Visible-Light-Mediated Dual Decarboxylative Coupling of Redox-Active Esters with α,β-Unsaturated Carboxylic Acids

Article abstract of DOI:10.1002/chem.201702200

An efficient visible-light-induced decarboxylative coupling between α,β-unsaturated carboxylic acids and alkyl N-hydroxyphthalimide esters has been developed. A wide range of redox-active esters derived from aliphatic carboxylic acids (1°, 2° and 3°) proved viable in this dual decarboxylation process, affording a broad scope of substituted alkenes in moderate to excellent yields with good E/Z selectivities. This redox-neutral procedure was highlighted by its mild conditions, operational simplicity, easy accessibility of carboxylic acids, and excellent functional-group tolerance.

Full text of DOI:10.1002/chem.201702200

A Journal of
Accepted Article
Title: Visible-Light-Mediated Dual Decarboxylative Coupling of Redox-
Active Esters with alpha,beta-Unsaturated Carboxylic Acids
Authors: Xin-Hua Duan, Jin-Jiang Zhang, Jun-Cheng Yang, and Li-Na
Guo
This manuscript has been accepted after peer review and appears as an
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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
to this Accepted Article as a result of editing. Readers should obtain
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: Chem. Eur. J. 10.1002/chem.201702200
Link to VoR: http://dx.doi.org/10.1002/chem.201702200
Supported by

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COMMUNICATION
Visible-Light-Mediated Dual Decarboxylative Coupling of Redox-
Active Esters with -Unsaturated Carboxylic Acids
α β
,
Jin-Jiang Zhang, Jun-Cheng Yang, Li-Na Guo, and Xin-Hua Duan*
Dedication ((optional))
Abstract: An efficient visible-light-induced decarboxylative coupling
between -unsaturated carboxylic acids and alkyl N-
and environmental friendliness.[6] Recently, MacMillan and Doyle
α,β
reported the first direct decarboxylative coupling of alkyl
carboxylic acids with aryl, vinyl and alkyl halides through
merging photoredox and nickel catalysis.[7] In fact, visible-light-
mediated decarboxylation of alkyl carboxylic acid derivatives can
be dated back to 1990s. Pioneering work developed by Okada
and co-workers revealed that redox-active NHP esters could
undergo decarboxylative Michael addition with electron-deficient
alkenes under photoredox catalytic conditions (Figure 1).[8] Later
on, Overman et al employed this radical decarboxylative
alkylation strategy for the construction of quaternary carbon
centers.[9] In 2015, Chen and co-workers disclosed visible-light-
induced reductive decarboxylative allylation and alkynylation of
the NHP esters, respectively.[10] More recently, the group of
König demonstrated an efficient metal-free decarboxylative
alkylation of electron-deficient alkenes with the NHP esters
under visible-light catalysis.[11] On the other hand, the oxidative
decarboxylative alkylation of cinnamic acids have also been well
documented, but the generation of these alkyl radical always
requires the employment of strong stoichiometric peroxides and
high temperature.[12a-f] Very recently, Chen has also reported a
photoredox catalyzed decarboxylative alkylation of vinyl
carboxylic acids using alkylboronic acids in the presence of
stoichiometric hypervalent iodine reagents.[12g] However, the
redox-neutral visible-light-induced decarboxylative alkylation of
hydroxyphthalimide esters has been developed. A wide range of
redox-active esters derived from aliphatic carboxylic acids (1o, 2o
and 3o) proved viable in this dual decarboxylation process, affording
a broad scope of substituted alkenes in moderate to excellent yields
with good E/Z selectivities. This redox-neutral procedure was
highlighted by its mild conditions, operational simplicity, easy
accessibility of carboxylic acids, and excellent functional-group
tolerance.
