as did heating
a solution of AlEt3 with B(C6F5)3 to 70 °C.4 Chen proposed
that the thermal and shock sensitive nature of unsolvated
Al(C6F5)3 derives from the ability of the compound to readily
decompose to form strong Al−F bonds and explosive
tetrafluorobenzyne. This was observed in the solid-state
structure of the compound which exists as a dimer [Al-
(C6F5)3]2 that displayed close intermolecular Al···F inter-
actions between the aluminum center of one molecule and an
ortho-fluorine atom of a second molecule.11
Literature reports of fluorinated triarylalanes with alternate
fluorine substitution patterns to Al(C6F5)3 are surprisingly
sparse (Figure 1). One example includes the use of Al(2,3,5,6-
ArF)3 for FLP assisted H2 and olefin activation.17 Additionally,
Al(4-ArF)3 has been used in optimizations toward sequential
retro-ene arylation and [3,3]-sigmatropic rearrangement/
nucleophilic arylation reactions.18,19
INTRODUCTION
■
Since the ground-breaking discovery of frustrated Lewis pairs
(FLPs) in 2006,1 the use of triarylboranes to aid organic
transformations has rapidly grown in the past 20 years.2,3
Interestingly though, the use of the heavier aluminum
analogues has received notably less attention. Pohlmann and
Brinckmann successfully prepared tris(pentafluorophenyl)-
alane [Al(C6F5)3] as the Et2O adduct in 1965,4 but no further
reports were made until the analogous THF adduct was
generated in 1995.5 Since these reports there have been
disputes in the literature regarding the Lewis acidity of
Al(C6F5)3 and whether it is a stronger Lewis acid than
B(C6F5)3, with claims by Lee et al. and Stahl et al. that the
latter is the much stronger Lewis acid.6,7 On the other hand, a
number of experimental and computational observations
contradict this view and support Al(C6F5)3 being the stronger
Lewis acid.8−10 A possible explanation for the disagreement in
the Lewis acidity of Al(C6F5)3 is its tendency to form strong
adducts with Lewis bases, which in turn quenches its reactivity.
In fact, it was not until 2016 that the unsolvated structure of
Al(C6F5)3 was reported by Chen et al., who achieved this by
transmetalation from B(C6F5)3 with AlEt3 in hexane.11
Since its first isolation as a THF adduct, Al(C6F5)3 has seen
a number of uses, including the modification of methylalumox-
ane,6 transfer polymerization,12 and methide abstraction.8
Lately, Al(C6F5)3 has been applied extensively toward FLP
chemistry, to undergo H2 activation and hydride transfer to
alkenes,13 C−H activation,14,15 and the activation of CO2.16
However, a note of caution has always been present when
working with Al(C6F5)3; Pohlmann and Brinckmann found
that attempts to sublime the crude mixture of AlCl3 and the
Given our group’s recent interest in the borane B(3,4,5-
ArF)3,20−22 we were inspired to synthesize its aluminum
analogue, in part due to the absence of ortho-positioned
fluorine atoms. We propose that the alane counterpart would
possess similar Lewis acidity as Al(C6F5)3, but devoid of ortho-
fluorine atoms, it may offer more stability.
Received: April 13, 2020
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Figure 1. Previously reported triarylalanes and this work.
Scheme 1. Synthesis of μ2-Dimethyl-bis[(3,4,5-trifluorophenyl)methyl-alane] from Partial Transmetallation
Herein, we report the synthesis of several new alanes bearing
fluorinated aryl rings and explore their structural properties. In
addition, we explore their reactivity as a catalyst for the
reduction of carbonyls, imines, and alkynes with the terminal
reductant pinacolborane (HBpin).
refinement of the single-crystal data revealed that an aluminum
dimer with bridging methyl groups and only one aryl group
had formed (2), as opposed to the expected Al(3,4,5-ArF)3
triarylalane (Scheme 1). Given the equivalence of the terminal
and bridging methyl groups in the compound, variable
1
temperature H NMR studies were performed. At −80 °C,
RESULTS AND DISCUSSION
the signals were found to resolve; however, results were
inconclusive due to the reduced solubility of the species (see
the Supporting Information).
■
Early attempts to synthesize unsolvated tris(3,4,5-
trifluorophenyl)alane, Al(3,4,5-ArF)3, procedurally mirrored
that of the previously reported unsolvated tris-
(perfluorophenyl)alane, Al(C6F5)3, by Chen.11 Trimethylalu-
minum (1 M in hexane) was added to B(3,4,5-ArF)3 (1) in
hexane and left undisturbed for 2 days at room temperature.
