Microwave-assisted one-pot diboration/Suzuki cross-couplings. A rapid route to tetrasubstituted alkenes

Article abstract of DOI:10.1016/j.tetlet.2008.06.017

Internal and terminal alkynes undergo rapid platinum(0)-catalyzed diboration with bis(pinacolato)diboron in dioxane to yield cis-1,2-bis(boryl)alkenes under sealed vessel microwave conditions. Subsequent addition of aryl bromides, base and a palladium catalyst to the reaction vial followed by resubjection to microwave conditions provides tetrasubstituted ethylenes in high yields via Suzuki cross-coupling of the boron intermediates.

Full text of DOI:10.1016/j.tetlet.2008.06.017

Tetrahedron Letters 49 (2008) 4831–4835
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Microwave-assisted one-pot diboration/Suzuki cross-couplings.
A rapid route to tetrasubstituted alkenes
Hana Prokopcová a, Jesús Ramírez b, Elena Fernández b, C. Oliver Kappe a,
*
a Christian Doppler Laboratory for Microwave Chemistry (CDLMC) and Institute of Chemistry, Karl-Franzens-University Graz, Heinrichstrasse 28, A-8010 Graz, Austria
b Dept. Química Física I Inorgànica, Universitat Rovira I Virgili, C/Marcel.lí Domingo s/n, 43007 Tarragona, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Internal and terminal alkynes undergo rapid platinum(0)-catalyzed diboration with bis(pinacolato)dibo-
ron in dioxane to yield cis-1,2-bis(boryl)alkenes under sealed vessel microwave conditions. Subsequent
addition of aryl bromides, base and a palladium catalyst to the reaction vial followed by resubjection
to microwave conditions provides tetrasubstituted ethylenes in high yields via Suzuki cross-coupling
of the boron intermediates.
Received 8 May 2008
Revised 30 May 2008
Accepted 3 June 2008
Available online 7 June 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Alkynes
Diboration
Palladium
Platinum
Suzuki reaction
Transition metal-catalyzed alkene and alkyne boration reac-
tions are attractive methods to produce alkyl- or alkenylboron
derivatives with defined regio- and stereochemistry.1 Organoboron
molecules are very useful reagents in organic synthesis, since the
carbon–boron bond can be cleaved in a variety of ways leading
to a wide range of different functional groups and interesting sub-
strates.1,2 During the past decades, considerable efforts have been
made to develop synthetic routes for the efficient preparation of
vicinal sp2 organodiboron derivatives.3–11 In 1993, Suzuki and
Miyaura reported the first addition of tetraalkoxydiboron reagents
to terminal and internal alkynes in the presence of a Pt(0) catalyst,
resulting in the formation of isomerically pure cis-1,2-bis(boryl)-
alkenes.3 This diboration reaction is typically performed using
bis(pinacolato)diboron (B2pin2) as a reagent in the presence of
3 mol % of Pt(PPh3)4 catalyst and involves heating of the reaction
mixture to 80 °C for 24 h in anhydrous DMF under inert atmo-
sphere (cf. Table 1). Subsequent research has demonstrated that
the use of more elaborate bis- or monophosphine4–6 or phos-
phine-free7,8 Pt(0)-based catalysts can often reduce the generally
required long reaction times and in some cases allows these
diborations to be performed also at lower temperatures, using less
polar solvents. In many instances, however, these methods are
restricted to the use of more active diboron reagents such as
bis(catecholato)diboron.4,7,8 The first heterogeneous Pt and Pd-
catalyzed diboration of alkynes using2 borametalloarenophanes
as diboron precursors has recently been described,9 and some
examples for the use of Co(0)10 and Cu(I)11 complexes as efficient
catalysts in diboration processes were also reported.
