Photocatalyzed cascade Meerwein addition/cyclization of: N -benzylacrylamides toward azaspirocycles

Article abstract of DOI:10.1039/c8ob00132d

A visible-light-induced cascade Meerwein addition/cyclization of alkenes involving C-F bond cleavage was developed. This method offers a rapid access to azaspirocyclic cyclohexadienones from N-benzylacrylamides via C-F bond cleavage applying H₂O as an external oxygen source, allowing for the incorporation of various aromatic moieties originating from aryldiazonium salts.

Full text of DOI:10.1039/c8ob00132d

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
benzene
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Photocatalyzed cascade Meerwein addition/cyclization of N-
benzylacryamides toward azaspirocycles†
Li Yuan,a† Sheng-Ming Jiang,a† Zeng-Zeng Li,a Yong Zhu,a Jian Yu,a Lan Li,a Ming-Zhu Li,a Shi Tang*a
and Rui-Rong Shengb,c
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A visible-light-induced cascade Meerwein Addition/cyclization of
alkenes involving C-F bond cleavage was developed. This method
offers rapid entry to azaspirocyclic cyclohexadienones from N-
benzylacrylamides via C-F bond cleavage applying H2O as the
external oxygen source, allowing for the incorporation of various
aromatic moieties orignated from aryldiazonium salts.
Previous work
a)
dearomatization with phenol derivatives as the starting arenes
R1
R1
R2
N
N
cat. [M]
refs.3h,4
5/6
R2
R -
reagents
+
(1)
f
O
RO
Rf
R = H, Me
Rf = CF3, CF2H, C4F9, etc.
b) spirocyclization of alkynes with TBHP as the oxidant
Ph
R1
Azaspirocycles are important structural motifs present in a
broad range of biologically active molecules,1 such as
pronuciferine,1d NO and ROS inhibitors,1e and Eryrhatinone.1f
Dearomatization reaction has been widely identified as a rapid
entry for synthesis of more valuable and complex
azaspirocyclic skeletons from simple aromatic molecules.2-4 In
this context, many research groups reported Lewis-acid-
catalyzed2 and transition-metal-catalyzed3 dearomative
cyclization toward azaspirocyclic ring systems, whereas only a
Ph
TBHP (4 equiv.)
cat. [Cu]
R
R1
alkynes
R
N
O
+
-H
(2)
O
refs.8
N
(aldehyde or ether)
O
R1
R1 = MeO, H, F
(only two examples involving C-F activation)
This work
c) C-F ativation with H2O as the terminal oxidant
R2
visible light
R1
R1
R2
N
N
Cat. [Ru]
H2O18 (4 equiv )
R3
O (3)
18O
-
F
O
+
ArN2+BF4
R3
Ar
alkenes
rt
Scheme
Cyclohexadienones.
1
Dearomative spirocyclization leading to azaspirocyclic
O
O
MeO
MeO
O
O
NMe
HO
O
MeO
O
spirocyclization is of high interest for the synthetic
practitioners.
OMe
N
N
O
O
O
N
H
On the other hand, expanding the diversity of starting
aromatic molecules for the dearomative reactions toward
desired azaspirocyclic cyclohexadienones has recently
attracted substantial attentions.6 Notably, these successes
were mainly limited to the utilization of phenol or its
derivatives as the starting aromatic molecules to introduce
O
spiro β-Lactams
(antibiotics)
(±)-Eryrhatinone
(curare like activity)
pronuciferine
(antihypertensive)
spirobacillene A
(NO and ROS inhibitor)
Figure 1 Selected examples of biologically active azaspirocycle molecules.
handful of examples employing the photocatalysis strategy are
disclosed for this purpose.4 Of note, photoredox catalysis has
recently emerged as a powerful synthetic strategy for the
assembly of diverse molecules for its merits such as eco-
carbonyl moieties without the requirement of external O-atom
4-6
(eqn (1), Scheme 1)
.
