Cobalt-Catalyzed Allylic Alkylation Enabled by Organophotoredox Catalysis

Article abstract of DOI:10.1002/anie.201902509

Co-catalyzed allylic substitution reactions have received little attention, arguably because of the lack of any known advantage of Co catalysis over either Rh or Ir catalysis. Described here is a general and regioselective Co-catalyzed allylic alkylation using an in situ catalyst activation by organophotoredox catalysis. This noble-metal-free catalytic system exhibits unprecedentedly high reactivities and regioselectivities for the allylation with an allyl sulfone, for the first time, representing the unique synthetic utility of the Co-catalyzed method compared to the related Rh- and Ir-catalyzed reactions.

Full text of DOI:10.1002/anie.201902509

A Journal of the Gesellschaft Deutscher Chemiker
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Chemie
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Accepted Article
Title: Cobalt-Catalyzed Allylic Alkylation Enabled by Organophotoredox
Catalysis
Authors: Koji Takizawa, Tomoyuki Sekino, Shunta Sato, Tatsuhiko
Yoshino, Masahiro Kojima, and Shigeki Matsunaga
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902509
Angew. Chem. 10.1002/ange.201902509
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10.1002/anie.201902509
Angewandte Chemie International Edition
COMMUNICATION
Cobalt-Catalyzed Allylic Alkylation Enabled by Organophotoredox
Catalysis
Koji Takizawa, Tomoyuki Sekino, Shunta Sato, Tatsuhiko Yoshino, Masahiro Kojima,* and Shigeki
Matsunaga*
Abstract: Co-catalyzed allylic substitution reactions have long
received little attention arguably because no characteristic advantage
of Co catalysis over Rh or Ir catalysis was ever known. Here we
describe a general and regioselective Co-catalyzed allylic alkylation
using an in situ catalyst activation by organophotoredox catalysis.
This noble-metal-free catalytic system exhibited unprecedentedly
high reactivity and regioselectivity for the allylation with an allyl sulfone,
for the first time representing a unique synthetic utility of a Co-
catalyzed method compared to the related Rh- or Ir-catalyzed
reactions.
Complementary to the Pd-catalyzed Tsuji-Trost reaction,[1]
catalytic allylic substitutions catalyzed by Rh or Ir complexes have
been considered privileged transformations in organic synthesis
due to their intrinsic tendency to afford synthetically versatile
branched products.[2,3] Substantial contributions to this area,
including catalysts with broader substrate scope, higher
regioselectivity and enantioselectivity, have been reported by
Tsuji,[4] Evans,[5] Takeuchi,[6] Helmchen,[7] Hartwig,[8] Carreira,[9]
You[10] and others,[11,12] realizing transformations hitherto
challenging with other metals (Scheme 1a). In addition, Breit
recently disclosed photoredox/Rh dual catalysis for the allylation
of amines,[13] demonstrating the prospects of group 9 metal
catalysis in the context of metallaphotoredox chemistry.[14,15] On
Considering these circumstances, our continuing interest in Co
catalysis[17] motivated us to devise a novel Co-catalyzed allylic
substitution reaction and its characteristic synthetic utility. We
hypothesized that low valent Co complexes generated in situ[18,19]
should facilitate the catalytic process, and that photoredox
the other hand, investigations into corresponding catalysis with
Co, i.e., the most abundant congener of group 9, have remained
underdeveloped for decades. Contrary to the potential
advantages of Co catalysis, which include lower cost and
relevance to the versatile Rh and Ir catalysts, literature surveys
on Co-catalyzed allylic alkylation with stabilized carbon
nucleophiles revealed only few examples with inferior outcomes
compared to Rh or Ir catalysis.[16] Allylic alkylations with sodium
malonate catalyzed by well-defined Co(I) complexes were
reported by Roustan in 1979,[16a] albeit that electrophiles were
limited to allyl chlorides and that the observed regioselectivity was
low (Scheme 1b). Iqbal studied CoCl2-catalyzed allylic alkylations
in 1993,[16b] but specific substituents on electrophiles were
necessary and a mechanism mediated by an allyl cation could not
be ruled out (Scheme 1c).
mediated reduction[19,20] which plays
a
key role in
photoredox/transition metal dual catalysis[14,21] might generate an
active Co catalyst with intriguing reactivity (Scheme 1d). Hence,
our reaction design is illustrated in Scheme 2. Under irradiation
with visible light, photoredox catalyst (PC) in its excited state
(PC*) oxidizes an electron donor (D). The reduced photocatalyst
(PC.-) transfers one electron to Co and returns to its ground state.