Notwithstanding these advances in transition metal-catalyzed
alkyne diboration chemistry,4–11 there is still need for improve-
ment since most of the more recently published methods apply
commercially unavailable catalysts or expensive boron reagents,
and have limited substrate scope. On the other hand, the resulting
cis-1,2-bis(boryl)alkenes 3 have been shown to be valuable starting
materials for the preparation of, for example, densely substituted
alkenes (such as the anticancer drug tamoxifen),12 chiral 2,20-bis-
bipyridines,13 and tetrathiothiophenes7 helicenes as novel push–
pull systems.14 In this Letter, we describe the rapid generation of
cis-1,2-bis(boryl)alkenes 3 and their application for the construc-
tion of tetrasubstituted alkene derivatives 6, utilizing a micro-
wave-assisted one-pot diboration/Suzuki cross-coupling reaction
sequence that builds on the original Suzuki–Miyaura protocol,3
involving commercially available Pt(PPh3)4 catalyst/bis(pinacola-
to)diboron reagent.
As a starting point for our investigation, we examined the dib-
oration of diphenylacetylene (1a) with bis(pinacolato)diboron (2),
using commercially available Pt(PPh3)4 as the catalyst system
(Table 1). All initial studies were performed applying controlled
single-mode microwave heating in sealed vessels.15 Screening of
reaction conditions for the diboration process focused on the
choice of solvent, different amounts of the Pt catalyst, and varia-
tions in reaction time and temperature. It was observed that in
general the diboration proceeded well in a variety of different
* Corresponding author. Tel.: +43 316 3805352; fax: +43 316 3809840.
E-mail address: oliver.kappe@uni-graz.at (C. O. Kappe).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.06.017

4832
H. Prokopcová et al. / Tetrahedron Letters 49 (2008) 4831–4835
Table 1
Effect of Pt catalyst concentration on the rate of the diboration reaction of alkynes 1 with bis(pinacolato)diboron (2)a
Entry
Pt(PPh3)4 (mol %)
Temperature (°C)
1a time (min)b
1a TOF (hꢀ1
)
1b time (min)b
1b TOF (hꢀ1
)
1
2
3
4
5
3
3
1
0.1
0.05
140
180
180
180
180
20
5
20
30
—
100
400
300
2000
—
2
1000
12,000
12,000
60,000
17,200
0.2
0.5
1
7
a
Reaction conditions: single-mode microwave irradiation (Biotage Initiatior 8 EXP 2.0, 400 W), 5 mL sealed Pyrex microwave vial, 0.3 mmol alkyne 1, 1.1 equiv
L dioxane, magnetic stirring, ca. 1 g SiC passive heating element, external IR temperature monitoring.
Reaction time (fixed hold time) required for full conversion (GC–MS) at the maximum set temperature (ramp time ca. 1 min).
bis(pinacolato)diboron (2), 0.05–3 mol % Pt(PPh3)4, 480
l
b
solvents such as MeCN, dioxane, THF or DME. The use of the orig-
inally applied3 DMF as solvent for our microwave-heated process
was not considered due to the known instability of DMF at the high
reaction temperatures typically used in sealed-vessel microwave
chemistry.15 Taking into account the second reaction step in our
anticipated one-pot diboration/Suzuki protocol, dioxane and MeCN
were selected as solvents of choice for subsequent optimization.
The distinct effect of catalyst concentration and reaction tem-
perature on the rate of diboration is highlighted in Table 1. For
diphenylacetylene (1a), for example, the catalyst-loading can be
reduced from the originally used 3 mol % to 0.1 mol %. While this
transformation was reported by Suzuki and Miyaura to require
24 h at 80 °C using 3 mol % of Pt(PPh3)4 catalyst,3 we find that
using sealed-vessel microwave heating the same diboration can
be completed in 20 min at 140 °C or in 5 min at 180 °C. This corre-
sponds to a significant increase in catalyst turnover frequency
(TOF) from 1.7 hꢀ1 to 400 hꢀ1. Reducing the amount of catalyst
from 3 mol % to 0.1 mol % the diboration requires 30 min for com-
by a control experiment applying conventional heating in a sealed
reaction vessel at a similar internal reaction temperature (for fur-
ther information, see the Supplementary data).