Aryl fluorides are readily available
molecules and function as versatile synthons (e.g., Sanger's
reagent) despite commonly high stability and chemical inertness
of C-F bond, which are promising to act as alternative starting
arenes in the dearomative spirocyclization via C-F cleavage.7
In this regard, a rare example involving the oxidative C-F
cleavage of alkynes (e.g., N-arylpropiolamides) was recently
disclosed by Li’s group.8 However, excess hazardous oxidant
friendliness,
sustainability,
convenience.5 Thus,
the
development of mild photocatalysis protocol in dearomative
TBHP (tert-butyl hydroperoxide) as the external oxygen source
was required to form the carbonyl group (eqn (2)). In addition,
these TBHP-mediated oxidative spirocyclization involving C-F
cleavage exhibited a very limited substrate scope, and only
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two examples using N-(p-fluoroaryl)propiolamides have been from
demonstrated in Li’s works.8a From
5 W blue LEDs (Table 1). Delightedly, the initial
a
diversification experiment using 5 mol% Ru(bpy)3Cl2 as the photocatalyst,
perspective, exploiting new starting arenes in the DMF as the solvent, and K3PO4 as the base led to the desired
dearomatization involving C-F cleavage, leading to product 3a in a yield of 48% (Table 1, entry 1). Inspired by the
azaspirocyclic cyclohexadienones, with clean and safe oxygen results, several typical photocatalyst such as fac-Ir(ppy)3,
source arguably remains synthetically interesting. As the Ru(bpy)3Cl2•6H2O and Eosin Y were subsequently investigated
continuation of our interest in photocatalysis and Meerwein (entries 2-4). Unfortunately, none of them gave better results
cyclization,9 we herein want to demonstrate a visible-light- than Ru(bpy)3Cl2, and Eosin Y was completely inefficient for
induced dearomative Meerwein cyclization of alkenes with the reaction. In order to improve the yield further, several
aryldiazonium salts involving C-F bond cleavage employing H2O bases were sequentially examined, and the results showed
as the oxygen source,10,11 allowing for the general synthesis of that K2CO3 was the optimal to increase the yield to 75% (entry
2-azaspiro[4.5]deca-6,9-diene-3,8-diones (eqn (3)). Notably, it 7), whereas the use of the K2HPO4, KH2PO4, KOAc, Et3N and etc.
is arguably of practically interest to introduce the carbonyl gave inferior yields (entries 5-10). In addition, the
group into azaspirocyclic core using clean, non-toxic and cost- photocatalyst Ru(bpz)3[PF6]2 with higher redox potential
effective water as an oxygen source by cost-effective Ru-based cannot improve the reaction yield further (entry 8). Next, the
photoredox catalysis.
Further solvent screening turned out that DMF was the
Our preliminary investigation was concentrated on the optimal medium among these solvents such as dioxane,
reactions between N-(p-fluorobenzyl)acrylamides 1a and 4- DMSO, CH3CN, CH2Cl2 (entries 11-14). Given that the visible-
nitrophenyldiazonium tetrafluoroborate 2a under irradiation
light irradiation and Ru catalyst are necessary for the
dearomative spirocyclization, two control experiments were
designed and conducted. Expectedly, the cyclization reactions
were found to be restrained seriously with the removal of
either the light irradiation or Ru catalyst (entries 15 and 16).
Remarkably, small amount of water plays significant role in the
reactions, and should function as an oxygen source in the C-F
carbonylation process. The conditions investigation showed
that 4 equiv. of H2O gave the best results, whereas no reaction
took place in the absence of H2O (for detail, see SI).
Interestingly, ethanol was also observed to be a viable oxygen
source instead of H20 in the reaction, and afforded a slightly
decreased reaction yield (entry 18).
Table 1 Optimization of reaction conditions for the synthesis of 3aa
With the optimal conditions in hand, we targeted to
investigate the scope of substrate in the dearomative
cyclization leading to azaspirocyclic cyclohexadienones
(Scheme 2). Initially, we screened the scope of aryldiazonium
yield of
3a (%)b
entry catalyst
base
solvent
1
2
Ru(bpy)3Cl2
K3PO4
DMF
DMF
48
36
12
0
Scheme 2 Substrate scope in the spirocycliaztion involving C-F bond cleavagea,b
fac-Ir(ppy)3
Ru(bpy)3Cl2•6H2O
Eosin Y
K3PO4
K3PO4
K3PO4
K2HPO4
KH2PO4
K2CO3
K2CO3
KOAc
Et3N
3
DMF
4
DMF
5
Ru(bpy)3Cl2
Ru(bpy)3Cl2
Ru(bpy)3Cl2
Ru(bpz)3[PF6]2
Ru(bpy)3Cl2
Ru(bpy)3Cl2
Ru(bpy)3Cl2
Ru(bpy)3Cl2
Ru(bpy)3Cl2
Ru(bpy)3Cl2
none
DMF
25
27
75
51
32
21
64
55
26
11
0
6
DMF
7
DMF
8
DMF
9
DMF
10
11
12
13
14
15
16c
DMF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
dioxane
DMSO
CH3CN
CH2Cl2
DMF
Ru(bpy)3Cl2
DMF
0
17d
18e
Ru(bpy)3Cl2
Ru(bpy)3Cl2
K2CO3
K2CO3
DMF
DMF
trace
69
a Reaction conditions: 1a (0.3 mmol), 2a (2 equiv.), photocatalyst (5 mol%), base
(3 equiv.), H2O (4 equiv.) and solvent (2 mL) were irradiated with a 5 W blue LEDs
at room temperaturefor 24 h. DMF = N,N-dimethyl formamide, DMSO = dimethyl
sulfoxide. b Yield of the isolated product. c Without light irradiation. d Without the
addition of external water. e Ethanol (4 equiv.) in stead of H2O.
