Merging Photoredox with Copper Catalysis: Decarboxylative Alkynylation of α-Amino Acid Derivatives

Article abstract of DOI:10.1021/acs.orglett.6b03888

A novel and efficient decarboxylative alkynylation of N-(acetoxy)phthalimides of α-amino acids with terminal alkynes has been developed by merging photoredox with copper catalysis at room temperature, and the reaction provided the valuable propargylamines in good to excellent yields with assistance of [Ru(bpy)₃]Cl₂/CuI and visible light. The simple protocol, mild reaction conditions, and high efficiency of this method make it an important strategy for synthesis of diverse molecules containing amino acid fragments.

Full text of DOI:10.1021/acs.orglett.6b03888

Letter
pubs.acs.org/OrgLett
Merging Photoredox with Copper Catalysis: Decarboxylative
Alkynylation of α‑Amino Acid Derivatives
Hao Zhang, Pengxiang Zhang, Min Jiang, Haijun Yang, and Hua Fu*
Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology (Ministry of Education), Department of Chemistry,
Tsinghua University, Beijing 100084, China
S
* Supporting Information
ABSTRACT: A novel and efficient decarboxylative alkynylation of N-
(acetoxy)phthalimides of α-amino acids with terminal alkynes has been
developed by merging photoredox with copper catalysis at room
temperature, and the reaction provided the valuable propargylamines in
good to excellent yields with assistance of [Ru(bpy)3]Cl2/CuI and visible light. The simple protocol, mild reaction conditions,
and high efficiency of this method make it an important strategy for synthesis of diverse molecules containing amino acid
fragments.
lkynes are versatile building blocks and intermediates in the
synthesis of organic materials and biologically active
decarboxylative couplings of carboxylic acids and their
derivatives as radical precursors.17 Waser and co-workers have
recently developed a decarboxylative alkynylation of carboxylic
acids using photoredox catalysis and ethynylbenziodoxolone
reagents.18 We have also developed some interesting visible-light
photoredox organic reactions.19 As part of our continuing study
on visible-light photoredox catalysis, we report herein the
synthesis of propargylamines from N-(acetoxy)phthalimides of
α-amino acids and terminal alkynes by merging photoredox with
copper catalysis at room temperature.
At first, decarboxylative coupling of 1,3-dioxoisoindolin-2-yl 2-
(diethylamino)-3-methylbutanoate (1a) with phenylacetylene
(2a) by merging photoredox with copper catalysis was chosen as
the model to optimize conditions including photocatalysts,
copper catalysts, solvents, and time. As shown in Table 1, four
copper catalysts were screened using 1.0 mol % of [Ru(bpy)3]Cl2
as the photoredox catalyst and dichloromethane (DCM) as the
solvent under argon atmosphere at room temperature for 6 h
(entries 1−4), and CuI provided the highest yield (entry 2). No
reaction occurred in the absence of copper catalyst (entry 5).
Other solvents were investigated (entries 6−9), and the results
showed that CH2Cl2 was optimal (compare entry 2 and entries
6−9). The reaction afforded 77% yield when [fac-(Ir(ppy)3] was
used as the photocatalyst (entry 10). Only a trace amount of
product 3a was observed in the absence of photocatalyst (entry
11). When the time was changed from 6 to 8 h, a similar yield was
provided (entry 12). The yield decreased in a short reaction time
(entry 13). The reaction did not work without irradiation of
visible light (entry 14).
