Photocatalyst-free hypervalent iodine reagent catalyzed decarboxylative acylarylation of acrylamides with α-oxocarboxylic acids driven by visible-light irradiation

Article abstract of DOI:10.1039/c5cc08253f

A hypervalent iodine(iii) reagent catalyzed carbonylarylation of acrylamides with α-oxocarboxylic acids driven by visible-light without a photoredox catalyst has been developed. The reactions generate the corresponding products in good yields at room temperature. Experiments indicate that a blue LED (450-455 nm) is the most effective energy for the cleavage of the oxygen-iodine bond to initiate the reaction. Mechanistic studies further demonstrate that the reaction undergoes a cascade decarboxylative radical addition/cyclization process along with releasing CO₂ and H₂.

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COMMUNICATION
Photocatalystꢀfree hypervalent iodine reagent catalyzed decarboxylative
acylarylation of acrylamides with αꢀoxocarboxylic acids driven by
visibleꢀlight irradiation
Wangqin Ji,a Hui Tan,a Min Wang,*a Pinhua Li,a and Lei Wang*a,b
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A
hypervalent
iodine
(III)
reagent
catalyzed
carbonylarylation of acrylamides with αꢀoxocarboxylic
acids driven by visibleꢀlight without photoredox catalyst
10 has been developed. The reactions generate the
corresponding products in good yields at room
temperature. Experiments indicate that blue LED
(450−455 nm) is the most effective energy to cleavage of
oxygenꢀiodine bond to initiate the reaction. Mechanistic
15 study further demonstrates that the reaction undergoes a
cascade decarboxylative radical addition/cyclization
process along with releasing CO2 and H2.
Oxindoles, an important class of nitrogenꢀcontaining heterocycles
20 with remarkable biological and medicinal properties, are the key
structural motifs in numerous natural products and biologically
active compounds.1 Moreover, oxindoles are valuable
intermediates in asymmetric synthesis, library design and drug
discovery.2 Therefore, the development of straightforward and
25 highly efficient methods for their preparation have received much
more attention. Recently, the difunctionalizations of activated
alkenes have been developed.3 In particular, oxidative 1,2ꢀ
difunctionalization of C=C bonds in arylacrylamides has attracted
the interest of synthetic chemists and is a fascinating approach to
30 3,3ꢀdisubstituted 2ꢀoxindoles. These reactions can be initiated by
the addition of either carbon radical or heteroatom radical to C=C
bonds followed by intramolecular radical cyclization. According
to the radical species, these transformations are divided into 1,2ꢀ
alkylarylation,4ꢀ6 1,2ꢀcarbonylarylation,7 1,2ꢀazidoarylation,8 1,2ꢀ
35 nitroarylation,9 1,2ꢀsulfonylarylation,10 1,2ꢀphosphorylarylation,11
and others,12 which allow the rapid access to diverse 3,3ꢀ
disubstituted 2ꢀoxindole scaffolds by simultaneous formation of
two C−C bonds or C−C/C−hetero bonds.
Scheme 1 The applications of hypervalent iodine reagents and
50 visibleꢀlight photoredox catalysts in organic transformations
oxocarboxylic acids.7b Despite these achievements obtained in
this area, the current methods require excessive oxidants,
expensive metals, or higher reaction temperature. To develop the
mild and environmentallyꢀbegin route to carbonylꢀcontaining
55 oxindoles is highly desirable.