Transition-metal catalyzed cross-coupling reactions involving
alkyl species have emerged as one of the powerful tools for
construction of Csp3−C bonds. While the easily available alkyl
halides containing
challenging substrates in such transformations due to their
difficulty of oxidative addition and ease of -hydrogen
β hydrogen atoms are one of the most
β
elimination.[1] During the past decade, significant progress has
been made in this field. So far, alkyl halides have been
developed to be efficient alkyl sources in Csp3−C bond formation
reactions.[1,2] In addition, a series of alkyl electrophiles including
cyclic anhydrides,[3a] alkyl ethers,[3b,c] styrenyl epoxides[3d] even
styrenyl aziridines,[3e] and cyclic sulfates[3f] have also been
successfully exploited to construct the Csp3−C bonds. Recently,
redox-active esters derived from alkyl carboxylic acids and N-
hydroxyphthalimide (NHPI) were demonstrated to be very
efficient alkyl electrophiles in alkyl cross-coupling reactions.[4,5]
For example, Baran and co-workers have successfully
developed a series of Ni-catalyzed alkyl-aryl and alkyl-alkyl
cross-coupling reactions using this redox-active esters as alkyl
sources.[4a-c] Simultaneously, the group of Weix has also
reported an efficient alkyl-aryl cross-electrophile coupling
reaction using the N-hydroxyphthalimide esters (NHP esters).[5]
Inspired by those results, we wish to develop a novel alkyl-vinyl
cross-coupling reaction using NHP esters as an alkyl source for
Csp3-Csp2 bond formation.[4e]
α,β-unsaturated carboxylic acids remains rare. On the basis of
these elegant results as well as our ongoing interest in
decarboxylative reactions,[6c,13] we report herein a concise
visible-light-mediated dual decarboxylative couping between
α,β-unsaturated carboxylic acids and redox-active NHP esters at
room temperature, which provides an efficient route to
substituted alkenes under redox-neutral conditions (Figure 1).
Previous Studies - Visible-light photocatalysis
EWG
O
O
O
Okada
Overman
Koenig
Chen
R
R
photoredox catalyst
or
N
Csp3
+
+
Csp3
Reductant
-CO2
EWG
O
O
X
In recent years, visible-light-mediated decarboxylative cross-
couplings of carboxylic acids and their derivatives have attracted
increasing attention owning to their sustainability, practicality
O
O
photoredox catalyst
N
Csp3
X
Ar
Csp
Chen
Reductant
-CO2
O
This Work
O
O
O
[a]
Jin-Jiang Zhang, Jun-Cheng Yang, Prof. Dr. Li-Na Guo, Prof. Dr.
Xin-Hua Duan
R
R1
photoredox catalyst
CO2
H
+
N
Csp3
Csp2
Ar
Without
Reductant
Department of Chemistry
O
School of Science and MOE Key Laboratory for Nonequilibrium
Synthesis and Modulation of Condensed Matter, Xi’an Jiaotong
University
-2CO2
Figure
1. Visible-light-mediated decarboxylative coupling of N-
No.28, Xianning West Road, Xi'an, China, 710049
E-mail: duanxh@xjtu.edu.cn
hydroxyphthalimide esters with different reactants
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
We commenced our studies with the optimization of the
coupling between N-acyloxyphthalimide 1a and 4-bromo
cinnamic acid (2a) under visible-light irradiation conditions
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Table 1. Optimization of the Reaction Conditions.[a]
Table 2. Scope of N-Hydroxyphthalimide Esters.
O
O
O
O
O
R
fac-Ir(ppy)3 (1 mol %)
Mg(ClO4 2 (2 equiv)
Br
Br
Br
O
N
Br
+
fac-Ir(ppy)3 (1 mol%)
+
)
N
R
Br
HO2C
solvent, additives
room temperature
visible light
HO2
C
NMP, rt
visible light
O
O
1a
3a
2a
1
3
2a
Entry
1
Solvent
NMP/H2O (4:1)
DMF/H2O (4:1)
dioxane/H2O (4:1)
THF/H2O (4:1)
DME/H2O (4:1)
DCE/H2O (4:1)
NMP/H2O (1:4)
H2O
Additives (equiv)
Yield [%][b]
42
Br
Br
Ts
N
O
-
2
-
11
3a, 64% (E/Z = 23:1)
3c, 77% (E/Z = 4.3:1)
3b, 63% (E/Z = 20:1)
3
-
19
Br
Br
Br
4
-
33
Boc
N
N
N
5
-
16
Boc
Bn
6
-
0
3d, 60% (E/Z = 7.4:1)
3e, 33% (E/Z > 99:1)
3f, 0%
7
-
36
Br
Br
Br
8
-
<5
O
9
NMP
-
44
Cl
O
Ph
10
11
12
13
14
15
16
17
18
NMP
Mg(ClO4)2 (1)
55
Cl
3g, 32% (E/Z = 6.3:1)
3h, 35% (E/Z = 6:1)
3i, 46% (E/Z = 6.5:1)
NMP
MgSO4 (1)
40
.
