Copper-catalyzed trifluoromethylation of alkenes: Synthesis of trifluoromethylated benzoxazines

Article abstract of DOI:10.1039/c5ob01196e

A simple base and ligand free copper catalyzed method for the construction of trifluoromethylated benzoxazines has been developed by using Umemoto's reagent. It involves the oxidative difunctionalization of alkenes through tandem C-O and C-CF₃ bond formations. Furthermore, synthesized benzoxazines were selectively converted into trifluoromethylated allylic and (E)-vinylic benzamides by the treatment of KO^tBu and CH₃Li, respectively.

Full text of DOI:10.1039/c5ob01196e

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Copper-Catalyzed Trifluoromethylation of Alkenes: Synthesis of
Trifluoromethylated Benzoxazines
Received 00th January 20xx,
Accepted 00th January 20xx
Sadhan Jana, Athira Ashokan, Shailesh Kumar, Ajay Verma and Sangit Kumar*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
simple base and ligand free copper catalyzed method for the Buchwald et al. had reported copper-catalyzed intramolecular
construction of trifluoromethylated benzoxazines has been developed by oxytrifluoromethylation of unactivated alkenes using Togni’s
using Umemoto’s reagent. It involves the oxidative difunctionalization of reagent for the synthesis of trifluoromethylated lactones.6
alkenes through tandem C-O and C-CF3 bond formations. Furthermore, Subsequently, intramolecular aminotrifluoromethylation7 and
synthesized
benzoxazines
were
selectively
converted
into carbotrifluoromethylation8 of simple alkenes have been
trifluoromethylated allylic and (E)-vinylic benzamides by the treatment of successfully established. Fu group had developed transition
KOtBu and CH3Li, respectively.
metal-free synthesis of trifluoromethylated oxazolines
exploiting CF3SO2Na as a CF3 source.9
Benzoxazines, N,O-containing six membered heterocycles are
present in many drug molecules and herbicides and also widely
used as building blocks for bioactive molecules. It shows interesting
biological and pharmaceutical properties such as progesterone
receptor (PR) modulators, anti-anxiety, anti-HIV, agonist, and
H
Cu-catalyst
[CuCF3]+
(1)
(2)
R
CF3
R
R
S
F3C
O
X
NH
antagonist activities (Fig.1).1,2
BF4
CF3
O
X
NC
CuI Catalyst
N
R
R
F3C
S
O
Scheme 1. Synthesis of trifluoromethylated of alkenes and benzoxazines
Cl
Cl
O
O
O
N
N
N
N
O
N
Construction of benzoxazine ring has been achieved by the
intramolecular cyclization of alkenes using electrophiles such
as Br+, I+, and KSCN, K2S2O8 combination.13,14 Benzoxazines
comprising CF3 group remained as an unexplored area.
Recently, Xiao et al. has demonstrated photocatalytic
oxytrifluoromethylation of N-allylamides for the synthesis of
Cl
H
H
H
O
Efavirenz
Anti-HIV acitivity
Progesterone RA
Etifoxine
psychiatric illness
Herbicides
Fig 1. Benzoxazine and related drugs
Incorporation of CF3 group in the organic molecule enhances
several biological properties such as solubility, lipophilicity,
metabolic stability, binding selectivity etc.3 As a result, several
trifluoromethylated heterocycles such as efavirenz and
celecoxib were used as potential drugs for the treatment of
HIV infection, arthritis, and spondylitis, respectively.4
trifluoromethylated
benzoxazines
using
expensive
Ru(bpy)3(PF6)2 catalyst and a base by radical pathway.15d
Copper-catalyzed trifluoromethylation of terminal alkenes
through allylic C-H bond activation has been accomplished by
Fu and Liu et al. exploiting copper catalyst, Umemoto’s
reagent, and 2,4,6-trimethyl pyridine reaction system (scheme
1, eq 1).10a In view of recent advances in Cu-catalyzed
As
a consequence, versatile methodologies are being
established for the introduction of CF3 moiety into
heterocycles and related organic molecules.5-12 However, the
functionalization
of
alkenes,16,17
we
envisioned,
trifluoromethylation of terminal alkene followed by
intramolecular addition of oxygen for the synthesis of
trifluoromethylated benzoxazines keeping allylic C-H bond
intact (eq 2). Herein, we present a simple base and ligand-free
Cu-catalyzed synthesis of trifluoromethylated benzoxazine
synthesis
of
CF3-containing
heterocycles
through
intramolecular cyclization has been less explored. In 2013,
heterocycles from N-(2-(prop-1-en-2yl)benzamide substrates
1
using Umemoto’s reagent 2 (scheme 1, eq 2). Synthesized
trifluoromethylated benzoxazines were also selectively
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converted into trifluoromethylated allylic and (E)-vinylic Cl, and Br favoured the trifluoromethylation which indeed led
benzamides.
good yields (74-92%) of 3e
-3h.