Recently, organic photochemical reactions, especially visibleꢀ
light induced organic transformations have emerged as one of the
most attractive research area due to its intrinsic characteristics of
mild, environmentally benign, operationally simple, infinitely
60 available source.15 Meanwhile, hypervalent iodine reagents (HIRs)
were found to be the effective reagents in organic synthesis,16 and
HIRs used for organic transformation by visibleꢀlight photoredox
catalysis have received considerable attention recently.16dꢀg For
example, Zhu described the formation of 3,3ꢀdisubstituted
65 oxindoles by visibleꢀlight photoredox catalysis in the presence of
facꢀIr(ppy)3 and PhI(OAc)2 (Scheme 1a).16d Subsequently,
Carbonylꢀcontaining oxindoles are considered to be privileged
40 frameworks in pharmaceutical agents, natural compounds, as well
as versatile intermediates in synthetic chemistry.13 As we known,
αꢀoxocarboxylic acids have been used as acylating reagents due
to their stability, easily preparation and high reactivity.14 In
general, αꢀoxocarboxylic acids are considered to undergo a
45 decarboxylative process to form acyl radicals along with
extrusion of CO2, and Duan developed
a AgNO3/K2S2O8
promoted 1,2ꢀcarbonylarylation of arylacrylamides using αꢀ
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Table 1. Optimization of the reaction conditionsa
Table 2. The scope of acrylamides and αꢀoxocarboxylic
acidsa
Entry
HIR (equiv)
PhI(OAc)2 (1.0)
PhI(OAc)2 (1.0)
PhI(OAc)2 (1.0)
PhI(OAc)2 (0.5)
PhI(OAc)2 (0.25)
BIꢀOAc (0.25)
BIꢀOH (0.25)
Yieldb(%) Entry
HIR (equiv) Yieldb(%)
1
2
3
4
5
6
7
8
46c
45d
60
47
32
78
61
15
9
0
BIꢀC≡CPh (0.25)
10
11
12
13
14
15
16
BIꢀOAc (0.35)
BI-OAc (0.20)
BIꢀOAc (0.15)
PhI(OAc)2 (0.25)
BIꢀOAc (0.20)
BIꢀOAc (0.20)
−
78
78
71
0e
0f
15g
BIꢀOTf (0.25)
0
aReaction conditions: NꢀmethylꢀNꢀphenylꢀmethacrylamide (1a, 0.20
mmol), 2ꢀoxoꢀ2ꢀphenylacetic acid (2a, 0.30 mmol), HIR (amount
indicated in Table 1), PhCl (1.0 mL) at room temperature under blue LED
(450−455 nm, 1.5 W) irradiation in air for 12 h. bIsolated yield. cWith
5
f
[Ru(bpy)3]Cl2·6H2O (2.0 mol%). dWith Eosin Y (2.0 mol%). eIn dark. 70
ºC in dark. g100 ºC in dark.
Chen developed
a series of visibleꢀlightꢀinduced organic
10 transformations with Ru(bpy)3(PF6)2/HIR photoredox catalytic
system (Scheme 1b and 1c).16eꢀg However, there is rare example
of the organic transformation driven by visibleꢀlight in the
presence of HIR catalyst without photoredox catalyst for saving
precious metal, such as Ru or Ir.16h Based on this understanding
15 and our recent works,17 herein, we report a remarkably mild HIRꢀ
catalyzed direct decarboxylative carbonylarylation of acrylamides
with αꢀoxocarboxylic acids under visibleꢀlight photolysis without
visibleꢀlight photoredox catalyst, which represents a novel and
energyꢀefficient approach to carbonylꢀcontaining oxindoles
20 (Scheme 1d).
Our initial investigation focused on the effect of photoꢀcatalyst
on the decarboxylative acylarylation of acrylamide with αꢀ
oxocarboxylic acid. The representative [Ru(bpy)3]Cl2·6H2O (2.0
mol%) and Eosin Y (2.0 mol%) were screened in a model
25 reaction of NꢀmethylꢀNꢀphenylmethacrylamide (1a) with 2ꢀoxoꢀ
2ꢀphenylacetic acid (2a) performed with PhI(OAc)2 (1.0 equiv)
and blue LED (450−455 nm, 1.5 W) irradiation at room
temperature for 12 h in PhCl, providing the product 3a in 46%
and 45% yields, respectively (Table 1, entries 1 and 2). When the
30 model reaction was carried out in the absence of photocatalyst, a
significant improved yield of 3a was obtained (entry 3). The yield
of 3a is decreased along with the less loading of PhI(OAc)2
(entries 4 and 5). Next, a variety of HIRs, including BIꢀOAc, BIꢀ
OTf, BIꢀOH and BIꢀC≡CPh, were examined. It is evident that BIꢀ
35 OAc was the most effective one, 3a was isolated in 78% yield
(entries 6−9). BIꢀOH and BIꢀOTf gave 3a in 61% and 15% yields,
and BIꢀC≡CPh was no effect. It was also found that the amount of
BIꢀOAc up to 35 mol% did not enhance the transformation, and
less than 20 mol% of BIꢀOAc resulted in less yield of the product
40 (entries 10−12). Specifically, no 3a was formed when the reaction
aReaction conditions: acrylamide (1, 0.20 mmol), αꢀoxocarboxylic
55 acid (2, 0.30 mmol), BIꢀOAc (0.040 mmol, 20 mol%), PhCl (1.0 mL)
under blue LED (1.5 W) irradiation at room temperature for 12 h in
air. bIsolated yield.
