Metal-free carbonylations by photoredox catalysis

Article abstract of DOI:10.1002/anie.201408516

The synthesis of benzoates from aryl electrophiles and carbon monoxide is a prime example of a transition-metalcatalyzed carbonylation reaction which is widely applied in research and industrial processes. Such reactions proceed in the presence of Pd or Ni catalysts, suitable ligands, and stoichiometric bases. We have developed an alternative procedure that is free of any metal, ligand, and base. The method involves a redox reaction driven by visible light and catalyzed by eosin Y which affords alkyl benzoates from arene diazonium salts, carbon monoxide, and alcohols under mild conditions. Tertiary esters can also be prepared in high yields. DFT calculations and radical trapping experiments support a catalytic photoredox pathway without the requirement for sacrificial redox partners.

Full text of DOI:10.1002/anie.201408516

Angewandte
Chemie
DOI: 10.1002/anie.201408516
Photocatalysis Hot Paper
Metal-Free Carbonylations by Photoredox Catalysis**
Michal Majek and Axel Jacobi von Wangelin*
[
7]
Abstract: The synthesis of benzoates from aryl electrophiles
and carbon monoxide is a prime example of a transition-metal-
catalyzed carbonylation reaction which is widely applied in
research and industrial processes. Such reactions proceed in the
presence of Pd or Ni catalysts, suitable ligands, and stoichio-
metric bases. We have developed an alternative procedure that
is free of any metal, ligand, and base. The method involves
a redox reaction driven by visible light and catalyzed by
eosin Y which affords alkyl benzoates from arene diazonium
salts, carbon monoxide, and alcohols under mild conditions.
Tertiary esters can also be prepared in high yields. DFT
calculations and radical trapping experiments support a cata-
lytic photoredox pathway without the requirement for sacrifi-
cial redox partners.
of a base. The first and last step can be viewed as formal
metal-centered two-electron redox reactions which result in
[
3]
an overall redox-neutral process (Scheme 1).
A
romatic esters are key building blocks in the synthesis of
fine chemicals, agrochemicals, pharmaceuticals, and materi-
Scheme 1. Pd-catalyzed versus photocatalyzed redox carbonylation.
[
1]
[2]
als. They can be prepared by various methods, most
importantly by the esterification of benzoic acids with
alcohols under Brønsted or Lewis acid catalysis at elevated
temperatures, or alternatively by the trimodular reaction of
an aromatic electrophile bearing a suitable leaving group,
gaseous carbon monoxide, and the alcohol in the presence of
transition-metal catalysts (mostly combinations of Pd or Ni
Here, we report an alternative metal-free and base-free
process which involves a hitherto unknown one-electron
redox mechanism that is driven by visible light in the presence
of an organic dye (Scheme 1, bottom). The following criteria
provided further stimuli for our explorations of such redox
carbonylations: 1) Reductive single electron transfer (SET)
processes with arene diazonium salts proceed even with mild,
nonmetallic, reducing agents because of their low redox
[
3]
complexes with phosphine ligands). Several metal-catalyzed
carbonylation reactions are being applied in industrial and
[
4]
academic syntheses of carbonyl compounds. Carbon mon-
oxide (CO) is abundantly available as a primary product from
the gasification of all carbon-based raw materials (oil, natural
gas, coal, biomass). The reaction mechanism of metal-
catalyzed carbonylations bears a close relationship to cross-
[
8,9]
potentials (ca. 0 V versus SCE).
2) The availability of low
lying s- and p-orbitals makes CO a good radical trap. 3) The
intermediate aryl radical can react with CO, while being
unreactive toward the alcohol.
[
3]
coupling reactions. Although aryl halides are the most
prominent class of electrophilic reagents in such reactions,
arene diazonium salts offer specific advantages because of
their ionic character, halogen-free preparation from anilines,
and incorporation of a very potent leaving group, dinitrogen
The utilization of visible light as an abundant source of
energy to enable chemical transformations has recently
experienced a renaissance, which is largely driven by new
[
10]
developments in the field of photoredox catalysis. Signifi-
cant effort has been devoted to visible-light-driven aromatic
substitutions of arene diazonium salts in the presence of
(
N ). Arene diazonium salts have been used in numerous
2
[5]
[11]
cross-coupling procedures, although there are only isolated
various photosensitizers. Tris(bipyridine)ruthenium(II) and
[6]
reports of carbonylations to generate benzoates. The gen-
erally accepted mechanism of metal-catalyzed carbonylations
involves reductive activation of the electrophilic aryl-X
species followed by CO insertion and nucleophilic displace-
ment by the alcohol in the presence of stoichiometric amounts
other metal complexes (Ir, Cu) have emerged as very
[
12]
powerful photocatalysts. However, the good coordinating
ability of CO ligands and the existence of numerous stable
carbonyl complexes of Ru (and other metals) discourage the
use of such organometallic catalysts for the proposed
[
13]
aromatic carbonylation process. Organic dyes, for example,
the cheap fluoresceins, display similar photocatalytic activity
[
*] Ing. M. Majek, Prof. Dr. A. Jacobi von Wangelin
Institute of Organic Chemistry, University of Regensburg
Universitaetsstrasse 31, 93040 Regensburg (Germany)
E-mail: axel.jacobi@ur.de
[14]
in some reactions
interference by the presence of carbon monoxide is known.
