Decarboxylative cross-coupling of mesylates catalyzed by copper/palladium systems with customized imidazolyl phosphine ligands

Article abstract of DOI:10.1002/anie.201208025

The activation of the inert C-O bonds in mesylates through the use of a new class of imidazolyl phosphines allows the decarboxylative coupling of aryl mesylates as well as polysubstituted alkenyl mesylates. Variation of the ligands leads to two complementary methods providing the corresponding biaryls and polysubstituted olefins in good yields. Copyright

Full text of DOI:10.1002/anie.201208025

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Angewandte
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DOI: 10.1002/anie.201208025
À
C C Coupling
Decarboxylative Cross-Coupling of Mesylates Catalyzed by Copper/
Palladium Systems with Customized Imidazolyl Phosphine Ligands**
Bingrui Song, Thomas Knauber, and Lukas J. Gooßen*
Decarboxylative cross-coupling reactions have recently
emerged as a powerful methodology for the regioselective
esterification of broadly available phenols or enolates with
inexpensive mesyl chloride or anhydride, often in substitution
patterns different to those of related organohalides.[15]
However, the higher basicity of mesylates in comparison
[1]
À
À
construction of C C and C heteroatom bonds. Their key
advantage over traditional cross-coupling reactions is that
they draw on stable and readily available carboxylate salts as
sources of carbon nucleophiles rather than expensive and
sensitive organometallic reagents. In the last decade, a rapidly
growing number of decarboxylative reactions have been
discovered including decarboxylative Heck reactions,[2] ally-
lations,[3] redox-neutral cross-coupling reactions,[4] oxidative
with, for example, triflates or even tosylates results in
[16]
À
a particularly stable C O bond that can only be cleaved
with a handful of extremely active cross-coupling catalysts.
Among the few catalysts that are capable of activating
mesylates are [NiCl2(PCy)2][17] and palladium complexes
bearing sophisticated ligands of the X-Phos, Brett-Phos, and
CM-Phos types (Figure 1).[18] The groups led by Percec,
Buchwald, and Kwong demonstrated that mesylates can be
used as substrates in traditional cross-couplings including
Suzuki–Miyaura,[19] Hiyama,[20] and Stille reactions,[21] as well
as in carbonylations and C–heteroatom bond-forming reac-
tions.[22]
coupling reactions,[5] C H arylations,[6] homocouplings,[7] and
À
Chan–Lam-type reactions.[8]
Redox-neutral decarboxylative cross-couplings mediated
by Cu/Pd or Ag/Pd bimetallic catalyst systems proved to have
a particularly broad scope with regards to both carboxylates
and carbon electrophiles. In this variant, the decarboxylation
step is mediated by a CuI [9] or AgI [10] catalyst, while a Pd
complex catalyzes the coupling with the carbon electrophile.
Whereas aryl bromides and iodides can be converted with
very simple ligand systems,[11] the activation of aryl chlor-
ides,[12] triflates,[13] and tosylates[14] requires the use of
sophisticated catalyst systems containing electron-rich,
bulky phosphine ligands. However, all attempts to develop
decarboxylative couplings of the notoriously hard-to-activate
methanesulfonates (mesylates) have failed so far (Scheme 1).
Aryl and alkenyl mesylates are particularly attractive
carbon electrophiles for preparative- and industrial-scale
syntheses, since they have the lowest molecular weight of all
sulfonate leaving groups. They are easily accessible by
Figure 1. State-of-the-art palladium ligands for mesylate activation.
In decarboxylative couplings, the use of mesylates is
a particular challenge because transmetalations are known to
proceed from copper or silver only to palladium but not to
nickel, precluding the use of this catalyst metal. Moreover,
particularly electron-rich ligands, which might promote the
oxidative addition of aryl/alkenyl mesylates onto palladium
catalysts, tend to interfere with the decarboxylation cata-
lyst.[12,13]
Scheme 1. Decarboxylative cross-coupling of aryl and alkenyl mesy-
lates.
