Visible-light-induced oxidative formylation of N-alkyl-N-(prop-2-yn-1-yl)anilines with molecular oxygen in the absence of an external photosensitizer

Article abstract of DOI:10.1039/c7cc03693k

Visible-light-induced oxidative formylation of N-alkyl-N-(prop-2-yn-1-yl)anilines with molecular oxygen in the absence of an external photosensitizer was developed and afforded the corresponding formamides in good yields under mild conditions. The investigation of the mechanism disclosed that both the starting material and the product act as photosensitizers, and ¹O₂ and O₂^- are generated through energy transfer and a single electron transfer pathway and play an important role in the reaction.

Full text of DOI:10.1039/c7cc03693k

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Visible-Light-Induced Oxidative Formylation of N-Alkyl-N-(prop-2-
yn-1-yl)anilines with Molecular Oxygen in the Absence of an
External Photosensitizer
Received 00th xxxx 2017
Accepted 00th xxxx 2017
DOI: 10.1039/x0xx00000x
Wangqin Ji,a Pinhua Li,a,* Shuai Yang,a and Lei Wanga,b,
*
www.rsc.org/
A visible-light-induced oxidative formylation of N-alkyl-N-(prop-2- salt as an photosensitizer,7 and Rueping reported the visible-light
photoredox (Ir-catalyst)-catalyzed
dimethyl-anilines with
phenylformamides as organic intermediates and formylation
reagents in Vilsmeier-Haak reactions.9
a
radical reaction of N,N-
leading to N-alkyl-N-
yn-1-yl)anilines with molecular oxygen in the absence of an
external photosensitizer was developed and afforded the
corresponding formamides in good yields under mild conditions.
Mechanism investigation disclosed that both the starting material
oxygen,8
A variety of visible-light induced organic transformations via
single electron transfer, energy transfer, or hydrogen atom
transfer process have recently received considerable
attention.10 Several C−C and C−heteroatom formations in the
presence of a photosensitizer under visible-light irradiation were
reported.11 In order to avoid the use of external photocatalysts,
photo-induced organic reactions by using substrate, or in-situ
generated electron donor-acceptor complex and possibly the
formed product as light-absorbing species were developed.12
On the basis of this understanding, herein, we report a visible-
light-induced oxidative formylation of N-alkyl-N-(prop-2-yn-1-
yl)anilines with molecular oxygen in the absence of an external
photosensitizer, affording the corresponding formamides in good
yields under ambient conditions. The protocol provides an efficient
transformation of activated methylene (CH2) into aldehyde via air
oxidation and selective cleavage of a C−C bond (Scheme 1d).
•−
and product act as photosensitizers, and 1O2 and O2 are
generated through energy transfer and single electron transfer
pathway and play important role in the reaction.
The ubiquitous and diverse of C−H bonds in organic molecules
makes their activation and functionalizations via new C−C and
C−heteroatom bond formations, providing
a novel synthetic
strategy for the constructions of organic compounds.1 However,
owing to their relatively inert property, the selective activation and
functionalization of C−H bonds is still a challenge in organic
synthesis. Besides C(sp2)−H activation,1 which is mainly conducted
through the use of transition-metal assisted transformations in the
presence of an appropriate directing group,1 activation of C(sp3)−H
bonds is more difficult. The general method of C(sp3)−H bonds
activation is via a free radical pathway.2 However, their high bond
energy makes them hard to be broken, and activating groups are
required, such as phenyl, nitrogen and oxygen. The generated
radical can be stabilized by the adjacent phenyl group or
heteroatoms through conjugated π system or the lone pair
electrons, and a variety of C(sp3)−H organic functionalizations were
developed on the basis of this strategy.3 Especially, the oxidation of
benzylic C(sp3)−H to ketones by molecular oxygen and further
reactions to give amide or ester derivatives in the presence of
transition-metal,4 or under transition-metal-free conditions were
established.5 On the other hand, there is the oxidation of
methylene C(sp3)−H bonds, which are activated by an aromatic
amino group and an electron-withdrawing group in the presence of
molecular oxygen, generating the corresponding ketones (Scheme
1a−c).6 Recently, Lei described a visible-light-induced oxidation of
benzylic C(sp3)−H to ketones using O2 in the presence of acridinium
a Department of Chemistry, Huaibei Normal University, Huaibei, Anhui 235000, P
R China; E-mail: leiwang@chnu.edu.cn; ppinhuali@126.com
Tel.: +86-561-380-2069; fax: +86-561-309-0518.
b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai 200032, P R China
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See DOI:
10.1039/x0xx00000x
Scheme 1. The oxidation of activated methylenes.