in the absence of photocatalyst was investigated with a variety of
60 acrylamides. The results were listed in Table 2. As can be seen
from Table 2, NꢀmethylꢀNꢀarylmethacrylamides (1) with both
electronꢀdonating and electronꢀwithdrawing groups, such as
methyl, methoxy, ethoxy, fluoro, chloro, bromo, iodo, phenyl,
and ester on the benzene rings reacted with 2a to afford the
65 corresponding products in good yields (Table 2, 3b−k, 3aa). The
reactions of 2a with NꢀmethylꢀNꢀarylꢀmethacrylamides (1)
containing a Me, MeO, or EtO group at the paraꢀposition of the
phenyl ring generated 70−75% yields of the products (3b−d). Nꢀ
MethylꢀNꢀarylꢀ methacrylamides (1) with a F, Cl, Br, I or EtOCO
70 group at the paraꢀposition of the benzene ring gave 66−77%
yields of the products (3e−h, 3aa). In addition, NꢀmethylꢀNꢀarylꢀ
methacrylamide (1) with a Ph group at the orthoꢀposition of the
aromatic ring afforded the product 3j in 69% yield. However, Nꢀ
methylꢀNꢀarylmethacrylamides attached amino or alcohol group
75 on the benzene rings could not react with 2a to generate the
desired products (3ab and 3ac). It should be noted that substrate 1
with an electronꢀdonating group (Me) at the metaꢀposition of
o
was performed at either room temperature or 70 C in dark, and
o
15% yield of 3a was obtained at 100 C in the absence of light
(entries 13−15). It is important to note that the addition of BIꢀ
OAc is essential to realize the reaction (entry 16). The further
45 optimization indicated that the reaction was generally completed
within 12 h at room temperature using 20 mol % of BIꢀOAc
under blue LED irradiation in chlorobenzene (Table S1 in
Supporting Information).
With the optimized reaction conditions in hand, the scope of
50 BIꢀOAc catalyzed decarboxylative acylarylation of acrylamides
with 2ꢀoxoꢀ2ꢀphenylacetic acid (2a) under visibleꢀlight irradiation
2
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O
the phenyl ring delivered a mixture of isomers 3k and 3k’ in
3:2 ratio with 70% yield. Moreover, ꢀethylꢀ
ꢀphenyl
Ph
N
Nꢀ
O
phenymethacrylamide,
methacrylamide,
N
ꢀ(
ꢀ(
ꢀ(
p
ꢀchlorobenzyl)ꢀ
N
N
N
N
p
m
ꢀmethylbenzyl)ꢀ
ꢀmethylbenzyl)ꢀ
N
N
ꢀphenylꢀ
ꢀphenylꢀ
3a
O
Ph
O
5
methacrylamide
and
H
O
methacrylamide underwent the reaction with 2a to generate
the anticipated products 3l in 70 84% yields. However, no
desired product was isolated when ꢀphenylmethacrylamide
reacted with 2a
I
O
N
−o
−
N
E
H
D
O
Trapped by
BHT
HRMS
Detected !
.
HO
O
O
I
10
In order to expand the scope of αꢀoxocarboxylic acids (2)
diverse array of 2ꢀoxoꢀ2ꢀ(substitutedꢀphenyl)acetic acids were
surveyed, also shown in Table 2. 2ꢀOxoꢀ2ꢀarylacetic acids with
either electronꢀdonating or electronꢀwithdrawing groups on the
phenyl rings of 2 could be applied to react with 1a, providing the
C
O
H2 GC Determined !
Ph 2a
O
Ph
O
CO2
Ph
O
N
O
B
O
O
O
O
I
1a
O
I
15 desired products 3p−z in moderate to good yields. The reactions
of substituted 2ꢀoxoꢀ2ꢀphenylacetic acids with an electronꢀ
donating group, such as methyl, ethyl or methoxy group at the
paraꢀposition of the benzene ring gave the desired products (3p−r)
in 70−78% yields. On the other hand, substituted 2ꢀoxoꢀ2ꢀ
20 phenylacetic acids with an electronꢀwithdrawing group including
fluoro, chloro or bromo group on the paraꢀposition of the phenyl
ring generated the corresponding products 3s–u in 63–66% yields.