NMP
MgCl2 6H2O (1)
27
Br
Br
Br
NMP
Mg(NO3)2 (1)
Mg(ClO4)2 (2)
Mg(ClO4)2 (3)
Mg(ClO4)2 (1)
Mg(ClO4)2 (1)
Mg(ClO4)2 (1)
34
64[c]
3j, 72%[a] (E/Z > 99:1)
3k, 78%[a] (E/Z > 99:1)
3l, 66%[a] (E/Z > 99:1)
NMP
[a] 1.0 equiv of Mg(ClO4)2 was used.
NMP
42
NMP
0[d]
0[e]
trace[f]
coupling with 4-bromocinnamic acid 2a (Table 2). The reactions
of various alkyl NHP esters proceeded efficiently to afford the
desired products 3 in moderate to good yields with good to
excellent E/Z selectivity (Table 2). Satisfactorily, secondary alkyl
NHP esters containing heteroatoms (O or N) on the aliphatic ring
also worked well to give the desired products in good yields (3b-
3d). It is noteworthy that the protecting groups on the N atom
exhibited a significant influence on the reaction efficiency. For
example, N-Boc- and N-Ts-protected substrates underwent the
decarboxylation process smoothly to furnish the corresponding
products 3c and 3d in 77% and 60% yields, respectively.
However, Bn-protected substrate 1f was inactive toward this
photoredox catalytic system. Besides secondary NHP esters,
the primary esters 1g-i were also effective substrates for this
reaction, albeit with relatively low yields. Notably, the carbonyl
group also survived in this procedure (3i). Compared with the
primary and secondary esters, the tertiary alkyl NHP esters
showed higher catalytic reaction activity, delivering the products
3j-l in 66-78% yields.
Subsequently, we explored the scope of acrylic acids 2 in this
decarboxylative reaction. As given in Table 3, a wide range of
cinnamic acids bearing electron-donating and -withdrawing
groups at the para-position of the phenyl ring all reacted
smoothly with 1d to give the corresponding alkenes 4b-g in
satisfied yields. More importantly, some functional groups such
as dimethylamino (4c), halogens (4d and 4e), and cyano (4g)
were well tolerated. Furthermore, the sterically congested ortho-
substituted cinnamic acids were also suitable substrates,
furnishing the corresponding products 4h and 4i in 65% and
42% yields, respectively. In addition, meta-substituted (2j and 2k)
and disubstituted (2l) cinnamic acids also resulted in satisfactory
yields. Furthermore, the cinnamic acid was successfully
NMP
NMP
[a] Reaction conditions: 1a (0.25 mmol, 1.0 equiv), 2a (0.375 mmol, 1.5 equiv),
additives and fac-Ir(ppy)3 (1 mol%), in 2.5 mL of solvent at room temperature
were irradiated by a 23 W compact fluorescent light (CFL) bulb for 36 h under
N2, E/Z = 23:1, the E/Z ratio was determined by 1HNMR spectroscopy of the
crude product. [b] Yield of isolated product. [c] Phthalimide was isolated as the
major byproduct in 97% yield. [d] No photocatalyst. [e] In the dark. [f] Under air.
(Table 1). To our delight, the desired decarboxylative coupling
product 3a was isolated in 42% yield in the presence of 1 mol%
fac-Ir(ppy)3 as photocatalyst (Table 1, entry 1).[14] Other Ir(III)
complexes such as Ir(ppy)2dtbbpy were less effective than the
fac-Ir(ppy)3 catalyst. In contrast, Ru(bpy)3Cl2, Cu(dap)2Cl and
some representative organic dyes including Eosin Y,
Fluorescein, Rhodamine B and 1,6-bis(dimethylamino)pyrene
were completely inactive (See the Supporting Information).