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For the optimization of the reaction conditions, a model reaction Table 1. Optimization of the reaction conditions.a
of N,N-di(prop-2-yn-1-yl)aniline (1a) was chosen in 0.20 mmol scale.
When the model reaction was carried out in CHCl3 at room
temperature under blue LED (420−425 nm, 1.5 W) irradiaꢀon for 18
h, the desired product 2a was isolated in 29% yield (Table 1, entry
Entry
1
Light source
Solvent
Yield (%)b
1). No reaction occurred when the model reaction was performed
in DME, dioxane, DMSO, NMP or DMC instead of CHCl3, and the
starting material 1a was recovered quantitatively (Table 1, entries
2−6). Less than 10% yield of 2a was obtained when the reaction was
conducted in CH3OH, or toluene (Table 1, entries 7 and 8). To our
delight, product 2a was generated in 76% yield using acetone as
solvent (Table 1, entry 9). The light source also plays an important
role in the reaction, and blue LED (420−425 nm) was the best one
for the reaction. However, LED (365−370 nm), LED (380−385 nm),
LED (390−395 nm) and blue LED (410−415 nm) were inferior owing
to the formation of side product in small amount under shorter
emission wavelengths, and blue LED (450−455 nm) and green LED
(530−535 nm) exhibited no efficiency for initiating the reaction
(Table 1, entries 11−16). In the absence of visible light, no product
was detected, indicating that visible light is essential in the reaction
(Table 1, entry 17). Further investigation indicated that the reaction
did not occur under nitrogen atmosphere, and the corresponding
product 2a was isolated in 71% yield under oxygen atmosphere
(Table 1, entries 18 and 19). Thus, the optimal reaction conditions
were found to be consisted of 1a (0.20 mmol) in acetone (2.0 mL)
and air atmosphere under 1.5 W blue LED (420−425 nm) irradiaꢀon
at room temperature for 18 h.
420−425 nm (1.5 W)
420−425 nm (1.5 W)
420−425 nm (1.5 W)
420−425 nm (1.5 W)
420−425 nm (1.5 W)
420−425 nm (1.5 W)
420−425 nm (1.5 W)
420−425 nm (1.5 W)
420−425 nm (1.5 W)
420−425 nm (1.5 W)
365−370 nm (1.5 W)
380−385 nm (1.5 W)
390−395 nm (1.5 W)
410−415 nm (1.5 W)
450−455 nm (1.5 W)
530−535 nm (1.5 W)
−
CHCl3
DME
29
0
2
3
Dioxane
DMSO
0
4
0
5
NMP
0
6
DMC
0
7
CH3OH
Toluene
Acetone
DCM
<10
<10
76
8
9
10
11
12
13
14
15
16
17
18
19
trace
58
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
63
64
70
trace
0
0c
traced
71e
420−425 nm (1.5 W)
420−425 nm (1.5 W)
aReaction conditions: N,N-di(prop-2-yn-1-yl)aniline (1a, 0.20 mmol), solvent
(2.0 mL) at room temperature under light irradiation in air for 18 h. bIsolated
yield. cIn dark. dN2 atmosphere. eO2 atmosphere.