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a Reaction conditions:
room temperature for 24 h. b Yield of the isolated product.
1 (0.3 mmol), 2 (2 equiv), Ru(bpy)3Cl2 (5 mol%), K2CO3 (3
products (e.g., 3c). The dearomatization of N-
(methoxybenzyl)acrylamides could also work without the H2O
as the additive, suggesting that the carbonyl group thereby
formed in these reactions should arise from the MeO
substituent (3z).4c
equiv.), H2O (4 equiv.) and DMF (2 mL) were irradiated with a 5 W blue LEDs at
tetrafluoroborates. Gratifyingly, the optimal conditions were
compatible with various aryldiazonium tetrafluoroborates with
2/4-NO2, 4-Cl, 3,4-2Cl, 3/4-CO2Me substituents, thereby
affording a series of azaspirocyclic cyclohexadienones in
In addition, we also investigated the reactivity of N-
benzylacrylamides with other para-substituents on the phenyl
moiety under the standard conditions (eqn (4)). In the
moderate to good yield (3a-3f). Next, we embarked on
investigating the scope of the N-(fluorobenzyl)acrylamides
with different substituents on the aromatic ring. Of note, the
cyclization of the benzylacrylamides bearing Br or Cl at 2/3-
position of aromatic ring proceeded smoothly, leading to
+
-
presence of p-ClC6H4N2 BF4 , these reactions did work to give
desired azaspirocyclic cyclohexadienones 3c via C-X (x = Cl, Br,
H) activation, but only in low yields (<32%), which suggested
that the dearomative spirocyclization involving C-F cleavage
would be more reactive to afford synthetically useful yields. In
addition, the reaction using N-(ortho-fluorobenzyl)acrylamide
as a substrate failed to afford the expected azaspirocycles via
the ortho-position C-F cleavage (eqn(5)), and yielded a small
amount of azaspirocycle via the C-H cleavage at the 4-position
as well (<10%).
corresponding diastereoisomers in moderate yields (3g and 3i
,
3h and 3j). To our delight, perfluorinated aryl ring were also
tolerated under the reaction system, affording the
corresponding polyfluoroazaspirocycle 3m via site-selective C-
F cleavage. Further screening revealed that a series of
substituents on the N-atom such as n-butyl, i-propyl, methyl,
benzyl and CH2CH2OTBS groups were also compatible with the
optimal conditions (3n-3r).
