Sunlight-promoted cyclization: Versus decarboxylation in the reaction of alkynoates with N -iodosuccinimide: Easy access to 3-iodocoumarins

Article abstract of DOI:10.1039/c6gc01027j

A sunlight-initiated radical conversion of aryl alkynoates to 3-iodocoumarins has been achieved using N-iodosuccinimide as the iodine source without the use of a catalyst or additive. Experimental, X-ray analysis, and computational studies indicate that the reaction under sunlight proceeds through iodination, spirocyclization and ring expansion to form the kinetic product. The sunlight-driven reaction provides a green pathway to especially valuable 3-halocoumarins derivatives.

Full text of DOI:10.1039/c6gc01027j

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DOI: 10.1039/C6GC01027J
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Sunlight‐promoted cyclization versus decarboxylation in reaction
of alkynoates with N‐iodosuccinimide: an easy access to 3‐
iodocoumarins
Received 00th January 20xx,
Accepted 00th January 20xx
Shengyang Ni,a Jia Cao,b,c Haibo Mei,a Jianlin Han,*a Shuhua Li*b and Yi Pan*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
A sunlight‐initiated radical process of aryl alkynoates has been great interests.
achieved by using N‐iodosuccinimide as iodine source without use
Aryl alkynoate has become an attractive and powerful
of any catalyst or additive. Experimental, x‐ray analysis, and organic intermediate in organic synthesis. It is mainly due to
computational studies indicate that the reaction under sunlight the easy introduction of several functional groups into
tends to proceed through iodination, spirocyclization and ring acetylene bond motif in aryl alkynoates.9 Furthermore,
expansion to form the kinetically controlled product. The sunlight‐ cascade intramolecular cyclization of aryl alkynoates renders
driven reaction provides a green pathway to especially valuable 3‐ this intermediate more attractive because of the application in
halocoumarins derivatives.
preparation of substituted heterocycles.10‐12 Recently, we
developed an iodination triggered cascade radical
decarboxylative migration reaction of alkynoates giving 1,1‐
Photoredox catalysis has been demonstrated to be powerful
and straightforward tools for carbon‐carbon and carbon‐
heteroatom bonds formation to assemble molecules with wide
range of structural varieties.1 The last decade has witnessed
significant achievements in the area of photocatalysis used in
synthesis of natural products and enantioselective catalysis.2
As an abundant, renewable and simple accessible light source,
sunlight can be applied in light activated reactions, therefore
bodes well for the development of green synthetic methods.3,4
In 2008, Yoon group developed an efficient Ru‐catalyzed [2+2]
enone cycloaddition reaction using the ambient sunlight.5
Recently, elegant works by Wang6 and Hashmi7 have shown
the possibility of radical alkynylation coupling reaction driven
by sunlight in the presence of hypervalent iodine reagent or
gold catalyst. Additionally, Jiao have demonstrated the
sunlight‐promoted aerobic oxidation of benzyl halides in the
presence of Ru(bpy)3Cl2.8 In the recent years, the demand in
the development of green organic methodologies is at an all‐
time high. In this regard, the sunlight‐driven organic reactions
without use of any catalyst or additive represent a topic of
o
diiodoalkenes at 160 C (Scheme 1a).13a In a continuation of
our work on cascade reaction of alkynoates and inspired by
these elegant sunlight irradiation works,5‐8 we envisioned that
the decarboxylative migration of alkynoates could proceed
under sunlight condition instead of high temperature (Scheme
1b). However, the product 1,1‐diiodoalkenes under sunlight
were not observed. Considering that sunlight‐irradiation at
room temperature is mild condition, the process might
proceed through ring expansion to form 3‐halocoumarins
product14 (Scheme 1b). Herein, we report the first example of
sunlight‐driven catalyst‐free cascade radical iodination and
cyclization of alkynoates with N‐iodosuccinimide (NIS) at room
a. School of Chemistry and Chemical Engineering, State Key laboratory of
Coordination Chemistry, Nanjing University, Nanjing, 210093, China. Email:
hanjl@nju.edu.cn, yipan@nju.edu.cn
b. Institute of Theoretical and Computational Chemistry, Nanjing University,
Nanjing, 210093, China. Email: shuhua@nju.edu.cn.
c. Shaanxi Key Laboratory of Chemical Reaction Engineering College of Chemistry &
Chemical engineering, Yan’an University, Yan’an, 716000, China.
