Visible Light Photocatalytic Radical Addition/Cyclization Reaction of o-Vinyl-N-Alkoxybenzamides for Synthesis of CF3-Containing Iminoisobenzofurans

Article abstract of DOI:10.1002/adsc.201800037

A visible light-induced photocatalytic radical addition/cyclization reaction of o-vinyl-N-alkoxybenzamides has been accomplished for efficient synthesis of diversely functionalized CF₃-containing iminoisobenzofurans. This external oxidant-free and mild protocol features ready availability of starting materials, excellent regioselectivity, good substrate scope and functional group tolerance. (Figure presented.).

Full text of DOI:10.1002/adsc.201800037

Title: Visible Light Photocatalytic Radical Addition/Cyclization Reaction
of o-Vinyl-N-alkoxyl-benzamides for Synthesis of CF3-Containing
Iminoisobenzofurans
Authors: Zhi-Cheng Liu, Quan-Qing Zhao, Jun Chen, Qiang Tang, Jia-
Rong Chen, and Wen-Jing Xiao
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
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the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800037
Link to VoR: http://dx.doi.org/10.1002/adsc.201800037

10.1002/adsc.201800037
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201 ((will be filled in by the editorial staff))
Visible Light Photocatalytic Radical Addition/Cyclization
Reaction of o-Vinyl-N-alkoxybenzamides for Synthesis of
CF3-Containing Iminoisobenzofurans
Zhi-Cheng Liu,+a Quan-Qing Zhao,+a Jun Chen,a Qian Tang,a Jia-Rong Chen,a,* and
Wen-Jing Xiaoa,b,*
a
CCNU-uOttawa Joint Research Centre, Hubei International Scientific and Technological Cooperation Base of Pesticide
and Green Synthesis, Key Laboratory of Pesticides & Chemical Biology, Ministry of Education, College of Chemistry,
Central China Normal University, 152 Luoyu Road, Wuhan, Hubei 430079, China
E-mail: chenjiarong@mail.ccnu.edu.cn; wxiao@mail.ccnu.edu.cn
b
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, 345 Lingling Road,
Shanghai 200032, China
These authors contributed equally to this work.
+
Received :(( will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
aminotrifluoromethylative cyclization reactions of
N-allylamides and unsaturated hydrazones.[4] These
transformations allowed for convenient synthesis of
various synthetically and biologically important
Abstract: A visible light-induced photocatalytic radical
addition/cyclization reaction of
o-vinyl-N-alkoxybenzamides has been accomplished for
efficient synthesis of diversely functionalized
CF3-containing iminoisobenzofurans. This external
oxidant-free and mild protocol features ready availability of
starting materials, excellent regioselectivity, good substrate
scope and functional group tolerance.
arylated
γ,γ-disubstituted
butyrolactones,
CF3-containg oxazolines and pyrazolines. Despite
these advances, it is still highly desirable to expand
the reaction mode of photoredox-catalyzed radical
addition/cyclization to include olefinic amides, which
would enable assembly of novel structurally diverse
heterocyclic scaffolds because of their inherent two
different nucleophilic sites.
Keywords:
visible
light;
photoredox
catalysis;
iminoisobenzofurans; radical cascade; heterocycles
Visible light-driven photoredox catalyzed radical
addition/cyclization reaction has received a great deal
of attention from the synthetic community during the
past decade,[1] owing to the unique redox properties of
photocatalysts and their prominent performances in
generation of various radicals.[2] Employing this
reaction mode, a wide range of alkenes bearing a
nucleophilic moiety could undergo facile radical
addition and subsequent intramolecular heteroatom
attack to enable construction of diversely
functionalized heterocycles at mild conditions. In
contrast to their traditional thermal variants, these
photo-induced reactions always displayed distinct
chemo- and regioselectivity. In this context, a series
of works from the groups of Glorius, Cho, Akita, Xia,
Xiao, and Chen disclosed that photogenerated aryl
and trifluoromethyl radicals enabled efficient cascade
radical addition/cyclization reactions, providing
access to various structurally diverse oxygen and
nitrogen heterocycles (Scheme 1a).[3,4] Our research
group has also developed a series of visible
light-driven photocatalytic arylation/lactonization of
olefinic carboxylic acids, as well as oxy- and
Scheme 1. Visible light-driven photocatalytic radical
addition/cyclization cascade of alkenes for heterocycle
synthesis.
