Chiral tetra-coordinate aluminum cation in catalysis

Article abstract of DOI:10.1021/acs.organomet.1c00036

Although the application of cationic aluminum complexes in catalysis has been extensively investigated, the incorporation of an aluminum cation into a chiral environment is less explored. In this study, a C₂-symmetric chiral tetra-coordinate aluminum cation [3]⁺ bearing two chiral amino alcohol-derived ligands was prepared from commercially available starting materials in two steps. The high Lewis acidity of [3]⁺ exceeding that of B(C₆F₅)₃ was verified by a competition binding experiment. Preliminary catalytic activity studies of [3]⁺ revealed that the tetra-coordinate aluminum cation is effective in accomplishing high conversion in all cases. However, the presence of a labile N?Al bond diminishes the enantioselectivity of the activation process.

Full text of DOI:10.1021/acs.organomet.1c00036

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Chiral Tetra-coordinate Aluminum Cation in Catalysis
Ching-Pei Hsu, Yi-Hung Liu, Ramalingam Boobalan, Ya-Fan Lin, Rong-Jie Chein,*
and Ching-Wen Chiu*
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ABSTRACT: Although the application of cationic aluminum
complexes in catalysis has been extensively investigated, the
incorporation of an aluminum cation into a chiral environment is
less explored. In this study, a C2-symmetric chiral tetra-coordinate
aluminum cation [3]+ bearing two chiral amino alcohol-derived
ligands was prepared from commercially available starting materials
in two steps. The high Lewis acidity of [3]+ exceeding that of
B(C6F5)3 was verified by a competition binding experiment.
Preliminary catalytic activity studies of [3]+ revealed that the tetra-
coordinate aluminum cation is effective in accomplishing high
conversion in all cases. However, the presence of a labile N−Al
bond diminishes the enantioselectivity of the activation process.
still limited. After the first chiral tetra-coordinate aluminum
cation (D) was prepared in 1996,39 several examples were
synthesized despite that only a few of their asymmetry
catalyses were reported. The tri-coordinate cation (E) is
thermally unstable and gave a low ee value in the hetero-Diels−
Alder catalysis.40 The binaphthyl-based aluminum cations (F)
and (G) are two of the very few examples of effective chiral
aluminum cation catalysts.41,42 They performed well in
catalyzing asymmetric aldol reaction and epoxide carbon-
ylation, respectively. Considering the scarcity of aluminum
cation catalysts in asymmetric synthesis, we sought to develop
chiral aluminum cations from readily available starting
materials and study their catalytic performance.
Since 1987, the oxazaborolidine-based Corey−Bakshi−
Shibata (CBS) catalyst has proven to be spectacular for
asymmetric catalysis in terms of efficiency and enantioselec-
tivity.43−46 Mechanistic studies suggested that the rigid, fused
ring system of the prolinol backbone is a crucial factor that
contributes to the high stereoselectivity of the CBS catalyst.47
In light of the success of the CBS catalyst and its related
compounds, the prolinol substituted aluminum complexes had
been prepared. In 2000, Sieler and co-workers reported a series
of chiral aluminum complexes with one or two (S)-(−)-α,α-
diphenyl-2-pyrrolidinyl-methanol ligands at the aluminum
center.48 However, the related cationic species and their
INTRODUCTION
■
Catalysts built upon the inexpensive, earth-abundant, and
nontoxic main-group elements have played a pivotal role in
organic synthesis for decades.1−7 In addition to the classical
Lewis acid/base catalysis, main-group catalysts featuring
reversible redox states have also emerged in recent years.8−11
Aluminum, the most abundant metal in the earth’s crust, is an
attractive candidate for catalysis applications.12−14 Over the
years, both chiral and achiral neutral aluminum compounds
have been explored in catalyzing a plethora of reactions, such
as hydroboration of C−E (E = C, N, O) unsaturated
bonds,15−20 cyanosilylation of carbonyl compounds,21−23
stereoselective glycosylation,24 and lactide polymerization.25,26
Besides the incorporation of halogenated substituents, turning
a neutral aluminum compound into a cationic species is also an
effective means to elevate the Lewis acidity of aluminum.27−29
However, the isolation of low-coordinate aluminum cations is
challenging due to the large radius of the Al atom and its
electron positive nature.30,31 A few selected examples are
depicted in Chart 1. [Et2Al]+ (A), isolated as a contact ion pair
