Metal-free reductive coupling of CO and CN bonds driven by visible light: Use of perylene as a simple photoredox catalyst

Article abstract of DOI:10.1039/c6cc05867a

Perylene, a simple polycyclic aromatic hydrocarbon, was used as a photoredox catalyst to enable the reductive coupling reaction of aromatic aldehydes, ketones, and an imine under visible-light irradiation using a white LED.

Full text of DOI:10.1039/c6cc05867a

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Metal-free reductive coupling of C=O and C=N bonds driven by
visible light: Use of perylene as a simple photoredox catalyst†
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
Shusuke Okamoto, Keita Kojiyama, Hiroki Tsujioka, and Atsushi Sudo*
DOI: 10.1039/x0xx00000x
www.rsc.org/chemcomm
Perylene, a simple polycyclic aromatic hydrocarbon, was used as a complex is sufficiently active to reduce a C=O double bond to
photoredox catalyst to enable the reductive coupling reaction of the corresponding ketyl radical, which then undergoes self-
aromatic aldehydes, ketones, and an imine under visible-light coupling to afford the corresponding vicinal diol. Besides,
irradiation using a white LED.
there have been several successful examples where ketyl
radicals generated by photoredox catalysis were efficiently
used for intra- and intermolecular coupling reactions with
Visible-light photoredox catalysis has attracted much attention
as a useful strategy for chemical transformations, affording
valuable molecules in safe, inexpensive, non-polluting, and
1
2
other functional groups and radical species.
Zhang et al. demonstrated a successful reductive coupling
1
thus potentially sustainable manners. Such visible-light
2
photoredox catalysis includes dehalogenation, C–C bond-
reaction of benzaldehyde under visible light using poly(p-
1
phenylene) as the photoredox catalyst. The extended
3
π-
2
a,3
4
trifluoromethylation, C–heteroatom
forming reactions,
bond-forming
conjugated system of poly(p-phenylene) was excited upon the
absorption of visible light to induce the photoredox catalysis.
Herein, we report a simplest system for the visible-light-
driven reductive coupling of aldehydes and ketones, where
5
reactions,
and
controlled
radical
6
polymerisation. Most of these reactions have been achieved
using metal complexes such as Ru(II) and Ir(II) polybipyridine
complexes as the photoredox catalysts; however, some
organic dyes have been revisited as easily accessible, cost-
perylene,
a commercially available polycyclic aromatic
hydrocarbon, was used as the photoredox catalyst (Fig. 1). The
essence of this photoredox catalysis is the ability of perylene
to mediate the oxidation of tertiary amines by absorbing
2
b,2c, 6a, 6b, 6c, 7
effective, and less toxic photoredox catalysts.
Among various C–C bond-forming reactions, the reductive
coupling reaction of aldehydes and ketones is attractive,
because it allows straightforward access to important
1
4
visible light at ∼430 nm, where single electron transfer (SET)
from amines to excited perylene (P*) affords an anion radical
of perylene (P˙−). Assuming that P˙− is sufficiently reductive to
allow the SET from P˙− to aldehydes and ketones, we
envisaged the photocatalysis described in this communication.
8
9
synthetic building blocks vicinal diols and chiral auxiliaries.
Highly efficient reductive coupling reactions have been
achieved using stoichiometric amounts of metals or low-valent
metal compounds with high reduction potentials such as
Mg(0), Ti(II), Zn(0), and Sm(II), whose productions require
1
0
significant amounts of energy. In contrast, the successful use
of visible light as an energy source for the coupling reactions
will lead to a much more cost-effective and environmentally
friendly access to vicinal diols. Recently, Nakajima et al.
visible light
R
R
R
P*
N
reported
a
visible-light-induced reductive coupling of
SET
aldehydes, ketones, and imines using an Ir(II) complex as the
1
1
R
N
photoredox catalyst. Therein, the complex absorbs visible
light to transform itself to the excited state that receives one
electron from a sacrificial tertiary amine. The resulting Ir
Pelylene
R
R
P
1/2
R2
R1
-O
O-
R1
R2
R1
R2
R1
R2
O
O-
Fig. 1 Visible-light-driven reductive coupling reaction of
aldehydes and ketones using perylene as the photoredox
catalyst
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So far,
Journal Name
a
perylene diimide derivative has been used Pyrene, an aromatic hydrocarbon with a more extended
π-
2a
successfully in photoredox reactions. However, to the best of
our knowledge, perylene itself has been used as a photoredox
catalyst only in atom transfer radical polymerisation under
system than anthracene, was also not an efficient photoredox
catalyst for the reductive coupling reaction. Its use afforded 2a
only in a poor yield (8%).
