Selective α-arylation of α,β-unsaturated imides mediated by a visible light photoredox catalyst

Article abstract of DOI:10.1039/c7cc04776b

Visible light-mediated α-arylation of α,β-unsaturated imides is achieved via aminium radicals generated from diarylalkylamines using a photoredox catalyst. On the basis of emission quenching experiments, a plausible pathway of the reaction is discussed.

Full text of DOI:10.1039/c7cc04776b

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
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DOI: 10.1039/C7CC04776B
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Selective α–arylation of α,β-unsaturated imides mediated by a
visible light photoredox catalyst
Yuki Ando, Takuto Kamatsuka, Hiroshi Shinokubo and Yoshihiro Miyake*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Visible light-mediated α-arylation of α,β-unsaturated imides is reduction of 2 are mediated by a single visible light photoredox
achieved via aminium radicals generated from diarylalkylamines catalyst (Scheme 1c).
using a photoredox catalyst. On the basis of emission quenching
experiments, a plausible pathway of the reaction is discussed.
N
N
Friedel–Crafts alkylation reactions of aromatic compounds are
a fundamental and reliable transformation for carbon–carbon
bond formations.1,2 Among them, Friedel–Crafts-type alkylation
of α,β-unsaturated carbonyl compounds with electron-rich
aromatics such as anilines has been widely used in organic
synthesis. In this reaction, aryl groups are exclusively
introduced at the β-position of α,β-unsaturated carbonyl
compounds because of the higher electrophilic nature at the β-
position. It is challenging to override this intrinsic reactivity of
α,β-unsaturated carbonyl compounds to achieve α-selective
addition reactions.
(a)
+
O
hν
O
α
(λ = 350 nm)
O
N
O
e–
H+
Ar
N
Ar
N
Ar
(b)
α-aminoalkyl
aminium radicals
radicals
O
O
In 2011, Hoffmann and co-workers have reported the
addition reaction of naphthylamines to the electron-deficient
double bond of furanones at the α-position (Scheme 1a).3 In
this reaction, the electron transfer from the photoexcited
naphthylamines to furanones is a key step. Consequently,
direct photoexcitation of arylamines with UV light is essential,
thus limiting the scope of applicable amine substrates. Recently,
we have explored the visible light-mediated generation and
synthetic use of α-aminoalkyl radicals (Scheme 1b).4-8 In this
process, photocatalytic single electron oxidation of amines
provides aminium radicals, which undergo fragmentation to
yield α-aminoalkyl radicals.4,6 We have envisaged that the use
of the photochemically generated aminium radicals with visible
light enables formal electron transfer from aryl amines to α,β-
unsaturated carbonyl compounds without direct photoexcitation
of amines. Here, we report selective α-arylation of α,β-
unsaturated imides 2, where both oxidation of amines 1 and
α
Me
N
R
N
O
O
O
photoredox
catalyst
Ar
+
(c)
R
N
O
hν
1
2
NMeAr
Scheme 1. (a) α-Arylation of furanones reported by Hoffmann's group.3 (b)
Fragmentation of aminium radicals to α-aminoalkyl radicals. (c) α-Arylation of α,β-
unsaturated imides mediated by photoredox catalysts (This work).
We conducted the reaction of methylphenyl(3,5-di-tert-
butylphenyl)amine (1a) with crotonyl oxazolidinone 2a (Table
1). A solution of 1a and 2a in the presence of 10 mol% of
[4a][BF4] and 2.0 equiv of Fe(OTf)2 (OTf = OSO2CF3) in
CH2Cl2 was illuminated with 300W Xe lamp at 10 °C for 18 h.
We isolated the corresponding α-arylation product 3a in 63%
yield, where para-isomer was obtained as a major product (p/o
= 5/1) (Table 1, entry 1). The use of photocatalyst 4b in place
of 4a also afforded 3a in a similar yield, while the use of 4c and
[Ru(bpy)3][BF4]2 (bpy = 2,2'-bipyridyl) as photoredox catalysts
dramatically decreased the yields of 3a (Table 1, entries 2-4).
