Carboxylation of Aromatic and Aliphatic Bromides and Triflates with CO2 by Dual Visible-Light–Nickel Catalysis

Article abstract of DOI:10.1002/anie.201706724

We report the efficient carboxylation of bromides and triflates with K₂CO₃ as the source of CO₂ in the presence of an organic photocatalyst in combination with a nickel complex under visible light irradiation at room temperature. The reaction is compatible with a variety of functional groups and has been successfully applied to the synthesis and derivatization of biologically active molecules. In particular, the carboxylation of unactivated cyclic alkyl bromides proceeded well with our protocol, thus extending the scope of this transformation. Spectroscopic and spectroelectrochemical investigations indicated the generation of a Ni⁰ species as a catalytic reactive intermediate.

Full text of DOI:10.1002/anie.201706724

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Accepted Article
Title: Carboxylation of Aromatic and Aliphatic Bromides and Triflates
with CO2 by Dual Visible Light - Nickel Catalysis
Authors: Qing-Yuan Meng, Shun Wang, and Burkhard König
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10.1002/anie.201706724
Angewandte Chemie International Edition
COMMUNICATION
Carboxylation of Aromatic and Aliphatic Bromides and Triflates
with CO2 by Dual Visible Light - Nickel Catalysis
Qing-Yuan Meng, Shun Wang, and Burkhard Kꢀnig*
Dedicated to Prof. Chen-Ho Tung on the occasion of his 80th birthday
Abstract: We report the efficient carboxylation of bromides and
triflates with K2CO3 as the source of CO2 using an organic
photocatalyst in combination with a nickel complex under visible light
irradiation at room temperature. The reaction is compatible with a
variety of functional groups and has been successfully applied to the
synthesis and derivatization of biologically active molecules.
Particularly, the carboxylation of unactivated cyclic alkyl bromides
proceeds well under our protocol extending the scope of this
Pd(OAc)2 and an Ir complex as catalyst and photosensitizer (PS),
respectively, to accomplish visible light-driven carboxylation of
aryl halides using i-Pr2NEt as the non-metallic reductant
(Scheme 1b, previous work).[12] However, only aromatic and
heteroaromatic halides were reacted. Herein, we presented a
cheap and mild method to realize carboxylation of bromides and
triflates with CO2 under visible light catalysis, i.e. NiBr2·glyme as
the catalyst, an organic photosensitizer and Hantzsch ester
(HEH) as the reductant. In addition to (hetero)aryl and primary
alkyl bromides, unactivated cyclic alkyl bromides react well
under our conditions (Scheme 1c, this work) overcoming
previous limitations.
transformation.
Spectroscopic
and
spectroelectrochemical
investigations indicate the generation of a Ni(0) species as catalytic
reactive intermediate.
Carbon dioxide (CO2), due to its abundance, low cost and
nontoxicity, has been regarded as an ideal and green one-
carbon (C1) building block for organic synthesis.[1] As one of the
most important conversions of CO2, carboxylation of carbon
nucleophiles provides a straightforward approach for accessing
carboxylic acid functional groups,[2] some of which have been
proved to be necessary to maintain their biological properties.[3]
Among these nucleophiles, organic (pseudo)halides have been
employed widely as coupling partners, thus avoiding the
application of traditionally used organometallic chemicals based
on organolithium or Grignard reagents. Martín and Correa
developed the catalytic carboxylation of readily available aryl
bromides with CO2 in the presence of Pd(OAc)2 as catalyst.[4]
Tsuji and co-workers applied NiCl2(PPh3)2 to accomplish the
carboxylation of more inert aryl and vinyl chlorides under a CO2
pressure of 1 atm at room temperature.[5] Later, a variety of
surrogates including sulfonates,[6] ester derivatives,[7] allylic
alcohols,[8] as well as ammonium salts[9] were devised to
complement the reaction scope. Notably, all of these systems
require stoichiometric metal reagents based on Et2Zn, Mn or Zn
powder as reducing agents (Scheme 1a, previous works).
Therefore, it is highly desirable to search for a new system that
utilizes non-metallic and mild reductants to realize carboxylation
of (pseudo)halides with CO2.