6
a
visible-light irradiation.
To isolate and characterise the product, the reaction
mixture was treated with acetic anhydride, pyridine, and 4-
The initial experiment was performed using benzaldehyde
BA) and triethylamine as the substrate and sacrificial reagent,
(
(
N,N-dimethylamino)pyridine (DMAP) to transform diol 1a into
respectively (Scheme 1). Acetonitrile was selected as the
solvent based on the successful photosensitised oxidation of
the corresponding diacetate 2a. Diacetate 2a was isolated by
preparative TLC (PTLC). The necessity of visible light, perylene,
and tertiary amine for the progress of the reaction was
confirmed as follows: Without irradiation (run 2) or without
the addition of triethylamine (run 4), no reaction occurred. In
the absence of perylene, no reaction occurred even if the
solution was irradiated (run 3).
1
4
tertiary amines using perylene in acetonitrile. To an
acetonitrile solution of benzaldehyde (initial concentration: 5
mM), perylene (8 mol%) and triethylamine (8 equiv) were
added, and visible light was irradiated using a commercially
available 8.5 W white LED as the light source (Table 1, run 1).
The experimental setup is shown in ESI. After the irradiation
for 16 h, the reaction mixture was analysed by thin-layer
chromatography (TLC), indicating the successful formation of
the coupling product 1a. Other aromatic hydrocarbons such as
anthracene and naphthacene did not catalyse the reaction.
After confirming the feasibility of our concept by the initial
experiments, the reaction was studied under varying reaction
conditions. By increasing the initial concentration of
benzaldehyde from 5 mM to 20 mM, the yield of the coupling
product increased (run 5 vs. run 1). An increase in the amount
of perylene was also effective in improving the yield of the
visible light
1
product (runs 5–7). The H NMR analysis of diacetate 2a
perylene
OH
obtained in run 7 showed that the dl:meso ratio was 50:50,
indicating that the ketyl radical formed from benzaldehyde by
SET underwent the coupling reaction without any
diastereoselectivity.
R N (8 equiv)
3
H
CH3CN
O
rt, 16 h, under Ar
OH
1a
BA
Then, several tertiary amines were screened (Table 1, runs
7
–12). The results show that the reaction efficiency and
diastereoselectivity significantly depend on the structure of
amine. When triethylamine was replaced with
diisopropylethylamine, the yield of 2a increased to 66%
without influencing the diastereoselectivity. N,N-
OAc
(
Ac) O, DMAP
2
pyridine
rt, 4 h
OAc
2
a
Dimethylaniline was also a suitable amine; its use afforded the
coupling product in a good yield. Interestingly, the product was
meso-rich. The reason for this selectivity is not clear at
present; however, as reported by Nakajima et al., an attractive
interaction occurs between aldehyde and the radical cation
formed by the SET from tertiary amine to the excited
Scheme 1
Table 1. Reductive coupling reaction of benzaldehyde
Amount
[
BA]
0
of
Yield
a
perylene (%)
b
run
R
3
N
dl:meso
(mM)
11
photoredox catalyst; thus, the steric and electronic nature of
(mol%)
amine would significantly affect the diastereoselectivity in the
coupling reaction. The use of N,N-diethylaniline decreased the
yield and diastereoselectivity. The use of cyclic amines such as
N-methylpiperidine and N-methylpyrrolidine led to poor yields
of the product. However, the use of latter led to exceptionally
high meso-selectivity. These results indicate that the
diastereoselectivity can be controlled by selecting an
appropriate tertiary amine.
1
5
5
Et
Et
Et
-
3
N
N
N
8
8
39
0
32:68
-
c
2
3
3
3
4
5
6
7
8
9
5
0
0
-
d
5
8
0
-
20
20
20
20
20
20
Et
Et
Et
3
3
3
N
N
N
8
49
35
52
66
63
37
47:53
50:50
49:51
48:52
31:69
50:50
4
12
12
12
12
i-Pr
PhNMe
PhNEt
2
NEt
Next, modified perylenes, 3-arylperylenes and 2,5,8,11-
tetra-tert-butylperylene, were investigated (Table 2). The 3-
15
2
1
0
2
arylperylenes were synthesised from 3-bromoperylene by
1
6
the Suzuki–Miyaura coupling reactions
with the
1
1
20
20
12
12
7
4
47:53
12:88
corresponding aryl boronic acids. Regardless of the
modification ranging from the attachment of an electron-
withdrawing substituent to an electron-donating substituent,
the coupling product was obtained in moderate yields. In other
words, various perylene derivatives can be easily designed by
attaching substituents on the perylene core and used as the
photoredox catalysts for the reductive coupling reaction.