and seemed more appropriate as no
[15]
Furthermore, CO only exhibits absorptions in the vacuum-
[16]
UV range below 180 nm.
[
**] This work was supported by the Graduate Program “Photokatalyse”
of the DFG (GRK 1626).
Our initial studies on the proposed photocatalytic carbon-
ylation with 4-methoxybenzenediazonium tetrafluoroborate
(1a) in methanol were carried out under reaction conditions
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201408516.
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[
a]
Table 1: Selected optimization experiments.
Entry
1
2
3
4
5
6
7
Deviation from the optimized conditions
Yield of 2a [%]
–
83
8
29
24
30
2
[
[
[
[
b]
b]
b]
b]
Rose bengal
eosin B
fluorescein
Scheme 3. Compatibility with various functionalized additives that
resulted in unaffected ester formation without competing conversion
of the additive.
[Ru(bpy) ]Cl2
3
without dye
dark reaction at 608C
<1
tolerated well, which is in contrast to the employment of
phenol (in large excess) as a reaction partner under otherwise
identical conditions, where complex product mixtures were
formed. Methylthio-bearing compounds did not affect the
[
a] Conditions: 1a (0.1 mmol), eosin Y (0.04 mmol), methanol (1 mL),
CO (50 bar), irradiation(LEDs, lmax =525 nm, 3.8 W), 188C, 4 h. [b] In
methanol/acetonitrile (1/1). bpy=bypyridine.
[
19]
ester formation, in agreement with a previous report, where
S-alkylated compounds were produced under similar condi-
tions. Interestingly, tert-butylisonitrile—a competent radical
that were reported for related photoredox Meerwein reac-
[
17]
tions.
25 nm, 3.8 W), solutions of 1a in methanol were treated
under an atmosphere of CO at room temperature to give
Under irradiation with green light (LED, lmax =
[
20]
5
trap in some reactions —was tolerated. Iodobenzene only
resulted in little interference, presumably by radical cleavage
of the weak ArꢁI bond. The addition of thiophenol led to
[
18]
methyl 4-methoxybenzoate (2a, Table 1).
Commercial eosin Y (4 mol%, employed as the disodium
salt) was used as the metal-free photoredox catalyst.
Unwanted dimerization (Ar ) and reduction (Ar-H) was
suppressed at higher dilutions by the higher relative concen-
trations of CO and the alcohol. Other dyes were less active
a large decrease in the yield, which is known from reports of
diazosulfide formation/photocatalytic cleavage under similar
[
21]
conditions. Biaryl sulfide (Ar-S-Ar) coupling was observed
in this case. The presence of aniline or N,N-dimethylaniline
resulted in the quenching of the excited state of the dye.
Azobenzenes were detected in small amounts. The addition of
triphenylphosphine led to inhibition as a result of quenching
of the excited dye. Accordingly, triphenylphosphine oxide was
2
(
entries 2–5). Lower pressures of CO resulted in low con-
version and competing hydrodediazotation (10–25%) and
biaryl coupling (ca. 5%). Eosin-free (entry 6) as well as dark
and thermal reactions (entry 7) produced only minimal
amounts of 2a. The optimized conditions were applied to
the synthesis of various alkyl benzoates (Scheme 2). Several
[
18]
formed.
tert-Butyl esters are notoriously difficult to obtain by
conventional esterification procedures because of their steric
[
1,2,22]
bulk.
Palladium-catalyzed carbonylation procedures also
fail to generate tert-butyl esters. The photocatalytic carbon-
ylation, however, produced tert-butyl benzoates in very good
yields, exceeding those of the less hindered methyl and
isopropyl esters. This is due to the lack of a-hydrogen atoms
within tert-butanol, which excludes undesired radical hydro-
gen abstraction and subsequent reductive reactions
(Scheme 4).