Other Lewis acids such as Zn(OTf)2 and Cu(OTf)2 did not
promote the reaction (Table 1, entries 5 and 6). Separately, we
confirmed that no formation of 3a was observed in the absence
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of visible light, photocatalyst, or Fe(OTf)2. Very recently, Yoon Table 2. Photocatalytic reactions of 1a with 2a
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DOI: 10.1039/C7CC04776B
and co-workers have reported the conjugate addition reaction of
O
O
[4a][BF4]
(10 mol%)
Fe(OTf)2
(2 equiv)
α-aminoalkyl radicals from α-silylamines to α,β-unsaturated
imides in the presence of photoredox catalysts and Lewis
acids.7e The present result is in sharp contrast to their reaction
system.
α
Me
N
O
O
R
N
O
Ar
+
R
N
O
CH2Cl2
10 °C, 18 h
visible light
2 (2 equiv)
3
1a
Table 1. Photocatalytic reaction of 1a with 2aa
NMeAr
(Ar = 3,5-tBu2C6H3)
O
O
photocatalyst
(10 mol%)
α
yield (%)b
49 3b
46 3c
47 3d
53 3e
0 3f
p/oc
6/1
8/1
8/1
8/1
–
Me
N
entry
2
O
O
N
O
Lewis acid
(2 equiv)
1
2
3
4
5
R = CH3CH2CH2 2b
R = PhCH2CH2 2c
R = p-MeC6H4CH2CH2 2d
R = p-ClC6H4CH2CH2 2e
R = Ph 2f
Ar
+
N
O
CH2Cl2
10 °C, 18 h
visible light
2a (2 equiv)
3a
1a
NMeAr
O
O
(Ar = 3,5-tBu2C6H3)
2g
N
O
6
23 3g
10/1
entry
photocatalyst
Lewis acid
Fe(OTf)2
Fe(OTf)2
Fe(OTf)2
Fe(OTf)2
Zn(OTf)2
Cu(OTf)2
yield (%)b
p/ob
5/1
5/1
10/1
7/1
–
1
2
3
4
5
6
[4a][BF4]
[4b][BF4]
[4c][BF4]
63(52)c
62
22
23
0
a
All reactions of 1a (0.10 mmol) with 2 (0.20 mmol) were conducted in the
[Ru(bpy)3][BF4]2
[4a][BF4]
presence of [4a][BF4] (0.01 mmol) and Fe(OTf)2 (0.20 mmol) in CH2Cl2 (1.0 mL)
with Xe-lamp irradiation at 10 °C for 18 h. bIsolated yield. cDetermined by 1H NMR.
[4a][BF4]
0
–
CF3
O
O
2+
[4a][BF4]
(10 mol%)
Fe(OTf)2
(2 equiv)
F
+
+
tBu
R
α
N
Ir
N
N
Ir
N
N
N
Me
N
O
O
N
O
N
N
N
N
N
F
Ru
N
Ar
+
F
N
O
Ar
N
Ar
CH2Cl2
N
tBu
R
10 °C, 18 h
visible light
2a (2 equiv)
3
1
F
CF3
4c
NMeAr
(Ar = 3,5-tBu2C6H3)
[Ru(bpy)3]2+
4a: R = H
4b: R = tBu
O
O
O
O
O
a
N
N
O
All reactions of 1a (0.10 mmol) with 2a (0.20 mmol) were carried out in the
ArMeN
presence of photocatalyst (0.01 mmol) and Lewis acid (0.20 mmol) in CH2Cl2 (1.0
mL) with Xe-lamp irradiation at 10 °C for 18 h. bDetermined by 1H NMR. cIsolated
yield.
Ph
NMeAr
Next, we conducted the photocatalytic reaction with various
alkenes 2 in the presence of 4a as a photocatalyst. α-Adduct 3b
was obtained in 49% yield from 2b (Table 2, entry 1). Alkenes
having arylethyl groups 2c–2e were also applicable: the
corresponding adducts 3c–3e were obtained in similar yields
(Table 2, entries 2-4). No reaction occurred with cinnamyl
oxazolidinone 2f (Table 2, entry 5). Unfortunately, the use of
chiral alkene 2g provided 3g in a decreased yield without
diastereoselectivity (diastereomeric ratio = 1/1) (Table 2, entry
6).
