α-Alkylation of Ketones with Alkenes Enabled by Photoinduced Activation of Silyl Enol Ethers in the Presence of a Small Amount of Water

Article abstract of DOI:10.1021/acs.orglett.1c01824

Under blue-light irradiation conditions with a photocatalyst [1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene], silyl enol ethers reacted with alkenes in the presence of a small amount of water to afford the α-alkylation products in good to high yields. A thiol cocatalyst was found to expand the substrate scope of the reaction.

Full text of DOI:10.1021/acs.orglett.1c01824

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Letter
α‑Alkylation of Ketones with Alkenes Enabled by Photoinduced
Activation of Silyl Enol Ethers in the Presence of a Small Amount of
Water
Tsubasa Hirata, Yoshihiro Ogasawara, Yasuhiro Yamashita,* and Shu Kobayashi*
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ABSTRACT: Under blue-light irradiation conditions with a
photocatalyst [1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene],
silyl enol ethers reacted with alkenes in the presence of a small
amount of water to afford the α-alkylation products in good to high
yields. A thiol cocatalyst was found to expand the substrate scope of
the reaction.
α-Alkylation reactions of carbonyl compounds provide one of
the most fundamental and important methodologies for
constructing target molecular skeletons.1 While alkyl halides
are often employed as electrophiles, alkenes without any
electron-withdrawing groups are readily available, inexpensive,
and more suitable electrophiles from the standpoint of atom
economy.2 However, they are sometimes difficult to employ in
the alkylation reactions of simple aldehydes and ketones
because of their less electrophilic nature. In 2000, Knochel and
co-workers reported base-catalyzed addition reactions of
ketones with styrene in the high-polarity solvent DMSO;
however, available alkenes were limited to styrene.3 In the
same year, Ishii and co-workers developed catalytic radical
addition reactions of ketones with alkenes using a Mn/Co-
dioxygen redox catalyst system; however, a corrosive acid,
acetic acid, was used as the solvent.4 In 2012, Kanai and co-
workers developed catalytic addition reactions of transition-
metal enolates with alkenes in which the reactivity was
improved but the available alkenes remained limited to
styrenes.5 In 2014, Dong and co-workers developed
Figure 1. Functionalization of silyl enol ethers via radical cation
formation.
coupling with heteroatom radicals (Figure 1a).12 Among them,
carbon−carbon (C−C) bond formation has been limited to
intramolecular reactions. Mattay and co-workers reported
photocatalytic cyclization of unsaturated silyl enol ethers.11
On the contrary, Ooi and co-workers recently reported β-
functionalization of silyl enol ethers by photoredox−Brønsted
base hybrid catalysis via allylic radical formation in an aprotic
solvent (Figure 1b).13 To the best of our knowledge, however,
intermolecular α-alkylation of silyl enol ethers with alkenes has
not yet been developed. Here, we describe our efforts to realize
rhodium-catalyzed α-alkylation of ketones with alkenes.6
A
fully catalytic protocol was achieved using a secondary amine
catalyst containing a directing group with a rhodium catalyst,
although a high temperature was required. More recently, in
2018, MacMillan and co-workers also reported catalytic
enantioselective α-alkylation of aldehydes using simple olefins.7
Silyl enol ethers are readily available, stable, easily isolable,
and storable enolate equivalents.8 They have been employed in
several reactions in organic synthesis, including Mukaiyama
aldol reactions and Michael reactions.9 On the contrary, silyl
enol ethers also form radical cations (RCs) by single-electron
oxidation (Figure 1), which further react with radical acceptors
at their α- or β-positions. α-Functionalization has been
conducted in solvents with water or alcohol additives via
formation of α-carbonyl radicals by desilylation; however, the
examples have been restricted to solvolysis,10 cyclization,11 and
Received: June 1, 2021
Published: July 15, 2021
https://doi.org/10.1021/acs.orglett.1c01824
© 2021 American Chemical Society
Org. Lett. 2021, 23, 5693−5697
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visible-light-induced α-alkylation of silyl enol ethers with
external alkenes under mild reaction conditions (Figure 1c).