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [Experimental procedures,
full spectroscopic data for compounds 3 and 4a, x‐ray analysis of 3j, computational
Scheme 1 Reaction of aryl alkynoates.
details, copies of 1H NMR and 13C NMR spectra.]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1‐3 | 1
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The reaction vial was packed with aluminium foil in the dark. d TBAI was used. e
temperature (Scheme 1b). This reaction could be carried out
without use of any oxidant or photocatalyst under simple
conditions by using NIS as iodine precursor,15 affording 3‐
iodocoumarins as products.
To probe the feasibility of this hypothesis, we initiated our
studies with phenyl 3‐phenylpropiolate 1a as a model coupling
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TBSI was used. f I2 was used.
DOI: 10.1039/C6GC01027J
the attempts to improve the efficiency of this reaction were
done through the variation of NIS loading amount, reaction
concentration and time. These results clearly showed that
using of 1.5 equiv of NIS, 2.5 mL of acetonitrile for 6 h was the
best choice (up to 98% yield, entries 12‐16). Finally, we would
like to mention here the attempts to use other iodine sources,
including TBAI, TBSI and I2, however, all of these iodine sources
did not lead to improved chemical yields (entries 17‐19).
partner and NIS
2 as iodine source for the cyclization reaction
at room temperature. We were gratified to find that when a
solution of 1a and NIS (2.0 equiv) in CH2Cl2 was exposed to
blue LEDs, a cyclization reaction occurred smoothly without
use of any photocatalyst and the expected adduct was isolated
in 80% yield (entry 1, Table 1). The scan of solvent found that
no improvements were obtained when the reaction was
conducted in acetone, THF, DMF and methanol (entries 2‐4, 6).
The result presented in entry 5 clearly suggested that
acetonitrile was the solvent of best choice, giving the desired
product 3a in nearly quantitative yield (96%). Subsequently,
the light sources, such white LED and fluorescent lamp, were
scanned, which gave no better results (entries 7 and 8). From
an environmental and practical point of view, we were
intrigued to use abundant sunlight for activating this chemical
transformation. Pleasingly, the reaction under sunlight could
complete within 6 h with excellent chemical yield (90%, entry
9). In sharp contrast, blue LEDs irradiated reaction resulted in
dramatically lower yield when the reaction was stopped at 6 h
(52%, entry 10), and no reaction happened when the reaction
was performed in the dark (entry 11). Inspired by this result,
With the optimized reaction conditions in hand our next
goal was the evaluation of potential generality of aryl
alkynoates
1 in this sunlight‐driven cyclization reaction. As
illustrated in Table 2, a variety of alkynoates that bear para‐
substitution on the aromatic ring of ester moiety can be
applied to give the 3‐iodocoumarin with good to excellent
yields
(3b‐3j, 66‐95% yield). The electronegativity of
substituted groups showed some effect on the efficiency of
the reaction. Generally, the substrates with electron‐donating
groups on ester moiety (3b‐3c) could work well to afford the
desired product within 6 h. The reactions needed longer time
to consume the starting material containing electron‐
withdrawing groups (fluoro 1d, chloro 1e, phenyl 1h and acetyl
1i), to give lower chemical yields. Similar trends were observed
in the reactions of substrates bearing substituted
phenylpropiolates (1k‐1n), giving rise to coumarin derivatives
