Decarboxylative and Denitrative Trifluoromethylation for the Synthesis of Cvinyl-CF3 Compounds with Togni (II) Reagent

Article abstract of DOI:10.1002/adsc.201500631

A highly efficient dimethylformamide (DMF)-promoted decarboxylative trifluoromethylation of α,β-unsaturated carboxylic acids with Togni (II) reagent under metal-free conditions has been developed. The reactions showed good yields, high stereoselectivities and excellent functional group tolerance. Mechanistic studies confirmed that free-radical processes were involved in this system since the CF₃ radical had been clearly trapped by scavengers. This method has been extended to the denitrative trifluoromethylation of β-nitrostyrenes in the presence of an iron(III) catalyst.

Full text of DOI:10.1002/adsc.201500631

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DOI: 10.1002/adsc.201500631
Decarboxylative and Denitrative Trifluoromethylation for the
À
Synthesis of Cvinyl CF3 Compounds with Togni (II) Reagent
Jing-jing Ma,a Wen-bin Yi,a,* Guo-ping Lu,a and Chun Caia
a
School of Chemical Engineering, Nanjing University of Science and Technology, Xiao Ling Wei Street, Nanjing 210094,
Peopleꢀs Republic of China
Fax: (+86)-25-8431-5030; phone: (+86)-25-8431-5514; e-mail: yiwenbin@mail.njust.edu.cn
Received: June 30, 2015; Revised: August 13, 2015; Published online: October 29, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500631.
Abstract: A highly efficient dimethylformamide
(DMF)-promoted decarboxylative trifluoromethyla-
tion of a,b-unsaturated carboxylic acids with Togni
(II) reagent under metal-free conditions has been
developed. The reactions showed good yields, high
stereoselectivities and excellent functional group
tolerance. Mechanistic studies confirmed that free-
radical processes were involved in this system since
the CF3 radical had been clearly trapped by scav-
engers. This method has been extended to the deni-
trative trifluoromethylation of b-nitrostyrenes in
the presence of an iron(III) catalyst.
prefunctionalized vinyl sulfonates,[8] vinylboron deriv-
atives[9] and by means of directing groups.
Carboxylic acids are ubiquitous compounds in or-
ganic chemistry, which are commercially available in
a large variety.[10] Transition metal-catalyzed decar-
boxylative trifluoromethylation is an attractive proto-
col for the synthesis of Cvinyl-CF3. Hu,[11] Liu,[12]
Maiti[13] and Duan[14] reported chronologically and in-
dependently the transition-metal-catalyzed trifluoro-
methylation of a,b-unsaturated carboxylic acids. Most
of these methods, however, suffered from several dis-
advantages, such as excess of reagents (overloaded
a,b-unsaturated carboxylic acids or trifluoromethyla-
tion reagents) and limited substrate scopes. Although
chemists were remarkably adept at decarboxylation
and direct trifluoromethylation, mainstream develop-
Keywords: decarboxylative reaction; denitrative re-
action; dimethylformamide (DMF); radical trifluor-
omethylation; Togni (II) reagent
The trifluoromethylation of organic compounds is
a significant topic of current interest in medicinal
chemistry.[1] Among them, compounds containing the
À
Cvinyl CF3 moiety have been shown to be useful fluo-
rinated substituents in medicinal chemistry over the
years (Figure 1).[2] Nevertheless, there are no natural-
À
ly occurring Cvinyl CF3 compounds, so it is not surpris-
ing that numerous efforts have been devoted to the
development of new methodologies for the prepara-
tion of these compounds.[3] More specifically, two
major approaches for the formation of these bonds
have emerged in last decade. One is the indirect cou-
pling of aryl haildes with highly volatile or pre-pre-
À
pared Cvinyl CF3 reagents, such as 1-iodo-3,3,3-
trifluoropropane,[4] (E)-trimethyl-(3,3,3-trifluoroprop-
1-enyl)silane[5] and 2-(trifluoromethyl)acrylicacid.[6]
The other is the direct trifluoromethylation reaction,
using unsaturated alkenes[7] or relatively expensive
and specifically prefunctionalized substrates such as
Figure 1. Molecules containing the Cvinyl-CF3 moiety.
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Jing-jing Ma et al.
Table 1. Optimization of the decarboxyltive trifluoromethy-
ments were still restricted to the metal-assisted ap-
proach, such as the pioneering use of photocatalysis
with metal complexes.[15]
lation conditions.[a]
Due to the low threshold residual tolerance of
metals in pharmaceuticals, it is highly desirable to
obtain them under transition metal-free conditions.
Recently, several studies on developing the decarbox-
ylation of a,b-unsaturated carboxylic acids for the for-
mation of alkenyl sulfones[16] and allylamines were re-
ported.[17] Furthermore, trifluoromethylation using
Togni (II) reagent [1-trifluoromethyl-1,2-benziodoxol-
3-(1H)-one] under metal-free conditions has attracted
attention in this field, such as using Bu4NI (tetrabutyl-
ammonium iodide) and TEMPONa (sodium 2,2,6,6-
tetramethylpiperidin-1-olate).[18] In the course of our
study with metal-free synthetic strategies,[19] we exam-
ined the metal-free decarboxylative trifluoromethyla-
tion and successfully found that DMF can promote
the decarboxylative trifluoromethylation of a,b-unsa-
turated carboxylic acids with Togni (II) reagent. This
method achieved the smooth and quantitative trans-
Entry CF3 (equiv.) Solvent[b] Temp.
