Copper-catalyzed fluoroalkylation of alkynes, and alkynyl & vinyl carboxylic acids with fluoroalkyl halides

Article abstract of DOI:10.1039/c7ob00641a

Copper-catalyzed fluoroalkylation of alkynes and alkynyl carboxylic acids has been achieved with high functional-group tolerance and excellent regio- and stereoselectivities. A variety of fluoroalkyl halides including ethyl bromodifluoroacetate can be employed. Additionally, an unprecedented decarboxylative fluoroalkylation of α, β-unsaturated carboxylic acids has been achieved via a radical pathway.

Full text of DOI:10.1039/c7ob00641a

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
benzene
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Copper-Catalyzed Fluoroalkylation of Alkynes, and Alkynyl & Vinyl
Carboxylic Acids with Fluoroalkyl halides
Received 00th January 20xx,
Accepted 00th January 20xx
Jing-Jing Ma and Wen-Bin Yi*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Previous work:
Cu, L5
X
N
Rf
R
H (COOH)
Zn dust
TFA
I
2000
Ramachandran
N
N
XRf
a)
Rf
R
IRf
+
R
R
X= Br, I
L5
R
Rf= CF2CO2Et, C2F5, C3F7, C4F9, C5F11,C6F13, C8F17, C10F21
Ru or Ir
hv
I
2012
Stephenson
52 examples
E/Z up tp 99:1
Cu, L5
R
Rf
R
+ IRf
b)
COOH
Rf
R
I
f
R
FeBr2
Cs2CO3
Copper-catalyzed fluoroalkylation of alkynes and alkynyl
carboxylic acids has been achieved with high functional-group
tolerance and excellent regio- and stereoselectivity. A variety of
fluoroalkyl halides including ethyl bromodifluoroacetate can be
employed. Additionally, an unprecedented decarboxylation
I
2014
Xu
Rf
Rf
+ IRf
+ IRf
c)
d)
R
R
R
R
I
2016
Vincent
CuLn/BP
Scheme 1. Copper-catalyzed or mediated strategies for the formation of vinyl-
fluoroalkylation of α, β-unsaturated carboxylic acids process has fluoroalkyl groups.
been achieved via a radical pathway.
halides across unsaturated double bonds.6 In contrast, less
Remarkable success witnessed in recent organofluorine
chemistry is obviously related to the widespread applications
of fluoroalkyl-containing compounds, such as pharmaceuticals,
agrochemicals, liquid crystals, dyes, polymers and functional
materials.1 In this field, a host of elegant approaches have
been developed for highly efficient and selective construction
progress has been made in the ATRA of fluoroalkyl halides
onto alkynes,7 mainly because of the difficulty in controlling
the regio- and stereoselectivity. The addition of fluoroalkyl
halides to alkynes has been achieved using radical initiators
such as AIBN,8a NaHCO3/Na2S2O4,8b Et3B,8c or light.8d However,
these reactions had a very limited substrate scope. In the past
decades, significant progress has been made in metal-
catalyzed fluoroalkylation of alkynes using iodofluoro-alkanes
(Scheme 1). In 2000, a zinc-catalyzed radical addition of
terminal alkynes using perfluoroalkyl iodides was reported by
Ramachandran (Scheme 1a).7d Stephenson et al. developed a
visible light-mediated ATRA of iodofluoroalkanes onto alkynes
using Ru or Ir catalysts in 2012 (Scheme 1b).7e Xu’ s group
reported a 1,2-addition of perfluoroalkyl iodides to alkynes in
the presence of iron catalyst (Scheme 1c).7g Very recently,
Vincent and his coworkers found a light-promoted radical
addition of alkynes with the help of copper/benzophenone
(BP) (Scheme 1d).7j In terms of substrate scope, only activated
iodides were liable to the reaction conditions and long
perfluoroalkyl chains (such as C8F17 and C10F21) were limited. In
addition, some protocols were restricted to poor regio- and
of
C
vinyl-CF3 bonds,2 whereas the analogous di- and
perfluoroalkylation for the formation of Cvinyl-fluoroalkyl bonds
have not been well developed.3 More specifically, drug
molecules with difluoromethyl group (CF2) often show
desirable changes to the physical and biological properties
since it is generally functionalize as an isopolar and isosteric
substitute for oxygen atom.4 Additionally, the introduction of
perfluoroalkyl motifs (Rf) has been involved in the
manufacturing processes of blockbuster drugs and highly
effective agrochemicals.5 Hence, more efficient and general
methodologies for the synthesis of di- and perfluoroalkylated
organic molecules are of great interest.
Recently, atom-transfer radical addition (ATRA) has stood
out as appealing method for the introduction of fluoroalkyl
stereoselectivities
.
