The reaction of tertiary anilines with maleimides under visible light redox catalysis

Article abstract of DOI:10.1002/adsc.201200608

Tertiary anilines can be prompted to react with N-aryl- and N-benzylmaleimides to form tetrahydroquinoline products under photocatalysis using visible light irradiation, the ruthenium or iridium complexes Ru(bpy) ₃Cl₂ or Ir(ppy)₂(dtbbpy)PF₆ as catalyst, and air as terminal oxidant.

Full text of DOI:10.1002/adsc.201200608

UPDATES
DOI: 10.1002/adsc.201200608
The Reaction of Tertiary Anilines with Maleimides under Visible
Light Redox Catalysis
a
a
a
a,
a,
Xuhui Ju, Dianjun Li, Weifei Li, Wei Yu, * and Fengling Bian *
a
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, Peopleꢀs Republic of China
Fax : (+86)-931-891-2582; e-mail: yuwei@lzu.edu.cn or bianfl@lzu.edu.cn
Received: July 11, 2012; Revised: September 17, 2012; Published online: December 4, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200608.
[
9]
[10]
[4d]
Abstract: Tertiary anilines can be prompted to
react with N-aryl- and N-benzylmaleimides to form
tetrahydroquinoline products under photocatalysis
using visible light irradiation, the ruthenium or iri-
et al., Reiser et al., and MacMillan et al. reveal
that thus formed a-aminoalkyl radicals are capable of
reacting with electron deficient alkenes or arenes. In
addition to these findings, we wish to report herein
dium
complexes
Ru
A
H
U
G
E
N
N
(bpy) Cl
or
Ir
A
H
U
G
R
N
N
(ppy)
that Ru ACHNUTGTRENNUNG( bpy) Cl or Ir ACHTUGNENTRUNG( ppy) ACHUNTGRENUN(G dtbbpy)PF -mediated
3
2
2
3 2 2 6
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(dtbbpy)PF as catalyst, and air as terminal oxidant.
photoredox catalysis can effect the reaction of tertiary
anilines with N-phenylmaleimide to form tetrahydro-
quinoline products. This transformation has recently
6
Keywords: aerobic oxidation; maleimides; photoca-
talysis; tertiary anilines; tetrahydroquinolines
been realized by using CuCl as catalyst and air as ox-
2
1
1
idant. The key steps for the formation of tetrahydro-
quinolines comprise the addition of a-aminoalkyl rad-
icals to the double bond and the subsequent free radi-
cal cyclization. The present visible light-induced pro-
Photo-induced electron transfer (PET) has proved to cess constitutes a very effective protocol with a low
[
1]
be of great value in synthetic organic chemistry.
Many reported PET processes make use of tertiary
loading level of catalyst.
We initiated this study by subjecting N,N,4-trime-
amines as electron donors, because these molecules thylbenzenamine (1a) and N-phenylmaleimide (2a) to
have low oxidation potentials. In this context, synthet- irradiation with two 40-W household fluorescent
ic applications have been found for the single electron lamps as the light source. Ru
transfer (SET) reactions between tertiary amines and (dtbbpy)PF were employed as catalyst to prompt the
photo-excited electron accepters, which involve the reaction.
addition of a-aminoalkyl radicals to alkenes as the As shown in Table 1, when a mixture of 2.0 equiv.
of 1a, 1.0 equiv. of 2a, and 5 mol% of Ru(bpy) Cl in
ACHTUNGTRENUNN(G bpy) Cl and Ir ACHTUNGTRENNUNG( ppy)
3 2 2
AHCTUNGTRENNUNG
6
[1,2]
key step.
ACHTUNGTRENNUNG
3
2
Recently PET reactions have rised to new promi- DMF was irradiated under an O atmosphere, the tet-
2
nence in organic synthesis, mostly due to the utiliza- rahydroquinoline product 3a was generated in 52%
tion of Ru(II) and Ir
photoredox catalyst to initiate organic reactions.
The advantages associated with Ru(II) and Ir(III) (Table 1, entry 2). On the other hand, the reaction did
ACHTUNGTRENNUNG( III) polypyridyl complexes as yield after 24 h (Table 1, entry 1). The yield was
[3]
raised to 75% by performing the reaction in air
AHCTUNGTRENNUNG
polypyridyl complexes not only allow the PET pro- not take place as indicated by TLC analysis if the re-
cess to take place under visible light irradiation, but action was performed under an argon atmosphere
also raise greatly the photochemical efficiency. The (Table 1, entry 3). When the molar ratio of 1a and 2a
synthetic potential of the so-called visible light photo- was adjusted to 1:1 and 1:2, the yield of 3a decreased
catalysis or redox catalysis has been greatly demon- considerably (Table 1, entries 7 and 8). It is interesting
[
4]
strated by the studies of MacMillan et al., Yoon to see that the reaction was improved when the load-
[5]
[6]
[7,8]
et al., Stephenson et al., and others.
Ir(III) polypyridyl complexes at their excited state 2 mol% (Table 1, entries 6 and 9). Using Ir
can easily oxidize tertiary amines to generate a-ami- (dtbbpy)PF as photocatalyst gave comparable results
Ru(II) and ing of Ru ACHTUNGTNERN(GU bpy) Cl was reduced to 3.5 mol% and
3 2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
ACHTUNGTRENNUNG( ppy)
2
AHCTUNGTRENNUNG
6
noalkyl radicals, which can be further converted to (Table 1, entries 10 and 11). The reaction was also
the iminiums via a second SET process. While many performed in CH CN and CH Cl , but the results
3
2
2
current studies in this area deal with the chemistry of were less satsifactory. Control experiments show that
[6d,f,7e–j,8a,c]
iminiums,
recent works by Nishibayashi
Adv. Synth. Catal. 2012, 354, 3561 – 3567
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3561

