Cu(0)/Selectfluor System-Mediated Mild Synthesis of Fluorinated Fluorenones from Nonaromatic Precursors (1,6-Enynes) Involving C-C Single Bond Cleavage

Article abstract of DOI:10.1021/acs.orglett.5b01110

A novel and facile method for the mild construction of fluorinated fluorenones from nonaromatic precursors (1,6-enynes) mediated by a Cu(0)/Selectfluor system has been successfully achieved. Preliminary mechanistic investigations indicate that the reactio

Full text of DOI:10.1021/acs.orglett.5b01110

Letter
pubs.acs.org/OrgLett
Cu(0)/Selectfluor System-Mediated Mild Synthesis of Fluorinated
Fluorenones from Nonaromatic Precursors (1,6-Enynes) Involving C−
C Single Bond Cleavage
Jian Zhang, Heng Wang, Shaobo Ren, Wei Zhang, and Yunkui Liu*
State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang University of Technology, Hangzhou
310014, P. R. China
S
* Supporting Information
ABSTRACT: A novel and facile method for the mild
construction of fluorinated fluorenones from nonaromatic
precursors (1,6-enynes) mediated by a Cu(0)/Selectfluor
system has been successfully achieved. Preliminary mechanistic
investigations indicate that the reaction may proceed via an
unprecedented annulation/C−C single bond cleavage/fluori-
nation sequence.
hanks to the unique properties of the fluorine atom, the
aromatic C−F bonds generally require aromatic precursors to
Tintroduction of the fluorine atom to organic molecules react with fluorinating agents.2 In contrast, the construction of
aromatic C−F bonds for the synthesis of fluoroarenes from
nonaromatic precursors and fluorinating agents has rarely been
documented.19,20 As our continued interest in developing
efficient approaches for the synthesis of fluoro-containing
molecules,21 we herein describe a novel and facile method for
the construction of aromatic C−F bonds to access fluorinated
fluorenones from nonaromatic precursors (1,6-enynes) and
Selectfluor involving a C−C single bond cleavage.
may significantly affect their original properties such as
solubility, biological activity, metabolic stability, and physical
properties.1 For example, fluorinated arenes are widely used as
pharmaceuticals, agrochemicals, and PET imaging reagents
owing to their desirable biological properties and unique
electronic characteristics.2 Thus, the development of efficient
and mild methods for the construction of aryl fluoride
containing compounds is of increasing interest for both
academic and industrial communities.3 The traditional synthetic
routes to fluorinated arenes, the Balz−Schiemann reaction,4
and Halex process,5 generally suffer from harsh reaction
conditions and substrate limitations. Recently, a variety of
new protocols have been developed for the synthesis of
fluoroarenes based on the aromatic C−F bond formation.2 In
particular, the coupling of an Ar−X (X = halo,6 OSO2CF3,7
I+Ar,8 BR2 (or BF3K),9 SiR3,10 SnR3,11 MgBr·LiCl,12 Ni,13 Li,14
etc.) with a nucleophilic or an electrophilic fluorinating agent in
the presence or absence of a transition metal has proven to be a
promising approach to the construction of aromatic C−F
bonds and synthesis of fluoroarenes. In addition, several
methods for the construction of aromatic C−F bonds to access
fluorinated arenes based on the transition-metal-catalyzed
direct aromatic C−H bond fluorination has also been explored
in recent years.15,21c Other approaches include the direct
transformation of phenols to aryl fluorides with a proper
fluorinating agent.16 All these reactions have attractive features
for the synthesis of fluorinated arenes. However, this research
on the formation of aromatic C−F bonds mainly focuses on the
cleavage of a C−X (X ≠ C, X = halo, O, N, B, Si, I, M(metal),
and H) bond. In contrast, to the best of our knowledge, only
very limited studies for the construction of aromatic C−F
bonds from C−C single bond cleavage have been reported.17,18
Besides, the dominant pathways for the construction of
Regarding research interest in the annulation of 1,n-enynes,22
we recently reported a Cu(0)/Selectfluor system-mediated
oxidative cyclization of 1,5-enynes to afford 3-formyl-1-
indenone derivatives via an annulation/C−C bond cleavage
process.23 Interestingly, when a 1,6-enyne 1a was subjected to
the reaction conditions (5 mol % Cu(0), 2 equiv of Selectfluor,
2 equiv of NaHCO3 in acetonitrile at 80 °C), an unexpected
fluorinated fluorenone 2a was obtained in 62% yield along with
a small amount of an annulated product 3a (Table 1, entry 1).