The reaction of trimethylsilyl (TMS) enol ether 2, derived
from cyclohexanone, with ethene-1,1-diyldibenzene (1a) was
first carried out in a mixed solvent prepared from acetonitrile
and methanol in the presence of 9,10-dicyanoanthracene
(DCA) as an organic photoredox catalyst, which can oxidize
silyl enol ethers under blue-light irradiation conditions (Table
1, entry 1).11a−d,14 However, instead of the desired product
result indicates that a protic Lewis base for trapping a silyl
cation might be essential for a higher yield.11a−d On one hand,
addition of 1 equiv of water to the reaction system was found
to be very effective as its inclusion led to a dramatically
improved yield (entry 4). The yield became lower when water
was employed in a solvent amount (entry 5). Considering the
stability of the TMS enol ether, the silyl group was changed to
a tert-butyldimethylsilyl group (TBS), which is a bulkier and
more stable silyl group, and the desired product was obtained
in higher yield (entry 6). Further optimization of the reaction
conditions revealed that a slight excess of water (1.2 equiv) was
more effective (entries 7 and 8). Finally, we assessed the
impact of decreasing the amount of 4CzIPN and shortening
the reaction time, and the reaction was found to proceed
within 2 h in the presence of only 2 mol % 4CzIPN (entry 9).
The scalability was tested, and a double-scale reaction also
afforded the desired product in high yield (entry 10).
a
Table 1. Optimization of Reaction Conditions
The substrate scope of the reaction was then examined
under the optimized reaction conditions (Table 2). First, a
a
Table 2. Scope of the Reaction with Respect to Alkenes
b
entry
1
R1
R2
product
yield (%)
4-MeC6H4
2-MeC6H4
4-(MeO)C6H4
4-BrC6H4
4-(MeO)C6H4
4-FC6H4
Ph
Ph
Ph
Ph
3ba
3ca
3da
3ea
3fa
3ga
3ha
3ia
90
73
89
69
87
91
75
76
81
62
90
64
68
78
56
65
42
82
78
56
b
b
,
c
entry
1
SiR3
PC
x
solvent
yield (%)
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
SiMe3
DCA
−
MeCN/MeOH
(17:3)
0
b
b
b
e
,
,
,
c
d
d
2
SiMe3
4CzIPN
−
−
MeCN/MeOH
(17:3)
MeCN
38
4-(MeO)C6H4
4-FC6H4
3
4
5
6
7
8
SiMe3
SiMe3
SiMe3
SitBuMe2
SitBuMe2
SitBuMe2
SitBuMe2
SitBuMe2
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
17
77
27
86
92
Ph
H
H
H
H
H
H
H
H
H
H
H
1.0 MeCN
−
1.0 MeCN
1.2 MeCN
2.0 MeCN
1.2 MeCN
1.2 MeCN
b
b
4-MeC6H4
4-tBuC6H4
4-PhC6H4
4-(MeO)C6H4
3-(MeO)C6H4
2-(MeO)C6H4
4-ClC6H4
4-AcC6H4
4-(NC)C6H4
2-Np
MeCN/H2O (9:1)
3ja
3ka
3la
b
b
b
b
e
85
3ma
3na
3oa
3pa
3qa
3ra
3sa
3ta
c
d
9
92 (90)
81 (80)
e
d
10
a
Reaction conditions: 1a (0.50 mmol), 2 (0.75 mmol), photocatalyst
(PC, 25 μmol), solvent (total of 5.0 mL), under blue LED (40 W)
b
b
b
b
b
b
b
1
irradiation, 45 °C, 12 h. The yield was determined by H NMR
c
analysis (CH2Br2 as the internal standard). In the presence of
4CzIPN (2 mol %), the reaction was carried out for 2 h. The yields
shown in parentheses are based on the isolated product. The reaction
d
Indene
-(CH2)4-
-(CH2)5-
,
,
f
f
e
3ua
was carried out on a double scale. Reaction conditions: 1a (1.00
mmol), 2 (1.50 mmol), 4CzIPN (20 μmol, 20 mol%), MeCN (10.0
mL), under blue LED (40 W, two light sources) irradiation, 45 °C, 24
h.
a
Reaction conditions: 1 (0.50 mmol), 2a (0.75 mmol), 4CzIPN (25
μmol), MeCN (5.0 mL), under blue LED (40 W) irradiation, 45 °C,
2 h. The shown yield was based on the isolated product. With 0.10
mmol (20 mol %) of benzenethiol (PhSH) added. The reaction was
performed for 48 h. The reaction was performed for 12 h. With 0.25
mmol (50 mol %) of benzenethiol (PhSH) added. Alkene (2.5 mmol,
5.0 equiv) and silyl enol ether (0.5 mmol, 1.0 equiv) were used.