Xuhui Ju et al.
UPDATES
[a]
Table 1. Screening of the conditions for the reaction of 1a and 2a.
[b]
Entry
Catalyst (mol%)
Oxidant
Solvent
Yield [%]
1
2
3
4
5
6
7
8
9
Ru
Ru
Ru
Ru
Ru
Ru
Ru
Ru
Ru
A
T
N
T
E
N
N
(bpy) Cl (5.0)
O2
air
none
air
air
air
air
air
air
air
DMF
DMF
DMF
52
75
N. D.
3
3
3
3
3
3
3
3
3
2
2
2
2
2
A
H
U
G
R
N
U
G
[
c]
[d]
A
H
U
G
R
N
U
G
A
H
U
G
R
N
U
G
CH CN
49
48
83[
3
A
H
U
G
R
N
U
G
DCM
DMF
DMF
DMF
DMF
DMF
DMF
DMF
A
H
U
G
R
N
U
G
A
H
U
G
R
N
N
(3.5)
2
e]
A
H
U
G
R
N
U
G
35
2
2
2
[f]
A
H
U
G
R
N
U
G
50
80
84
A
H
U
G
R
N
U
G
10
11
12
Ir
Ir
A
T
N
T
E
N
N
(ppy)
A
H
U
G
R
N
U
G
2
6
[c]
[d]
A( ppy)
H
U
G
R
N
U
G
A
H
U
G
R
N
U
G
none
air
N. D.[
2
6
d]
none
N. D.
[
a]
0.5 mmol of 1a and 0.25 mmol of 2a were used unless otherwise specified. The reaction time was 12 h except for the cases
of entries 1, 4 and 5 where the reaction time was 24 h.
Isolated yield based on the compound used in lesser amount.
The reaction was performed under a strict argon atmosphere.
[
[
[
[
[
b]
c]
d]
e]
f]
3
0
0
a was not detected by TLC analysis.
.25 mmol of 1a and 0.25 mmol of 2a were used.
.25 mmol of 1a and 0.5 mmol of 2a were used.
the presence of photocatalyst is necessary for the de- tion as the key steps. It is generally accepted that the
sired reaction to proceed (Table 1, entry 12).
oxidative CÀH functionalizations of tertiary amines
The optimized conditions (Table 1, entry 6) were under visible light photoredox catalysis are initiated
then applied to the reaction of 2a with variously sub- by the single electron transfer between the photo-ex-
stituted tertiary N-methylanilines, and the results are cited catalyst and amines, which would first generate
summarized in Table 2. N,N-Dimethylanilines incor- a-aminoalkyl radicals. Besides this mechanism, com-
porating methyl, methoxy, and halogen atoms on the petitive pathways might exist considering that singlet
phenyl ring reacted with 2a to afford the correspond- oxygen can be generated from quenching of the
[13]
ing tetrahydroquinolines 3 in good yields, except for photo sensitizers by triplet molecular oxygen. The
j. The reaction of compound 1g with 2a resulted in reaction between singlet oxygen and amines may also
1
the formation of 3g-1 and 3g-2, with 3g-1 being the generate a-aminoalkyl radicals. However, our control
major product. The preference for 3g-1 reflects the experiment indicates that singlet oxygen does not
ortho-regioselectivity of free radical addition to aryl play an important role as the reaction proceeded
rings.
CÀC coupling took place exclusively at the N-methyl porphyrin (TPP)-sensitized conditions (Scheme 1).
carbon (Table 2, entries 8 and 9), consistent with the Further evidence to support this scenario is provided
[11,12]
When 1h and 1i were used as substrate, the rather slowly and with low yield under tetraphenyl-
[14]
[
11]
result reported by Miura et al.
Besides compound 2a, maleimides 2b–2g reacted as Ru
well with 1a as shown in Table 3. However, compound show that N,N-dimethylaniline (1b) can efficiently
a failed to react with the olefins shown in Figure 1 to quench the emission of Ru(bpy) Cl in the presence
deliver the tetrahydroquinoline products. In these of air.
cases, compound 1a decomposed mostly under the re- To shed more light on the mechanistic details, we
action conditions. Performing the reaction at elevated examined both inter- and intramolecular deuterium
temperature failed to render any improvement. isotope effects, and the results are shown in
We believe that the formation of tetrahydroquino- Scheme 2. The (k /k ) value for the reaction of 1b
by the fluorescence quenching experiments with
AHCTUNGTRENNUNG
(bpy) Cl (Figure 2). The Stern–Volmer studies
3
2
1
ACHTUNGTRENNUNG
3
2
H
D inter
lines involves the addition of a-aminoalkyl radicals to was determined to be 4.0, larger than those expected
the double bond and subsequent free radical cycliza- for the oxidation of 1b through electron/proton trans-
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Adv. Synth. Catal. 2012, 354, 3561 – 3567

