Asymmetric Arylation of 2,2,2-Trifluoroacetophenones Catalyzed by Chiral Electrostatically-Enhanced Phosphoric Acids

Article abstract of DOI:10.1021/acs.orglett.8b00900

A series of highly reactive metal-free chiral phosphoric acids possessing positively charged phosphonium ion substituents are reported and have been applied to Friedel-Crafts alkylations of indoles and 2,2,2-trifluoromethyl aryl ketones. These catalysts a

Full text of DOI:10.1021/acs.orglett.8b00900

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Asymmetric Arylation of 2,2,2-Trifluoroacetophenones Catalyzed by
Chiral Electrostatically-Enhanced Phosphoric Acids
Jie Ma and Steven R. Kass*
Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States
S
* Supporting Information
ABSTRACT: A series of highly reactive metal-free chiral phos-
phoric acids possessing positively charged phosphonium ion sub-
stituents are reported and have been applied to Friedel−Crafts
alkylations of indoles and 2,2,2-trifluoromethyl aryl ketones.
These catalysts are orders-of-magnitude more active and have
similar or better enantioselectivities than their noncharged ana-
logues. High tolerance to a range of substrates with electron-
withdrawing and electron-donating substituents was also observed.
ndoles are among the most important type of nitrogen-
containing heterocyclic scaffolds, as they are widely found in
nitro groups are sometimes incorporated into phenyl rings at the
3 and 3′ positions of BINOL-derived phosphoric acids.18,19
In previous studies from our laboratory, positively charged ion
centers were found to enhance the catalytic reactivities of phe-
nols,20 thioureas,21 and phosphoric acids22 by orders of mag-
nitude. This approach was also used to obtain good to excellent
enantioselectivities for the alkylation of indoles with trans-β-
nitrostyrene derivatives using chiral thioureas.23 As a result, we
decided to develop a series of BINOL-based phosphoric acid cat-
alysts possessing 3,3′-phosphonium ion substituents (Figure 1)
I
natural products,1 pharmaceuticals,2 and synthetic chemicals.3
Most of these indole-based compounds possess special chemical
properties as well as bioactivities, and thus, they have attracted
widespread attention.4 Fluorinated species are another important
class of organic materials since the small size and high electro-
negativity of a fluorine atom lead to compounds with remarkably
different properties than their fluorine-free analogues.5 A com-
bination of fluorine substituents on indole frameworks leading to
novel bioactivities and pharmaceutical properties made the
synthesis of such compounds inviting targets.6,7
A number of strategies have been devised to incorporate
fluorine atoms into indole derivatives.7c,8 Given the electron-rich
nature and the enhanced reactivity in electrophilic alkylations
(especially at the C-3 position) of this heterocycle,9 one of the
most straightforward methods for synthesizing these derivatives
is to carry out a Friedel−Crafts alkylation with an indole and a
fluorine-containing electrophile.10 This type of transformation is
also a powerful method for the formation of carbon−carbon
bonds that can be applied to construct optically active com-
pounds.11 In general, these reactions proceed in good yields and
high enantioselectivities with suitable catalysts. For the specific
case of Friedel−Crafts alkylations of indole derivatives with
2,2,2-trifluoromethyl aryl ketones, inorganic bases,12 cinchona
alkaloids,13 guanidine,14 trifluoromethanesulfonic acid,15 and
phosphoric acids16 were reported to be successful catalysts. Most
of these studies, however, focused on either racemic products or
bis-coupled adducts. There are only a few reports on asymmetric
alkylations, and these required either long reaction times (i.e., up
to 96 h)16 or harsh reaction conditions (i.e., up to 9 kbar pressure)13
presumably because it is a challenge to trigger the alkylation and
terminate it at the monoarylated stage.15,16 More effective
enantioselective catalysts, consequently, are still needed for this
transformation.
