Photoredox-induced three-component Oxy-, Amino-, and Carbotrifluoromethylation of enecarbamates

Article abstract of DOI:10.1021/ol500374e

A photoredox-catalzyed trifluoromethylation of enecarbamates process is reported. This pathway uses Togni's reagent as the CF₃ source and follows a radical/cationic pathway. Under the optimized conditions using [Ru(bpy)₃(PF₆)₂] as the photocatalyst, a wide range of substituted enecarbamates can readily be difunctionalized by means of various O, N, and C nucleophiles.

Full text of DOI:10.1021/ol500374e

Letter
pubs.acs.org/OrgLett
Photoredox-Induced Three-Component Oxy‑, Amino‑, and
Carbotrifluoromethylation of Enecarbamates
Aude Carboni,† Guillaume Dagousset,† Emmanuel Magnier,*,‡ and Geraldine Masson*,†
́
†Centre de Recherche de Gif, Institut de Chimie des Substances Naturelles, CNRS, 91198 Gif-sur-Yvette Cedex, France
‡Institut Lavoisier de Versailles, UMR 8180, Universite
́
de Versailles-Saint-Quentin, 78035 Versailles Cedex, France
S
* Supporting Information
ABSTRACT: A photoredox-catalzyed trifluoromethylation of enecarbamates
process is reported. This pathway uses Togni’s reagent as the CF3 source and
follows a radical/cationic pathway. Under the optimized conditions using
[Ru(bpy)3(PF6)2] as the photocatalyst, a wide range of substituted enecarbamates
can readily be difunctionalized by means of various O, N, and C nucleophiles.
Scheme 1. Photocatalyzed Three-Component Reaction of
β-Trifluoromethyl amines have been recognized as important
structural motifs in many bioactive compounds.1 The presence
of CF3 enhances the hydrophobicity, metabolic stability, and
bioavailability of the amines, making them useful intermediates
in the synthesis of fluorinated amino acids and peptidomi-
metics.2 In spite of the prime importance of β-trifluoromethyl
amines in the identification of novel drugs, there are few direct
syntheses of these amines.3 Indeed, in most synthetic works,
the reagents already bear a trifluoromethyl group, such as
trifluoromethylated alkenes,1d,e,2a,b,d,e,4 ketones,5 or epoxides.1c
Recently, Loh et al. reported an elegant Cu-catalyzed
oxytrifluoromethylation of α-substituted enamides.3b Although
this process is effective, the method is restricted to terminal
enamides, leading to the formation of quaternary carbon-
containing β-trifluoromethylated α-amidoether products. For
this reason, establishing methods that can directly incorporate
the CF3 group6 at the β-position of a variety of amines
continues to be actively pursued.
Enecarbamates10 and Rational Design for
Trifluoromethylation of Enecarbamates by Photoredox
Catalysis
enecarbamates in the presence of photoredox catalysts 5
(Scheme 1, eq 2). Herein, we wish to report an efficient process
giving rise to key and original trifluoromethyl amines. Direct
intermolecular oxy-, amino-, and carbotrifluoromethylation of a
wide range of substituted enecarbamates is described in this
paper.
