Visible Light Accelerated Vinyl C–H Arylation in Pd-Catalysis: Application in the Synthesis of ortho Tetra-substituted Vinylarene Atropisomers

Article abstract of DOI:10.1002/cjoc.201700618

A visible light accelerated C–H functionalization reaction in palladium-catalyzed arylation of vinyl arenes with diaryliodonium salts is reported in the absence of additional photosensitizer. The kinetic isotope effect (k_H/k_D) was ch

Full text of DOI:10.1002/cjoc.201700618

Visible Light Accelerated Vinyl C-H Arylation in Pd-Catalysis:
Application in the Synthesis of ortho Tetra-substituted Vinylarene
Atropisomers
Jia Feng,+ Bin Li,+ Julong Jiang, Mingkai Zhang, Wenbai Ouyang, Chunyu Li, Yao Fu* and Zhenhua
Gu*[a]
Dedication ((optional))
Abstract: A visible light accelerated C-H functionalization reaction in
palladium-catalyzed arylation of vinyl arenes with diaryliodonium salts
is reported in the absence of additional photosensitizer. The kinetic
isotope effect (kH/kD) was changed from 3.6 (under darkness) to 1.1
with irradiated by visible light, which indicated that the C-H
functionalization step was the rate determining step under darkness
and significantly accelerated by the irradiation of visible light. Finally
the synthesis of ortho tetra-substituted vinylarene atropisomers with
high enantiospecificity was realized via this protocol.
intermediate would give a higher energetic species. As a result,
the activation energy of the reaction was lowered by ΔΔE
(Scheme 2b).[11,12] Herein we report our efforts in direct vinyl C-H
arylation via the photo and palladium synergistic catalysis.
The synthesis of tetra-substituted olefins is gaining much
attention in synthetic organic chemistry since there are important
applications in chemistry, medicinal chemistry and life science.
However, their synthesis are of great challenges not only because
of the highly steric congestion of four substituents, but also the
difficulties in control of the stereochemistry. Some approaches
have been applied in the construction of tetra-substituted olefins.
De novo synthesis via Wittig-type reaction[1] and palladium-
catalyzed Heck reaction, carbene-involved reaction[2] invariably
suffered either its low reactivity or poor stereoselectivity (Scheme
1a). Functionalization of alkynes[3] and 2,3-allenals[4] is a very
useful protocol for the synthesis of stereodefined tetra-substituted
olefins (Scheme 1b). Cross-metathesis of alkenes is a feasible
method for construction of fully-substituted alkenes, which,
however, is usually limited to the intramolecular reaction for
construction of cyclic compounds.[5]
Scheme 1. Approaches for the Synthesis of Tetrasubstituted Olefins.
Direct functionalization of vinyl C-H is a robust method for the
construction of poly-substituted alkenes.[6] Two strategies were
usually employed in the previous studies in this area. The first is
the use of activated olefins, such as styrenes, vinyl ethers (esters),
enamines, α-oxoketene dithioacetates (Scheme 2a).[7] The
second frequently used strategy is coordination-directed C-H
functionalization.[8] Albeit these great successes, the synthesis of
tetra-substituted olefins via these strategies is still challenging,
such as limited directing groups or poor chemical selectivity (vinyl
vs. allylic C-H bonds). Recently, organic syntheses reap huge fruit
from visible light-promoted reactions.[9] Till now an army of visible
light-promoted C-H functionalization were developed.[10] We
Scheme 2. Approaches for Vinyl C-H Functionalization.
reasoned that irradiation of
a ground-state substrate or
During the research on the Pd-catalyzed arylation reaction of
substrate 1a with diphenyliodonium salt,[13] we serendipitously
found that the reaction proceeded much fast in day time than in
the night. Further investigation indicated that irradiation with
visible light significantly accelerated the reaction rate, and indeed
all subsequent optimizations were conducted under the irradiation
of 36 W lamp. Survey the palladium source found that Pd(OAc)2
was better than PdCl2 or Pd2(dba)3 (Table 1, entries 1-3). In the
absence of NaHCO3, the reaction did proceed albeit with a
relative lower conversion (entry 4). Bidentate ligand dppp
[a]
J. Feng, B. Li, Dr. J. Jiang, M. Zhang, W. Ouyang, C. Li, Prof. Dr. Y.
Fu and Prof. Dr. Z. Gu
Department of Chemistry
University of Science and Technology of China
96 Jinzhai Road, Hefei, Anhui 230026, P. R. China
E-mail: zhgu@ustc.edu.cn; fuyao@ustc.edu.cn
Prof. Dr. Y. Fu and Prof. Dr. Z. Gu
Hefei National Laboratory for Physical Sciences at the Microscale
These authors contributed equally to this work.
