Thiophenol-Catalyzed Visible-Light Photoredox Decarboxylative Couplings of N-(Acetoxy)phthalimides

Article abstract of DOI:10.1021/acs.orglett.6b03300

We have developed visible-light photoredox decarboxylative couplings of N-(acetoxy)phthalimides without an added photocatalyst in which simple and commercially available thiophenols are used as the effective organocatalysts, and 4-(trifluoromethyl)thiophenol shows optimal catalytic activity. Three representative decarboxylative examples were chosen including one amination and two C-C bond couplings to confirm efficacy of the visible-light photoredox reactions, and the results exhibited that they performed very well at room temperature. The interesting discovery should provide a novel and environmentally friendly strategy for visible-light photoredox transformation of organic molecules.

Full text of DOI:10.1021/acs.orglett.6b03300

Letter
pubs.acs.org/OrgLett
Thiophenol-Catalyzed Visible-Light Photoredox Decarboxylative
Couplings of N‑(Acetoxy)phthalimides
Yunhe Jin, Haijun Yang, and Hua Fu*
Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology (Ministry of Education), Department of Chemistry,
Tsinghua University, Beijing 100084, P. R. China
S
* Supporting Information
ABSTRACT: We have developed visible-light photoredox
decarboxylative couplings of N-(acetoxy)phthalimides without
an added photocatalyst in which simple and commercially
available thiophenols are used as the effective organocatalysts,
and 4-(trifluoromethyl)thiophenol shows optimal catalytic
activity. Three representative decarboxylative examples were
chosen including one amination and two C−C bond couplings
to confirm efficacy of the visible-light photoredox reactions,
and the results exhibited that they performed very well at room
temperature. The interesting discovery should provide a novel
and environmentally friendly strategy for visible-light photo-
redox transformation of organic molecules.
he acquirement of energy from visible light is a highly
economical and environmentally friendly strategy of
some interesting visible-light photoredox organic reactions.13
More importantly, a visible-light photoredox decarboxylative
arylthiation of N-(acetoxy)phthalimides has been well estab-
lished in the absence of an added photocatalyst.14 As our
continuing study on the visible-light photoredox catalysis, we
report thiophenol-catalyzed visible-light photoredox decarbox-
ylative couplings of N-(acetoxy)phthalimides at rt, in which
three representative decarboxylative examples including an
intramolecular amination and two intermolecular C−C bond
couplings are described without an added photocatalyst.
Initially, intramolecular visible-light photoredox decarbox-
ylative amination of Boc-Val-OPht (Pht = phthalimide) (1d)
leading to 2d was used as the model to optimize conditions
including catalysts, bases, and solvents (Table S1, Supporting
Information (SI)). The results showed that the optimal
photoredox conditions are as follows: 10 mol % 4-(trifluoro-
methyl)thiophenol (TFTP) (G) as the organocatalyst, Cs2CO3
as the base, and DMF as the solvent under an Ar atmosphere
and irradiation of visible light with 40 W compact fluorescent
light (CFL). Subsequently, we investigated the substrate scope
on the decarboxylative amination of various N-protected amino
acid active esters (1). As shown in Table 1, active esters of eight
neutral amino acids (Boc-AA-OPht, AA = amino acid) including
natural and unnatural amino acids provided satisfactory yields
(see 2a−h). N,O-Protected amino acid active esters (Boc-
Ser(OtBu)-OPht and Boc-Thr(OtBu)-OPht) with hydroxyl on
the side chains were tested, and the corresponding products 2i
and 2j were obtained in 83% and 95% yields, respectively. Boc-
Met-OPht afforded target product 2k in 71% yield. Boc-
T
promoting chemical transformations. A century ago, Ciamician
had already realized organic reactions irradiated with visible
light.1 However, the fact that most common organic molecules
can not absorb light of visible wavelengths seriously limits the
development of photochemical processes. Fortunately, various
sensitizers including photocatalysts can absorb photons in the
visible range to form excited species capable of activating
organic substrates, and the efficiency of reactions usually
depends on the photocatalyst systems.2 In 2008 and 2009, the
seminal works by MacMillan,3 Yoon,4 and Stephenson5
demonstrated the tremendous potential of photoredox catalysis
in organic synthesis. Since then, the visible-light photoredox
catalysis has become a powerful methodology with develop-
ment of diverse and efficient photocatalysts, and various novel
and useful reactions were disclosed under very mild
conditions.2−5 The previous photocatalysts mainly include
both transition-metal complexes6 such as ruthenium or iridium
polypyridyl complexes and organic dyes.7 Aside from classical
visible-light-mediated photoredox catalysis, some novel trans-
formations have recently been reported with dual catalytic
systems8 merging photoredox catalysis with other catalyses
including organocatalysis9 and transition-metal catalysis,10 but
the reactions do not proceed using either catalyst in isolation.
