Consecutive one-pot sonogashira-glaser coupling sequence- Direct preparation of symmetrical diynes by sequential Pd/Cu catalysis

Article abstract of DOI:10.1002/ejoc.201001472

Sonogashira coupling and the catalytic Glaser coupling are both catalyzed by the Pd-Cu complex couple and can be concatenated to a consecutive sequentially Pd/Cu-catalyzed process in a one-pot fashion, and air oxygen serves as the only oxidant in the second step. In a pseudo-four-component synthesis, a broad variety of symmetrically substituted 1,4-bis(hetero)aryl-1,3-butadiynes are obtained in good to excellent yields. Interestingly, the presence of iodide ions has been found to be advantageous over other halides to trigger the Pd/Cu-catalyzed Glaser step, and Pd and Cu species, as well as triethylamine as a base, are prerequisite for both couplings, which proceed with higher efficiency if performed in a one-pot sequence. Two catalytic alkyne coupling reactions were combined in a consecutive sequentially Pd/Cu-catalyzed process to furnish a novel pseudo-four-component synthesis of a variety of symmetrically substituted 1,4-bis(hetero)aryl-1,3-butadiynes in goodyields. Pd and Cu species and triethylamine (base) are essential for the couplings, which proceed with higher efficiency if performed in a one-pot sequence. Copyright

Full text of DOI:10.1002/ejoc.201001472

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201001472
Consecutive One-Pot Sonogashira–Glaser Coupling Sequence – Direct
Preparation of Symmetrical Diynes by Sequential Pd/Cu Catalysis
Eugen Merkul,[a] Dominik Urselmann,[a] and Thomas J. J. Müller*[a]
Dedicated to Prof. Dr. Henning Hopf on the occasion of his 70th birthday
Keywords: Alkynes / C–C coupling / Copper / Multicomponent reactions / Palladium
Sonogashira coupling and the catalytic Glaser coupling are
both catalyzed by the Pd–Cu complex couple and can be con-
catenated to a consecutive sequentially Pd/Cu-catalyzed
process in a one-pot fashion, and air oxygen serves as the
only oxidant in the second step. In a pseudo-four-component
synthesis, a broad variety of symmetrically substituted 1,4-
bis(hetero)aryl-1,3-butadiynes are obtained in good to excel-
lent yields. Interestingly, the presence of iodide ions has been
found to be advantageous over other halides to trigger the
Pd/Cu-catalyzed Glaser step, and Pd and Cu species, as well
as triethylamine as a base, are prerequisite for both cou-
plings, which proceed with higher efficiency if performed in
a one-pot sequence.
published. Although these transformations avoid the use of
free terminal alkynes, they still require the application of
sophisticated organoboron or organosilicon substrates.
A reliable, quick and general approach to generate ter-
minal alkynes starting from easily available (hetero)aryl ha-
lides employs the Pd/Cu-cocatalyzed Sonogashira–Hagi-
hara cross coupling, followed by subsequent deprotec-
tion.[12] Since this catalytic system matches with the condi-
tions of the Glaser-type couplings mentioned above,[9] the
development of a sequentially catalyzed route for the syn-
thesis of 1,4-disubstituted 1,3-butadiynes utilizing the same
catalyst couple for alkynylation and oxidative coupling lies
at hand (Scheme 1). The isolation of intermediate terminal
alkynes becomes dispensable. To the best of our knowledge,
this obvious and straightforward concept has never been
realized.
Introduction
1,4-Disubstituted 1,3-butadiynes are recurring building
blocks in natural products and analogues,[1] molecular elec-
tronic devices,[2] or optical materials[3] and serve as interme-
diates in the synthesis of a variety of heterocycles.[4] Tradi-
tionally, copper(I)- or copper(II)-catalyzed Glaser,[5] Eglin-
ton,[6] or Hay[7] coupling reactions and their numerous
modifications[8] utilizing terminal alkynes as substrates are
applied for the synthesis of symmetrical 1,3-diynes. Even
more efficient transformations rely on catalytic systems
consisting of combined palladium(0) or palladium(II) and
copper(I) sources.[9] Recently, a ligand-, palladium-, amine-
and even oxidant-free procedure, which uses copper nano-
particles as a catalyst, has been reported.[8a] Despite numer-
ous efficient strategies developed for Glaser-type acetylene
dimerizations,[10] the major drawback of utilizing terminal
alkynes as substrates is their sensitivity towards polymeriza-
tion and their occasionally tedious purification. Moreover,
some of the terminal alkynes proved to be quite unstable,
which limits their shelf life and complicates their synthesis.