After this time crystals suitable for single-crystal X-ray
diffraction had formed. The 1H NMR spectrum of these
crystals showed the expected aromatic signal at δ = 6.87 ppm,
but more significantly, an upfield singlet resonance at δ =
−0.39 ppm was also present, with an integral ratio of 6:2
compared to the aromatic signal. Unexpectedly, structural
The formation of 2 can be rationalized as a partial
transmetalation reaction in which only one of the aryl groups
from the starting borane has transferred, leaving two methyl
groups from the trimethylaluminum still bonded. Repeating
this transmetalation reaction at 40 °C still failed to give
complete transfer of all three aryl groups to the aluminum
center. Recent work by Stammler et al. on the bonding of
Al2Me6 suggests that the bridging CH3 groups are assignable to
two highly ionic 2e,3c bonds with tetra-coordinate aluminum
atoms.23 Similar dimeric species containing 2e,3c bonds have
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been recorded from incomplete transmetalation of B(C6F5)3
with AlMe3.24
Figure 2. Solid-state structure of μ2-dimethyl-bis[(3,4,5-
trifluorophenyl)methyl-alane]. H atoms are omitted for clarity, and
thermal ellipsoids are drawn at 50% probability.
Inspection of the solid-state structure of dimer 2 (Figure 2),
which crystallizes in the triclinic space group P1 with half a
̅
molecule in the asymmetric unit, revealed an Al(1)···Al(1′)
distance of 2.599(2) Å. The bridging Al(1)−C(1) bond was
measured at 2.096(3) Å, which is appreciably longer than the
terminal Al(1)−C(2) bond (1.940(5) Å). Interestingly, the
Al(1)−C(1)′ bridging bond length is longer than that of
Al(1)−C(1), measuring 2.134(3) Å and showing asymmetry in
the dimer.
Measuring the bond angles around the aluminum center
shows a C(2)−Al(1)−C(3) bond angle of 116.7(2)°, which is
approximately 7° lower than what is seen in Al2Me6. However,
the C(1)−Al(1)−C(3) bond angle measures 106.5(1)°, which
is comparable to that seen in Al2Me6.23
Although the transmetalation reaction did not give the
desired 3,4,5-ArF derived triarylalane, use of the novel B(2,3,4-
ArF)3 triarylborane (3, Figure 3, top) under the same
conditions gave the expected unsolvated Al(2,3,4-ArF)3 alane
(4) (Scheme 2) from the transmetalation reaction. Initially 4
was characterized using single-crystal X-ray diffraction, where it
crystallizes in the orthorhombic Pbca space group, with one
molecule present in the asymmetric unit (Figure 3, bottom). 4
could also be isolated as the THF adduct.
The solid-state configuration revealed the expected triar-
ylalane structure, with the three 2,3,4-fluorinated aryl groups
coordinated to the aluminum center. The geometry of
Al(2,3,4-ArF)3 is similar to that of the parent borane, in that
it is near trigonal planar, with C−Al−C bond angles measuring
115.0(2)°, 121.5(2)°, and 115.9(2)°.
Figure 3. Top: Solid-state structure of B(2,3,4-ArF)3. Bottom: Solid-
state structure of Al(2,3,4-ArF)3. H atoms are omitted for clarity, and
thermal ellipsoids are drawn at 50% probability.
1-bromo-3,4,5-trifluorobenzene and aluminum trichloride in
ether solvent (Scheme 3). Upon workup, the 1H NMR
spectrum in C6D6 showed the expected aryl proton signal at δ
= 6.99 ppm, but in addition it also showed stoichiometric ether
present, with signals appearing at δ = 3.00 ppm and δ = 0.28
ppm. The latter is much more upfield compared to free
uncoordinated ether. The 19F NMR spectrum shows the
expected two signals in a 2:1 ratio at δ = −153.5 ppm and δ =
−160.7 ppm, respectively.
