Copper-catalyzed gem -difluoroolefination of diazo compounds with TMSCF3 via C-F bond cleavage

Article abstract of DOI:10.1021/ja409941r

A novel Cu-catalyzed gem-difluoroolefination of diazo compounds is described. This transformation starts from readily available TMSCF₃ and diazo compounds, via trifluoromethylation followed by C-F bond cleavage, to afford the desired 1,1-difluoroalkene products.

Full text of DOI:10.1021/ja409941r

Communication
pubs.acs.org/JACS
Copper-Catalyzed gem-Difluoroolefination of Diazo Compounds with
TMSCF3 via C−F Bond Cleavage
Mingyou Hu, Zhengbiao He, Bing Gao, Lingchun Li, Chuanfa Ni, and Jinbo Hu*
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Ling-Ling
Road, Shanghai 200032, China
S
* Supporting Information
thylation of α-diazo esters using pregenerated “CuCF3”.20
ABSTRACT: A novel Cu-catalyzed gem-difluoroolefina-
tion of diazo compounds is described. This transformation
starts from readily available TMSCF3 and diazo com-
pounds, via trifluoromethylation followed by C−F bond
cleavage, to afford the desired 1,1-difluoroalkene products.
Under the strictly anhydrous conditions, 1,1-difluoroalkenes
could be obtained in moderate yields,20 which indicates that the
2,2,2-trifluoroethylcopper intermediate (A) could undergo β-
fluoride elimination (Scheme 1, R2 = COOR).21 Based on this
Scheme 1. Proposed Cu-Catalyzed gem-Difluoroolefination
of α-Diazo Esters
luorine-containing organic molecules often possess ele-
Fvated reactivity, lipophilicity, and bioactivity compared to
their nonfluorinated counterparts.1 For instance, 1,1-difluor-
oalkenes are structurally unique organofluorine compounds,
which are used as enzyme inhibitors via an addition−
elimination mechanism.2 The gem-difluorovinyl functionality
is also considered as a bioisostere of the carbonyl group in drug
design.3 The gem-difluorovinyl moiety could be converted to
other useful fluorinated functionalities such as trifluoromethyl,4
difluoromethyl,3 monofluoroalkenyl,5 and difluoromethylenyl
groups.6 Direct CCF2 double bond construction from
aldehydes and ketones,5,7 pyrolysis of difluoromethylene-
containing compounds,8 and cross-coupling with gem-difluor-
ovinyl building blocks4b,9 are conventional methods for the
synthesis of 1,1-difluoroalkenes.
finding, we envisaged that a substoichiometric amount of
copper might be able to catalyze the gem-difluoroolefination of
diazo compounds (Scheme 1). However, we quickly realized
that two issues have to be considered before a catalytic version
of Cu-assisted gem-difluoroolefination can be achieved: (a) how
to enable 1 to efficiently react with the in situ generated
“CuCF3” to give A in the presence of only a catalytic amount of
Cu(I) (Scheme 1, step 1);20 and (b) how to facilitate an
efficient β-fluoride elimination of A to regenerate the CuF
catalyst (Scheme 1, step 2).21b
Recently, transition-metal-catalyzed fluorination,10 fluoroal-
kylation,11 and fluoroarylation12 have attracted much attention
in synthetic organic chemistry, owing to the mild reaction
conditions and excellent functional-group tolerance in these
transition-metal-assisted reactions. However, the transition-
metal-mediated CCF2 double bond formation reaction still
remains a challenging task. Although the olefin metathesis
reaction proved to be a powerful approach for olefin
synthesis,13 its application in the synthesis of fluorinated olefins
has been less successful.14 Grubbs and co-workers attempted
the reaction between 1,1-difluoroethene and a ruthenium
carbene catalyst, and they found that only a small amount of
β,β-difluorostyrene was formed under the stoichiometric
reaction conditions.15 On the other hand, diazo compounds,
an important type of carbene precursors, are widely used in
olefination reactions;16 however, there is still a lack of
transition-metal-catalyzed efficient gem-difluoroolefination of
diazo compounds.17
Our investigation started with addressing the first issue.
