Direct Photoassisted α-Trifluoromethylation of Aromatic Ketones with Trifluoroacetic Anhydride (TFAA)

Article abstract of DOI:10.1002/adsc.201801305

Direct α-Trifluoromethylation of acetophenone derivatives was achieved by using trifluoroacetic anhydride (TFAA) as the trifluoromethyl source and pyridine-N-oxide (Py?O) as activator and oxidant under visible light irradiation and tris-(2,2′-bipyridine)ruthenium(II) hexafluorophosphate (Ru(bpy)₃(PF₆)₂) as the photocatalyst. Different acetophenone derivatives could be converted to the corresponding α-CF₃ derivatives with high selectivity. Extensive mechanistic investigation revealed the formation of vinyl trifluoroacetate as the key intermediate for this transformation. (Figure presented.).

Full text of DOI:10.1002/adsc.201801305

Title: Direct Photoassisted α-Trifluoromethylation of Aromatic Ketones
with Trifluoroacetic Anhydride (TFAA)
Authors: Somnath Das, A. Stephen K. Hashmi, and Thomas Schaub
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201801305
Link to VoR: http://dx.doi.org/10.1002/adsc.201801305

10.1002/adsc.201801305
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Direct Photoassisted α-Trifluoromethylation of Aromatic
Ketones with Trifluoroacetic Anhydride (TFAA)
Somnath Das,a A. Stephen K. Hashmib and Thomas Schauba,c*
a
Catalysis Research Laboratory (CaRLa), Im Neuenheimer Feld 584, 69120 Heidelberg, Germany.
Organisch-Chemisches Institut, Heidelberg University, Im Neuenheimer Feld 270, 69120 Heidelberg,
b
Germany.
c
BASF SE, Synthesis and Homogeneous Catalysis,67056 Ludwigshafen, Germany
Fax: (+ 49)-621-60-664-8957; e-mail: thomas.schaub@basf.com
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. Direct α-Trifluoromethylation of acetophenone synthesis of such molecules have been reported,
derivatives was achieved by using trifluoroacetic anhydride
(TFAA) as the trifluoromethyl source and pyridine-N-oxide
(Py-O) as activator and oxidant under visible light
either directly from the ketones or from other starting
materials.[9] The most common strategies for the
synthesis
of
α-trifluoromethylated
carbonyl
irradiation
and
tris-(2,2ˊ-bipyridine)ruthenium(II)
compounds from carbonyl molecules usually are
based on the two steps shown in Scheme 1a. Enolates
derived from the ketones are reacted with either a
radical CF3 or an electrophilic CF3 source to provide
the α-CF3 products.[10][11]
hexafluorophosphate (Ru(bpy)3(PF6)2) as the photocatalyst.
Different acetophenone derivatives could be converted to
the corresponding α-CF3 derivatives with high selectivity.
Extensive mechanistic investigation revealed the formation
of vinyl trifluoroacetate as the key intermediate for this
transformation.
Keywords: Trifluoromethylation; Fluorinated organic
compounds; Photoredox catalysis; Aromatic ketone;
Trifluoroacetic anhydride.
Fluorinated organic compounds are particularly
important molecules because of their widespread
application in pharmaceuticals and agrochemicals. In
fact, 20-30% of modern pharmaceuticals and
agrochemicals contain at least one fluorine atom.[1]
One approach of incorporating fluorine atom in
organic molecules is via the installation of
trifluoromethyl (CF3) group which can have a
profound influence on the pharamacokinetic
properties of the molecules.[2] Many elegant
approaches for the trifluoromethylation of organic
molecules[3] using electrophilic,[4] nucleophilic[5] and
radical[6] sources are now available. Despite this,
most of the reagents available for this purpose are
expensive and a large-scale application would be
limited due to availability and costs. Thus, the
development of new trifluoromethylation methods
applying cheap and abundant CF3 sources will be
very useful in synthetic chemistry.[7]
Scheme 1. a) Trifluoromethylation of ketones using radical
or electrophilic CF3 sources as reported in the literature; b)
photoredox C-H trifluoromethylation of aromatics using
TFAA as CF3 source as reported by Stephenson’s group; c)
our work on the direct α-trifluoromethylation of
acetophenones via the in situ formation of vinyl
trifluoroacetate.
