Visible-light-induced intramolecular sp3 C-H oxidation of 2-alkyl-substituted benzamides for the synthesis of functionalized iminoisobenzofurans

Article abstract of DOI:10.1039/c9cc07791j

We have developed a protocol for the synthesis of functionalized iminoisobenzofurans by means of visible-light-induced intramolecular cyclization reactions of 2-alkyl-substituted benzamides. This one step-economical protocol, which involves intramolecular sp³ C-O bond formation, features mild reaction conditions, exclusive chemoselectivity, and high yields.

Full text of DOI:10.1039/c9cc07791j

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DOI: 10.1039/C9CC07791J
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3
Visible-Light-Induced Intramolecular sp C−H Oxidation of 2-Alkyl-
Substituted Benzamides for Synthesis of Functionalized
Iminoisobenzofurans
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a,b
Lingang Wu , Yanan Hao , Yuxiu Liu and Qingmin Wang *
DOI: 10.1039/x0xx00000x
We have developed a protocol for the synthesis of functionalized Scheme 1. Syntheses of isoindolinones and iminoisobenzofurans by
iminoisobenzofurans by means of visible-light-induced intramolecular selective aminative or oxidative cyclization of 2-
intramolecular cyclization reactions of 2-alkyl-substituted alkyl-substituted benzamides.
benzamides. This step-economical protocol, which involves
intramolecular sp C-O bond formation, features mild reaction
previous works: oxidative N-cyclization of 2-alkylbenzamides
3
a) Kondo et al., Miura et al., Kumar et al.
conditions, exclusive chemoselectivity, and high yields.
O
O
Ar
[Cu] or I2/[O]
N
H
Ar
Ar
Ar
N
Ar
Isobenzofurans are prevalent in biologically active compounds and
[
1]
natural products. In addition, these heterocycles can serve as
multipurpose synthetic building blocks and as precursors of reactive
intermediates. Iminoisobenzofurans are particularly important
members of this class of compounds, and the development of
methods for their synthesis has attracted considerable research
b) Shi et al.
ArI(cat)
O
O
[
O]
OMe
N
H
N
OMe
Ar
ArI(III)
[
2]
c) Laha et al., Xiao et al.
attention. Iminoisobenzofurans are generally synthesized by Pd-
[
3]
O
O
catalyzed coupling of 2-alkylbenzamides or arynes formed in
photocatalyst
visible light
OMe
N
[
4]
Ar
H
R
Ar
N OMe
situ,
by Ag- and Au-catalyzed cycloisomerization of 2-
1
R1
R2
[
5]
R2
This work: oxidative O-cyclization of 2-alkylbenzamides
alkylbenzohydroxamic acid derivatives, and by cyclization of
olefinic amides.
[
6]
R1
N
O
Me
OH
OH
O
R1
photocatalyst
visible light
N
H
R
O
HO
HO
Ar
Ar
O
NC
3
OH
R
R3
R2
O
O
R2
OH
OH
O
chemoselective sp3 C-H oxidation
high yield
mild conditions
OH
N
F
HBr
HO
R = Me or CH2OMe
Flavimycins
Phenolphthalein
Citalopram hydrobromide
The use of selective intramolecular oxidative amination reactions
of the sp C-H bonds of 2-alkyl-substituted benzamides for the
MeO
MeO
3
O
O
OH
Me
O
synthesis of isoindolinones has been thoroughly studied. The
research groups of Kondo and Miura separately reported the
HO
Elmonin
OMe
Me
Elmenol
3
development of copper-catalyzed sp C-H aminative cyclization
reactions of 2-alkyl-N-arylbenzamides for the synthesis of N-aryl-
isoindolinones. In addition, Kumar’s group successfully utilized 2-
Figure 1. Examples of medicinally active compounds and natural
products containing isobenzofurans.