Scheme 1. Carboxylation of bromides.
The donor-acceptor dye 4CzIPN[13] and HEH were chosen as
photosensitizer and stoichiometric reductant (Table S1);
NiBr2·glyme combined with neocuproine was employed as co-
catalyst. When a solution of bromobenzene with the above
components in MeCN was irradiated with blue LEDs in the
presence of K2CO3 as the source of CO2, the desired product
benzoic acid was formed after work up with aqueous HCl, albeit
in low yield (Table S1, entry 1). Then solvent screening indicated
that DMF was much more effective than CH3CN, DMSO and
DMA (Table S1, entries 2-4). Considering that the source of CO2
may have an influence on the formation of benzoic acid, we
investigated other carbonates, but inferior results were obtained
to K2CO3 (Table S2, entries 1-5). Optimization of the amount of
K2CO3 had a negligible effect on the reactivity (Table S2, entries
6-8). Obviously, CO2 generated in situ from K2CO3 partially
escapes from the solution into the gas phase, thus reducing the
CO2 concentration in solution. Therefore, a balloon filled with
CO2 was used, which increased the product yield to 45% (Table
The rapid development of photocatalysis[10] has also brought
new impetus to carboxylation with CO2.[11] In 2015, Murakami
and co-workers disclosed the first example of a novel and direct
carboxylation of benzylic Csp3-H bonds under UV irradiation.[11a]
In contrast, Jamison and co-workers reported a photoredox
activation of CO2 and amines to synthesize -amino acid
derivatives in moderate to excellent yields under UV light
irradiation.[11c] And very recently, Iwasawa and Martín employed
[*]
Dr. Q.-Y. Meng, S. Wang, Prof. B. Kꢀnig
Institute of Organic Chemistry, Faculty of Chemistry and Pharmacy,
University of Regensburg, D-93040 Regensburg (Germany)
E-mail: burkhard.koenig@ur.de
Supporting information for this article is given via a link at the end of
the document.
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S1, entry 5). Then a variety of photosensitizers including organic
dyes, Ru(II)- and Ir(III)-based metal complexes were screened
for this transformation, but only [Ir(dF-CF3-ppy)2(t-Bu-bpy)]PF6
gave a comparable result (Figure S1). Other investigated ligands
and catalysts provided inferior efficiencies than neocuproine and
NiBr2·glyme (Table S2, entries 9-13). Further increasing the
amount of HEH showed better performance (Table S1, entry 6,
Table S2, entries 14 and 15). H2O had a deleterious effect on
this reaction (Table S1, entry 7) and we anticipated that the
addition of molecular sieves (MS) facilitates the transformation
(Table S1, entry 8). Gratifyingly, 71% yield of benzoic acid was
obtained by subtle change of the concentration (Table S1, entry
9, Table S2, entries 16 and 17). Control experiments revealed
that all reaction components were essential to promote this
transformation (Table S2, entries 18-21).
method was also compatible with naphthalene and hetero-
aromatic rings including benzothiophene, benzothiazole, as well
as quinoline (2y, 2z, 2aa and 2ab). Only hydrogenation adducts
have been detected as the by-products and no dimerization
adducts were observed (Table S3).
Table 2. Scope of alkyl bromides for carboxylation[a]
Table 1. Scope of aryl bromides for carboxylation[a]
[a] All reactions were carried out with alkyl bromides (0.2 mmol), K2CO3
(0.2mmol), 4CzIPN (0.002 mmol), NiBr2·glyme (0.02 mmol), neocuproine (0.04
mmol), HEH (0.4 mmol) and 50 mg 4Å MS in anhydrous DMF (4 mL),
irradiation with blue LEDs at room temperature with a CO2 balloon for 24 h. [b]
1 equiv. CsF was added.
Next, we explored the substrate scope of alkyl bromides for
this protocol. As shown in Table 2, all primary alkyl bromides
react well to give the corresponding products with yields ranging
from 38% to 69% (4a-4e). Compounds 4a and 4b are suitable
for a subsequent synthesis of unsymmetric dicarboxylates and
cross-coupling reactions. Tridecanoic acid 4c is an important
starting material for the production of surfactants and spices.