A
molecules.1 Sonogashira coupling is a classical and powerful
method for the synthesis of alkynes containing aryl C(sp2)−
C(sp) bonds.2 However, it still is a challenge to construct alkyl
C(sp3)−C(sp) bonds. Recently, the alkyl C(sp3)−C(sp) bond
formation has been addressed by the development of
nucleophilic3 and electrophilic reagents,4 but the substrate
scope is limited thus far. Propargylamines are high value building
blocks in organic synthesis, and they are considered as key
intermediates for the preparation of many nitrogen-containing,
biologically active compounds and important structural elements
of natural products and therapeutic drug molecules.5 The most
conventional methods for their synthesis are via transition-metal-
catalyzed,6 especially copper-catalyzed,7 three-component cou-
plings of aldehydes, amines, and terminal alkynes. Carboxylic
acids and their derivatives are ubiquitous in organic molecules,
natural products, and biomasses, and some notable decarbox-
ylative examples have recently been developed such as
decarboxylative Heck coupling,8 aldol additions,9 and asym-
metric enolate alkylations.10 Tunge and co-workers reported an
interesting intramolecular decarboxylative C(sp3)−C(sp) cou-
pling from ester derivatives.11 α-Amino acids widely occur in
nature, and their selective functionalization attracts much
attention. Liang and Li cooperatively developed a copper-
catalyzed decarboxylative coupling of sp3-hybridized carbon
atoms of α-amino acids.12 Recently, visible-light photoredox
catalysis has attracted much attention, and it has emerged as a
powerful activation protocol in chemical transformations.13
Sanford and co-workers described the development of a mild
method for the cross-coupling of arylboronic acids with CF3I via
the merger of photoredox and Cu catalysis.14 Rueping’s group
developed the alkynylation of tertiary amines with terminal
alkynes in the presence of a photoredox catalyst and a metal
catalyst.15 Very recently, Hashmi and co-workers have developed
an efficient gold-catalyzed photoredox α-C(sp3)−H alkynylation
of tertiary aliphatic amines under irradiation of sunlight.16 In
addition, some progress has been achieved in photoredox
After obtaining the optimized, dual catalytic conditions, we
investigated substrate scope on the decarboxylative alkynylation
of N-(acetoxy)phthalimides (1) with terminal alkynes (2). As
shown in Table 2, various terminal alkynes were tested using 1a
as the partner, and the results showed that both aliphatic and
Received: December 29, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b03888
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Optimization of Conditions for Decarboxylative
Coupling of 1,3-Dioxoisoindolin-2-yl 2-(Diethylamino)-3-
methylbutanoate (1a) with Phenylacetylene (2a) by Merging
Table 2. Decarboxylative Alkynylation of α-Amino Acid
Derivatives (1) with Terminal Alkynes (2) by Merging
a
Photoredox with Copper Catalysis
a
Photoredox with Copper Catalysis
photo-
catalyst
time
(h)
yield
b
entry
Cu catalyst
solvent
CH2Cl2
(%)
1
2
A
A
A
A
A
A
A
A
A
B
Cu(CH3CN)4PF6
CuI
6
6
6
6
6
6
6
6
6
6
6
8
4
6
70
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
ClCH2CH2Cl
CH3CN
CHCl3
89
3
CuBr
72
4
CuSCN
NR
c
5
NR
6
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
75
62
17
55
77
7
8
9
DMF
10
11
12
13
14
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
d
trace
89
A
A
A
80
e
NR
a
Reaction conditions: irradiation of visible light with 40 W CFL and
argon atmosphere, 1,3-dioxoisoindolin-2-yl 2-(diethylamino)-3-meth-
ylbutanoate (1a) (0.6 mmol), phenylacetylene (2a) (0.3 mmol),
photocatalyst (3 μmol), Cu catalyst (0.03 mmol), solvent (2.0 mL), rt
b
c
(∼25 °C), 4−8 h in a sealed Schlenk tube. Isolated yield. No
d
e
addition of copper catalyst. No addition of photocatalyst. No light.
NR = No reaction. CFL = compact fluorescent light.
aromatic alkynes were effective substrates (see 3a−l). Sub-
sequently, N-(acetoxy)phthalimides containing different amino
acid units were investigated, and the substrates with aliphatic
amino acid side chains (see 3m−s) provided yields higher than
those with a phenyl side chain (see 3t−v). We investigated
diverse substrates with different N-substituents on the amino
acids, and they all gave the corresponding propargylamines in
good to excellent yields (see 3w−ad). The decarboxylative
alkynylation by merging photoredox with copper catalysis
exhibited some functional group tolerance, including ethers
(see 3d, 3i, 3p, 3v, 3z, 3aa, and 3ac), C−F bond (see 3e), C−Cl
bonds (see 3f, 3g, 3o, 3s, and 3ad), ester (see 3j), and amides
(see 3k and 3l).
As shown in Scheme 1, we attempted a gram-scale synthesis of
compound 3d under the standard conditions. Reaction of 1a
with 4-ethynylanisole 2d provided the target product 3d in a high
yield (86%). Therefore, the present method is very effective for
decarboxylative alkynylation of α-amino acid derivatives with
terminal alkynes.
a
Reaction conditions: irradiation of visible light with 40 W CFL and
argon atmosphere, N-(acetoxy)phthalimide of α-amino acid (1) (0.6
mmol), alkyne (2) (0.3 mmol), [Ru(bpy)3Cl2] (3 μmol), CuI (0.03
mmol), CH2Cl2 (2.0 mL), rt (∼25 °C), 6−24 h in a sealed Schlenk
b
tube. Isolated yield.