with the weakly coordinating dodecacarborate anion, is the first
example of a di-coordinate aluminum cation, which has been
applied in olefin polymerization and CO2 reduction.32,33 Since
then, some other di-coordinate aluminum cations have also
been reported.34−36 Aluminum cations with a coordination
number of three or four are still highly Lewis acidic, as the N-
NacNac-stabilized tri-coordinate hydrido aluminum cation (B)
can catalyze hydrosilylation of alkene,37 and the THF
coordinated aluminum cation (C) is highly efficient in
promoting aldehyde dimerization.38 However, compared with
the well-developed neutral chiral aluminum compounds,
asymmetric induction involving aluminum cation catalysts is
Received: January 20, 2021
Published: April 22, 2021
https://doi.org/10.1021/acs.organomet.1c00036
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Chart 1. Selected Aluminum Cations
Scheme 1. Synthesis of [3][B(C6F5)4] and
[3][Al(OC(CF3)3)4]
were generated from hydride abstraction of 1 and chloride
abstraction of 2, respectively (Scheme 1).
The distinct yellow color of the trityl cation faded upon
addition of [Ph3C][B(C6F5)4] to a DCM solution of 1. The
formation of ethylene and trityl methane was unambiguously
1
identified in the H NMR spectrum of the reaction mixture.
On the other hand, precipitation of silver chloride was
observed after mixing 2 and Ag[Al(OC(CF3)3)4] in DCM.
According to the 1H NMR integration, [3]+ was formed
quantitatively in both cases. The C2-symmetry of the cation
was corroborated by the detection of only one set of prolinol
signals. Comparing to the penta-coordinate neutral complexes
1 and 2, the pyrrolidine NH signal is significantly downfield
shifted in [3]+ (Δδ = 1.04 ppm from 1, Δδ = 0.57 ppm from 2,
Figure S5), consistent with the formation of cationic species.
Moreover, the observation of one set of 19F NMR signals for
[3][B(C6F5)4] and [3][Al(OC(CF3)3)4] implies the lack of
strong cation-anion interaction in DCM. Although all
spectroscopic characterizations suggest the successful gener-
ation of [3]+ in high yield, all attempts to isolate [3]+ from the
reaction mixture failed due to its instability in the condensed
phase. [3]+ decomposed upon removing DCM or adding
nonpolar solvents. From a solution consisting of the
decomposition products of [3][Al(OC(CF3)3)4], we obtained
a single crystal of [4][Al(OC(CF3)3)4]2, a dicationic di-
aluminum complex (Figure S46). The structure of [4]2+
contains two distinct aluminum atoms linked by one
bridging-N,O ligand and two μ2-O ligands. The isolation of
[4]2+ suggests that the decomposition of [3]+ may involve
ligand redistribution/rearrangements between two Al atoms.
DFT Calculation. To have more information in the
electronic property of the tetra-coordinate aluminum cation,
the molecular structure of [3]+ was optimized with the DFT
method in the B3LYP/6-311G** basis set. The optimized
structure of [3]+ bears an exposed aluminum center in a highly
distorted tetrahedron geometry with wide N−Al−N and O−
Al−O bond angles of 138.7° and 139.1°, respectively (Figure
1a). As demonstrated in the square planar tetra-coordinate
silicon complexes,51,52 the Lewis acidity of the central silicon
atom originated from the low-lying empty 3p orbital at silicon,
whose overlaps with the Si-ligand σ-bonds are symmetry
forbidden. Such geometrical constraint is also applicable to the
catalytic applications remain unexplored. We noticed that the
complexes bearing two prolinol ligands adopt a C2-symmetric
geometry with a penta-coordinate aluminum atom. As the
incorporation of C2-symmetric ligands has been proven to be a
highly effective strategy in the field of asymmetric catalysis,49,50
we considered creating such a chiral environment around the
central aluminum cation to be a rational design (H). The two
intramolecular amine-Al coordinations help to stabilize the
central aluminum cation and maintain the C2-symmetry of the
catalyst. Herein, we report the synthesis of a chiral tetra-
coordinate aluminum cation with two chiral prolinol-derived
ligands coordinated at the aluminum metal in a C2-symmetric
arrangement. Lewis base competition studies reveal that the
tetra-coordinate aluminum cation possesses Lewis acidity
exceeding that of the well-known strong Lewis acid, tris-
(pentafluorophenyl)borane (BCF). Further reactivity studies
indicate that the chiral tetra-coordinate aluminum cation is
capable of catalyzing asymmetric hydroboration of ketone,
Diels−Alder reaction, and Michael addition reaction with good
efficiency and moderate enantioselectivity.