This unique construction of fluorinated arenes from non-
aromatic precursors based on C−C single bond cleavage
encouraged us to further optimize the reaction conditions
(Table 1; also see Table S1 in the Supporting Information). It
was found that the reaction performed better at 45 °C even
without a base (92% yield of 2a, entry 9 vs 4). However,
attempts to conduct the reaction at 25 °C gave a low yield of 2a
(entry 5). Among several copper species tested, the use of
Cu(0) powder gave the highest yield of 2a and selectivity
(entry 9, Table 1; also see Table S1). The reaction failed to give
the fluorinated product 2a if the amount of Selectfluor was less
than 1 equiv (entries 6, 7). Control experiments showed that
the copper species was indispensable for the reaction (entry 8).
Received: April 16, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01110
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Letter
a
Table 1. Optimization of Reaction Conditions
Scheme 1. Scope of the Annulation/Fluorination of 1
b
yield (%)
2a
entry
catalyst
base
solvent
3a
c
1
Cu(0)
Cu(0)
Cu(0)
Cu(0)
Cu(0)
Cu(0)
Cu(0)
−
Cu(0)
Cu(0)
Cu(0)
Cu(0)
Cu(0)
NaHCO3
Na2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
−
−
−
−
−
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
62
33
65
90
13
0
5
c
2
6
c
3
28
2
4
d
5
14
26
0
e
6
f
7
0
8
0
0
g
9
92(70 )
3
h
10
91
0
2
i
11
solvent
MeCN
MeCN
0
j
12
0
0
k
13
−
42
26
a
All reactions were carried out with 1a (0.2 mmol), catalyst (5 mol %
based on 1a), Selectfluor (2 equiv), base (2 equiv), in solvent (2 mL)
b
at 45 °C for 1.5 h unless otherwise noted. Determined by GC using
c
d
dodecane as an internal standard. The temperature is 80 °C. The
e
f
temperature is 25 °C. Using 1 equiv of Selectfluor. In the absence of
Selectfluor. Isolated yield. Using 3 equiv of Selectfluor. DMSO,
g
h
i
Encouraged by the successful construction of fluoroarenes
from 1,6-enynes 1 based on C−C single bond cleavage, we
envisioned that 1,6-enyne 4 bearing a C−Si bond should be
more easily cleaved and fluorinated than 1 (eq 1). When 4 was
DMF, DCE, toluene, and 1,4-dioxane were used as the solvent,
respectively. Selectfluor was replaced by 1-fluoropyridinium tetra-
j
fluoroborate and N-fluorobenzenesulfonimide (NFSI), respectively.
k
Selectfluor was replaced by 1-chloromethyl-4-fluoro-1,4-diazoniabi-
cyclo[2.2.2]octanebis(hexafluorophosphate) (F-TEDA-PF6).
A series of solvents were investigated, and MeCN was identified
as the best choice of solvent (entry 11 vs 9). Among several
fluorinating agents screened, Selectfluor showed the most
effectiveness for the formation of 2a (entries 12, 13 vs 9).