Next, to further explore the application of this cyclization
system, the dearomatization reactivity of N-benzylacrylamides
with a para-methoxyl substituent is also investigated under
the optimal conditions (Scheme 3). Expectedly, a series of
substituents such as t-butyl, n-butyl, i-propyl, Me, and benzyl
on N-atom of the acrylamides moiety was well tolerated and
furnished a series of azaspirocyclic cyclohexadienones in
moderate to good yields (Scheme 3). Aryldiazonium
tetrafluoroborates irrespective of with electron-donating
substituents (e.g., MeO, CH3) or electron-withdrawing
substituents (e.g., Cl and NO2) were compatible with the
optimal conditions. Of note, the yields in the dearomatization
were commonly higher comparing with above-mentioned
examples involving C-F bond cleavage leading to the same
Finally, the O18-labelled control experiment was designed
and conducted (eqn (6)). Expectedly, we found that the
azaspirocyclic cyclohexadienone 3a containing O18-atom
thereby obtained was increased as the main product with the
addition 4 equiv of H218O in the reaction mixture, which
suggested that the oxygen atom of newly-formed carbonyl
group should originate from H2O in the C-F cleavage process
(for detail, see SI).11,12
Scheme
3
Dearomative
Meerwein
cyclization
of
N-(p-
methoxyl)benzylacrylamidesa,b
R4
R2
R2
R1
N
Ru(bpy)3Cl2(5 mol %)
K2CO3 (3 equiv)
N
R3
+
O
R3
O
MeO
O
DMF, H2O (4 equiv,)
rt, 24 h, 5 W LEDs
N2BF4
R4
1
3
2
R2 = n-Bu, i-pr, Me, Benzyl
N
O
N
O
N
O
O
O
O
On the basis of above experimental results and the
literature, a rational reaction mechanism is proposed in
Scheme 4.3-12 First, photoexcitation of [Ru(bpy)3]2+ by light
irradiation generates the excited state *[Ru(bpy)3]2+, and it is
then oxidizes to [Ru(bpy)3]3+ by ArN2 BF4 (Ar = p-NO2C6H4),
thereby leading to a radical Ar• via a single-electron-transfer
(SET) process.13 Subsequently, the radical Ar• adds onto the
Cl
Me
R4
R4 = MeO, 3v,65%
R4 = CH3, 3w, 47%
R4
88%
R4 = Cl,
3c,
R4 = MeO,
3u,
83%
3s,
50%
R4 = CH3, 3t, 55%
+
-
N
N
O
O
N
O
O
O
O
Cl
C=C bond of 1a to give intermediate
is transformed into intermediate
cyclization process. In the presence of Ru(III) species, the
intermediate is then oxidized to the carbocation , along
with the Ru(III) species simultaneously reducing to its initial
state Ru(II) species. Finally, the carbocation reacts with H2O
A
. Then the intermediate
Cl
OMe
3x,
51%
3z,
87%
83%c
3y
, 48%
A
B
via a dearomative
NO2
a Reaction conditions:
1
(0.3 mmol),
2
(2 equiv.), Ru(bpy)3Cl2 (5 mol%), K2CO3 (3
equiv.), H2O (4 equiv.)and DMF (2 mL) were irradiated with a 5 W blue LEDs at
room temperature for 24 h. b Yield of the isolated product. c Without the addition
of H2O.
B
C
C
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via the cleavage of C-F bond.12 Finally, the
is transformed to desired 3a with the
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intermediate
E
E
abstraction of hydrogen atom under the aid of base.
Scheme 4 Proposed mechanism for the formation of 3a
2
3
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Conclusions
In summary, we have disclosed a visible-light-induced
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dearomative
Meerwein
cyclization
of
N-(p/o-
fluorobenzyl)acrylamide via C-F bond cleavage applying water
as the oxygen source for the first time. The spirocyclization
reactions exhibit excellent compatibility with a wide scope of
N-(fluorobenzyl)acrylamide and aryldiazonium salts, allowing
for
rapid
construction
of
various
azaspirocyclic
cyclohexadienones under mild conditions (e.g., room
temperature). The use of readily available aryldiazonium salts
as aryl radical source, and environment-friendly water as the
external oxygen source, in combination with clean and atomic-
economy Ru-based photoredox catalysis, make this protocol
high attractive for organic and medicinal practitioner, therein
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a
valuable complementary entry to 2-
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Conflicts of interest
There are no conflicts to declare.
Synthesis of organic fluorides, see: (a) Q. Liu, C. Ni, and J. Hu,
Nati. Sci. Rev., 2017, 4, 303; (b) M. G. Campbell and T. Ritter,
Acknowledgements
Chem. Rev., 2015, 115, 612; C-F activation: (c) F. Sanger,
Biochem. J. 1945. 39, 507. (d) X. Lu, Y. Wang, B. Zhang, J.-J.
Pi, X.-X. Wang, T.-J. Gong, B. Xiao and Y. Fu, J. Am. Chem.
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Financial support from the National Natural Science
Foundation of China (No.21662013
, 21462017), Hunan
Provincial Natural Science Foundation of China (No.
2018JJ1020), and the Scientific Research Fund of Hunan
Provincial Education Department (No. 16A171). Dr. R.-L. Sheng.
thanks the ARDITI-Agência Regional parao Desenvolvimento da
,
8
9
(a) X.-H. Ouyang, R.-J. Song, Y. Li, B. Liu and J.-H. LI, J. Org.