RESULTS AND DISCUSSION
■
Synthesis of Aluminum Cation. In Sieler’s work, 1 was
obtained from the reaction of (tBu-Cp)Et2Al with 2 equiv of
amino alcohol.48 In our modified procedures, 1 and 2 can be
readily prepared from the commercially available alkyl alanes
and prolinol ligand to obtain the penta-coordinate complexes
in one step with high yield (Scheme 1). The crystal structure
of 2 (Figure S45) shows a similar C2-symmetric structure with
the reported structural data of 1. The tetra-coordinate
aluminum cations [3][B(C6F5)4] and [3][Al(OC(CF3)3)4]
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reversible. Dissolution of crystalline [5][Cl] in CD2Cl2
resulted in a mixture consisting of [5][Cl], 2, and OPEt3
([5][Cl]:2:OPEt3 = 1:0.85:0.85). In the reaction mixture
consisting of a 1:1 ratio of OPEt3 and [3][B(C6F5)4],
[5][B(C6F5)4] (δ = 73.9 ppm) is the only one species
detected in the solution. The 31P NMR signal at 73.9 ppm
suggests that [3][B(C6F5)4] possesses comparable Lewis
acidity to BCF (δ = 77.1 ppm). Since the Gutmann−Beckett
method is susceptible to the steric and electronic factors of
both the Lewis acids and the probe molecules,56 we decided to
determine the relative Lewis acidity between [3][B(C6F5)4]
and BCF with competition experiments.57,58 As shown in
Figure 2b, the addition of [3][B(C6F5)4] to the BCF-OPEt3
adduct in CD2Cl2 resulted in the transfer of OPEt3 from BCF
to [3]+. However, no detectable change was identified in the
mixture prepared from [5][B(C6F5)4] and BCF (Figure S28),
confirming the higher Lewis acidity of [3]+.
Figure 1. (a) Computed global minimum structure of [3]+ and (b)
isodensity plot of LUMO of [3]+.
tetra-coordinate aluminum cation with the Lewis acidity of the
central Al atom associated with the planarization degree of the
complex (Figure S50).53,54 Accordingly, significant contribu-
tion of the empty 3p orbital of aluminum in the lowest
unoccupied molecular orbital (LUMO) of [3]+ is captured by
theoretical computation (Figure 1b), suggesting that the Al
atom in [3]+ could be extremely acidic.
Acidity Determination. To address the Lewis acidity of 1,
2, and [3]+, we have conducted the Gutmann−Beckett
experiment.55 No coordination between 1 and triethylphos-
phine oxide (OPEt3) was observed due to the nondissociable
nature of the ethyl substituent. When equimolar amounts of
OPEt3 and 2 were mixed, two 31P resonances corresponding to
coordinated and noncoordinated OPEt3 were detected. The
new 31P NMR signal that appeared at 73.9 ppm is attributed to
the coordinated cationic complex [5][Cl], which was
corroborated by single crystal X-ray diffraction analysis.
[5][Cl] displays a trigonal bipyramidal geometry at the Al
atom with the two nitrogen atoms sitting at the axial position
(Figure 2a). The formation of [5][Cl] was found to be
To quantify the Lewis acidity of [3]+, the solvation corrected
fluoride ion affinity (FIA) of [3]+, BCF, Al(C6F5)3, Al(OC-
(CF3)3)3, and SbF5 was computed with the BP86-D3(BJ)
method and def2-SVP basis set with the conductor-like
polarizable continuum model (CPCM) for DCM solva-
tion.59,60 As summarized in Table S3, [3]+ (480 kJ/mol)
displays higher affinity toward fluoride than BCF (409 kJ/
mol), which is consistent with the base competition experi-
ment. With an FIA value comparable to that of SbF5 (489 kJ/
mol), Al(C6F5)3 (493 kJ/mol), and Al(OC(CF3)3)3 (503 kJ/
mol), we can conclude that [3]+ is indeed a strong Lewis acid.