Yield
[%]
E/
[8C]
Z[d]
1
2
3
Togni (II)
Togni (II)
Togni (II)
Togni (II)
Togni (II)
Togni (II)
Togni (II)
Togni (II)
Togni (II)
Togni (I)
CF3SO2Na
CF3SO3Na
TMSCF3
HCONH2 120
TMEDA 120
N.D.[c]
N.D.[c]
42
98
98
87
71
37
–
–
DMAc
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
120
120
120
100
80
95/5
100
100
91/9
72/28
56/44
–
100
–
–
4
5[e]
6
7
8
9
10
11
12
13
14
60
r.t.
trace
13
120
120
120
120
120
N.D.[c]
N.D.[c]
N.D.[c]
N.D.[c]
–
–
CF3SO3TMS DMF
À
formation to various Cvinyl CF3 products without tran-
sition metal catalysts.
[a]
Unless otherwise stated, all reactions were carried out
with 1b (0.2 mmol), CF3 reagent (1.0 equiv.), solvent
(2.0 mL) for 24 h, GC yield.
TMEDA=tetramethylethylenediamine, DMAc=N,N-di-
methylacetamide, DMF=N,N-dimethylformamide.
Not detected by GC-MS.
The E/Z ratio was determined by GC-MS on crude prod-
ucts.
Under an argon atmosphere.
Nitroolefins are useful intermediates in organic syn-
thesis and are important structural units that can be
used as starting materials for many classes of com-
pounds.[20] Denitrative reactions of b-nitrostyrenes
with halides,[21a,b] thiols,[21c] and olefins,[21d] generating
alkenes or vinyl sulfides had been achieved until now.
Denitrative trifluoromethylation of b-nitrostyrenes
under Togni (II)/DMF conditions has been also ach-
ieved by our group which is a new protocol for the
[b]
[c]
[d]
[e]
À
synthesis of Cvinyl CF3 compounds.
Under the optimized reaction conditions, various
We initially chose the trifluoromethylation of 4- a,b-unsaturated carboxylic acids were tested to exam-
methylcinnamic acid (1b) as the model reaction to op- ine the scope of this system (Table 2). The reaction
timize suitable conditions (Table 1). After screening proved to be general and amenable to a range of
of solvents (entries 1–4), the best result was achieved structurally diverse substrates, and the desired prod-
by using 1 equiv. Togni (II) as trifluoromthylating re- ucts (2a–2cc) were obtained in good yields. In general,
agent in DMF at 1208C for 24 h, giving (E)-1-methyl- both electron-rich and electron-deficient substrates
4-(3,3,3-trifluoroprop-1-enyl)benzene (2b) in 98% reacted well and a wide variety of functional groups
yield (entry 4). Almost no difference was found when was covered, including methyl (2b), hydroxy (2c),
the reaction was run under an argon atmosphere phenyl (2d), methoxy (2e–2g), halogens (2h–2k), nitro
(entry 5). Only a trace of product was detected at (2l), carboxyl (2n). We found that substrates with
room temperature by GC-MS (entry 9), indicating a substituent (such as OMe) on the aromatic ring at
that it was a temperature-dependent reaction. Fur- any position of the ring (para-, meta- and ortho-) re-
thermore, as the temperature increased, the E config- acted efficiently with almost no difference in the yield
uration of product was obviously improved and this (2e–2g). It was worth noting that we firstly reported
might relate to the stability of the E isomer at high the decarboxylative trifluoromethylation of a-substi-
temperature (entries 6–8). Togni (I) reagent (1-tri- tuted a,b-unsaturated carboxylic acids (2p–2v), which
fluoromethyl-1,3-dihydro-3,3-dimethyl-1,2-benzioxo-
had a very important application in the biological ac-
dole) was also screened, but no obvious product peak tivity of medicines (Figure 1). Furthermore, acids with
was found (entry 10). When using other trifluorome- heteroaryl rings including furan, thiophene, and pyr-
thylating reagents (entries 11–14), such as CF3SO2Na, role, are also viable substrates in the current reaction,
À
CF3SO3Na, TMSCF3 [trimethyltrifluoromethyl)silane] giving the corresponding Cvinyl CF3 compounds (2w–
and CF3SO3TMS (trifluoromethyl trimethylsilanesul- 2y) in 97%, 93% and 92% yields, respectively. Alkyl-
fonate), no corresponding product 2b was obtained.
substituted a,b-unsaturated carboxylic acids, such as
4-ethoxy-4-oxobut-2-enoic acid, fumaric acid, also re-
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Decarboxylative and Denitrative Trifluoromethylation
Table 2. Decarboxylative trifluoromethylation of a,b-unsaturated carboxylic acids.[a]
[a]
Reaction conditions: 1 (0.2 mmol), Togni (II) (1.0 equiv.) in 2.0 mL DMF, ar 1208C for 24 h, isolat-
ed yield, only the E isomer formed.
