Dual role of ethyl bromodifluoroacetate in the formation of fluorine-containing heteroaromatic compounds

Article abstract of DOI:10.1039/c8cc04298e

An efficient one-pot cascade process via unprecedented quadruple cleavage of BrCF₂COOEt with primary amines to afford valuable fluorine-containing heterocycles is described, in which BrCF₂COOEt plays a dual role as a C1 synthon and a difluoroalkylating reagent for the first time. Mechanistic studies supported by DFT calculations suggest that a base plays an active role in the formation of the key intermediate isocyanides generated in situ from primary amines and difluorocarbene.

Full text of DOI:10.1039/c8cc04298e

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Dual Role of Ethyl Bromodifluoroacetate in the Formation of
Fluorine-containing Heteroaromatic Compounds
Xingxing Ma,a Shaoyu Mai,a Yao Zhou,a Gui-Juan Chengb and Qiuling Song*a,c
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
An efficient one-pot cascade process via unprecedented
quadruple cleavage of BrCF2COOEt with primary amines to afford
valuable fluorine-containing heterocycles is described, in which
BrCF2COOEt plays dual role as C1 synthons and difluoroalkylating
reagent for the first time. Mechanistic studies supported by DFT
calculations suggest that base plays an active role to form the key
intermediate isocyanides, generated in-situ from primary amines
and difluorocarbene.
C-F bonds are one of the strongest bonds that are ever formed
in substances,1 which make them inert and chemically stable.
Therefore, activation of C-F bonds2 has gained significant
attentions. The activation of C-F bonds on arylfluorides has long
been studies2a,2b,2d, but research on C(sp3)-F bonds scission2c,3 is
relatively rare, strong nucleophiles, air- and moisture sensitive
organometallic reagents (organolithium or Grignard reagents) or
high temperature, have been employed.2h,4 Thus cleavage of C-F
bonds under mild and operational simple conditions still remains a
fundamental and ongoing issue.
Scheme 1
compounds;
compound
.
A
) Single and double cleavage in halodifluoromethyl
) Quadruple cleavage in halodifluoromethyl
) Cascade approaches to access valuable products
Halodifluoromethyl compounds (halo = Br, I or Cl) have
been widely employed as difluoroalkylating reagents5 as well
as difluorocarbene synthons,6 and many difluoroalkylation or
difluoromethylation reactions have been developed with them
via either single C-X bond cleavage or double C-X and C-R
bonds cleavage (Scheme 1A).5,6 However, despite the great
advances on difluoroalkylation/difluorometylation with
halodifluoromethyl compounds, quadruple cleavage on the
same carbon atom leading to a novel C1 source, to our
knowledge, has not yet been reported (Scheme 1B). Recently,
our research group has developed several difluoroalkylation
B
C
from arylamines with ethyl bromodifluoroacetate.
reactions with BrCF2COOEt via radical process with Cu/B2pin2
catalytic system.5k-5n,7 Herein, we report two unusual cascade
reactions via unprecedented quadruple cleavage of
BrCF2COOEt with primary amines to access various
phenanthridines and benzothiazoles (Scheme 1C). These novel
transformations have several significant merits: (1) these
reactions proceed via an isocyanide intermediate which are
widely used in organic synthesis.8 Notably, these are the first
examples in which isocyanides were formed in situ, and
subsequently fully converted into valuable products.; (2) two
molecules of BrCF2COOEt were involved in the synthesis of 6-
difluoroacetate phenanthridines and 2-difluoroacetate
benzothiazoles: one serves as C1 source by the cleavage of two
C-F bonds, one C-Br bond and one C-COOEt bond, another one
acts as a normal difluoroalkylation reagent. To our knowledge,
the current protocol represents the first example for using
a.Institute of Next Generation Matter Transformation, College of Chemical
Engineering at Huaqiao University, 668 Jimei Blvd, Xiamen, Fujian, 361021, P. R.
China. E-mail: qsong@hqu.edu.cn.
b.Warshel Institute for Computational Biology, School of Science and Engineering,
The Chinese University of Hong Kong, Shenzhen, 2001 Longxiang Road, Shenzhen
518172, P. R. China.
c. State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai 200032, P R China.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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BrCF2COOEt as a dual role in one reaction. (3) a novel yl)aniline were also amenable to this reaction, giving the products
mechanism is proposed via DFT calculations, which represents 3aa and 3ab in decent yields. Most remarkably, for 2-(1H-pyrrol-1-
a highly attractive and efficient strategy for both isocyanide yl)aniline, the cascade process didn’t stay at mono-substitution,
chemistry and fluorine chemistry, thus might promote the instead, further difluoroalkylation occurred on another C-H bond
discovery of new methodologies with this unusual C-F bond which adjacent to N-atom of pyrrole ring, leading to 6,9-
cleavage strategy.
difluoroalkylative product 3ab when 4 equivalents of BrCF2COOEt
We commenced our study by utilizing 2-phenylaniline (1a) and was used.