The Reaction of Tertiary Anilines with Maleimides under Visible Light Redox Catalysis
Table 2. The reaction of tertiary N-methylanilines with 2a.
[a]
Entry
Amine (1)
Product(s) 3/Yield [%]
1
2
1
2
3
4
5
6
7
1a, R =4-Me, R =Me
3a/83
3b/81
3c/62
3d/82
3e/72
3f/67
3g-1+3g-2 (2:1)/89
1
2
1b, R =H, R =Me
1
2
1c, R =4-MeO, R =Me
1
2
1d, R =4-F, R =Me
1
2
1e, R =4-Cl, R =Me
1
2
1f, R =4-Br, R =Me
1
2
1g, R =3-Me, R =Me
1
2
8
9
1
1h, R =4-Me, R =Bn
3h/57
3i/80
3j/20
1
2
1i, R =H, R =Et
1
2
0
1j, R =H, R =Ph
[
a]
Isolated yield based on 2a.
Table 3. The reaction of 1a with N-substituted maleimides 2
[a]
Entry
R
2
3/Yield [%]
1
2
3
4
5
6
R=4-Me-C H -
2b
2c
2d
2e
2f
2g
3ab/66
3ac/85
3ad/98
3ae/89
3af/80
3ag/68
6
4
R=4-MeO-C H -
6
4
R=4-F-C H -
6
4
R=4-Cl-C H -
6
4
R=4-Br-C H -
6
4
R=Bn
[
a]
Isolated yield.
[15]
fer process. Therefore, in the present cases, the hy- might form via hydrogen atom abstraction by the
drogen atom transfer (HAT) mechanism cannot be HOOC radical. This mechanism explains reasonably
ruled out for the formation of a-aminoalkyl radicals. the observed intermolecular deuterium isotope effects
Recently, Wu and co-workers reported a detailed in the present case.
study on the eosin Y bis(tetrabutylammonium salt)
TBA-eosin Y)-mediated photochemical cross dehy- literature, a mechanism is proposed for the reaction
drogenative coupling of tetrahydroisoquinolines with of 1 and 2 (Scheme 3). In this mechanism, the reac-
On the basis of our experiments and the relevant
(
[8c]
nucleophiles. They proposed that besides the elec- tion is initiated by the single electron transfer from
tron/proton transfer process, the a-amino radicals amine 1 to the photo-excited Ru(bpy) Cl , which re-
AHCTUNGTRENNUNG
3
2
Adv. Synth. Catal. 2012, 354, 3561 – 3567
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
3563