GC yield (due to high volatility).
Substrate for (E)-3-(perfluorophenyl)acrylic acid (due to the product reacting with DMF).
[b]
[c]
acted smoothly to give the corresponding products reoselectivities (E configuration) in the presence of
(2z–2cc) in good yields. Notably, the reactions were ferric acetylacetonate [Fe(acac)3] as catalyst.
found to be highly stereoselective with only the E
isomer of the product being obtained.
To gain some mechanistic insights into the current
decarboxylative trifluoromethylation reactions, a set
Our next goal was to extend the method to the de- of comparative experiments was designed as shown in
nitrative trifluoromethylation process. Denitrative tri- Scheme 1. The reaction between Togni reagent (II)
fluoromethylation of b-nitrostyrenes afforded a low and carboxylic acid 1b gave the desired product 2b in
yield of the expected product under Togni (II)/DMF 95% yield (Table 2), while a similar reaction using 1-
conditions because of the unactivated nitro group. methyl-4-vinylbenzene failed. This result indicated
Some metal catalysts (see Table S-2 in the Supporting that the presence of a carboxylic acid group in the
Information) were added to address this issue. Fortu- substrate played an important role for the success of
À
nately, iron-catalyzed denitrative trifluoromethylation the efficient Cvinyl CF3 forming reaction, and implied
was achieved via a similar radical process. After that the reaction process involved decarboxylation
a quick screening of the reaction conditions, we found followed by trifluoromethylation. To better under-
that
denitrative
trifluoromethylation
products stand the mechanism of this transformation, a series
(Table 3) were achieved in good yields and high ste- of experiments was also carried out. No desired prod-
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Jing-jing Ma et al.
Table 3. Iron-catalyzed denitration trifluoromethylation of
b-nitrostyrenes in the Togni (II)/DMF system.[a]
Scheme 2. Mechanism experiments.
Scheme 3. Radical trapping experiments.
curred when the CF3 radical was trapped by TEMPO
in the presence of an Fe(III) catalyst. This result indi-
cated that the iron catalyst stimulated the denitrative
reaction but did not promote the generation of the
CF3 radical.
[a]
Reaction conditions: 3 (0.2 mmol), Fe(acac)3 (1.0 equiv.),
Togni (II) (1.0 equiv.) in 2.0 mL DMF, at 1208C for 24 h,
isolated yield, only the E isomer formed.
Although the role of DMF was not clear, it certain-
ly promoted the decomposition of Tongi reagent (II)
and thus initiated the reaction as proved by the fact
that the compound 8 could be significantly deter-
1
mined by GC-MS and verified by crude H NMR. We
preferred a plausible radical mechanism as shown in
Scheme 4.[12] High temperature promoted the interac-
tion of DMF and Togni (II), which was accompanied
by the homolytic cleavage of the hypervalent iodine
reagent to obtain the CF3 radical. The radical then co-
Scheme 1. Control experiments.
uct was detected from the reaction in the presence of ordinated to the carboxylic acid or b-nitrostyrene
TEMPO (3 equiv.) and 1,1-diphenylethylene functionality to generate the intermediate 9 or 10. Fi-
(3 equiv.) as radical scavengers (Scheme 2), which nally, facile removal of CO2 led to the desired decar-
points towards a radical mechanism. As a result, 4 boxylative trifluoromethylated product 2. The mecha-
(15%) and 5 (34%) were detected by GC-MS. This nism of denitrative triflluoromethylation was similar
observation was consistent with the hypothesis that to that of decarboxylative trifluoromethylation and
the reaction proceeds via a radical pathway. Further- the difference was that the iron catalyst activated the
more, additional direct evidence for the radical mech- nitro group and thus was likely to assist in the elimi-
anism was obtained by trapping the CF3 radical with nation of NO2.
the radical clocks diethyl 2,2-diallylmalonate[22]
and
In conclusion, we have developed a convenient de-
N,N-diallyl-4-methylbenzenesulfonamide[23] carboxylative trifluoromethylation reaction of various
(Scheme 3), where the desired cyclization products 6 a,b-unsaturated carboxylic acids using the Togni (II)/
and 7 were produced and detected by 19F NMR. Simi- DMF system. The advantages such as good yields,
lar mechanism experiments had been performed on high stereoselectivities, excellent functional group tol-
the denitrative trifluoromethylation of aryl-substitut- erance make this transition metal-free trifluoromethy-
ed nitroethenes. Compared to the situation without lation strategy useful and practical for the stereospe-
a metal catalyst, almost no obvious difference oc- cific synthesis of CF3-substituted E-alkenes. Mecha-
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Decarboxylative and Denitrative Trifluoromethylation
Scheme 4. Proposed reaction mechanisms.
nistic investigations revealed that this reaction pro-
ceeded via a radical pathway. The method has been
extended to the denitrative trifluoromethylation of
aryl-substituted nitroethenes in the presence of an
iron(III) catalyst.
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