Visible-Light-Induced Three-Component Intermolecular Trifluoromethyl-Alkenylation Reactions of Unactivated Alkenes

Article abstract of DOI:10.1002/adsc.202001434

Herein, we describe a practical protocol for efficient, mild, visible-light-induced three-component intermolecular trifluoromethyl-alkenylation reactions of unactivated alkenes. The protocol has good functional group tolerance and a broad substrate scope. Using this protocol, we not only introduced a trifluoromethyl group into alkenes but also converted unactivated alkenes to styrene-based activated alkenes, in addition to accomplishing late-stage functionalization of pharmaceutical intermediates. (Figure presented.).

Full text of DOI:10.1002/adsc.202001434

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DOI: 10.1002/adsc.202001434
Visible-Light-Induced Three-Component Intermolecular
Trifluoromethyl-Alkenylation Reactions of Unactivated Alkenes
Yuan-Qiang Guo,a Kaihua Wang,a Ruiguo Wang,a Hongjian Song,a Yuxiu Liu,a
and Qingmin Wanga, b,*
a
State Key Laboratory of Elemento-Organic Chemistry, Research Institute of Elemento-Organic Chemistry, College of Chemistry,
Nankai University, Tianjin 300071, People’s Republic of China
E-mail: wangqm@nankai.edu.cn
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, People’s Republic of China
b
Manuscript received: November 17, 2020; Revised manuscript received: January 4, 2021;
Version of record online: Februar 4, 2021
Dedicated to the 100th anniversary of Chemistry at Nankai University.
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.202001434
cluding transition-metal-catalyzed reactions, visible-
light-mediated reactions, reactions involving both
Abstract: Herein, we describe a practical protocol
for efficient, mild, visible-light-induced three-com-
photoredox and transition-metal catalysts, and electro-
ponent intermolecular trifluoromethyl-alkenylation
catalytic fluoroalkylation reactions.[4] Of these meth-
reactions of unactivated alkenes. The protocol has
ods, one of the most attractive involves difunctionali-
good functional group tolerance and a broad
zation of alkenes with CF3 radical (such as the
substrate scope. Using this protocol, we not only
Umemoto reagent,[5] Togni reagents,[6] Langlois’
introduced a trifluoromethyl group into alkenes but
also converted unactivated alkenes to styrene-based
reagent,[7] CF3SO2Cl,[8] CF3I,[9] CF3Br,[10] TMSCF3,[11]
CF3SO2NHNHBoc,[12] (CF3CO)2O[13] and (bpy)
activated alkenes, in addition to accomplishing late-
Cu(CF3)3[14]) (Scheme 1A). Using these reagents, many
stage functionalization of pharmaceutical intermedi-
research groups have achieved various difunctional
ates.
modifications of the styrenes and unactivated alkenes
through trifluoromethyl radicals. These research results
can be divided into carbo-CF3,[15] amino-CF3,[16] oxo-
Keywords: Photocatalysis; Three components; Un-
activated alkenes; Trifluoromethylation; Alkenylation
CF3,[17] thio-CF3,[18] halo-CF3.[19] and so on.
In the past few decades, considerable progress has
been made in the development of methods capable of
synthesizing CF3-containing molecules, with particular
The development of fluorine chemistry has great emphasis on the direct trifluoromethylation of alkenes.
significance for organic synthesis because introduction These remarkable achievements can help us effectively
of a fluorine atom or fluorinated moiety into a
molecule can improve its physical, chemical, or bio-
logical properties.[1] For example, replacing a hydrogen
atom or methyl group with a trifluoromethyl group
(CF3) markedly alters a molecule’s lipophilicity,
absorbability, metabolic stability, hydrophobicity, and
binding selectivity.[2]
Intermolecular difunctionalization of alkenes, a
transformation that forms two chemical bonds in a
single reaction, is an efficient, atom-economical
method for constructing complex organic molecules
from inexpensive, readily available starting materials.[3]
Over the past few decades, a number of methods for
alkene difunctionalization have been developed, in-
Scheme 1. Trifluoromethylative difunctionalization of alkenes.
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convert alkenes into various trifluoromethyl-containing irradiation of 1a (1 equiv.), 2a (2 equiv.), 3a
molecules by forming two chemical bonds in one (2 equiv.), and [Ir(dF(CF3)ppy)2(dtbbpy)]PF6 (2 mol%)
reaction. However, there is still a lack of effective in 10:1 (v/v) ethyl acetate/H2O (0.1 M) under argon
bifunctional reaction methods that convert unactivated afforded an excellent yield of 4a (see the Supporting
alkenes into styrene-based activated alkenes. This Information for screening details). When the solvent
reaction product can be used as an olefin synthon for mixture was changed to 10:1 (v/v) acetone/H2O
other types of difunctionalization reactions. Very (0.1 M), the yield decreased slightly (entry 6), and
recently, Yang and co-workers reported iron-catalyzed different solvent mixtures gave greatly reduced yields
trifluoromethyl-alkenylation reactions of unactivated (entries 7–9). When the reaction was carried out under
olefins with a substituted cinnamic acid as the alkenyl air, the yield of the product was low (entry 10), which
source.[20] Many substrates can undergo trifluorometh- indicates that oxygen interfered with the reaction.
yl-alkenylation by means of this method, but the range Control experiments showed that in the absence of a
and quantity of unactivated alkenes are still insuffi- photocatalyst or light, none of the desired product was
cient. In addition, because of the harsh reaction detected (entries 11 and 12).
conditions, the method cannot be used for late-stage
functionalization of pharmaceutical intermediates.