Asymmetric Catalysis Studies. Encouraged by the
observed high Lewis acidity of [3]+, we started to examine
its application in Lewis acid-catalyzed transformations.
Asymmetric hydroboration of ketones is a useful reaction for
constructing optically active alcohols.61 Though high yield and
high enantioselectivity have been documented,19,43,44,62 we
consider hydroboration of acetophenone as a platform to
evaluate the performance of the chiral tetra-coordinate
aluminum cation. For neutral complexes 1 and 2, low
conversion of acetophenone 6 was observed (Table 1, entries
1 and 2), which is consistent with the low Lewis acidity of 1
and 2. When the catalyst was changed to [3][B(C6F5)4], the
reaction reached completion with some degree of enantiose-
lectivity within several hours under a similar condition (Table
1, entry 3). A shorter reaction time and better ee value were
obtained when Me2S·BH3 was utilized as the hydride source
instead of HBPin (Table 1, entry 4). Replacing the solvent
from DCM to DCM/THF (1/1, v/v) resulted in a complete
racemized product (Table 1, entry 5). The lack of
Figure 2. (a) Crystal structure of [5][Cl] and (b) time-course 31P
NMR of competition experiment in DCM.
Table 1. Optimization of Hydroboration Conditions
a
b
entry
solvent
CDCl3
CDCl3
CD2Cl2
CD2Cl2
catalyst
borane
HBPin
HBPin
t (h)
conv. (%)
ee (%)
1
2
3
4
5
6
1
2
6
6
6
0.5
0.5
0.5
trace
trace
85
N.A.
N.A.
14
30
0
[3][B(C6F5)4]
[3][B(C6F5)4]
[3][B(C6F5)4]
[3][Al(OC(CF3)3)4]
HBPin
c
Me2S·BH3
Me2S·BH3
Me2S·BH3
99 (95)
CD2Cl2/THF
CD2Cl2
99
99
34
a
b
c
1
Conversions were determined by H NMR spectroscopy. Determined by chiral HPLC analysis. Isolated yield.
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enantioinduction of [3]+ could be attributed to the
dissociation of the Al−Npyrrolidine bond upon THF coordina-
tion, causing the collapse of the fused ring system and the loss
of enantioselectivity. Besides, comparable yield and ee value
were obtained when [3][Al(OC(CF3)3)4] was employed,
suggesting that the catalytic performance of [3]+ is
independent of the counteranion (Table 1, entry 6).
effect on the cation-anion interaction. The results revealed that
[3][B(C6F5)4] has a higher degree of aggregation in C7D8 than
in CD2Cl2 (see SI). Noticeably, when AlCl3 was used as the
halophile for in situ generation of [3]+ from 2, an extremely
poor result was observed (Table 2, entry 4). A stoichiometry
reaction of 2 and AlCl3 monitored with 1H NMR spectroscopy
revealed the formation of several unidentified species (Figure
S13). Therefore, the use of noncoordinating anions is critical
in exploring the catalytic applications of the tetra-coordinate
cationic aluminum complex.
In addition to Diels−Alder reaction, we have also examined
the asymmetric Michael addition of (E)-1-phenyl-2-buten-1-
one 10 and indole. Previously, the reaction has been performed
using neutral chiral aluminum salen complexes.67,68 In contrast
to the earlier report, where the addition of 2,6-lutidine is
critical in realizing high yield and enantioselectivity, the added
base significantly hampered the chemoselectivity of the
reaction with negligible influence on the enantioselectivity of
the Michael addition product (Table 3, entries 1 and 3). The
isolated yield of 11 considerably decreased in the presence of
2,6-lutidine. Such a result is consistent with our previous
observations that the catalytic reactivity of [3]+ is diminished
when coordinating bases are introduced to the system.