Figure 1. Catalysts used in this work.
that were prepared using reported synthetic methods.24 These
large groups provide needed steric bulk and two acidity-enhancing
positively charged ion centers. Their performance in the
In general, there is a good correlation between reactivity
and the acidity of related organocatalysts.17 For this reason,
electron-withdrawing substituents such as trifluoromethyl or
Received: March 19, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00900
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Friedel−Crafts reactions of indole derivatives with 2,2,2-
trifluoromethyl aryl ketones are reported herein.
5 mol % catalyst at 0 °C in the presence of 3 Å MS.
The triphenylphosphonium containing derivative 2a gave a
much lower yield (30%) and selectivity (77% ee) (entry 13),
while the tri(p-ethylphenyl) and tri(p-propylphenyl)phosphonium
derivatives 2c and 2d gave similar results to 2b (entries 14−15).
Interestingly, the diphenyl(p-tolyl)phosphonium ion tagged
catalyst 2e, which is sterically similar to 2a and electronically in
between 2a and 2b, afforded the product with a reduced yield and
ee relative to 2b of 68% and 84%, respectively. These results
suggest that the large difference between 2a and 2b is due to
electronic effects brought about by replacing a hydrogen atom
with a methyl group on each of the phenyl rings attached to the
phosphorus atom. A lower temperature experiment at −30 °C
was also performed with 3 Å MS, and a 3% increase in ee to 90%
was observed (entry 17). A further drop in the temperature to
−50 °C was impractical due to reduced catalyst solubility.
Reaction media were examined next with catalyst 2b, and the
results are summarized in Table 2. Solvents were found to have a
This investigation was initiated by examining the reaction of
indole (3a) with 2,2,2-trifluoroacetophenone (4a) using the
tri(p-tolyl)phosphonium ion containing phosphoric acid at the
3,3′-positions of the (R)-BINOL backbone (i.e., 2b) along with
the noncoordinating tetrakis(3,5-bis(trifluoromethyl)phenyl)-
borate (BArF −) counterion; more common and interacting
4
anions lead to less active catalysts.20−22 At room temperature in
CH2Cl2 with a 5 mol % catalyst loading, both the desired and
undesired mono- and bis-coupled products (5a and 6, respec-
tively) were formed, respectively (eq 1). This led to a loss of yield
a
Table 2. Screening of Reaction Solvents
for the former species, as previously has been reported with other
strong acid catalysts,12,15 and the enantioselectivity was also poor
(Table 1, entry 1). To our delight, when the reaction temperature
b
c
entry
solvent
time (h) yield of 5a (%) ee (%)
1
2
3
4
5
6
7
8
CH2Cl2
CHCl3
C6H5Cl
DCE
22
22
22
22
20
20
20
20
92
37
91
89
86
81
88
89
87
67
62
63
84
83
83
85
Table 1. Optimization Conditions for the Reaction of Indole
a
with 2,2,2-Trifluoroacetophenone
1:1 CH2Cl2/C6H6
b
c
entry
cat. t (°C)
additive
time (h) yield of 5a (%)
ee (%)
1:1 CH2Cl2/C6H5CH3
1:1 CH2Cl2/1,3-(CH3)2C6H4
1:1 CH2Cl2/CCl4
1
2
3
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2a
2c
2d
2e
2b
20
0
5
48
90
87
92
87
79
58
81
86
83
81
87
30
91
86
68
85
17
85
86
87
66
80
85
83
80
80
80
87
77
86
86
84
90
22
96
22
22
21
21
20
20
20
20
20
22
22
22
22
40
a
−30
0
Experiments performed with 0.140 mmol of indole, 0.168 mmol of
2,2,2-trifluoroacetophenone, 0.007 mmol of catalyst 2b, and 5 mg of
3 Å MS in 0.25 mL of solvent at 0 °C. Isolated yield. Determined by
d
4
3 Å MS
4 Å MS
5 Å MS
13X MS
Na2SO4
NaCl
d
b
c
5
0
d
6
0
chiral HPLC.
d
7
e
0
large influence on the reactivity and enantioselectivity. For exam-
ple, CHCl3 (entry 2) led to both a lower yield and enantiose-
lectivity than CH2Cl2 (entry 1). Chlorobenzene (entry 3) and
1,2-dichloroethane (DCE, entry 4) gave similar results (91% and
89% yields along with 62% and 63% ee), neither of which is as
good as CH2Cl2. Several mixtures using CH2Cl2 and aromatic
solvents (i.e., benzene, toluene, m-xylene) or CCl4 were also
tested (entries 5−8). None of these screenings led to better
results than when CH2Cl2 was used.
