Copper-Catalyzed Decarboxylative Atom Transfer Radical Addition of Iododifluoroacetate to Alkynyl Carboxylic Acids

Article abstract of DOI:10.1021/acs.orglett.6b02216

The first example of copper-catalyzed decarboxylative atom transfer radical addition of alkynyl carboxylic acids has been developed with a readily available fluoroalkyl halide. This novel protocol has demonstrated a unique difunctionalization of nonterminal alkynes with a broad substrate scope and excellent functional-group tolerance. Mechanistic investigations revealed that the catalytic cycle was initiated by the attack of a difluoroalkyl radical to an in situ generated alkynylcopper species.

Full text of DOI:10.1021/acs.orglett.6b02216

Letter
pubs.acs.org/OrgLett
Copper-Catalyzed Decarboxylative Atom Transfer Radical Addition
of Iododifluoroacetate to Alkynyl Carboxylic Acids
Gang Li, Yi-Xuan Cao, Chen-Guang Luo, Yi-Ming Su, Yan Li, Quan Lan, and Xi-Sheng Wang*
§
,†
§,†
†
†
†
†
,†,‡
†
Department of Chemistry, University of Science and Technology of China, Hefei 230026, China
‡
Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Chinese Academy of Sciences,
Shanghai Institute of Organic Chemistry, Shanghai, 200032, China
*
S Supporting Information
ABSTRACT: The first example of copper-catalyzed decarboxylative
atom transfer radical addition of alkynyl carboxylic acids has been
developed with a readily available fluoroalkyl halide. This novel
protocol has demonstrated a unique difunctionalization of nonterminal alkynes with a broad substrate scope and excellent
functional-group tolerance. Mechanistic investigations revealed that the catalytic cycle was initiated by the attack of a
difluoroalkyl radical to an in situ generated alkynylcopper species.
luorine-containing structural motifs play important roles in
biologically active compounds, especially pharmaceuticals
we envisioned that alkynyl carboxylic acids, readily available
coupling partners in metal-catalyzed decarboxylative coupling
reactions, could be feasible precursors to terminal unsaturated
F
9
and agrochemicals, due to the drastic enhancement of lip-
ophilicity, metabolic stability, and bioavailability relative to the
parent molecules via the incorporation of fluorine atoms or
compounds (Scheme 1b). Herein, we report the first example of
copper-catalyzed decarboxylative atom transfer radical addition
of alkynyl carboxylic acids, initiated by fluoroalkyl radicals
generated by copper catalysts, affording fluoroalkylated alkenyl
1
fluorine-containing groups. As a result, the selective introduc-
tion of fluorine or fluorinated groups onto the particular position
10
in small organic molecules has been developed as a powerful
iodides in a highly efficient way.
2
strategy in drug design. Accordingly, the development of
Our study commenced with 3-phenylpropiolic acid (1a) used
as the model substrate and ethyl iododifluoroacetate (2) as the
fluoroalkyl coupling partner in the presence of a catalytic amount
of CuCl (10 mol %) in 1,4-dioxane at 100 °C. The desired
product 3a was obtained successfully in a good yield (84%) using
1,10-phenanthroline (20 mol %) as the ligand (entry 1, Table 1).
Reducing the amount of 1,10-phenanthroline to 10 mol % gave a
slightly lower yield (entry 2). Next, a variety of copper species,
efficient fluoroalkylation methods, especially mediated or
catalyzed by transition-metal catalysts, has attracted extensive
3
research attention in recent decades.
Starting from the pioneering work by Kharasch and co-
4
workers, atom transfer radical addition (ATRA) has been
developed as a reliable method to introduce two functional
5
groups onto unsaturated C−C bonds in one step. Typically
6
7
initiated by radical initiators or by transition-metal catalysis,
including cuprous salts (CuBr, CuI, Cu O) and cupric salts
2
ATRA represents a practical synthetic path to difunctionalization
of alkenes or alkynes with activated halides in a highly atom-
economic way and provides the corresponding alkyl or vinyl
halides as important intermediates for further transformation
[
Cu(acac) , Cu(OAc) , Cu(OTf) ] were examined carefully,
2 2 2
which indicated both Cu(I) and Cu(II) worked well in this
catalytic system (entries 1−9), and Cu O gave the best yield
(
2
90%) (entry 5). Accordingly, increasing the amount of Cu O to
2
(Scheme 1a). However, the unsaturated acceptors have been
1
0 mol % resulted in a slightly higher yield (entry 11).