During the screening process of the above reaction, we
noticed that the addition of boranes, followed by ketone, is
essential. When reverse addition was applied, the coordination
of ketone to [3]+ was identified (Figures S29 and S30) and an
extremely slow reaction rate was observed (see the Supporting
Information (SI)). This suggests that ketone hydroboration
might proceed through borane activation rather than ketone
activation. We confirmed the interaction of [3]+ and borane by
1
2
monitoring the H and H NMR (Figures S31−S33) of the
reaction between [3][B(C6F5)4] and Me2S·BD3 (see SI). Thus,
we decided to examine whether cation [3]+ can catalyze
reactions that involve carbonyl activation. Asymmetric Diels−
Alder reaction of 2,2,2-trifluoroethyl acrylate 8 and cyclo-
pentadiene (CpH) was chosen as another platform to evaluate
the catalytic performance of [3]+. As summarized in Table 2,
Table 2. Optimization of Diels−Alder Reaction Conditions
Effect of Al-N Interaction and Counteranion. As
demonstrated in the OPEt3 competition reaction and the
catalytic applications, the tetra-coordinate aluminum cation
[3]+ is indeed a powerful Lewis acid that can effectively
catalyze hydroboration of ketone, Diels−Alder reaction, and
Michael addition. However, no ee value higher than 50% has
been accomplished thus far. The reason behind the inability of
[3]+ to achieve high stereoselectivity is very likely to be the
result of labile Al−Npyrrolidine coordination. To verify the
hypothesis, we decided to further weaken the Al−Npyrrolidine
interaction by introducing a methyl group at the nitrogen.
Although the N−Al bond distance of 12 (Figure S47) is only
slightly elongated (0.07 Å) comparing to 2 (Figure S45), the
catalytic performance of the corresponding aluminum cation
generated from 12 and Ag[Al(OC(CF3)3)4] dropped dramat-
ically (see SI). This result may be viewed as a support for the
crucial role of the rigid fused ring system of chiral prolinol-
derived catalysts for effective enantioinduction. However, the
presence of a secondary hydrogen bonding interaction
involving the N−H moiety cannot be completely ruled out.69
Besides, we have also examined the effect of a counteranion.
Treatment of 2 with AgNTf2 resulted in the formation of
a
b
entry
solvent
catalyst
t (h) conv. (%)
ee (%)
1
2
3
4
DCM
toluene
DCM
DCM
[3][B(C6F5)4]
[3][B(C6F5)4]
[3][Al(OC(CF3)3)4]
2+AlCl3
2
20
3
99
87
99 (97)
32
4
42
c
27
30
1
a
Conversions were determined by crude 1H NMR spectroscopy;
endo:exo > 99.9:0.01 for all entries. Determined by chiral GC
b
c
analysis. Isolated yield.
the reaction reached completion in less than 3 h at −78 °C in
the presence of 9 mol % of [3][B(C6F5)4] or [3][Al(OC-
(CF3)3)4] (Table 2, entries 1 and 3). Akin to that observed in
ketone hydroboration, no noticeable anion effect was observed,
suggesting the lack of contact ion pair during the reaction.
However, a dramatically reduced reactivity and enantioselec-
tivity was obtained when a less polar solvent was applied,
potentially due to the inability of [3][B(C6F5)4] to form the
solvent-separated ion pair in toluene (Table 2, entry 2).63−66
DOSY NMR experiments were performed to verify the solvent
1
complex 13 in high yield. The H signals of 13 are markedly
upfield shifted compared to that of [3]+, suggesting the
Table 3. Optimization of Michael Addition Conditions
a
b
c
d
entry
additive
catalyst
conv. (%)
yield (%)
ee (%)
1
2
3
4
2,6-lutidine
none
2,6-lutidine
none
[3][B(C6F5)4]
[3][B(C6F5)4]
[3][Al(OC(CF3)3)4]
[3][Al(OC(CF3)3)4]
99
99
99
99
20
85
35
83
27
39
29
39
a
b
c
d
1
10 mol %. Conversions were determined by H NMR spectroscopy. Isolated yield. Determined by chiral HPLC analysis.