[+]
Supporting information for this article is given via a link at the end of
the document.
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been through the copyediting, typesetting, pagination and proofreading process, which
may lead to differences between this version and the Version of Record. Please cite this
article as doi: 10.1002/cjoc.201700618
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completely inhibited the reaction, while (4-FC6H4)3P produced a
considerable amount of side product, which was identified as
aromatized compound 3a (entries 5-6). Organic base Et3N or
strong base tBuOLi produced inferior results (entries 7-8).
Surprisingly, upon the addition of photosensitizers, such as A, B
and Ru(bipy)3Cl2, the efficiency of this transformation somehow
decreased (entries 9-11). In the absence of palladium source, no
conversion was observed either in the presence of
Ru(bipy)2(PF6)2 or in the absence of photosensitizer.
[a] The reactions were performed with 1a (0.10 mmol), Ph2IOTf (1.1 equiv),
Pd(OAc)2 (5 mol%), PPh3 (5 mol%), and NaHCO3 (1.1 equiv) in DCE at 70 oC.
Table 1. Reaction Condition Optimization[a]
Table 3. Substrate Scope
Entry
ligand
Base
Additive
Yield of
Yield of
(5 mol%)
2a/%
3a/%
1[b]
2[c]
3
PPh3
NaHCO3
NaHCO3
NaHCO3
-
-
37 (43)
-
PPh3
-
30 (66)
-
PPh3
-
91
-
4
PPh3
-
72 (78)
-
5
dppp
NaHCO3
NaHCO3
Et3N
-
-
-
6
(4-FC6H4)3P
PPh3
-
61
-
22
-
7
-
8
PPh3
tBuOLi
NaHCO3
NaHCO3
NaHCO3
-
<5
14
47
-
-
9
PPh3
A
35
-
10
11
PPh3
B
PPh3
Ru(bipy)3Cl2
-
[a] The reactions were performed with 1 (0.10 mmol), Ar2IOTf (1.1 equiv),
Pd(OAc)2 (5 mol%), PPh3 (5 mol%), and NaHCO3 (1.1 equiv) in dichloroethane
[a] The reactions were performed with 1a (0.10 mmol), Ph2IOTf (1.1 equiv),
Pd(OAc)2 (5 mol%), ligand (5 mol%), and base (1.1 equiv) in dichloroethane
at 70 oC under the irradiation of 36W light bulb. The numbers in parentheses
are the conversions. [b] Pd2(dba)3 was used. [c] PdCl2 was used.
b
at 70 oC under the irradiation of 36W light bulb. A small amount of mono-
arylation product (~3%) was detected.
Subsequently, the generality of this reaction was investigated
under optimized conditions. The 3,4-dihydronaphthalene moiety
could be elaborated with halides (Table 3, 2b-d), methoxyl group
(2h). The diaryliodonium was compatible with halogen
substituents (2f-g), however currently the ortho-substituted
diaryliodoniums were not suitable arylation reagents (See SI).
The phosphine oxide was not indispensable: 2j and 2k were
isolated in 91% and 62%, respectively, at a slight elevated
temperature or prolonged reaction time. Compounds 2j and 2k
were isolated in 36% and 44% under darkness, indicating that
they were less depend on the irradiation than the corresponding
phosphine oxides. The reactions with indene derivative also
provided satisfactory results (2l), and reaction of the 2H-
chromene analogue completed within 4 h albeit with a dropped
yield (2m). Replacing the aryl rings by a cyclopropyl group
resulted poor reactivity of the substrate, and no desired arylated
product 4a was detected. Disappointedly, these results gave no
The investigation of wavelength effect was also performed by
the use of a parallel reactor irradiated from the bottom of the
Schlenk tube (Table 2). All the different wavelength light we used
was able to promote the reaction (5 watts for 6 h). However white
(300-850 nm) and green (530 nm) light gave the highest efficiency
(entries 1-6), though it was surprise to find that 1a, Ph2IOTf, or
palladium catalyst have no absorption at the region of wavelength
larger than 500 nm in UV-Vis spectra [see the Supporting
Information (SI) for details]. The reaction with white and green
light irradiation could give 67% and 38% yield, respectively, even
decreasing the irradiation power to 1 watt and quenched the
reaction after 1 h (entries 7-9).