With the optimized conditions in hand, the tolerance of the
reaction towards structural modification of the starting material
was explored. First, a variety of N,N-di(prop-2-yn-1-yl)anilines (1)
were examined, as in Scheme 2. The results indicate a broad
tolerance of the reaction towards substituents on the aromatic
rings. When N,N-di(prop-2-yn-1-yl)anilines with an electron-
donating group, such as Me, Et, iPr, tBu or MeO at the para-position
of the benzene rings underwent the oxidative formylation with
molecular oxygen to generate the corresponding products (2b−2f)
in 63−76% yields. Substrates 1 bearing 3-Me or 3,5-(Me)2 on the
aromatic rings afforded the desired products 2g and 2h in 70% and
74% yields, respectively. Furthermore, N,N-di(prop-2-yn-1-
yl)anilines that possessed an electron-withdrawing group, including
F, Cl, Br, I, COCF3, CO2Et, CN or C6H5 at the para-position of the
phenyl rings reacted with oxygen to provide the anticipated
products (2i−2p) in 74−83% yields. Moreover, the oxidative
formylation of substrates 1 substituted by 3-F, 3-Cl, 3-Br, or 3-
(C≡CH) on the benzene rings with molecular oxygen gave the
corresponding products (2q−2t) in 75−78% yields. Interestingly, the
reactions of 1 with 3,4-(Cl)2, or 3,5-(Cl)2 on the anilines afforded the
products 2u and 2v in 73% and 75% yields, respectively. The
oxidative carbonylation of N,N-di(prop-2-yn-1-yl)naphthalen-2-
amine with oxygen proceeded well and generated the product 2w
in 71% yield. Also, N-mono(prop-2-yn-1-yl) derivatives such as N-
methyl-N-(prop-2-yn-1-yl)aniline, N-ethyl-N-(prop-2-yn-1-yl)aniline,
bromophenyl)amine,
and
N-methyl-N-(prop-2-yn-1-yl)-(4-
bromophenyl)amine were further examined to react with oxygen,
providing the corresponding products 2ab–2ad in 75–81% yields.
However, no desired product (2ae) was detected when N-(tert-
butyl)-N-(prop-2-yn-1-yl)-(3-bromophenyl)amine was used owing to
the effect of steric hindrance. When N-benzyl derivative was
selected as substrate, no oxidative formylation product (2af) was
isolated owing to an alkyl radical near to aromatic ring would be
formed firstly, then a further oxidation would undergo in other way.
4-Nitro-N,N-di(prop-2-yn-1-yl)aniline, 1-(prop-2-yn-1-yl)-1H-indole,
and N-(prop-2-yn-1-yl)-N-(p-toluenesulfonyl)aniline failed to give
the anticipated products (2ag–2ai) and in all cases only of the
starting material was recovered. It maybe resulted from their strong
electron withdrawing effect.
To expand the substrate scope, N,N-disubstituted anilines (3),
such as N,N-diallylaniline, N,N-di(but-2-yn-1-yl)aniline, N,N-di(3-
phenylprop-2-yn-1-yl)aniline, N,N-di(3-(p-bromophenyl)prop-2-yn-
1-yl)aniline, and 4-bromo-N,N-di(but-3-yn-1-yl)aniline were
examined as substrates for the photosensitized reaction with
oxygen, and the results are shown in Scheme 3. When methylene
groups of substrates are activated by a carbon-carbon double bond
or a carbon-carbon triple bond, their oxidative formylations with
molecular oxygen proceeded smoothly to generate the
corresponding products (4a–4d) in 60−74% yields. Meanwhile, a
lower yield of product 4e was obtained starting from N,N-di(but-3-
yn-1-yl)aniline, in the structure of which the activated methylene
group is separated by an additional carbon atom from the carbon-
carbon triple bond.
N-butyl-N-(prop-2-yn-1-yl)aniline,
N-phenyl-N-(prop-2-yn-1-
yl)aniline underwent the reaction smoothly to afford the desired
products 2x−2aa in 70−76% yields. N-Methyl-N-(prop-2-yn-1-yl)-(3-
chlorophenyl)amine, N-methyl-N-(prop-2-yn-1-yl)-(3-
2 | Chem. Commun., 2017, 00, 1-4
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R
N
R
O2 in air (1.0 atm)
visible-light irradiation
420-425 (nm)
much higher intensity (Figure S9c and S9d). On the other hand,
N
O
1
these signals of O2 adduct with TEMP was also recorded when the
R1
R1
H
acetone, r.t., 18 h
above solution was irradiated with LED (420−425 nm), but no signal
was found without light irradiation (Figure S10). These results
1
2
R1 = H, 2a, 76%
•−
indicated that there are active species O2 and 1O2 formed in the
R1 = Me, 2b, 63%
R1 = Et, 2c, 72%
R1 = iPr, 2d, 76%
Me
N
O
Me
N
O
N
O
reaction system. In addition, ultraviolet-visible absorption spectra
of substrates and their corresponding products demonstrated that
they absorb the light (420−425 nm) and act as photosensitizers
(Figure S11−17), avoiding the need for any external photocatalyst.