Solvent screening revealed that NMP/H2O (4:1, NMP = N-
methylpyrrolidone) was superior to other mixed solvents (entries
1-6). Changing the ratio of NMP/H2O from 4:1 to 1:4 did not
significantly affect the yield of product 3a (entry 7). It was found
that using NMP as the sole solvent also resulted in a similar
yield compared (entry 9). To further enhance the reaction
efficiency, some additives were next examined. Satisfactorily,
addition of 1.0 equiv of Mg(ClO4)2 significantly enhanced the
yield of the desired product 3a (entry 10).[15] However, other
magnesium salts were found to be inferior for this transformation
(entries 11-13). Increasing the amount of Mg(ClO4)2 to 2.0 equiv
further enhanced the yield to 64%, while further increasing the
amount of additive led to a diminished yield (entries 14 and 15).
Other Lewis or Brønsted acids such as AlCl3, ZnCl2, AcOH or
HCl were also tested, but none of them gave higher yields than
Mg(ClO4)2. Furthermore, benziodoxole reagent (BIOH) was also
proved to be ineffective for this reaction (see the Table S1 in
extended to other α,β-unsaturated carboxylic acids such as 3,3-
diphenylacrylic acid (2m) and 3-methyl-3-phenylacrylic acid (2n),
providing the corresponding trisubstituted alkenes 4m and 4n in
63% and 68% yields, respectively. The alkyl NHP esters 1 could
also react smoothly with different cinnamic acids to give the 1,2-
disubstituted alkenes 4o-q in moderate to good yields. To our
delight, 3-(4-methylbenzoyl)-2-propenoic acid (2r) was also an
effective substrate in this protocol, and the desired product 4r
Supporting Information).
A series of contrast experiments
revealed that both the photocatalyst and light were essential for
the success of this decarboxylative coupling reaction (entries 16
and 17). Of note, the reaction must be carried out under an inert
atmosphere (entry 18).
With the optimal reaction conditions in hand, we then
examined the scope of the N-hydroxyphthalimide esters in the
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ox
was isolated in 50% yield [Eq. (1)]. In contrast, when 3,3-
dimethylacrylic acid (2s) was subjected to the reaction, only N-
(p-tolylsulfonyl)piperidine (4s’) was isolated as major product
and no desired coupled product 4s was detected [Eq. (2)].[16]
cinnamic acid 2b, E1/2 = +2.01 V vs. SCE in MeCN).[17] In
addition, treatment of vinyl carboxylic acid 2a with TEMPO under
standard conditions did not afford any alkene-TEMPO adduct
[Eq. (7)], which excludes a vinyl radical intermediate through
direct decarboxylation of cinnamic acids in this transformation.[12]
Based on these results and previous reports,[8-11] a possible
reaction mechanism was proposed in Scheme 1. Initially, the
photocatalyst PC is excited to a strongly reducing excited state
(PC*) by visible light irradiation. Next, a single electron transfer
(SET) from this species to NHP esters 1 generates PC+ and a
radical anion I, which then fragments followed by extrusion of
CO2, thereby producing an alkyl radical and the phthalimide
anion II. Subsequently, the alkyl radical attacks the C=C bond of
acrylic acids 2 to give the radical III. Deprotonation of III with II to
Table 3. Scope of
α,β
-Unsaturated Carboxylic Acids and N-
Hydroxyphthalimide Esters.