Table 2. Wavelength Effect Investigation[a]
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information regarding the possibility of radical pathway. The use
of acyclic substrate, diarylation product 4b was formed
predominantly even with only 1.1 equiv of diaryliodonium salt.
less than 10% conversion after being stirred at 70 oC for 24 h (see
SI).
Benzene radical cation signals in 1,2-dichloroethane solution
was detected by the EPR analysis, which might be generated
from phenyl radical. The experimental signal well matched the
simulated results (Figure 1d). Experimentally, the addition of
radical scavengers galvinoxyl and TEMPO, partially inhibited the
reaction, and galvinoxyl was a more effective inhibitor (Scheme
3). The isotope effect was around 1.1 with irradiation while the
reaction showed a typical primary isotope effect (kH/kD~3.6) under
darkness. It means that the C-H functionalization step is rate
determining step under darkness, which could be immensely
accelerated by visible light irradiation to become a fast step.
Recently, we developed a protocol for synthesis optically
active vinylarene atropisomers.[14] However, it was found that the
flexibility of the cyclohexene ring leads to these molecules have a
relatively lower rotation barrier than the corresponding biaryl
compounds. Studies on the thermal stability of axial chirality found
that the half-life of optically pure 1a is around 40 h at 80 oC, and
Ea is 28.7 kcal mol-1 (See SI). To gain high chirality retention the
reaction temperature is better to be performed below 80 oC.
Remarkably, this visible light-promoted vinyl C-H arylation
provided an ideal method for the functionalization of these
compounds. Thus, the synthesis of ortho-tetrasubstituted
vinylarene atropisomers was probed and in all the cases high
chirality retention was observed (Table 4). Electron-rich
substituents on the diaryliodonium salt diminished the reaction
rates (Table 4, compare 2s, 2t with 2a, 2n-p). No conspicuous
effect on the enantioselectivity was found for the identity of the
phosphine oxide and substituents on dihydronaphthalene moiety
(2u-2B).
(a)
(b)
Table 4. Synthesis of ortho-Tetrasubstituted Atropisomers
Pd(OAc)2, PPh3
R
R
NaHCO3 (1.1equiv), DCE
36W lightbulb, 70 oC, 6 h
Ar'
+
Ar'2IOTf
P(O)Ar2
P(O)Ar2
1
2
(c)
(d)
Me
X
R
Figure 1. (a) Initial rate vs [Pd(OAc)2]; (b) Conversion vs Time at different ratio
of Pd/PPh3. (c) Irradiation control experiments. (d) Experimental and simulated
EPR spectra for the reaction mixture at room temperature. Microwave
frequency: 9074.603 MHz; Microwave power: 2.0 mW.
P(O)Ph2
P(O)Ph2
P(O)Ph2
2a
(R)- , X = H, 99%, 99%ee
(R)-2n, X = F, 93%, 98%ee
2o
(R)- , X = Cl, 89%, 99%ee
[b]
2q
(R)- , R = Me, 86%, 98%ee
(R)-2r, R = tBu, 86%, 99%ee
(R)-2s, R = OMe, 75%, 99%ee[b]
2t
(R)- , 66%, 97%ee
2p
(R)- , X = Br, 88%, 97%ee
Me
Pd(OAc)2, PPh3
NaHCO3, DCE, 70 o
(R)-2v, R = Me, 84%, 85%ee (86%ee)[b]
(R)-2w, R = Ph, 63%, 87%ee (87%ee)[b]
(R)-2x, R = CF3, 74%, 86%ee (91%ee)[b]
Ph
C
O
H/D
Me
P
+
Ph2IOTf
2a
P(O)Ph2
P(O)Ph2
R
1.1 equiv
[b]
2y
(R)- , R = F, 71%, 90%ee (90%ee)
t-Bu
t-Bu
[b]
2u
(R)- , 86%, 99%ee
1a or 1a-d
O
O
R
OMe
5
k
H/kD
substrate
light Additive (2.0 equiv)
yield/%
6
F
t-Bu
t-Bu
Ph
O
P
1a
1a
On
On
Galvinoxyl
TEMPO
-
-
6 (31% conv.)