In search for an effective catalyst system for the desired
decarboxylative arylation of mesylates, we chose the reaction
of potassium ortho-nitrobenzoate (1a) with 2-naphthyl mesy-
late (2a) as a model. The desired biaryl (3aa) was formed only
in trace amounts when a phenanthroline/copper decarbox-
ylation catalyst was used in combination with various
palladium complexes. Neither Buchwaldꢀs S-Phos nor Brett-
Phos, nor Kwongꢀs CM-Phos proved to be effective.
In the coupling reaction using the CM-Phos ligand,
significant amounts of protodecarboxylation products were
observed, indicating that this ligand does not inhibit the
decarboxylation step. As its ability to activate aryl mesylates
is well-documented, we reasoned that the transmetalation
from copper to palladium must be the critical step in the
overall catalytic process depicted in Scheme 2.[23]
[*] B. Song, Dr. T. Knauber, Prof. Dr. L. J. Gooßen
FB Chemie-Organische Chemie, TU Kaiserslautern
Erwin-Schrçdinger-Strasse, Geb. 54
67663 Kaiserslautern (Germany)
E-mail: goossen@chemie.uni-kl.de
Homepage: http://www.chemie.uni-kl.de/goossen
[**] We thank the DFG (SFB-TRR 88 “3Met”), Saltigo GmbH, Bayer
Science & Education Foundation (fellowship to B.S.), and NanoKat
for funding. We also thank Dr. U. Bergstrꢀßer and Dr. H. Kelm for
the crystal structure analysis, and D. Hackenberger for technical
assistance.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201208025.
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Scheme 2. Decarboxylative cross-coupling of 2-naphthyl mesylate.
In the Stille reaction, where the transmetalation step is
often limiting, it has been found that the use of tri-2-
furylphosphine rather than triphenylphosphine enhances
reaction rates by two orders of magnitude.[24,25] This ligand
features a potentially coordinating heteroatom in close
proximity to the phosphorus, which enhances p-back-dona-
tion. It has also been reported that only one phosphine should
be present in transmetalations to palladium.[26]
Scheme 3. Study of ligand performance in the decarboxylative coupling
of aryl mesylates. Reaction conditions: 1a (0.3 mmol), 2a (0.6 mmol),
[Pd(acac)2] (5.0 mol%), L (7.5 mol%), Cu2O (2.5 mol%), 1,10-phenan-
throline (5.0 mol%), NMP (2.0 mL), 1708C, 16 h. Yields determined by
GC analysis using n-tetradecane as the internal standard. Cy=cyclo-
hexyl, iPr=isopropyl, tBu=tert-butyl. [a] 50% yield of protodecarbox-
ylation product was observed.
As a lead structure for ligand design, we thus chose
a benzimidazolyl skeleton rather than the indolyl system of
CM-Phos. This ligand class, which was first reported by
Altenbach[27] and has recently been used by Kwong in Suzuki
couplings,[28] should have reduced s-donor strength, offer
a weakly coordinating group that may aid an associative
transmetalation step, and provide enhanced p-back-donation.
We synthesized various imidazolyl phosphines by a sequence
of cyclization, alkylation, and phosphonation reactions,[29] and
systematically evaluated their activities in decarboxylative
cross-couplings of aryl mesylates (Scheme 3).
In the decarboxylative cross-coupling of 1a with 2a, the
new ligands L1–L15 directly proved to be effective. A clear
trend was evident for the influence of the phosphorus
substituents (L1–L3). The moderately bulky, electron-rich
dicyclohexyl-substituted phosphine showed the highest activ-
ity (L3), whereas tert-butyl and phenyl groups were less
effective. As a substituent on the imidazole nitrogen (R3),
more sterically demanding groups such as isopropyl (L4),
phenyl (L5), and methoxymethyl (L6) had a beneficial effect.