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(m, Fo), −164.3 (t, Fp), −168.3 (t, Fm). Because of the presence of
Ph3CH byproduct, the aromatic 1H and 13C signals could not be
resolved. The elemental analysis was not performed due to the
instability of [3][B(C6F5)4].
existence of an Al-NTf2 interaction in the solution. The
stereoselectivity of complex 13 is slightly decreased comparing
to that of [3]+, emphasizing the critical role of noncoordinating
anions in catalytic application of aluminum cations (see SI).
Generation of [3][Al(OC(CF3)3)4]. To a J. Young NMR tube
wrapped with aluminum foil were added 2 (11.3 mg, 19.9 μmol) and
Ag[Al(OC(CF3)3)4] (21.4 mg, 19.9 μmol), followed by CD2Cl2 (0.5
mL). The tube was vigorously shaken, and the precipitation of AgCl
was observed. The formation of [3][Al(OC(CF3)3)4] in quantitative
yield was verified by 1H NMR. Because of the instability of
[3][Al(OC(CF3)3)4], the complex was freshly prepared and then
used as catalyst without purification. The 1H NMR and 13C{1H}
NMR spectra are similar to those of [3][B(C6F5)4]. 19F{1H} NMR
(CD2Cl2): δ −75.7 (s, CF3). 27Al NMR (CD2Cl2): δ 34.7 (s,
Al(OC(CF3)3)4). The elemental analysis was not performed due to
the instability of [3][Al(OC(CF3)3)4].
CONCLUSION
■
In conclusion, chiral tetra-coordinate aluminum cations
[3][B(C6F5)4] and [3][Al(OC(CF3)3)4] could be readily
obtained in two steps from commercially available starting
materials. With Lewis acidity exceeding that of BCF, the
catalytic performances of [3]+ in asymmetric hydroboration of
ketone, Diels−Alder reaction, and Michael addition have been
investigated. In all cases, high conversion, but moderate ee
value, was observed, suggesting a better control of the chiral
environment around the central aluminum atom is required.
The dissociation of Al−Npyrrolidine and the cation-anion
association were found to be disadvantageous for asymmetric
induction. To address the stereoselectivity issue, we are now
working on tetra-coordinate aluminum cations that feature
nondissociable prolinol-derived ligands.
General Procedure of Hydroboration of Acetophenone 6.
To a J. Young NMR tube containing 10.0 μmol of catalyst
([3][B(C6F5)4] was freshly prepared from 5.6 mg of 1 and 9.2 mg
of [Ph3C][B(C6F5)4]; [3][Al(OC(CF3)3)4] was freshly prepared
from 5.7 mg of 2 and 10.2 mg of Ag[Al(OC(CF3)3)4]) in 0.5 mL of
solvent was added HBPin (21.8 μL, 150 μmol) or BH3·SMe2 (83.3
μL, 1.8 M in toluene, 150 μmol), followed by acetophenone 6 (11.7
1
μL, 100 μmol). The reaction was monitored by H NMR. Upon full
EXPERIMENTAL SECTION
consumption of the starting material, Et2O (2 mL) and sat.
NaHCO3(aq) (2 mL) were added. The mixture was extracted with
Et2O (3 × 2 mL), and the collected organic layer was dried over
Na2SO4. After the solid was filtered off, the organic layer was
concentrated and purified by column chromatography (EtOAc/
■
General Procedures. All manipulations involving the air- and
moisture-sensitive compounds were carried out under a nitrogen
atmosphere using the standard Schlenk and glovebox technique.
Toluene, hexane, and DCM were purified by passing through a
molecular sieve-packed solvent purification system and stored over
activated 4 Å molecular sieves. CDCl3 and CD2Cl2 were degassed and
dried over activated molecular sieves. Pentane, C6D6, and C7D8 were
dried by distillation over Na/K-benzophenone, followed by vacuum
transfer, and stored over activated 4 Å molecular sieves. All
commercially available reagents were used as received unless
otherwise stated. Indole was purified by sublimation under vacuum.