BrCF2COOEt as model substrates for optimization of the reaction
The successful examples encouraged us to further apply our
(see SI for details). After many attempts, to our delight, the use of dual-role strategy of BrCF2COOEt for the construction of 2-
Cu(OTf)2/1,10-phen catalytic system, the tandem transformation difluoroacetate benzothiazoles or benzoxazoles from 2-thiol aniline
could afford the desired product 3a in 82% yield by using Na2CO3 as or 2-hydroxy aniline, since these types of compounds are prevalent
base in CH3CN at 120 oC. Under the optimal reaction conditions, the scaffolds in many bioactive molecules,10 the incorporation of
substrate scope of amines in this novel cascade process was fluorine atom in the above two types of heterocyclic compounds
subsequently investigated (Scheme 2). Firstly, various amines with might significantly alter the property of these compounds. With
different substituents on the Ar2 ring were investigated, where C-H these in mind, initial attempts with above two types of anilines as
bond activation occurred. Various groups of Ar2 ring were well starting materials under our standard conditions were conducted.
compatible, rendering the target molecules in good to excellent Unfortunately, no desired 2-difluoroacetate benzazoles and
yields (3a-3q). The structure of 3a was unambiguously confirmed by benzothiazoles were obtained, and only trace amount of
X-ray crystallographic analysis.9 Of note, it is possible to provide two corresponding 2-difluoroacetate target molecules were detected.
isomers when the substrates bearing substituent on the meta- Inspired by our recent progress on the radical cyclization of 2-
position of the Ar2 ring are used. However, gratifyingly, excellent alkynyl thioanisoles,11 we changed 2-thiol aniline into 2-amino
regioselectivity was observed with these substrates in our thioanisole. To our delight, 2-amino thioanisole 4a could smoothly
transformation, and the products with C-H cyclization occurring convert into target 2-difluoroacetate benzothiazole 5a under the
para- to the substitute group was predominant (3r-3t), which standard conditions. A brief survey on the parameters of the
5k-5n
suggested that the C-H activation was sensitive to the steric reaction indicated that 30 mol% of B2pin2
environment. Subsequently, the substituent effect on the Ar1 ring
could significantly
promote the transformation (see SI), rendering 2-difluoroacetate
Scheme 2. The substrate scope of amines for the synthesis of 6- benzothiazoles (5) in decent yields with good functional group
difluoroacetate phenanthridinesa
tolerance (Scheme 3).
Scheme 3. 2-Difluoroacetate benzothiazoles formation from 2-
aminothioanisoles and ethyl bromodifluoroacetate with the latter
one serving as dual rolesa
Cu(OTf)2 (10 mol%)
1,10-phen (12 mol%)
SMe
Br
S
COOEt
+
R
R
F
F
COOEt
N
Na2CO3 (4 equiv)
B2pin2 (30 mol%)
NH2
F
F
5
2
4
CH3CN, 80 o
C
S
S
N
S
COOEt
COOEt
COOEt
N
MeO
N
F
F
F
F
EtO
F
F
F
5a, 78% (47%)b
5b, 77%
5c, 82%
S
N
S
N
S
N
COOEt
COOEt
COOEt
Me
F
Br
F
F
F
F3CO
F
F
5d, 85%
5e, 79%
5f, 67%
S
N
F
S
COOEt
COOEt
N
F
F
F
F
F
5g, 75%
5h, 61%
a Reaction conditions: 4 (0.2 mmol), ethyl bromodifluoroacetate (2) (3 equiv), Cu(OTf)2 (10 mol%),
1,10-phen (12 mol%), B2pin2 (30 mol%), Na2CO3 (4 equiv), CH3CN (2 mL) under N2 atmosphere
at 80 oC for 24 h. Isolated yields. b 2-(benzylthio)aniline as a substrate.
In order to understand the source of the extra carbon, different
difluoromethyl compounds and solvent were further screened (SI).
The formation of 6-substituted phenanthridine 3a and the trace
amount of 6 were observed under the optimized conditions. When
(bromodifluoromethyl)phosphonate 7 was subjected to the above
reaction, isocyanide 6 was obtained in 43% isolated yield with 31%
of amine 1a recovered (Scheme 4b). In addition, corresponding
isocyanides 8 was obtained in 45% isolated yield, with 24% of
starting material amines 1g recovered in the absence of Cu catalyst
was explored. Not surprisingly, the cascade transformations
proceeded smoothly as well and delivered the corresponding
products (3u-3z) in 71-86% yields. To our delight, the heterocyclic
amines such as 2-phenylpyridin-3-amine and 2-(1H-pyrrol-1-
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ligand, satisfactory yields of isocyanide 6 was obtained with both
BHT and TEMPO cases (Scheme 5c). Above results suggested the
formation of isocyanide is
a radical free process, while the
subsequent cyclization and difluoroalkylation step should be a SET
pathway. In order to trap the possible intermediate (SI), carbene
scavenger benzimidazole (9) was added to the reaction for the
synthesis of 6-difluoroacetate phenanthridine with 1g as the
substrate, notably, 3g was obtained in 24% yield along with the
formation
of
1-(difluoromethyl)-1H-benzo[d]imidazole
10.
