Green Photo-Organocatalytic C?H Activation of Aldehydes: Selective Hydroacylation of Electron-Deficient Alkenes

Article abstract of DOI:10.1002/chem.201705634

Selective C?H activation is an area of growing importance. Metal-free C?H activation of branched aldehydes mediating the hydroacylation of electron-deficient alkenes is an attractive transformation, but is limited by selectivity issues, especially in the case of α,α-disubstituted aldehydes. Herein, we report a green, cheap, versatile, and easily reproducible selective hydroacylation of alkenes utilizing phenylglyoxylic acid as the photocatalyst and common household bulbs for irradiation, leading to products in excellent yields and selectivities. The reaction mechanism was also studied to account for the high selectivity.

Full text of DOI:10.1002/chem.201705634

A Journal of
Accepted Article
Title: Green PhotoOrganocatalytic C-H Activation of Aldehydes:
Selective Hydroacylation of Electron Deficient Alkenes
Authors: Giorgos N. Papadopoulos, Errika Voutyritsa, Nikolaos
Kaplaneris, and Christoforos G. Kokotos
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To be cited as: Chem. Eur. J. 10.1002/chem.201705634
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10.1002/chem.201705634
Chemistry - A European Journal
DOI: 10.1002/chem.201xxxxxx
█ PhotoOrganocatalysis
Green PhotoOrganocatalytic C-H Activation of Aldehydes:
Selective Hydroacylation of Electron Deficient Alkenes
Giorgos N. Papadopoulos,[a] Errika Voutyritsa,[a] Nikolaos Kaplaneris[a] and Christoforos G. Kokotos*[a]
Abstract: Selective C-H activation is an area of growing selective hydroacylation of alkenes utilizing phenylglyoxylic
importance. Metal-free C-H activation of branched aldehydes acid as the photocatalyst and common household bulbs for
mediating the hydroacylation of electron deficient alkenes is irradiation leading to products in excellent yields and
an attractive transformation, limited by selectivity issues, selectivities. The reaction mechanism was also studied in
especially in the case of α,α-disubstituted aldehydes. Herein, order
to
account
for
the
high
selectivity.
we report a green, cheap versatile and easy reproducible
Introduction
During the last decade, photoredox catalysis has emerged
as a viable solution in longstanding problems.[1] In general,
transition metal catalysts are employed, having the advantage of
tuning their electronic properties via ligand manipulation.
PhotoOrganocatalysis constitutes a cheaper alternative.[2] We
have recently developed a photoorganocatalytic protocol that is
easy to operate employing cheap household lamps and a cheap,
organic and commercially available molecule as the catalyst.[3] To
explore the limits of this methodology and expand its potential, it
was necessary to identify a problematic photochemical reaction
that our mechanistic scenario could provide a solution and
elucidate its activation mode.
A practical strategy for the construction of highly desirable
C-C bonds is the hydroacylation of electron deficient olefins
(Scheme 1). Usually, metal-catalyzed processes[4] or the Stetter
reaction (N-heterocyclic carbenes, NHCs)[5] have been proposed
for this process (Scheme 1, A). Alternatively, a direct and metal-
free acyl radical generation from the aldehyde could provide a
desirable route. Indeed, several approaches based on
peroxides/UV light, photogeneration of radicals, aerobic,
autooxidation and photoinitiation have been proposed (Scheme 1,
B).[6-8] Although these methodologies are excellent for linear
Scheme 1. Approaches for the hydroacylation of electron
aldehydes, the use of branched aldehydes suffers from poor
selectivity. In the latter case, acyl radicals from branched
aldehydes undergo rapid decarbonylation to produce stable and
nucleophilic alkyl radicals, which react with the electron deficient
deficient olefins.
alkenes leading to alkylated byproducts. Thus, only limited
reports for the hydroacylation of branched aldehydes exist.
Fagnoni employed tetrabutylammonium decatungstate
(TBADT) as an effective photocatalyst, which under UV irradiation
led to significant decarbonylation (low selectivity), that was
[a]
Dr. G. N. Papadopoulos, E. Voutyritsa, N. Kaplaneris, Prof. Dr. C. G.
Kokotos
partially solved by lowering the reaction temperature (<-20 C).[7]
o
Laboratory of Organic Chemistry, Department of Chemistry,
National and Kapodistrian University of Athens, Panepistimiopolis
15771, Athens, Greece.