The formed products formamides absorb longer-wavelength light to
activate molecular oxygen more efficiently, and the reaction rate is
accelerated so that the entire oxidative formylation is self-
sustaining in an autocatalytic fashion (Figure S18). When 2 mol% of
rose bengal or 1 mol% of Ir(ppy)3 was used as the sensitizer, the
reaction of 1a could underwent smoothly and afforded the desired
product 2a in 73% and 76% isolated yield, respectively.
H
H
H
R1
=
tBu, 2e, 73%
R1
Me
R1 = MeO, 2f, 73%
2g, 70%
2h, 74%
R1 = F, 2q, 76%
R1 = Cl, 2r, 75%
R1 = Br, 2s, 78%
R1 = C CH, 2t, 75%
R1 = F, 2i, 78%
R1 = Cl, 2j, 79%
R1 = Br, 2k, 81%
R1 = I, 2l, 78%
O
R1
N
O
N
H
R1 = COF3, 2m, 81%
R1 = EtO2C, 2n, 74%
R1 = CN, 2o, 83%
H
R1
Cl
Cl
N
O
R1 = C6H5, 2p, 80%
H
2u, 73%
R
N
Cl
N
O
R = Me, 2x, 76%
R = Et, 2y, 71%
R = nBu, 2z, 70%
R = Ph, 2aa, 75%
N
O
O
H
H
H
2w, 71%
Cl 2v, 75%
Me
Me
N
Me
N
O
Cl
Br
N
O
O
N
O
O
H
H
H
H
Br
2ac, 77%
2ab, 75%
2ad, 79%
2ae, ND
Ts
N
N
H
O
N
O
N
Scheme 3. The scope of other N,N-disubstituted anilines (3).
H
H
O
O2N
H
2af, 0
2ai, 0
2ag, 0
2ah, 0
O2 in air (1.0 atm)
TEMPO (2.0 equiv)
Scheme 2. The scope of N-alkyl-N-(prop-2-yn-1-yl)anilines (1).
N
N
H
N
(a)
+
hv (420 425 nm)
acetone, r.t., 18 h
O
O
NC
N
NC
NC
To gain mechanistic insights into this reaction, a radical scavenger,
2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) was added into the
reaction of N,N-di(prop-2-yn-1-yl)(4-cyanophenyl)amine (1o) under
air atmosphere, the oxidative formylation was completely inhibited,
along with the formation of its adduct 5 with TEMPO (Scheme 4a),
which was detected by HRMS (high resolution mass spectroscopy)
(Figure S1, ESI), demonstrating the formation of an aminomethyl
radical during the reaction although it may be in a small amount. In
addition, 18O2 labeling experiments were performed, also shown in
Scheme 4. When the irradiation of substrate 1o was performed
under 18O2 atmosphere, 2o-18O was isolated in 74% yield and was
confirmed by HRMS (Figure S7, ESI) with no formation of unlabeled
2o (Scheme 4b). Moreover, when 1o was irradiated in the presence
of air (16O2) and H218O (10 equiv), 2o was obtained in 73% yield and
no 2a-18O was detected (Scheme 4c, Figure S8 of ESI). These results
indicated that oxygen element in the products comes from
molecular oxygen in air.
2o, 0%
1o
5
HRMS-detected !
18O2 (1.0 atm)
N
H
N
16O
2o, 0%
H
N
(b)
(c)
+
18O
18O-2o, 74%
hv
(420 425 nm)
acetone, r.t., 18 h
NC
NC
NC
NC
NC
1o
O2 in air (1.0 atm)
H218O (10.0 equiv)
N
N
H
N
H
+
18O
18O-2o, 0%
16O
2o, 73%
hv (420 425 nm)
acetone, r.t., 18 h
NC
1o
Scheme 4. The control experiments.