Therefore, a synthetic route that avoids the isolation of
these compounds would be highly desirable.
Scheme 1. Concept of a Sonogashira–Glaser coupling sequence.
In the past years some methodologies for the preparation
of 1,3-butadiynes starting from alkynyltrifluoroborates,[11a]
alkynylboronates,[11b] or alkynylsilanes[11c–11e] have been
Concatenating the Sonogashira and the Glaser coupling
reactions into a one-pot sequence is especially advan-
tageous from an economical point of view but also because
of practical considerations. Herein, we report a sequence of
a Pd/Cu-cocatalyzed Sonogashira coupling of iodo arenes
with trimethylsilylacetylene (TMSA), followed by in-situ
cleavage of the silyl protective group and subsequent Gla-
ser-type homocoupling of the generated terminal alkynes
[a] Institut für Organische Chemie und Makromolekulare Chemie,
Heinrich-Heine-Universität Düsseldorf,
Universitätsstr. 1, 40225 Düsseldorf, Germany
Fax: +49-211-81-14324
E-mail: ThomasJJ.Mueller@uni-duesseldorf.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201001472.
238
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Consecutive One-Pot Sonogashira-Glaser Coupling Sequence
to form symmetrical 1,4-disubstituted 1,3-butadiynes under combination of both precatalysts, a slight increase in the
aerobic conditions in a one-pot fashion. This resulting se- isolated yield of product 2a could be observed (Entry 1).
quence represents another unique showcase for sequentially Another experiment, which uses 2 mol-% PdCl2(PPh3)2,
Pd/Cu-catalyzed processes.[13]
4 mol-% CuI, and 1.5 equiv. KF starting from commercially
available trimethyl(phenylethynyl)silane, gave 55% of diyne
2a within 28 h of reaction time. This implies that no fluoro-
silanate complexes are involved in the transmetalation step.
Surprisingly, the yields of the Glaser coupling step alone
were much lower than those from the complete sequence,
which additionally includes Sonogashira and deprotection
steps. Bearing in mind that the apparent difference between
the conditions applied in the sequence and in the separated
Glaser coupling step is the generation of 1 equiv. trieth-
ylammonium iodide in the Sonogashira coupling step, we
scrutinized the influence of halide anions on the Glaser
coupling step. Thus, upon addition of 1.0 equiv. potassium
fluoride, the yield of the isolated diyne 2a increased dramat-
ically relative to the yield obtained without additives (En-
tries 1 and 4). Furthermore, the yield continuously in-
creased from fluoride (82%) through chloride (86%) and
bromide (92%) to iodide (95%) (Entries 4–7). To the best
of our knowledge, the effect of halide anions on the Glaser
coupling has neither been observed nor investigated. Al-
though the reason for this behavior is not clear at this stage,
the high yield obtained by the addition of iodide may ex-
plain the high efficiency of the developed sequence, since
ammonium iodide generated in the Sonogashira step could
promote the Glaser coupling step. Moreover, this prelimi-
nary investigation shows that the catalytic system
PdCl2(PPh3)2/CuI/NEt3 is essential for both Sonogashira
and Glaser coupling reactions to proceed efficiently, since
the absence of one of these components diminishes the yield
dramatically (Entries 8–10). These findings clearly emphas-
ize the benefits of combining Sonogashira and Glaser cou-
plings into a one-pot sequence.
Results and Discussion
At the outset of our investigations, we reacted iodo-
benzene (1a) with TMSA (1.5 equiv.) under standard
Sonogashira conditions [PdCl2(PPh3)2, CuI, and NEt3 as a
base][14] to form trimethyl(phenylethynyl)silane in a smooth
reaction within 1 h at ambient temperature. After the reac-
tion vessel was opened, potassium fluoride and methanol
were added to cleave the TMS protective group. Simulta-
neously, upon subjecting to aerobic atmosphere and further
stirring at room temperature, the Glaser-type coupling pro-
ceeded smoothly. The desired 1,4-diphenylbuta-1,3-diyne
(2a) was isolated in 81% yield [Equation (1)].