With the optimized reaction conditions in hand, a variety of
1,6-enynes 1 with different substitution patterns were
synthesized and examined for the annulation/fluorination
reaction (Scheme 1). It was found that the electronic properties
of the R1 group had a significant effect on the outcome of the
reaction. When the R1 group included aryl rings (1f−1o) or
electron-donating groups (1a, 1c−1e, and 1p−1v), the reaction
proceeded smoothly and the desired fluorinated products were
obtained in modest to good yields. However, substrates 1 were
reluctant to be annulated when R1 was an electron-deficient
group (e.g., Cl, F, and CF3, not listed in Scheme 1). On the
other hand, the electronic natures of R2 groups had little
influence on the annulations/fluorination process, and the
corresponding fluorinated fluorenones could be obtained in
moderate to good yields (45%−82%, 2p−2v). Generally, the
reaction showed high chemoselectivity to afford 2 rather than 3
as the predominant products (2:3 ≥ 1.5:1−95:5; 2a−2h, 2k,
2n, 2p, 2q, and 2r−2v). However, in several cases, non-
fluorinated products 3 accounted for a large proportion of the
total yield (2i/3i, 2j/3j, 2l/3l, 2m/3m, and 2o/3o).
Fortunately, in these cases, 2 and 3 could be separated by
high performance liquid chromatography (HPLC). Several
substituents including halo, alkyl, aryl, and methoxy groups
were compatible with the optimized reaction conditions.
subjected to the standard conditions, the process of annulation
and C−Si bond cleavage did occur, but the reaction gave a
nonfluorinated product 5 as the exclusive product in 58% yield
while the desired 6 was not detected.
In order to gain some insight into the reaction pathways,
several preliminary mechanistic studies on the annulation/
fluorination reaction of 1a were conducted (Scheme 2; see the
Supporting Information). First, the intramolecular kinetic
isotope effects (KIE) were investigated by using 1a-D as the
substrate under the standard reaction conditions (eq 2, Scheme
2). An intramolecular competitive KIE of 1.0 was obtained,
Scheme 2. Preliminary Mechanistic Studies Based on 1a
B
DOI: 10.1021/acs.orglett.5b01110
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Letter
suggesting that the cleavage of the C−H bond is not the rate-
determining step. Second, the direct conversion of the
preparative 3a to 2a was carried out under the standard
conditions, but no formation of 2a was detected while the
starting substrate was recovered (eq 3, Scheme 2), demonstrat-
ing that 3a was not likely an intermediate for the formation of
2a. In addition, when 1a was subjected to the standard reaction
conditions except under an argon atmosphere, the reaction also
gave the target product in 65% yield, indicating that dioxygen
was not involved in the reaction (eq 4, Scheme 2).
because most of such [4 + 2] cyclizations require high
temperatures;29 yet it could not be completely excluded.
In summary, we have achieved the construction of aromatic
C−F bonds based on a C−C single bond cleavage in the
tandem annulation−fluorination of 1,6-enynes. The present
method for the synthesis of fluoroarenes features (1) the use of
nonaromatic precursors, (2) mild reaction conditions, and (3)
the use of inexpensive copper species as the catalyst.
Furthermore, the resulting fluorinated arenes containing both
fluorenone30 and fluorinated aryl moieties2 potentially may
have biological and pharmaceutical activities as well as optical
and electronic properties. Further studies to gain a better
understanding of the mechanism of the present reaction are
currently undertaken in our laboratory.
At the present stage, the exact mechanism for the formation
of 2a has not yet been clearly understood. On the basis of the
above-mentioned results and the previous literature,23−29
a
possible mechanism is proposed in Scheme 3. According to our
ASSOCIATED CONTENT
Scheme 3. Proposed Mechanism
■
S
* Supporting Information
Experimental details, full characterization data, charts of the
mechanistic studies, and copies of NMR spectra for compounds
2 and 3. The Supporting Information is available free of charge
on the ACS Publications website at DOI: 10.1021/
acs.orglett.5b01110.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ykuiliu@zjut.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the Natural Science Foundation of China
(Nos. 21172197 and 21372201) and the Opening Foundation
of Zhejiang Key Course of Chemical Engineering and
Technology, Zhejiang University of Technology for financial
support.
REFERENCES
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Furthermore, an alternative pathway for the formation 2a via [4
+ 2] cyclization of 1,6-eneyne 1a followed by a fluorination/C−
C single bond cleavage/aromatization process is not likely
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