Cyclopentadiene was freshly cracked from its dimer at a temperature
above 170 °C. Rac-7,70 Rac-9,71 Rac-11,72 and 1073 was synthesized
according to literature reports. NMR spectra were recorded on a
Bruker AVIII-400 FT-NMR spectrometer (1H, 400.2 MHz; 13C,
100.6 MHz; 11B, 128.4 MHz; 19F, 376.5 MHz; 27Al, 104.3 MHz; 31P,
162.0 MHz). Chemical shifts (δ) are given in ppm and are referenced
to the signals of the residual solvent (1H NMR: CDCl3, 7.26 ppm;
CD2Cl2, 5.32 ppm. 13C NMR: CDCl3, 77.16 ppm; CD2Cl2, 53.84
ppm). Elemental analyses were performed on a Heraeus varioIII-
NCH elemental analyzer. Flash chromatography separations were
performed on SiliaFlash G60 (0.060−0.200 mm) mesh silica gel.
Analytical thin-layer chromatography (TLC) was performed on
Merck 60 F254 silica gel plates. Visualization was performed using
a UV lamp or cerium ammonium molybdate stain. Enantiomeric
excess values were determined by HPLC using a CHIRALCEL OD
column (250 mm, 4.6 mm), or by GC analysis using an MN Scientific
Hydrodex-β-6TBDM capillary chiral column (25 m, 0.25 mm).
Generation of [3][B(C6F5)4]. CD2Cl2 (0.5 mL) was added to a J.
Young NMR tube containing 1 (11.2 mg, 20.0 μmol) and
[Ph3C][B(C6F5)4] (18.4 mg, 19.9 μmol). The tube was vigorously
shaken until the yellow color of [Ph3C]+ faded. 1H NMR showed that
the yield of [3][B(C6F5)4] was quantitative. Since [3][B(C6F5)4] is
unstable upon concentrating the solution and decomposes in the
solution after 24 h, it was not isolated. Still, it could be characterized
by NMR spectroscopy immediately after the generation of the cation.
For further reaction involving [3][B(C6F5)4], it was freshly prepared
1
hexane = 3/10) to afford 1-phenylethanol 7 as a colorless oil. H
NMR (CDCl3): δ 7.41−7.31 (m, 4H), 7.31−7.25 (m, 1H), 4.89 (q,
3
3JH‑H = 6.6 Hz, 1H), 2.07 (br s, 1H), 1.50 (d, JH‑H = 6.5 Hz, 3H).
13C{1H} NMR (CDCl3): δ 145.9, 128.6, 127.6, 125.5, 70.5, 25.3.
HPLC analysis:74 OD column (210 nm), method: nHex:IPA = 98:2,
flow 1.0 mL/min, tS = 15.41 min, tR = 20.36 min. All the analytical
data were in accord with those reported in the literature.74
General Procedure of Diels−Alder Reaction of 2,2,2-
Trifluoroethyl Acrylate 8. The precursor 1 or 2 (50.0 μmol) and
hydride abstractor or halophile (45.0 μmol) were weighed into a
Schlenk tube in a glovebox. Afterward, solvent (1 mL) was added, and
the resulting solution was cooled to −78 °C. Then, 2,2,2-
trifluoroethyl acrylate 8 (63.4 μL, 0.500 mmol) was added in one
portion and followed by dropwise addition of CpH (126 μL, 1.50
mmol). The reaction was monitored by TLC. Upon full consumption
of the starting material, the reaction was quenched with triethylamine
(300 μL). The resulting mixture was then passed through a SiO2 gel
plug. To extract the product, the silica gel was further washed with
DCM (∼50 mL). The combined DCM filtrate was evaporated to give
a crude compound, which was purified by column chromatography
(pentane/DCM = 100/2−100/3) to afford 2,2,2-trifluoroethyl
bicyclo[2.2.1]hept-5-ene-2-carboxylate 9 as a colorless liquid. 1H
NMR (CDCl3): δ 6.21 (dd, 3JH‑H = 5.8, 3.1 Hz, 1H), 5.92 (dd, 3JH‑H
=
5.8, 2.8 Hz, 1H), 4.48 (dq, 2JH‑H = 12.7, 3JH‑F = 8.5 Hz, 1H), 4.35 (dq,