The scope of this transformation was examined at 0 °C using
5 mol % 2b, CH2Cl2 as the solvent, and 3 Å MS as a drying agent
(eq 2). As summarized in Table 3, a wide variety of substituents
including electron-withdrawing and electron-donating groups on
either reactant were tolerated. With a few exceptions, excellent
yields (82−94%) and good enantioselectivities (79−91% ee)
were obtained at 0 °C (entry 1−19). Halide substituents at the
5-positon of indole did not affect the selectivity (85−87% ee)
(entries 1−2) relative to the parent compound (i.e., R3, R4 = H),
whereas electron-donating groups at the 5- and 6-positions led to
reduced enantioselectivities of 79% and 48% ee (entries 3−4).
Interestingly, while 6-methylindole (3e) gave the worst product
enantiomeric purity (48% ee), its isomer 7-methylindole (3h)
provided the highest enantioselectivity (91% ee, entry 7). This
difference should be quite helpful in developing computational
models for the mechanism of this reaction. Other substituents at
the 7-position of indole (Cl, Br, Et) had no impact on the
enantiomeric excess (entries 5−6 and 8), but 7-bromoindole
(3g) slowed the reaction leading to approximately half the yield
over the same time period. Introduction of electron-withdrawing
8
0
e
9
0
e
10
0
MgSO4
CaCl2
e
11
0
df
,
12
13
14
15
16
17
0
3 Å MS
3 Å MS
3 Å MS
3 Å MS
3 Å MS
3 Å MS
d
d
d
d
d
g
g
0
0
0
0
−30
a
Experiments performed with 0.140 mmol of indole, 0.168 mmol of
2,2,2-trifluoroacetophenone, 0.007 mmol of catalyst in 0.25 mL of
b
c
d
CH2Cl2. Isolated yield. Determined by chiral HPLC. 5 mg of MS
were used. 1.0 equiv of salt was used. 0.014 mmol catalyst was
employed. Starting material was present.
e
f
g
was reduced to 0 °C, the formation of the bis-coupled byproduct
was nearly eliminated and a dramatic increase in the ee from 17%
to 85% was observed (entry 2). A further decrease to −30 °C
only led to a 1% increase in the ee at the expense of a much longer
reaction time (entry 3), and this result was surpassed by adding
3 Å molecular sieves (MS) at 0 °C (i.e., 87% ee, entry 4). Other
MS (4 Å, 5 Å, and 13X) decreased both the yield and the
enantioselectivity (entries 5−7). Attempts to improve this
transformation with inorganic salts (Na2SO4, NaCl, MgSO4,
and CaCl2) as drying agents also led to poorer results (entries
8−11). An increase in the catalyst loading to 10 mol % did
not improve the enantioselectivity further (entry 12) so the
other phosphonium ion containing species were screened with
B
DOI: 10.1021/acs.orglett.8b00900
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(28%) and ee (64%, entry 18), while 2-chloro-2,2-difluoroace-
tophenone (4l) also gave a low yield (36%) but without much
difference in the enantioselectivity (i.e., an 84% ee was observed,
entry 19). For the two reactions that gave ≥90% ee at 0 °C
(entries 7 and 9), a decrease in temperature to −30 °C resulted in
improved selectivities of 95% and 93% ee (entries 20−21). The
former transformation was further examined with a decreased
catalyst loading of 1.5 mol %, and the enantioselectivity was
preserved with a measured ee of 96% (entry 22). A larger scale
reaction was also performed with six times more material and
reactant weights in excess of 100 mg along with a catalyst loading
of 1.5 mol %. This led to similar results to the small-scale
transformation and afforded a 96% yield and a 96% ee (entry 23).