BocNH is tolerated, but the product 4e is only obtained in low
yield. Remarkably, non-activated cyclic alkyl bromides can be
converted into the corresponding products in moderate yields
(4f-4h).[15] Furthermore, the reaction scope was extended to aryl
triflates. It is noteworthy that electron-rich aryl triflates showed
better efficiencies than electron-deficient substrates (Table 3, 2a,
2b and 2g), which is an inverse trend compared to aryl bromides
(Table 1, 2a, 2b and 2g). For chlorides, only substrates bearing
electron-withdrawing substituents on the aryl ring gave moderate
to good yields of the desired products (Table S4).
[a] All reactions were carried out with aryl bromides (0.2 mmol), K2CO3
(0.2mmol), 4CzIPN (0.002 mmol), NiBr2·glyme (0.02 mmol), neocuproine (0.04
mmol), HEH (0.4 mmol) and 50 mg 4Å MS in anhydrous DMF (4 mL),
irradiation with blue LEDs at room temperature with a CO2 balloon for 24 h.
With the optimum reaction conditions established, we
investigated the scope of this protocol (Table 1). A broad range
of para-(2b-2l) or meta-substituted (2m-2r) arenes reacted with
CO2 smoothly to afford carboxylic acids in moderate to excellent
yields (47-93%). It is noteworthy that many reactive groups
including ether (2c, 2e and 2n), alcohol (2d), fluoro (2f), chloro
(2g), cyano (2h and 2p), ester (2i and 2r), ketone (2j and 2q),
trifluoromethyl (2k) and Boc-protected amine (2o) are tolerated
under the reaction conditions. Intriguingly, ortho-substituted aryl
bromides with an ester group could be used as substrate to give
the desired product with good efficiency (2s), which was difficult
in previous reported systems.[5] Next, di-substituted aryl rings
were investigated, affording carboxylic acids in moderate to
good yields (48-74%). Particularly, the obtained 3,5-difluoro-
benzoic acid has been regarded as an important precursor for
the synthesis of liquid crystalline materials.[14] This carboxylation
Table 3. Scope of aryl triflates for carboxylation[a]
[a] All reactions were carried out with aryl triflates (0.2 mmol), K2CO3
(0.2mmol), 4CzIPN (0.002 mmol), NiBr2·glyme (0.02 mmol), neocuproine (0.04
mmol), HEH (0.4 mmol) and 50 mg 4Å MS in anhydrous DMF (4 mL),
irradiation with blue LEDs at room temperature with a CO2 balloon for 24 h.
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Finally, some synthetic applications of this Ni-based
photocatalytic transformation are demonstrated by the concise
synthesis of MCPB, which exhibits potent biological activity
(Scheme 2, equation 1).[16] Moreover, estrone could be
derivatized into the desired carboxylic acid 7 directly in a
moderate yield without column separation (Scheme 2, equation
2).
generation of LNiBr2 (L = neocuproine) in situ during the initial
process (Figure 1a). Moreover, 77% yield of benzoic acid was
produced when LNiBr2 (L = neocuproine) was used as the
catalyst instead of NiBr2·glyme and neocuproine (Scheme S1,
equation 3).
We next investigated the initial electron transfer during the
process by fluorescence quenching and electrochemical
experiments. The luminescence of 4CzIPN [E1/2(*P/P-) = +1.35 V
vs SCE][13b] at max = 537 nm in degassed DMF solution was
red
readily quenched by HEH (E1/2
= +0.887 V vs SCE),[17]
showing a Stern-Volmer kinetic with a rate of 0.61 M-1 (Figure
S3a). In addition, NiBr2·glyme (E1/2 = -1.70 V vs Fc/Fc+ ，
Ox
Figure S4b) can also quench the luminescence of 4CzIPN
[E1/2(+P/*P) = -1.04 V vs SCE][13b] with a rate of 0.18 M-1 (Figure
S3b). Introduction of neocuproine as the ligand increased the
rate slightly (0.20 M-1, Figure S3c) due to the in situ generation
of LNiBr2 (L = neocuproine) with a relatively positive reduction
potential (E1/2 = -1.27 V vs Fc/Fc+ for LNiBr2, Figure S4a and
Ox
Scheme 2. Photocatalytic carboxylation in multistep synthesis.