Isobenzofurans are an important class of
heterocyclic scaffolds that are prevalent in numerous
natural products and biologically active compounds.[5]
1
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10.1002/adsc.201800037
Advanced Synthesis & Catalysis
They can also serve as versatile building blocks and that the observed reactivity was due to the visible
precursors for formation of reactive intermediates. In light-driven photoredox catalyzed activation of
particular, iminoisobenzofurans have recently become substrates. Notably, in the presence of stoichiometric
increasingly
attractive
synthetic
targets.[6] radical scavenger TEMPO, the reaction was
Representative synthetic methods toward these completely suppressed (entry 15); this finding implies
compounds include Pd-catalyzed three-component that the reaction involves a radial process.[15] Notably,
coupling of in situ-formed arynes,[7] Pd-catalyzed Umemoto reagent 2b was also proved to be suitable
cyclization/coupling of o-alkynylbenzamides,[8] Ag- for the reaction, giving 3a in 75% yield (entry 16).
and
Au-catalyzed
cycloisomerization
of
o-alkynylbenzohydroxamic acid derivatives,[9] as well Table 1. Identification of optimal conditions.[a]
as transition metal-free oxyfluorination and
thiocyanooxygenation of olefinic amides.[10] Despite
these advances, to our knowledge, there is no report
on the practical, general, and catalytic synthesis of
fluoroalkylated iminoisobenzofurans.[1,11] On the basis
of our continued interest in the photoredox-catalyzed
synthesis of heterocycles,[4,12] herein, we disclosed a
visible
light-driven
photocatalytic
radical
addition/cyclization of o-vinyl-N-alkoxybenzamides
(Scheme 1b). The reaction provides an external
oxidant-free, and practical approach to diversely
functionalized iminoisobenzofurans.
It has been well documented that the introduction
of fluorinated functional group, such as CF3, into
carbo- and heterocycles can bring about significant
improvement of their lipophilicity, metabolic stability
and bioavailability.[13] Thus, our optimization study
began by investigating the radical addition/cyclization
Yield
[%][b]
Entry Photocatalyst
Base
Solvent
1
Ru(bpy)3(PF6)2 NaHCO3 CH3CN
84
80
54
2
3
fac-Ir(ppy)3
[Ir][c]
NaHCO3 CH3CN
NaHCO3 CH3CN
NaHCO3 CH3CN
cascade
of
the
readily
accessible
o-vinyl-N-methoxyl-benzamide 1a using Umemoto’s
reagent[14] 2a as a CF3 radical precursor (Table 1).[15]
Under our previous conditions that enabled a
successful visible light-induced photocatalytic radical
oxytrifluoromethylation of N-allylamides,[4b] the
model reaction of 1a and 2a proceeded smoothly
through a highly regioselective O-cyclization mode,
giving rise to iminoisobenzofuran 3a in 84% yield
(Table 1, entry 1). The (Z)-configuration of the
product 3a was unambiguously determined based on
its X-ray crystal structure.[16] Notably, we did not
detect any N-cyclization product, isoindolin-1-one 3a’.