Analysis of the unit cell of 4 shows short Al···F contacts,
where the ortho-fluorine from one aryl group interacts with the
aluminum center on a neighboring triarylalane. This donation
from the ortho-fluorine creates a long chain arrangement in the
packing structure of 4, where this bridging ortho-fluoride
interaction is repeated (Figure 4). This Al···F interaction is
strong, as seen by the Al···F distance of 2.034(3) Å. As a
consequence of this specific chain formation, π-stacking
between two of the aryl groups in neighboring alanes (one
from each triarylalane) is permitted, with the two aryl plane
distances measuring 3.401(5) Å. The long chain arrangement
of 4 is different to that seen in the unsolvated Al(C6F5)3, where
Chen reports a dimeric packing structure of the alane through
ortho-fluorine atoms.11
The solid-state structure of triarylalane 5 was obtained, and
structural refinement found that it crystallizes in the triclinic P1
̅
space group with one molecule in the asymmetric unit. The
solid-state structure also showed that the triarylalane exists as
an etherate adduct, Al(3,4,5-ArF)3·OEt2 (5) (Figure 5), as was
1
suggested from the H NMR spectrum. The coordination of
ether means that unlike in 4, 5 adopts a near tetrahedral
geometry, with O−Al−C bond angles of 104.1(2)°, 104.5(2)°,
and 107.1(2)°.
Density functional theory (DFT) was then employed to
better understand the structural properties possessed by these
triarylalanes. Initially, geometry optimization and vibrational
frequency calculations on the unsolvated structures were
Due to difficulties in the synthesis of Al(3,4,5-ArF)3 by
transmetalation, a Grignard method was instead adopted from
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Scheme 2. Synthesis of Al(2,3,4-ArF)3 from Transmetallation
Figure 5. Solid-state structure of Al(3,4,5-ArF)3·OEt2. H atoms are
omitted for clarity, and thermal ellipsoids are drawn at 50%
probability.
Figure 4. Chain arrangement of Al(2,3,4-ArF)3 molecules in the unit
cell; dashed bond represents neighboring Al···F contacts. H atoms are
omitted for clarity.
Table 1. Fluoride Ion Affinity Values
structure
FIA (kJ mol−1
)
relative FIA (%)
Al(3,4,5-ArF)3
Al(2,3,4-ArF)3
B(3,4,5-ArF)3
B(2,3,4-ArF)3
Al(C6F5)3
511
501
427
404
541
459
95
93
79
75
100
85
undertaken using the theory M06-2X/cc-pVDZ. Natural bond
orbital (NBO) analysis showed a significantly greater buildup
of positive charge at the aluminum center than at the boron
center in the analogous triarylboranes, which led us to believe
these triarylalanes would show enhanced Lewis acidic behavior
(see the Supporting Information). To quantify this, fluoride
ion affinity (FIA) calculations were performed (Table 1).
As the fluoride ion is relatively small and highly basic, it will
interact with most Lewis acids.25 At this level of theory, the
well-known strong Lewis acid B(C6F5)3 had a FIA of 459 kJ
mol−1, similar to that previously reported.8,26,27 Proceeding
with this, the triarylalanes Al(3,4,5-ArF)3 and Al(2,3,4-ArF)3
were found to have a FIA of 511 and 501 kJ mol−1,
respectively, whereas their triarylborane counterparts B(3,4,5-
ArF)3 and B(2,3,4-ArF)3 produced values of 427 and 404 kJ
mol−1 accordingly. What is interesting about these values is
B(C6F5)3
that the triarylalanes appear to be approximately 15% more
Lewis acidic than their triarylborane counterparts.
With Al(3,4,5-ArF)3·OEt2 (5) in hand, its reactivity was
probed by using it in the first reported example of triarylalane-
assisted hydroboration reduction catalysis. For the optimiza-
tion conditions, the hydroboration of acetophenone with
HBpin was performed (Table 2). Initial use of 5 mol %
catalytic loading at room temperature proved kinetically slow,
as did increasing the catalytic loading to 10 mol % 5. However,
Scheme 3. Synthesis of Al(3,4,5-ArF)3·OEt2 via the Grignard Reaction
D
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a
sources found that neither 9-BBN nor HBcat were reactive
toward the hydroboration of acetophenone.
Table 2. Optimization of Hydroboration Catalysis
Proceeding with the optimum conditions of 10 mol %
precatalyst 5, 1.2 equiv of HBpin, and 70 °C in CDCl3, the
substrate scope was expanded to determine the versatility and
suitability of 5 as a catalyst for hydroboration reduction
(Scheme 4).
catalyst loading
(mol %)
T
(°C)
borane source
(equiv)
time
(h)
b
entry
yield
First, aldehydes were readily reduced within 2 h and
obtained as alcohols in high isolated yields up to 97% (6a−d),
with little discrepancy in tolerance between electron with-
drawing, electron donating, and bulky substrates. Likewise,
ketones and aldimines were readily reduced, with smooth
conversion to the product and excellent isolated yields upon
hydrolysis workup (7a−d, 8a−d). We then investigated the
reduction of C−C multiple bonds. Preliminary studies with the
olefins styrene, alpha-methylstyrene, 4-chlorostyrene, and 4-
(trifluoromethyl)styrene were promising, showing good
conversions (>90%) in 24−48 h. Finally, terminal alkynes
were also hydroborated efficiently into their corresponding
boronate esters (9a−d), with bulky, along with electron-
withdrawing and electron-donating, substituents being toler-
ated. The internal alkynes diphenylacetylene and 1-phenyl-1-
propyne showed no conversion under these conditions. Note
boronate esters 9a−d were stable even under basic conditions.