α-CF3 carbonyl compounds are valuable building
blocks for the synthesis of various complex CF3
containing molecules.[8] Different approaches for the
1
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10.1002/adsc.201801305
Advanced Synthesis & Catalysis
d)
Several reports on the α-C-H trifluoromethylation
as the catalyst. The reaction was carried out in the dark.
e) No photocatalyst.
of carbonyl compounds appeared in the last years.
Macmillan et al achieved an α-trifluoromethylation of
ketones via the reaction of silyl enol ether and CF3I as
the CF3 source under photoassisted conditions.[12]
Recently, Kamimura et al. have shown another
interesting protocol for the synthesis of α-CF3
ketones.[13] Vinyl triflates derived from a ketone can
be rearranged to α-CF3 carbonyl compounds in the
presence of a radical initiator, e.g. triethyl borane
(BEt3). The same rearrangement can also be
performed by photocatalysis as shown by Liu et al.[14]
Despite these advancements, most of the reported
procedures use CF3-sources which are expensive and
all these procedures are based on a two-step approach
for the trifluoromethylation of ketones.
To start these investigations, 4-fluoroacetophenone
2a was used as the model substrate and
Ru(bpy)3(PF6)2 as the photocatalyst. The use of 2 eq.
of Py-O and TFAA at room temperature did not
provide any significant conversion of the substrate 2a
after 24 h. Twice the amount of TFAA/Py-O
provided 30% conversion. Unfortunately, we
obtained a mixture of both the α-CF3 product 3a as
well as the trifluoromethylation product 5 (Table 1,
entries 1-2). Interestingly, increasing the temperature
from RT to 50 °C increased the conversion and the
selectivity towards the α-CF3 product 3a (entry 3).
Increasing the temperature further to 65 °C, provided
reasonable conversion and good selectivity to the
α-CF3 product 3a (entry 4). Changing the
photocatalyst from Ru(bpy)3(PF6)2 to Ru(bpy)3(Cl)2
under otherwise similar condition, gave a slightly
lower yield (entry 5). The conversion could be further
increased by using 5.5 eq. of TFAA and 5.0 eq. of
Py-O (entry 6). Under these conditions, no aromatic
Recently, Stephenson’s group reported
a
photoredox trifluoromethylation protocol using
trifluoroacetic anhydride (TFAA) as CF3 source and
pyridine N-oxide 1 (Py-O) as the activator for TFAA
as well as the oxidant.[15] Using Ru(bpy)3(Cl)2 as the
photocatalyst, they achieved trifluoromethylation of a
series of electron-rich aromatics and heteroaromatics
under mild conditions (Scheme 1b). TFAA is a cheap
and easily available CF3 source, thus expansion of
this protocol could be valuable for the synthesis of
other CF3-containing molecules. Inspired by their
trifluoromethylation product
5
was observed.
Addition of external bases e.g. K3PO4 or Na2CO3 did
not improve the conversion (see SI Table S1, page 3
for full optimisation and screening). Control
experiments showed no conversion to the product in
the absence of light or catalyst (entries 7-8).
report,
we
were
wondering
if
this
trifluoromethylation approach could be extended for
the synthesis of other CF3-containing building blocks.
In
particular,
the
target
was
the
α-
trifluoromethylation of acetophenone derivatives. We
envisioned that in the presence of TFAA and Py-O,
the trifluoroacetate 4 can be generated in situ, which
can react with the CF3 radical under photochemical
condition to provide the α-CF3 product 3. Herein, we
report
this
direct
trifluoromethylation
of
acetophenone derivatives using the TFAA/Py-O
system under photoassisted condition.
Table 1. Initial screening results for the α-
trifluoromethylation of 4-fluoroacetophenone.
TFAA/Py-O
Entrya)
Temp. Conversionb)
3a/5
(eq.)
Scheme 2. Substrate scope for the α-trifluoromethylation
of acetophenones under photoassisted conditions using
TFAA as CF3 source and Ru(bpy)3(PF6)2 as the
photocatalyst. Isolated yields after the chromatographic
purification.