Our initial investigations focused on the Ru-photocatalyzed
trifluoromethylation of benzyl N-vinylcarbamate 1a with
Umemoto’s reagent 4a in the presence of methanol 3a
irradiated by blue LEDs. As described in Table 1, we were
able to obtain the expected three-component trifluoromethy-
lated adduct 6a in 19% yield. Replacing 4a with a more reactive
nitro-substituted derivative 4b resulted only in the degradation
of 4b and enecarbamate 1a. Various sources of CF3 radical were
then tested. While no reaction took place with potassium or
Photoredox7 trifluoromethylation of alkenes has attracted
considerable attention over the past few years.3b,8 Various
substrates such as standard alkynes,8e alkenes,8c,j,n,q styrenes,8g,h
allylic alcohols and amines,8k enolates,8g or acryl amides8o are
generally involved. Specifically, enecarbamates have never been
used as substrates for this type of process, although they are a
promising starting material for the synthesis of β-trifluor-
omethyl amines.9
Recently, our group became interested in the C−H
functionalization of enamides and enecarbamates.10 Notably,
we could efficiently perform a photocatalyzed α,β-difunction-
alization of enecarbamates through a three-component radical/
cationic reaction11,12 between enecarbamate 1, alkyl halide 2,
and alcohol 3 (Scheme 1, eq 1).10a Based on this experience as
well as on our knowledge of electrophilic trifluoromethylating
reagents,13,14 we envisioned the direct trifluoromethylation of
Received: January 21, 2014
Published: February 12, 2014
© 2014 American Chemical Society
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Letter
Table 1. Survey of Reaction Conditions for the
Photocatalyzed Oxytrifluoromethylation of 1a
Scheme 2. Substrate Scope of the Oxytrifluoromethylation of
Enecarbamates 1
a
a c
−
b
entry
4
R
photoredox cat.
6
yield (%)
1
4a
4b
4c
4d
4e
4f
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
Ru(bpy)3(PF6)2 (5a)
6a
6a
6a
6a
6a
6a
6a
6a
6a
6a
6a
6a
6a
6a
6b
6c
6d
19
2
5a
c
3
5a
4
5a
5
5a
83
d
6
5a
86
7
4e
4e
4e
4e
4e
4e
4e
4e
4e
4e
4e
Ru(bpy)3Cl2 (5b)
70
77
8
Ir(ppy)2(dtbbpy)(PF6) (5c)
e
9
5a
77
10
11
12
13
14
15
16
17
f
f,g
f,g
5a
5a
5a
5a
5a
5a
h
36
c,i
a
Reaction conditions: enecarbamate 1 (0.10 mmol), 4e or 4g (0.12
mmol), 5a (0.05 equiv) in MeOH. 3a (2.0 mL) irradiated at rt for 3 h.
71
50
iPr
H
b
j
Yields referred to chromatographically pure product. Yields in
69
c
parentheses are obtained when 1 mol % of 5a is used. dr determined
a
General conditions: 1a (0.10 mmol), CF3 source (0.12 mmol), 5
by 19F NMR analysis of crude mixtures. Because of the presence of
b
(0.05 equiv) in MeOH (2.0 mL) irradiated at rt for 3 h. Yields
referred to chromatographically pure product. Trace of product 6a
and degradation of 1a were observed. Yield determined by NMR
spectroscopy. With 1 mol % of 5a. 1a was recovered. Without any
irradiation. With 2 equiv of Et3N. With 2 equiv of 1,3-
dinitrobenzene. With a 1:1 mixture of THF/H2O as solvent. dtbbpy
d
rotamers, it was impossible to assign the syn/anti assignments. With
10 mol % of Ph3P. Starting from (E)-enecarbamate. Starting from
(Z)-enecarbamate.
c
e
f
d
e
f
g
h
i
j
which has not previously been reported.17 Pleasingly, the side
reaction was suppressed by the addition of a catalytic amount of
Ph3P as an iodine trap,18 and the desired β-trifluoromethyl
amines were produced with similar yields. Unexpectedly, both
diastereomers (Z)-1e and (E)-1e of benzyl prop-1-en-1-
ylcarbamate led to the corresponding product 6e in similar
yield, albeit with a reverse diastereoselectivity. A variety of
protecting and functional groups bonded to the nitrogen of 1,
including benzylthiourea, benzylthiocarbamate, and Boc, were
well tolerated, as well as the functionalized enecarbamate 1n
derived from Garner’s alcohol. The cyclic enecarbamate 1j and
enethiourea 1k also reacted successfully to afford compounds 6j
and 6k. They are a direct intermediate, after reduction of the
aminal function, to the very rare but promising, in terms of
bioactivity, 3-(trifluoromethyl)pyrrolidine.19 Interestingly, the
enecarbamate 1o bearing a free hydroxy group reacted
successfully to afford the intramolecular oxytrifluoromethylated
product 6o. Finally, we showed that this method can also be
applied to the synthesis of β-perfluoroalkylated amino
derivatives. Thus, the hypervalent iodine reagent 4g bearing a
C2F5 substituent was prepared according to the literature
procedure20 and gave the expected adduct 6p in 65% yield by
using the optimized conditions.
