Operationally simple hydrotrifluoromethylation of alkenes with sodium triflinate enabled by Ir photoredox catalysis

Article abstract of DOI:10.1039/c6cc01944g

We report herein a single component Ir photoredox catalyst which is capable of catalyzing the hydrotrifluoromethylation of terminal alkenes and Michael acceptors with sodium triflinate (Langlois reagent) in methanol under irradiation at room temperature. Various synthetically useful functional groups, including ester, amide, ether, aldehyde, sulfone, ketone and aryl boronate, are well tolerated in this reaction.

Full text of DOI:10.1039/c6cc01944g

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DOI: 10.1039/C6CC01944G
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Operationally Simple Hydrotrifluoromethylation of Alkene with
Sodium Triflinate Enabled by Ir‐Photoredox Catalysis
Lei Zhu,*a Lian‐Sheng Wang,a Bojie Li,a Boqiao Fu,a Cheng‐Pan Zhang*b and Wei Li
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
We reported herein a single component of Ir‐photoredox catalyst
is capable to catalyze hydrotrifluoromethylation of terminal
alkenes and Michael acceptors with sodium triflinate (Langlois
reagent) in methanol under an irradiation condition at room
temperature. Various synthetically useful functional groups
including ester, amide, ether, aldehyde, sulfone, ketone and aryl
boronate can be well tolerated in this reaction.
Due to the significantly improving effect of trifluoromethyl
substituent on biological properties of small molecules by
increasing its lipophilicity and metabolic stability,1 new
methodologies to introduce trifluoromethyl onto small
molecules has attracted intensive attention from synthetic
chemists.2 After extensive exploration to introduce
trifluoromethyl substituent onto unsaturated3 and allylic
carbon4 atoms, efforts to construct C(sp3)‐CF3 bond appears its
emergency5. The methods to install trifluoromethyl group onto
terminal C(sp3) atom in a aliphatic chain include reactions of
trifluoromethyl reagents6 with alkyl halides,7 alkyl boronic
Figure 1. Photoredox‐catalyzed hydrotrifluoromethylation of alkene.
al.9b Hydrotrifluoromethyaltion of alkenes using low‐cost
Langlois reagent6d is more appealing. Nicewice et al. reported
acridinium photoredox catalyst combining with a thiophenol
additive
is
capable
to
catalyze
alkene
hydrotrifluoromethylation with Langlois reagent for an ample
scope of olefins.9c Although reported, the development of new
operationally
simple
catalytic
system
for
acids8
and
terminal
alkene9,
among
which
hydrotrifluoromethylation with low cost reagent is still
desirable. In this work we discovered that a simple component
of Ir‐catalyst in methanol under irradiation is capable to
catalyze hydrotrifluoromethylation of alkenes using Langlois
reagent. This reaction exhibits excellent anti‐Markovnikov
selecticvity, and can also be extended to electron deficient
Michael acceptors including α,β‐unsaturated sulfone, ester,
amide and ketone. The advantages of this method include
operational simplicity and good functional group compatibility.
Our study started by choosing (hex‐5‐en‐1‐yloxy)benzene
hydrofunctionalization of terminal olefin is most appealing due
to its atom economy and generation of less waste comparing
with substitution reactions.10 Photoredox catalysis has been
demonstrated as a powerful method to generate radical
species without the necessity of using stoichiometric redox
agents, and to tame the reactivity of active radical in a
controllable manner, which was recently applied for
trifluorometlation.11,12 Meanwhile, hydrotrifluoromethylation
of alkenes was first achieved by using a ruthenium photoredox
catalyst and expensive Umemoto reagent by Gouverneur et
(
1a) as model substrate. It was found after exposing a reaction
mixture composed of 1a (0.25 mmol), Langlois reagent (0.75
mmol) and Ir‐photoredox catalyst in a mixture of DMF and
water for 24 h, anti‐Markovnikov type hydrotrifluoro‐
methylation product was detected in 19% yield by GC‐MS
(entry 1). Screening the solvent (entry 2‐9) revealed that
methanol is the best solvent, while using other solvents mixed
with water or using other alcohols as solvent all gave inferior
results.
a. College of Chemistry and Materials Science, Hubei Engineering University, Hubei,
432000, China.
Email: Lei.zhu@hbeu.edu.cn
b. School of Chemistry, Chemical Engineering and Life Science, Wuhan University of
Technology, 205 Luoshi Road, Wuhan, 430070, China.
Email: cpzhang@whut.edu.cn
† Electronic Supplementary Information (ESI) available: Experimental procedures
and product characterizations. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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Table 1. Optimization of reaction conditionsa
Table 2. Investigation of different trifluoromethyl sourcesa
DOI: 10.1039/C6CC01944G
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a Reaction condition: 1a (0.25 mmol), 2 (0.5 mmol), photoredox catalyst (2.0
mol %), MeOH (3.0 mL), irradiation with 36 W blue LEDs at room
temperature for 24 h under argon.