Xuhui Ju et al.
UPDATES
Figure 1. Olefins that failed to react with 1a under the pres-
ent conditions.
Scheme 1.
Scheme 3. Proposed reaction mechanism.
oxidation by O . The catalyst Ru ACTHUNGTERNN(UNG bpy) (II) is regener-
3
2
ated from Ru
ACHTUNGTNERUNNG( bpy) (I) via aerobic oxidation, with
3
concomitant formation of superoxide radical
[
6d,8c]
anion.
The protonation of superoxide radical
anion would generate the HOOC radical. The HAT
process between HOOC and 1 opens another route to
get access to A. HOOC might be generated as well
Figure 2. Emission quenching experiments with Ru
in the presence of air.
ACHTUNGTNRENUNG( bpy) Cl
3 2
from the oxidation of radical C by O .
2
It should be noted that, in the works of Nishibaya-
[
9]
[10]
shi et al. and Reiser et al., the photocatalyst was
sults in the generation of a-aminoalkyl radical A. proposed to be recovered via the single electron oxi-
Ru(bpy) (II)* is reduced to Ru(bpy) (I) at the same dation of their reduced forms by the a-carbonyl radi-
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
ACHTUNGTRENNUNG
3
3
time. A reacts with 2 to generate B, and the latter cals derived from the addition of a-aminoalkyl radi-
then undergoes cyclization to form radical C. C is cals to the a,b-unsaturated carbonyl compounds, as
converted to tetrahydroquinoline 3 following rapid the reactions in their cases were conducted in the ab-
Scheme 2. Inter- and intramolecular deuterium isotope effects.
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Adv. Synth. Catal. 2012, 354, 3561 – 3567

