A visible light photoredox catalyzed carbon radical-mediated generation of: Ortho -quinone methides for 2,3-dihydrobenzofuran synthesis

Article abstract of DOI:10.1039/c9cc00727j

A visible light photoredox-catalyzed carbon radical-mediated strategy for in situ formation of ortho-quinone methides from 2-vinyl phenols is described. This strategy enables a multicomponent cyclization reaction of 2-vinyl phenols, Umemoto's reagent, and

Full text of DOI:10.1039/c9cc00727j

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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A visible light photoredox catalyzed carbon radical-mediated
generation of ortho-quinone methides for 2,3-dihydrobenzofuran
synthesis
Received 00th January 20xx,
Accepted 00th January 20xx
Fan Zhou,a Ying Cheng,a Xiao-Peng Liu,a Jia-Rong Chen,a* and Wen-Jing Xiaoa,b
DOI: 10.1039/x0xx00000x
www.rsc.org/
A visible light photoredox-catalyzed carbon radical-mediated
strategy for in situ formation of ortho-quinone methides from 2- evolved to be a powerful platform for chemists to design
vinyl phenols is described. This strategy enables
various valuable chemical reactions involving radical species.10
Over the past decade, visible light-driven photocatalysis has
a
multicomponent cyclization reaction of 2-vinyl phenols, In connection with our ongoing project on heterocycle
Umemoto’s reagent, and sulfur ylides, providing access to synthesis,11 we recently started a program towards developing
trifluoromethylated 2,3-dihydrobenzofurans.
new cyclization reaction of aza-o-QMs, wherein the aza-o-QMs
are generated in situ from 2-vinyl-subsituted anilines by a
visible light photocatalytic radical-mediated strategy (Scheme
1a).12 Herein, we describe the development of a new strategy
for the in situ generation of o-QMs from 2-vinyl phenols using
Umemoto’s reagent as CF3 radical source; as such, we
developed an efficient multicomponent reaction of 2-vinyl
phenols, Umemoto’s reagent, and sulfur ylides for the first
time (Scheme 1b). This mild and redox-neutral protocol
provides practical access to valuable trifluoromethylated 2,3-
dihydrobenzofurans.13
Ortho-quinone methides (o-QMs) are a highly polarized and
reactive species class that have a vicinal exocyclic methylene
and a carbonyl group.1-3 Owing to their inherent driving force
to facilely rearomatize, o-QMs can readily under Michael
addition with appropriate nucleophiles as well as cycloaddition
or cyclization with a variety of dienophiles or dipolars. As a
result, over the past decades, numerous methods and
precursors have been developed to generate structurally
diverse o-QMs in situ to overcome the limitations often
involved in the reactions of isolable o-QMs. Classical protocols
for o-QM generation are typically performed by chemical (acid
or base-promoted elimination,4 oxidation,5 tautomerization,6
or nucleophilic addition1e,7), thermolysis,8 or photochemical
processes.9 However, many of these early traditional
approaches utilized high temperatures, UV irradiation, or
harsh reaction conditions. Recently, a wide range of catalytic
asymmetric transformations of in situ formed o-QMs have
been achieved by using transition metals, phase-transfer
catalysts, Brønsted acids and bases, or N-heterocyclic carbenes
under mild conditions.3 While these advances greatly
enhanced the synthetic potential of o-QMs, development of
mechanistically distinct strategies for controlled generation of
o-QMs in stiu from readily accessible substrates, especially
with simultaneous incorporation of important functional
handles, is still in high demand.
(a) Visible light-driven generation of aza-o-QMs and development of multicomponent reaction
-our previous work
R
"redox-neutral"
PC
O
photoredox catalyst
O
R
X
+
+
S
NH
Ts
R1
visible light, base, room temp.
R1
N
H
SET
O
reduction
S
R
R1
radical addition
R
R
R
R
SET
oxidation
base
NH
Ts
N
N
NH
Ts
Ts
Ts
I
II
III
III'
(b) Visible light-driven generation of o-QMs and development of multicomponent reaction
-this work
PC
CF3
fac-Ir(4'-CF3ppy)3 (2 mol%)
R1
S
O
+
R1
2a
CF3
OTf
3 W blue LEDs, TFA (20 mol%)
CF3CH2OH, rt
OH
R2
O
O
1
"redox-neutral"
S
R2
3
4
Scheme 1 Visible light photoredox-catalyzed radical-mediated
in situ formation of aza-o-QMs and o-QMs, and reaction design.