1
2
3
4
5
6
7
8
9
10
0
5
10
0
25
25
25
70
70
70
70
70
70
70
HBpin (1.2)
HBpin (1.2)
HBpin (1.2)
HBpin (1.2)
HBpin (1.2)
HBpin (1.2)
HBpin (1.2)
HBpin (1.0)
HBcat (1.2)
<5%
<5%
<5%
<5%
68%
>95%
>95%
85%
24
24
24
24
24
6
6
24
24
5
10
10
10
10
10
c
<5%
<5%
9-BBN (1.2)
b
24
1
a
Acetophenone (0.2 mmol, 24 mg). Conversion determined by H
NMR spectroscopy with internal mesitylene standard (0.1 mmol, 14
c
mL). Benzene-d6 solvent instead of CDCl3.
use of 10 mol % 5 at 70 °C gave quantitative conversion to the
boronate ester in 6 h. Switching the solvent from CDCl3 to
benzene-d6 gave no deleterious effect, but moving from 1.2
equiv to stoichiometric HBpin did. Testing alternative borane
a
Scheme 4. Hydroboration of Aldehydes, Ketones, Aldimines, and Alkynes with HBpin using 5
a
1
Time taken to reach quantitative conversion by H NMR spectroscopy. Isolated yields in parentheses.
E
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procedure 1, using magnesium turnings (0.49 g, 20 mmol, 1.0
equiv), 5-bromo-1,2,3-trifluorobenzene (2.4 mL, 20 mmol, 1.0 equiv),
and BF3·OEt2 (0.8 mL, 6.7 mmol, 0.33 equiv). Yield: 0.38 g, 0.93
EXPERIMENTAL SECTION
■
General Experimental. Unless stated otherwise, all reactions
were carried out under an atmosphere of dinitrogen using standard
Schlenk and glovebox techniques. With the exception of THF, Et2O,
and deuterated solvents, all solvents used were dried by passing them
through an alumina column incorporated into an MB SPS-800 solvent
purification system, degassed, and finally stored in an ampule fitted
with a Teflon valve under a dinitrogen atmosphere. THF was dried
over molten potassium for 3 days and distilled over argon, whereas
Et2O was dried over sodium wire and benzophenone before being
distilled over argon. Deuterated solvents were dried over calcium
hydride, distilled, freeze−pump−thawed degassed, and stored over 3
Å molecular sieves in a glovebox. Starting materials were purchased
1
mmol, 14%. H NMR (400 MHz, C6D6, 295 K) δ/ppm: 6.43−6.38
(m, 6H, Ar−H). 13C{1H}NMR (126 MHz, C6D6, 295 K) δ/ppm:
156.8−156.0 (m, 3C, Ar), 154.2−153.5 (m, 3C, Ar), 141.6−141.3
(m, 3C, Ar), 139.1−138.8 (m, 3C, Ar), 132.3 (dd, 2JFC = 12.9 Hz, 3JFC
= 7.7 Hz, 3C), 112.8 (dd, 2JFC = 16.7 Hz, 3JFC = 3.1 Hz, 3C). 11B{1H}
NMR (128 MHz, C6D6, 295 K) δ/ppm: 62.6 (s). 19F{1H} NMR (376
3
4
MHz, C6D6, 295 K) δ/ppm: −121.6 (dd, JFF = 20.9 Hz, JFF = 12.4
3
4
Hz, 3F), −126.0 (dd, JFF = 20.9 Hz, JFF = 12.4 Hz, 3F), −160.8 (t,
3JFF = 20.9 Hz, 3F). HRMS (EI+) [M]+ [C18H6BF9]+: calcd 404.0419;
found, 404.0410. EA calcd for C18H6BF9: C, 53.51; H, 1.50; N, 0.00.