However, it is a great challenge to develop visible-light
photoredox reactions without an added photocatalyst thus far.
On the other hand, carboxylic acids are abundant and
inexpensive biomass-derived platform molecules, and their
conversion into high-value products such as medicinally related
molecules and biofuels represents an important goal.11
Recently, some efficient decarboxylative couplings have been
developed under photoredox catalysis.12 We have also reported
Received: November 3, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b03300
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Organic Letters
Letter
visible-light photoredox decarboxylicative couplings. Sunlight is
a nearly inexhaustible source of clean energy, and its use in
sustainable organic synthesis has gained considerable attention.
We attempted decarboxylicative amination of 1d irradiated with
sunlight, and the result showed that the reactivity was similar to
that irradiated with visible light (Scheme 1b).
Table 1. Intramolecular Visible-Light Photoredox
Decarboxylative Amination of N-Protected-AA-OPht (1)
a
under Catalysis of TFTP (G)
Next, photoredox decarboxylative coupling of carboxylic acid
active esters (R2COOPht) (1) with various olefins (3)
including α,β-unsaturated amides, ketones, and esters was
investigated in the presence of TFTP (G) as the organocatalyst
(Table 2). For active esters of common organic carboxylic acids,
Table 2. Visible-Light Photoredox Decarboxylative Coupling
of Active Esters R2CO-OPht (1) with Alkenes (3) under
a
Catalysis of TFTP (G)
a
Reaction conditions: Ar atmosphere and irradiation of visible light, N-
protected AA-OPht (1) (0.15 mmol), TFTP (G) (15 μmol), Cs2CO3
(0.075 mmol), DMF (1.5 mL), temperature (rt, ∼25 °C), time (4−10
b
h) in a sealed Schlenk tube. Isolated yield. CFL = compact
t
fluorescent light. Boc = tert-butyloxycarbonyl. Bu = tert-butyl. Cbz =
benzyloxycarbonyl.
Lys(Boc)-OPht displayed high reactivity (2l). Boc-glutamic acid
and glutamine active esters afforded 2m and 2n in 67% and 52%
yields, respectively. Another N-protective group, benzyloxy-
carbonyl (Cbz), was attempted, and Cbz-Val-OPht showed
similar reactivity to Boc-Val-OPht (2d and 2o). The
decarboxylicative amination exhibited tolerance of some
functional groups including amides, ethers (2i and 2j), thioether
(2k), and ester (2m). In previous research, arylthiolation of N-
(acetoxy)phthalimides without NH at the β-position of ester
was found.14 Here, we attempted the reaction of Boc-Ala-OPht
(1a) with 4-(trifluoromethyl)thiophenol (TFTP) (1.2 equiv)
under standard conditions, but no arylthiolation product was
observed. One key reason is that the present substrate (1a)
contains β-NH, and its arylthiolation product can further react
with phthalimide to form 2a.
As shown in Scheme 1a, intramolecular decarboxylicative
coupling of 1d (1.08 g) on gram scale was performed under the
standard conditions. Interestingly, the reaction provided the
target product (2d) in a high yield (0.882 g, 92%) with a small
amount (2.2%) of 1,2-bis(4-(trifluoromethyl)phenyl)disulfane
(BTFPD) as the byproduct appearing from the dimerization of
TFTP.15 Therefore, the present method is very effective for
a
Reaction conditions: Ar atmosphere and irradiation of visible light,
R2CO−OPht (1) (0.225 mmol), alkene (3) (0.15 mmol), TFTP (G)
(15 μmol), Cs2CO3 (0.075 mmol), Hantzsch ester (HE) (0.225
mmol), DMF (1.5 mL), temperature (rt, ∼ 25 °C), time (3−9 h) in a
b
sealed Schlenk tube. Isolated yield. Boc = tert-butyloxycarbonyl. tBu =
n
tert-butyl. Bu = normal-butyl.