Caddick employed an aldehyde autooxidation methodology,
which again led to mixture of products for branched aldehydes
and prolonged reaction times.[8] The best solution was given by
E-mail: ckokotos@chem.uoa.gr
Maruoka employing
a
hypervalent iodine compound as
a
Supporting information for this article is given via a link at the end of
the document.
1
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10.1002/chem.201705634
Chemistry - A European Journal
DOI: 10.1002/chem.201xxxxxx
photoinitiator.[9] Maruoka identified the optimum hypervalent
iodine compound to promote this reaction with excellent
selectivity towards the acyl radical,[9a] studied the reaction
mechanism[9b] and provided
a
diastereoselective method.[9c]
Unfortunately, the catalyst is not commercially available.
Results and Discussion
We questioned whether our photoorganocatalyst could
provide a solution (Scheme 2). We initially investigated the
reaction between 1a and diethyl maleate. A variety of known
photoinitiators (5a-c) revealed the problematic nature of the
reaction, leading to moderate yields and low selectivities (3a:4a).
Phenylglyoxylic acid (5d) outperformed all other compounds
providing 3a in almost quantitative yield and in excellent
selectivity in short reaction time (4 h). It is quite evident the high
selectivity obtained in this difficult reaction. Further optimization
studies proved that the reaction can be performed in water and
can be driven by sunlight (21 September 2016, 10:00-14:00,
Athens, Greece, 37.97° N, 23.72° E).[10] In the absence of either
the catalyst or light, the reaction did not proceed.
Scheme 3. Substrate scope of the photocatalytic hydroacylation
of 2a.
Scheme 2. Optimization of the photocatalytic synthesis of 3a.
Having established the optimum reaction conditions, we
turned our attention to the exploration of the substrate scope.
First, a series of aldehydes were tested (Scheme 3). Branched
aldehydes led to similarly high yields and excellent selectivities
(3a-g). Linear aliphatic aldehydes led to a single product in good
to excellent yields, while various functionalities were well
tolerated (3h-n). Maruoka never reported the use of aromatic
aldehydes.[9] Our protocol provides an excellent solution to this
end, since moderate yields can be obtained (3o, 3p).
Next, we turned our attention to the alkene (Scheme 4).
Other aliphatic esters were used successfully (6a-c). One of the
main disadvantages of the existing methodologies involve low-
yielding reactions in the presence of other weak C-H bonds
[benzylic (BDE 89.8 kcal/mol) or α-ether fragments (BDE 92.0
kcal/mol)]. Our protocol afforded products 6d-i in high yields. A
current limitation constitutes the use of amides of maleic acid that
are unreactive, while the use of methyl acrylate, methyl crotonate
and N-phenyl maleimide leads to polymerization. When dibenzyl
fumarate was utilized, product 6f was isolated in slightly lower
yield than starting from dibenzyl maleate. Other electron deficient
olefins led to moderate yields (6j-n).
Unlike common photosensitizers, phenylglyoxylic acid must
be promoting the reaction via a different reaction mechanism in
order to account for the difference in the selectivity. Thus, the
reaction mechanism was studied, starting from fluorescence
Scheme 4. Substrate scope of the photocatalytic hydroacylation
of olefins.
2
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10.1002/chem.201705634
Chemistry - A European Journal
DOI: 10.1002/chem.201xxxxxx
(EDA) complex. EDA complexes are known in literature since the
1950s,[11,12] but only recently Melchiorre and others have identified
them as active species in photochemical reactions.[13,14] An EDA
complex sometimes is recognized, when upon addition of the two
components, an increase in the UV absorbance of the mixture is
observed.[13] In an effort to understand the use of phenylglyoxylic
acid as the photocatalyst, we performed a number of UV-Vis and
NMR studies. Upon mixing phenylglyoxylic acid and diethyl
maleate in Petroleum Ether (P. E.) or H2O (best solvents for our
protocol), no increase in the UV absorbance was observed,
excluding the possibility of an EDA complex formation (Figure 1,
A). Irradiation of diethyl maleate did not lead to any changes.
