Introducing a new radical trifluoromethylation reagent

Article abstract of DOI:10.1039/c5cc00905g

Perfluoro-3-ethyl-2,4-dimethyl-3-pentyl radical (PPFR) is a persistent radical stable at room temperature, but easily decomposes at 90 °C to produce a CF₃ radical which is able to react with a variety of aromatic compounds to afford the corresponding trifluoromethyl derivatives, usually as mixtures of regioisomers in good to excellent overall yields.

Full text of DOI:10.1039/c5cc00905g

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Introducing a new radical trifluoromethylation
reagent
Cite this: DOI: 10.1039/x0xx00000x
Azusa Sato,a Jianlin Han,b Taizo Ono,c Alicja Wzorek,d José Luis Aceñae and
Vadim A. Soloshonok*ef
Received 00th January 2012,
Accepted 00th January 2012
Dedicated to Professor Iwao Ojima on the occasion of his 70th birthday.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Perfluoro-3-ethyl-2,4-dimethyl-3-pentyl radical (PPFR) is a PPFR
1 as trifluoromethylation reagent features several
persistent radical stable at room temperature, but easily benefits, including controllable rate of CF3 radical release,
decomposes at 90 ºC to produce CF3 radical which is able to totally metal-free one-step process under operationally
react with a variety of aromatic compounds to afford the convenient conditions, which, most likely, bodes well for PPFR
corresponding trifluoromethyl derivatives, usually as
mixtures of regioisomers in good to excellent overall yields.
1 widespread, general application.
F
T 1/2
90 ºC: 6.08 h
100 ºC: 1.83 h
F3C
F3C
CF3
F3C
F3C
F
The profound impact of fluorine on modern pharmaceutical
industry is well recognized and has been the subject of
numerous publications.1 In particular, CF3-containing drug
candidates usually show improved efficacy, enhanced
membrane permeability and, most importantly, significantly
higher stability towards oxidative degradation.1,2 Consequently,
in the recent years the development of trifluoromethylation
methodology has been quite an intensive area of research.
Approaches based on electrophilic,3 nucleophilic4 and radical5
supply of a trifluoromethyl group are presently well-established
and offer an exciting wealth of methodological choices. The
latter approach has received exceptional attention6 as it can be
conducted under operationally convenient conditions7 and can
be readily scaled up.8 One may agree that all available sources
of trifluoromethyl radical have been already completely
exhausted9-19 and the current research is mainly being focused
on refining the reaction conditions and chemo/regio-selectivity.
Therefore, the discovery and introduction of new reagents
capable of generating CF3 radical might be of high scientific
and methodological significance.
F
F
CF3
+
F
CF3
F
CF3
CF3
F
CF3
F
PPFR 1
2
3
Scheme 1 Structure and reactivity of persistent perfluoroalkyl radical PPFR
1
Persistent perfluoroalkyl radical PPFR
1 was first reported
in 198520 and can be easily prepared by fluorination (F2) of
industrial perfluoropropylene trimer in good yields (up to ca.