Entry
Catalyst
Additive (2 eq.)
Solvent
Yieldb
The effectiveness of Ir[dF(CF3)ppy]2(dtbpy)PF6 [E1/2 red (*IrIII/IrII)
red
1
2
3
4
Ir‐cat.1
Ir‐cat.1
Ir‐cat.1
Ir‐cat.1
/
/
/
/
DMF/H2O=1:1
DMSO/H2O=1:1
MeOH/H2O=1:1
CH3CN/H2O=1:1
19%
14%
41%
23%
= +1.21 V, E1/2 (IrIII/IrII) = ‐1.37 V vs. SCE]11e compared with
other photoredox catalyst can be rationalized from its redox
potentials. The redox potential of Langlois reagent is 1.05 V (vs.
SCE).13 From these redox potentials, it is reasonable that
Ir[dF(CF3)ppy]2(dtbpy)PF6 is capable to oxidize triflinate anion,
while Ru(bpy)3Cl2 and Ir(ppy)3 are unable to oxidize triflinate
anion to generate trifluoromethyl radical. Compared with the
reaction system reported by Nicewicz et al., it is noteworthy
that our reaction only needs a single component of Ir‐catalyst,
5
6
Ir‐cat.1
Ir‐cat.1
/
/
CH2Cl2/H2O=1:1
MeOH
28%
61%
7
8
Ir‐cat.1
Ir‐cat.1
Ir‐cat.1
Ir‐cat.1
/
EtOH
i‐PrOH
t‐BuOH
MeOH
49%
38%
11%
28%
/
/
while in Nicewice’s system,
a thiophenol additive is
necessary.9c This observation may also be rationalized by
comparing the redox potential of the two catalyst. The
acridinium catalyst [E1/2red (*A/A‐) = +2.06 V, Eox (A/A‐) = ‐0.57 V
vs. SCE]14 used in Nicewicz’s reaction is strong enough to
oxidize triflinate, but compared with Ir[dF(CF3)ppy]2(dtbpy)PF6,
the reduction power of its reduced form is much weaker. From
these potential values, it is reasonable that alkyl 2‐
trifluoromethyl methylene radical is able to oxidatively
regenerate Ir[dF(CF3)ppy]2(dtbpy) PF6, but unable to
regenerate acridinium catalyst. We also screened other
possible trifluoromethyl sources (Table 2), such as
9
11
K2HPO4
12
Ir‐cat.1
formic acid
MeOH
42%
13
14
Ir‐cat.1
Ir‐cat.1
benzoic acid
phenol
MeOH
MeOH
trace
58%
15
Ir‐cat.1
Ir‐cat.1
Ir‐cat.1
Ir‐cat.1
Ir‐cat.1
Ru(bpy)3Cl2
fac‐Ir(ppy)3
Ir‐cat.1
/
TsOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
46%
51%
52%
64%
81%
N.R
16
17c
18d
19d,e
20d,e
21d,e
22d,e,f
23d,e
H2O
/
/
/
/
/
/
/
‐
trifluoroacetate, CF3 B(OMe)3K+ and TMSCF3/KF. However, only
CF3SO2Na is effective.
With the optimal reaction condition in hand, we tested the
olefin scope. The hydrotrifluoromethylation exhibits excellent
regioselectivity. Anti‐Markovnikov selectivity was exclusively
obtained for terminal olefins. This reaction also possesses
good functional‐group‐compatibility, as demonstrated in Table
3, ether (3c), aldehyde (3g), methyl ketone (3h), sulfide (3k),
ester (3n) and amide (3o) are all well tolerated. Aryl chloride
N.R
N.R
N.R
a Reaction condition: 1a (0.25 mmol), 2a (0.75 mmol), photoredox catalyst
(2.0 mol %), solvent (2.0 mL), irradiation with 36 W blue LEDs at room
temperature for 24 h under argon. b GC yields using benzophenone as an
internal standard. c Using 1.0 mol % of Ir‐cat.1. d 2 (0.5 mmol). e Solvent (3.0
mL). f without irridiation.
(
3d), aryl bromide (3e), aryl pinacol boronate (3i) and
especially aryl iodide (3f) are all compatible, which are useful
for further modification through cross‐coupling reactions.
Photoredox catalyzed deiodination15 was not observed.