Then, we continued to screen other reaction
parameters to improve the yield. A brief evaluation of
several other commonly used photocatalysts showed
that Ru(bpy)3(PF6)2 was still superior over others
(Table 1, entries 1 vs 2-4). In accordance with our
previous studies,[4] the addition of base is also critical
to the reaction. An array of weak bases, such as
Na2CO3, K2CO3 and K2HPO4 all proved to be
compatible with the reaction with the exception of
strong base NaOH (entries 5-8); and NaHCO3 still
proved to be the best one of choice. With the
combination of photocatalyst Ru(bpy)3(PF6)2 and base
NaHCO3, we further examined a series of solvents
including CHCl3, MeOH and toluene; however,
significant decrease of yield was observed in these
cases (Table 1, entries 9-11). Without base, the
reaction also proceeded well, but resulting in a slight
decrease of yield (entry 12). As expected, control
experiments that were carried out in the absence of
photocatalyst or visible light irradiation led to none of
the desired product (entries 13 and 14), suggesting
4
MesAcrClO4
54
5
6
7
8
Ru(bpy)3(PF6)2 Na2CO3
Ru(bpy)3(PF6)2 NaOH
Ru(bpy)3(PF6)2 K2CO3
Ru(bpy)3(PF6)2 K2HPO4
Ru(bpy)3(PF6)2 NaHCO3 CHCl3
Ru(bpy)3(PF6)2 NaHCO3 MeOH
Ru(bpy)3(PF6)2 NaHCO3 toluene
CH3CN
CH3CN
CH3CN
CH3CN
81
trace
59
75
48
22
17
81
trace
trace
trace
75
9
10
11
12[d]
13[e]
14[f]
15[g]
16[h]
Ru(bpy)3(PF6)2
-
-
CH3CN
NaHCO3 CH3CN
Ru(bpy)3(PF6)2 NaHCO3 CH3CN
Ru(bpy)3(PF6)2 NaHCO3
Ru(bpy)3(PF6)2 NaHCO3
CH3CN
CH3CN
[a]
Reaction conditions: 1a (0.1 mmol), 2a (0.11
mmol), photocatalyst (2 mol%), base (2.0 equiv.,
0.2 mmol) in solvent (2.0 mL) under irradiation
of 3 W blue LEDs at room temperature under Ar
atmosphere. N.R. = no reaction.
Isolated yield after flash chromatography.
[Ir] = Ir[dF(CF)3(ppy)]2(dtbbpy)PF6
Without NaHCO3.
Without photocatalyst.
Without visible light irradiation.
With addition of TEMPO (2.0 equiv).
Using 2b as a CF3 radical precursor.
[b]
[c]
[d]
[e]
[f]
.
[g]
[h]
With the optimal conditions established (Table 1,
entry 1), we proceeded to investigate the substrate
generality and limitation of our reaction by reacting a
representative set of o-vinyl-N-alkoxybenzamides 1
with 2a on a 0.3 mmol scale. As highlighted in Table
2, the protocol demonstrated broad substrate scope
2
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10.1002/adsc.201800037
Advanced Synthesis & Catalysis
and good functional group tolerance. For instance, as
Finally, structural variation of the amide moiety
for the R1 group in the vinyl moiety, in addition to 1a, was also simply studied. Replacing the methyl ether
a range of substrates 1b-1e bearing a weak with a isopropyl (1m) or benzyl (1n) group was well
t
electron-donating (e.g., Me, Bu) or withdrawing tolerated, as the desired products 3m and 3n were
substituents (e.g., Cl) at the para- or meta-position of obtained in 72% and 68% yields, respectively. It
the aryl group worked well to give the corresponding should be noted that sunlight could also be used as the
products 3b-3e in 44-72% yield. The steric hindrance light source instead of blue LEDs; efficient synthesis
of the phenyl ring has no influence on the reaction of 3a could be achieved with 76% yield after 2 h of
either; the reaction of multi-substituted substrate 1f total sunlight exposure.[15]
proceeded smoothly to furnish a 71% yield of 3f.