The low valent Co complex thus generated then engages with an
[a]
K. Takizawa, T. Sekino, S. Sato, Dr. T. Yoshino, Dr. M. Kojima, Prof.
Dr. S. Matsunaga
Faculty of Pharmaceutical Sciences
Hokkaido University
Kita-ku, Sapporo 060-0812, Japan
E-mail: m-kojima@pharm.hokudai.ac.jp;
smatsuna@pharm.hokudai.ac.jp
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201902509
Angewandte Chemie International Edition
COMMUNICATION
allylic electrophile to form a π-allyl cobalt intermediate. A
subsequent attack of the nucleophile to the π-allyl cobalt
intermediate would then afford the allylated product and
regenerate the low valent cobalt catalyst.
Based on this proposal, we started our investigation with the
evaluation of cobalt complexes (Table 1, entries 1–3). We
observed that combination of CoBr2 and dppp enables the
regioselective alkylation of allyl carbonate 1a with sodium
[a] Reactions were run with 1a (0.071 mmol), Na2a (2.0 equiv), CoBr2 (10
mol%) in THF (0.05 M) at room temperature for 12 hours under blue LED
irradiation unless otherwise noted. [b] Determined by 1H NMR. [c] MS4A
(200 g/mol) was added. [d] Reactions were run with 1a (0.30 mmol), 2a (2.0
equiv), CoBr2 (1.0 mol%) in THF (0.2 M) for 16 hours. [e] Isolated yield. HEH:
diethyl 1,4-dihydro-2,6-dimethyl-3,5-pyridinedicarboxylate.
The substrate scope of the Co-catalyzed allylic alkylation with
respect to electrophiles is summarized in Table 2. Branched allyl
carbonates with alkyl substituents (1a–1d) afforded the branched
products in excellent yield and regioselectivity, irrespective of the
size of alkyl group (entries 1–4). Electrophiles with unsubstituted
(1e), electron-rich (1f) or -deficient (1g–1j) aryl groups were also
successfully employed in the desired C-C bond formation (entries
5–10). Notably, the current conditions are compatible with the C-
Br bond in 1j (entry 10). Substituents at the ortho- (1k) or meta-
position (1l) did not affect the outcome of the reaction (entries 11–
12). Even a heteroarene-substituted substrate (1m) underwent
the allylic alkylation in high regioselectivity (entry 13). Linear allyl
carbonates with methyl (1n) or phenyl (1o) groups exclusively
afforded the branched product (entries 14, 15), suggesting
negligible “memory effect”[24] in regioselectivity for this system.
Allyl carbonates containing a 4-methoxyphenyl group (1p) or
extended π system (1q, 1r) produced the product in good yield
while maintaining the excellent regioselectivity (entries 16–18). In
terms of leaving group, allyl esters with benzoyl (1s) or 4-
chlorophenylcarbonyl (1t) group afforded 3aa in excellent yield,
providing effective alternatives to methyl carbonates (entries 19–
20). Substitution with an allyl sulfone (1u) selectively delivered the
branched product (entry 21; vide infra).
dimethyl
malonate
Na2a
in
the
a
presence
photocatalyst and
of
[Ir{dF(CF3)ppy}2(dtbbpy)][PF6] 4a as
Hantzsch ester (HEH) as an electron donor under blue LED
irradiation (entry 2). Note that branched product 3aa was obtained
in high selectivity, suggesting similarity of this system to the Rh
and Ir catalysis. Although Ir-based photosensitizers always
demonstrated the best performance in previous studies,[19] the
metal-free organophotocatalysts 4b and 4c[22] have shown even
better results, comprising a noble-metal-free catalytic system
(entries 4–5). An evaluation of electron donors revealed that a
i
coordinating amine decreases the regioselectivity while Pr2NEt
demonstrated superior performance to that of HEH (entries 5–7).