With optimized conditions for rapid alkyne diborations in hand,
the usefulness of this efficient entry to cis-1,2-bis(boryl)alkenes
was demonstrated by electrophilic fluoro-deboronation of the
resulting bis(boryl)alkene 3a, using Selectfluor (4) as reagent
according to our recently published procedure (Scheme 1).17 While
fluorination of 3a leading to a,a-difluorinated carbonyl compound
5 under conventional conditions required 15 h at room tempera-
ture for completion,17 the high-temperature microwave protocol
was considerably faster. Using MeCN as solvent, the diboration of
diphenylacetylene (1a) using 1.1 equiv of bis(pinacolato)diboron
reagent (2) and 3.0 mol % of Pt(PPh3)4 catalyst progressed to full
conversion after 15 min at 120 °C. In a subsequent reaction step,
3.0 equiv of Selectfluor (4) and 2.5 equiv of NaHCO3 were added
into the microwave reaction vial. After resealing, the reaction mix-
ture was subjected to microwave-heating for an additional 20 min
at 100 °C. After basic workup and column chromatography, the
desired 2,2-difluoro-1,2-diphenylethanone 5 was obtained in 89%
isolated yield (Scheme 1).17
We next focused on the development of a high-speed one-pot
diboration/Suzuki cross-coupling protocol. We initially considered
the possibility to use a Pt(0) catalyst for both the diboration and
the Suzuki cross-coupling step. While Pt-catalyzed cross-coupling
reactions of organoboronic acids have been reported in the litera-
ture, the general success of Pt catalysts in these transformations
is rather limited, as both oxidative addition and reductive elimina-
tion are slower as compared to the corresponding Pd complexes.18
pletion with
a further increase in catalyst efficiency (TOF
2000 hꢀ1). For the more reactive 1-phenyl-2-butyne starting mate-
rial 1b,12 full conversion to cis-1,2-bis(boryl)alkene 3b can be
achieved with only 0.05 mol % of the Pt catalyst at 180 °C in
7 min (entry 5). Applying the more typically used 3 mol % catalyst
loading the diboration can be completed within 10 s at 180 °C. Pre-
vious diboration studies applying conventional heating at 80 °C
have reported reaction times around 24 h for 1-phenyl-2-butyne
(1b).3,12
Both diborations are very clean without any indication of
byproduct formation. Conversions can easily be monitored by
GC–MS and the purity/identity of the resulting cis-1,2-bis(boryl)-
alkenes 3 was further established by 1H NMR spectroscopy. We
have found that using a sealed microwave vial diborations of this
type can be performed without inert gas atmosphere in nonde-
gassed commercially available dioxane. One of the best sets of con-
ditions for the diboration of both internal alkyne substrates 1a,b
utilized 1.1 equiv of bis(pinacolato)diboron (2), 0.1 mol % of
Pt(PPh3)4 as a catalyst at 180 °C (5–6 bar) for 30 min (for 1a) or
1 min (for 1b) in dioxane using controlled microwave heating.
Since dioxane is a virtually microwave transparent solvent,15 the
use of a silicon carbide-passive heating element was required in
order to achieve reaction temperatures >120 °C.16 While the
observed rapid transition metal-catalyzed diborations are
impressive, it should be emphasized that the increase in reaction
rate on going from conventional heating to microwave heating is
probably the consequence of a purely thermal effect as confirmed
Scheme 1. One-pot diboration/fluorination reaction.

H. Prokopcová et al. / Tetrahedron Letters 49 (2008) 4831–4835
4833
After extensive experimentation involving a variety of different
solvent systems, bases, and reaction conditions (see the Supple-
mentary data) the concept of a unified Pt catalytic system for our
diboration/Suzuki cross-coupling protocol had to be abandoned.
This was mainly a consequence of the fact that the Suzuki reaction
generally requires aqueous basic conditions.19 Due to the compar-
atively slow Pt-catalyzed Suzuki cross-coupling, the competing
deboronation processes could not be prevented.
