Copper-mediated 1,2-bis(trifluoromethylation) of arynes

Article abstract of DOI:10.1039/c8sc03754j

We herein describe an unprecedented 1,2-bis(trifluoromethylation) of arynes with [CuCF₃] in the presence of an oxidant DDQ. The method allows the rapid construction of a new class of 1,2-bis(trifluoromethyl)arenes in one-step from aryne precurs

Full text of DOI:10.1039/c8sc03754j

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Copper-Mediated 1,2-Bis(trifluoromethylation) of Arynes
Xinkan Yang and Gavin Chit Tsui*
Received 00th January 20xx,
Accepted 00th January 20xx
We herein describe an unprecedented 1,2-bis(trifluoromethylation) of arynes with [CuCF3] in the presence of an oxidant
DDQ. The method allows the rapid construction of a new class of 1,2-bis(trifluoromethyl)arenes in one-step from aryne
precursors under mild conditions. Its synthetic utility has been demonstrated in the preparation of bis(trifluoromethylated)
molecules with potential pharmaceutical and material applications. Mechanistic studies indicated the presence of an o-
trifluoromethyl aryl radical intermediate via CF3 group transfer from [CuCF3] to the aryne.
DOI: 10.1039/x0xx00000x
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arynes whereas we reported
a
copper-mediated
Introduction
trifluoromethylation-allylation protocol,4 both methods
successfully realized vicinal difunctionalization of arenes
involving C-CF3 bond and C-I/C-C bond construction in one-step
from aryne intermediates. On the other hand, to install two
identical vicinal functional groups onto arenes, metal-catalyzed
aryne insertion to a σ-bond has been utilized. Under Pd, Cu or
Pt catalysis, arynes/hetarynes can insert into heteroatom-
heteroatom bonds such as Si-Si, Sn-Sn and B-B bonds to provide
1,2-bis(functionalized) arenes with high synthetic utility
(Scheme 1b).5 However, such method is not amenable to the
preparation of bis-CF3 products due to the fact that an infeasible
aryne insertion to a "CF3-CF3" bond would be required.6 We
herein describe a new approach for 1,2-bis(functionalization) of
arenes by reacting the aryne intermediate with an
organometallic reagent twice, in this case [CuCF3], thereby
achieving an unpresented 1,2-bis(trifluoromethylation) of
arynes (Scheme 1c).
Arynes are versatile reactive intermediate for the rapid
synthesis of multifunctionalized arenes.1 In particular,
multicomponent reactions of arynes in the presence of a
transition metal can provide easy access to diverse 1,2-
difunctionalized arenes.1a-b The two new bonds are formed in
one step on the aryne intermediate to install orthogonal
functional groups adjacent to each other (Scheme 1a). This
strategy has only been recently applied by Hu's group and us to
the synthesis of highly functionalized trifluoromethylated
arenes, which are important building blocks in widely used
pharmaceuticals and agrochemicals.2 Hu and co-workers
developed a silver-mediated trifluoromethylation-iodination3 of
Results and discussion
During our investigation of the trifluoromethylation-allylation
of aryne precursor 1a using the fluoroform-derived [CuCF3],4 we
observed that the reaction produced an unexpected 1,2-
bis(trifluoromethyl)arene product 2a (30% yield) when open to
air without the electrophile allylbromide (Table 1, entry 1). The
[CuCF3] reagent was prepared from CuCl, t-BuOK and CF3H as a
solution in DMF according to Grushin's procedure,7,8 and
stabilized with Et3N·3HF. The oxidative condition was crucial for
the formation of 2a (Table 1, entry 2). Screening of various
oxidants revealed that DDQ (2,3-dichloro-5,6-dicyano-1,4-
benzoquinone) was capable to increase the yield (58%) (Table 1,
entries 3-7).
A major side product was the mono-
Scheme 1. Transition Metal-Catalyzed/-Mediated 1,2-Difunctionalization of Arynes
and Applications in Trifluoromethylation Reactions.
trifluoromethylated arene (regioisomeric mixture), its
formation could be reduced by using DMSO as a co-solvent (1:1
ratio), thus further improving the yield (77%) (Table 1, entry 8).