[
7]
alkyl-N-arylbenzamides
in
transition-metal-free
selective
3
intramolecular oxidative sp C-N coupling reactions to generate N-
[
8]
aryl-isoindolinones (Scheme 1a). Furthermore, in an elegant
study, Shi and co-workers efficiently constructed C–N bonds via
3
iodoarene-catalyzed stereospecific intramolecular sp
amination reactions of
(
C-H
2-cyclohexyl-N-methoxybenzamides
Scheme 1b). In recent years, visible light photocatalysis has
a. State Key Laboratory of Elemento-Organic Chemistry, Research Institute of
Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin
[
9]
3
00071, People’s Republic of China. E-mail: wangqm@nankai.edu.cn
emerged as powerful technique to facilitate activation of organic
molecules and engineer new chemical processes since the seminal
studies from the groups of MacMillan, Yoon, and Stephenson.
b.
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin 300071, People’s Republic of China
[10]
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Such a catalytic strategy has also proven to be efficient accesses to
[
11]
N-radicals and the application of visible light for photocatalytic
activation of amides represents a valuable approach to facilitate the
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Journal Name
formation of amidyl radicals.[12] The groups of Laha, Xiao, and Zeng are given. [c] Reaction time is 36 h and 60 h, resVpieewcAtritvicelelyO.nl[idne]
3
reported visible-light-catalyzed or electrochemical benzylic sp C-H Reaction performed in the absence of light. DOI: 10.1039/C9CC07791J
amination reactions of 2-alkyl-N-methoxybenzamides (Scheme
[
13]
1
c).
The C–H amidation through an amidyl radical has been
We began our study by evaluating the oxidative intramolecular
studied well, however, to our knowledge, chemoselective cyclization of 2-isopropyl-N-phenylbenzamide (1a) in several
3
intramolecular sp C-H O-cyclization, as opposed to N-cyclization, solvents with various oxidants and photocatalysts (Table 1). We
under mild conditions with high yields has not been reported. were delighted to find that in the presence of 1.5 mol% of
Herein, we disclose
intramolecular sp C-H oxidation of 2-alkyl-substituted benzamides.
a
3 2 2
protocol for visible-light-induced Ru(bpy) Cl ·6H O as a photocatalyst and acetoxybenziodoxole (BI-
3
OAc) as an oxidant, reaction of 1a in dichloroethane under
irradiation with a 5 W blue LED proceeded smoothly and
chemoselectively via an O-cyclization pathway to give
iminoisobenzofuran 2a in 51% yield (entry 1). None of the N-
cyclization product, 3,3-dimethyl-2-phenylisoindolin-1-one (2a’),
was detected. Screening of various solvents (MeOH, DMSO, and
MeCN) revealed that MeCN was the most effective (entries 1–4).
None of the other oxidants we tested afforded the desired product
in good yield (entries 5–7). Evaluation of two other photocatalysts
Table 1. Optimization of reaction conditions for O-cyclization of 2-
[
a]
isopropyl-N-phenylbenzamide.
O
O
N
Ph
photocatalyst (1.5 mol %)
oxidant (1.5 equiv)
N Ph
Ph
N
H
O
solvent (2.5 mL), 48 h
blue LED
2a'
1a
2a
O-cyclization
N-cyclization
not detected
3 2 2
showed that Ru(bpy) Cl ·6H O was superior (entries 8 and 9). The
changes of amount of solvent or oxidant could not give a better
entry
photocatalyst
oxidant
solvent
yield (%)[b] result (entries 10–13). Similarly, prolonging or shortening the
reaction time did not lead to a better yield (entry 14). Control
experiments demonstrated that the reaction failed to proceed in
the absence of oxidant, photocatalyst, or light (entries 15–17).