With the optimized reaction conditions (Table 1,
entry 1) in hand, we evaluated the substrate scope with
We hypothesized that we could use Langlois’ respect to alkene 1 (Scheme 2). Unactivated terminal
reagent (CF3SO2Na) to achieve visible-light-mediated aliphatic alkenes could be smoothly transformed into
intermolecular trifluoromethyl-alkenylation reactions the corresponding trifluoromethyl-alkenylation prod-
between unactivated alkenes and vinyl sulfones under ucts (4a–4f) in moderate to good yields with modest
mild conditions. Such a method not only would have a Z/E ratios. Enol ethers, enol esters, and common alkyl
wide substrate scope but also could be used for late- ethers also afforded the target products (4g–4k) in
stage functionalization of drug molecules containing moderate yields with modest Z/E ratios. A free
unactivated alkenes (Scheme 1B).
hydroxyl group and a tosyl-protected hydroxyl group
To test our hypothesis, we chose 4-phenyl-1-butene
(1a), CF3SO2Na (2a), and (E)-(2-(phenylsulfonyl)
vinyl)benzene (3a) as model substrates to optimize the
reaction conditions (Table 1). By screening several
photocatalysts (entries 1–5), we found that [Ir(dF(CF3)
ppy)2(dtbbpy)]PF6, fac-Ir(ppy)3, and Eosin Y gave
target product 4a. We were delighted to find that
Table 1. Optimization of reaction conditions.[a]
Entry Variation from standard conditions
Yield (%)[b]
1
2
3
4
none
81
fac-Ir(ppy)3 as photocatalyst
EosinY as photocatalyst
Ru(bpy)3Cl2 ·6H2O as photocatalyst
9[c]
7[c]
NR[c]
5
Mes-Acr+-Me ClO4À as photocatalyst NR[c]
6
7
8
9
10
11
12
10:1 acetone/H2O as solvent
10:1 MeCN/H2O as solvent
10:1 THF/H2O as solvent
10:1 toluene/H2O as solvent
under air
78
trace
23
11
20
0
no photocatalyst
no light
0
[a] Reaction conditions, unless otherwise noted: 1a (0.3 mmol),
2 a (0.6 mmol), 3 a (0.6 mmol), and [Ir(dF(CF3)ppy)2
(dtbbpy)]PF6 (2 mol%) in 10:1 (v/v) ethyl acetate/H2O
(0.1 M) were irradiated with 30 W blue LEDs at room
temperature under argon for 36 h.
Scheme 2. Substrate scope with respect to alkene 1 [a]Reactions
were performed under the conditions listed in entry 1 of
Table 1, unless otherwise noted; isolated yields are provided.
[b]The reaction was carried out with 4 mmol of 1a. [c]Z-4p was
also produced, but it is inseparable from unknown impurities
and no pure Z-4p can be obtained.
[b] Isolated yields are provided. NR=no reaction.
[c] 3 mL DMSO (0.1 M) used as solvent.
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were tolerated (4l and 4m). Various nitrogen-contain- ortho or para position to give the corresponding
ing functional groups-an amide (4n), an imide (4o), trifluoromethyl-alkenylation products (5e–5i) in mod-
and a carbazole (4p)-remained intact under the mild erate yields with modest Z/E ratios. Moreover, fused-
reaction conditions. Unfortunately, however, styrene- ring aromatic-hydrocarbon-, pyridine-, and thiophene-
type olefins were not amenable to the reaction. To based vinyl sulfones also reacted smoothly to give
demonstrate the scalability of the protocol, we con- target products 5j–5l in moderate yields. Substrates
ducted a reaction using 4 mmol alkene 1a, which with an electron-withdrawing atom (À F, À Cl or À Br) in
afforded 0.76 g 4a (62% yield). The excellent func- the para-position or have an electron-withdrawing
tional group tolerance of the reaction suggested that it pyridine ring gave better yields than electron-donating
t
could be used for late-stage functionalization of group (e.g., À Me, À OMe, À Bu or À OCOMe). We were
bioactive molecules. To evaluate this possibility, we pleased to find that acetylene sulfones could also be
selected methyl eugenol, dicamba derivatives, and the used to achieve the trifluoromethyl-alkynylation reac-
drug molecules menthol derivatives, ibuprofen deriva- tion, affording target compounds 5m–5p in moderate
tives, and boldenone undecylenate as substrates. to good yields.