Table 1 Optimization of the reaction conditions a
F3C
F3C
F3C
F3C
O
F3C
O
O
O
Cu(I) (20 mol %)
N
N
N
+
+
Solvent (1 mL)
80 oC, N2
NH
S
N
Ph
NH
NO2
R
3e
3b
3c
(92%)
Ph
BF4
R, Me (59%)
R, OMe (50%)
3d
(54%)
Ph
CF3
O
O
F3C
3a
1a
2
4a
F3C
F3C
O
O
O
O
O
N
OMe
entry
catalyst + CF3 Source
solvent
Yield of 3ab
N
N
N
N
1
2
3
4
5
6
7
8
CuI + Togni’s reagent
CuI + TMSCF3
CuI +Shreeves’ reagent DCE
CuI + 2
- + 2
CuTc + 2
CuCN + 2
[Cu(CH3CN)4]BF4 + 2
[Cu(CH3CN)4]PF6 + 2
CuI + 2
DCE
DCE
30b, 45c
ND
37
52
ND
17
26
5
6
52
F
X
3i
3g
3h
(46%)
X, Cl (76%)
X, Br (74%)
3f
(79%)
F3C
F3C
F3C
DCE
DCE
DCE
DCE
DCE
DCE
DMF
DMAc
NMP
DMSO
DMSO
O
O
X
Cl
NO2
N
N
3j
3k (91%)
3l X, OMe (trace)
X
(67%)
3m
X, Cl (74%)
9
F3C
F3C
F3C
Br
10
11
12
13
14
X
O
O
N
CuI + 2
CuI + 2
CuI + 2
Cu
58
39
68
18
N
Cl
3n
3q
(trace)
X, Cl (71%)
3o X, CF3 (ND)
3p
3r (72%)
X, CH3 (ND)
F3C
aAll reactions were carried out using 0.2 mmol of 1a, 0.35 mmol of 2 in 1 mL
solvent at 80 oC in a Schlenk tube under nitrogen and progress of the reaction
was monitored by TLC upto 35 h. bPercentage yield was determined by 19F-
NMR using fluorobenzene as an internal standard. cYield of 4a, ND = Not
detected, CuTc = Thiophene-2-carbonyloxylate copper(I)
F3C
F3C
O
O
O
O
N
S
N
N
N
N
3t
(53%)
3s (52%)
3u (49%)
F3C
F3C
F3C
We initially examined the reaction of N-(2-(prop-1-en-2-yl)
benzamide 1a with CuI (20 mol %) and various
trifluoromethylating reagents in DCE (Table 1, entries 1-4).
TMSCF3 noticed to be sluggish and the formation of desired
product was not realized. Togni’s reagent gave a mixture of
desired trifluoromethylated product 3a in low yield (30%)
along with 45% allylic trifluoromethylated product 4a (entry 1,
O
O
O
N
N
3w (73%)
3x
(74%)
3v (72%)
Scheme 2. Synthesis of CF3-benzoxazines: scope with respect to amide and aniline rings
Structures of CF3-benzoxazines 3f and 3z are also established
by single crystal structure study (Fig 2, for 3z and
crystallographic details, please see SI page S202-S232). meta-
Methoxy substituted benzoxazine 3i was obtained in moderate
yield (46%) whereas Cl and NO2 substituted benzoxazines 3j
and 3k were obtained in 67 and 91% yields, respectively. The
reaction of meta-di-OCH3-subtituted substrate noticed to be
sluggish and only trace of respective trifluoromethylated
benzoxazine 3l was isolated. On the other hand, meta-di-
chloro-substituted benzoxazine 3m was obtained in 74% yield.
Table 1). Shreeves’ reagent, a triflate analogue of
37% yield of 3a. The use of Umemoto’s reagent
further increase in the yield by 15% of the desired product 3a
2
provided
2, led to
.
The presence of copper is crucial for trifluoromethylation as
the reaction failed to provide even traces of 3a in the absence
of copper (entry 5, Table 1). Although, various Cu sources
(Table 1, entries 6-9, 14), ligands, and bases were screened
(See SI, Table S1, page S5-S6 for detailed optimization study),
CuI alone was noticed to be effective. The change in the
solvent from DCE to DMAc, DMF, and DMSO (Table 1, entries
10-13) led to further improvement in the yield and best yield
(68%) was obtained in DMSO (entry 13, Table 1).
Next, substrate scope of trifluoromethylation of the alkenes
using 20 mol% of CuI at 80 oC in DMSO was studied. The
substitution in the amide ring of 1a was explored (Scheme 2).