Found: C, 53.31; H, 1.51; N, 0.00.
from commercial suppliers and used as received. H, 13C{1H}, 19F,
1
General Procedure 2. The parent borane (1.0 equiv) was
suspended in hexane (3 mL). To this, trimethylaluminum (1.0 equiv)
was added dropwise and the reaction was left undisturbed for 4 days.
During this time, crystals of the alane were developed. The solvent
level was reduced by removal in vacuo and placed in the freezer at −40
°C to ensure all the product had crystallized out. The hexane solvent
was removed via pipet, and the crystals were briefly dried in vacuo to
give the product.
and 11B NMR spectra were recorded on a Bruker Avance 400 or 500
MHz spectrometer. Chemical shifts are expressed as parts per million
(ppm, δ) and are referenced to CDCl3 (7.26/77.16 ppm) or C6D6
(7.16/128.06 ppm) as internal standards. Multinuclear NMR spectra
were referenced to BF3·Et2O/CDCl3 (11B) and CFCl3 (19F). The
description of signals includes s = singlet, d = doublet, t = triplet, q =
quartet, sept = septet, ov dd = overlapping doublet of doublets, and m
= multiplet. All coupling constants are absolute values and are
expressed in Hertz (Hz). IR spectra were measured on a Shimadzu IR
Affinity-1 photospectrometer. The description of signals includes s =
strong, m = medium, w = weak, sh = shoulder, and br = broad. Mass
spectra were measured by the School of Chemistry in Cardiff
University on a Waters LCT Premier/XE or a Waters GCT Premier
spectrometer. Elemental analysis was performed by Dr. Nigel Howard
at Cambridge University.
Synthesis of μ2-Dimethyl-bis[(3,4,5-trifluorophenyl)methyl-
alane] (2). Caution! This compound is potentially shock and thermally
sensitive; appropriate care should be taken.
μ2-Dimethyl-bis[(3,4,5-trifluorophenyl)methyl-alane] was synthe-
sized in accordance with general procedure 2, using tris(3,4,5-
trifluorophenyl)borane (210 mg, 0.51 mmol, 1.0 equiv) and
trimethylaluminum (0.25 mL, 0.51 mmol, 1.0 equiv, 2.0 M solution
1
in hexanes). Yield: 158 mg, 0.42 mmol, 83%. Mp: 105−110 °C. H
Note 1: Caution! The aluminum compounds prepared in this
manuscript are potentially shock and thermally sensitive due to the
potential formation of benzyne intermediates. Appropriate care
should be taken.
Note 2: Due to the potential for benzyne formation, lack of in-
house elemental analysis (EA) facilities, and the closure of our
laboratory as well as the external EA facilities due to the COVID-19
pandemic, elemental analysis of some of the compounds was not
performed.
General Procedure 1. The triarylboranes were synthesized in a
procedure adapted from Lancaster28 whereby magnesium turnings
(1.0 equiv) were suspended in diethyl ether (50 mL). The appropriate
bromobenzene (1.0 equiv) was slowly added dropwise, and an ice
bath was added to ensure that the reaction did not reflux. 1,2-
Dibromoethane was added if the Grignard reaction was slow to
initiate. After it stirred at ambient temperature for 1 h, the mixture
was transferred via filter cannula to a stirred solution of BF3·OEt2
(0.33 equiv) in toluene (30 mL). The diethyl ether solvent was
removed in vacuo, and the resulting toluene solution was heated to
reflux for 3 h. After this time the reaction was allowed to stir for 16 h
at ambient temperature, after which the solvent was removed in vacuo.
The resulting solid was subjected to a 2-fold sublimation (120 °C, 1 ×
10−3 mbar), whereupon the pure borane was collected as a white
crystalline solid.
NMR (500 MHz, C6D6, 295 K) δ/ppm: 6.87 (ov dd, J = 7.0 Hz, 4H,
Ar−H), −0.39 (s, 12H, CH3). 13C{1H} NMR (126 MHz, C6D6, 295
K) δ/ppm: 151.5 (ddd, 1JFC = 256 Hz, 2JFC = 12.3 Hz, 3JFC = 1.6 Hz,
1
2
4C, mC), 141.4 (dt, JFC = 254 Hz, JFC = 12.3 Hz, 2C, pC), 138.5
(2C, C−Al), 121.9 (dd, 2JFC = 11.6 Hz, 3JFC = 3.0 Hz, 6C, oC), −7.9
(4C, CH3). 19F NMR (376 MHz, C6D6, 295 K) δ/ppm: −135.3 (dd,
3
3JFF = 19.8 Hz, JFH = 5.8 Hz, 4F, mF), −158.2 (s, 2F, pF). IR νmax
(cm−1): 1516 (m), 1387 (m), 1302 (m), 1198 (w), 1088 (m), 1030
(m), 878 (m), 849 (m), 654 (br, m), 579 (br, m). Note:
approximately 8% impurity was observed in the 19F NMR spectrum
due to the parent borane and other unidentified partially trans-
metalated species.