strictly limited to terminal alkenes and alkynes and, thus, are still
hampered by the instability and low availability of the
corresponding unsaturated substrates. In our continuous effort8s
to develop transition-metal-catalyzed fluoroalkylation methods,
Furthermore, a general survey of ligands was also performed,
which showed bidentate nitrogen ligands were the optimal
choice, and 1,10-phenanthroline was still the best ligand (entries
1
3−17). To improve the yield further, we assessed the effect of
reaction temperature and solvents on the conversion and found
that no higher yield could be obtained by decreasing or
increasing the temperature or changing the solvent (entries
Scheme 1. Atom Transfer Radical Addition with Fluoroalkyl
Halides
1
8−23). Finally, several control experiments were performed,
which showed only a 25% yield of the desired product 3a was
afforded when ligands were excluded (entry 24), and no 3a was
obtained without a copper catalyst (entry 26).
Received: July 27, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02216
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
,
b
,
ab
Table 1. Optimization of Conditions
Scheme 2. Substrate Scope of Alkynyl Acids
entry
cat. (mol %)
ligand (mol %)
yield (%)
1
2
3
4
5
6
7
8
9
CuCl (10)
CuCl (10
CuBr (10)
CuI (10)
1,10-phen (20)
1,10-phen (10)
1,10-phen (20)
1,10-phen (20)
1,10-phen (20)
1,10-phen (20)
1,10-phen (10)
1,10-phen (20)
1,10-phen (20)
1,10-phen (10)
1,10-phen (20)
84
75
53
41
90
80
76
64
62
84
96
46
57
43
84
90
86
90
92
88
77
82
79
25
84
0
Cu O (5)
2
Cu(acac) (10)
2
Cu(acac) (10)
2
Cu(OAc) (10)
2
Cu(OTf) (10)
2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
Cu O (5)
2
Cu O (10)
2
Cu O (10)
2
PPh (20)
3
Cu O (10)
2
dppp (20)
Cu O (10)
2
Pyr (20)
Cu O (10)
2
tby (20)
Cu O (10)
2
TMEDA (20)
DMEDA (20)
1,10-phen (20)
1,10-phen (20)
1,10-phen (20)
1,10-phen (20)
1,10-phen (20)
1,10-phen (20)
−
Cu O (10)
2
c
Cu O (10)
2
d
e
f
Cu O (10)
2
Cu O (10)
2
Cu O (10)
2
g
h
Cu O (10)
2
Cu O (10)
2
Cu O (10)
2
i
Cu O (10)
2
1,10-phen (20)
−
1,10-phen (20)
a
Unless otherwise noted, the reaction conditions were as follows: 1a
0.2 mmol, 1.0 equiv), 2 (0.3 mmol, 1.5 equiv), 1,4-dioxane (1.0 mL),
(
b
c
d
e
f
N . Isolated yield. 90 °C. 110 °C. Et O was used as solvent. THF
was used as solvent. MeCN was used as solvent. CH Cl was used as
solvent. No NaOAc was added
2
2
g
h
2
2
i
With the optimized reaction conditions in hand, we next
focused on broadening the scope of alkynyl carboxylic acids 1
with this atom transfer radical addition protocol, as shown in
Scheme 2. A large number of para-substituted arylpropiolic acids
installed with either electron-donating substituents, such as
OMe, Me, SMe, and Ph (1b−1e), or electron-withdrawing
substituents, such as C(O)Me, NO , CO Me, CH OH, F, and Cl
a
Unless otherwise noted, the reaction conditions were as follows: 1
O (10 mol %),
,10-phen (20 mol %), NaOAc (0.3 mmol, 1.5 equiv), 1,4-dioxane (1.0
(0.2 mmol, 1.0 equiv), 2 (0.3 mmol, 1.5 equiv), Cu
1
mL), 100 °C, 12 h, N . Isolated yield. The ratios in parentheses are
E/Z ratios, determined by F NMR spectroscopy of crude products.