1
2
3
4
5
6
7
8
9
Ru(bpy)
Ru(bpy)
Ru(bpy)
3
3
3
Cl
Cl
Cl
2
2
2
·6H
·6H
·6H
2
2
2
O
O
O
BI-OAc
BI-OAc
BI-OAc
BI-OAc
BI-OH
BIO
DCE
51
13
24
81
10
0
MeOH
DMSO
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
Table 2. Substrate scope with respect to the 2-alkylbenzamide.[a]
AcO
O
R1
N
I
O
O
Ru(bpy)3Cl2
6H2O (1.5 mol %)
R1
Ru(bpy)
3
Cl
2
·6H
2
O
O
O
O
N
BI-OAc (1.5 equiv)
R4
R4
O
H
2
R
MeCN (2.5 mL), 48 h
blue LED
R2
R3
BI-OAc
R3
Ru(bpy)
Ru(bpy)
Ru(bpy)
3
3
3
Cl
Cl
Cl
2
·6H
2
·6H
2
·6H
2
2
1
2
2
Cl
O
2
0
Br
N
O
N
Ph
N
O
O
[Ir(dtbbpy)(ppy)
2
][PF
6
]
BI-OAc
BI-OAc
47
20
2c, 70%[b]
2
a, 81%
2b, 82%
CCDC 1945647
Me
+
-
tBu
Ph
Mes-Acr-Me ClO
4
N
N
O
N
N
Me
O
O
O
Ru(bpy)
3
Cl
2
·6H
·6H
2
O
O
BI-OAc
1.25 equiv)
2g, 80%
1
1
0
1
74
80
2e, 78%
2f, 64%
2d, 83%
(
Me
N
O
OMe
Cl
Cl
N
O
N
O
N
O
Ru(bpy)
3
Cl
2
2
BI-OAc
1.75 equiv)
MeCN
(
i, 74%[b]
2j, 55%
2k, 84%
2
2
h, 69%
N
Ph
N
Ph
N
Ph
F
N
Ph
1
1
2
3
Ru(bpy)
Ru(bpy)
Ru(bpy)
Ru(bpy)
3
3
3
3
Cl
Cl
Cl
Cl
2
·6H
2
·6H
2
·6H
2
·6H
2
O
2
O
2
O
2
O
BI-OAc
BI-OAc
BI-OAc
-
MeCN
2.0 mL)
MeCN
3.0 mL)
MeCN
63
N
Ph
O
Br
O
O
(
(
O
O
N
Boc
78
2m, 75%
2n, 82%
2o, 77%
2p,85%
2l, 86%
1
4[c]
65, 81
N
Ph
N
Ph
Ph
N
O
Ph
N
Ph
N
O
Cl
Br
Me
O
O
O
Cl
1
5
6
MeCN
MeCN
MeCN
0
0
0
2q, 73%
2r, 83%
2s, 80%
2t, 70%
2u, 40%
1
—
BI-OAc
BI-OAc
N
Ph
O
1
7[d]
Ru(bpy)
3
Cl
2
·6H
2
O
2v, 0%
[
a] General conditions, unless otherwise noted: 1a (0.25 mmol),
photocatalyst (0.00375 mmol), oxidant (0.375 mmol), and solvent
2.5 mL) at room temperature under Ar atmosphere. DCE,
dichloroethane; BI-OAC, acetoxybenziodoxole. [b] Isolated yields
[
a] Reactions were performed on a 0.25 mmol scale, and isolated
(
yields are given. [b] Reaction time is 60 h.
2
| J. Name., 2012, 00, 1-3
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COMMUNICATION
With the optimal conditions established (Table 1, entry 4), we
investigated the substrate generality and the limitations of the
reaction (Table 2). First, we examined the effect of substituents on
the phenyl group attached to the N atom of o-alkylbenzamides 1.
Substrates bearing para substituents with various electronic
properties were well tolerated, affording (Z)-3,3-dimethyl-N-
phenylisobenzofuran-1(3H)-imines 2b–2f in good to excellent
yields. For 2c, notably, a good yield could be obtained with an
extended reaction time. The configuration of product 2b was
View Article Online
DOI: 10.1039/C9CC07791J
Scheme 3. Proposed mechanism.
O
N
Ph
Ph
N
H
O
1a
-OAc
hv
2a
RuII*
BI.
RuII
[
14]
- H+
O-cyclization
unambiguously determined by means of X-ray crystallography.
BIOAc
RuIII
o-IC6H4CO2H
O
Substrates with meta- and ortho-substituents, afforded oxidative
products 2g and 2h, respectively, in good yields. Benzamides with a
polysubstituted phenyl group (1i), a naphthyl group (1j), and a
methoxy group (1k) on the N atom were also compatible with this
protocol. In addition, oxidation reactions of substrates with cyclic
tertiary C–H bonds (1l and 1m) worked well. Notably, a Boc-
protected piperidyl substrate was acceptable, affording spiro
product 2n in 82% yield. We next evaluated the influence of
substituents on the phenyl ring of the benzamide moiety.