O
O
R
R1
R1
fac-Ir(ppy)3 (1 mol %)
CO2H
O
N
+
R
Mg(ClO4)2 (2 equiv)
Ar
Ar
NMP, rt
O
visible light
1
2
4
N
Ts
N
Ts
N Ts
Me
N
Me
Me
4c, 68% (E/Z = 29:1)
4a, 69% (E/Z = 6.2:1)
4b, 66% (E/Z = 39:1)
give phthalimide and
β-radical carboxylate IV, which might be
N
Ts
N
Ts
N
Ts
Cl
F3C
oxidized by PC+ to a radical V. Then intermediate V could
F
release CO2 to deliver the desired product. Oxidative quenching
4d, 88% (E/Z = 20:1)
4e, 78% (E/Z = 4:1)
4f, 41% (E/Z = 14:1)
of fac-Ir(ppy)3 generates the oxidizing species ([Ir(ppy)3]+ (E1/2
IV/III
R2
R3
N
Ts
Ts
N
Ts
N Ts
= 0.77 V vs. SCE),[14] which is suited to single-electron oxidation
of radical anion IV to deliver the radical intermediate V (for alkyl
carboxylate, E1/2ox = +0.80 ∼ +1.2 V vs. SCE). [7b,18]
NC
4g, 86% (E/Z = 8.7:1)
4h, R2 = Me, 65% (E/Z = 30:1)
4i, R2 = Cl, 42% (E/Z = 7:1)
4j, R3 = Me, 58% (E/Z = 9:1)
4k, R3 = Cl, 52% (E/Z = 15:1)
Cl
standard
Ts
N
conditions
N
N
Ts
N
Ts
+
Br
3c
1c
+
(3)
N
Cl
Ph
Ph
TEMPO (1 equiv)
O
HO2
C
38%
Ph
Me
2a
5, 42%
4l, 79% (E/Z = 5.5:1)
4m, 63%
4n, 68% (E/Z = 11:1)
4q, 69%[a] (E/Z > 99:1)
O
O
O
O
standard
Ts
Ts
N
conditions
N
MeO
MeO
Me
+
TEMPO
N
N
(4)
O
MeO
4p, 52% (E/Z = 9:1)
O
4o, 75%[a] (E/Z > 99:1)
1c
5, 48%
[a] 1.0 equiv of Mg(ClO4)2 was used.
standard
conditions
O
O
+
2a
(5)
N
Br
O
Br
O
O
O
O
Me
standard
conditions
1m
3m, 19%
3m', Not detected
Ts
N
+
N
CO2
H
(1)
N
Me
Ts
O
O
O
O
standard
conditions
O
1c
Ts N
2r
4r, 50%, E/Z > 99:1
Ts
N
(6)
+
N
CO2Et
EtO2
C
O
O
O
O
standard
conditions
1c
6a
7a, 0%
Me
Ts
N
+
N
CO2
H
N
Ts
H
N
Ts (2)
Me
Me
Me
standard
conditions
Br
O
Br
+
TEMPO
(7)
1c
4s', 95%
N
2s
4s, 0%
HO2
C
O
To gain mechanistic insight into this process, some control
2a
8, Not detected
experiments were conducted (Also see the Supporting
Information). As shown in Eq. 3, when TEMPO, a typical radical
scavenger was added to the reaction of 1c with 2a, 3c was
obtained in only 38% yield, along with 42% yield of alkyl-TEMPO
adduct 5 isolated. In addition, the reaction of TEMPO with redox-
active ester 1c also directly afforded the adduct 5 in 48% yield
[Eq. (4)]. These results suggest that alkyl radical intermediate
might be involved in this transformation. Moreover, treatment of
cyclopropylmethyl substituted NHP ester 1m with 2a under the
standard conditions furnished the ring-opening product 3m in
19% yield [Eq. (5)]. The formation of 3m also suggests a radical
process. It should be mentioned that the alkyl substituted
cinnamic acid could not be detected in all cases under the
optimized reaction conditions. Furthermore, when ethyl
cinnamate 6a was employed instead of the cinnamic acid, no
reaction occurred, indicative of the necessity of the COOH group
PC*
reductant
1
O
N
O
Visible Light
Photoredox
Catalytic
Cycle
SET
O
O
R
+
N
R
CO2
R
PC+
O
I
O
II
2
PC
oxidant
SET
R
R
R1
O
R1
R1
R
3 or 4
CO2
CO2H
Ar
O
Ar
Ar
CO2
V
IV
III
phthalimide
II
Scheme 1. Proposed Mechanism.
In summary, we have realized an efficient visible-light-
mediated redox-neutral, and decarboxylative coupling between
the alkyl NHP esters and α,β-unsaturated carboxylic acids. This
sequential decarboxylation procedure provided a facile and
efficient method for the preparation of structurally diverse
alkenes. Remarkably, this redox-neutral protocol features mild
conditions, simple operation, good functional group tolerance
and good to excellent stereoselectivity.