On the basis of the above investigation, a possible reaction
mechanism is proposed in Scheme 5. First, substrate 1 is excited by
LED (420−425 nm) irradiation to generate the excited species 1*,
which acts as a photosensitizer. The formed 1* then undergoes an
energy transfer process with triplet oxygen (3O2) in ground-state to
generate highly reactive excited-state singlet oxygen (1O2) along
with the regeneration of 1 in the ground state. The obtained singlet
1O2 reacts with substrate 1 via a single electron transfer (SET)
pathway to afford a free radical cation A and O2•−, which further
reacts with 1 to give A and O22−. The formed A loses a proton to
afford an aminomethyl radical B, which reacts with triplet oxygen
3O2 to give a peroxide radical C. The C undergoes an intramolecular
radical addition to form a four-membered peroxide D, which was
trapped by TEMPO (Figure S2, ESI). The formed D further reacts
To determine the active species of oxygen in the reaction,
capturing agents, 5,5-dimethyl-pyrroline-N-oxide (DMPO) and
•−
2,2,6,6-tetramethylpiperidine (TEMP) were used to trap O2 and
1O2, respectively, and the recorded electron-spin resonance (ESR)
spectra were presented in Figure S9 (ESI). There was no signal when
DMPO was added into a solution of 4-bromo-N,N-di(prop-2-yn-1-
yl)aniline (1k) in air-saturated acetone in the absence of light
•−
(Figure S9a). However, a strong characteristic signal of O2 adduct
with DMPO was obtained when the above solution was irradiated
with LED (420−425 nm) (Figure S9b). The same characteristic signals
with O2 to give E via Path a, followed by an abstraction of HO• to
3
•−
intermediate F, which was detected by HRMS (Figure S3). Finally,
of O2 were observed upon prolonged irradiation, however with
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(e) J. Ye and M. Lautens, Nature Chem., 2015,
Wilson, R. Berg, P. Ryberg and J. F. Hartwig, J. Am. Chem. Soc., 2015,
137, 6742. For C(sp3) H activation, see: (g) H. Jiang, J. He, T. Liu and
J.-Q. Yu, J. Am. Chem. Soc., 2016, 138, 2055; (h) B. Wang, W. A. Nack,
G. He, S.-Y. Zhang and G. Chen, Chem. Sci., 2014, , 3952; (i) H.
7, 863; (f) T. Lee, T. W.
two reverse [2+2] cycloadditions of F afford the desired product 2
along with the generation of CO2 gas, which was determined by FT-
IR (Figure S5), and 4-bromobenzonic acid in 20 % isolated yield. On
the other hand, the obtained D reacts with HO• to afford G (it was
confirmed by HRMS, as in Figure S4) through Path b, which further
undergoes a reverse [2+2] cycloaddition to generate product 2, and
CO gas (detected by FT-IR, Figure S5), and 4-bromobenaldehyde,
which was confirmed by GC-MS (Figure S6). Obviously, product 2
also acts as photosensitizer, thereby completing the catalytic cycle.
In summary, we have developed a visible-light-induced oxidative
formylation of N,N-di(prop-2-yn-1-yl)anilines with molecular oxygen
in the absence of any additional photosensitizer. Investigations
support a mechanism whereby that both the starting material and
product act as photosensitizers upon excitation using blue LED to
−
5
Richter and O. G. Mancheño, Angew. Chem. Int. Ed., 2012, 51, 8656.
C. Liu, D. Liu and A. Lei, Acc. Chem. Res., 2014, 47, 3459.
2
3
For selected radical mediated C−H activations, see: (a) L. Zhang, Z.
Hang and Z.-Q. Liu, Angew. Chem. Int. Ed., 2016, 55, 236; (b) J. Ke, Y.
Tang, H. Yi, Y. Li, Y. Cheng, C. Liu and A. Lei, Angew. Chem. Int. Ed.,
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For selected recent examples, see: (a) F.-T. Du and J.-X. Ji, Chem. Sci.,
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3
photochemically promote ground-state oxygen O2 to excited-state
oxygen 1O2 via an energy transfer process. EPR results
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,
•−
demonstrated the formation of 1O2 and O2 via a single electron
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5
For selected recent examples, see: (a) J. K. Laha, K. P. Jethava and S.
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This protocol provides
a simple and mild transformation of
activated methylenes (Csp3–H bonds) into aldehydes via air
oxidation and the selective cleavage of carbon-carbon bonds. We
tried to isolate intermediates E and F, but failed. A detailed
investigation on the mechanism is underway in our laboratory.
6
(a) C. Huo, Y. Yuan, M. Wu, X. Jia, X. Wang, F. Chen and J. Tang,
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Scheme 5. The possible reaction mechanism.
We gratefully acknowledge the National Natural Science
Foundation of China (21572078, 21372095) for financial support.
12 (a) E. Arceo, I. D. Jurberg, A. Álvarez-Fernaández and P. Melchiorre,
Notes and references
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1
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4 | Chem. Commun., 2017, 00, 1-4
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