When the substitution on the phosphorus-bound aryl ring was
increased using alkoxy groups in the manner of a Brett-Phos
ligand (R2 = OMe, L7), the yields dropped. Increasing the
electron density of the benzimidazole nitrogens by methyl
substituents (R4) on the ring improved the yields (L10). A
tetrahydrobenzimidazole fragment (L11), which would also
have a high electron density on the nitrogens, was nearly as
effective as L10. Further optimization of the N-substituents
(R3) on the final ligand core led to another step-up in the
yields (L12–L15), with n-octyl being the most effective
residue.
À
(2.042 ꢁ) than the Pd Cs bond in the Pd–CM-Phos complex
(1.986 ꢁ),[16] confirming the targeted labile chelating coordi-
nation mode.
The reaction conditions were systematically optimized
using the solid ligand L10 (Table S1 in the Supporting
Information). A step-up in yields to 68% was achieved by
changing the solvent to a 1:3 mixture of NMP and mesitylene
and changing to [Pd(dba)2] as the Pd precursor. Further
experiments confirmed that [Pd(dba)2] and Cu2O are the
most effective precatalysts. Silver decarboxylation co-cata-
lysts can also be used but are less effective. Among the
N ligands, the best results were obtained with 3,4,7,8-tetra-
methyl-1,10-phenanthroline (Me4-phen). Control experi-
ments revealed that the decarboxylative coupling reaction
requires both palladium and copper to proceed. Under these
optimized conditions, the use of ligand L14, a viscous oil, still
made a positive difference. Up to 79% yield was obtained
with the catalyst generated in the reaction solution and almost
quantitative yields were obtained using the preformed PdCl2–
L14 complex. The reaction is easy to perform also in
a laboratory microwave reactor.
We next investigated the scope of the new transformation.
Various aromatic carboxylate salts were successfully coupled
with 2-naphthyl mesylate using a catalyst system generated in
situ from [Pd(dba)2] (5 mol%), L14 (12 mol%), Cu2O
(2.5 mol%), and 3,4,7,8-tetramethyl-1,10-phenanthroline
(5 mol%) within 30 min at 1808C in a laboratory microwave
reactor. As illustrated in Table 1, various ortho-substituted
arenecarboxylate salts were converted into the corresponding
biaryls in high yields (3aa–3ea). Even the highly sterically
demanding substrate 1g gave satisfactory yields. Potassium
ortho-methoxybenzoate was converted only in low yield,
while non-ortho-substituted benzoates did not react (3ha).
In the crystal structure of the optimal catalyst system
(Figure S1 in the Supporting Information),[30] the palladium
center is coordinated by both amino and phosphino groups in
À
a distorted square–planar geometry. The Pd N bond is longer
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Table 1: Decarboxylative coupling of aryl and heteroaryl carboxylates with
aryl mesylates could also be coupled, albeit in moderate yields
(3ab–3ae).
aryl mesylates.[a]
We considered the cross-coupling of alkenyl mesylates to
be of even higher interest than that of aryl mesylates as these
compounds are easily accessible by esterification of enolates
in substantially greater structural diversity than alkenyl
halides. We were pleased to find that only minor adjustments
were necessary to extend this reaction concept to this
substrate class (Tables S2 and S3). Remarkably, L8 with
electron-withdrawing chloro substituents on the benzimida-
zolyl backbone was most effective as the ligand for palladium,
and 2,2’-bipyridine for copper. With the catalyst system
generated in situ from Pd(OAc)2/L8 and Cu2O/2,2’-bipyri-
dine, a broad array of arenecarboxylate salts were coupled
with various alkenyl mesylates in high yields within only 1 h
(Table 2). Among the products are several that would be hard
to access otherwise, for example tetrasubstituted olefins and
vinyl arenes.