3h: 57% yield
(from 1b)
3i: 21% yield
(from 1c)
O
O
O
O
N
O
N
O
ArMeN
3j
3j’
3j+3j’: 22% yield (3j/3j’ = 5/2)
(from 1d)
NMeAr
Scheme 2. Photocatalytic reactions of 1 with 2a. All reactions of 1 (0.10 mmol) with 2a
(0.20 mmol) were carried out in the presence of [4a][BF4] (0.01 mmol) and Fe(OTf)2
(0.20 mmol) in CH2Cl2 (1.0 mL) with Xe-lamp irradiation at 10 °C for 18 h.
Other amines 1b–1d were examined in the photocatalytic α-
arylation of 2a (Scheme 2). The corresponding adduct 3h was
obtained in 57% yield as a sole product from 1b. Treatment of
1c afforded the adduct 3i in 21% yield. The reaction of 1c
yielded 3i,9 in which the bond formation occurred selectively at
4-position of the naphthyl ring. In the reaction of 1d, 3j and 3j'
were obtained in 22% yield as a mixture of regioisomers.
In order to get mechanistic insights on photo-induced
electron transfer, we investigated emission quenching process
of 4a in the presence of 1a or 2a in CH2Cl2. The quenching rate
constant for 1a was measured to be 5.43 × 109 M–1s–1 by using
Stern–Volmer plot. In sharp contrast, no quenching of 4a with
2a occurred at all in the presence and the absence of Fe(OTf)2.
These results clearly support that oxidation of 1a by the excited
state of 4a to generate radical cation species is an initiation step
in this reaction system.
2 | J. Name., 2012, 00, 1-3
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4
5
K. Nakajima, Y. Miyake, Y. Nishibayashi, Acc. ChVeimew. ARrteicsl.e,O2n0li1n7e,
Accordingly, a plausible reaction pathway is proposed in
Scheme 3. Oxidation of amine 1 by the photoexcited iridium
catalyst *[IrIII] provides the aminium radical A and the reduced
iridium catalyst [IrII]. Reduction of alkene 2 with [IrII] affords
the radical anion B, which is then protonated to yield the radical
C.10 Finally, radical–radical coupling between A and C results
49, 1946.
DOI: 10.1039/C7CC04776B
For recent reviews on organic transformations by using
photoredox catalysts, see: (a) J. M. R. Narayanam and C. R. J.
Stephenson, Chem. Soc. Rev., 2011, 40, 102; (b) C. K. Pier, D. A.
Rankic and D. W. C. MacMillan, Chem. Rev., 2013, 113, 5322;
(c) T. Koike and M. Akita, Synlett, 2013, 24, 2492; (d) D. M.
Schultz and T. P. Yoon, Science, 2014, 343. 1239176.
(a) Y. Miyake, N. Nakajima and Y. Nishibayashi, J. Am. Chem.
Soc., 2012, 134, 3338; (b) Y. Miyake, Y. Ashida, K. Nakajima
and Y. Nishibayashi, Chem. Commun., 2012, 48, 9528; (c) Y.
Miyake, K. Nakajima and Y. Nishibayashi, Chem. Eur. J., 2012,
18, 16473; (d) Y. Miyake, K. Nakajima and Y. Nishibayashi,
Chem. Commun., 2013, 49, 7854; (e) Y. Miyake, Y. Ashida, K.
Nakajima and Y. Nishibayashi, Chem. Eur. J., 2014, 20, 6120; (f)
K. Nakajima, M. Kitagawa, Y. Ashida, Y. Miyake and Y.
Nishibayashi, Chem. Commun., 2014, 50, 8900.
in the formation of the α-arylation product
3 after
deprotonation. On the basis of electrochemical studies using
differential pulse voltammetry, the reduction potential of 2a
was determined to be –2.32 V (vs SCE in THF). On the other
hand, the reduction potential increased to –1.34 V by the
addition of Fe(OTf)2. These results demonstrate that reduced
photocatalyst [IrII] (–1.38 V)9 can reduce 2a to B with the aid
of Fe(OTf)2, which lowers the LUMO level of 2 by
coordination.
6
7
For examples on visible light-mediated synthetic utilization of α-
aminoalkyl radicals, see: (a) A. McNally, C. K. Prier and D. W.