2
2
2
2
(
1f−1k), gave good to excellent yields of the corresponding
b
difunctionalization products in our new catalytic system. It is
worth noting that normally unstable functional groups such as
2
19
CH OH could be well tolerated in this transformation.
2
Meanwhile, meta-substituted arylpropiolic acids have also been
successfully decarboxylated, and an atom transfer radical was
added to afford the desired product (3l−3o). Notably, steric
hindrance in the ortho-substituted substrates has also no effect in
this catalytic system, and satisfactory yields were afforded in all
cases (3p−3v). Moreover, di- and trisubstituted arylpropiolic
acids could also afford the desired products (3w−3y).
Intriguingly, good to excellent E/Z selectivities were observed
for all cases in which phenylpropiolic acid or its derivatives were
examined, which demonstrated the excellent stereoselectivity of
this new catalytic transformation. Remarkably, decarboxylative
atom transfer radical addition of heteroarene-based propiolic
acids also proceeded smoothly to afford the corresponding
functional styrenes with acceptable yields, albeit with relatively
lower E/Z selectivity (3z). Finally, alkylated propiolic acid 1aa
was compatible with this catalytic system, giving difunctional
alkyl olefin 3aa with a moderate yield and stereoselectivity.
To gain insights into the mechanism of this iodofluoroalky-
lation, a series of control experiments were then carried out. First,
when 1.0 equiv of 2,2,6,6-tetramethyl-1-piperidinyloxy
(TEMPO), a radical scavenger, was added into the standard
reaction conditions, none of the desired product 3a was obtained,
1
9
while TEMPO-captured product 4 could be detected by
F
NMR and GC-MS (Scheme 3, eq 1). This revealed a possible
radical pathway was involved in the catalytic cycle. The
fluoroalkyl radical path was also implicated by successful trapping
of the EtO CF C· radical using β-pinene under the standard
2
2
conditions, which afforded the ring-opened diene 5 as an
B
DOI: 10.1021/acs.orglett.6b02216
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3. Preliminary Mechanistic Studies
D was quenched by proton to afford the desired iodofluoro-
alkylated product 3a and regenerate the copper catalyst.
To demonstrate the synthetic potential of this newly
developed method, fluorinated alkyl iodide 3a was next studied
to evaluate its reactivity toward the known palladium-catalyzed
cross-coupling reactions that would form a variety of
fluoroalkylated alkenes (Scheme 5). Upon treatment of 3a
Scheme 5. Synthetic Transformations of 3a
with phenylboronic acid, alkenylboronic acids, and 1-decyne in
the presence of a catalytic amount of palladium species, the
fluorinated alkenyl iodide 3a was coupled with aryl, alkenyl, and
alkynyl structural motifs smoothly, which clearly show the value
of the method to the synthesis of fluorine-containing compounds
or intermediates.
The synthetic potential of this protocol was further elaborated
by successful application to the late-stage iododifluoro-
methylation of biologically active compounds. As shown in
Scheme 6, the subjection of estrone-derived propiolic acid 12
isomeric mixture (Scheme 3, eq 2). Next, when 10 mol % of
preprepared (phenylethynyl)copper 6 was added into the
reaction system instead of Cu O, the desired 3a was obtained
2
smoothly in 97% yield, which indicated 6 may serve as a key
intermediate in the cycle (Scheme 3, eq 3). While the subjection
of ethynylbenzene 7 into the standard conditions could also
afford 3a in good yield, we then tried to reveal the real radical
acceptor. Accordingly, stoichiometric, preprepared (phenyl-
ethynyl)copper 6 gave almost the same moderate yields with
or without addition of HOAc, which supposedly was produced
via abstraction of a proton from propiolic acid with NaOAc and
could quench the copper species 6 to ethynylbenzene 7 (Scheme
Scheme 6. Decarboxylative Iododifluoromethylation of
Estrone-Based Propiolic Acid 12
3
, eqs 3 and 5). These results showed the quenching step of
(phenylethynyl)copper 6 to ethynylbenzene 7 may not be
involved in the catalytic cycle.