On the basis of these studies, we herein describe a high
regio- and stereoselectivity fluoroalkylhalogenation of alkynes,
as well as alkynyl carboxylic acids with ethyl bromodifluoro-
acetate (BrCF2CO2Et, 2a), ethyl iododifluoroacetate (ICF2CO2Et,
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2b) and iodoperfluoroalkanes, which requires only catalytic iododifluoroacetate (2b), leading to 85% yield of A17 in the
amount of copper and inexpensive ligand.9 Our method allows presence of CuI as catalyst. Phenylacetylene derivatives with
the introduction of both fluoroalkyl and halogens functional methyl (A18), butyl (A19), methoxyl (A20), halogens (A21
-A23)
groups into alkynes without generating a byproduct from the and trifluoromethyl (A24) substituents were well-suited
homologous fluoroalkylative sources. Therefore, it gives substrates for the corresponding fluoroalkyl products. Notably,
prominence to higher atom economical. As a source of it was no detrimental effect as the introduction of meta-amino
difluorinated moieties, CF2CO2Et specie is readily available and substitutes (A26), as well as other functional group (A25). It
can be further modified into other fluoroalkyl-containing was worth mentioning that heteroaryl acetylenes (pyridine
groups. During the preparation of our manuscript, the and thiophene) were also functionalized in decent yields (A27
-
decarboxylative difluoroalkylation of α, β-unsaturated A28). Moreover, inactive aliphatic acetylenes were suitable
carboxylic acids and alkynyl carboxylic acids were developed substrates giving the corresponding outcomes, albeit in lower
by Wang’s group.10 To enrich the diversity of Cvinyl-fluoroalkyl yields (A29
-A31). The incorporation of perfluoroalkyl motifs
species, we extented a decarboxylative perfluoroalkylation of (Rf) into organic compounds endow them with large potential
α, β-unsaturated carboxylic acids with corresponding for applications in several research fields. In addition,
fluoroalkyl halides.
perfluoroalkyl motifs are more suitable for large-scale
synthesis compared with other small fluorous species. Not only
C2F5-C5F11I, but C6F13I, C8F17I, C10F21I could be efficiently
transformed into desired outcomes in the presence of both
Table 1. Scope of the addition reaction of XRf to alkynes. a
.
aryl- and heteroarylacetylenes (A32-A38). The addition is E
selective10b,11 with an E/Z ratio up to 99:1.
In recent years, alkynyl carboxylic acids have attracted great
attention in the synthetic community as terminal acetylene
equivalents due to their intrinsic properties, such as no
unpleasant smell, solids, easy to handle and store.12 To test
whether our transformation would be applicable to this class
of substrates, phenylpropiolic acid was submitted to the
reaction conditions. To our delight, A1 was obtained in 78%
yield (Table 2). The scope of our transformation mirrored our
previous results, 2a
BrC8F17 could be applied to this decarboxylative addition
reaction. variety of functional groups were covered,
including methyl (A2 and A18), methoxyl (A5 and A20), halos
A7 A9 A21 A23).
, 2b, iodoperfluoroalkyl reagents and
A
(
-
,
-
Table 2. Scope of the decarboxylative addition of XRf to alkynyl carboxylic acids. a
a Method I: 1 (0.25 mmol), 2 (1.5 equiv.), CuCl (15 mol%), L5 (30 mol%), MeCN
(1.0 mL) under argon atmosphere for 24 h, Isolated yield. Method II: 1 (0.25
mmol), 2 (1.5 equiv.), CuI (15 mol%), L5 (30 mol%), MeCN (1.0 mL) under argon
atmosphere for 24 h, Isolated yield. Q = CF2CO2Et.
With the optimal conditions in hand (see the Supporting
Information Table S1-S3), we investigated the scope of the
reaction (Table 1). First, a broad scope of phenylacetylene
derivatives with 2a was evaluated. The reaction proceeded
smoothly with aromatic rings bearing a variety of functinonal
groups, including methyl (A2), halogens (A7-A9), cyano (A10),
ester (A11) or CF3 (A12). The substitution patterns on the
a Unless otherwise stated, all reactions were carried out with 3 (0.25 mmol), 2
(1.5 equiv.), CuCl (15 mol%), L5 (30 mol%), MeCN (1.0 mL) under argon
atmosphere for 24 h, Isolated yield. b Using CuI instead of CuCl. Q = CF2CO2Et.
aromatic ring have minor influence on the outcome of the
reaction since ortho-and meta-substituted phenylacetylenes
(Me and F, A13-A16) were obtained in good yields. To further
demonstrate the synthetic utility of such an approach,
reactions have been performed on 10 mmol scale and
Carboxylic acids are pervasive compounds in organic
chemistry, which are commercially available in
a large
corresponding compounds (A1, A3 and A4) were obtained in
variety.13 This copper-catalyzed method was then extended to
82%, 72% and 68%, respectively. An important extension of
this protocol was carried out by using ethyl
perfluoroalkylation of α, β-unsaturated carboxylic acids.