Previously, an excess amount of water or CuI was added to
enhance the reactivity of pregenerated “CuCF3” to efficiently
react with 1.20 However, under the catalytic reaction conditions
where “CuCF3” was in situ generated from CuI/CsF/TMSCF3
(Scheme 1, step 1), the addition of water jeopardized the
−
desired reaction by protonating the in situ generated “CF3 ”
from TMSCF3/CsF. We also examined the possibility of using
extra CuI to promote the reaction between 1 and in situ
generated “CuCF3”. When we conducted Cu-catalyzed gem-
difluoroolefination with ethyl phenyldiazoacetate (1a) as a
model substrate using 20 mol % of CuI and 10 mol % of CsF,22
to our surprise and disappointment, no desired product 2a was
detected, and trifluoromethylated product 2a′ was obtained in
7% yield (eq 1a). After further optimization of the reaction
Trifluoromethylcopper (CuCF3), which is often pregener-
ated prior to the desired reactions, is a powerful trifluor-
omethylating agent.18 Recently, much effort has been devoted
to developing CuCF3-mediated trifluoromethylations with the
use of a substoichiometric amount of copper.19 Previously, we
reported a water-promoted and copper-mediated trifluorome-
Received: September 26, 2013
Published: October 30, 2013
© 2013 American Chemical Society
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Table 1. Survey of Reaction Conditions
a
entry
initiator
solvent
yield (%)
1
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
KF
1,4-dioxane/NMP (10:1)
1,4-dioxane
85
21
62
82
80
70
64
59
2
2
3
1,4-dioxane/NMP (20:1)
1,4-dioxane/NMP (15:1)
1,4-dioxane/NMP (8:1)
1,4-dioxane/NMP (5:1)
1,4-dioxane/NMP (2:1)
1,4-dioxane/NMP (1:1)
NMP
4
5
6
parameters (using 1,4-dioxane/NMP as solvent), 2a was
formed in only 33% yield (eq 1b, for more detailed information,
see SI). Thereafter, we envisaged another plausible approach to
address the first issue, namely, using a more reactive diazo
compound to react with “CuCF3”. It is well-known that diaryl
diazomethanes possess higher reactivity than α-diazo esters.23
When diphenyl diazomethane (3a) was utilized as a substrate
and subjected to the pregenerated CuCF3 without adding any
promoter (water or CuI), we were delighted to find that 1,1-
difluoroalkene 4a was obtained in 63% yield after 20 h without
the formation of any trifluoromethylated product 4a′ (eq 2a).
More interestingly, when 3a was subject to the standard water-
promoted trifluoromethylation reaction conditions (44 equiv of
water were added),20 3a was consumed in 5 min and 4a was
obtained in 63% yield, with the trifluoromethylation product
4a′ being formed in only 18% yield (eq 2b). These results
indicate that 3a can directly react with unactivated “CuCF3”
(from CuI/CsF/TMSCF3), and the resulting 2,2,2-trifluoroe-
thylcopper intermediate A (Scheme 1) readily undergoes β-
fluoride elimination to afford 1,1-difluoroalkene 4a (even in the
presence of an excess amount of water!), which nicely address
the aforementioned issues.
7
8
9
b
10
11
12
13
14
15
16
17
18
19
20
21
1,4-dioxane/NMP (10:1)
1,4-dioxane/NMP (10:1)
1,4-dioxane/NMP (10:1)
1,4-dioxane/NMP (10:1)
1,4-dioxane/NMP (10:1)
1,4-dioxane/NMP (10:1)
1,4-dioxane/NMP (10:1)
THF/NMP (10:1)
10
c
77
69
47
8
TBAT
TMAF
tBuOK
CsF
CsF
CsF
CsF
CsF
none
50
d
64
64
34
61
DCM/NMP (10:1)
toluene/NMP (10:1)
1,4-dioxane/NMP (10:1)
1,4-dioxane/NMP (10:1)
e
<5%
0
a
The reactions were carried out on 1 mmol scale. The yields were
b
determined by 19F NMR with PhCF3 as an internal standard. 2.5 mol
c
d
% of CsF was used. 10 mol % of CsF was used. 5 mol % of CuCl was
e
used as the catalyst. Without a Cu-catalyst.
generally gave lower yields. When the reaction temperature was
decreased, the reactions with 3e, 3f, and 3i proceeded smoothly
to give corresponding products 4e, 4f, and 4i, respectively, in
higher yields. However, in the case of bis(4-dimethylamino)-
phenyl diazomethane (3l), no desired product 4l was obtained.
This might be attributed to the strong electron-donating amino
group making the carbenic carbon more electron-rich through
p−π conjugation and thus difficult to undergo migratory
insertion into the Cu-CF3 single bond.24 On the contrary, those
with electron-withdrawing substituents on the arenes generally
gave higher yields (Table 2, 4m−4x). It is noteworthy that no
aromatic trifluoromethylation occurred in the reactions with
substrates bearing a F, Cl, or Br substituent (Table 2, 4m−4v).
The reaction was found to be not sensitive to the ortho-, meta-,
or para-substituents in the aromatic rings. Note that this
reaction represents arguably the most general gem-difluoroolefi-
nation method for the synthesis of 2,2-diaryl-1,1-difluoroole-
fins.5,7−9 When alkyl-substituted diazo compound 3y was used,
the product yield was significantly lower, probably due to the
side reactions of the diazo compounds such as self-
condensation and 1,2-H shifts.25
Encouraged by these results, we soon examined the catalytic
version of the reaction between 3a and TMSCF3 (Table 1). To
our delight, when 5 mol % of CuI and 5 mol % of CsF were
applied, 3a readily underwent gem-difluoroolefination to afford
4a in 85% yield (Table 1, entry 1). When 1,4-dioxane was used
as solvent, the reaction was sluggish (entry 2). Further
examination revealed that the use of a mixed solvent system
of 1,4-dioxane and NMP (entries 1, 3−8) was better than solely
using 1,4-dioxane (entry 2) or NMP (entry 9), and the optimal
ratio of 1,4-dioxane to NMP was found to be 10:1. When
lowering the loading of CsF to 2.5 mol %, the yield decreased
sharply (entry 10). However, the yield was also slightly
decreased when more CsF (10 mol %) was used (entry 11).