In contrast to the results obtained using a Pt catalyst, the corre-
sponding Pd-catalyzed bis-Suzuki cross-couplings involving cis-
1,2-bis(boryl)alkenes 3 and aromatic halides3,12–14 performed very
well under microwave conditions (Table 2). Building on our previ-
ous experience, the screening of the reaction conditions for the
coupling of bis(pinacol)ester 3a with aromatic halides was
performed by tuning all the main factors: solvent, base, type and
concentration of the Pd catalyst, aromatic halide, reaction temper-
ature, and time. Based on the successful use of dioxane in the
diboration step (Table 1), a 3:1 mixture of dioxane/H2O was rapidly
identified as a very suitable reaction medium for these Suzuki cou-
plings. In conjunction with KOH as a strong base for activation of
the diboron reagent, rapid cross-couplings were generally ob-
served. As far as the palladium catalyst is concerned, most of the
traditional catalyst/ligand systems used for Suzuki cross-couplings
such as Pd(PPh3)4, Pd(OAc)2, Pd(OAc)2/PPh3, Pd(OAc)2/tributyl-
phosphine, Pd(OAc)2/dppf, PdCl2(dppf)/dppf, and immobilized Pd
sources (Fibrecat 1001) provided useful conversions and high
selectivities.20 For the specific selected examples discussed herein,
the use of Pd(PPh3)4 and Pd(OAc)2/PPh3 generally provided opti-
mum results. Catalytic loadings apparently play a very important
role in controlling the selectivity between the desired bis-Suzuki
cross-coupling product 6a, the mono-Suzuki coupling product 8a
and the deboration byproducts 7a and 9a (Table 2). For very low
catalyst loadings (0.001 mol %), the major product in the reaction
of bis(pinacol)ester 3a with iodobenzene is the deboration by-
product 9a (Table 2, entry 1), while the use of 0.1 mol % of the same
catalyst allows a 93% conversion to the desired cross-coupling
product 6a (Table 2, entry 6). Following reaction optimization of
the Suzuki cross-coupling starting from isolated bis(pinacol)ester
3a, a further fine tuning of the reaction conditions was performed
combining both reaction steps, the Pt-catalyzed diboration and the
Pd-catalyzed Suzuki cross-coupling in one sequence. Using the
crude reaction mixture of bis(pinacol)ester 3a for the Suzuki cou-
pling step required an increase of the catalytic loading of Pd in
comparison to experiments staring from pure bis(pinacol)ester
3a (compare entries 6 and 7). Ultimately, the reaction proceeded
most effectively using 1 mol % of Pd(OAc)2, 2 mol % of PPh3 as
ligand, 2 equiv of bromobenzene, and 6 equiv of KOH in dioxane/
H2O 3:1 as a solvent. Microwave heating at 140 °C for 30 min pro-
vided full conversion and 97% HPLC selectivity for the desired
product 6a, leading to a 94% isolated yield of pure product (Table
2, entry 10).21 In comparison, conventionally processed Suzuki
cross-couplings involving bis(pinacol)esters of type 3 and aryl ha-
lides require 12 h at 90 °C for completion.3
Table 2
Optimization of reaction conditions for the one-pot diboration/Suzuki cross-couplinga
With optimized conditions for a microwave-assisted one-pot
diboration/Suzuki cross-coupling protocol in hand, we then
focused our attention on the scope and limitations of this protocol
and to the generation of a small collection of 1,2-cis-substituted
alkenes 6 (Table 3). For this purpose, we selected eight terminal
and internal alkynes 1a–h as starting materials in combination
with seven different substituted bromobenzene derivatives. As
can be seen from the data presented in Table 3, some minor adjust-
ments in both catalyst loading and reaction time were necessary in
the diboration step for the less reactive, aromatic internal alkynes
1d–h and for terminal alkyne 1h. Gratifyingly, no changes were
required for the Suzuki procedure and in all 12 cases the reaction
proceeded to completion within 30 min at 140 °C. In three cases
(Table 3, entries 3, 7, and 12) small amounts (<15%) of the corre-
sponding trans alkene-isomer was detected by NMR spectroscopy
(see Supplementary data). The constitution of the major cis-isomer
was confirmed by 2D NMR experiments (NOESY, HMBC). Impor-
tantly, all of the utilized alkynes were transformed to the desired
final cis-alkene derivatives 6 within ca. 0.5–1.5 h overall processing
time. The isolated yields after column chromatography were be-
tween 21% and 98%.22
In conclusion, we have developed a microwave-assisted one-
pot diboration/Suzuki cross-coupling protocol that allows the
rapid synthesis of tri- and tetrasubstituted ethylenes. Taking
advantage of sealed-vessel microwave processing at high temper-
atures both steps in the sequence were dramatically improved.