Reaction at room temperature was equally effective (Table 1,
entry 9). Different DMF/DMSO ratios were tested showing that
*Department of Chemistry, The Chinese University of Hong Kong, Shatin, New
Territories, Hong Kong SAR. E-mail: gctsui@cuhk.edu.hk.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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fluoride was needed to generate arynes from
1
Table 1. Optimization Studies for 1,2-Bis(trifluoromethylation) of Aryne Precursor
1a[a]
addition of Et3N·3HF as a stabilizer to the [DCOuCI: F103].1r0e3a9g/Ce8nStC(0in37s5i4tuJ
generating KF with t-BuOK).7 Moderate to good yields were
obtained for symmetrical and unsymmetrical 1,2-
bis(trifluoromethyl)arenes (2a-g). Functional groups such as
acetal (2d), chloro (2l), bromo (2p, 2q), allyl (2n) and even silyl
(2m) were tolerated. Hydroxy group (2r), on the other hand,
was not compatible. Substituents adjacent to the reaction
centre (2o, 2p) generally caused lower yields than remote
substituents (2j, 2h). Oxabicyclic compounds containing the bis-
CF3 moiety were also synthesized in reasonable yields (2s-u),
they could serve as useful substrates for asymmetric ring-
opening reactions11 towards drug analogue preparation. The
bis-CF3 polyaromatic compound (2v) was also synthesized
which may exhibit interesting material properties; the lower
yield was mainly due to the solubility issue of the precursor. In
some cases, larger amounts of DMSO were required to inhibit
the formation of mono-CF3 side products (2e, 2f, 2k, 2t). Several
products were very volatile, and their NMR yields were
entry
1[c]
2
oxidant
air
solvent
DMF
temp (oC)
yield (%)[b]
50
50
50
50
50
50
50
50
rt
30
0
none
DMF
3
BQ
DMF
4
4
Cu(OAc)2
AgOAc
PhI(OAc)2
DDQ
DMF
7
5
DMF
26
26
58
77
78
62
77
6
DMF
7
DMF
8[d]
9[d]
10[e]
11[f]
DDQ
DMF/DMSO
DMF/DMSO
DMF/DMSO
DMF/DMSO
DDQ
DDQ
rt
DDQ
rt
[a] Unless specified otherwise, reactions were carried out using 1a (0.1 mmol),
[CuCF3] (0.4 mmol in 1.0 mL DMF), oxidant (0.2 mmol) and DMF (1.0 mL), under
argon. [b] Determined by 19F NMR analysis using benzotrifluoride as the
internal standard. [c] Reaction was open to air. [d] DMF:DMSO = 1.0:1.0 mL. [e]
DMF:DMSO = 1.0:0.5 mL. [f] DMF:DMSO = 1.0:2.0 mL.
a larger amount of DMSO was generally beneficial for the
reaction, and at 1:2 ratio (DMF/DMSO) the mono-CF3 side
products could be completely supressed (Table 1, entries 10-11).
Other reaction parameters such as additives, co-solvents and
reagent equivalents were screened with no further
improvement.8 The reaction design requires at least two
equivalents of [CuCF3] due to its role as a "carrier of CF3", we
found that four equivalents were necessary to provide the
highest yield. However, this stable reagent can be prepared
from inexpensive copper and CF3 sources at scale7d (fluoroform
is an industrial byproduct and commercially available at
<$0.10/mol),9 therefore justifying its use in excess.