Substrates bearing a methyl group or a halogen atom produced
iminoisobenzofurans 2o–2t in good yields. A substrate with a
secondary C–H bond, that is, 2-ethyl-substituted compound 1u,
underwent oxidation to afford 2u, but the yield was relatively low
O
O
Ph
N
Ph
Ph
H
N
1,5-HAT
N
H
C
A
B
On the basis of these observations and literature reports, we
propose that the reaction was initiated by single-electron transfer
from photoexcited Ru(bpy)
2+
3+
3
* to BI-OAc to generate Ru(bpy)
3
and
.
[15]
BI (Scheme 3). Amidyl radical A were generated when amide 1a
. [12a]
was oxidized by BI .
Subsequently, a 1,5-hydrogen atom transfer
[16]
(
HAT) reaction provided C-centered radical B.
oxidized by Ru(bpy)
then nucleophilic O-cyclization delivered target product 2a.
Whereas the exact mechanism of chemoselectivity O-cyclization is
unclear at the moment, mild condition and longer reaction time
may indicate that O-cyclization is advantageous process
thermodynamically compared with N-cyclization.
Radical B was
3+
3
to afford carbocation intermediate C, and
[6c],[6d]
(
40%). However, a substrate containing a primary C–H bond (1v)
was incompatible with this protocol; most of the starting material
was recovered unchanged. To explore the synthetic utility of the
protocol, we transformed imino-group-containing product 2a to
3
,3-dimethylisobenzofuran-1(3H)-one
by
means
of
acid
[
6b]
hydrolysis.
In conclusion, we have developed a protocol for visible-light-
induced intramolecular sp C–H oxidation reactions of a variety
3
Scheme 2. Mechanistic experiments.
of 2-alkyl-substituted benzamides to afford functionalized
iminoisobenzofurans. This mild reaction exhibited exclusive
chemoselectivity and high yields, indicating that it has great
potential utility for the preparation of a wide variety of
iminoisobenzofurans. Further studies aimed at evaluating the
bioactivity of the products and broadening the synthetic
applications are in progress in our laboratory.
O
Ph
N
H
O
N
Ph
[M + H]+
Ru(bpy)3Cl2
BI-OAc (1.5 equiv)
6H2O (1.5 mol %)
Ph
Calc. 395.2693
Found: 395.2689
N
O
N
H
O
MeCN (2.5 mL), TEMPO (2.0 equiv)
blue LED, 48 h
1a
2a, trace
4, detected by HRMS
O
N
Ph
Ru(bpy)3Cl26H2O (1.5 mol %)
BI-OAc (1.5 equiv)
Ph
N
H
O
MeCN (2.5 mL), BHT (2.0 equiv)
blue LED, 48 h
1a
2a, trace
Conflicts of interest
There are no conflicts to declare.
O
[M + Na]+
Calc. 508.0380
Ph
N
H
O
N
Ph
Found: 508.0375
Ru(bpy)3Cl26H2O (1.5 mol %)
BI-OAc (1.5 equiv)
Ph
O
I
N
H
O
O
DCE (2.5 mL)
blue LED, 48 h
1a
2a, 51%
6, detected by HRMS
Acknowledgements
Having explored the substrate scope, we turned our attention to We are grateful to the National Key Research and Development
the mechanism (Scheme 2). Control experiments showed that the Program of China (2018YFD0200100) and the National Natural
oxidative cyclization of amide 1a was impeded by the presence of a Science Foundation of China (21732002, 21672117, 21772102)
radical inhibitor (2,2,6,6-tetramethyl-1-piperidinyloxy [TEMPO] or for generous financial support for our programs.
butylated hydroxytoluene [BHT]); most of the starting material was
recovered unchanged. Byproducts 4 and 5 were observed by mass
spectrometry (see the Supporting Information for details), Notes and references
suggesting that an alkyl radical had been generated. In addition,
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4 CCDC 1945647 (2b) contains supplementary crystallographic
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R1 DOI: 10.1039/C9CC07791J
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sp C-H amination
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