The amount of the electron donor can be reduced to 30 mol%
without loss of reactivity (entry 8) while the addition of MS4A
afforded 3aa in nearly quantitative yield under preservation of the
regioselectivity (entry 9). The catalyst loading can be reduced to
as low as 1.0 mol% (Co complex) and 0.10 mol% (photocatalyst)
when using 1.0 equivalent of iPr2NEt (entry 10), which represents
a robust protocol for large scale reactions.[23]
Table 1. Evaluation of the reaction parameters for the Co-catalyzed allylic
alkylation[a]
Table 2. Reaction scope with respect to electrophiles[a]
Entry
Electrophile 1
Product 3
Yield (%)
b/l[b]
Entry
Ligand
PC
Additive
Yield
(%)[b]
< 5
41
b/l[b]
(mol%)
(mol%)
4a (1.0)
4a (1.0)
4a (1.0)
4b (1.0)
4c (1.0)
4c (1.0)
(equiv)
1
2
3
4
5
6
dppe (10)
dppp (10)
dppb (10)
dppp (10)
dppp (10)
dppp (10)
HEH (0.5)
HEH (0.5)
HEH (0.5)
HEH (0.5)
HEH (0.5)
DABCO
–
1
2
3
4
5
6
7
8
R1 = Me (1a)
R1 = nPr (1b)
R1 = iBu (1c)
R1 = CH2CH2Ph (1d)
R1 = Ph (1e)
3aa
3ba
3ca
3da
3ea
3fa
90
91
93
98
94
94
97
92
> 20/1
> 20/1
> 20/1
> 20/1
> 20/1
> 20/1
> 20/1
> 20/1
> 20/1
< 5
59
–
> 20/1
> 20/1
2.1/1
64
35
R1 = 4-Me-C6H4 (1f)
R1 = 4-CF3-C6H4 (1g)
(0.5)
7
dppp (10)
dppp (10)
dppp (10)
dppp (1.0)
4c (1.0)
4c (1.0)
4c (1.0)
4c (0.10)
iPr2NEt (0.5)
iPr2NEt (0.3)
iPr2NEt (0.3)
iPr2NEt (1.0)
72
> 20/1
> 20/1
> 20/1
> 20/1
3ga
3ha
R1
= 4-CO2Me-C6H4
8
74
(1h)
9[c]
10[c,d]
97
91 (90)[e]
9
R1 = 4-Cl-C6H4 (1i)
R1 = 4-Br-C6H4 (1j)
R1 = 2-Cl-C6H4 (1k)
R1 = 3-Cl-C6H4 (1l)
3ia
3ja
3ka
3la
> 99
85
> 20/1
> 20/1
> 20/1
> 20/1
10
11
12
90
94
13
82
10/1
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10.1002/anie.201902509
Angewandte Chemie International Edition
COMMUNICATION
(1.0 equiv) was added. [d] CoBr2 (5.0 mol%), ligand (5.0 mol%) and 4c (0.50
mol%) were used. [e] dr = 1.0/1. [f] dr = 1.2/1. [g] dr = 1.5/1.
14
15
16
17
18
R2 = Me (1n)
R2 = Ph (1o)
R2 = 4-OMe-C6H4 (1p)
3aa
3ea
3pa
3qa
3ra
78
80
75
73
88
> 20/1
> 20/1
> 20/1
> 20/1
> 20/1
Among all these reactions, the regioselective alkylation of allyl
sulfone 1u deserves special attention. Allyl sulfones are
synthetically valuable electrophiles due to their ambiphilic
nature.[26] However, branch-selective substitutions of allyl
sulfones remain highly challenging in metal-catalyzed allylic
substitution reactions.[27,28] Indeed, a comparison of the reactivity
of the catalytic alkylation with 1u revealed that only the Co-
catalyzed system delivered the branched product in high yield,
while Rh[5a] or Ir[6a] catalysts afforded modest conversions and/or
regioselectivity under the corresponding conditions (Table 4).
Table 4. Comparison of the reactivity using allyl sulfone 1u[a]
R2 = 4-Ph-C6H4 (1q)
R2 = 2-naphthyl (1r)
19
20
LG = OBz (1s)
LG = OCO(4-Cl-C6H4)
(1t)
3aa
3aa
95
96
> 20/1
> 20/1
21[c]
LG = SO2Ph (1u)
3aa
91
> 20/1
[a] Reactions were run with 1 (0.30 mmol), 2a (2.0 equiv), CoBr2 (1.0 mol%),
dppp (1.0 mol%), 4c (0.10 mol%), iPr2NEt (1.0 equiv) and MS4A (200 g/mol)
in THF (0.2 M) at room temperature for 16 hours under blue LED irradiation
unless otherwise noted. [b] Determined by 1H NMR analysis of the crude
reaction mixture. [c] CoBr2 (2.5 mol%), dppp (2.5 mol%), 4c (0.25 mol%)
and iPr2NEt (2.0 equiv) were used.
The substrate scope with respect to nucleophiles is
summarized in Table 3. Malonate esters with larger alkyl
substituents (2b, 2c) underwent allylation in high yield and
absolute regioselectivity (entries 1, 2). The sterically more
hindered nucleophile 2d also afforded satisfactory results in the
presence of LiCl[25] (entry 3). Allylation of dibenzyl malonate
derivatives (2e–2g) with 1n efficiently proceeded in high yield and
branch selectivity (entries 4–6). Malononitrile (2h), cyanoacetic
acid derivatives (2i, 2j), and phenylsulfonylacetonitrile (2k) were
coupled with 1t in good yield under absolute regioselectivity
(entries 7–10).
Entry
1
Conditions (mol%)
Yield (%)[b]
95 (91)[c]
b/l[b]
CoBr2 (2.5), dppp (2.5), 4c (0.25),
iPr2NEt (200), MS4A
THF, rt, Blue LED
Rh(PPh3)3Cl (2.5), P(OPh)3 (10)
THF, 30 °C
> 20/1
2
3
28
49
6.1/1
[Ir(cod)Cl]2 (1.25), P(OPh)3 (5.0)
THF, 30 °C
> 20/1
[a] See the Supporting Information for experimental details. [b] Determined
by 1H NMR analysis of the crude reaction mixture. [c] Isolated yield.