The scope of the reaction was subsequently investigated
using various 2-(trimethylsilyl)aryl triflates
1 as the aryne
precursors (Scheme 2). Although many methods are available
for generating aryne intermediates,1f the 2-(trimethylsilyl)aryl
triflates 1
, developed by Kobayashi and co-workers in 1983,10
remain the most convenient and widely used precursors owing
to the mild conditions (usually when exposed to fluoride) and
broad synthetic applications.1a-b They are either commercially
available or can be prepared in a few steps at scale according to
known procedures. In our reaction, conveniently no extra
Scheme 2. Scope of 1,2-Bis(trifluoromethyl)arenes 2. [a] General conditions: 1 (0.4
mmol), DDQ (0.8 mmol), [CuCF3] (1.6 mmol in 4.0 mL DMF), DMSO (8.0 mL), under
argon. Isolated yields. [b] DMF:DMSO = 4.0:4.0 mL. [c] DMF:DMSO = 4.0:12.0 mL.
[d] Yield determined by 19F NMR analysis using benzotrifluoride as the internal
standard.
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Scheme 3. 1,2-Bis(trifluoromethylation) of Estrone Derivative
determined (2j, 2l, 2o). Pharmaceutical compounds containing
two trifluoromethyl groups on arenes are well-precedented,12
however, the class of 1,2-bis(trifluoromethyl)arenes has been
much less represented possibly due to the difficulty in their
synthesis. Traditional methods often involved hazardous
conditions (using SF4 and HF) and lengthy procedures from
trifluoromethylated building blocks.13 Our approach is more
operationally simple and general for synthesizing structurally
diverse 1,2-bis(trifluoromethyl)arenes. Furthermore, aryne
precursor 1w derived from estrone6 was successfully converted
into the 1,2-bis(trifluoromethylated) derivative 2w (Scheme 3)
demonstrating relevance of the current method to the
modification of biologically active compounds.
Scheme 5. Mechanistic Studies.
(Scheme 4b). The intramolecular reaction afforded compound
5
from 2p (Scheme 4c), which is a trifluoromethylated analogue
of 9-fluorenone that has been recently shown as an effective
metal-free photocatalyst.17 Finally, Sonogashira cross-coupling
of 2q with terminal alkyne afforded compound 6, which could
provide access to bis(trifluoromethylated) benzofurans via
cyclization of the -OMe group.18
The above 1,2-bis(trifluoromethyl)arenes
2 are useful
intermediates for further transformations and their synthetic
applications were explored (Scheme 4). Deoxygenation of
compound 2u14 directly led to the bis(trifluoromethylated)
9,10-diphenylanthracene derivative 3. UV-Vis absorption and
cyclic voltammetry (CV) studies showed decreased HOMO and
LUMO energy levels compared with the parent compound
(Scheme 4a).8 This "tuning" effect by the bis-CF3 groups could
have potential applications in the development of organic
semiconductors.15 Under protolytic defluorination protocols,16
To gain more insights into the reaction mechanism, additional
studies were performed (Scheme 5).8 In the 19F NMR
experiment, we observed that the peak of the initial [CuICF3]7a
quickly disappeared after adding DDQ and stirred for 5 min at rt,
indicating a facile oxidation of [CuICF3] presumably to [CuIICF3]7c
trifluoromethylated ketone
4 could be obtained from 2e
by DDQ (Scheme 5a). Subjecting mono-CF3 compound
7 to the
standard conditions did not give any bis-CF3 products 2a or 2c
,
thus ruling out the C-H trifluoromethylation pathway (Scheme
5b).19 Adding a known radical scavenger TEMPO (2,2,6,6-
tetramethyl-1-piperidinyloxy)20a to the reaction of 1d under
standard conditions dramatically decreased the yield of 2d
(Scheme 5c, 7% vs. 78% cf. Scheme 2). Significant amounts of
Scheme 6. Proposed Mechanism.
Scheme 4. Further Transformations of 1,2-Bis(trifluoromethyl)arenes 2.
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mono-CF3 product (29%) and CF3-containing dimer product
(12%) were detected, however, only trace TEMPO-CF3 adduct
was observed. Also, styrene derivatives20b-c were added to the
standard conditions with 1d to trap any CF3-adducts, but only
trace amounts were detected. These studies suggested that CF3
radicals were not likely to be present in the reaction. On the
other hand, a radical clock experiment20d using substrate 1x
Acknowledgements
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DOI: 10.1039/C8SC03754J
This work was supported by the Research Grants Council of
Hong Kong (CUHK 24301217) and the Chinese University of
Hong Kong (the Faculty Strategic Fund for Research from the
Faculty of Science and the Direct Grant for Research 4053276).