To compare the reactivity and enantioselectivity of our
charged BINOL phosphoric acid derivative 2b with noncharged
analogues, three different structures (1a−1c) were used. These
species included the unsubstituted BINOL phosphoric acid (1a),
a sterically similar structure with triphenylsilyl substituents at the
3- and 3′-positions (1b), and 3,3′-bis(2,4,6-triisopropylphenyl)-
BINOL phosphoric acid (1c). The last of these derivatives is
known as TRIP and is one of the most successful phosphoric acid
catalysts that has been reported.25 7-Methylindole (3h) and
2,2,2-trifluoroacetophenone (4a) in CH2Cl2 at −30 °C with a
1.5 mol % catalyst loading were used as the test reaction, and
the results after 65 h are given in Table 4. The least sterically
a
Table 3. Reaction Scope
b
c
entry
substrate
adduct
time (h)
yield (%)
ee (%)
1
3b + 4a
3c + 4a
3d + 4a
3e + 4a
3f + 4a
3g + 4a
3h + 4a
3i + 4a
3a + 4b
3a + 4c
3a + 4d
3a + 4e
3a + 4f
3a + 4g
3a + 4h
3a + 4i
3a + 4j
3a + 4k
3a + 4l
3h + 4a
3a + 4b
3h + 4a
3h + 4a
5b
5c
5d
5e
5f
26
26
20
20
28
28
14
20
20
20
20
28
20
20
20
20
20
24
28
18
42
65
70
85
82
84
83
87
41
92
85
92
88
94
86
90
89
91
85
88
28
36
96
91
93
96
85
87
79
48
87
88
91
88
90
89
89
85
88
85
86
86
88
64
84
95
93
96
96
2
a
3
Table 4. Catalyst Comparisons
4
b
c
entry
catalyst
yield (%)
ee (%)
5
1
2
3
4
1a
1b
1c
2b
1
1
6
5g
5h
5i
9
93
97
96
7
43
93
8
9
5j
a
10
11
12
13
14
15
16
17
18
19
5k
5l
Experiments performed with 0.140 mmol of 7-methylindole,
0.168 mmol of 2,2,2-trifluoroacetophenone, 1.5 mol % of the desired
catalyst, and 10 mg of 3 Å MS in 0.25 mL of CH2Cl2 at −30 °C.
5m
5n
5o
5p
5q
5r
5s
5t
b
c
Isolated yield after 65 h. Determined by chiral HPLC.
hindered parent structure with hydrogen atoms at the 3 and
3′-positions (1a) was ineffective. Only a 1% yield was obtained,
and this material was essentially racemic (i.e., 1% ee, entry 1).
The catalyst 1b with two triphenylsilyl substituents only gave a
yield of 9%, but this product was formed in 93% ee (entry 2).
TRIP is more reactive, but only resulted in a 43% yield. The
observed ee of 97%, however, is quite impressive (entry 3). Our
charged phosphonium ion tagged phosphoric acid catalyst 2b
proved to be superior in that an excellent yield of 93% was
obtained along with a 96% ee (entry 4). Assuming second-order
kinetics for these processes,26 one can estimate that 2b is more
reactive than 1a−1c by factors of 690, 70, and 10, respectively,
but still is as selective as the best of these noncharged species.
To investigate the role of hydrogen bonding between the
phosphoryl oxygen and indole substrate, a reaction lacking this
interaction was performed with 1-methylindole and 2,2,2-
trifluoroacetophenone at 0 °C (eq 3). The yield after 18 h was
d
20
5h
5j
d
21
de
,
22
23
5h
5h
f
a
Experiments performed with 0.140 mmol of indole, 0.168 mmol of
2,2,2-trifluoroacetophenone, 5 mol % of 2b and 5 mg of 3 Å MS in
0.25 mL of CH2Cl2 at 0 °C. Isolated yield. Determined by chiral
HPLC. Reaction run at −30 °C with 10 mg of 3 Å MS. 1.5 mol %
catalyst. Larger scale reaction with 0.840 mmol of 7-methylindole,
1.008 mmol of 2,2,2-trifluoroacetophenone, 1.5 mol % of 2b and
30 mg of 3 Å MS in 1.50 mL of CH2Cl2 at −30 °C; 247.5 mg of
product was produced.