Reactions of these molecules gave the corresponding
trifluoromethyl-alkenylation products (4q–4u) in mod- carried out several control experiments. When the
erate yields. radical scavenger TEMPO (2,2,6,6-tetramethyl-piper-
To gain mechanistic insight into the process, we
Next we carried out reactions of a broad range of idine nitroxide) was added to the model reaction
functionalized sulfones 3 with 4-phenyl-1-butene (1a) system, the desired coupling was completely shut
and CF3SO2Na (2a) under the optimized conditions down, and the corresponding radical-trapping product
(Scheme 3). Vinyl sulfones with a halogen atom (F, Cl, was detected by high-resolution mass spectrometry
or Br) in the para position smoothly gave the (Scheme 4A). This result indicates that a trifluorometh-
corresponding trifluoromethyl-alkenylation products yl radical formed from CF3SO2Na and rapidly added to
(5b–5d) in good yields with modest Z/E ratios. The the alkene to form a new alkyl radical intermediate.
reaction also proceeded smoothly with phenylvinyl Radical clock experiments using 1,5-cyclooctadiene
sulfones bearing an electron-donating group at the (6a) and (1S)-(À )-β-pinene (7a) under the standard
conditions afforded cyclized product 6b (Scheme 4B)
and ring-opened product 7b (Scheme 4C), respec-
tively, results that further support the free-radical
mechanism we have proposed. Through controlled
experiments, we found that isomer E will be converted
to Z isomer, but not completely converted (can see the
Supporting Information for more details).[21] We also
monitored the changes of the E/Z isomer ratio during
the reaction by H NMR. Through H NMR, we found
that at the beginning of the reaction, the thermody-
Scheme 3. Substrate scope with respect to sulfone 3 [a]Reactions
were performed under the conditions listed in entry 1 of
Table 1, unless otherwise noted; isolated yields are provided.
[b]The solvent was 10:1 (v/v) acetone/H2O (0.1 M).
Scheme 4. Control experiments [a]Reactions were carried out
under the conditions listed in entry 1 of Table 1; isolated yields
are provided.
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namically more stable E isomer was the main product,
but as the reaction progressed, the thermodynamically
unstable Z isomer became the main product (can see
the Supporting Information for more details). Unfortu-
nately, we have no method to improve stereoselectiv-
ity.
Experimental Section
To a 15 mL glass vial was added [Ir(dF(CF3)ppy)2(dtbbpy)]PF6
(6.7 mg, 0.006 mmol, 2 mol%), alkene 1 (0.3 mmol, 1 equiv.),
2a (93.6 mg, 0.6 mmol, 2 equiv.), sulfone
3 (0.6 mmol,
2 equiv.) and ethyl acetate:H2O (3 mL, V/V=10:1). The
reaction mixture was degassed by bubbling with argon for 10 s
with an outlet needle and the vial was sealed with PTFE cap.
The mixture was then stirred rapidly and irradiated with a 30 W
Blue LED (approximately 5 cm away from the light source) at
room temperature for 36 h. When the reaction is completed,
extracted with ethyl acetate, washed with brine, dried over
anhydrous sodium sulfate, concentrated in vacuo, and purified
by column chromatography (hexane/ethyl acetate) to afford the
target compound.
On the basis of our mechanistic experiments and
literature reports,[22] we propose the mechanism de-
picted in Scheme 5. Photoexcited *[Ir(dF(CF3)
red
ppy)2(dtbbpy)]PF6 [E1/2
(*IrIII/IrII)= +1.21 V vs
SCE; τ=2.3 μs][23] undergoes a single-electron-transfer
reaction with CF3SO2Na to form a trifluoromethyl
radical and IrII. The radical reacts with the alkene to
form new alkyl radical intermediate A, which then
attacks the vinyl sulfone to form benzyl radical B.
Radical B releases the desired product C and an open-
shell phenyl sulfonyl radical. Finally, the phenyl
sulfonyl radical (E1/2red = +0.50 V vs SCE) undergoes
a single-electron-transfer reaction with the reduced
photocatalyst IrII (E1/2 [IrIII/IrII]=À 1.37 V vs SCE) to
close the catalytic cycle.[22b] Theoretically, the gener-
ated phenylsulfonyl radicals may react with an alkene
during the reaction. However, we did not find this
addition product, probably because the phenylsulfonyl
radicals react with the reduced photocatalyst IrII at a
faster rate than the reaction with an alkene.
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (21732002, 22077071) for generous financial support for
our programs.
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In conclusion, we have developed a practical
protocol for efficient, mild, visible-light-induced three-
component intermolecular trifluoromethyl-alkenylation
reactions of unactivated alkenes. Using this protocol,
we not only introduced a trifluoromethyl group into
alkenes but also converted unactivated alkenes to
styrene-based activated alkenes. The excellent func-
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protocol a broad substrate scope and make it suitable
for late-stage functionalization of pharmaceutical inter-
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