3k‐3n) in 70‐91% yields. In order to investigate the
regioselectivity of the cyclization, we turned our attention to
(
Table 1 Optimization of the reaction conditions.a
R2
R2
O
I
H
O
sunlight
rt, 6 h
N
I
R1
+
R1
O
O
O
O
3
1
2
I
I
I
I
I
F
O
O
Cl
O
O
O
O
Entry
NIS
(equiv)
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.5
2.5
1.5
1.5
1.5
1.5
1.5
1.5
Light source
Time
(h)
24
24
24
24
24
24
24
24
6
6
6
6
6
6
6
4
6
Solvent (mL)
Yield
(%)b
80
83
21
63
96
54
90
H3C
O
O
O
O
3e, 75% yielda
3d, 81% yielda
3b, 95% yield
3a, 98% yield
3c, 85% yield
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Blue LED
Blue LED
Blue LED
Blue LED
Blue LED
Blue LED
White LED
Fluorescent lamp
Sunlight
Blue LED
‐‐
DCM (2.0)
Acetone (2.0)
DMF (2.0)
I
I
I
I
I
O
O
O
F3C
O
O
Br
O
O
MeO
O
O
Ph
O
O
THF (2.0)
3i, 73% yieldb
3f, 72% yield
3g, 66% yield
3h, 73% yielda
3j, 80% yield
MeCN (2.0)
MeOH (2.0)
MeCN (2.0)
MeCN (2.0)
MeCN (2.0)
MeCN (2.0)
MeCN (2.0)
MeCN (2.0)
MeCN (2.0)
MeCN (1.5)
MeCN (2.5)
MeCN (2.5)
MeCN (2.5)
MeCN (2.5)
MeCN (2.5)
CH3
Br
Cl
F
I
O
O
78
90
I
I
I
I
O
O
H3C
O
O
O
Br
O
Cl
O
O
F
O
O
52
3o, 86% yieldc
3k, 74% yielda
NRc
97
3n, 91% yield
3l, 77% yielda
3m, 70% yield
Sunlight
Sunlight
Sunlight
Sunlight
Sunlight
Sunlight
Sunlight
Sunlight
H
I
83
95
98
84
I
I
I
MeO
I
O
O
O
O
O
O
O
O
O
O
3r, 90% yieldc
3s, 0% yield
3p, 82% yieldc
3q, 91% yield, 3q1:3q2 = 2:1
NRd
Tracee
25f
CH3
I
I
I
I
6
6
O
O
O
O
O
O
O
O
3v, <10% yieldd
3u, 89% yield
3w, 10% yieldd
Cl
Me
3t, 83% yield
a Reaction conditions: 1a (0.2 mmol), NIS, solvent, under air. b Isolated yield.
c
2 | J. Name., 2012, 00, 1‐3
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Scheme 2 Substrate scope of the sunlight‐driven reaction. (Reaction conditions: 1 (0.2
mmol), NIS 2 (0.3 mmol), acetonitrile (2.5 mL), at room temperature, under air for 6 h.
result clearly discloses that the migration of the VeieswteArrticmleoOinelitnye
happens in this process.
DOI: 10.1039/C6GC01027J
a
Isolated yield based on 1. 12 h. b 18 h. c Only one isomer was found which was
determined by 1H NMR. d 24 h)
Based on our experimental observation and previous
reports,10‐14 we then investigated the possible reaction
mechanism for this reaction (Scheme 5). Initially, the iodine
radical is generated from NIS under the sunlight irradiation.
Then the addition of the iodine radical to the C–C triple bond
meta‐substituted aryl phenylpropiolates. For substrates 1q‐1r
with meta‐substituents, the reactions proceeded very well
with excellent yields as well as obvious regioselectivity.
Especially, in the case of 1r with meta‐OMe, almost one isomer
was observed (3r, >10:1 ratio). It is interesting to note that
benzodioxole and naphthyl groups had no clear impact on the
reactivity or regioselectivity, and the transformations afforded
the expected site‐selective products with 86% and 82% yield
respectively (3k‐3l). Aliphatic substrates, with methyl (1t) and
ethyl‐substituted acetylenic acid (1u), also could work rather
of alkynoate
intermediate
spirocyclization to form the intermediate
1
leads to the intermediate
proceeds through an intramolecular
, which undergoes
the cleavage of C–O bond to give the carboxyl radical . Along
path a, the cyclization of intermediate generates the
compound . The deprotonation of by NIS would give the
product
. As reported,13 along an alternative path b,
intermediate can also convert into the intermediate 10 by
releasing a CO2 molecule. Finally, the intermediate 10 reacts
with NIS to form the final 1,1‐diiodoalkene product 11
5. Subsequently, the
5
7
8
8
9
9
3
8
well affording excellent yields within 6 h (3t‐3u, 83‐89%).