2JH‑H = 12.7, 3JH‑F = 8.5 Hz, 1H), 3.26 (br s, 1H), 3.06−3.01 (m, 1H),
2.94 (br s, 1H), 1.98−1.92 (m, 1H), 1.47−1.41 (m, 2H), 1.30 (d,
3JH‑H = 8.2 Hz, 1H). 13C{1H} NMR (CDCl3): δ 173.3, 138.3, 132.2,
123.2 (q, 1JC‑F = 277.2 Hz), 60.2 (q, 2JC‑F = 36.6 Hz), 49.8, 46.0, 43.1,
42.7, 29.4. GC analysis:75 Hydrodex-B-6TBDM column (110 °C, 25
psi), tminor = 9.97 min, tmajor = 11.67 min. All the analytical data were
in accord with the spectra reported in the literature.75
General Procedure of Michael Addition of (E)-1-Phenyl-2-
buten-1-one 10. To a J. Young NMR tube containing freshly
generated [3][B(C6F5)4] (10.0 μmol, prepared from 5.6 mg of 1 and
9.2 mg of [Ph3C][B(C6F5)4] in 0.5 mL of CD2Cl2) or [3][Al(OC-
(CF3)3)4] (10.0 μmol, prepared from 5.7 mg of 2 and 10.2 mg of
Ag[Al(OC(CF3)3)4] in 0.5 mL of CD2Cl2) were added indole (17.6
mg, 150 μmol), 2,6-lutudine (1.2 μL, 10.4 μmol), and (E)-1-phenyl-2-
buten-1-one 10 (14.6 mg, 100 μmol). The reaction was monitored by
1H NMR. Upon full consumption of the starting material, Et2O (2
1
and used without purification. H NMR (CD2Cl2): δ 4.82−4.71 (m,
2H, NHCHCH2), 3.83 (t, 2H, JH‑H = 6.8 Hz, NH), 2.95−2.79 (m,
3
2H, NHCH2CH2), 2.63−2.44 (m, 2H, NHCH2CH2), 2.17−2.03 (m,
2H, NHCH2CH2), 2.01−1.87 (m, 2H, NHCH2CH2), 1.86−1.76 (m,
2H, NHCHCH2), 1.76−1.64 (m, 2H, NHCHCH2). 13C{1H} NMR
(CD2Cl2): δ 78.7 (COAl), 69.8 (NHCHCH2), 47.9 (NHCH2CH2),
26.9 (NHCHCH2), 23.8 (NHCH2CH2). 11B NMR (CD2Cl2): δ
−16.2 (s, B(C6F5)4). 19F{1H} NMR (CD2Cl2): δ −133.9 to −134.3
mL) and sat. NaHCO3(aq) (2 mL) were added. The mixture was
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extracted with Et2O (3 × 2 mL), and the combined organic layer was
dried over Na2SO4. After the solid was filtered off, the organic
solution was concentrated and purified with column chromatography
(hexane/Et2O = 10/1−4/1) to give 1-phenyl-3-(1H-indol-3-yl)-
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* Supporting Information
The Supporting Information is available free of charge at
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Corresponding Authors
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Rong-Jie Chein − Institute of Chemistry, Academia Sinica,
Taipei 11529, Taiwan; Email: rjchein@chem.sinica.edu.tw
Ching-Wen Chiu − Department of Chemistry, National
Taiwan University, Taipei 10617, Taiwan; orcid.org/
0000-0001-7201-0943; Email: cwchiu@ntu.edu.tw
Authors
Ching-Pei Hsu − Department of Chemistry, National Taiwan
University, Taipei 10617, Taiwan
Yi-Hung Liu − Department of Chemistry, National Taiwan
University, Taipei 10617, Taiwan
Ramalingam Boobalan − Institute of Chemistry, Academia
Sinica, Taipei 11529, Taiwan
Ya-Fan Lin − Department of Fragrance and Cosmetic Science,
Kaohsiung Medical University, Kaohsiung 80708, Taiwan;
orcid.org/0000-0001-6541-8334
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.1c00036
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Ministry of Science and
Technology of Taiwan (MOST 107-2113-M-002-007-MY3)
and the Innovative Joint Program of National Taiwan
University and Academia Sinica (NTU-AS-109L104302).
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