Photoredox-catalysed redox-neutral trifluoromethylation of vinyl azides for the synthesis of α-trifluoromethylated ketones

Article abstract of DOI:10.1039/c6cc10035j

A redox-neutral, mild, and simple protocol is developed for the synthesis of α-trifluoromethylated ketones from vinyl azides under transition-metal-free conditions. In the presence of organic photoredox catalyst N-methyl-9-mesityl acridinium and sodium trifluoromethanesulfinate, a broad range of substituted vinyl azides were found to react smoothly upon visible-light irradiation, readily furnishing the corresponding products in satisfied yields.

Full text of DOI:10.1039/c6cc10035j

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DOI: 10.1039/C6CC10035J
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Photoredox-Catalysed Redox-Neutral Trifluoromethylation of
Vinyl Azides for the Synthesis of α–Trifluoromethylated Ketones
Hai-Tao Qin,a,† Shu-Wei Wu,a,† Jia-Li Liua and Feng Liu*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A redox-neutral, mild, and simple protocol is developed for the
synthesis of α-trifluoromethylated ketones from vinyl azides
under transition-metal-free conditions. In the presence of organic
photoredox catalyst N-methyl-9-mesityl acridinium and sodium
trifluoromethanesulfinate, a broad range of substituted vinyl
azides were found to react smoothly upon visible-light irradiation,
readily furnishing the corresponding products in satisfied yields.
light photoredox systems. N-Methyl-9-mesityl acridinium
Mes-Acr+ ) with excited-state reduction potential (E1/2red*) >
(
, 3
1.8 V (vs. SCE),12,13 as one of the acridinium species, is actually
sufficient for single electron oxidation of the Langlois reagent.
We wondered if it was possible to achieve α-
trifluoromethylated ketones using Langlois reagent and mesityl
acridinium with visible-light irradiation.
The trifluoromethyl (CF3) moiety, with chemical stability and
electron withdrawing character, is a useful substituent for
organic compounds.1 For example, Introduction of a CF3 group
can lead to profound changes on the metabolic stability,
lipophilicity, and bioavailability of potential drug candidates.1
Therefore, the development of novel methods for efficient,
selective and mild incorporation of a CF3 group into diverse
skeletons has emerged as a pivotal objective in organic
synthesis.2 Radical trifluoromethylation, especially, via
photoredox catalysis, has received great attention. A variety of
agents, such as CF3I,3 CF3SO2Cl,4 Togni reagent,2b,5 Umemoto
reagent,6 Langlois/Baran reagents (CF3SO2Na,7 (CF3SO2)2Zn8),
and Ruppert-Prakash reagent9 (TMSCF3) could serve as CF3
radical precursors. Among these reagents, the commercially
available, low cost, and easy-handling Langlois reagent has
been extensively utilized as the source of CF3 radical by single
electron oxidation.7,10 However, there have been only few
reports referring to a photoredox catalyst in combination with
Langlois reagent towards generating a CF3 radical to date
(Scheme 1).10 The oxidation potential of Langlois reagent is
H
Cat. Ir-photoredox
or organic photoredox
CF3
R2
(1)
R1
+ CF3SO2Na
R1
ref. 10b-d
R2
H
O
OH
or
MeO-H or
F3C
F3C
H
CF3
Ar
N
N
CF3SO2Na
ArN2 BF4
Cat. Ru-photoredox
R2
+
(2)
CF3
R1
R1
ref. 10a
R2
Scheme 1 Visible-light-induced trifluoromethylation with CF3SO2Na.
As versatile building blocks, in general, α-trifluoromethylated
carbonyl compounds are prepared by electrophilic or radical
trifluoromethylation of enolates, silyl enol ethers, or enamines
which are obtained from corresponding carbonyl compounds in
advance or in situ,14 or via oxidative keto-trifluoromethylation of
alkenes5b,7f,15 in a radical process. A few other unique protocols
were also established.16 Vinyl azides17 with high intrinsic reactivity
have been used as versatile synthons for various transformations.9a-
1.05
V
(vs. SCE).10d,11 Organic dyes, such as Fukuzumi
acridiniums,10d,12 have been widely adopted as catalytic visible-
a.Jiangsu Key Laboratory of Translational Research and Therapy for Neuro-Psycho-
Diseases and Department of Medicinal Chemistry, College of Pharmaceutical
Sciences, Soochow University, 199 Ren-Ai Road, Suzhou, Jiangsu 215123, People’s
Republic of China. E-mail: fliu2@suda.edu.cn
b,18
For example, radical trifluoromethylation of vinyl azides with
TMSCF3 and PhI(OAc)2 allows preparation of α-trifluoromethyl
azines, followed by hydrolysis to give α-trifluoromethylated
ketones.9a In this reaction, the iminyl radical intermediate was
generated, followed by immediate dimerization to yield the azines
(Scheme 2). It should be noted that the above method requires
super-stoichiometric amounts of strong oxidant PhI(OAc)2 which is
often not compatible with sensitive functional groups. In
b.Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032,
People’s Republic of China.