2.2/2.0
4.4/4.0
4.4/4.0
4.4/4.0
4.4/4.0
5.5/5.0
4.4/4.0
4.4/4.0
RT
RT
-
-
1/3
3.5/1
9/1
12/1
100/-
-
1
2
3
4
30%
34 %
57%
52%
85%
0%
50 °C
65 °C
65 °C
65 °C
65 °C
65 °C
5c)
6
7d)
With the optimised reaction conditions for the α-
trifluoromethylation of 2a in hand, we next examined
the substrate scope for this method. Different
acetophenone derivatives were screened under the
optimised reaction conditions. Similar isolated yields
were obtained for the different 4-halo-substituted
acetophenones to their corresponding CF3 products.
8e)
0%
-
a)
b)
0.25 mmol of substrate was used in 1.5 mL MeCN.
Conversion refers to the total conversion of 2a to products
(3a + 5) and was determined from GC after work up with
c)
an aq. NaHCO3 solution. Ru(bpy)3(Cl)2.6H2O was used
2
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10.1002/adsc.201801305
Advanced Synthesis & Catalysis
For p-CF3 acetophenone, the conversion was only
2,6-di-tBu-4-Me pyridine as the base[16] (Scheme 3).
When 4 was treated under photoassisted conditions in
the absence of TFAA, in contrast to the vinyl triflates
as shown by Lie et al,[14]no rearrangement to 3a
could be observed. Also, no product was formed
when 4 was treated under photoassisted conditions in
the presence of Py-O. The product formation only
occurred when both reagents TFAA and Py-O were
present in the reaction mixture. This observation
could explain the requirement for an excess of both
the reagents TFAA and Py-O for achieving good
yields of the α-CF3 product.
1
44% (from H NMR) and the isolated yield of the
product 3d was 30%. Other acetophenone derivatives
gave similar yields of the products as shown in
Scheme 2; e.g. for 3-chloro and 2-Me acetophenones,
the yield obtained for the corresponding α-CF3
products 3e and 3f were 51% and 64% respectively.
While acetophenone as substrate gave the
corresponding product 3g in 60% yield,
2-acetopnaphthone provided only 33% of the
corresponding product 3h. Also, in the case of
indanone, the product 3j was obtained in very low
yield. When cyclohexanone or dimethyl malonate
were used as substrates, no corresponding α-CF3
products were obtained under these conditions.
Table 2. Optimisation study for the vinyl trifluoroacetate 4
formation in the presence of TFAA and Py-O.
Entrya) TFAA (eq.) Py-O (eq.) Temp. Conversionb)
4
2
5
5
5
5
5
1
1
1
0
2
3
4
RT
2%
34%
60%
0%
89%
97%
quant.
1
2
3
4
5
6
7
65 °C
65 °C
65 °C
65 °C
65 °C
65 °C
a)
All reactions were carried out in 0.25 mmol scale in 1.5
mL of CD3CN.
product and were measured from H NMR of the crude
reaction mixture.
b)
Conversions are given relative to the
1
Scheme 3. Synthesis of vinyl trifluoroacetate 4 and
subsequent control reactions.
To investigate the role of TFAA and Py-O for this
transformation, the initial step of the reaction in the
dark which involves the formation of vinyl
trifluoroacetate 4 was examined. As shown in Table 2,
no vinyl trifluoroacetate 4 is formed at room
temperature, which explained the low conversion
under ambient conditions. Higher temperatures
accelerate the formation of 4 and the formation of 4
was completely stopped in the absence of pyridine N-
oxide. Almost 5.0 eq. of TFAA and 4.0 eq. of Py-O
are required for the quantitative conversion of the
substrate 2a to the vinyl trifluoroacetate 4. This can
explain the necessity of the demand for the excess of
TFAA and Py-O to obtain reasonable conversions
and yields.
Interestingly, this vinyl trifluoroacetate 4 reacted
smoothly under photoassisted condition to provide
the α-CF3 product 6 at room temperature (Scheme 3a,
also see SI Figure S1, page 10 for more details). As
from the first step there was already Py-O and TFAA
present in the crude reaction mixture, this experiment
could not provide any details about the role of Py-O
and TFAA for the second step of the reaction.
Based on the observations described above and
following literature reports, the mechanism as shown
in Scheme 4 can be proposed, exemplified by 4-
fluoro acetophenone 2a as the substrate. First, vinyl
trifluoroacetate 4 is formed from the acetophenone 2a.