90%). This procedure has been reproduced by us on a kilogram
scale rendering compound
1 readily available on truly large
scale. PPFR is stable at room temperature in the open air and
totally inert to the dimerization, O2, halogens, aqueous acid or
base, and its presence and concentration in a reaction mixture
can be monitored simply by routine GC, like it is in the case of
common organic compounds. However, at 80-120 ºC it
1
undergoes β-scission releasing CF3 radical
(Scheme 1). While this property has been known for quite some
time, PPFR was considered as merely a chemical curiosity
3 and olefin 2
1
and its applications remained unexplored.21 Only very recently,
the Ameduri group has reported a series of publications
Here we introduce perfluoro-3-ethyl-2,4-dimethyl-3-pentyl
radical (PPFR)
1 (Scheme 1) as a non-conventional source of
showing application of PPFR
1 to initiate various radical
trifluoromethyl radical and report preliminary data on its
application for direct trifluoromethylation of unfunctionalized
arenes. We demonstrate that the use of the benchtop stable
polymerization processes.22 On the other hand, opportunity to
use PPFR
1
as a new radical trifluoromethylation reagent has
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never been considered. To explore this exciting synthetic reaction mixture at 90 ºC for 24 h. All the reactions reported in
prospect we decide first to study the trifluoromethylation of Table 2 were conducted under these standard conditions to
toluene. Under arbitrary conditions, using toluene as a reactant allow for comparison of the data obtained. Under these
and a solvent, the mixture with PPFR
1 was heated at 90 ºC for conditions the trifluoromethylation of benzene 4b (entry 2)
24 h. As shown in Table 1 (entry 1) the target products, CF3- occurred with excellent outcome allowing preparation of
toluenes, were determined in the reaction mixture with a trifluoromethyl derivative 5b in excellent 93% yield. Similarly
combined yield of 42%. Regardless of the relatively low yield, good results were obtained in a series of mono-alkyl-substituted
it was an important conceptual breakthrough as this result benzenes (entries 3-8). Typically, the chemical yields in these
clearly demonstrated that PPFR
trifluoromethylating reagent.
1
can perform as radical trifluoromethylations were above 80%, occurring with
moderate preference for -substitution on the phenyl ring. It is
o
interesting to note that the steric bulk of the alkyl group had no
clear impact on neither reactivity nor substitution pattern. For
Table 1 Optimization of trifluoromethylation reaction using PPFR 1
example, trifluoromethylation of
mixture of CF3-derivatives 5d in 84% yield and ratio of
- as 1.7/1.0/1.6 (entry 4). On the other hand, significantly
bulkier benzene 4e, containing iso-propyl group afforded 86%
yield of a -, -, - mixture of products 5e in 1.7/1.0/1.0 ratio
n
-propyl benzene 4d gave a
PPFR 1
o
-,m-
CF3
solvent, N2
,p
90 ºC, 24 h
o
m p
Entry
Toluene
(equiv.)
20
1
1
PPFR
Solvent
Yield
(%)a
42c
59
94
65
56
88
90
0
Isomer ratio
(p/m/o)b
(equiv.)
(entry 5). Similar trends were observed in the reactions of
benzenes 4f-h (entries 6-8) giving rise to CF3-products 5f-h in
1
2
3
1
2
3
3
3
2
3
3
none
DCE
DCE
none
DCE
DCE
DCE
DCE
2.5/1.0/2.2
2.4/1.0/1.7
3.0/1.0/1.5
2.6/1.0/1.3
3.0/1.0/1.5
2.3/1.0/1.2
3.0/1.0/1.5
83-86% yields and moderate preference for
o-substitution.
4
1
1
1
1
Radical trifluoromethylation of a series of di-alkyl-substituted
benzenes 4i-l also gave rather excellent results (entries 9-12).
5d
6e
7e
8f
For example,
5i in a ratio of 1.3/1 and good 82% yield (entry 9). A bit lower
yield of 75% was recorded in the trifluoromethylation of
o-xylene 4i furnished two isomeric CF3-products
1
m
-
a Yields determined by 19F NMR analysis of the crude reaction mixtures and
based on toluene. b Determined by integration in the 19F NMR spectrum. c
Yield based on PPFR. d Reaction performed under O2 atmosphere. e 48 h
reaction time. f 1 equiv. of TEMPO was added to the reaction mixture.
xylene 4j, resulting in the formation of three isomeric
derivatives 5j (entry 10). Synthetically very attractive outcome
was obtained in the reaction of
product 5k (entry 11) in very good 81% yield. Interestingly,
trifluoromethylation of -Bu-toluene 4l proceeded with
p-xylene 4k which gave a single
Encouraged by this exciting result, we tried next to optimize
the reaction conditions. Application of 1,2-dichloroethane
(DCE) as a solvent and using only one equivalent of toluene
allowed us to enhance the yield to 59% (entry 2). Further
improvement in the reaction outcome was achieved using 3
p-t
excellent chemical yield and reasonably good regio-selectivity.