secondary and tertiary carboxylic acid derivatives showed higher
reactivity than primary ones (compare 4a−k). Lithocholic acid
derivatives show diverse pharmaceutical activity.16 However,
modification of lithocholic acid through C−C bond formation
via a decarboxylative process is difficult by using a classical
synthetic method. Herein, photoredox decarboxylative coupling
of lithocholic acid active ester with N-phenylacrylamide was
performed well, and the corresponding product (4l) was
obtained in 62% yield. For α,β-unsaturated amides with aryl, the
substrates containing electron-donating groups on the aryl rings
afforded higher yields than those containing electron-with-
drawing groups (4c−i). Various N-protected amino acid active
esters were also used as the radical sources (4m−r), and
different acceptors, α,β-unsaturated amides (4c−i and 4s),
ketones (4t and 4u), and esters (see 4v and 4w), were suitable.
Furthermore, we tested decarboxylative coupling of carbox-
ylic acid active esters (1) with substituted 2-isocyanobiphenyls
Scheme 1. (a) Gram Scale Preparation of 2d under the
Standard Conditions; (b) Decarboxylation of 1d Irradiated
with Sunlight
B
DOI: 10.1021/acs.orglett.6b03300
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Organic Letters
Letter
(5) in the presence of organocatalyst TFTP (G). As shown in
Table 3, the reactivity of eight different substituted 2-
Therefore, a plausible mechanism is proposed in Scheme 2
according to the results above and our previuos investigations.14
Table 3. Visible-Light Photoredox Decarboxylative Coupling
of Active Esters R2CO-OPht (1) with Substituted 2-
Isocyanobiphenyls (5) under Catalysis of TFTP (G)
Scheme 2. Possible Mechanism for the Three Visible-Light
Photoredox Decarboxylative Couplings
a
a
Reaction conditions: Ar atmosphere and irradiation of visible light,
R2CO−OPht (1) (0.375 mmol), substituted 2-isocyanobiphenyl (5)
(0.15 mmol), TFTP (G) (15 μmol), Cs2CO3 (0.075 mmol), DMF
(1.5 mL), temperature (rt, ∼25 °C), time (4−8 h) in a sealed Schlenk
Here, Boc-Val-OPht (1d) was chosen as the example to explain
the mechanism. Treatment of arylthiol with a base (Cs2CO3)
affords ArSCs and CsHCO3. There are two resonance
structures of 1d and A-1 in the solution (Note: the charge
transfer occurs from N to O in N-(acetoxy)phthalimide to form
A-1 with assistance of Cs2CO3, but no evidence was found for
metal to ligand charge transfer between Cs+ and the N-acetoxy-
phthalimide to date),19 and complexation of A-1 with Cs2CO3
affords A-2. Irradiation of A-2 as a photosensitizer with visible
light provides the excited-state A-3,14 and an electron in the
ArS− anion transfers to the phthalimide group of A-3 to form
two radicals ArS• and A-4 (Note: the control experiment in
Scheme S2a(B) showed that the reaction provided a low yield
without thiophenol, so the procedure of a single electron
transfer from the ArS− anion to phthalimide in A-3 to form A-4
is a key step). Decarboxylation of A-4 provides A-5 and radical
A-6 freeing CO2, and reaction of A-5, radicals A-6 and ArS•
yields imine A-7 and phthalimide A-8 regenerating catalyst
anion ArS−. Finally, nucleophilic addition of A-7 to A-8 affords
the target product (2) (demonstrated by Scheme S2b). For the
formation of product 4, reaction of ArS• with Hantzsch ester
(HE) donates A-9 regenerating catalyst anion ArS−. Meanwhile,
Michael addition of A-6 to α,β-unsaturated alkene (3) leads to
radical A-10, and treatment of A-10 with A-9 affords the target
product (4) leaving a pyridine derivative A-11. For the
formation of product 6, addition of A-6 to substituted 2-
isocyanobiphenyl (5) forms radical A-12, and intramolecular
cyclization of A-12 gives A-13. Treatment of radical ArS• with
A-13 produces cation A-14 regenerating catalyst anion ArS−.