1
2
a
b
Yield of 2a isolated by preparative TLC (PTLC). Determined
1
by the H NMR analysis of 2a. Without irradiation. No amine
c
d
was added.
2
| J. Name., 2013, 00, 1-4
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Table 2. Reductive coupling reaction of benzaldehyde using
a
modified perylenes as the photoredox catalysts
R2
R2
Scheme 2
Table 3.
Substrate
b
)
Yield (% of 2a
R1
a
1
Yield
(%)
product
b
R2
Modified perylene
R2
1
2
R =Ph, R =H
48
dl:meso = 35:65)
56
(
(
(
(
1
R =4-(CF )C H , R =H
2
51
1b (dl:meso = 50:50)
3
6
4
dl:meso = 40:60)
1
R =4-(CH
2
, R =H
65
5
6
4
3
O)C
6
H
4
dl:meso = 51:49)
1
c
(dl:meso = 81:19)
(dl:meso = 70:30)
1
R =H, R =tert-Bu
2
55
dl:meso = 53:47)
a
[
BA]
0
= 20 mM; amount of modified perylene = 12 mol%;
NEt = 8 equiv; irradiated using an 8.5 W white
b
LED at rt for 16 h. Yield of 2a isolated by PTLC.
6
amount of i-Pr
2
1
d
e
The protocol using perylene as the photoredox catalyst was
extended to the reductive coupling reactions of several
aromatic aldehydes (Table 3). The corresponding coupling
6
7
8
products 1b–1e were obtained in moderate-to-good yields.
1
(dl:meso = 53:47)
Notably, the less electron-deficient 4-halo-substituted
benzaldehydes underwent the coupling reaction more
efficiently than those bearing electron-donating methyl and
methoxy groups. Particularly, 4-chlorobenzaldehyde with a
chlorobenzene moiety often susceptive to photoredox-
3
1f
2
,11
catalysed reductive dehalogenation underwent the reaction
efficiently. The by-products formed from such reductive
dehalogenation were not detected by TLC. Interestingly, the
presence of a methoxy group and fluorine atom in the
substrates led to the dominance of dl-products over meso-
products, indicating that the introduction of heteroatoms with
23
1
g
b
(dl:meso = 46:54)
a
1
Determined by H NMR. Isolated by silica gel column
a higher electronegativity would enable efficient stereocontrol chromatography.
in the reaction.
To extend this protocol to the photoredox catalysis of Another aromatic ketone, acetophenone, also underwent the
reductive coupling reactions, those of aromatic ketones were coupling reaction. As expected from the higher reduction
1
7
investigated (Scheme 2). When benzophenone was irradiated potential of acetophenone compared to benzaldehyde, the
with visible light under the optimized conditions for the yield of the corresponding coupling product 1g was not
reductive coupling reaction of benzaldehyde, the satisfactory, demonstrating that the photoredox catalysis of
corresponding coupling product 1f was obtained in 38% yield. acetophenone using perylene is challenging. However, side
The yield was much lower than that expected from the lower reactions such as the reduction of acetophenone to 1-
reduction potential of benzophenone compared to phenylethanol were not observed, indicating that the
benzaldehyde, making the SET to the former much easier than efficiency can be improved by further optimisation of the
1
7
that to the latter. In addition to the formation of 1f, 1,1- conditions.
diphenylmethanol was obtained in 28% yield, indicating that
Finally, the applicability of the photoredox catalysis using
the coupling reaction of the anion radical formed by SET to perylene to the reductive coupling reaction of imine was
benzophenone in the photoredox catalysis was competed by H investigated (Scheme 3). An imine
3 that can be easily
radical abstraction from H radical donors by the anion radical. synthesized from benzaldehyde and benzylamine was selected
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a
simple polyaromatic
7
hydrocarbon, was used as the photoredox catalyst for the
reductive coupling reactions of C=O and C=N double bonds.
The use of perylene enabled the efficient progress of the
reactions using visible light emitted by an 8.5 W white LED as
the energy source. In addition to the metal-free and energy-
saving aspects, the present perylene-mediated photoredox
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We gratefully acknowledge the financial support of the
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3
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