a
Key Laboratory of Pesticide
& Chemical Biology, Ministry of Education;
International Joint Research Center for Intelligent Biosensing Technology and
Health; College of Chemistry, Central China Normal University, 152 Luoyu Road,
Wuhan, Hubei 430079, China. E-mail: chenjiarong@mail.ccnu.edu.cn
On the basis of our previous visible light-driven generation
of aza-o-QMs,12 and the biological significance of CF3 moiety,
we initially investigated the model reaction of 2-vinyl phenol
b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, 345 Lingling Road, Shanghai 200032,China
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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1a, Umemoto’s reagent 2a,14 and sulfur ylide 3a in various withdrawing (e.g., Br, Cl) functional groups at the para- or
View Article Online
DOI: 10.1039/C9CC00727J
solvents with [Ru(bpy)3]Cl2·6H2O as photocatalyst under ortho-position of phenyl ring were all well tolerated; and the
irradiation of 3 W blue LEDs (Table 1).15 To our delight, the corresponding products 4ab-4ai were obtained in 55-71%
reaction indeed worked to give the desired product with yields. Notably, these products bearing halogen functional
variable yields; and protic CF3CH2OH proved to be superior to groups would allow further downstream diversification.
others, giving 4aa in 42% NMR yield with exclusive trans- Moreover, the reaction of disubstituted sulfur ylides 3j also
selectivity (entry 4). With CF3CH2OH as the reaction media, we proceeded well to give the expected product 4aj in 58% yield.
then continued to evaluate several other commonly used The fused aromatic group substituted substrates such as 3k
photocatalysts (entries 5-9), and identified fac-Ir(4’-CF3ppy)3 to also proved to be suitable for the reaction, furnishing a 62%
be the best candidate, affording 4aa with 73% NMR yield yield of 4ak. Again, the reactions with furan- and thiophene-
(entry 9).15 Further optimization studies with respect to substituted substrates 3l and 3m worked well to produce 4al
additives and substrate ratios were also conducted; and the and 4am in 68% and 61% yields, respectively. Remarkably, the
optimal reaction conditions were finally identified to be: 2 mol% precursor of sulfur ylide, sulfonium bromide 5 can be used
fac-Ir(4’-CF3ppy)3 as photocatalyst, 1.1 equiv of Umemoto’s directly, and the product 4aa was still isolated in 50% yield.
reagent 2a and 2.0 equiv of sulfur ylide 3a (relative to the Finally, the structure of 4af was also ambiguously determined
amount of 1a), 20 mol% TFA as the additive in trifluoroethanol by X-ray crystallographic analysis, confirming its trans-
at room temperature by irradiation of 3 W blue LEDs, and 4aa configuration.18 It should be noted that all of the 2,3-
was isolated in 80% yield (entry 11). Finally, control dihydrobenzofurans were formed as thermodynamically more
experiments revealed that both visible light irradiation and stable and single diastereomers. When the reaction of 1a was
photocatalyst are critical to reaction, though light alone could carried out on a 1.0 mmol scale, product 4aa was still isolated
still promote the reaction to some extent.16 In accordance with in 76% yield. As a limitation of this method, reactive sulfur
our previous study,12 no desired product could be detected ylide or sulfur ylide containing nitrogen group are not suitable
when using Togni’s reagent (2b) as CF3 radical source (result for the reaction.
not shown).17
Table 2 Scope of sulfur ylidesa,b
Table 1 Condition optimizationa
CF3
fac-Ir(4'-CF3ppy)3 (2 mol%)
O
S
CF3
+
3 W blue LEDs, TFA (20 mol%)
CF3CH2OH, degas, rt, 6 h
CF3
2a
OTf
OH
fac-Ir(4'-CF3ppy)3 (2 mol%)
S
O
O
Ar
O
1a
+
()-4aa-4am
3 W blue LEDs
CF3CH2OH, degas, rt, 6 h
S
CF3
OTf
2a
OH
3
Ar
O
Ph
O
1a
CF3
CF3
O
CF3
CF3
()-4aa
S
3a
Ph
O
O
O
Entry
1
Photocatalyst
Solvent
DCM
Yieldb (%)
36
O
O
O
O
[Ru(bpy)3]Cl2·6H2O
[Ru(bpy)3]Cl2·6H2O
[Ru(bpy)3]Cl2·6H2O
[Ru(bpy)3]Cl2·6H2O
Eosin Y
2
Toluene
13
20
OMe
Me
Ph
()-4ad, 63%
()-4aa, 80% (76%)d
()-4ab, 55%
()-4ac, 60%
CF3
CF3
CF3
CF3
3
DMF
O
O
O
O
4
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
42
O
O
O
O
5
53
6
fac-Ir(ppy)3
51
Br
Cl
F
()-4ah, 53%
CN
()-4ae, 71%
()-4af, 65%
()-4ag, 66%
CF3
CF3
7
49
[Ru(bpz)3]PF6
CF3
CF3
O
O
O
8
[Ir(dtbbpy)(ppy)2]PF6
fac-Ir(4’-CF3ppy)3
fac-Ir(4’-CF3ppy)3
fac-Ir(4’-CF3ppy)3
57
F
O
O
O
F
O
O
9
10c
11c,d
73
O
77
()-4al, 68%
F
()-4ai, 62%
()-4aj, 58%
()-4ak, 62%
81(80)
CF3
O
CF3
O
a
Reaction conditions: 1a (0.20 mmol), 2a (0.22 mmol), 3a (0.30 mmol), and
photocatalyst (2.0 mol %) in CF3CH2OH (2.0 mL) at room temperature under
the irradiation of 3 W blue LEDs for 6 h. b Yields were determined by 1H NMR
O
S
O
S
Ph
O
c
Br
using 1,3,5-trimethoxybenzene as an internal standard. With 20 mol%
X-ray crystal
structure of 4af
()-4aa, 50%c
5
CF3CO2H as additive. d 0.40 mmol 3a was used.