(1)
To gain insight into the mode of action of the catalytic
system in the final Glaser coupling step, we subjected com-
mercially available phenylacetylene to different combina-
tions of precatalysts and additives used for the entire se-
quence to form the diyne 2a in a single step [Equation (2),
Table 1].
(2)
The presented sequence is preparatively strikingly simple
and utilizes commercially available and stable reagents and
precatalysts without fancy ligands or additives. The use of
air as an oxidant additionally supports the sustainable as-
pect of the method (Scheme 2). With these mild conditions
for the sequence in hand, the substrate scope was examined
and a novel pseudo-four-component synthesis of symmetri-
cal 1,3-butadiynes 2 was established (Figure 1). All reac-
tions were carried out on a 2-mmol scale with respect to
(hetero)aryl iodide 1. The structures of compounds 2 were
unambiguously supported by combustion analysis, NMR
spectroscopy, and mass spectrometry.
Table 1. Influence of precatalysts and additives in the Glaser-type
coupling step.[a]
Entry Conditions
Time [h] Yield of 2a [%][b]
1
2
3
4
5
6
7
8
9
10
no additives
24
25
25
26
26
26
26
26
26
58
8
no PdCl2(PPh3)2, no additives
no CuI, no additives
1.0 equiv. KF
1.0 equiv. NH4Cl
1.0 equiv. NaBr
1.0 equiv. NaI
1.0 equiv. NaI, no NEt3
1.0 equiv. NaI, no CuI
36
82
86
92
95
5
41
7
1.0 equiv. NaI, no PdCl2(PPh3)2 26
[a] All reactions were carried out on a 2-mmol scale with 2 mol-%
PdCl2(PPh3)2, 4 mol-% CuI, and NEt3 (1.0 equiv.) in a mixture of
THF (5 mL) and methanol (5 mL). The mixture was stirred at
room temperature for the indicated time. The conditions were var-
ied or adjusted by additives as indicated. [b] Yield of isolated and
purified compound 2a.
Whilst use of CuI iodide as a single catalyst gave only
very low amounts of the desired compound 2a (Entry 2),
application of the palladium precatalyst led to the forma-
Scheme 2. Optimized Sonogashira–Glaser coupling sequence for
tion of an increased amount of diyne 2a (Entry 3). Upon the synthesis of 1,4-bis(hetero)aryl-1,3-butadiynes 2.
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239

E. Merkul, D. Urselmann, T. J. J. Müller
SHORT COMMUNICATION
Figure 1. Synthesized diynes 2 (isolated yields). Me = methyl, Boc = tert-butoxycarbonyl, Bn = benzyl, Et = ethyl.
The substituent pattern of the obtained 1,4-bis(hetero)- the described sequence showed no notable amounts of by-
aryl-1,3-butadiynes 2 (Figure 1) clearly supports that the products in the TLC in nearly all examples mentioned
precursor (hetero)aryl halides 1 can be electroneutral, elec- above. Thus, isolation of the desired 1,3-butadyines 2
tron rich, as well as electron poor. Substituents in the ortho-, turned out to be remarkably quick and easy. It should be
meta- and para position, as well as the higher-substituted noted that the reaction times (see Supporting Information,
trimethoxyphenyl group or the dimethyluracil derivative, Table S2) are not optimized and might be considerably
are tolerated. Overall, a large variety of functional groups shorter than indicated. The yields of the isolated diynes 2
in the (hetero)aryl iodides 1, such as halides, nitro, cyano, are fair to excellent regardless of the electronic nature or
ester, amide, carbamate, urea, and, even unprotected hy- substituent pattern of the applied iodides. The whole se-
droxy and amino groups, can be carried through the se- quence is performed at ambient temperature. Indeed, many
quence without difficulties. The functional group tolerance applications in medicinal chemistry and material science
and the possibility to react different types of electronically can be envisioned.
diverse six- and five-membered heterocyclic iodides are re-
By considering the results obtained from our preliminary
markable for such a simple sequence. In most cases, the studies (vide supra), the mechanistic rationale of this Sono-
isolated target compounds 2 are stable crystalline solids that gashira–Glaser coupling sequence can be outlined as fol-
can be conveniently purified by column chromatography or lows (Scheme 3). In the Sonogashira cycle, driven by the
recrystallization. The crude reaction mixtures obtained by catalytic Pd0/CuI pair, the cross-coupling of (hetero)aryl io-
Scheme 3. Mechanistic sketch of the consecutive one-pot Sonogashira–Glaser coupling sequence.