Finally, reaction of A-14 with A-5 donates the target product
(6) freeing phthalimide (A-8).
b
t
tube. Isolated yield. Boc = tert-butyloxycarbonyl. Bu = tert-butyl.
isocyanobiphenyls (5) was first investigated using Boc-Val-
Opht (1d) as the partner, and the reactions provided the
corresponding phenanthridines in 64−94% yields (6a−h). The
tested carboxylic acid active esters include various derivatives of
N-protected amino acids (6a−m) and common carboxylic acids
(see 6n−q). An active ester containing alkene provided a lower
yield (6q), and other substrates displayed higher reactivity (6a−
p). An active ester of pentapeptide Boc-Gly-Gly-Gly-Gly-Met-
OPht was also used in the photoredox reaction. Interestingly,
the conjugate (6r) containing peptide and phenanthridine was
obtained in 58% yield. The visible-light photoredox decarbox-
ylicative coupling exhibited tolerance of some functional groups
including amides, ethers (6c and 6j), C−Cl bond (6d, 6e, and
6h), CF3 (6f), ester (6l), and thioether (6r). Phenanthridines
widely occur in a variety of natural alkaloids17 and show diverse
biological and pharmaceutical activities.18 The present method
affords an efficient and practical protocol for synthesis of diverse
phenanthridines.
In our previous research, we explored the mechanism on the
visible-light photoredox decarboxylative arylthiation of N-
(acetoxy)phthalimides in the presence of Cs2CO3.14 Similarly,
to determine the mechanism in the present decarboxylative
couplings, we surveyed UV−visible absorption spectra of
cyclohexanecarboxylic acid active ester (AE), Hantzsch ester
(HE), 4-(trifluoromethyl)thiophenol (TFTP), and 1,2-bis(4-
(trifluoromethyl)phenyl)disulfane (BTFPD) in the absence or
presence of Cs2CO3 and performed the corresponding Stern−
Volmer fluorescence quenching and radical-trapping experi-
ments (Figures S2−S6 and Scheme S1, SI). In addition, some
control experiments were performed (Scheme 2S, SI).
In summary, we have developed thiophenol-catalyzed visible-
light photoredox decarboxylative couplings of N-(acetoxy)-
C
DOI: 10.1021/acs.orglett.6b03300
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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phthalimides in which simple and commercially available
thiophenols are used as the effective organocatalysts, and 4-
(trifluoromethyl)thiophenol shows optimal catalytic activity.
Three representative decarboxylative examples including one
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couplings performed well at room temperature with excellent
tolerance of functional groups.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b03300.
Reaction optimization, synthetic procedures, character-
1
ization data, and H, 13C NMR spectra (PDF)
(11) (a) Gallezot, P. Chem. Soc. Rev. 2012, 41, 1538. (b) Straathof, A.
J. J. Chem. Rev. 2014, 114, 1871.
(12) (a) Teply, F. Collect. Czech. Chem. Commun. 2011, 76, 859.
́
AUTHOR INFORMATION
(b) Okada, K.; Okamoto, K.; Oda, M. J. Am. Chem. Soc. 1988, 110,
8736. (c) Okada, K.; Okamoto, K.; Oda, M. J. Chem. Soc., Chem.
Commun. 1989, 1636. (d) Okada, K.; Okamoto, K.; Morita, N.; Okubo,
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K.; Morita, N.; Oda, M. Chem. Lett. 1993, 22, 2021. (f) Okada, K.;
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■
Corresponding Author
*E-mail: fuhua@mail.tsinghua.edu.cn.
ORCID
Hua Fu: 0000-0001-7250-0053
Notes
̀
(g) Cano, M.; Fabrias, G.; Camps, F.; Joglar, J. Tetrahedron Lett. 1998,
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors would like to thank Dr. Haifang Li in this
department for her great help in analysis of high resolution mass
spectrometry, and the National Natural Science Foundation of
China (Grant No. 21372139) for financial support.
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