Scheme 1. Synthesis of Compound 3d on Gram Scale by
Merging Photoredox with Copper Catalysis under Standard
Conditions
To explore the mechanism for the visible-light photoredox
decarboxylative alkynylation, the reaction of 1,3-dioxoisoindolin-
2-yl 2-(diethylamino)-3-methylbutanoate (1a) with phenyl-
acetylene (2a) was performed in the presence of 2 equiv of
2,2,6,6-tetramethyl-1-piperidinyloxy as the radical-trapping
agent under the standard conditions, and no reaction occurred,
which demonstrated that the reaction could undergo a free-
B
DOI: 10.1021/acs.orglett.6b03888
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
radical intermediate process. We investigated the types of the
radicals produced during the reaction by electron spin resonance
(ESR). As shown in Figure 1a, a solution of active ester 1a (100
two groups of radical signals were observed; they were from
adducts of a carbon or nitrogen radical with DMPO
(corresponding to I and IV, respectively, in Scheme 2). A sextet
peak signal (marked with the stars) with a g value of 2.003, AN =
1.44 mT, AH = 2.11 mT was assigned to be from an adduct of a
carbon radical with DMPO.21
A plausible mechanism for the decarboxylative alkynylation is
proposed in Scheme 2 according to the results above and the
previous research.7,13−19 Irradiation of Ru(bpy)3 with visible
2+
light gives the excited-state [Ru(bpy)32+]*, and the photoexcited
catalyst was reduced by the tertiary amine in 1 to provide
Ru(bpy)3+ and radical cation I.22 A single electron transfer from
Ru(bpy)3+ to phthalimide in another 1 leads to radical anion II,
regenerating catalyst Ru(bpy)32+, with subsequent cleavage of
phthalimide anion III and carbon dioxide from II to provide
carbon radical IV. IV is then oxidized by an excited catalyst
[Ru(bpy)32+]* to regenerate the Ru(bpy)3+ state and deliver an
iminium ion V.23 Meanwhile, treatment of terminal alkyne with
CuI in the presence of III yields VII, freeing phthalimide VI and
I−, and nucleophilic attack of VII to V affords the desired target
product 3,7,23 liberating the copper catalyst.
In summary, we have developed a novel and efficient
decarboxylative alkynylation of α-amino acid derivatives with
terminal alkynes by merging photoredox with copper catalysis at
room temperature. Valuable propargylamines were prepared in
good to excellent yields with tolerance of various functional
groups. The method exhibited some advantages, including
readily available α-amino acid derivatives and terminal alkynes as
the staring materials, a simple protocol, mild reaction conditions,
high efficiency, and no addition of extra base and additive. The
present method provides a convenient and practical strategy for
dual catalysis by merging photoredox with transition-metal
catalysis, and we believe that it will find wide applications in
organic synthesis.
Figure 1. Investigations of mechanism for the decarboxylative
alkynylation by ESR. ESR spectra of the mixtures under different
conditions: (a) Capillary with solution of active ester 1a (100 mM),
Ru(bpy)3Cl2 (1 mM), CuI (10 mM), DMPO (100 mM), and CH2Cl2
(200 μL) was irradiated under Ar for 1 min and then detected by ESR.
(b) Capillary with solution of active ester 1a (50 mM), Ru(bpy)3Cl2 (1
mM), and CuI (10 mM) in CH2Cl2 (100 μL) was irradiated under Ar for
5 min, and then DMPO (100 mM) in DCM (100 μL) was injected with
a syringe, and the resulting solution was detected by ESR.
mM), Ru(bpy)3Cl2 (1 mM), CuI (10 mM), 5,5-dimethyl-1-
pyrroline N-oxide (DMPO) (100 mM), and CH2Cl2 (200 μL)
was bubbled with Ar for 5 min, and a small amount of solution
was transferred into a capillary. The capillary was irradiated with a
40 W CFL bulb for 1 min and analyzed by ESR. A quartet peak
signal (marked with triangles) was observed with a g value of
2.003, AN = 1.33 mT, AH = 1.21 mT, and it was presumed to be
from an adduct of a nitrogen radical (corresponding to the
nitrogen radical of I in Scheme 2) with DMPO.20,21 As shown in
Figure 1b, a capillary with a solution of active ester 1a (50 mM),
Ru(bpy)3Cl2 (1 mM), and CuI (10 mM) in CH2Cl2 (100 μL)
was irradiated with a 40 W CFL bulb for 5 min under Ar
atmosphere, and then DMPO (100 mM) in CH2Cl2 (100 μL)
was injected into the solution with a syringe. The resulting
mixture was transferred into a capillary and analyzed by ESR, and
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b03888.
Scheme 2. Plausible Mechanism for the Decarboxylative
Alkynylation by Merging Photoredox with Copper Catalysis
Experimental details, NMR data (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: fuhua@mail.tsinghua.edu.cn.
ORCID
Hua Fu: 0000-0001-7250-0053
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the National Natural Science Foundation of
China (Grant No. 21372139) for financial support.
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