2 | J. Name., 2012, 00, 1-3
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Consistent with our expectation, the screening protocol in
Finally, to rationalize the reaction pathway, two control
Table 1 also worked for α, β-unsaturated carboxylic acids. experiments were performed (Scheme 3). The reaction was
After screening, the desired product B1 was isolated in 39% completely quenched in the presence of 2,2,6,6-tetramethyl-1-
yield by using 2.0 equiv. of pentafluoroethyl iodide. Nitro (B2), piperidinyloxy (TEMPO) and 1,4-benzoquinone, while trapped
methoxyl (B3), trifluoromethyl (B4), halogens such as bromine products
5 and 6
were obtained as expected. [16] These results
and chlorine (B5 and B6), hydroxyl (B7) groups were all indicated that a difluoroalkyl radical might be involved in this
compatible with various iodoperfluoro-alkanes (Table 3). process.
Gratifyingly, acids with heteroaryl ring including pyrrole,
While the exact reaction mechanism is currently unclear, on
thiophere and furan, were also viable substrates in the current the basis of the above experiments and relevant results
reaction, giving Cvinyl-Rf compounds (B8-B10) in 51%, 32% and reported, a plausible mechanism was proposed in Scheme 4.
36%, respectively. According to coupling constants of The catalytic cycle was initiated by the situ generated copper
hydrogen atoms on vinyl group, the decarboxylative (I) species I, which could be oxidized by fluoroalkyl halides (2)
perfluoroalkylation was inclined to E selectivity. [11b]
through a single-electron transfer to afford the fluoroalkyl
radical and copper (II) species II (Scheme 4a).17 Subsequently,
the fluoroalkyl radical attacked the terminal carbon atom of
Table 3. Scope of the decarboxylation of IRf to α, β-unsaturated carboxylic acids. a
alkynes to afford the carbon radical III 17d
XRf to afford the desired product and regenerated the
,
III was quenched by
A
fluoroalkyl radical (Scheme 4b).7g The decarboxylative fluoro-
alkylation mechanism of alkynyl carboxylic acids was slight
different, the ligand abstracted an acidic proton from
carboxylic acids, generating the corresponding intermediate
V
under the removal of carbon dioxide.10b The following radical
addition promoted the formation of intermediate VII, and the
final product was obtained with the help of IV (Scheme 4c).
The mechanism of α, β-unsaturated carboxylic acids was
similar to (c), due to the unique properties of IX, the product
B
was formed under addition/elimination process (Scheme
4d).10a,17f
a Unless otherwise stated, all reactions were carried out with 4 (0.25 mmol), IRf
(2.0 equiv.), CuI (15 mol%), L5 (30 mol%), MeCN (1.0 mL) under argon
atmosphere for 24 h, Isolated yield.
Next, the synthetic usefulness of CF2CO2Et-containing
compounds was investigated. It is well-known difluoro-
methylated alkynes are important intermediates for the
preparation of valuable fluorinated compounds and have
become a type of useful building blocks.14 As such, it is of great
importance to explore simple and efficient methods to obtain
CF2 groups. To this end, Csp-CF2H was obtained and the results
are shown in Scheme 2.15
Scheme 4. Proposed reaction mechanism.
Scheme 2. Transformation of CF2CO2Et species.
In summary, a facile, broad scope and high regio- and
stereoselectivity copper-catalyzed fluoroalkylation method is
reported. This methodology provides a streamlined access to
di- and perfluoroalkylated organic compounds starting from
alkynes or alkynyl carboxylic acids. Furthermore,
a
decarboxylation perfluoroalkylative of α, β-unsaturated
carboxylic acids process has been achieved in a similar radical
process, which can be used to sterospecifically prepare
perfluoroalkyl-substituted E-alkenes. The synthetic utility of
CF2CO2Et-containing compounds was investigated for the
transformation to Csp-CF2H species.
Scheme 3. Trapping experiments.
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Castet, S. A. Denisov, N. D. McClenaghan, D. Lastédécouères
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Acknowledgements
8
9
We gratefully acknowledge the National Natural Science Foundation
of China (21476116), the Fundamental Research Funds for the
Central Universities (30916011102), Natural Science Foundation of
Jiangsu (BK20141394) and Qing Lan Project for financial support.
The price of 99% CuI and L5 from Sigama: $1.04/g for CuI and
$0.61/mL for L5.
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