At same time, 2ꢀoxoꢀ2ꢀarylacetic acids attached a methyl group
on the metaꢀposition or orthoꢀposition of the benzene ring
25 afforded the desired products 3v and 3w in 74% and 69% yields,
respectively. It is important to note that the substrate introduced
two strong electronꢀdonating groups (RO) on the aromatic ring of
2ꢀoxoꢀ2ꢀarylacetic acid (for example, 2ꢀ(benzo[d][1,3]dioxolꢀ5ꢀ
yl)ꢀ2ꢀoxoacetic acid) reacted with 1a to provide the
30 corresponding product 3x in 75% yield. 2ꢀ(Naphthalenꢀ1ꢀyl)ꢀ2ꢀ
oxoacetic acid and 2ꢀ(naphthalenꢀ2ꢀyl)ꢀ2ꢀoxoacetic acid were
used to react with 1a to generate the products 3y and 3z in 60%
and 58% yields, respectively. However, the reactions were
limited to furoylformic acid and 2ꢀthienylglyoxylic acid under the
35 optimal reaction conditions. Furthermore, 2ꢀoxoꢀ2ꢀaliphatic
acetic acid and its derivatives, such as 4ꢀmethylꢀ2ꢀoxopentanoic
acid and oxalic acid 1ꢀethyl ester failed under the recommended
reaction conditions owing to the less stable of aliphatic acyl
radicals compared with aromatic ones.
N
O
by
pped
Tra
A
TEMPO
OAc
HOAc
TEM
PO
BI-OAc
O
N
O
Ph
4
, 70%
Scheme 2. The Proposed Mechanism
40
A plausible mechanism of BIꢀOAc catalyzed and visibleꢀlight
driven decarboxylative acylarylation of acrylamides with αꢀ
oxocarboxylic acids was depicted in Scheme 2. The reaction
processes probably proceed as follows; (i) the formation of an
intermediate A,18 by a transesterification of BIꢀOAc with 2ꢀoxoꢀ
70
Scheme 3. The control experiments
In summary, an efficient approach for the direct
decarboxylative acylarylation of acrylamides with αꢀ
oxocarboxylic acids has been developed. The reaction was based
45 2ꢀphenylacetic acid (2a); (ii) the generation of iodanyl radical
C16h,19 and benzoyl radical B from the homolytic cleavage of A
under blue LED irradiation along with releasing CO2 (FTꢀIR
analysis of the resulting CO2 gas, Supporting Information);17a,b
(iii) the formation of an intermediate D through a free radical
50 addition16d,17c of formed B to the C=C bond of 1a and followed
by intramolecular radical cyclization; (ix) the generation of the
product 3a by a hydrogen atom abstraction of D along with
forming intermediate E, which reacts with 2a to afford A for next
cycle run and H2 (GC analysis of the forming H2 gas, Supporting
55 Information). It is important to note that benzoyl radical B
derived from the formed A in situ was trapped by TEMPO
75 on a BIꢀOAc catalyzed process and was driven by visibleꢀlight in
the absence of photocatalyst. This acylarylation tolerates a series
of substituted groups and affords the desired carbonylꢀcontaining
oxindoles in good yields. Experiments showed that blue LED is
the most effective energy source to initiate the reaction. The
80 reaction mechanism investigation indicated that the reaction
undergoes a cascade decarboxylative radical addition/cyclization
process along with releasing CO2 and H2. Further study on the
detail reaction mechanism and application of HIRs are underway.
We gratefully acknowledge the National Natural Science
85 Foundation of China (Nos. 21572078, 21202057) and the Natural
Science Foundation of Anhui (No. 1308085MB25) for financial
support of this work.
(2,2,6,6ꢀtetramethylꢀ1ꢀpiperidinyloxy),
affording
the
corresponding adduct 4 in 70% yield under the standard reaction
conditions.20 On the other hand, the in situ generated iodanyl
60 radical
C
was trapped by BHT (2,6ꢀdiꢀtertꢀbutylꢀ4ꢀ
Notes and reference
Department of Chemistry, Huaibei Normal University, Huaibei, Anhui
methylphenol)21 as in Scheme 3, providing the anticipated
product 5 in 31% yield and 6 (HRMS analysis, SI). In order to
further support the proposed mechanism, the related control
experiments were conducted (SI). These results imply that A, as
65 an important intermediate may involve in the reaction.
a
90 235000, P R China; E-mail: leiwang@chnu.edu.cn
b
State Key Laboratory of Organometallic Chemistry, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, P
R China
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See DOI: 10.1039/b000000x/
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