Irradiation of phenylglyoxylic acid in H2O led to an increase in the
absorption in the range of 310-390 nm (Figure 1, B). Noteworthy,
irradiation of phenylglyoxylic acid with diethyl maleate in H2O
showed the same increase in the absorption (310-390 nm, Figure
1, C).[10] When a solution of PhCOCOOH, diethyl maleate and
heptanal in H2O (reaction mixture) was irradiated, this increase in
the absorbance in the UV-Vis-spectra is occurring, however with
a lower rate (Figure 1, D). An exciplex is an excited state complex,
“non-bonding” in the ground state, that cooperatively emits a
photon.[1f,12] It is known that phenylglyoxylic acid in water is
A
B
photodecomposed
leading
to
benzaldehyde.[15]
The
photodecomposition is fast and the increase in the absorbance in
the solution of PhCOCOOH or PhCOCOOH with diethyl maleate
(Figure 1, B and C) can be attributed to the UV-Vis properties of
the products of the photodecomposition. However, it seems that
an exciplex between PhCOCOOH and diethyl maleate is
formed.[3e] When a compound that triggers a hydrogen atom
transfer (HAT) (heptanal) is present, this photodecomposition is
slower (Figure 1, D).
C
The reaction was also probed by the use of NMR
spectroscopy (Figure 2). 13C NMR spectra of either diethyl
maleate or phenylglyoxylic acid exhibited no differences
before and after irradiation.[10] The irradiation was performed
in water, extracted, dried and NMR experiments were
performed in CDCl3. In the case of phenylglyoxylic acid, after
irradiation, benzaldehyde is formed, which exists in the
D
solution with PhCOCOOH (1H and 13C-NMR) (Figure 2,
and
).[10] The mixture of phenylglyoxylic acid and diethyl
A
B
maleate presented all expected signals. After irradiation of
the reaction mixture for 2 h, additional signals are present
that can be attributed to benzaldehyde (193 ppm).[10] As
observed in the UV-Vis experiments, 1H and 13C-NMR
confirm that in the case of the reaction mixture
(PhCOCOOH, diethyl maleate and heptanal in H2O),
benzaldehyde is present, but it forms in a lesser extent than
Figure 1
) and diethyl maleate (0.25
absorption of phenylgyoxylic acid (10-2
after consecutive irradiation. C. UV-Vis absorption of
phenylgyoxylic acid (10-2
) and diethyl maleate (0.25 ),
recorded in H2O after consecutive irradiation. D. UV-Vis
absorption of phenylgyoxylic acid (10-2
), diethyl maleate
(0.25 ) and heptanal (0.5 ) in H2O after consecutive
irradiation.
.
A. UV-Vis absorption of phenylgyoxylic acid (10-2
) in Pet. Ether. B. UV-Vis
), recorded in H2O
M
M
M
M
M
when the HAT donor is absent (Figure 2,
C
).[10] These two
M
M
M
facts hint that phenylglyoxylic acid, by itself, is incapable of
initiating the HAT process and has to form the exciplex with
diethyl maleate to do so.
quenching studies.[10] No fluorescence quenching was observed
with heptanal, which states that the acyl radical is not generated
directly by the excited phenylglyoxylic acid, as it occurs in all
other literature precedents employing photoinitiators, while
quenching is observed with esters of maleic acid. Both diethyl
maleate and fumarate quench phenylglyoxylic acid.
Next UV-Vis experiments were performed. The association
of an electron rich molecule with an electron acceptor can lead to
the formation of an aggregate, called electron donor-acceptor
In a recent report, Yoon employed quantum yield
measurement as a mechanistic tool.[16] Using potassium
ferrioxalate as the actinometer,[16,17] we calculated the
quantum yield of the reaction in various wavelengths and in
all cases, quantum yield values were above 1, indicating a
chain propagation mechanism.[10] The low quantum yield
above 400 nm hints that irradiation below 400 nm is
productive for the reaction. This was confirmed with a cut-off
experiment below 400 nm.[10]
3
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10.1002/chem.201705634
Chemistry - A European Journal
DOI: 10.1002/chem.201xxxxxx
as well as the new peaks in 1H and 13C-NMR after irradiation,
can be attributed to benzaldehyde.[15] Fluorescence
quenching experiments verifies the fact that excited
phenylglyoxylic acid cannot initiate the HAT process by itself.