On the basis of the above results and previous reports,
plausible mechanism involving a copper-initiated decarboxylative
fluoroalkyl radical addition catalytic cycle was proposed (Scheme
11
into the standard conditions afforded the fluorine-containing
vinyl iodide 13 in good yield. While fluoroalkylated 13 with an
embedded iodine atom could be further converted to different
fluorine-containing estrone derivatives, this outcome indeed
showed the great potential of this iodofluoroalkylation method in
drug discovery and screening.
a
4
). The catalytic cycle was initiated by copper-catalyzed
Scheme 4. Plausible Mechanism
In conclusion, we have developed a novel copper-catalyzed
decarboxylative atom transfer radical addition of alkynyl
carboxylic acids. Compared with the known ATRA reactions,
this transformation demonstrated the first example of iodo-
fluoroalkylation of nonterminal alkynes with a broad substrate
scope and excellent functional-group tolerance. Mechanistic
investigations revealed that the catalytic cycle was initiated by the
attack of a difluoroalkyl radical to the in situ generated
alkynylcopper species. Further investigations into the detailed
mechanism and application of this protocol to modify bio-
logically active compounds are still ongoing in our laboratory.
decarboxylation of phenylpropiolic acid 1a, which afforded the
(
phenylethynyl)copper species A. Meanwhile, reduction of
ASSOCIATED CONTENT
Supporting Information
■
fluoroalkyl iodide 2 is catalyzed by copper via a single-electron
transfer to afford the fluoroalkyl radical B, which attacked the
(
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b02216.
phenylethynyl)copper species A to give the vinyl radical C. The
carbon radical C could abstract an iodine atom from R I to result
f
in formation of intermediate D, and regenerating of the Rf ·
radical through a chain transfer mechanism. Finally, intermediate
Experimental procedure and characterization of all new
compounds (PDF)
C
DOI: 10.1021/acs.orglett.6b02216
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Cahard, D.; Besset, T.; Poisson, T.; Pannecoucke, X. Synthesis 2014, 46,
AUTHOR INFORMATION
Corresponding Author
■
*
1
(
859. (i) Che, C.; Zheng, H.; Zhu, G. Org. Lett. 2015, 17, 1617.
j) Cheung, C. W.; Hu, X. Chem. - Eur. J. 2015, 21, 18439. (k) Wang, Y.-
Q.; He, Y.-T.; Zhang, L.-L.; Wu, X.-X.; Liu, X.-Y.; Liang, Y.-M. Org. Lett.
015, 17, 4280. Photoredox catalysts ATRA reactions: (l) Nguyen, J.
E-mail: xswang77@ustc.edu.cn.
Author Contributions
2
D.; Tucker, J. W.; Konieczynska, M. D.; Stephenson, C. R. J. J. Am.
Chem. Soc. 2011, 133, 4160. (m) Wallentin, C.-J.; Nguyen, J. D.;
Finkbeiner, P.; Stephenson, C. R. J. J. Am. Chem. Soc. 2012, 134, 8875.
§
G.L. and Y.-X.C. contributed equally.
Notes
(
n) Iqbal, N.; Jung, J.; Park, S.; Cho, E. J. Angew. Chem., Int. Ed. 2014, 53,
The authors declare no competing financial interest.
5
2
2
39. (o) Knorn, M.; Rawner, T.; Czerwieniec, R.; Reiser, O. ACS Catal.
015, 5, 5186. (p) Tang, X.-J.; Dolbier, W. R., Jr. Angew. Chem., Int. Ed.
015, 54, 4246. (q) Bagal, D. B.; Kachkovskyi, G.; Knorn, M.; Rawner,
ACKNOWLEDGMENTS
■
This research was supported by the National Basic Research
Program of China (973 Program 2015CB856600), the National
Science Foundation of China (21522208, 21372209), and the
Excellent Young Scientist Foundation of Anhui Province
T.; Bhanage, B. M.; Reiser, O. Angew. Chem., Int. Ed. 2015, 54, 6999.
(r) Riente, P.; Pericas, M. A. ChemSusChem 2015, 8, 1841. (s) Cao, M.-
Y.; Ren, X.; Lu, Z. Tetrahedron Lett. 2015, 56, 3732.
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