In this case two products 5l were obtained in a ratio of 2.9/1.0
with a preference for the less sterically constrained isomer
(entry 12). Rather exceptional in this study result was observed
in the trifluoromethylation of mesitylene 4m (entry 13) which
gave a mixture of mono- and bis-trifluoromethylated products
5m in a ratio of 2.8 (mono)/1.0 (bis) and excellent (94%)
equivalent excess of reagent
1 (entry 3). In this case the target
CF3-toluenes were obtained in excellent 94% yield. The use of
solvent and inert (N2) atmosphere were found to be essential for
high yields as the reactions conducted without a solvent (entry
4) or in the presence of O2 (entry 5) gave the products with
noticeably reduced chemical yields. On the other hand, a lower
chemical yield. This outcome suggested that reagent
1 may
hold some more synthetic potential for performing bis-
trifluoromethylation of highly electron-rich arenes. Next we
decided to study several examples of the trifluoromethylation of
halogen-substituted arenes (entries 14-16). In sharp contrast
with the alkyl-substituted arenes discussed above, reactions of
chloro- 4n (entry 14) and bromobenzenes 4o (entry 15)
occurred with noticeably lower chemical yields of 52 and 57%
respectively. These results suggested that trifluoromethylation
of benzenes containing electron-withdrawing substituents with
excess (2 equiv.) of the reagent
1 and longer reaction time (48
h) gave rather promising results (entry 6) allowing preparation
of the target trifluoromethylation products in very good 88%
yield. By contrast, the attempt to improve the 94% yield (entry
3) by using longer reaction time was unsuccessful and the CF3-
products were obtained with a bit lower chemical yield (entry
7). Finally, we demonstrated that in the presence of TEMPO
(entry 8) the reaction did not take place at all, clearly indicating
that the process under study is a radical trifluoromethylation.
With the optimized reaction conditions in hand our next
reagent
1 may require particular optimization of the reaction
conditions. Quite agreeably with possible electronic substituent
effect on the chemical yields, the reaction of 2-bromo-1,3-
dimethylbenzene 4p gave products 5p (entry 16) with typically
good for this study 89% yield. One may assume that in this case
two methyl groups overpowered the negative effect of the
bromine atom. Finally, we studied the trifluoromethylation of
naphthalene 4r, which proceeded in rather good both yield and
regio-selectivity for α-position (entry 17).
goal was the study of potential generality of reagent
1 for
trifluoromethylation of various unfunctionalized arenes. Most
successful and representative results are collected in Table 2.
As discussed above, the best results in trifluoromethylation
of toluene 4a (entry 1) were obtained using: 3 equivalents of
reagent 1, DCE as a solvent, N2 atmosphere and stirring the
2 | J. Name., 2012, 00, 1-3
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13 (m)f
94
2.8/1.0
(1.0/0)c
(1.0/0)d
(1.0/2.6)g
(a/ab)
Table 2 Substrate scope of radical trifluoromethylation reaction using
reagent 1
(78)c
(70)d
(94)g
a
CF3
b
PPFR 1
CF3
(3 equiv)
14 (n)
15 (o)
52
2.0/1.8/1.0
(a/b/c)
b
b
R
R
a
CF3
c
c
DCE, N2
90 ºC, 24 h
Cl
Cl
4
5
57
Entry
Substrate
Major product
Isomer ratiob
1.5/1.3/1.0
(a/b/c)
Yield
(%)a
a
CF3
1 (a)
94
3.0/1.5/1.0
(2.7/1.4/1.0)c
(a/b/c)
c
Br
Br
(81)c
a
CF3
b
16 (p)
17 (q)
89
3.2/1.0
(4.0/1.0)d
(a/b)
b
a
CF3
(75)d
2 (b)
3 (c)
93
CF3
(87)c
(81)d
Br
Br
84
6.8/1.0
(4.8/1.0)c
(a/b)
CF3
(70)c
73
4.2/3.1/1.0
(a/b/c)
a
c
a
b
CF3
b
Et
Et
a Yields determined by 19F NMR analysis of the crude reaction mixtures. b
Determined by integration in the 19F NMR spectrum. c Data in parentheses
reported in ref. 23. d Data in parentheses reported in ref. 16. e Data in
parentheses reported in ref. 24. f A mixture of mono- and bis-
trifluoromethylated products (a:ab) was obtained. g Data in parentheses
reported in ref. 25.