The Reaction of Tertiary Anilines with Maleimides under Visible Light Redox Catalysis
Table 4. Irradiation of 1k and 2a under an argon atmos- provide evidence that intermediate B can not convert
[a]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
phere.
Ru ACHTUNGTERNNUG( bpy) (I) to Ru ACHTUNGTERNNUN(G bpy) (II).
3 3
As shown in Table 2, the reaction of 1h and 1i with
2a generated exclusively a-aminomethyl radicals re-
sulting from deprotonation at the N-methyl group. It
was of our interest to see if N,N-diethylaniline and
other N-methyl-lacking tertiary anilines would under-
go similar reaction under the present irradiation con-
ditions. The work of Hirano and Miura et al. indicated
that the CuCl /O system failed to effect the reaction
Entry Catalyst
Reaction time
h]
Recovered S.M.
[b]
[
[%]
1
2
3
4
Ru
Ru
none
none
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(bpy) Cl 12
80 (2a), 77 (1k))
trace
65 (2a), 35 (1k)
trace
3
2
A( bpy) Cl 48
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3
2
12
48
[
[
a]
2.0 equiv. of 1k and 1.0 equiv. of 2a were used.
2
2
b]
[11]
Besides the recovered starting materials, a high polar of N,N-diethylaniline with 1a.
complex mixture was detected after reaction.
N,N-diethylaniline (4) and 1-phenylpyrrolidine (5) as
aꢀs reaction partners. It turns out that both 4 and 5
can react with 2a to generate the corresponding tetra-
So next we chose
2
sence of oxygen. However, the present reaction did hydroquinolines 6 and 7, albeit in only moderate
not proceed in the absence of oxygen. We speculate yields (Scheme 4) (3 equiv. of 4 and 5 were used to
that it is probably because the electron-withdrawing guarantee the complete conversion of 2a). Both com-
effect of the amido carbonyl group in intermediate B pounds 6 and 7 were obtained as roughly 1:1 diaste-
is not strong enough to render radical B to be re- reoisomers.
duced by Ru
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(bpy) (I). To verify this hypothesis, an
When compound 1l was subjected to the reaction
3
experiment was conducted with N,N,2,6-tetramethyla- conditions, the expected reaction did not take place,
niline (1k) as the substrate. It was expected that if with only the starting materials being recovered. To
radical B could oxidize Ru
tion product would form as the two ortho-methyl a more powerful oxidant as its excited state,
ACHTUNGTRENNGNU( bpy) (I), the simple addi- effect the reaction, Ir ACHTUNGERTUNNNG( ppy) ACUTHTGNRENNUGN( dtbbpy)PF , which is
3
2
6
[17]
was
groups would prevent the cyclization step from com- used as photocatalyst under the otherwise same con-
peting with the redox process between B and ditions. As expected, the reaction happened, generat-
[16]
Ru ACHTUNGTRENNUNG( bpy) (I). We found that under an argon atmos- ing tetrahydroquinoline product 3l in yield of 44%
3
phere, 1k did not react with 2a to yield the addition (Scheme 5). Like the reaction of 1h and 1i, it was the
product (Table 4). After 12 h of irradiation, most of N-methyl carbon that connected with the double
the starting materials remained. Prolonging the reac- bond of 2a in the coupling step. Besides 3l, a mixture
tion time to 48 h resulted in the disappearance of of compounds 8 and 9 was also obtained. These two
most 1k and 2a. Still, no addition product was detect- products might be from radical E, whose formation
ed. Control experiments showed that the loss of 1k can be accounted for by the enhanced acidity of the
and 2a was not caused by the Ru
photochemical process, as the phenomenon was the E.
ACHTUNGENTRNUNG( bpy) (II)-mediated N-methylene hydrogen and the captodative effect in
3
same in the absence of Ru
A
H
U
G
R
N
U
G
In summary, we have demonstrated that the
3
Ru
A
H
U
G
R
N
U
G
A
H
U
G
R
N
U
G
A
H
U
G
E
N
N
(dtbbpy)PF -mediated visible
3
2
2
6
Scheme 4.
Adv. Synth. Catal. 2012, 354, 3561 – 3567
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asc.wiley-vch.de
3565

Xuhui Ju et al.
UPDATES
Scheme 5.
light photochemistry provides an effective method for Liu of State Key Laboratory of Applied Organic Chemistry,
the preparation of tetrahydroquinoline compounds Lanzhou University for helpful discussions.
from tertiary anilines and N-arylmaleimides. This pro-
tocol is advantageous in terms of high catalytic effi-
ciency and easy operation. This work serves as a new
illustration that visible light photocatalysis will en-
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[
[
2
3] For reviews, see: a) T. P. Yoon, M. A. Ischay, J. Du,
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(
filled with CaCl ) and magnetic stir bar were added tertiary
2
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4
solution (15 mL). The aqueous layer was extracted with
EtOAc (10 mLꢂ3). The combined organic layers were
washed with brine (15 mLꢂ4) and dried with anhydrous
Na SO . The solvent was removed under reduced pressure,
[
2
4
and the residue was treated with silica gel chromatography
to give products 3. Compounds 3 could be further purified
by recrystalization from CH Cl and hexane.
2
2
1
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The authors thank Professor Li-Zhu Wu of Key Laboratory
of Photochemical Conversion and Optoelectronic Materials,
Technical Institute of Physics and Chemistry, and Dr. Qiang
3566
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Adv. Synth. Catal. 2012, 354, 3561 – 3567

The Reaction of Tertiary Anilines with Maleimides under Visible Light Redox Catalysis
4
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