= di-tert-butyl-2,2′-bipyridine.
sodium trifluoromethanesulfinate 4c,d (Langlois reagent),
Togni’s reagents 4e and 4f15 reacted very smoothly and
afforded 6a in 83% and 86% yield (entries 5 and 6),
respectively.16 After screening different Ru and Ir photo-
catalysts, [Ru(bpy)3(PF6)2] 5a was found to be the most
efficient catalyst, since we could decrease its loading to 1 mol %
without any significant loss of reactivity (entries 9 vs 5). The
stoichiometry of the reaction and the solvents were also
optimized (see the Supporting Information). Other alcohols
(EtOH (3b), iPrOH (3c)) and water were also found to be
suitable reaction partners, affording the corresponding
trifluoromethylated aminals or hemiaminals in slightly lower
yields (entries 15−17).
With the optimized reaction conditions in hand, the scope of
this Ru(bpy)3(PF6)2-catalyzed three-component oxytrifluoro-
methylation of enecarbamates was investigated (Scheme 2). To
our delight, a wide range of β-substituted enecarbamates
bearing either a linear or branched alkyl group reacted
efficiently to afford the expected trifluoromethylated adducts
6e−j in good to excellent yields (60−92%). However, in some
cases it was found that the trifluoromethylated products 6 were
contaminated with impurities (<10%) that were attributable to
the iodinated products 7. This side product 7 resulted from the
iodination of 1 with iodine generated during the reaction,
Encouraged by this result, we turned our efforts toward
extending this radical/cationic multicomponent protocol to the
challenging intermolecular amino- and carbotrifluoromethyla-
tion of enecarbamates 1. In contrast to the oxytrifluoromethy-
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Organic Letters
Letter
lation, there have been only few reports of the amino- and
carbotrifluoromethylation of alkenes.21 The first example of
photocatalyzed aminotrifluoromethylation was reported re-
cently by Koike and Akita et al.,8h and photocatalyzed
carbotrifluoromethylation of alkenes has been limited to
intramolecular aryltrifluoromethylation.8o While the develop-
ment of such reactions is difficult because of possible
competition between the nitrogen or carbon nucleophile with
MeOH, we reasoned that under appropriate conditions amino-
and carbotrifluoromethylation may also proceed. In our initial
experiment, we were pleased to find that irradiation of a THF/
H2O solution of enecarbamate 1e, Togni’s reagent 4e (1.2
equiv) and NaN3 (5 equiv) with 5a as photocatalyst afforded
the aminotrifluoromethylated product 8a in 45% isolated yield,
without the formation of the corresponding hemiaminal 6 (R =
H). However, a partial decomposition of 4e and traces of the
corresponding β-iodo compound were observed during the
reaction. Fortunately, the yield was improved to 65% with 2
equiv of 4e and 10 mol % of Ph3P. A variety of functionalized
and β-substituted enecarbamates were subjected to these
conditions, furnishing the corresponding α-azido-β-CF3 com-
pounds 8a−d in good yields (65−79%, Scheme 3), which are
mechanism can be proposed on the basis of the above results
(Scheme 4). First, irradiation with visible light excites
Scheme 4. Proposed Mechanism for the Radical/Cationic
Trifluoromethylation of Enecarbamates
2+
2+
Ru(bpy)3 into a strong reductant species *Ru(bpy)3
,
which performs a single electron transfer (SET) to generate
•CF3 from Togni’s reagent 4e.8i,o Subsequent regioselective
addition of electrophilic •CF3 to enecarbamate 1 leads to the α-
amido radical 10, which can be rapidly oxidized into N-
10a,23
3+
acyliminium cation 11 by SET from Ru(bpy)3
.