This must be the reason accounting for the difference in the
selectivity that phenylglyoxylic acid is exhibiting versus
normal photoinitiators. Thus, exciplex formation between
phenylglyoxylic acid and dialkyl maleate is necessary for the
reaction to occur. Addition of the initial acyl radical from
aldehyde to the dialkyl maleate affords a new radical
species, which via a radical propagation mechanism affords
the desired product. The differences observed in UV-Vis
experiments, depending on the solvent employed, can be
attributed to how easily the exciplex performs the HAT
process (with the solvent, when aldehyde is present or not)
and how fast decomposition is occurring and to which
product.
A.
B.
C.
Scheme 5. Proposed reaction mechanism.
Conclusion
In conclusion, mechanistic studies reveal that phenylglyoxylic
acid forms an exciplex with diesters of maleic acid upon
irradiation. This exciplex in the presence of a HAT donor can
outperform the inherent photodecomposition of phenylglyoxylic
acid. This can be employed in introducing a simple, cheap and
efficient photoorganocatalytic protocol for the C-H activation of
aldehydes and their reaction with electron deficient alkenes
solving the previously reported issues with the selectivity of this
transformation. This method relies on a small organic molecule
and cheap household lamps or sunlight that can provide an
excellent solution to a longstanding problem. This new exciplex-
based mechanism constitutes a new catalysis concept, which can
open new avenues in photocatalysis and is currently explored in
other reactions.
Figure 2. A. 13C NMR spectrum of PhCOCOOH in CDCl3. B.
13C NMR spectrum of PhCOCOOH in CDCl3 after irradiation
for 2 h. C. 13C NMR spectrum of a mixture of PhCOCOOH,
diethyl maleate and heptanal in CDCl3 after irradiation for 2
h.
Based on the above experiments, a proposed reaction
mechanism is depicted in Scheme 5. Upon irradiation, the
excited phenylglyoxylic acid forms an exciplex with dialkyl
maleate via a photoinduced electron transfer (PET). This
exciplex is probably a radical ion pair, as depicted in Scheme
5, resembling the exciplex formed between benzophenone
and alkyl amines.[1f,18] This exciplex seems to be in
equilibrium with another ion pair between PhCOCOOH and
water.[15b] When a HAT donor (aldehyde) is not present,
Experimental Section
photodecomposition
is
occurring
fast
leading
to
benzaldehyde. In the presence of a HAT donor (aldehyde), a
HAT process with the aldehyde affords the initial acyl radical
and a radical of PhCOCOOH that slowly decomposes to
benzaldehyde. The increase in the absorbance in Figure 1,
General Procedure: In a glass vial with a screw cap containing phenylglyoxilic
acid (15 mg, 0.10 mmol) in H2O or P.E. (1 mL), alkene (0.50 mmol) and
4
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10.1002/chem.201705634
Chemistry - A European Journal
DOI: 10.1002/chem.201xxxxxx
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aldehyde (1.00 mmol) were added consecutively. The vial was sealed with a
screw cap and left stirring under household bulb irradiation (2 x 15W household
lamps) for 4-20 h. After completion of the reaction, the product was isolated by
flash silica chromatography of the crude mixture.
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The authors gratefully acknowledge the Latsis Foundation
for financial support through the programme “ΕΠΙΣΤΗΜΟΝΙΚΕΣ
ΜΕΛΕΤΕΣ 2015” (PhotoOrganocatalysis: Development of new
environmentally-friendly methods for the synthesis of compounds
for the pharmaceutical and chemical industry). The authors would
also like to thank Dr. Maroula Kokotou for her assistance in
acquiring HRMS data and Prof. V. Constantinou from the
Agricultural University of Athens for access to the fluorescence
spectrometer. E.V. would like to thank the National Scholarship
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Keywords: Hydroacylation • Photoorganocatalysis • Green
chemistry • Exciplex • C-H activation
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DOI: 10.1002/chem.201xxxxxx
Entry for the Table of Contents
FULL PAPER
PhotoOrganocatalysis. An exciplex
solving the selectivity issues.
█ PhotoOrganocatalysis
G. N. Papadopoulos, E. Voutyritsa,
N. Kaplaneris, C. G. Kokotos
■■ – ■■
Green PhotoOrganocatalytic C-H
Activation of Aldehydes:
Selective Hydroacylation of
Electron Deficient Alkenes
6
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