For the Pt-catalyzed diboration reaction 1?3, reaction times were
reduced from 12 h using conventional heating at 80 °C to less than
a minute using microwave processing at 180 °C for a selection of
seven internal and one terminal alkyne. At the same time, we have
shown that the amount of Pt catalyst can be reduced from 3 mol %
to 0.1 mol % without loss of efficiency. Combined with the higher
reaction rates, this leads to a significant increase in catalyst turn-
over frequencies. Similarly, in the Pd-catalyzed Suzuki cross-cou-
pling step 3?6, the necessary reaction times of 12 h using
Entry PhX
Catalyst
Catalyst (mol %) Conv.b 7a/8a/9a/6a Yieldc (%)
1
2
3
4
PhI
PhI
PhI
PhI
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
0.001
0.004
0.008
0.01
0.02
0.1
22/0/65/13
3/0/28/69
0/0/21/79
0/0/18/81
0/0/17/83
0/0/7/93
—
—
—
—
—
—
—
56
5
PhBr Pd(PPh3)4
PhBr Pd(PPh3)4
PhBr Pd(PPh3)4
PhBr Pd(OAc)2/
PPh3
PhBr Pd(OAc)2/
PPh3
PhBr Pd(OAc)2/
PPh3
6
7d
8d
0.1
0.1/0.2
3/7/53/37
0/0/20/80
9d
0.2/0.4
1/2
0/0/15/85
0/0/3/97
72
94
10d
a
Single-mode microwave irradiation (Biotage Initiatior 8 EXP 2.0, 400 W), 5 mL
sealed Pyrex microwave vial, magnetic stirring, external IR temperature monitor-
ing. For conditions for step 1 (diboration), see Table 1 entry 4. For step 2 (Suzuki
reaction): 2 equiv of PhX, 6 equiv of KOH, 0.001–1 mol % Pd, dioxane/H2O 3:1
(600 lL), 140 °C, 30 min. For more details, see Ref. 21.
b
Product distribution refers to relative peak area (%) ratios of crude HPLC–UV
(215 nm) traces.
c
Isolated product yield after column chromatography.
Data for one-pot diboration/Suzuki cross-coupling method.
d

4834
H. Prokopcová et al. / Tetrahedron Letters 49 (2008) 4831–4835
Table 3
One-pot diboration/Suzuki cross-coupling of alkynes 1 with arylbromidesa
Entry
1
R1
R2
Pt(PPh3)4 (mol %)
Timeb (min)
R3
Yieldc (%)
1
2
3
4
5
6
7
8
9
1a
1a
1b
1b
1b
1c
1c
1d
1e
1f
Ph
Ph
Et
Et
Et
Pr
Pr
p-CF3Ph
p-CF3Ph
Ph
p-OMePh
H
H
H
H
H
H
H
H
0.1
0.1
0.1
0.1
0.1
0.1
0.1
2
1
2
1
1
30
30
1
1
1
H
94
73
98
69
93
73
96
59
21
66
62
60
m-CN
p-OMe
m-Me
p-CN
H
p-OMe
p-OMe
p-Cl
3
3
p-OMe
p-CF3
OMe
OMe
OMe
60
60
60
30
30
10
11
12
p-CHO
p-CHO
H
1g
1h
a
Single-mode microwave irradiation (Biotage Initiatior 8 EXP 2.0, 400 W), 5 mL sealed Pyrex microwave vial, magnetic stirring, external IR temperature monitoring. For
conditions, for step 1 (diboration), see Table 1, entry 4; for step 2 (Suzuki reaction), see Table 2, entry 10. For more details, see the Supplementary data.
b
Hold time required for full diboration (GC–MS) at 180 °C (ramp time ca 1 min).