† These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: Experimental details,
characterization data, and copies of 1H and 13C NMR spectra for all compounds. See
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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continuation of our research interest in transformation of vinyl reaction time (Table 1, entry 2). It should be noted that this
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azides19 and radical trifluoromethylation,20 it was envisioned that a protocol did also work well under the air, DoOnlIy: 1l0e.1a0d3i9n/gC6toCCa10s0li3g5hJt
redox-neutral radical trifluoromethylation of vinyl azides could decrease of the yield (Table 1, entry 3). We next examined the
serve as a useful methodology to achieve α-trifluoromethylated influence of light source. The results showed that blue LEDs gave
ketones directly in a mild redox process in which Langlois reagent the highest yield (Table 1, entries 1, 4 and 5). In order to quench the
acts as reductant and the iminyl radical intermediate generated iminyl anion B (Scheme 2), various weak base or weak acids as
from vinyl azides could play as oxidant, though an H-atom proton donor were added. To our surprise, the chemical yields
abstraction process would also happen. Consequently, we believed dropped (Table 1, entries 6−9). Other photocatalyst, such as Na2-
that the interception of the iminyl radical A via single electron Eosin Y, Rhodamine B, or Riboflavin was also tested, affording 2a in
reduction by 4 to circumvent dimerization would furnish α- much lower yields (Table 1, entry 5 vs. entries 10−12). Control
trifluoromethylated ketones when using N-Methyl-9-mesityl experiments were performed in the absence of the photoredox
acridinium perchlorate (3) as the photoredox catalyst (Scheme 2).
catalyst or in the dark revealed that both the catalyst and
photoirradiation are crucial for this radical trifluoromethylation
(Table 1, entries 13 and 14).
Previous work:
Table 1 Optimization of reaction conditions a
R
F3C
N
N3
O
PhI(OAc)2, CsF
Me3SiCF3
3N HCl
O
N3
N
CF3
R
photocatalyst (2.5 mol%)
CF3SO2Na (2.0 equiv)
R
CF3
CF3
R
azines
two steps
CF3
Br
Br
with srong oxidant
1,4-dioxane (wet), LEDs
2a
1a
N
CF3
R
entry
1
LEDs
photocatalyst
Mes-Acr+
Mes-Acr+
Mes-Acr+
Mes-Acr+
Mes-Acr+
Mes-Acr+
Mes-Acr+
Mes-Acr+
Mes-Acr+
Na2-Eosin Y
Rhodamine B
Riboflavin
Mes-Acr+
−
additiveb
yield (%)c
65
A
white
white
white
green
blue
blue
blue
blue
blue
blue
blue
blue
−
−
This work:
2d
3e
4
−
65
N3
3
Cat. , CF3SO2Na
O
−
52
one step
in a redox-neural fasion
CF3
R
R
blue LEDs
−
53
2
1
5
−
72
hydrolysis
Mes
N
6
KHCO3
< 5
55
NH
R
CF3
N2
CF3
C
7
HOAc
n
o
i
t
c
a
r
8
NH4OAc
37
s
b
a
-
Me
H
ClO4
N
9
NH4Cl
63
N
3, Mes-Acr
CF3
CF3
R
10
11
12
13f
14g
−
−
−
−
−
33
R
B
A
Mes
25
3
4
29
hv
N
0
CF3
Me
4
3*
blue
0
a1a (0.20 mmol), catalyst (2.5 mol%), CF3SO2Na (0.4 mmol), 1,4-dioxane (2 mL),
Argon balloon, 5 W LEDs, 36-60 h. b2.0 equivalent of additive cIsolated yield.
dMes-Acr+ (5 mol%), 24 h. eOpen flask, under the air. fIn the dark. gNo photoredox
catalyst.
CF3SO2Na
Scheme 2 Radical trifluoromethylation of vinyl azides.