Notably, the reaction was also tolerant of other
Moreover, substrates 1g-1i with R1 group as linear or simple alkyl and aryl groups on the benzamide
cyclic aliphatic functional group proved to be suitable nitrogen atom. For instance, both of the normal
for the reaction, affording 3g-3i in good yields amides 1o (N-Bn) and 1p (N-Ph) reacted well with
(61-82%). As shown in the cases of substrates 1j-1l, Umemoto reagent 2b under the standard conditions,
the presence of a methyl or fluoro group in the giving the corresponding products 3o and 3p in 78%
para-position or a chloro group in the meta-position and 79% yields, respectively [Eq. 1]. Moreover, the
with respect to the vinyl substituent did not affect the reaction of substrate 1q bearing an internal alkene
reaction either, and the expected iminoisobenzofurans moiety also proceeded smoothly to give the desired
3j-3l were isolated in 64-68% yields.
product 3q with moderate diastereoselectivity; and the
major isomer could be obtained in 56% yield by
column chromatographic purification [Eq. 2].
Table 2. Scope of the o-Vinyl-N-alkoxybenzamides.[a,b]
To further demonstrate the potential of our
protocol, we examined a series of other commercially
available radical precursors (Scheme 2). For example,
2-bromo-2,2-difluoroacetate ester 2b could serve as a
competitive fluoroalkyl source to participate in the
desired reaction efficiently, when using fac-Ir(ppy)3
as the photocatalyst and Na2HPO4 as the base,
furnishing the difluoroalkylation product 4 in 83%
yield (Scheme 2a). Moreover, the reaction with other
haloalkane 2c and bromochloroform 2d also reacted
well with 1a to give products 5 and 6 in 72% and
73% yields, respectively (Scheme 2b and 2c).
[a]
Reaction conditions:
1 (0.3 mmol), 2a (0.33
mmol), Ru(bpy)3(PF6)2 (2 mol%), NaHCO3 (2.0
equiv., 0.6 mmol) in CH3CN (6.0 mL) under
irradiation of
3
W
blue LEDs at room
temperature under Ar atmosphere.
Isolated yield after flash chromatography.
Under sunlight irradiation.
[b]
[c]
3
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10.1002/adsc.201800037
Advanced Synthesis & Catalysis
Scheme 2. Examination of other radical precursors.
by 1a was observed at 350 nm in CH3CN in the
presence of or without base NaHCO3. In sharp
contrast, in the case of 2a, significant decrease of
luminescence emission intensity was oberved with or
without NaHCO3. These findings suggested that the
reaction might proceed through a visible light-driven
oxidative quenching cycle.
With the ester group as a synthetically versatile
handle, the product 4 could be readily transformed to
alcohol 7 in 81% yield upon reduction with NaBH4
(Scheme 3a). Interestingly, treatment of 4 with base
resulted in
a
facile hydrolysis/intramolecular
On the basis of these mechanistic studies and
cyclization cascade, leading to formation of
fluorinated γ-lactone 8 in 95% yield (Scheme 3b).
Such a scaffold might be useful in the pharmaceutical
related literature,[11] we proposed
a
plausible
redox-neutral process for the current photocatalytic
radical addition/cyclization reaction (Scheme 4). First,
a single electron transfer from the excited state
photocatalyst *[Ru(bpy)3]2+, formed reversibly upon
visible light irradiation, to the Umemoto reagent 2a
occurs to generate the highly reactive CF3 radical and
the oxdized form of the photocatalyst [Ru(bpy)3]3+.
Next, the CF3 radical undergoes a radical addition
across the vinyl moiety to furnish benzyl radical
intermediate A, which can be further converted to
relatively stable benzylic cation B by the oxidizing
state [Ru(bpy)3]3+, closing the photocatalytic cycle
(path a). Finally, nucleophilic O-cyclization results in
formation of the product 3a. Another pathway
involving transformation of A to B by radical chain
propagation can not be ruled out at the current stage,
but should not be the predomidant way, as continuous
irradiation is critical to the reaction. Moreover, we
determined the quantum yield of the reaction of 1a to
be 1.32, which suggests that the photocatalytic cycle
should be the predominant pathway.[17]
and
agrochemical
fields.[18]
Surprisingly,
Pd/C-catalyzed hydrogenation of product 3o in HOAc
gave rise to trifluoromethylated benzamide 9 in 75%
yield, which should be formed via an acid-promoted
ring-opening/hydrogenation sequence (Scheme 3c).[15]
Scheme 3. Derivatization of products 4 and 3o.