Other Lewis base activators for TMSCF3, such as KF,
tetrabutylammonium triphenyldifluorosilicate (TBAT), tetra-
t
methylammonium fluoride (TMAF), and BuOK, were found
to be inferior to CsF (entries 12−15). CuCl was not as effective
as CuI when used to promote the gem-difluoroolefination
reaction (entry 16). The use of other solvents, such as THF/
NMP, DCM/NMP, and toluene/NMP, resulted in lower yields
(entries 17−19). It should be noted that, in the absence of
either a Cu(I) catalyst or CsF, the reaction was significantly
inhibited (entries 20−21).
With the optimized reaction conditions in hand (Table 1,
entry 1), we further examined the substrate scope and
limitation of this reaction. The reactions with various diaryl
diazomethanes proceeded smoothly to give 1,1-difluoroalkenes
in moderate to excellent yields (Table 2). The substrates with
electron-donating substituents on the arenes (Table 2, 4d−4l)
When TMSCF2CF3 was employed alternatively as a
fluoroalkylating reagent, 1,1,1,2-tetrafluoro-3,3-diphenylpro-
pene (5a) was obtained in 39% yield (eq 3). The relatively
lower yield of 5a is probably due to the fact that the in situ
−
generated pentafluoroethyl anion (CF3CF2 ) readily undergoes
β-fluoride elimination to give tetrafluoroethene (CF2CF2,
detected by 19F NMR and GC-MS in this case). However,
when pregenerated CuCF2CF3 was used, 5a could be obtained
in 70% yield (eq 4).
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Table 2. Cu-Catalyzed gem-Difluoroolefination of Diazo
Compound 3
Scheme 2. Proposed Mechanism of Cu-Catalyzed gem-
Difluoroolefintion of Diazo Compounds
species A undergoes a fast β-fluoride elimination to give gem-
difluoroolefin product 4 and CuF, and CuF further reacts with
TMSCF3 to regenerate CuCF3 (Scheme 2).
In summary, we have developed a novel Cu-catalyzed gem-
difluoroolefination of diazo compounds. This new synthetic
protocol enables the efficient preparation of a variety of
structurally diverse 1,1-difluoroalkenes, which promises to find
applications in medicinal chemistry and materials science
related fields. It was found that, even without the activation
by water or CuI,20 the “CuCF3” species (generated from CuI/
CsF/TMSCF3) could still directly react with diaryl diazo-
methanes. Furthermore, the resulting 1,1-diaryl-2,2,2-trifluor-
oethylcopper intermediate A underwent facile β-fluoride
elimination to give 1,1-difluoroalkene. Not only do these two
important factors facilitate an efficient Cu(I)-catalyzed gem-
difluoroolefination process, they also provide intriguing new
insights into the unique reaction between “CuCF3” and diazo
compounds.20 Moreover, this olefination process provides a
proof of concept for the use of β-fluoride (or other leaving
groups) elimination to realize a catalytic olefination via a Cu-
carbene intermediate.26 Further exploration of this new
synthetic protocol is currently underway in our laboratory.
a
All reactions were performed by using 1.0 mmol of 3, 0.05 mmol of
CuI and CsF unless otherwise noted. The yields referred to isolated
b
yields. The reaction was performed at the melting point of the mixed
c
solvent (6 °C). The yield was determined by 19F NMR with PhCF3 as
ASSOCIATED CONTENT
* Supporting Information
■
d
an internal standard. The reaction was performed on 2.0 mmol scale
with 0.1 mmol of CuI and CsF.
S
Experimental procedures and characterization for all new
compounds. This information is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
jinbohu@sioc.ac.cn
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Based on the aforementioned results, we propose a plausible
mechanism for the Cu-catalyzed gem-difluoroolefination of
diazo compounds (Scheme 2). TMSCF3 first reacts with CuI
and CsF to generate CuCF3, and the latter species reacts with
diazo compound 3 to form Cu-carbene intermediate B.
Intermediate B undergoes carbene migratory insertion into
the Cu−CF3 single bond to give intermediate A.20 Then the
■
This work was supported by the National Basic Research
Program of China (2012CB821600, 2012CB215500), the
National Natural Science Foundation of China (21372246,
20825209), and MSD China R&D Postdoc Fellowship (M.H.).
We thank Dr. Teruo Umemoto (Visiting Professor at SIOC)
for helpful discussions.
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