However, no desired product was observed when phenyl
propiolate 1s was used as substrate. Finally, the substrates
with ortho‐methyl and chloro on the benzene ring of ester
moiety were examined, however, almost no desired product
3v and 3w were found even the reaction time was prolonged
to 24 h. It is mainly because that the steric hindrance of ortho‐
substituent makes the spirocyclization step difficult.
.
Interestingly, 11 was not detected under the current sunlight
irradiation experimental condition.
To highlight the utility of this cyclization reaction, we
decided to demonstrate some further methodological
potential of these derivatives
3 in the synthesis of the cores of
pharmacologically active molecules. Hartwig amination of 3‐
iodocoumarin 3a with p‐anisidine catalyzed by Pd2(dba)3 and
Xantphos
afforded
the
corresponding
3‐(4‐
methoxyphenylamino)‐4‐phenyl‐coumarin16 4a in 72% yield
(Scheme 3).
Scheme 5 Proposed mechanism.
Scheme 3 Transformation of 3a.
To understand the experimental observations, the free
energy profiles of three possible reaction paths of 1a are
obtained with UB3LYP calculations. The obtained results are
depicted in Figure 1 (for computational details, see SI). The
optimized geometries of all the species along the reaction
pathways are given in Fig S1 (see SI). The results indicate that
To get insight into the reaction mechanism, a control
intermediate‐trapping experiment with the addition of the
radical‐trapping reagents 2,2,6,6‐tetramethyl‐1‐piperidinyloxy
(TEMPO) was carried out (Scheme 4). The formation of desired
product 3a was suppressed (see SI), and the result indicates
that the transformation may proceed via a radical course.
the energy barrier of the rate‐limiting step along path
higher than those of path and . Hence, path should be
excluded. This result is in line with the experiment (no radical
intermediate was detected). For path and path , these
two pathways are the same before the compound is formed.
The species is converted into along path via the transition
state TS8‐9 (with a barrier of 8.83 kcal/mol), and converted into
10 and CO2 along path via TS8‐10 (with a barrier of 11.93
kcal/mol), respectively. The formation of 10 and CO2 is
exothermic by 16.03 kcal/mol, and the formation of is
exothermic by 1.28 kcal/mol. Thus, the formation of the
c is
a
b
c
6
a
b
8
8
9
a
Scheme 4 Mechanism Study.
b
To confirm the chemical structure of the product, the X‐ray
analysis of 3j was performed (see SI). The product 3j is 3‐iodo‐
4‐phenyl‐7‐(trifluoromethyl)‐2H‐chromen‐2‐one instead of 3‐
9
intermediate
9
is the kinetically controlled product. Once the
iodo‐4‐phenyl‐6‐(trifluoromethyl)‐2H‐chromen‐2‐one.
This
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species is formed, its further deprotonation can easily lead to
the formation of 3a, which is the main product detected
experimentally at room temperature. The experimental results
Journal Name
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Fig. 1 Gibbs free energy profile for the reaction calculated in acetonitrile solution (The
free energy barrier ΔG is relative to that of the reactant 1a and iodine radical).
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Conclusions
To conclude, a sunlight‐driven catalyst‐free cascade radical
iodination and cyclization reaction of aryl alkynoates with N‐
iodosuccinimide as iodine source was reported. The reactions
were carried out under convenient conditions via exposing the
reaction mixture to the sun. Experimental, x‐ray analysis, and
computational studies indicate that the reaction under
sunlight tends to proceed through iodination, spirocyclization
and ring expansion to form the kinetically controlled product.
The sunlight‐driven reaction provides a green pathway to
especially valuable 3‐halocoumarins derivatives.
We gratefully acknowledge the financial support from the
National Natural Science Foundation of China (Nos. 21102071
and 21472082). The Jiangsu 333 program (for Pan) and
Changzhou Jin‐Feng‐Huang program (for Han) are also
acknowledged.
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4 | J. Name., 2012, 00, 1‐3
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DOI: 10.1039/C6GC01027J
A sunlight-initiated radical process of aryl alkynoates has been achieved by using NIS as iodine source without use
of any catalyst or additive. Experimental, x-ray analysis, and computational studies indicate that the reaction under
sunlight tends to proceed through iodination, spirocyclization and ring expansion to form the kinetically controlled
product. The process is especially valuable for 3-halocoumarins syntheses and a process than several conventional
oxidative reactions.