Regarding the mechanism, substitution of an enantiomerically
enriched electrophile (S)-1e (> 99% ee) occurred by an overall
retention of configuration in reduced enantiopurity (40% ee),
indicating competitive η3-η1-η3 interconversion of the allylcobalt
before addition of nucleophiles.[29] In addition, light/dark
experiments supported our mechanistic hypothesis as well as the
beneficial effect of the irradiation throughout the reaction.[29]
In summary, we have developed an organophotoredox-enabled
Co catalyst system for allylic alkylation. This noble-metal-free[30]
system exhibits uniformly high selectivity to afford branched
products for a series of C-C bond forming reactions between
allylic electrophiles and soft nucleophiles. In addition, the high
reactivity for the branch-selective substitution of an allyl sulfone is
highly characteristic for this catalytic system, representing the first
Co-catalyzed allylic substitution reaction with unique synthetic
utility compared to the related Rh- or Ir-catalyzed reactions.
Table 3. Reaction scope with respect to nucleophiles[a]
Entry
Electrophile
Pronucleophile 2
Product
Yield
(%)
b/l[b]
1
3
1
1a
1a
1a
1n
1n
R3 = Et (2b)
R3 = iPr (2c)
R3 = tBu (2d)
R3 = Bn (2e)
R3 = CH2(4-Me-
C6H4) (2f)
R3 = CH2(4-Cl-
C6H4) (2g)
3ab
3ac
3ad
3ne
3nf
77
89
97
94
92
> 20/1
> 20/1
16/1
2
3[c]
4[d]
5[d]
Investigations
of
enantioselective
reactions[31]
and
> 20/1
> 20/1
comprehensive study of substrate scope are currently in progress
in our group.
6[d]
1n
3ng
85
> 20/1
Acknowledgements
This work was supported in part by JSPS KAKENHI Grant
Number JP15H05802 in Precisely Designed Catalysts with
Customized Scaffolding, JSPS KAKENHI Grant Number
JP17H03049 and JP18H06097. We thank Prof. Dr. Mikako
Ogawa and Dr. Hideo Takakura in Hokkaido University for their
support in the determination of the quantum yield.
7[d]
8[d]
9[d]
10[d]
1t
1t
1t
1t
R4 = CN (2h)
3th
3ti
80
> 20/1
> 20/1
> 20/1
> 20/1
R4 = CO2Et (2i)
R4 = CONMe2 (2j)
R4 = SO2Ph (2k)
64[e]
70[f]
78[g]
3tj
3tk
[a] Reactions were run with 1 (0.30 mmol), 2 (2.0–2.2 equiv, pretreated with
2.0 equiv of NaH), CoBr2 (1.0 mol%), dppp (1.0 mol%) and 4c (0.10 mol%),
iPr2NEt (1.0 equiv) and MS4A (200 mg/mmol) in THF (0.2 M) at room
temperature for 16 hours under blue LED irradiation unless otherwise noted.
[b] Determined by 1H NMR analysis of the crude reaction mixture. [c] LiCl
Keywords: allylic substitution • photoredox catalysis • cobalt •
sulfone • dual catalysis
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10.1002/anie.201902509
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COMMUNICATION
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photocatalyst, electron donor and visible light) are indispensable for this
reaction. In addition, when Zn powder or In powder was employed in
i
place of 4c and Pr2NEt under the relevant conditions, no 3aa was
obtained. See the Supporting Information for details.
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[29] See the Supporting Information for mechanistic investigations including
the substitution of an enantioenriched substrate, the determination of the
quantum yield (Φ = 0.13), and the light/dark experiments.
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5.3/1). See the Supporting Information for details.
[31] In our preliminary study toward the enantioselective reaction, (S)-3ea
was obtained in 89% ee as shown below. See the Supporting Information
for details.
.
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Entry for the Table of Contents
COMMUNICATION
Co-catalyzed allylic alkylation enabled
by organophotoredox catalysis is
described. In addition to its broad
Koji Takizawa, Tomoyuki Sekino,
Shunta Sato, Tatsuhiko Yoshino,
Masahiro Kojima,* Shigeki Matsunaga**
substrate
regioselectivity, this noble-metal-free
system exhibited unprecedented
performance with respect to branch-
selective substitution of an allyl
sulfone. Thus, this study is the first
representation of a unique synthetic
scope
and
excellent
Page No. – Page No.
Cobalt-Catalyzed Allylic Alkylation
Enabled by Organophotoredox
Catalysis
utility of Co catalyzed
allylic
substitution compared to the related
Rh- or Ir-catalyzed reactions.
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