b
c
d
e
(3aa), methanol adduct 4 was obtained [69% yield (see the
Supporting Information)]. The side reaction might be caused
by oxidation of the employed alkene [E(1a+•/1a) = +1.81 V vs
SCE] by DCA [E(PC*/PC−•) = +1.97 V vs SCE].15,16 To
address this issue, 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyano-
benzene (4CzIPN), which possess a lower reduction potential
[E(PC*/PC−•) = +1.35 V vs SCE],16 was next employed as a
photocatalyst. Under these conditions, we were pleased to find
that the desired ketone adduct 3aa was obtained without
formation of 4, albeit in moderate yield (entry 2). The reaction
was then conducted in acetonitrile to suppress solvolysis of the
silyl enol ether;10 however, the yield decreased (entry 3). This
f
range of alkenes were employed in the reaction system. The
reaction with ethene-1,1-diyldibenzene, bearing a methyl group
at the 4-position of its aromatic ring, proceeded with excellent
yield (3ba, entry 1). However, the desired adduct was obtained
in low yield when a methyl group was substituted at the 2-
position. To overcome this problem, a catalytic amount of
benzenethiol (PhSH), which was expected to work as a
hydrogen-atom transfer (HAT) agent, was added, and the yield
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was improved to 73% (3ca). An alkene substrate bearing an
electron-donating group, such as a 4-methoxy group, was also
suitable, and the reaction proceeded in high yield (3da). The
reaction using an alkene substituted with a bromo group was
also accelerated by PhSH, and the desired adduct was obtained
in good yield (3ea). In addition to monosubstituted alkenes,
alkenes having dimethoxy or difluoro groups at their 4- and 4′-
positions reacted smoothly to afford the desired products in
high yields in the presence of PhSH (3fa and 3ga,
respectively). Notably, the scope of the reactions with respect
to alkenes was expanded to styrene analogues as well as several
simple alkenes using PhSH. The reaction using styrene
proceeded to afford the desired adduct in good yield (3ha).
When styrenes bearing an alkyl or phenyl group were
employed, the desired products were obtained in moderate
to high yields (3ia−3ka). Styrenes substituted with electron-
donating groups such as a methoxy group were also suitable
(3la−3na). When 4-chlorostyrene was employed, the reaction
proceeded in good yield (3oa). The reactions with styrenes
having electron-withdrawing groups, such as carbonyl and
cyano groups, proceeded in good yields (3pa and 3qa,
respectively). When 2-vinylnaphthalene was employed, the
reaction proceeded in moderate yield (3ra). An internal alkene
such as indene also reacted with the silyl enol ether in good
yield (3sa). It was also found that simple alkenes that do not
have phenyl groups, such as methylenecyclopentane and
methylenecyclohexane, were suitable and the desired products
were obtained in moderate to good yields (3ta and 3ua,
respectively).
Next, the scope of the reaction with silyl enol ethers was
investigated (Table 3). The silyl enol ethers derived from the
cyclic ketones, cyclopentanone and cycloheptanone, reacted
smoothly to afford the desired products 3ab and 3ac,
respectively, in excellent yields. The silyl enol ethers derived
from methyl-substituted cyclohexanones were also applicable,
and good to high yields were obtained (3ad and 3ae). An
advantage of using silyl enol ethers is that bulkier sites can be
alkylated, whereas less constricted sites are selectively alkylated
in conventional systems.7b When the silyl enol ether derived
from 3-methylcyclohexanone was employed, product 3ae was
obtained with high diastereoselectivity. The silyl enol ethers
derived from acyclic ketones were also examined. While the
silyl enol ether derived from 3-pentanone reacted smoothly to
afford desired product 3af in good yield, the silyl enol ether
derived from propiophenone gave desired product 3ag in good
yield. The reactions of the silyl enol ethers derived from
acetophenone and tert-butyl methyl ketone were unsuccessful
under the conditions (see the Supporting Information for
details). It is noted that a wide variety of substrates can be
applied in this system.