Scheme 2. General conditions and scope of the photoredox carbon-
ylation.
functional groups and electron-poor and -rich substituents
(
nitro, chloro, bromo, esters, benzylic protons) were tolerated
in the substrates. A screening of various additives (Scheme 3)
showed significant tolerance of halogen-containing electro-
philes, acidic protons, p-nucleophiles, and electron-deficient
arenes, without erosion of the ester yield or consumption of
[
18]
the additive. The presence of 1 equiv phenol (pK ꢀ 9) was
Scheme 4. Photocarbonylation to tert-butyl benzoates.
a
2
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We have probed the effectiveness of the metal-free
photocarbonylation in the context of the synthesis of 2w,
a low-odor, low-volatility, and low-viscosity ester produced on
multiton scales for applications as a coalescent in the
formulation of latex paints (Velate 368), a plasticizer in the
manufacture of key polymers, as a sunscreen, and an
antiperspirant ingredient in cosmetics (Finsolv EB), as a tex-
tile dye carrier for the treatment of synthetic fibers, and as
ature (up to 708C) gave no carbonylation product, which
excludes homolytic bond cleavage of the starting material to
an aryl radical under these conditions. Instead, the thermal
dark reaction of 1a in methanol at 708C produced small
amounts of the corresponding methyl ether and aryl fluoride
(Scheme 7). This observation is consistent with previous
reports of unsuccessful attempts to thermolytically carbon-
[
26]
ylate diazonium salts.
[
23]
a solvent for cellulose ethers. Under standard conditions, 2-
ethylhexyl benzoate (2w) was obtained in 58% yield from the
reaction of PhN BF4 with 2-ethylhexanol and CO under
2
irradiation with green light (Scheme 5). Acetonitrile was used
Scheme 7. Dark reactions of 4-anisyldiazonium salt (1a).
We have performed detailed mechanistic studies to prove
the postulate of a light-driven reduction/oxidation cascade
which is devoid of any sacrificial redox partner. On the basis
Scheme 5. Synthesis of the technical ester 2w.
[
11,17,27]
of related literature reports
and our own findings we
propose the following operating mechanism (Scheme 8): The
electron-deficient arene diazonium salt (I) accepts one
electron from the electron-rich, dianionic, photoexcited
state of eosin Y (EY*). This single-electron transfer (SET)
as a cosolvent to solubilize the diazonium salt in the fatty
alcohol. Methyl 4-anisate (2a) is another technical product
with applications as a food and flavor ingredient. Technical
applications of catalytic reactions are always associated with
the search for an effective technique for catalyst separation.
results in the release of dinitrogen (N ) and generation of aryl
2
radical II. This initiation step is thermodynamically favored
due to the more positive redox potential of the I!II
5
ꢁ1
ꢁ1
The intensely colored eosin Y (e ꢀ 10 m cm , 530 nm,
ethanol) was removed from the reaction mixture by
[
24]
[9]
+
reduction (ca. 0 V versus SCE) compared with the EYC !
EY* reduction (ꢁ1.1 V versus SCE). The aryl radical II then
[
25]
adsorption on basic alumina. UV/Vis spectra documented
the complete removal of the dye, while < 1% of the product
was lost in this operation (Scheme 6).
[
28]
rapidly reacts with CO to afford acyl radical III.
The
addition of the radical trap TEMPO (2,2,6,6-tetramethylpi-
peridinyloxyl) results in the formation of adducts with both
The mechanistic proposal of a visible-light-driven dye-
catalyzed process was supported by the following experi-
ments: The product yields severely dropped when no photo-
catalyst was present in the reaction and/or under dark
conditions. Reactions in the dark and at increased temper-
[18]
radical species (II, III) being observed. We further propose
that III undergoes rapid one-electron oxidation to give the
highly electrophilic acylium ion IV. Reaction thereof with the
alcohol affords the benzoate ester V. The back electron
transfer (BET) is presumed to be fast, as no adduct of acyl
radical III with electron-rich p-donors (anisole, furan, sty-
rene) was detected. This assumption can be instructively
substantiated by thermodynamic data: The redox potential of
C+
the EY !EY reduction is ꢁ0.8 V (versus SCE). The
potential of the III!IV redox couple is not available
experimentally, since both species are short lived. However,
an estimation of this redox potential could be derived from
[18,29]
DFT calculations,
which provided values between
ꢁ
0.14 V and 0.82 V (versus SCE) depending on the substitu-
tion pattern on the aromatic ring (Scheme 9). Therefore, the
BET step is thermodynamically feasible. Calculation of the
redox potential of the back electron transfer for the 2-
nitroaroyl radical species gave a prohibitive value of 1.7 V
(versus SCE). Indeed, 2-nitrobenzenediazonium tetrafluoro-
borate did not afford the desired ester under carbonylation
conditions. The addition of TEMPO to the reaction resulted
in the formation of the benzoyl–TEMPO adduct. Both
experiments provide further evidence of the postulated
mechanism involving the carbonylation of the intermediate
Scheme 6. Dye removal with basic alumina. Bottom: corresponding
absorption spectra before (blue) and after (red) work-up.
