Palladium-Catalyzed Radical Cascade Iododifluoromethylation/Cyclization of 1,6-Enynes with Ethyl Difluoroiodoacetate

Article abstract of DOI:10.1021/acs.orglett.5b02068

A novel and convenient Pd-catalyzed radical cascade iododifluoromethylation/cyclization of 1,6-enynes with ethyl difluoroiodoacetate is demonstrated. The proposed transformation presents high stereoselectivity under mild and facile reaction conditions, th

Full text of DOI:10.1021/acs.orglett.5b02068

Letter
pubs.acs.org/OrgLett
Palladium-Catalyzed Radical Cascade Iododifluoromethylation/
Cyclization of 1,6-Enynes with Ethyl Difluoroiodoacetate
Yu-Qi Wang, Yu-Tao He, Lu-Lu Zhang, Xin-Xing Wu, Xue-Yuan Liu, and Yong-Min Liang*
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. China
S
* Supporting Information
ABSTRACT: A novel and convenient Pd-catalyzed radical
cascade iododifluoromethylation/cyclization of 1,6-enynes with
ethyl difluoroiodoacetate is demonstrated. The proposed trans-
formation presents high stereoselectivity under mild and facile
reaction conditions, thereby allowing an efficient access to a variety
of iodine-containing difluoromethylated pyrrolidines. A possible radical pathway for the transformation is proposed on the basis
of the results of control experiments and relevant literature reviews.
ntroducing fluoroalkyl groups into organic molecules often
significantly changes the physical and biological properties of
Scheme 1. Summary of the Present Research and Our New
Transformation of Iododifluoromethylation/Cyclization of
1,6-Enynes
I
the resultant molecules and greatly improves their lipophilicity,
metabolic stability, and bioavailability.1 Thus, fluorine-contain-
ing compounds have gained ground in the fields of agrochemical
and pharmaceutical production as well as materials science.2 In
particular, the difluoromethylene group (CF2), which acts as a
bioisostere for mimicking the electronic features of an ethereal
oxygen atom as well as a lipophilic hydrogen-bond donor, has
been regarded as a potential structural motif because of its unique
properties.3 Traditional strategies for synthesizing difluoroalky-
lated compounds often involve deoxofluorination of aldehydes
or ketones with (diethylamino)sulfur trifluoride (DAST) or
Deoxofluor.4 Over the past few decades, several methods for
incorporating the difluoromethylene group (CF2) into organic
molecules by transition-metal-catalyzed as well as free-radical-
mediated difluoromethylation reactions have been developed.5
For instance, the Hartwig6 and Prakash7 groups independently
reported the copper-mediated nucleophilic difluoromethylation
of aryl iodides with TMSCF2H and n-Bu3SnCF2H. Amii and co-
workers described a sequential three-step copper-catalyzed
approach (including cross-coupling, hydrolysis, and decarbox-
ylation) to synthesize difluoromethyl aromatic and heteroar-
omatic compounds.8 Baran developed convenient difluorome-
thylations of heterocycles with the use of zinc difluoromethane-
sulfinates (DFMS).9 Recently, a range of intriguing researching
studies have reported the construction of difluoromethyl-
containing compounds through a radical addition process of
alkenes. In 2012, Liu and co-workers described an iron-catalyzed
decarboxylative difluoromethylation of cinnamic acids with zinc
difluoromethanesulfinates (DFMS).10 In 2014, Zhang developed
a palladium-catalyzed radical fluoroalkylation of alkenes with
fluoroalkyl bromides (Scheme 1a).11 Wang reported palladium
or iron-catalyzed radical aryldifluoromethylation/cyclization of
various N-arylacrylamides to form useful difluoromethylated
oxindole derivatives (Scheme 1b).12
difluoromethyl source, respectively.13 Despite the obvious
benefits of the aforementioned difluoromethylation strategies,
a concise, moderate, and economical difluoromethylation
strategy for synthesizing complex organic compounds remains
highly desired.
As a series of good radical receptors, 1,6-enynes have been
considered as valuable and versatile scaffolds in medical and
biological chemistry and may be used to establish efficient
methods to construct pyrrolidines and other five-membered
heterocycles.14 The tandem cyclization of 1,6-enynes to
incorporate two important functional groups has gained
increased interest because of the aforementioned advantages.15
In particular, iodine atoms, which occur in large amounts of
organic intermediates and natural products, can be widely used
for cross-coupling reactions and nucleophilic substitutions.
