Visible-Light-Promoted Photoredox Syntheses of α,β-Epoxy Ketones from Styrenes and Benzaldehydes under Alkaline Conditions

Article abstract of DOI:10.1021/acs.orglett.5b02629

A range of styrenes and benzaldehydes were smoothly combined to form α,β-epoxy ketones under the synergistic actions of photocatalyst Ru(bpy)₃Cl₂, tert-butyl hydroperoxide (t-BuOOH), cesium carbonate (Cs₂CO₃), and visible light irradiation. The process likely proceeds through visible-light-enabled photocatalytic generations of acyl radicals as key intermediates.

Full text of DOI:10.1021/acs.orglett.5b02629

Letter
pubs.acs.org/OrgLett
Visible-Light-Promoted Photoredox Syntheses of α,β-Epoxy Ketones
from Styrenes and Benzaldehydes under Alkaline Conditions
Jing Li and David Zhigang Wang*
Laboratory of Chemical Genomics, School of Chemical Biology and Biotechnology, Shenzhen Graduate School of Peking University,
Shenzhen 518055, China
S
* Supporting Information
ABSTRACT: A range of styrenes and benzaldehydes were smoothly combined to form α,β-epoxy ketones under the synergistic
actions of photocatalyst Ru(bpy)3Cl2, tert-butyl hydroperoxide (t-BuOOH), cesium carbonate (Cs2CO3), and visible light
irradiation. The process likely proceeds through visible-light-enabled photocatalytic generations of acyl radicals as key
intermediates.
α,β-Epoxy ketones are important intermediates and precursors
in modern synthetic organic chemistry.1 Meanwhile, alkenes
and aldehydes are both abundant chemical feedstocks and
organic building blocks, and their direct functionalizations
represent remarkably versatile means to access complex
molecular entities.2 As α,β-epoxy ketones are a structurally
formally oxidative combination of their corresponding alkenes
and aldehydes,3−7 not surprisingly, conceptual design as well as
experimental realization of such an approach stands out as a
highly sought after synthetic goal. Within this context, a radical
chemistry-based scenario seems to serve as the logical
connection between α,β-epoxy ketones syntheses8−11 and
visible light photoredox catalysis, which has witnessed
tremendous growth in recent years in uncovering both new
catalysis concepts and robust synthetic applications12−15 and
has particular strengths in initiating radical-based mechanistic
networks. In turn, such opportunities serve as the key
motivation for us to tackle a visible light-mediated protocol
for the direct and mild merging of alkene and aldehyde partners
to produce desired α,β-epoxy ketone product.
stimulation, photocatalysis, and external oxidation, thereby
generating relatively long-lived acyl radicals species.16 Sub-
sequent radical addition to alkenes would accomplish C−H to
C−C bond conversion and then furnish new acyl-functionalized
radicals, followed by radical coupling with an alkylperoxy radical
leading to the key intermediate β-peroxy ketones that are labile
for producing α,β-epoxy ketones under alkaline conditions. The
choice of an external oxidant is crucial in this design scenario
since it should conceivably operate as a multifunctional reagent,
i.e., as a radical precursor, a hydrogen acceptor, and an oxygen
donor. Thus, the challenge in experimental demonstration of
this proposal evidently relies on screening and identifying
optimal reaction conditions that are capable of nurturing
simultaneously all of the above-mentioned events.
With 1-chloro-4-vinylbenzene 1f and benzaldehyde 2a as the
model substrates and a 45 W household bulb as the visible light
source, we initially discovered that the desired α,β-epoxy
ketone 3f was produced in 39% yield (entry 1, Table 1) in a
solvent of MeCN (concn = 0.05 M) at room temperature,
together with Ru(bpy)3Cl2 (0.02 equiv), t-BuOOH (4.0 equiv,
5.5 M in decane), and K2CO3 (2.0 equiv) under an argon
balloon atmosphere for 36 h. When an organic base Et3N
(entry 2) was employed, the reaction was found to be
completely inhibited. To our delight, the yield of 3f was
improved to 61% when Cs2CO3 was employed as the base
(entry 3). As a comparison, a lower loading of Cs2CO3 was
shown to be less effective (46% yield, entry 4), and the Lewis
acid Mg(ClO4)2 completely inhibited the reaction (entry 5).
Interestingly, when 4 Å molecular sieves were employed as the
additive, a respectable 82% yield of 3f was observed (entry 6).
Furthermore, the solvent effects were shown to be significant in
the reaction, and the product was all furnished in lower yields
As shown in Scheme 1, our design concept envisions that a
hydrogen abstraction event occurring on an aldehyde substrate
might be initiated via synergistic interactions of visible light
Scheme 1. Visible-Light-Promoted Photoredox Preparations
of α,β-Epoxy Ketones from Styrenes and Benzaldehydes
under Alkaline Conditions
Received: September 11, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b02629
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Letter
Table 1. Identification of the Optimal Reaction Conditions
Scheme 2. Reactivity Screenings on Styrenes
e
f
entry
base
additive
solvent
oxidant
yield (%)
1
2
3
K2CO3
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DMSO
DMF
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
(BzO)2
39
trace
61
Et3N
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
a
4
46
b
5
Mg(ClO4)2
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
trace
82
6
7
trace
37
8
9
toluene
CH2Cl2
Et2O
trace
trace
21
10
11
12
13
14
15
16
MeOH
THF
trace
35