b
c
d
e
f
groups (F, Cl, Br, and CF3) to the para position of 2,2,2-
trifluoroacetophenone gave slight improvements of 1−3% in the
ee (entries 9−11 and 13), whereas an electron-donating methyl
group at the same location lowered the enantioselectivity by an
equivalent amount (entry 12). Halogen substituents at one or
both of the meta positions in 2,2,2-trifluoroacetophenone had
little, if any, impact on the reaction outcome (entries 14−17).
Perfluoroacetophenone (4k) was found to give both a low yield
71%, and the product was formed with a 24% ee. These results
show that the selectivity is significantly lower and that the yield is
C
DOI: 10.1021/acs.orglett.8b00900
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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These findings indicate that indole reacts 2.6 times faster than
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In summary, we demonstrated a successful application of chiral
electrostatically enhanced phosphoric acids to a Friedel−Crafts
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b00900.
(12) Liu, X. D.; Wang, Y.; Ma, H. Y.; Xing, C. H.; Yuan, Y.; Lu, L.
Tetrahedron 2017, 73, 2283−2289.
Experiment procedures, NMR spectra, and HPLC traces
(PDF)
(13) (a) Kasztelan, A.; Biedrzycki, M.; Kwiatkowski, P. Adv. Synth.
Catal. 2016, 358, 2962−2969. (b) Biedrzycki, M.; Kasztelan, A.;
Kwiatkowski, P. ChemCatChem 2017, 9, 2453−2456.
(14) Bandini, M.; Sinisi, R. Org. Lett. 2009, 11, 2093−2096.
(15) Wang, Y.; Yuan, Y.; Xing, C. H.; Lu, L. Tetrahedron Lett. 2014, 55,
1045−1048.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: kass@umn.edu.
ORCID
(16) (a) Nie, J.; Zhang, G. W.; Wang, L.; Fu, A. P.; Zheng, Y.; Ma, J. A.
Chem. Commun. 2009, 2356−2358. (b) Wang, T.; Zhang, G.-W.; Teng,
Y.; Nie, J.; Zheng, Y.; Ma, J.-A. Adv. Synth. Catal. 2010, 352, 2773−2777.
(17) Kaupmees, K.; Tolstoluzhsky, N.; Raja, S.; Rueping, M.; Leito, I.
Angew. Chem., Int. Ed. 2013, 52, 11569−11572.
Steven R. Kass: 0000-0001-7007-9322
Notes
(18) Akiyama, T.; Morita, H.; Itoh, J.; Fuchibe, K. Org. Lett. 2005, 7,
2583−2585.
The authors declare no competing financial interest.
(19) (a) Akiyama, T.; Itoh, J.; Yokota, K.; Fuchibe, K. Angew. Chem.,
Int. Ed. 2004, 43, 1566−1568. (b) Zhou, F. T.; Yamamoto, H. Angew.
Chem., Int. Ed. 2016, 55, 8970−8974.
ACKNOWLEDGMENTS
■
Generous support from the National Science Foundation (CHE-
1665392) and the Petroleum Research Fund (55631-ND4) as
administered by the American Chemical Society is gratefully
acknowledged.
(20) Samet, M.; Buhle, J.; Zhou, Y. W.; Kass, S. R. J. Am. Chem. Soc.
2015, 137, 4678−4680.
(21) Fan, Y.; Kass, S. R. Org. Lett. 2016, 18, 188−191.
(22) Ma, J.; Kass, S. R. Org. Lett. 2016, 18, 5812−5815.
(23) Fan, Y.; Kass, S. R. J. Org. Chem. 2017, 82, 13288−13296.
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Pericas, M. A. ACS Catal. 2016, 6, 7647−7651. (b) Reid, J. P.;
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