Product
Yield [%]
86
Product
Yield [%]
73
3aa
3ja
88
50
3ba
3ka
85
84
62
0
The range of carboxylate salts that were successfully
converted includes ortho-substituted benzoates and hetero-
cyclic carboxylates. Even sterically demanding derivatives
gave good results. The reaction is broadly applicable with
regard to the alkenyl mesylates. Sterically hindered vinyl
mesylates (5d–f) generated from highly substituted ketones
reacted as well as unsubstituted vinyl mesylates. Even
conjugated alkenyl mesylates were converted in moderate
yields (6ah). No experiments have yet been performed with
stereoisomerically pure alkenyl mesylates, but when E/Z
mixtures were used as starting materials, it appears as if the
ratio between the E- and Z-configured products increases
slightly in the course of the reaction.
3ca
3da
3la
3na
80[b]
27
3ea
3ab
In summary, the use of customized imidazolyl phosphines
for the first time allows the use of aryl and alkenyl mesylates
in decarboxylative cross-coupling reactions. This is a decisive
step in leading this relatively young reaction concept to
synthetic maturity. Further optimization of the catalyst
system is currently underway in our group.
50[b]
26[d,e]
3ga
3ha
3ia
3ac
3ad
3ae
17[c]
28[d,e]
Experimental Section
An oven-dried, nitrogen-flushed 10 mL microwave vessel was
charged with potassium 2-nitrobenzoate (1a, 103 mg, 0.50 mmol),
bis(dibenzylideneacetone)palladium(0)
5.0 mol%), copper(I) oxide (1.81 mg, 0.0125 mmol, 2.5 mol%),
3,4,7,8-tetramethyl-1,10-phenanthroline (5.9 mg, 0.025 mmol,
(14.4 mg,
0.025 mmol,
45
35[d,e]
5.0 mol%), and 2-naphthyl mesylate (2a, 167 mg, 0.75 mmol,
1.5 equiv). A solution of L14 in NMP (0.06m, 1 mL, 0.06 mol,
12 mol%) and mesitylene (3 mL) was added by syringe. The mixture
was then stirred for 5 min at 508C, treated with microwave irradiation
at 1808C for 30 min at a maximum power of 100 W, and then cooled
with air-jet cooling. The mixture was diluted with HClaq (1n, 20 mL),
extracted with ethyl acetate (3 ꢂ 20 mL), washed with water and
brine, dried over MgSO4, filtered, and concentrated. The residue was
purified by column chromatography (SiO2, ethyl acetate/hexane
gradient) yielding 3aa [CAS-No. 94064-83-2] as a yellow solid
(214 mg, 86%).
[a] Reaction conditions: 1 (0.5 mmol), 2 (0.75 mmol), [Pd(dba)2]
(5.0 mol%), L14 (12.0 mol%), Cu2O (2.5 mol%), 3,4,7,8-tetramethyl-
1,10-phenanthroline (5.0 mol%), NMP (1.0 mL), mesitylene (3.0 mL),
mW at 1808C/100 W/30 min; yields of isolated products. [b] 1/2
ratio=2:1. [c] Yield determined by GC. [d] NMP/mesitylene=1:5.
[e] [Pd(acac)2] was used instead of [Pd(dba)2].
Control experiments revealed that the phosphine ligand
impedes the decarboxylation of these derivatives. Hetero-
cyclic carboxylates were smoothly transformed into biaryls in
moderate to high yields (3ia–3la). Besides 2-naphthyl, other
Received: October 4, 2012
Revised: November 6, 2012
Published online: February 5, 2013
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86
Product
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54
6aa
6ja
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82
84
83
6ba
6ca
6ab
6ac
86[b]
83
85
80
40[c,d]
6ma
6da
6ad
6ae
63
73
6ea
6af
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22
42
54[e]
6 fa
6ga
6ag
6ah
21[f]
[a] Reaction conditions: 1 (0.5 mmol), 5 (0.75 mmol), Pd(OAc)2
(5.0 mol%), L8 (12.0 mol%), Cu2O (2.5 mol%), 2,2’-bipyridine
(5.0 mol%), NMP (1.0 mL), mesitylene (3.0 mL), 1708C, 1 h; yields of
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Keywords: carboxylic acids · copper · decarboxylative coupling ·
mesylates · palladium
.
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[30] CCDC 904069 contains the supplementary crystallographic data
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