Very recently, the Cho and Zhu groups reported visible light
photoredox-catalyzed radical difluoroalkylations of alkenes and
allylic alcohols with ethyl bromodifluoroacetate (EBDFA) as the
Received: July 19, 2015
© XXXX American Chemical Society
A
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Letter
Notably, chemists have exerted efforts to develop different
strategies to introduce iodine and fluorinated groups in one
step.16 Recently, our group disclosed several radical cyclization
reactions of 1,6-enynes for the synthesis of trifluoromethyl/nitro
substituted heterocycles and phosphorated fluorine derivative-
s.15a,b,17 The synthesis of iodine/difluoromethylene-containing
pyrrolidines via a radical addition pathway has never been
explored. Therefore, the goal of the present work is to establish a
safe and green strategy for introducing iodine and difluoro-
methylene groups to 1,6-enynes. With previous studies as bases
and with the 1,6-enyne substrates as our focus, herein we report
in this paper the palladium-catalyzed radical-mediated iododi-
fluoromethylation/cyclization of 1,6-enynes to yield iodine/
difluoromethylene substituted pyrrolidines and other five-
membered heterocycle derivatives (Scheme 1c).
formation reached the highest reaction activity of the desired
product 2a (92%) at 50 °C (Table 1, entry 13). Additionally, no
product was formed without palladium catalyst or ligand (Table
1, entry 14). Finally, the use of 1a (1.0 equiv), PdCl2(PPh3)2 (10
mol %), DPE-Phos (20 mol %), and Cs2CO3 (2.0 equiv) with
ethyl difluoroiodoacetate (2.0 equiv) in the presence of dioxane
(2.0 mL) under an argon atmosphere at 50 °C for 10 h was
considered the optimal reaction conditions.
With the optimized reaction conditions in hand (Table 1,
entry 13), the substrate scope of 1,6-enynes was then tested. As
depicted in Scheme 2, 1,6-enynes with both electron-donating
Scheme 2. Substrate Scope of the Iododifluoromethylation/
a b
,
Cyclization of 1,6-Enynes
Initially, our investigation was carried out by employing 1,6-
enyne 1a as the pilot substrate with the commercially available
reagent ethyl difluoroiodoacetate (2.0 equiv) as difluoromethyl
source, in the presence of PdCl2(MeCN)2 (10 mol %), DPE-
Phos (20 mol %), and K2CO3 in dioxane (2.0 mL) at 25 °C under
an argon atmosphere. Gratifyingly, our expected product 2a was
isolated in 43% yield after 10 h (Table 1, entry 1). Meanwhile the
a
Table 1. Optimization of Reaction Conditions
b
yield
entry
[Pd] (mol %)
ligand (mol %)
base (equiv)
(%)
1
PdCl2(MeCN)2 (10) DPE-Phos (20) K2CO3 (2.0)
43
2
Pd(dba)2 (10)
DPE-Phos (20) K2CO3 (2.0)
DPE-Phos (20) K2CO3 (2.0)
DPE-Phos (20) K2CO3 (2.0)
32
3
Pd(OAc)2 (10)
37
4
PdCl2(PPh3)2 (10)
PdCl2(PPh3)2 (10)
PdCl2(PPh3)2 (10)
PdCl2(PPh3)2 (10)
PdCl2(PPh3)2 (10)
PdCl2(PPh3)2 (10)
PdCl2(PPh3)2 (10)
PdCl2(PPh3)2 (10)
PdCl2(PPh3)2 (10)
PdCl2(PPh3)2 (10)
PdCl2(PPh3)2 (0)
46
5
PPh3 (20)
K2CO3 (2.0)
K2CO3 (2.0)
K2CO3 (2.0)
K2CO3 (2.0)
N.R.
trace
33
6
dppb (20)
7
X-Phos (20)
XantPhos (20)
8
39
9
DPE-Phos (20) Na2CO3 (2.0)
DPE-Phos (20) KOAc (2.0)
DPE-Phos (20) K3PO4 (2.0)
DPE-Phos (20) Cs2CO3 (2.0)
DPE-Phos (20) Cs2CO3 (2.0)
trace
64
10
11
12
66
73
c
13
92
14
DPE-Phos (0)
Cs2CO3 (2.0)
N.R.
a
Reaction conditions: 1a (0.2 mmol), ethyl difluoroiodoacetate (2.0
equiv) in anhydrous dioxane (2.0 mL) under an argon atmosphere at
b
c
25 °C for 10 h. Isolated yields. At 50 °C.
a
All of the reactions were carried out in the presence of 1 (0.2 mmol),
PdCl2(PPh3)2 (10 mol %), DPE-Phos (20 mol %), ethyl
difluoroiodoacetate (2.0 equiv), and Cs2CO3 (2.0 equiv) in dioxane
molecular structure of 2a was unambiguously determined by X-
ray crystallographic analysis (see the Supporting Information).
Inspired by this result, different palladium catalysts were varied
for this transformation. Among the catalysts surveyed, we found
that PdCl2(PPh3)2 was the most efficient, promoting an
increasing yield to 46% (Table 1, entries 1−4). In contrast to
DPE-Phos, other ligands including diphosphines and mono-
phosphines failed to show more catalytic reactivity (Table 1,
entries 5−8). A subsequent survey on several representative
bases indicated that Cs2CO3 was the most suitable catalyst,
producing product 2a in 73% yield (Table 1, entries 9−12). To
improve the yield further, various solvents were examined, and
dioxane exhibited the best performance for the reaction. The
adjustments in reaction temperature revealed that this trans-
b
(2.0 mL) under an argon atmosphere at 50 °C for 10 h. Isolated
c
yields. The ratios of Z/E isomers were determined by 19F NMR
spectroscopy. Structures of the major isomers are shown.
and electron-withdrawing groups on the phenyl ring (1a−m)
were well tolerated in this protocol and furnished excellent yields
of the corresponding iododifluoromethylated pyrrolidines (2a−
m) with good stereoselectivities and no evident electronic effect.