4c). No quenching was observed with bromobenzene as the
Ox
estimated potential difference is unfeasible (E1/2 ˂ -2.0 V vs
SCE).[18] Therefore, both reductive and oxidative quenching of
the excited state of 4CzIPN may occur, while the former is faster
than the latter. Furthermore, spectroelectrochemistry was
employed to investigate a solution containing NiBr2·glyme and
neocuproine. At the beginning, a new absorption between 600-
800 nm assigned to the one electron reduced species was
observed with decreasing potential. Subsequently, another new
absorption between 450-600 nm appeared, which can be
ascribed to the two electron reduced species (Figure 1b). Using
LNiBr2 (L = neocuproine) instead, the same phenomenon was
obtained, showing absorptions between 600-800 nm and 450-
600 nm, which might correspond to LNi(I)Br and LnNi(0) (L =
neocuproine), respectively (Figure 1c). Indeed, monitoring the
UV-vis spectrum also showed the generation of these
characteristic absorptions upon irradiation with blue LEDs in the
presence of HEH, whereas the absence of HEH led to no
change of the absorption (Figure 1d).
Preliminary mechanistic studies were performed. When
radical inhibitors such as butylated hydroxytoluene (BHT), 9,10-
dihydroanthracene (DHA) or 1,1-diphenylethylene (DPE) were
added to the system, the reactions proceeded well. The use of
TEMPO as an additive reduced the reactivity, presumably acting
as an interfering oxidant (Scheme S1, equation 1). The amount
of the ligand neocuproine could be decreased to equimolar
amounts to NiBr2·glyme, maintaining still 61% yield of the final
product (Scheme S1, equation 2). Accordingly, we suggest that
NiBr2·glyme and neocuproine form the initial catalyst in a ratio of
1:1, which was confirmed by Job’s method (Figure S2). On the
other hand, when we mixed neocuproine with NiBr2·glyme in
DMF solution, characteristic absorptions of LNiBr2 (L
neocuproine) were observed, which also supported the
=
Figure 1. Spectroscopic investigations. (a) UV-vis spectrum of NiBr2·glyme
(1.010-4 M), L (1.010-4 M), LNiBr2 (1.010-4 M), mixture of NiBr2·glyme
(1.010-4 M) and L (1.010-4 M) in DMF. (b) Spectroelectrochemical absorption
spectrum of the mixture of NiBr2·glyme (2.510-3 M) and L (2.510-3 M) in DMF
as a function of the change of voltage. (c) Spectroelectrochemical absorption
spectrum of LNiBr2 (2.510-3 M) in DMF as a function of the change of voltage.
(d) UV-vis spectrum of the catalytic system before and after illumination. (L:
neocuproine).
Scheme 3. Plausible reaction mechanism
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3). First, ligand exchange occurs between NiBr2·glyme and
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In conclusion, we have developed a new, efficient and mild
strategy to realize carboxylation of bromides and triflates with
CO2 generated in situ from K2CO3 at room temperature by
combining visible light catalysis with Ni catalysis. A broad scope
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to Ni(0)Ln promoting the catalytic cycle. Further mechanistic
studies are under way in our laboratory.
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Financial support from the German Science Foundation (DFG)
(GRK 1626, Chemical Photocatalysis) is acknowledged. We
thank Dr. Rudolf Vasold (University of Regensburg) for his
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Keywords: visible light • Ni catalysis • CO2 • carboxylation•
photocatalysis
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COMMUNICATION
Qing-Yuan Meng, Shun Wang, and
Burkhard Kꢀnig*
Page No. – Page No.
Carboxylation of Aromatic and
Aliphatic Bromides and Triflates with
CO2 by Dual Visible Light - Nickel
Catalysis
An efficient and mild carboxylation of bromides and triflates with K2CO3 as the
source of CO2 using an organic fluorophore in combination with a nickel complex
under visible light irradiation is reported. The present protocol can tolerate a variety
of functional groups. Non-activated cyclic alkyl bromides react well. The synthesis
and derivatization of biologically active molecules demonstrate the applications of
the method in multistep synthesis.
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