Isolated product yield after column chromatography.
c
5. Pt(PPh3)2[B(OR2)]2: Lesley, G.; Nguyen, P.; Taylor, N. J.; Marder, T. B.; Scott, A. J.;
conventional heating at 90 °C were reduced to 30 min at 140 °C
using sealed-vessel microwave heating. The combined sequence
allows the efficient preparation of densely functionalized ethyl-
enes of the tamoxifen type.
Clegg, W.; Norman, N. C. Organometallics 1996, 15, 5137–5154.
6. Pt(PCy3)(g
2-C2H4)2: Thomas, R. L.; Souza, F. E. S.; Marder, T. B. J. Chem. Soc.,
Dalton Trans. 2001, 1650–1656.
7. Pt(COD)Cl2: Mann, G.; John, K. D.; Baker, R. T. Org. Lett. 2000, 2, 2105–2108.
8. Pt(0)–NHC complexes: Lillo, V.; Mata, J.; Ramírez, J.; Peris, E.; Fernández, E.
Organometallics 2006, 25, 5829–5831.
9. Braunschweig, H.; Kupfer, T.; Lutz, M.; Radacki, K.; Seeler, F.; Sigritz, R. Angew.
Chem., Int. Ed. 2006, 45, 8048–8051.
Acknowledgments
10. Adams, C. J.; Baber, R. A.; Batsanov, A. S.; Bramham, G.; Charmant, J. P. H.;
Haddow, M. F.; Howard, J. A. K.; Lam, W. H.; Lin, Z.; Marder, T. B.; Norman, N.
C.; Orpen, A. G. Dalton Trans. 2006, 1370–1373.
11. Lillo, V.; Fructos, M. R.; Ramírez, J.; Braga, A. A. C.; Maseras, F.; Díaz-Requejo,
M. M.; Pérez, P. J.; Fernández, E. Chem. Eur. J. 2007, 13, 2614–2621.
12. (a) Brown, S. D.; Armstrong, R. W. J. Am. Chem. Soc. 1996, 118, 6331–6332; (b)
Brown, S. D.; Armstrong, R. W. J. Org. Chem. 1997, 62, 7076–7077; (c)
Wenckens, M.; Jacobsen, P.; Vedsø, P.; Huusfeldt, P. O.; Gissel, B.; Barfoed,
M.; Brockdorff, B. L.; Lykkesfeldt, A. E.; Begtrup, M. Bioorg. Med. Chem. 2003, 11,
1883–1899.
This work was supported by a grant from the Christian Doppler
Society (CDG). C.O.K. thanks the ‘Bundesministerium für Wissens-
chaft und Forschung’ for an Acciones Integradas grant (9/2006). E.F.
thanks the CTQ2007-60442/BQU for financial support, the General-
itat de Catalunya for providing J.R. with a fellowship, and MEC for
Acciones Integradas HU2005-0028. Professor Klaus Zangger is
thanked for help with 2D NMR spectra.
13. Perret-Aebi, L.-E.; von Zelewsky, A. Synlett 2002, 773–774.
14. (a) Baldoli, C.; Bossi, A.; Giannini, C.; Licandro, E.; Maiorana, S.; Perdicchia, D.;
Schiavo, M. Synlett 2005, 1137–1141; (b) Licandro, E.; Rigamonti, C.; Ticozzelli,
M. T.; Monteforte, M.; Baldoli, C.; Giannini, C.; Maiorana, S. Synthesis 2006,
3670–3678.
Supplementary data
Supplementary data associated with this article (experimental
procedures, NMR spectra) can be found in the online version at,
doi:10.1016/j.tetlet.2008.06.017.
15. Kappe, C. O. Angew. Chem., Int. Ed. 2004, 43, 6250–6284 and references cited
therein.
16. Silicon carbide (SiC) passive heating elements absorb microwave energy and
subsequently transfer the generated thermal energy through conduction and
convection to the reaction mixture and therefore allow the efficient heating of
nonpolar, low microwave-absorbing media: Kremsner, J. M.; Kappe, C. O. J. Org.
Chem. 2006, 71, 4651–4658.