This step requires both reagents TFAA and Py-O as
well as higher temperature. This step is a light
independent reaction. The second step is the
photoassisted cycle, where the formed vinyl
trifluoroacetate 4 will trap the CF3 radical formed
from the pyridinium intermediate A as proposed by
Stephenson’s group.[15] Oxidation of this radical by
Ru(III) will form the carbocation D. This will be
captured by the trifluoroacetate anion to form the
trifluoroacetyl-protected acetal 6. This compound
upon work up will give the product 3a.
To examine the role of TFAA/Py-O for the formation
of the product 3a from the vinyl trifluoroacetate 4,
the vinyl trifluoroacetate 4 was prepared in situ using
3
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10.1002/adsc.201801305
Advanced Synthesis & Catalysis
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Scheme 4. Mechanistic proposal for the photoassisted
α-trifluoromethylation of acetophenones with TFAA
exemplified by 4-fluoroacetophenone as the substrate.
In summary, we achieved the direct α-
trifluoromethylation of aromatic ketones under
photoassisted conditions. This method utilizes TFAA
as CF3 source and pyridine-N-oxide as oxidant.
Because of the low cost and the availability of these
materials, we expect that this method will find
numerous applications for the synthesis of α-CF3
ketones.
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Experimental Section
In a 4 mL screw-capped vial with a rubber septum, 4-
fluoroacetophenone (60 µl, 0.5 mmol), pyridine-N-oxide
(240 mg, 2.5 mmol), Ru(bpy)3(PF6)2 (13.0 mg, 0.015
mmol) were dissolved in 2 mL dry MeCN. The vial was
closed and degassed by bubbling argon for 30 sec.
Trifluoroacetic anhydride (480 µl, 3.5 mmol) was added to
the reaction mixture and stirred for 16 h at 65 °C under
blue light irradiation (435-445 nm, 500 mA, 5-6 watts).
After this time, the reaction mixture was quenched by
adding 10 mL sat. aq. NaHCO3 solution and extracted with
EtOAc (3 X 15 mL). The organic layer was dried using
anhydrous MgSO4 and concentrated in vacuo. The pure
product was obtained after the chromatographic
purification as colorless oil. Yield: 64%; 66 mg. Rf = 0.22
[5] Selected recent reports for nucleophilic trifluoro-
methylations: a) B. Musio, E. Gala, S. V. Ley, ACS
Sustain. Chem. Eng. 2018, 6, 1489-1495; b) J. A. Pike,
J. W. Walton, Chem. Commun. 2017, 53, 9858-9861; c)
G. K. S. Prakash, F. Wang, Z. Zhang, R. Haiges, M.
Rahm, K. O. Christe, T. Mathew, G. A. Olah, Angew.
Chem. Int. Ed. 2014, 53, 11575-11578; Angew. Chem.
2014, 126, 11759-11762.
1
(5% EtOAc in pet ether); H NMR (301 MHz, CDCl3) δ
8.00 – 7.92 (m, 1H), 7.21 – 7.13 (m, 1H), 3.76 (q, J = 9.9
Hz, 1H); 13C NMR (76 MHz, CDCl3) δ 188.11 (q, J = 2.5
Hz), 166.37 (d, J = 257.1 Hz), 132.30 (dq, J = 3.2, 1.7 Hz),
131.17 (d, J = 9.6 Hz), 123.86 (q, J = 277.0 Hz), 116.20 (d,
J = 22.2 Hz), 42.14 (q, J = 28.4 Hz); 19F NMR (283 MHz,
CDCl3) δ -62.00 (s), -102.87 (s).
[6] Selected recent reports for radical trifluoromethylation:
a) S. Zhou, T. Song, H. Chen, Z. Liu, H. Shen, C. Li,
Org. Lett. 2017, 19, 698-701; b) L. Wu, F. Wang, X.
Wan, D. Wang, P. Chen, G. Liu, J. Am. Chem. Soc.
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L. Liang, R. Lu, P. Chen, Z. Lin, G. Liu, Angew. Chem.
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Acknowledgements
CaRLa (Catalysis Research Laboratory) is being co‐financed by
the Ruprechts‐Karls‐University Heidelberg (University of
Heidelberg) and BASF SE.
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