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
0
(t-BuO)2
K2S2O8
0
0
c
17
t-BuOOH
t-BuOOH
0
d
18
0
19
0
a
b
Scheme 3. Reactivity Screenings on Benzaldehydes
Cs2CO3 (0.5 equiv) was used. Mg(ClO4)2 (1.0 equiv) was used.
c
d
e
f
No light. No Ru(bpy)3Cl2. t-BuOOH 5.5 M in decane. Yield was
1
determined by H NMR.
under otherwise identical reaction conditions when MeCN was
replaced by dimethyl sulfoxide (DMSO), dimethylformamide
(DMF), toluene, CH2Cl2, Et2O, MeOH, or tetrahydrofuran
(THF) (entries 7−13). As anticipated, the choice of an oxidant
was evidently most critical, and the employment of benzoyl
peroxide (BPO), (t-BuO)2, and K2S2O8 all drastically inhibited
the reactivity (entries 14−16). Subsequently, control experi-
ments operated under the absence of either a photocatalyst, or
visible light, or an oxidant (entries 17−19) unambiguously
pointed to complete inhibition of the reactivity. Thus, these
screening results helped establish the optimal reaction
conditions to be the following: 0.02 equiv of Ru(bpy)3Cl2, 2
wt % of activated powdered 4 Å molecular sieves, 4.0 equiv of t-
BuOOH, and 2.0 equiv of Cs2CO3 in the solvent of MeCN and
under a balloon−argon atmosphere at room temperature.
With the identified optimal reaction conditions in hand, we
next sought to examine the scope of aromatic alkenes 1. As
shown in Scheme 2, the transformations generally proceed to
produce the corresponding α,β-epoxy ketones 3 in moderate to
good isolated yields (51−85%). Neither electron-withdrawing
nor donating properties, or the substitution patterns (ortho,
meta, para) on the aryl rings (3a−3o), seemed to have a
significant influence on the reaction efficiency. Moreover, the
aryl ring with benzyl chloride (3o), without substitution (3p),
or with more conjugation (3q) was each shown to be
comparably competent substrate. Remarkably, 1,1-disubstituted
alkenes bearing α-methyl (3r) or phenyl substituent (3s) and
alkenes featuring heteroatom (3t) or pentafluorinations (3u)
were all found to be well accommodated, thereby significantly
expanding the reaction scope and synthetic usefulness.
-donating properties and the substitution patterns (ortho, meta,
para) on the aryl rings, as their corresponding products (4a−k)
were all consistently produced in 61−82% isolated yields.
Furthermore, an aryl ring with heteroatom (4l) was also
successful in this protocol, allowing direct access to desired
product in 83% isolated yield.
The synthetic application of this new visible-light-promoted
photoredox C−H functionalization technology was further
demonstrated through a gram-scale preparation of α,β-epoxy
ketone. As illustrated in Scheme 4, under the standard
conditions, 1-chloro-4-vinylbenzene 1f (0.9 g) and benzalde-
hyde 2a were smoothly converted to 3f (1.1 g) in 66% isolated
yield. This representative transformation helps manifest
Scheme 4. Synthetic Application in Gram-Scale Preparation
We next focused our attention on the scope of the aromatic
aldehydes 2. As exemplified in Scheme 3, a range of substrates
were readily implementable, and the reactivities were confirmed
to be broadly compatible, regardless of electron-withdrawing or
B
DOI: 10.1021/acs.orglett.5b02629
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Letter
practical value that this mthod may offer for rapid and reliable
access of α,β-epoxy ketone substances and under very mild
reaction conditions.
Mechanistic experiments were next conducted to help shed
light on the potential reaction pathways. As summarized in
Scheme 5, using 1-chloro-4-vinylbenzene 1f and benzaldehyde
possibility of C−H functionalization and visible light photo-
redox catalysis, we have established a straightforward protocol
for α,β-epoxy ketone syntheses that directly employed
abundantly available styrenes and benzaldehydes as substrates
and operated under very mild conditions. Of central
importance to this discovery is likely the identification of
multiple synergistic parameters allowing for reliable generations
of acyl radical intermediates. Further mechanism studies and
new developments will thus be continuously explored and
reported in due course.
Scheme 5. Experimental Probes on Reaction Mechanism
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b02629.
2a as the model reaction, the addition of a widely known
radical-scavenger 2,2,6,6-tetramethyl-1-piperidinyloxy
(TEMPO) led to complete inhibition of the intended reactivity,
implying strongly involvement of radical intermediates.
Furthermore, structure 5 was detected by high-resolution
mass spectrum (HRMS) analysis of the reaction mixture when
the base Cs2CO3 was absent. Finally, tert-butyl benzoperoxoate
6 was isolated when neither 1f nor Cs2CO3 was added to the
reaction.
Experimental procedures and spectral data for all new
compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: dzw@pkusz.edu.cn.
Notes
The observed reactivities coupled with the above inves-
tigations collectively point to a plausible mechanistic network
shown in Scheme 6. The sequence would be initiated through
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the NSFC (Grant Nos. 20972008 and 21290180 to
D.Z.W.), the national “973 Project” of the State Ministry of
Science and Technology (Grant No. 2013CB911500 to
D.Z.W.), the Shenzhen Bureau of Science and Technology,
and the Shenzhen “ShuangBai Project” for financial support.
Scheme 6. Proposed Mechanistic Network
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