17. (a) Ramírez, J.; Fernández, E. Synthesis 2005, 1698–1700; (b) Ramírez, J.;
Fernández, E. Tetrahedron Lett. 2007, 48, 3841–3845.
18. (a) Bedford, R. B.; Hazelwood, S. L. Organometallics 2002, 21, 2599–2600; (b)
Oh, C. H.; Lim, Y. M.; You, C. H. Tetrahedron Lett. 2002, 43, 4645–4647; (c) Lillo,
V.; Mata, J. A.; Segarra, A. M.; Peris, E.; Fernandez, E. Chem. Commun. 2007,
2184–2186.
19. Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457–2483.
20. For microwave-assisted Suzuki reactions, see: (a) Appukkuttan, P.; Van der
Eycken, E. Eur. J. Org. Chem. 2008, 1133–1155; (b) Leadbeater, N. Chem.
Commun. 2005, 2881–2902.
21. Typical procedure (Table 2, entry 10): A 5 mL microwave process vial was
charged with a stir bar and a SiC heating element (ca. 1 g). To the vessel were
References and notes
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Chem. 2008, 12, 405–423; (f) Miura, T.; Murakami, M. Chem. Commun. 2007,
217–224.
2. (a) Brown, H. C. Organic Synthesis via Boranes; Wiley-Interscience: New York,
1975; (b) Pelter, A.; Smith, K.; Brown, H. C. Borane Reagents; Academic Press:
New York, 1988; (c) Onak, T. Organoborane Chemistry; Academic Press: New
York, 1975; (d) Matteson, D. S. Stereodirected Synthesis with Organoboranes;
Springer: Berlin, 1995; (e) Kaufmann, D. E.; Matteson, D. S. Science of Synthesis,
Boron Compounds; Thieme: Stuttgart, 2005.
3. (a) Ishiyama, T.; Matsuda, N.; Miyaura, N.; Suzuki, A. J. Am. Chem. Soc. 1993, 115,
11018–11019; (b) Ishiyama, T.; Matsuda, N.; Murata, M.; Ozawa, F.; Suzuki, A.;
Miyaura, N. Organometallics 1996, 15, 713–720.
added bis(pinacolato)diboron
Pt(PPh3)4 stock solution (100
0.1 mol %), and diphenylacetylene (1a) (53.5 mg, 0.3 mmol). After the addition
of dioxane (380 L) the reaction vessel was sealed and the mixture was
subsequently heated in a microwave reactor at 180 °C for 30 min. After cooling
2
(83.8 mg, 0.33 mmol), freshly prepared
l
L, 0.003 M solution in dioxane, 0.0003 mmol,
4. Pt(PPh3)2(
Ed. 1996, 35, 1501–1503.
g
2-C2H4): Maderna, A.; Pritzkow, H.; Siebert, W. Angew. Chem., Int.
l

H. Prokopcová et al. / Tetrahedron Letters 49 (2008) 4831–4835
4835
and opening of the sealed vial, bromobenzene (63.2
prepared Pd(OAc)2 stock solution (100 L, 0.03 M solution in dioxane, 1 mol %),
freshly prepared PPh3 stock solution (20 L, 0.3 M solution in dioxane,
2 mol %), and KOH (200 L, 9 M solution in H2O) were added and the
reaction mixture subsequently heated under sealed vessel microwave
conditions at 140 °C for 30 min. After cooling, H2O (60 mL) was added and
the crude reaction mixture was extracted with CHCl3 (3 ꢁ 20 mL). The
combined organic phases were dried over MgSO4 and the solvent was
l
L, 0.6 mmol), freshly
removed under reduced pressure. Purification by flash chromatography on
silica gel (dichloromethane/hexanes) provided 93.7 mg (94%) of tetraphenyl-
acetylene 6a as a white solid, mp 226–228 °C.
l
l
l
22. In addition to bis-Suzuki reactions of bis(pinacol)esters 3 with aryl bromides,
we have also attempted sequential mono-Suzuki cross-coupling protocols. In
agreement with previous reports (Ref. 12), we find these transformations to be
rather unselective and to produce mixtures of the corresponding regioisomers
(see Supplementary data for further information).