Synthesis of Tris(2,3,4-trifluorophenyl)alane (4). Caution! This
compound is potentially shock and thermally sensitive; appropriate care
should be taken.
Tris(2,3,4-trifluorophenyl)alane was synthesized in accordance
with general procedure 2, using tris(2,3,4-trifluorophenyl)borane
(300 mg, 0.74 mmol, 1.0 equiv), and trimethylaluminum (0.37 mL,
0.74 mmol, 1.0 equiv, 2.0 M solution in hexanes). Yield: 243 mg, 0.58
mmol, 78%. Crystals suitable for single-crystal X-ray diffraction were
collected from the hexane solution. Mp: 145−147 °C. 1H NMR (400
MHz, C6D6, 295 K) δ/ppm: 6.74 (br s, 3H, Ar−H), 6.53 (dd, 3JFH
=
14.8 Hz, 3JHH = 8.5 Hz, 3H, Ar−H). 13C{1H} NMR (126 MHz, C6D6,
295 K) δ/ppm: 156.9 (d, 1JFC = 235 Hz, 3C, Ar), 152.8 (d, 1JFC = 252
1
2
2
Synthesis of Tris(3,4,5-trifluorophenyl)borane (1). Tris(3,4,5-
trifluorophenyl)borane was synthesized according to general
procedure 1, using magnesium turnings (3.0 g, 0.125 mol, 1.0
equiv), 5-bromo-1,2,3-trifluorobenzene (14.9 mL, 0.125 mol, 1.0
equiv), and BF3·OEt2 (5.1 mL, 42 mmol, 0.33 equiv). Yield: 2.70 g,
6.67 mmol, 16%. Spectroscopic analyses agree with literature values.22
1H NMR (500 MHz, CDCl3, 295 K) δ/ppm: 7.19−7.16 (m, 6H, Ar−
H). 13C{1H} NMR (126 MHz, C6D6, 295 K) δ/ppm: 152.3−150.2
(m, 6C, Ar), 144.2−141.9 (m, 3C, Ar), 136.4 (3C, C−B), 122.8−
121.2 (m, 6C, Ar). 11B{1H} NMR (160 MHz, CDCl3, 295 K) δ/ppm:
64.5 (s). 19F NMR (471 MHz, CDCl3, 295 K) δ/ppm: −133.3 (dd,
3JFF = 20.0 Hz, 3JFH = 7.2 Hz, 6F, mF), −158.2 (tt, 3JFF = 20.0 Hz, 4JFH
= 6.8 Hz, 3F, pF). EA calcd for C18H6BF9: C, 53.51; H, 1.50; N, 0.00.
Found: C, 53.27; H, 1.41; N, 0.00.
Hz, 3C, Ar), 139.4 (ddd, JFC = 255 Hz, JFC = 21.4 Hz, JFC = 14.7
Hz, 3C, Ar), 132.1 (s, 1C, Ar), 130.7 (s, 1C, Ar), 113.5 (d, 2JFC = 14.9
Hz, 3C, Ar). 19F NMR (376 MHz, C6D6, 295 K) δ/ppm: −115.1 (s,
3F, Ar−F), −132.9 (s, 3F, Ar−F) −162.1 to −162.2 (m, 3F, Ar−F).
HRMS (ES+): [M]+ [C18H6AlF9]+: calcd 420.0141; found 419.3153.
EA calcd for C18H6AlF9: C, 51.45; H, 1.44; N, 0.00. Found: C, 51.49;
H, 1.43; N, 0.00.
Characterization of Tris(2,3,4-trifluorophenyl)alane·THF (4·THF).