O
The hypothesis was first examined using 4-bromopheny vinyl
azide 1a as the model substrate. Initially, various solvents (used
directly without dehydration) were screened upon irradiation by
white LEDs (light emitting diodes) in the presence of catalytic
amount of 3 (2.5 mol%), as might be expected, affording the
desired α-trifluoromethylated ketone 2a in various yields, as well as
identifying 1,4-dioxane as the best solvent in terms of chemical
yield (see supporting information). An increase in the loading of
Mes-Acr+ did not improve the chemical yield but shortening the
H3C
H3C
N
N
NH
COOH
Cl
Et
COONa
Br
N
O
Br
HO
Et
N
O
N
NaO
O
O
Et
Et
HO
OH
Br
Br
OH
Riboflavin
Rhodamine B
Na2-Eosin Y
2 | J. Name., 2012, 00, 1-3
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With the optimal reaction conditions in hand, we aimed to define
COMMUNICATION
Control experiments were performed to gain moVreiewinAsrtiigclhetOsnloinne
the scope of this protocol. As shown in Scheme 3, a broad range of the reaction mechanism (Scheme 4). In tDheOI:p1r0e.1s0e3n9c/eC6oCfCr1a0d0i3c5aJl
substituted α-trifluoromethylated ketones were readily furnished in scavenger TEMPO, no desired product was found from the reaction
moderate to good yields. The reaction was found to tolerate a mixture, along with the formation of the adduct TEMPO-CF3 5 only
series of different functional groups on phenyl ring, giving the in 1% yield based on 19F NMR (Scheme 4, eq 1).10b,21 This
corresponding α-trifluoromethylated acetophenone derivatives in observation supports the involvement of a radical process. We
satisfactory yields (46−77% yield, 2a−p). Investigations of the further conducted the reaction of 6 under the standard conditions,
substrate scope also demonstrated that both electron-rich and affording the cyclization product 7 in 74% (Scheme 4, eq 2). The
electron-deficient arenes bearing diverse groups at para, meta, or result could confirm that this reaction involves the α-CF3 iminyl
ortho position, ranging from OMe (2e, 2m and 2o) to Ac (2g) and radical A intermediate.
CHO (2h), could be employed in good yields. In addition, the steric
effect did not hinder the reaction (2l and 2m) and this protocol
could be extended to non-terminal vinyl azides (2n and 2o), giving
N3
3 (2.5 mol%)
CF3SO2Na (2.0 equiv)
the desired products in good yields. It is of note that substrates
N
O
(1)
(2)
2a
not detected
+
TEMPO (2.0 equiv)
1,4-dioxane, blue LEDs
containing alkynyl (2i) or thienyl moiety (2q) were compatible with
the reaction conditions. Not only aryl but alkyl vinyl azides could be
subject to this radical trifluoromethylation, achieving the
corresponding α-trifluoromethylated ketones in useful yields
(63−71% yield, 2r−v). Furthermore, the chemical yields were not
compromised when functionalized groups were installed (2r−u vs.
2v).
Br
CF3
5 (1%)
1a
N
N3
CF3
3 (2.5 mol%)
CF3SO2Na (2.0 equiv)
1,4-dioxane, blue LEDs
7 (74%)
6
O
3
(2.5 mol%)
N3
CF3SO2Na (2.0 equiv)
Scheme 4 Control experiments.
CF3
R2
R1
Br
R1
1,4-dioxane (wet), blue LEDs
R2
O
O
O
In conclusion, we have outlined a novel and simple protocol for
the preparation of α-trifluoromethylated ketones from vinyl azides
via organic photoredox catalysis upon visible-light irradiation in a
redox-neutral manner. The commercially available, low cost, and
easy-handling Langlois reagent serves as CF3 radical source. This
mild catalytic reaction demonstrates a broad substrate scope and
high functional group tolerance, as well as displaying potential for
further applications in medicinal and agrochemical research.
CF3
CF3
CF3
R
Ph
2a
: R = Br, 72%
2j
, 67%
2i
, 54%
CF3
2b
2c
: R = Cl, 66%
: R = F, 67%
2d
: R = H, 59%
2e: R = OMe, 64%
2f
Br
O
O
O
O
CF3
CF3
Me
: R = Ph, 46%
2g
2h
: R = Ac, 69%
2l
2m
, 68%
, 67%
: R = CHO, 77%
2k, 65%
Acknowledgements
O
O
O
O
Grateful for the financial support from the National Natural
Science Foundation of China (Grant No. 21302134), the
Natural Science Foundation of Jiangsu (Grant No. BK20130338),
and the Priority Academic Program Development of Jiangsu
Higher Education Institutions (PAPD).
CF3
CF3
CF3
CF3
O
Ph
S
2q
2p
, 62%
, 70%
2o, 68%
2n, 65%
O
O
O
O
O
O
CF3
N
CF3
N
CF3
O
O
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Scheme 3 Substrate scope for α-trifluoromethylated ketones. Reaction
conditions: A mixture of
(0.20 mmol), CF3SO2Na (0.4 mmol) and Mes-Acr+
(0.005 mmol, 2.5 mol%) in 1,4-dioxane (2 mL) was irradiated by blue LEDs
2
1
3
at rt for 36-60 h. Isolated yields.
This journal is © The Royal Society of Chemistry 20xx
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This could be rationalized regarding the potential vs. SCE: Eox
(CF3SO2Na) = 1.05 V, Eox (TEMPO) = 0.76 V.
8
9
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4 | J. Name., 2012, 00, 1-3
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