Alkenes with electron donating substituents such as CH3,
OCH3, and tBu at para-position of benzamide ring, gave
trifluoromethylated benzoxazines 3b-3d in moderate yields
Fig 2. ORTEP diagrams of 3f and 5d
(50-59%) whereas electron withdrawing groups such as NO2 F,
2 | J. Name., 2012, 00, 1-3
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Similarly, the substrate with ortho-chloro substitution in amide trifluoromethylated (E)-vinyl benzamides 5c and 5d
,
ring, gave desired trifluoromethylated benzoxazine 3n in good respectively. Structure of 5d is also established by X-ray single
yield (71%). ortho-Methyl and trifluoromethyl substituted crystal study (Figure 2). It seems that KOtBu removes the less
substrates failed to yield trifluoromethylated benzoxazines 3o hindered methyl proton whereas CH3Li prefers abstraction of
and 3p and recovered quantitatively and this could be due to highly acidic proton adjacent to CF3 group.
the steric effect of CH3 and CF3 which may prevent the In the mechanistic consideration, copper(I) can form Cu(III)CF3
coordination of –CONHPh group with the CuCF3 (vide infra).
intermediate by the reaction of Umemoto’s reagent 2 (Scheme
On the other hand, bromo-substitution in the aniline ring para 5).10a The formation of Cu(I)CF3 intermediate is also possible by
to NH has negative effect on the trifluoromethylation reaction the disproportionation of Cu(I) into Cu(II) and Cu(0) followed
as only trace of 3q was obtained. Substrate with chloro by the reaction of Cu(0) with Umemoto’s reagent (please see
substituent in the aniline ring, para to alkene, underwent SI, page S8-S13 for details).12e,18 Nevertheless, both Cu(I)CF3
trifluoromethylation
smoothly
and
yielded
chloro and Cu(III)CF3 could act as trifluoromethylating reagent.12e
Ligand exchange with alkenamide substrate would lead to the
substrate –copper intermediate I.19
benzoxazines 3r in 72% yield.
Scheme 3. Substrate scope with regards to olefins
Next, substrates with various N-aromatics such as naphthyl
heteroaromatics; thiophenyl, furanyl, and pyridinyl were
subjected for trifluoromethylation reaction. Indeed, reaction
also showed compatibility with naphthyl and heteroaryl
containing substrates and respective trifluoromethyl
benzoxazines 3s-3v were obtained in 49-72% yields.
Scheme 5. Mechanism for Cu-catalyzed trifluoromethylation
Substrates not only with various aryl benzamides but also with
alkyl amides such as methyl and tert-butyl substituents were
also tolerated and yielded trifluormethylated benzoxazines 3w
and 3x in 73 and 74% yields, respectively.
Intramolecular coordination of alkene followed by the
formation of Heck-type four-membered ring would generate
transition state II. This could allow the formation of C-CF3 bond
leading to III, subsequent reductive elimination may give
benzoxazine, by concomitant release of catalyst. It seems
unlikely that the reaction proceeds via radical pathway
because reaction mixture failed to give any signal in EPR
spectrum.
The non-reactive nature of ortho-methyl and trifluoromethyl
substituted substrates (vide supra, 3o and 3p) and formation
of trifluoromethylated allylic product 4bb could be rationalized
Scheme 4. Further modification of CF3-benzoxazines
based on the proposed intermediates I-III. It seems that allylic
After studying various N-aryl, alkyl and benzamide substrates,
C-2 substituted alkenes were explored in the copper-catalyzed
C-CF3 and C-O bond formation reaction (Scheme 3). Alkenes
with H and Ph substituents at C-2 position provided excellent
yields (90 and 94%) of trifluoromethylated benzoxazines 3y
trifluoromethylation of alkene through allylic C-H bond
activation process10a also occurred along with the formation of
new C-CF3 and C-O bonds. This may be due to the steric effect
of iso-propyl substituent, therefore, gave a mixture of C-CF3
and C-O bond forming product 3bb and trifluoromethylated
and 3z
benzoxazines (3a, vide supra), 3aa, and 3bb were obtained in
68, 74, and 35% yields, respectively. In the case of 3bb
. C-2 substituted methyl, ethyl, and iso-propyl
allylic product 4bb
.
In Conclusion, we have developed a simple, ligand and base
free copper catalyzed method for the construction of
trifluoromethylated benzoxazines by using Umemoto’s
reagent. As the reaction involves mild conditions,
trifluoromethylated benzoxazines with functionalities such as
nitro and bromo could be constructed which are useful for the
,
formation of trifluoromethylated uncyclized product 4bb in
45% yield was also observed.
Next, the synthetic utility of trifluoromethylated benzoxazines
was explored (Scheme 4). An addition of KOtBu to
benzoxazines 3a
,
3e, and 3k provided trifluoromethylated
5a, and 5b, respectively. Interestingly,
later
stage
modifications.
Synthetic
utility
of
allylic benzamides 4a
,
trifluoromethylation reaction has also been demonstrated by
treatment of 3a and 3y with CH3Li provided
converting CF3-containing benzoxazines into allylic and (E)
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vinylic benzamides. The stereoselective synthesis utilizing
chiral ligand and copper catalytic system as well as biological
study of trifluoromethylated benzoxazines are under
investigation in our laboratory.
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S.K thanks DRDO New Delhi and IISER Bhopal for generous
funding. SJ, SK and AV acknowledge IISER Bhopal and UGC New
Delhi, respectively, for fellowship. SK also thank to Ch. Durga
Prasad for proofreading of the manuscript.
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4 | J. Name., 2012, 00, 1-3
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