Final
nucleophilic trapping by alcohol 3, NaN3, or KCN affords the
corresponding trifluoromethylated adducts 6, 8, or 9. It is also
worth noting that in the case of amino- and carbotrifluor-
omethylation the desired product 8 or 9 was not observed on
addition of NaN3 or KCN sequentially,24 thus suggesting that
the equilibrium between hemiaminal 6 (R = H) and N-
acyliminium cation 11 does not take place during the reaction
and that 11 is directly trapped by NaN3 or KCN.
Scheme 3. Substrate Scope of the Intermolecular Amino-
and Carbotrifluoromethylation of Enecarbamates 1
a c
−
In conclusion, a photoredox-induced three-component
synthesis of β-trifluoromethyl amines has been developed.
The procedure is suitable for the completely regioselective
synthesis of a wide variety of functionalized oxytrifluoromethy-
lated carbamates. Importantly, this radical/ionic process has
also been adapted and broadened to new N and C nucleophiles.
To the best of our knowledge, this method represents the first
intermolecular photoinduced carbotrifluoromethylation of
carbon−carbon double bonds. Extension of this method, to
more elaborated substrates for the preparation of potentially
bioactive molecules, is currently underway in our laboratory
and will be reported in due course.
a
General conditions: 1 (0.10 mmol), 4e (2−2.5 equiv), NaN3 or KCN
(5 equiv), 5a (0.05 equiv), and Ph3P (0.1 equiv) in a 1:1 mixture of
b
THF/H2O (2.0 mL) irradiated at rt for 12 h. Yields referred to
c
chromatographically pure product. dr determined by 19F NMR
ASSOCIATED CONTENT
analysis of crude mixtures. Because of the presence of rotamers, it was
impossible to assign the syn/anti assignments.
■
S
* Supporting Information
Detailed experimental procedures and spectral data for all new
compounds are provided. This material is available free of
charge via the Internet at http://pubs.acs.org.
direct precursors of the corresponding α-amino-β-CF3
compounds. Similarly, KCN could be efficiently used instead
of NaN3, leading to cyanotrifluoromethylated precursors of
amino acids.22 This analogous photoinduced intermolecular
carbotrifluoromethylation of double bonds remains unexplored.
Fortunately, as shown in Scheme 3, all the reactions were also
found to proceed smoothly under similar conditions to afford
the corresponding α-cyano-β-CF3 compounds 9a−d in accept-
able yields.
The following control experiments were carried out to gain
some mechanistic insight. No reaction took place in the
absence of irradiation and/or [Ru(bpy)3(PF6)2] 5a. Moreover,
the formation of 6 was inhibited in the presence of radical
scavengers such as TEMPO, suggesting that a radical/cationic
process is involved in this reaction. Although the mechanism of
this transformation is not yet completely clear, a possible
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: geraldine.masson@cnrs.fr.
*E-mail: emmanuel.magnier@uvsq.fr.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank ICSN and CNRS for financial support and doctoral
fellowships to A.C. G.D. thanks Labex Charmmmat for a
postdoctoral fellowship. Lucy Cooper is acknowledged for
improvement of the English usage in the manuscript.
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Letter
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(23) Oxidation of 10 by Togni’s reagent 4e to regenerate •CF3
(radical chain propagation) was excluded, as the reaction requires
continuous irradiation.
(24) In this case, the reaction leads only to the corresponding
hemiaminal 6 (R = H).
6209. (l) Mizuta, S.; Engle, K. M.; Verhoog, S.; Galicia-Lop
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ez, O.;
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