In most cases, excellent Z/E product ratios were obtained. It is
noteworthy that when the ortho-position was substituted, the
reaction occurred smoothly to undergo an iododifluoromethy-
lation/cyclization process with low stereoselective ratios of Z/E
B
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Letter
isomer. Considering this phenomenon, a hypothesis on the
significant effect of steric hindrance influenced on the stereo-
selectivity is proposed. The reaction occurred mildly when 2-
naphthalenyl was attached to the triple bond 1n. In addition, a
series of heterocyclic groups or no group attached to the triple
bond (1o−r) were compatible with the reaction conditions,
resulting in good yields and stereoselectivities. When unsub-
stituted allylic substrate 1s and enyne 1v were applied under the
standard conditions, only trace amounts of 2s and 2v were
detected, whereas enyne with a phenyl substituent 1t afforded
the product 2t in a decreased yield of 21%. Based on these cases,
we indicated that the substituents in the internal position of the
alkene affected the stereoselectivity dramatically. After subse-
quent investigation of the scope of this reaction, we found that
the carbon-tethered (1w, 1x) and oxygen-tethered (1y, 1z) 1,6-
enynes as well as methacrylamide (1u) could also be converted
into the desired products in moderate yields. Finally, the
molecular structures of 2m and 2p were unambiguously
confirmed through X-ray crystallography (see the Supporting
Information for data for 2m and 2p).
On the basis of the experimental results and previous
literature, a plausible mechanism involving the difluoromethyl
radical pathway was proposed (Scheme 4). First, ethyl
Scheme 4. Possible Mechanism
To gain insight into the mechanism of this reaction, a series of
mechanistic studies were performed (Scheme 3). When 2.0 equiv
Scheme 3. Control Experiments for Mechanistic Studies
difluoroiodoacetate is reduced by Pd(0) to afford the CF2
radical and Pd(I). The addition of the CF2 radical to the double
bond of 1,6-enyne A generates the radical intermediate B, which
can be subsequently activated by Pd(I) to form C, and then an
intramolecular 5-exo-dig cyclization follows to produce the
Pd(II) iodine complex D. Finally, D undergoes reductive
elimination to afford the final product and to regenerate the
catalyst Pd(0).
In order to further evaluate the practicability of this method,
the reaction of enynes 1a and ethyl difluoroiodoacetates was
enlarged to a gram scale under the optimal conditions, and the
desired product 2a was obtained in 70% yield (eq 1 in Scheme 5).
Scheme 5. Gram-Scale Experiment and Palladium-Catalyzed
Coupling Reaction
of TEMPO (2,2,6,6-tetramethylpiperidinyloxy, a well-known
radical scavenger) were added to the reaction system, no product
was detected, and the TEMPO−CF2COOEt adduct was
observed by GC−MS with a yield of 47%. BHT (2,6-di-tert-
butyl-4-methyl-phenol), a radical inhibitor, was used under the
standard conditions for the reaction, and the resultant
difluoroalkylated product 2a was isolated with low yield (42%).
The yield of product 2a decreased by 22% when a stoichiometric
amount of ethene-1,1-diyldibenzene (4b) was added as a radical
scavenger, and the 5b−6b mixture was obtained in 49% yield. In
addition, the reaction of N,N-diallyl-4-methylbenzenesulfona-
mide 4a with ethyl difluoroiodoacetate under the standard
reaction conditions resulted in the cyclized product 5a with a
yield of 64% (dr >20:1). The control experiments above
demonstrated the involvement of the difluoromethyl radical in
this process.
Moreover, iododifluoromethylated product 2y can be further
elaborated by using the Sonogashira reaction.18 The Sonogashira
coupling of 2y afforded the corresponding product 3y in 82%
yield (eq 2 in Scheme 5).
In conclusion, we have developed a palladium-catalyzed radical
cascade iododifluoromethylation/cyclization of 1,6-enynes. This
reaction provides an efficient protocol for preparing a variety of
useful iododifluoromethylated pyrrolidines that may be used as
potential intermediates in organic synthesis and medicinal
chemistry. Compared with traditional difluoromethylation
methods, our reaction system simultaneously introduces the
pharmaceutically active group (CF2) and takes advantage of the
well-known cross-coupling utility of iodine. The mechanistic
C
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b02068.
(11) Feng, Z.; Min, Q.-Q.; Zhao, H.-Y.; Gu, J.-W.; Zhang, X. Angew.
Chem., Int. Ed. 2015, 54, 1270.
Detailed experimental procedures and spectral data for all
new compounds (PDF)
Crystallographic data for 2a (CIF)
Crystallographic data for 2m (CIF)
Crystallographic data for 2p (CIF)
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AUTHOR INFORMATION
Corresponding Author
■
*E-mail: liangym@lzu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Science Foundation (NSF21272101,
21472074, and 21472073) for financial support. We also
acknowledge support from the “111” Project (J1103307) and
the Program for Changjiang Scholars and Innovative Research
Team in University (IRT1138).
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