()-4am, 61%
a
Reaction conditions: 1a (0.20 mmol), 2a (0.22 mmol), 3 (0.4 mmol),
With the optimal reaction conditions in hand, we first
examined the substrate scope of sulfur ylides 3 by reacting
them with 2-vinyl phenol 1a and Umemoto’s reagent 2a on 0.2
mmol scale. As shown in Table 2, variation of the electronic
properties and substitution pattern of the aromatic ring within
the sulfur ylides had no obvious effect on the reaction
efficiency. For example, in addition to 3a, a set of sulfur ylides
3b-3i with electron-donating (e.g., MeO, Me, Ph) or electron-
CF3CO2H (20 mol%) and fac-Ir(4’-CF3ppy)3 (2.0 mol%) in CF3CH2OH (2.0
mL) at room temperature under irradiation of 3 W blue LEDs for 6 h. b
Isolated yield. c Sulfonium bromide 5 (2.0 equiv) and NaOH (4.0 equiv)
were used. d 1.0 mmol scale.
Next, we turned our attention to briefly study the substrate
scope of 2-vinyl phenols 1 in combination with substrates 2a
2 | J. Name., 2012, 00, 1-3
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and 3a (Table 3). The reactions of substrates 1b-1e showed photogenerated CF3 radical to 2-vinyl phenol 1a, might be
View Article Online
DOI: 10.1039/C9CC00727J
that a variety of functional groups were well tolerated, involved as an intermediate in the reaction.
including methyl, methoxy, and halides (e.g., Cl, Br); and the
desired products 4ba-4ea were obtained with 41-68% yields.
CF3
standard conditions
S
O
Notably, substrate 1f containing nitro group that is usually
incompatible with photoredox conditions, could also
participate in the reaction, though with low yield of 4fa. The
reaction with sterically encumbered disubstituted 2-vinyl
phenols 1g and 1h again worked well to confer products 4ga
and 4ha in 58% and 61% yields, respectively. Finally, we also
examined iodoacetonitrile 6 as a carbon radical source to react
with 2-vinyl phenol 1a and sulfur ylide 3a. The reaction worked
well to furnish the cyanoalkylated 2,3-dihydrobenzofuran 7 in
70% yield, when using Ru(bpy)3Cl2·6H2O as photocatalyst.
+
2a
TEMPO (2.0 equiv)
or BHT (2.0 equiv)
no 4aa was detected
CF3
OTf
OH
O
O
Ph
1a
S
3a
()-4aa, 28%
Ph
Key intermediate
CF3
OH
CF3
O
TEMPO
N
OH
1a-A
8 (detected by HRMS)
Scheme 3 Mechanistic studies.
On the basis of the above experimental results and our
previous studies,11d,12 we proposed a photoredox-catalyzed
radical-mediated process for the reaction using 1a, 2, and 3a
as model substrates (Scheme 4). First, upon visible light
irradiation, the ground state photocatalyst fac-IrIII(4’-CF3ppy)3
is transformed to its photoexcited state *fac-IrIII(4’-CF3ppy)3.19
Then, a single electron transfer (SET) event occurs between
*fac-IrIII(4’-CF3ppy)3 and Umemoto’s reagent 2a to give CF3
radical and the oxidized form of the photocatalyst, fac-IrIV(4’-
CF3ppy)3. Addition of the electrophilic CF3 radical to the alkene
moiety of 2-vinyl phenol 1a gives rise to relatively stable
benzylic radical 1a-A. Such a radical species could be further
oxidized by the oxidizing state fac-IrIV(4’-CF3ppy)3 via a SET
process to form benzylic cation 1a-B, and close the
photocatalytic cycle by release of ground state photocatalyst.