56
Galvinoxyl
Ph
O
P
Ph
O
P
Me
1a or 1a-d
On
Off
-
-
1.05-1.14
3.6
-
[a]
N
O
Me
1a
1a
or -d
-
Me
[a] The reaction was conducted at 90 oC.
TEMPO
Me
(R)-2z, 97%, 90%ee (90%ee)[b]
(R)-2A, 68%, 95%ee (95%ee)[b] (R)-2B, 87%, 88%ee (88%ee)[b]
[a] The reaction was conducted at 0.10 mmol scale of 1. The ee values of
starting materials are 99% unless stated otherwise. [b] Reaction time is 12 h.
The ee values in parentheses refer to starting materials.
Scheme 3. Control Experiments
Primary kinetic studies found the first-order dependence of
the reaction rate on the concentration of Pd(OAc)2, while zeroth
order dependence on Ph2IOTf (Figure 1a).[15] Switching the
catalyst-ligand ratio from 1:1 to 1:2 increased the initial rate, while
the rate slightly decreased by further addition of PPh3 (Figure 1b).
With alternating periods of irradiation or darkness, very slow
reaction rate was observed at dark stage, which excluded a
radical chain reaction process (Figure 1c). Under darkness, it got
Bringing together these experimental results, as well as the
DFT calculation, a plausible radical-involved catalytic cycle was
proposed in Scheme 4,[16] albeit the energy transfer process could
not be ruled out. The irradiation of Ph2IOTf gave excited
[Ph2IOTf]*, which split to two radical species I and II.[17] DFT
calculation revealed that the homolysis of Ph2I+ is a highly
endothermic process, which, however could be readily overcome
by absorbing visible light (See SI for details). Radical addition of
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II to 1 would afford III, which might associate with catalyst PdII to
give a new palladium species IV. The oxidation of IV by I through
single-electron-transfer gave PhI and V, which showed significant
carbon cation character by DFT calculation and readily formed
benzylic cation VI and Pd(II). The formation of PhI was
unambiguously confirmed by GC-Mass with the comparison of
authentic iodobenzene. Finally, V would give the olefin product 2
via base-mediated elimination (See SI for details). It is fair to state
that under darkness, the C-H functionalization step was the rate
determining step, while the irradiation of visible light changed the
reaction pathway and C-H functionalization became a fast step.
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Scheme 4. Plausible Catalytic Cycle.
In conclusion, we report a visible light promoted C-H
functionalization reaction in palladium-catalyzed arylation of vinyl
arenes, where no photosensitizer is required. The rate
determining step (C-H functionalization step) was accelerated to
be a fast step via the irradiation of visible light, which confirmed
by the change of kinetic isotope effect (kH/kD) from ~3.6 (darkness)
to ~1.1 (irradiation). Remarkably, this method enabled the
efficient synthesis of ortho tetra-substituted vinylarene
atropisomers under mild conditions with high enantiospecificity.
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Acknowledgements ((optional))
This work was supported by the '973' project from the MOST of
China (2015CB856600), NSFC (21472179, 21622206). We thank
Prof. Jihu Su (USTC) for the EPR analysis and simulation, and
Prof. Dong Xue (Shaanxi Normal University) for the use of photo
parallel reactor in his group.
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Keywords: palladium • visible light • C-H functionalization •
isotope effect • atropisomer
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COMMUNICATION
Jia Feng, Bin Li, Julong Jiang, Mingkai
Zhang, Wenbai Ouyang, Chunyu Li, Yao
Fu* and Zhenhua Gu*
Page No. – Page No.
Visible Light Accelerated Vinyl C-H
Arylation in Pd-Catalysis: Application
in the Synthesis of ortho Tetra-
Palladium-catalyzed vinyl C-H functionalization with diaryliodonium was promoted
by the irradiation of visible light. Under darkness the C-H activation step is the rate
determining step, and kinetic isotope effect (KIE) is around 3.6. With the aid of
visible light, the C-H activation step was significantly accelerated and KIE was
changed to 1.1.
substituted Vinylarene Atropisomers
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