240
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Consecutive One-Pot Sonogashira-Glaser Coupling Sequence
(48), 87 (35). C20H14O4 (318.3): C 75.46, H 4.43; found C 75.22, H
4.70.
dide 1 and TMSA furnishes the TMS-protected (hetero)aryl
alkyne 3, which is deprotected with fluoride to give the cor-
responding terminal alkyne 4. Alkyne 4 (2 equiv.) now en-
ters the Glaser cycle, which is triggered by the catalytic PdII/
CuI pair. CuI ions are involved in transmetalation to PdII,
and thus a dialkynyl PdII complex is generated, which fur-
nishes the desired 1,3-butadiyne 2 upon reductive elimi-
nation. Moreover, CuI is readily oxidized to CuII by atmo-
spheric oxygen. In analogy to the Wacker oxidation,[15] an
intercepting CuI/CuII cycle fueled by oxygen is ultimately
responsible for reoxidizing Pd0 back to PdII. Interestingly,
the same mechanism that leads to the unwanted by-product
of the Sonogashira coupling in the initial activation step[16]
now becomes the modus operandi to form the desired di-
ynes 2.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures and spectroscopic data for the com-
pounds prepared are presented.
Acknowledgments
The authors cordially thank Merck Serono KGaA, Darmstadt for
the financial support of this research.
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Experimental Section
Synthesis of 2m: A mixture of methyl 2-iodobenzoate (1m) (535 mg,
2.00 mmol), PdCl2(PPh3)2 (28 mg, 0.04 mmol, 2 mol-%) and CuI
(16 mg, 0.08 mmol, 4 mol-%) was dissolved in dry degassed THF
(5.00 mL) in a screw-cap Schlenk vessel with septum. After ad-
dition of TMSA (0.43 mL, 3.00 mmol) and dry triethylamine
(0.55 mL, 4.00 mmol), the solution was stirred at room temperature
(water bath) for 1 h until complete conversion (monitored by TLC).
KF (236 mg, 4.00 mmol) and methanol (5.00 mL) were then added,
and the reaction mixture was stirred in air (the reaction vessel was
opened) for 22 h. After completion of the reaction, as indicated by
TLC, the mixture was filtered and adsorbed on Celite®, and after
removal of the solvents in vacuo, the residue was purified by col-
umn chromatography on silica gel by using petroleum ether (boil-
ing range 40–60 °C)/ethyl acetate = 10:1 (Rf = 0.15) to give, after
drying in vacuo, 1,4-bis(2-methylbenzoyl)buta-1,3-diyne (2m)
(296 mg; 93%) as a yellow oil. Upon suspension in n-pentane, son-
ication in ultrasound bath, filtration and drying in vacuo, an ana-
1
lytically pure yellow solid was obtained. M.p. 59–60 °C. H NMR
(500 MHz, CDCl3): δ = 3.97 (s, 6 H), 7.40–7.44 (m, 2 H), 7.48–
7.52 (m, 2 H), 7.65–7.69 (m, 2 H), 7.97–8.00 (m, 2 H) ppm. 13C
NMR (125 MHz, CDCl3): δ = 52.4 (CH3), 78.9 (Cquat), 81.5 (Cquat),
122.5 (Cquat), 128.8 (CH), 130.6 (CH), 131.8 (CH), 132.7 (Cquat),
135.2 (CH), 166.1 (Cquat) ppm. EI-MS (70 eV): m/z (%) = 318
[M]+ (46), 303 [M – CH3]+ (32), 285 (61), 275 (25), 272 (28), 259
[M – C2H3O2]+ (74), 258 (34), 257 (40), 204 (28), 202 [C16H10]+
(49), 200 [C16H8]+ (25), 189 (35), 188 (31), 187 (60), 176 (41), 144
(100), 133 (25), 127 (32), 114 (37), 105 (27), 101 (35), 100 (51), 88
Eur. J. Org. Chem. 2011, 238–242
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E. Merkul, D. Urselmann, T. J. J. Müller
SHORT COMMUNICATION
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[16]
The formation of diynes is a well-known “plague” of the Sono-
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Received: October 29, 2010
Published Online: December 9, 2010
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