Table 3 Scope of 2-vinyl phenolsa,b
CF3
fac-Ir(4'-CF3ppy)3 (2 mol%)
S
O
R
+
R
3 W blue LEDs, TFA (20 mol%)
CF3CH2OH, degas, rt, 6 h
CF3
OTf
2a
OH
O
O
Ph
1
()-4ba-4ha
S
Ph 3a
CF3
CF3
CF3
CF3
Me
Cl
O
Br
O
O
O
O
Ph
O
O
O
Ph
Ph
CF3
Ph
MeO
()-4ba, 41%
()-4ca, 64%
()-4da, 63%
()-4ea, 68%
CF3
Cl
Br
I
CN
CF3
O
O
6
CN
O2
N
O
O
O
O
Ph
Ph
O
Ph
Cl
()-4ga, 58%
Br
()-4ha, 61%
O
Ph
()-7, 70%c
()-4fa, 20%
a
Intermediate 1a-B undergoes
a facile deprotonation to
Reaction conditions: 1 (0.20 mmol), 2a (0.22 mmol), 3a (0.4 mmol),
generate transient o-QM intermediate 1a-C.4,12 A final formal
[4+1] annulation between 1a-C and sulfur ylide 3a deliver the
corresponding product 4aa. The additive TFA might facilitate
the annulation by activation of the o-QM intermediate 1a-C or
sulfur ylide by H-bonding,4,20 and avoiding the SET oxidation of
2-vinyl phenol. The exact role of TFA is still unclear at the
moment.
CF3CO2H (20 mol%) and fac-Ir(4’-CF3ppy)3 (2.0 mol%) in CF3CH2OH (2.0
mL) at room temperature under irradiation of 3 W blue LEDs for 6 h. b
Isolated yield. c With Ru(bpy)3Cl2·6H2O (2.0 mol%) as catalyst in CH2Cl2.
Based on our previous enantioselective [4+1] annulation of
(R)-BINOL-derived sulfur ylides and α,β-unsaturated imines,[11f]
we preliminarily examined the enantioselective version using
chiral sulfur ylide 3b’ under the standard conditions (eqn (1)).
Although the reaction proceeded with low conversion
probably due to steric hindrance; the strategy indeed worked
and product 4ab was isolated with 25% ee.
S
1a
OH
radical
CF3
SET
S
2a-A
addition
CF3 OTf
2a
*fac-IrIII(4'-CF3ppy)3
fac-IrIV(4'-CF3ppy)3
CF3
SET
CF3
OH
standard
fac
-IrIII(4'-CF3ppy)3
conditions
O
S
CF3
1a-A
=
(1)
S
S
*
+
CF3
OTf
2a
OH
CF3
O
[4+1]
annulation
O
CF3
Ar
4ab, 18% yield, 25% ee
O
1a
-H+
*
S
O
3b', Ar = 4-MeOC6H4
Ar
OH
O
Ph
S
O
Ph
4aa
3a
1a-C
1a-B
Then, we conducted a series of control experiments with
model substrates, including 2-vinyl phenol 1a, Umemoto’s
reagent 2a and sulfur ylide 3a. In the presence of
stoichiometric radical scavengers, TEMPO or 2,6-di-tert-butyl-
4-methylphenol (BHT), significant or complete inhibition of the
model reaction was observed (Scheme 3), indicating the
radical property of the process. Upon addition of TEMPO, a 28%
yield of product 4aa was observed, and the radical trapping
product 8 also was detected by HRMS.16 This result suggests
that the benzylic radical 1a-A, formed upon addition of
Scheme 4 The proposed mechanism.
In conclusion, we have developed a visible light-driven
carbon radical-mediated strategy for in situ formation of o-
QMs from 2-vinyl phenols for the first time. This redox-neutral
protocol enables a multicomponent reaction of 2-vinyl phenols,
Umemoto’s reagent, and sulfur ylides, providing practical
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We are grateful to the NNSF of China (No. 91856119,
21622201, 21820102003 and 21772053), the Science and
Technology Department of Hubei Province (2016CFA050 and
2017AHB047), and the Program of Introducing Talents of
Discipline to Universities of China (111 Program, B17019) for
support of this research.
9
Conflicts of interest
There are no conflicts to declare.
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