Chem,. 2014, 79, 4582; (b) W.-T. Wei, R.-J. Song, X.-H.
Ouyang, Y. Li, H.-B. Li and J.-H. Li, Org. Chem. Front., 2014, 1,
484;
For representative examples, see, photoredox catalysis: (a) S.
Tang, Y.-L. Deng, J. Li, W.-X. Wang, G.-L. Ding, M.-W. Wang
and R. -L. Sheng, J. Org. Chem., 2015, 80, 12599; (b) S. Tang,
L. Yuan, Z.-Z. Li, L.-N. Wang, G.-X. Huang and R. Sheng,
Tetrahedron Lett., 2017, 58, 329; (c) S. Tang, L. Yuan, Z. Li ,
Z.-Y. Peng , Y.-L. Deng , L.-N. Wang, G.-X. Huang and R.-L.
Investigação Tecnologia
e Inovação through the project
M1420-01-0145-FEDER-000005-Centro de Química da
Madeira-CQM+ (Madeira 14-20) for the sponsorship.
Notes and references
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Organic & Biomolecular Chemistry
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DOI: 10.1039/C8OB00132D
Journal Name
COMMUNICATION
Sheng, Tetrahedron Lett., 2017, 58, 2127; Meerwein
reactions: (d) S. Tang, D. Zhou, Y. Wang, Eur. J. Org. Chem.,
2014, 3656; (e) S. Tang, D. Zhou, Y. Deng, Z. Li, Y. Yang, J. He
and Y. Wang, Sci China Chem., 2015, 58, 684.
10 For selected examples on Meerwein reactions, see: (a) S.
Mahouche-Chergui, S. Gam-Derouich, C. Mangeney and M.
M. Chehimi, Chem. Soc. Rev., 2011, 40, 4143; (b) A. Roglans,
A. Pla-Quintana and M. Moreno-Mañas, Chem. Rev., 2006,
106, 4622; (c) D. P. Hari and B. König, Angew. Chem. Int. Ed.,
2013, 52, 4734; (d) A. L. J. Beckwith and G. F. Meijs, J. Org.
Chem., 1987, 52, 1922.
11 (a) J.M. Risley and R. L. Van Etten, J. Am. Chem. Soc., 1979,
101, 252-253; (b) Jean E. Parente, John M. Risley, Robert L.
Van Etten, J. Am. Chem. Soc., 1984, 106, 8156-8161
12 (a) G. Zhang, X. Hu, C. Chiang, H. Yi, P. Pei, A. K. Singh and A.
Lei, J. Am. Chem. Soc., 2016, 138, 12037; (b) X. Hu, G. Zhang,
F. Bu and A. W. Lei, ACS Catal., 2017, 7, 1432.
13 For selected examples on visible-light-induced reactions
involving aryldiazonium salts by Ru catalysis or others, see:
(a) D. Xue, Z.-H. Jia, C. J. Zhao, Y.-Y. Zhang, C. Wang and J.
Xiao, Chem. Eur. J., 2014, 20, 2960; (b) X.-L. Yu, J.-R. Chen, D.-
Z. Chen and W.-J. Xiao, Chem. Commun., 2016, 52, 8275; (c)
D. P. Hari, D. P. Schroll and B. König, J. Am. Chem. Soc., 2012,
134, 2958. (d) S. J. Kwon and D. Y. Kim, Org. Lett., 2016, 18,
4562. (e) W. Guo, H.-G. Cheng, L.-Y. Chen, J. Xuan, Z.-J. Feng,
J.-R. Chen, L.-Q. Lu and W.-J. Xiao, Adv. Synth. Catal., 2014,
356, 2787; (f) Z, Gonda, F, Béke, O, Tischler, M, Petró, Z,
Novák and B. L. Tóth, Eur. J. Org. Chem., 2017, 2112.
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Organic & Biomolecular Chemistry
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DOI: 10.1039/C8OB00132D
visible light
R2
R1
R1
R2
Cat. Ru
N
ꢀ
N
H2O
-
N2+BF4
O
R3
+ Ar
O
F
O
room temperature
R3
Ar
28 examples, up to 87% yields
water as the terminal oxidant or oxygen source
highly efficient C-F bond activation
atomic-economy photoredox catalysis
mild reaction conditions and broad substrate scope