Tertiary aniline structure was not compatible (3l), probably
due to the strong reductive quenching ability of aniline.16
Although styrene type substrate is unreactive (3m), 1,1‐
disubstituted olefin is also amenable substrate, as
demonstrated by 3p. Although internal olefins were not
suitable substrate and delivered a complicated mixture, a
symmetric cyclic alkene, cyclopent‐3‐ene‐1‐carboxylate gave
hydrotrifluoro‐methylation product in 66% yield with a
diastereoselectivity of 5:1, in which trans‐isomer was the
major isomer (3q). Remarkably, heterocycles such as furan and
Adding proton source to enhance the acidity of the reaction
mixture failed to further increase the yield of
hydrotrifluoromethylation product (entry 11‐16). Reducing the
loading of the photoredox catalyst to 1 mol% caused a slight
decrease in yield. The best yield was achieve by reducing the
amount of Langlois reagent to 2.0 equivalent and further
diluting the reaction mixture (0.083M) (Entry 19). The reaction
did not proceed either without irradiation (entry 22) or in the
absence of the photoredox catalyst (entry 23). Ru(bpy)3Cl2 [E1/2
(*RuII/RuI) = +0.77 V, E1/2 (RuII/RuI) = ‐1.33 V vs. SCE]11e
red
red
and Ir(ppy)3 [E1/2 red (*IrIII/IrII) = +0.31 V, E1/2 red (IrIII/IrII) = ‐2.19 V
thiophene are well tolerated (3r, 3s)
.
vs. SCE]11e are totally ineffective for this reaction (entry 20, 21).
2 | J. Name., 2012, 00, 1‐3
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Table 3. Reaction scope of unactivated alkenesa,b
outer‐sphere radical mechanism involved (Figure 3). When
deuterated methanol was applied as solvent, it was found that
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DOI: 10.1039/C6CC01944G
deuterium atom was incorporated into the internal position of
olefin (Figure 4). Since hydrotrifluoromethylation can also be
t‐
detected when using BuOH as solvent (table 1, entry 9), it is
possible that MeOH just works as a protonation reagent, but
does not involve in the redox cycle.19
Table 4. Reaction scope of Michael acceptorsa,b
a Reaction condition: 1a (0.50 mmol), 2a (1.0 mmol), Ir‐cat.1 (2.0 mol %),
MeOH (3.0 mL), irradiation with 36 W blue LEDs at room temperature for
24 h under argon. b Yields based on isolated products.
Figure 2. Modification of an estrone derivative.
a Reaction condition: 1a (0.25 mmol), 2a (0.50 mmol), Ir‐cat.1 (2.0 mol %), MeOH
(3.0 mL), irradiation with 36 W blue LEDs at room temperature for 24 h under
argon. b Yields based on isolated products.
Figure 3. Radical‐trapping experiment.
We were delighted to find this reaction is not only suitable
for electronically unbiased terminal alkene, but also can be
applied to Michael acceptors.17 It should be noted that alkene
of Michael acceptor type was not demonstrated in the report
by Gouverneou et al.,9b and was demonstrated as a relatively
low yield example in Nicewicz’s report.9c Table 4 demonstrates
the scope for Michael acceptors. α,β‐Unsaturated sulfone (3t),
Figure 4. Isotope‐labelling experiment.
Figure 5 demonstrates the possible mechanism. The
sulfinate anion reductively quenchs excited *Ir(III) catalyst to
generate trifluoromethyl radical. The trifluoromethyl radical
reacts with olefin to generate a relatively stable methylene
radical. The methylene radical possessing a strong electron‐
withdrawing CF3 group adjacent to the radical center may have
higher oxidative potential compared with normal secondary
carbon radical,20 and can oxidize Ir(II) to regenerate Ir(III) (E1/2
red (IrIII/IrII) = ‐1.37 V vs. SCE) to complete the catalytic cycle.21
In summary, we discovered that a single component of Ir‐
ester (3u
substrates. Substituent at the α‐position of the Michael
acceptor are tolerable (3v 3w 3x), but a methyl substituent at
, 3v), amide (3w), ketone (3x) are all amenable
,
,
the β‐position (internal olefin) shuts down the reaction (3z).
Cyclic Michael acceptor, 1‐phenyl‐1H‐pyrrole‐2,5‐dione, a
substrate demonstrated by Rueping et al. recently,9d also
worked under our reaction condition, but delivered the
product in only 21% yield (3y).
The synthetic utility of this reaction is further demonstrated
by applying it to modify an estrone derivative. The
trifluomethylated estrone derivative was obtained in good
yield (Figure 2). Simple radical‐trapping experiments were
conducted. The addition of 2,2,6,6‐tetramethylpiperidine 1‐
oxyl (TEMPO) or styrene kills this reaction,4c,18 supporting an
photoredox
catalyst
is
capable
to
catalyze
hydrotrifluoromethylation of simple alkene and Michael
acceptors with Langlois reagent in methanol under an
irradiation condition. Various synthetically useful functional
groups including ester, amide, ether, aldehyde, sulfone,
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Figure 5. Proposed mechanism.
We acknowledge the financial support from the National
Natural Science Foundation of China (No. 21304032), the
Natural Science Foundation of Hubei Province of China (No.
2014CFB570) and Hubei Engineering University.
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using a reported method (see the SI), which suggests that a
short chain radical mechanism is also probably concomitant.
According to suggestion of one referee, a pathway involving
hydrogen abstraction from methanol to alkyl radical
generating methoxy radical followed by oxidation of Ir(II) by
methoxy radical to finish the photoredox cycle can not be ruled
out.
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