Interestingly, the yield of 10 was increased to 62% and trace
amount of the desired product 3g was detected when the reactant
was corresponding isocyanide 8 (Scheme 5d). All the three control
experiments clearly indicated that difluorocarbene (:CF2) was
generated in situ during this transformation.
On the basis of the above results and literature, the reaction is
proposed to occur by an isocyanide intermediate which was
generated from a difluorocarbene and primary amine with the help
of base (Scheme 6). To investigate the formation of difluorocarbene
and isocyanide intermediate, DFT calculations were performed12 (SI).
Computational results suggest that the BrCF2COOEt reacts with
Na2CO3 to generate BrCF2COONa which undergoes further
decarboxylation and debromination processes to afford
difluorocarbene. The activation free energy of the generation of
difluorocarbene is 26.1 kcal/mol which can be overcome under the
reaction condition. Subsequently, difluorocarbene reacts with
amine substrate in the presence of Na2CO3. Computations reveal
that the formation of C-N bond, the N-H and C-F bonds activation
Scheme 4. Control experiments to figure out the key intermediate
for this transformation.
and ligand (Scheme 4c). These results implied the extra carbon in
both 3a and 6 should come from the carbon which is bound to two
fluorine atoms in BrCF2COOEt and base itself could promote the
formation of isocyanide slowly, yet Cu salt and ligand will lead to
the sequential cyclization and difluoroacetylation of isocyanide. Not
surprisingly, the formation of phenanthridine actually could
accelerate the speed for the generation of isocyanide. Based on the
above experiments, isocyanide might be the key intermediate for
this cascade transformation. To verify our hypothesis, the
isocyanides 8 was subjected to the optimized conditions with
BrCF2COOEt (SI). Gratifyingly, the corresponding desired product 3g
was obtained in good yield (Scheme 4d). These results clearly
demonstrated that isocyanides are the key intermediates for this
tandem transformation.
by Na2CO3 occur in
a concerted way (TS3), leading to N-
(fluoromethylene)arylaminium (P), NaHCO3 and NaF. Then P easily
decomposes to isocyanide M via N-H and C-F bonds cleavage by
Na2CO3. Calculation results indicate the formation of isocyanide
intermediate from difluorocarbene and amine with the assistance
Scheme 5. Control experiments and radical trapping experiments.
Scheme 6. Plausible reaction mechanisms
of Na2CO3 is quite facile (ꢀGTS3‡ = 11.3 and ꢀGTS4‡ =1.3 kcal/mol).
Strong interaction between the sodium atom of Na2CO3 and fluorine
atom is observed in the preceding intermediate of TS3 and TS4,
respectively, which elongates C-F bond by about 0.1 Å compared to
normal C-F bonds. Thus the pre-activation of C-F bond by Na2CO3
may account for the facile C-F bond cleavage and low activation
free energies of TS3 and TS4. Once isocyanide M is generated,
Radical trapping experiments were then carried out under
various conditions in order to thoroughly understand the
mechanism of this transformation (Scheme 5a-c). For
phenanthridine synthesis, when aryl amine 1a or the corresponding
isocyanide 6 was exposed to the standard conditions in the
presence of radical scavenger (BHT or TEMPO), no desired product
was obtained (Scheme 5a-5b). Next, when reaction was performed
with aryl amine 1a and BrCF2COOEt in the absence of Cu salt and
sequential reaction between
M
and BrCF2COOEt takes place,
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,
We have developed a novel and unusual C-F bond activation
from BrCF2COOEt and primary amines, rendering straightforward
synthetic methods toward two types of valuable heteroaromatic
rings. These protocols proceed via an isocyanide intermediate
which is generated in situ between BrCF2COOEt and primary amines
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Acknowledgements
Financial support from the National Natural Science Foundation
(21772046), Program of Innovative Research Team of Huaqiao
University (Z14X0047), the Recruitment Program of Global Experts
(1000 Talents Plan), the Natural Science Foundation of Fujian
Province (2016J01064) are gratefully acknowledged. We also thank
Instrumental Analysis Center of Huaqiao University for analysis
support X. M. thanks the Subsidized Project for Cultivating
Postgraduates’ Innovative Ability in Scientific Research of Huaqiao
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