1H NMR (400 MHz, CDCl3, 295 K) δ/ppm: 7.10−7.00 (m, 3H, Ar−
H), 6.99−6.88 (m, 3H, Ar−H), 4.39 (s, 6H, THF), 2.64−2.13 (m,
6H, THF). 13C{1H} NMR (101 MHz, CDCl3, 295 K) δ/ppm: 156.5
(ddd, 1JFC = 232.6 Hz, 2JFC = 6.6 Hz, 3JFC = 2.9 Hz, 3C, oC−F), 152.2
1
(ddd, JFC = 249.5 Hz, 2JFC = 10.0 Hz, 3JFC = 3.9 Hz, 3C, pC), 139.4
1
2
2
(ddd, JFC = 253.9 Hz, JFC = 22.3 Hz, JFC2 = 14.4 Hz, 3C, mC−F),
132.5−132.2 (m, 3C, oC−H), 126.2 (d, JFC = 51.5 Hz, 3C, iC),
113.4 (d, 2JFC = 15.5 Hz, 3C, mC−H), 74.0 (s, 4C, THF), 25.7 (s, 4C,
Synthesis of Tris(2,3,4-trifluorophenyl)borane (3). Tris(2,3,4-
trifluorophenyl)borane was synthesized according to general
F
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Article
THF). 19F NMR (376 MHz, CDCl3, 295 K) δ/ppm: −117.89 (dd,
CONCLUSIONS
■
4
3
3JFF = 26.9 Hz, JFF = 8.3 Hz, 3F, pF), −135.21 (dd, JFF = 19.1 Hz,
In conclusion, we have reported the synthesis of Al(3,4,5-
ArF)3·Et2O, which was prepared from the corresponding
Grignard solution. Attempts to synthesize the triarylalane
from transmetalation resulted in the formation of the bridging
dimer μ2-Al(3,4,5-ArF)Me2. On the other hand, the synthesis
of the novel triarylalane Al(2,3,4-ArF)3 proceeded smoothly
from the parent borane B(2,3,4-ArF)3. The solid-state
structures of these compounds were obtained, and Al(2,3,4-
ArF)3 was found to form an extended chain structure via Al···F
strong contacts. Lastly, Al(3,4,5-ArF)3·Et2O was found to be an
efficient catalyst for hydroboration reduction, with wide
tolerance toward carbonyls, imines, and alkynes. Further
studies are underway in our laboratory to explore the full
potential of these triarylalanes in catalysis.
3
4
4JFF = 8.3 Hz, 3F, oF), −163.21 (dd, JFF = 26.9 Hz, JFF = 19.1 Hz,
3F, mF).
Synthesis of Tris(3,4,5-trifluorophenyl)alane etherate (5). Cau-
tion! This compound is potentially shock and thermally sensitive;
appropriate care should be taken.
In an ice bath, a Grignard solution was prepared by adding
magnesium turnings (486 mg, 20 mmol, 1.0 equiv) to 5-bromo-1,2,3-
trifluorobenzene (2.4 mL, 20 mmol, 1.0 equiv) in diethyl ether (20
mL). 1,2-Dibromoethane (0.5 mL) was added dropwise to initiate the
reaction. Once the reaction had initiated the ice bath was removed
and the reaction was left to stir at ambient temperature for 1 h. Note
that if the Grignard solution starts to reflux, it needs to be placed back
in the ice bath. During this time a three-necked flask equipped with a
reflux condenser was set up and degassed. Aluminum trichloride (888
mg, 6.7 mmol, 0.33 equiv) and toluene (20 mL) was added to the
three-necked flask. An ice bath was added to the three-necked flask,
and the Grignard solution was transferred to the mixture via filter
cannula. The resulting reaction was allowed to warm to room
temperature, after which the ether solvent was removed in vacuo. The
toluene solution was heated to reflux for 3 h, during which time the
reaction turned golden yellow and a precipitate formed. The reaction
was then cooled to ambient temperature; the reflux condenser was
removed (a glass stopper added), and then the mixture was stirred at
ambient temperature for a further 16 h. After this, the solution was
filtered via a filter cannula and the toluene solvent was removed in
vacuo. The resulting solid was washed with pentane and dried in vacuo
to give the etherate product tris(3,4,5-trifluorophenyl)alane as an off-
white powder (1.58 g, 3.2 mmol, 48%). Crystals suitable for single-
crystal X-ray diffraction were grown from a saturated solution of
toluene with a few drops of pentane added and cooled to −40 °C.
Due to the potential for benzyne formation, the product was not
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01076.