Table 3. Scope of the Reaction with Respect to Silyl Enol
Ethers
a
a
Reaction conditions: 1a (0.50 mmol), 2 (0.75 mmol), 4CzIPN (10
μmol), MeCN (5.0 mL), under blue LED (40 W) irradiation, 45 °C,
b
2 h. The shown yields were based on the isolated product. With 25
c
μmol (5 mol %) of 4CzIPN employed. With 0.10 mmol (20 mol %)
d
of benzenethiol (PhSH) added. The reaction was performed for 12
e
h. The reaction was performed for 48 h.
To gain insight into the reaction mechanism, several
investigations were conducted (Figure 2). When the reaction
was carried out under dark conditions, none of the desired
product was obtained and unreacted alkene 1a was recovered
(Figure 2a). Similarly, the reaction did not proceed at all in the
absence of a photoredox catalyst (Figure 2b). These results
imply that the photoredox catalyst, activated by visible-light
irradiation, plays a key role in the reaction. Deuterium labeling
experiments were also conducted to identify the proton source
(Figure 2c). When deuterated acetonitrile (MeCN-d3) was
employed as a solvent, almost no deuterium was incorporated
into the obtained product. On the contrary, when D2O was
used as an additive, the desired product containing deuterium
Figure 2. Mechanistic study.
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Letter
at its benzylic position was afforded. Judging from these results,
the proton source is not acetonitrile but water.
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
The proposed mechanism is shown in Figure 3. First, silyl
enol ether 2a is oxidized by an activated photoredox catalyst
AUTHOR INFORMATION
Corresponding Authors
■
Shu Kobayashi − Department of Chemistry, School of Science,
̅
The University of Tokyo, Tokyo 113-0033, Japan;
orcid.org/0000-0002-8235-4368; Phone: +81-3-
58414790; Email: shu_kobayashi@chem.s.u-tokyo.ac.jp
Yasuhiro Yamashita − Department of Chemistry, School of
Science, The University of Tokyo, Tokyo 113-0033, Japan;
Phone: +81-3-58414791; Email: yyamashita@chem.s.u-
tokyo.ac.jp
Authors
Tsubasa Hirata − Department of Chemistry, School of Science,
The University of Tokyo, Tokyo 113-0033, Japan
Yoshihiro Ogasawara − Department of Chemistry, School of
Science, The University of Tokyo, Tokyo 113-0033, Japan
Figure 3. Proposed reaction mechanism.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01824
(4CzIPN*) under single-electron transfer (SET) conditions to
afford the corresponding cation radical I. After removal of the
silyl cation by water, α-carbonyl radical II is formed, which
easily reacts with an alkene to afford radical intermediate III.11
When the reduced catalyst (4CzIPN−•) has sufficient oxidation
potential to give a single electron to intermediate III, the
reaction proceeds through path A. After reduction of III, the
formed intermediate IV is immediately protonated to afford
the desired product 3. When 4CzIPN−• does not have the
oxidation potential to reduce radical intermediate III, a thiol
cocatalyst is required as a HAT catalyst (path B).7a Radical
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by the Agency for Medical
Research and Development (S.K.) and Japan Society for the
Promotion of Science KAKENHI Grant JP 17H06448 (Y.Y.).
T.H. thanks the MERIT program, The University of Tokyo,
for financial support.
intermediate III [BDE = 357.3
6.3 kJ/mol for H-
CH(Me)Ph] abstracts the hydrogen atom from benzenethiol
(PhS-H; BDE = 349.4 4.5 kJ/mol) to afford desired product
3 and the thiyl radical.17 The formed thiyl radical is reduced by
4CzIPN−• and immediately protonated to regenerate benze-
nethiol. Thereby, the reactions proceed in the presence of
catalytic amounts of 4CzIPN and benzenethiol.
In summary, we developed photoinduced α-alkylation
reactions of silyl enol ethers with alkenes. Under blue-light
irradiation conditions with 4CzIPN, the alkylation reactions
proceeded smoothly in the presence of a small amount of water
to afford the desired products in good to high yields. By using a
thiol as a HAT catalyst, the substrate scope was expanded not
only to styrene derivatives but also to other alkyl-substituted
alkenes. To the best of our knowledge, this is the first example
of photoinduced intermolecular α-alkylation reactions of silyl
enol ethers with alkenes. Further applications of this
methodology to other electrophiles are ongoing in our
laboratory.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01824.
■
sı
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Experimental procedures, characterization data, and
crystal data (PDF)
Accession Codes
CCDC 2080743 contains the supplementary crystallographic
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