Professor Qian Miao is also thanked for the use of their
equipment.
gave both bis-CF3 product 2x and cyclized product 8, therefore
hinting the intermediacy of a transient aryl radical (Scheme 5d).
Based on the above studies and literature examples, we
propose the following mechanism for the 1,2-
bis(trifluoromethylation) of arynes (Scheme 6). The initial
fluoroform-derived [CuICF3] is quickly oxidized by DDQ to
[CuIICF3].7c The [CuIICF3] is capable of transferring a CF3 group to
Notes and references
1
a) For reviews on aryne chemistry, see: (a) R. A. Dhokale and
S. B. Mhaske, Synthesis, 2018, 50, 1; (b) M. Feng and X. Jiang,
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Winkler and W. Sander, Angew. Chem. Int. Ed., 2003, 42, 502.
For reviews on the synthesis and applications of
trifluoromethylated arenes, see: (a) C. Alonso, E. Martínez de
Marigorta, G. Rubiales and F. Palacios, Chem. Rev., 2015, 115,
1847; (b) T. Liu and Q. Shen, Eur. J. Org. Chem., 2012, 2012,
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2011, 111, 4475.
aryne
A , supported by the
resulting in an aryl radical species B 21
radical clock experiment (cf. Scheme 5d) and our own
observation of CF3 group transfer to alkenes with [CuIICF3] for
generating alkyl radicals.22 Intermediate
B
reacts with a second
equivalent of [CuIICF3] presumably leading to a CuIII-CF3 species
C 20b
Final reductive elimination affords the 1,2-
bis(trifluoromethyl)arene product . Related reactions of aryl
radicals and [CuCF3] to form aryl-CF3 bonds have been
reported.23 There also exists the possibility that intermediate
may arise via carbocupration24 processes with [CuCF3] under
oxidative conditions. For instance, aryne may undergo
carbocupration with
[Cu(CF3)n]25 species to give
Alternatively, an arylcopper intermediate may be formed first,
which then reacts with another molecule of [CuIICF3] leading to
. It is difficult to pinpoint the exact pathway at the moment
.
2
2
3
C
A
a
C.
D
(a) Y. Zeng, L. Zhang, Y. Zhao, C. Ni, J. Zhao and J. Hu, J. Am.
Chem. Soc., 2013, 135, 2955; (b) Y. Zeng and J. Hu, Chem. Eur.
J., 2014, 20, 6866; (c) Y. Zeng and J. Hu, Synthesis, 2016, 48,
2137.
C
due to the complicated nature of the fluoroform-derived [CuCF3]
reagent, especially in oxidized forms. A major side reaction was
4
5
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E via protodemetallation of
C
(t-BuOH is present in the reagent and can act as a proton
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elimination.26
C thus favouring reductive
6
7
Conclusions
In conclusion, a novel 1,2-bis(trifluoromethylation) of arynes
using [CuCF3] has been developed. By employing 2-
(trimethylsilyl)aryl triflates as aryne precursors, structurally
diverse
1,2-bis(trifluoromethyl)arenes
with
potential
Ed., 2012, 51, 7767; (d) V. V. Grushin, Chemistry Today, 2014
32, 81.
,
applications in pharmaceutical and material areas can be
synthesized in one-step under mild and safe conditions. Notably
the sole source of CF3 in all of these valuable compounds is the
inexpensive industrial waste fluoroform. New mechanistic
insights will further the field of copper-mediated/-catalyzed
trifluoromethylation-functionalization of arynes, and related
studies are currently ongoing in our laboratory.
8
9
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Conflicts of interest
There are no conflicts to declare
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A novel 1,2-bis(trifluoromethylation) of arynes using [CuCF3] is described.