Scheme 4. Proposed catalytic cycle.
In conclusion, we have developed a room
temperature visible light-induced photocatalytic
Figure 1. Fluorescence quenching of Ru(bpy)3(PF6)2.
radical
addition/cyclization
reaction
of
o-vinyl-N-alkoxybenzamides, providing efficient
access to diversely substituted iminoisobenzofurans.
This external oxidant-free and mild protocol features
ready availability of starting materials, excellent
regioselectivity, good substrate scope and functional
group tolerance. Further studies to modulate the
regioselectivity are currently underway in our group.
In principle, both o-vinyl-N-methoxyl-benzamide
1a and 2a are capable of quenching the excited state
photocatalyst *[Ru(bpy)3]2+ via
a
reductive
quenching[12] or oxidative quenching process,[1c,11]
respectively. To assist the understanding of the
mechanism, we conducted a series of luminescence
quenching experiments with 1a and 2a (Figure 1).[15]
No direct quenching of the excited state *[Ru(bpy)3]2+
4
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10.1002/adsc.201800037
Advanced Synthesis & Catalysis
Experimental Section
Chen, Q. Wei, Q.-Q. Zhao, L.-Q. Lu, W.-J. Xiao,
Chem. Commun. 2015, 51, 3537-3540; c) Q. Wei,
J.-R. Chen, X.-Q. Hu, X.-C. Yang, B. Lu, W.-J.
Xiao, Org. Lett. 2015, 17, 4464-4467.
General Procedure for the Radical Addition/Cyclization
Reaction of o-Vinyl-N-alkoxybenzamides: To a 10 mL
Schlenk tube equipped with a magnetic stirring bar was
charged with 1a (0.30 mmol, 76 mg), 2a (0.33 mmol, 113
mg) and NaHCO3 (0.60 mmol, 51 mg) in 6 mL of CH3CN.
The mixture was degassed using the “freeze–pump–thaw”
technique (3 times) and then irradiated with 3 W blue
LEDs at room temperature under Ar. Upon completion of
the reaction as monitored by TLC, the crude reaction
mixture was concentrated and directly subjected to column
chromatography, and eluted with petroleum ether and ethyl
[5] a) J. K. Harper, A. M. Arif, E. J. Ford, G. A. Strobel,
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Acknowledgements
We are grateful to the NNSFC (Nos. 21622201, 21472058,
21472057, and 21772053), the Distinguished Youth Foundation
of Hubei Province (No. 2016CFA050), the Science and
Technology Department of Hubei Province (2017AHB047), and
CCNU (Nos. CCNU17TS0011 and CCNU16JCZX02) for support
of this research. The Program of Introducing Talents of
Discipline to Universities of China (111 Program, B17019) is
also appreciated.
Noyori,
Organometallics 2014, 33, 3500-3507.
[8] a) J.-J. Wang, Z.-Y. Yan, C.-M. Tan, X. Wang, F. Li,
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10.1002/adsc.201800037
Advanced Synthesis & Catalysis
COMMUNICATION
Visible Light Photocatalytic Radical
Addition/Cyclization Reaction of
o-Vinyl-N-alkoxybenzamides for Synthesis of
CF3-Containing Iminoisobenzofurans
Adv. Synth. Catal. Year, Volume, Page – Page
Zhi-Cheng Liu,+a Quan-Qing Zhao,+a Jun Chen,a
Qian Tang,a Jia-Rong Chen,a,* and Wen-Jing
Xiaoa,b*
7
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