C. MacMillan, Science, 2011, 334, 1114; (b) P. Kohls, D. Jadhav,
G. Pandey and O. Reiser, Org. Lett., 2012, 14, 672; (c) S. Zhu, A.
Das, L. Bui, H. Zhou, D. P. Curran and M. Rueping, J. Am. Chem.
Soc., 2013, 135, 1823; (d) Z. Zuo, D. T. Ahneman, L. Chu, J. A.
Terrett, A. G. Doyle and D. W. C. MacMillan, Science, 2014,
345, 437; (e) L. R. Espelt, I. S. McPherson, E. M. Wiensch and T.
P. Yoon, J. Am. Chem. Soc., 2015, 137, 2452; (f) D. Uraguchi, N.
Kinoshita, T. Kizu and T. Ooi, J. Am. Chem. Soc., 2015, 137,
13768; (g) J. J. Murphy, D. Bastida, S. Paria, M. Fagnoni and P.
Melchiorre, Nature, 2016, 532, 218.
For examples on synthetic utilization of α-aminoalkyl radicals
under the UV-light irradiation, see: (a) U.-C. Yoon and P. S.
Mariano, Acc. Chem. Res., 1992, 25, 233; (b) N. Hoffmann, S.
Bertrand, S. Marinković and J. Pesch, Pure Appl. Chem., 2006,
78, 2227; (c) D. Harakat, J. Pesch, S. Marinković and N.
Hoffmann, Org. Biomol. Chem., 2006, 4, 1202; (d) A. Bauer, F.
Westkämper, S. Grimme and T. Bach, Nature, 2005, 436, 1139.
Me
N
Ar
1
O
O
A
*[IrIII]
R
N
O
H+
H
visible
light
[IrII]
3
NMeAr
[IrIII]
D
H
O
O
O
2
O
8
9
H+
R
N
O
R
N
O
C
B
Scheme 3. Plausible reaction pathway.
In summary, we have developed selective α-arylation of
α,β-unsaturated imides mediated by visible light photoredox
catalysts. Both oxidation of amines and reduction of α,β-
unsaturated imides are important steps and the appropriate
combination of photoredox catalysts with Lewis acids realizes
formal electron transfer from amines to alkenes without direct
photoexcitation of amines. α-Selectivity in the present reaction
is in sharp contrast to Friedel–Crafts type β-arylation of alkenes
under thermal conditions.
Crystallographic data for 3j: C32H40N2O3, Mw
= 500.66,
monoclinic, P21/c, a = 24.5341(10), b = 10.0047(4), c =
11.5533(5) Å, β = 97.7210(10), V = 2810.1(2) Å3, Z = 4, R =
0.0627 (I > 2.0 σ(I)), Rw = 0.1115 (all data), GOF = 1.070.
Crystallographic data for 3j has been deposited with the
Cambridge Crystallographic Data Centre as supplementary
publication no. CCDC-1556653.
10 F. O. Garces, K. A. King and R. J. Watts, Inorg. Chem., 1988, 27,
3464.
This work was supported by Grant-in-Aids for Scientific
Research on Innovative Areas “Advanced Molecular
Transformations by Organocatalyst” (No. 2304)”(JSPS
KAKENHI Grant 26105725) from MEXT, Japan.
Notes and references
1
G. A. Olah, R. Krishnamurti and G. K. S. Prakash, Friedel-Crafts
Alkylation in Comprehensive Organic Synthesis, Vol. 3; (eds.; B.
M. Trost and I. Fleming,), Pergamon, Oxford, 1991, 293.
For recent reviews, see: (a) K. A. Jørgenen, Synthesis, 2003,
1117; (b) M. Bandini, A. Melloni and A. Umani-Ronchi, Angew.
Chem. Int. Ed. 2004, 43, 550; (c) M. Bandini, E. Emer, S.
Tommasi and A. Umani-Ronchi, Eur. J. Org. Chem., 2006, 3257.
(a) R. Jahjah, A. Gassama, F. Dumur, S. Marinković, S. Richert,
S. Landgraf, A. Lebrun, C. Cadiou, P. Sellès and N. Hoffmann, J.
Org. Chem., 2011, 76, 7104; (b) N. Hoffmann, J. Phys. Org.
Chem., 2015, 28, 121; (c) M. Oelgemöller and N. Hoffmann, Org.
Biomol. Chem., 2016, 14, 7392.
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