■
sı
Catalysis information, NMR spectra, XRD data, and
DFT calculations (PDF)
Accession Codes
CCDC 1996144−1996147 and 2022498 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1
sublimed. Mp: 126−134 °C. H NMR (400 MHz, C6D6, 295 K) δ/
AUTHOR INFORMATION
Corresponding Author
ppm: 6.99 (ov dd, J = 7.0 Hz, 6H, Ar−H), 3.00 (q, 3JHH = 7.0 Hz, 4H,
■
3
CH2), 0.28 (t, JHH = 7.0 Hz, 4H, CH3). 13C{1H} NMR (101 MHz,
1
2
Rebecca L. Melen − Cardiff Catalysis Institute, School of
Chemistry, Cardiff University, Cardiff CF10 3AT, Cymru/
Wales, U.K.; orcid.org/0000-0003-3142-2831;
Email: MelenR@cardiff.ac.uk
C6D6, 295 K) δ/ppm: 151.4 (dd, JFC = 254 Hz, JFC = 11.8 Hz, 6C,
mC), 140.4 (m, 3C, C−Al), 140.3 (dt, 1JFC = 252 Hz, 2JFC = 11.8 Hz,
3C, pC), 120.2 (dd, JFC = 11.1 Hz, JFC = 4.2 Hz, 6C, oC), 68.2 (s,
2C, CH2), 12.5 (s, 2C, CH3). 19F NMR (376 MHz, C6D6, 295 K) δ/
ppm: −153.5 (d, JFF = 19.9 Hz, 6F, mF), −160.7 (t, JFF = 19.9 Hz,
3F, pF). IR νmax (cm−1): 1601 (m), 1512 (s), 1387 (s), 1302 (s),
1267 (w), 1223 (w), 1190 (w), 1148 (w), 1088 (s), 1028 (s), 1015
(s), 887 (m), 878 (m), 849 (s), 772 (m), 745 (m), 710 (m), 600 (s),
584 (s), 519 (s), 503 (sh). HRMS (EI+) [M-OEt2]+ [C18H6AlF9]+:
calcd 420.0141; found 419.1153.
2
3
3
3
Authors
Darren M. C. Ould − Cardiff Catalysis Institute, School of
Chemistry, Cardiff University, Cardiff CF10 3AT, Cymru/
Wales, U.K.
Jamie L. Carden − Cardiff Catalysis Institute, School of
Chemistry, Cardiff University, Cardiff CF10 3AT, Cymru/
Wales, U.K.
General Procedure 3. In accordance with the literature known
procedure,3 the necessary aldehyde (10 mmol) was dissolved in
CH2Cl2 (10 mL) along with 3 Å molecular sieves. To this, the
required amine (10 mmol) was added. The reaction was left at
ambient temperature for 2 h, at which point MgSO4 was added with
subsequent filtration. Volatiles were removed in vacuo to leave the
pure imine in quantitative yields. Full synthetic procedures and
characterization for each imine can be found in the Supporting
Information.
Rowan Page − Cardiff Catalysis Institute, School of Chemistry,
Cardiff University, Cardiff CF10 3AT, Cymru/Wales, U.K.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01076
Author Contributions
‡These authors contributed equally to this work.
General Procedure 4. In an NMR tube, pinacol borane (35 μL,
240 μmol, 1.2 equiv) and the substrate (200 μmol, 1.0 equiv) were
combined in deuterated chloroform (0.7 mL). To this, tris(3,4,5-
trifluorophenyl)alane etherate (10 mg, 20 μmol, 0.1 equiv) was
added, and the NMR tube was sealed. The mixture was heated to 70
Notes
The authors declare no competing financial interest.
Information about the data that underpins the results
presented in this article, including how to access them, can
be found in the Cardiff University data catalogue at http://doi.
org/10.17035/d.2020.0114501208.
1
°C, and conversion was monitored via in situ H NMR spectroscopy
until the desired boronate ester had been formed in >95% yield. Upon
completion of the reaction, the catalyst was removed (and for
compounds 6−8, the boronate ester was hydrolyzed) by washing with
1 M NaOH (3 × 10 mL) and was further purified using flash column
chromatography. Full synthetic procedures and characterization for
each reduction product can be found in the Supporting Information.
ACKNOWLEDGMENTS
R.L.M. would like to acknowledge the EPSRC for an Early
Career Fellowship for funding (EP/R026912/1). J.L.C. would
■
G
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Article
Alkoxyenamides for the Synthesis of Tert -Alkylamines. Org. Lett.
2013, 15, 3374−3377.
like to acknowledge the EPSRC for funding (EP/L016443/1).
D.M.C.O. would like to acknowledge Professor Jeremy M.
Rawson (University of Windsor) for assistance with refinement
of X-ray structures.
(20) Yin, Q.; Soltani, Y.; Melen, R. L.; Oestreich, M. BArF -
3
Catalyzed Imine Hydroboration with Pinacolborane Not Requiring
the Assistance of an Additional Lewis Base. Organometallics 2017, 36,
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