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by an organic dye. Alkyl benzoates
can be prepared from arene diazo-
nium salts, CO, and alcohols at room
temperature under irradiation with
visible light. The reaction uses cata-
lytic eosin Y as a cheap photosensi-
tizer. Mechanistic studies support the
sequential operation of SET reduc-
tion, carbonylation, and back elec-
tron transfer to give aroylium cations,
which undergo rapid addition to
alcohols. Unlike with metal-catalyzed
carbonylations, tert-butyl esters can
be prepared in good yields. The
general method has been applied to
the synthesis of industrial intermedi-
ates.
Scheme 8. Mechanism of the visible-light-driven eosin-catalyzed carbonylation of arene diazonium
salts.
Experimental Section
General procedure: A vial (3 mL) was
charged with a magnetic stir bar, the arene
diazonium salt (0.1 mmol), and eosin Y (0.04 mmol) under N , and
2
capped with a rubber septum. Dry solvent (1 mL, alcohol or mixture
with MeCN if the solubility of the diazonium salt was poor) was
added. The vial was purged with N2 (5 min) in the dark and
transferred to a reactor containing a Quartz window bottom (Parr
Instr.; Figure 1). The septum was punctured with a needle. The
Scheme 9. The back electron transfer (BET): DFT calculations on the
prohibitive redox process to aryl cations (top left), the operating BET
to give acylium ions (bottom left), and supportive preparative experi-
ments (right).
+
2
-nitroaryl radical, but the BET to EYC is thermodynamically
Figure 1. High-pressure photoreactor.
[
21]
prohibited.
The reaction of aryl cations with CO is known from acid-
reactor was sealed, placed on a magnetic stirrer, and slowly filled with
[
30]
catalyzed Gatterman-Koch carbonylations,
but can be
CO (50 bar). The reaction was irradiated with external LEDs (lmax
=
excluded on the basis of DFT calculations. The carbonylation
of aryl cations (VI, from thermal heterolysis of the arene
diazonium) was already disproven (Scheme 7). A potential
back electron transfer with EYC at the aryl radical stage (II)
can likewise be refuted. The DFT-derived reduction poten-
525 nm, 3.8 W). After 4 h at 188C, the gas was released and the vial
retrieved. Water (5 mL) was added to give an emulsion, which was
extracted with ethyl acetate (2 ꢀ 5 mL). The organic phases were
+
washed (5 mL brine) and dried (Na SO ). Volatiles were evaporated
2 4
and the residues purified by chromatography on SiO gel.
2
tials of the VI/II couples of our substrates are 1.7–2.3 V
Received: August 28, 2014
Revised: September 23, 2014
Published online: && &&, &&&&
[
18]
(
versus SCE).
These values are prohibitively high in
+
comparison with the oxidizing power of EYC (ꢁ0.8 V
versus SCE) and thus exclude the intermediacy of aryl
cations.
This new procedure enables metal-free and base-free
carbonylations through an photoredox mechanism catalyzed
Keywords: carbonylation · esters · organocatalysis ·
photoredox catalysis · redox reaction · visible light
.
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Chemie
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[
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Angewandte
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Communications
Photocatalysis
M. Majek,
A. Jacobi von Wangelin*
&&&& — &&&&
Metal-Free Carbonylations by Photoredox
Catalysis
A metal-free and base-free carbonylation
has been developed which affords pri-
mary, secondary, and tertiary alkyl ben-
zoates under irradiation with visible light
in the presence of eosin Y as a photo-
catalyst. The mechanism has been stud-
ied by spectroscopic, theoretical, and
preparative methods, and appears to
involve intermediate aryl and aroyl radical
species as well as a light-driven one-
electron redox cycle without any sacrificial
redox partner.
6
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