[Eq. (6)]. Given the strong reduction potential of the catalyst fac-
IV/*III
Ir(ppy)3 (E1/2
= -1.73 V vs. SCE), it is sufficient to reduce the
red
ester 1a (E1/2 = -1.2 V vs. SCE in MeCN) to afford an alkyl
radical.[8-10,17] Meanwhile, cinnamic acid can not be oxidized by
*III/II
the excited *Ir (III) complex (E1/2
= +0.31 V vs. SCE)[14]
because of their comparatively high oxidation potentials (for
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10.1002/chem.201702200
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COMMUNICATION
Soc. 2016, 138, 2174; b) T. Qin, J. Cornella, C. Li, L. R. Malins, J. T.
Edwards, S. Kawamura, B. D. Maxwell, M. D. Eastgate, P. S. Baran,
Science 2016, 352, 801; c) J. Wang, T. Qin, T.-G. Chen, L. Wimmer, J.
T. Edwards, J. Cornella, B. Vokits, S. A. Shaw, P. S. Baran, Angew.
Chem. 2016, 128, 9828; Angew. Chem. Int. Ed. 2016, 55, 9676; d) X.
Lu, B. Xiao, L. Liu, Y. Fu, Chem. Eur. J. 2016, 22, 11161. e) During
review of our manuscript, a Ni-catalyzed decarboxylative alkenylation
reaction was reported, see: J. T. Edwards, R. R. Merchant, K. S.
McClymont, K. W. Knouse, T. Qin, L. R. Malins, B. Vokits, S. A. Shaw,
D.-H. Bao, F.-L. Wei, T, Zhou, M, D. Eastgate, P. S. Baran, Nature
2017, DOI:10.1038/nature22307
Experimental Section
Representative Procedure for the Decarboxylative Coupling of
Unsaturated Carboxylic Acids with Redox-Active Esters
α β
, -
A 10 mL oven-dried Schlenk-tube equipped with a magnetic stir
bar under a nitrogen atmosphere was charged with fac-Ir(ppy)3
(1.64 mg, 1 mol%) and Mg(ClO4)2 (0.5 mmol, 2.0 equiv). Then
NHP ester 1 (0.25 mmol, 1.0 equiv), α,β-unsaturated carboxylic
acids 2 (0.375 mmol, 1.5 equiv) in 2.5 mL of NMP were added
by syringe. The tube was then sealed and the mixture was
stirred under the irradiation of a 23 W compact fluorescent light
(CFL) bulb at room temperature for 36 h. The mixture was then
quenched with H2O and transferred to a separatory funnel with
EtOAc (2 mL). The aqueous layer was extracted with EtOAc (3 x
5 mL), then the organic phases were combined and washed with
brine (10 mL), dried over Na2SO4, and evaporated under
reduced pressure. The resulting residue was purified by column
[5]
[6]
K. M. M. Huihui, J. A. Caputo, Z. Melchor, A. M. Olivares, A. M.
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D. J. Weix, J. Am. Chem. Soc. 2016, 138, 5016.
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E. R. Jarvo, Angew. Chem. 2016, 128, 1362; Angew. Chem. Int. Ed.
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D. W. C. MacMillan, Angew. Chem. 2015, 127, 8040; Angew. Chem. Int.
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11200.
chromatography
on
silica
gel
(gradient
eluent
of
EtOAc/petroleum ether: 1/100 to 1/50) to give the corresponding
products 3 or 4 in yields listed in Table 2 and 3.
Acknowledgements
Financial support from Natural Science Basic Research Plan in Shaanxi
Province of China (No. 2016JZ002) andthe Fundamental Research
Funds of the Central Universities (No.2015qngz17 and xjj2016056) are
greatly appreciated. We also thank the reviewers for their comments and
suggestions in the reaction mechanism.
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Entry for the Table of Contents (Please choose one layout)
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Text for Table of Contents
Jin-Jiang Zhang, Jun-Cheng Yang, Li-
Na Guo, and Xin-Hua Duan*
O
O
O
photoredox catalyst
R
+
R1
OH
R1
Page No. – Page No.
N
R
O
O
2CO2
phthalimide
28 examples
Visible-Light-Mediated Dual
Decarboxylative Coupling of Redox-
R = alkyl
R1 = aryl, acyl
up to 88% yield
Active Esters with
Carboxylic Acids
α β
, -Unsaturated
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