4 (d)
5 (e)
80
84
1.7/1.6/1.0
(a/b/c)
c
a
a
CF3
b
b
b
n-Pr
n-Pr
1.7/1.0/1.0
(a/b/c)
c
CF3
Considering the data presented in Table 2, one may agree
that compound
trifluoromethylation of unfunctionalized arenes. Notably, the
chemical yields of the target products are generally higher
1 works really well as a reagent for radical
-Pr
-Pr
i
i
6 (f)
7 (g)
8 (h)
86
86
1.8/1.6/1.0
(a/b/c)
c
a
a
CF3
5
(entries 1, 2, 9-13, 16 and 17) as compared with the data
reported for the known trifluoromethylation reagents.16,23-25
In conclusion, this work has revealed that perfluoro-3-ethyl-
-Bu
-Bu
n
n
2.5/1.5/1.0
(a/b/c)
b
CF3
c
c
2,4-dimethyl-3-pentyl radical (PPFR)
successfully used as new reagent for radical
trifluoromethylation of unfunctionalized arenes. We
1 (Scheme 1) can be
a
s-Bu
t -Bu
s-Bu
83
1.7/1.0/0
(7.7/4.8/1.0)e
(a/b/c)
b
(81)e
a
CF3
demonstrate that the application of the reagent
1 offers some
beneficial features, such as high chemical yields and
operationally convenient conditions, underscoring its high
synthetic potential.
We thank IKERBASQUE, Basque Foundation for Science,
and the Basque Government (SAIOTEK S-PE13UN098) for
financial support.
t-Bu
9 (i)
82
1.3/1.0
(1.4/1.0)c
(2.0/1.0)d
(a/b)
(65)c
(72)d
a
CF3
b
10 (j)
75
4.2/3.1/1.0
(5.2/3.5/1.0)c
(2.0/1.0/0)d
(a/b/c)
c
(76)c
(77)d
a
CF3
Notes and references
a Department of Chemistry, Tokyo Women’s Medical University, 8-1
Kawada-cho, Shinjuku-ku, 162-8666 Tokyo, Japan
b School of Chemistry and Chemical Engineering, Nanjing University
210093 Nanjing, China
b
11 (k)
12 (l)
81
CF3
(76)c
(77)d
c National Institute of Advanced Industrial Science and Technology (AIST),
2266-98 Anagahora, Shimoshidami, Moriyama, Nagoya, Aichi, 463-8560,
Japan
93
2.9/1.0
(5.0/1.0)d
(a/b)
b
t-Bu
(78)d
d Institute of Chemistry, Jan Kochanowski University in Kielce,
Świętokrzyska 15G, 25-406 Kielce, Poland
t-Bu
a CF3
e Department of Organic Chemistry I, Faculty of Chemistry, University of the
Basque Country UPV/EHU, Paseo Manuel Lardizábal 3, 20018 San
Sebastián, Spain. Fax: +34 943-015270; Tel: +34 943-015177; E-mail:
vadym.soloshonok@ehu.es
e IKERBASQUE, Basque Foundation for Science, María Díaz de Haro 3,
48013 Bilbao, Spain
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†
Electronic Supplementary Information (ESI) available: Experimental
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