Palladium-catalyzed hydroarylation of alkynes with arenediazonium salts

Article abstract of DOI:10.1021/ol800266e

The palladium-catalyzed hydroarylation of arenediazonium tetrafluoroborates with alkynes in the presence of triphenylsilane affords stereoselectively hydroarylation products in moderate to high yields. The reaction tolerates a variety of substituents including keto, ester, cyano, and nitro groups and can be performed as a one-pot procedure generating the arenediazonium salt in situ. With ethyl phenylpropynoate as the starting alkyne, the hydroarylation affords ethyl (2)-2-arylcinnamates stereo- and regioselectively.

Full text of DOI:10.1021/ol800266e

ORGANIC
LETTERS
2008
Vol. 10, No. 8
1597-1600
Palladium-Catalyzed Hydroarylation of
Alkynes with Arenediazonium Salts
Sandro Cacchi,* Giancarlo Fabrizi, Antonella Goggiamani, and Daniela Persiani
Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente AttiVe,
UniVersita` degli Studi “La Sapienza”, P.le A. Moro 5, 00185 Rome, Italy
sandro.cacchi@uniroma1.it
Received February 6, 2008
ABSTRACT
The palladium-catalyzed hydroarylation of arenediazonium tetrafluoroborates with alkynes in the presence of triphenylsilane affords
stereoselectively hydroarylation products in moderate to high yields. The reaction tolerates a variety of substituents including keto, ester,
cyano, and nitro groups and can be performed as a one-pot procedure generating the arenediazonium salt in situ. With ethyl phenylpropynoate
as the starting alkyne, the hydroarylation affords ethyl (Z)-2-arylcinnamates stereo- and regioselectively.
After the first palladium-catalyzed arylation of olefins with
arenediazonium salts reported by Matsuda et al.,1 arenedi-
Scheme 1
azonium salts have been used as aryl partners in a variety
of palladium-catalyzed reactions including Mizoroki-Heck
reactions,2 Suzuki-Miyaura2g,3 and Stille4 cross-couplings,
and carbonylation reactions.2h,i,5 They have also been used
as precursors of sulfinic acids6 and boronic esters.7 Indeed,
their higher reactivity, the utilization of mild conditions,
aqueous medium, their availability from inexpensive anilines,
and the absence of added bases in several applications make
them an attractive alternative to aryl halides or triflates.8
Nevertheless, very little has been done with alkynes. To the
best of our knowledge, the sole attempt to involve alkyne
partners in a reaction with arenediazonium salts was made
by Geneˆt et al.,9 who described the palladium-catalyzed
cross-coupling of potassium 1-hexenyltrifluoroborate with
p-toluenediazonium tetrafluoroborate. However, only small
amounts of the desired cross-coupling derivative were ob-
served, the starting diazonium salt being quantitatively
reduced.
be a useful tool for the preparation of substituted olefins.
With alkynes containing nucleophilic and electrophilic
centers close to the carbon-carbon triple bond, the reaction
affords cyclic derivatives.10c The intramolecular version of
the reaction has also been described.11 The extension of the
hydroarylation protocol to arenediazonium salts would widen
significantly the synthetic scope of this chemistry. Herein
we report the results of this study.
The reaction of diphenylacetylene with p-methoxyben-
zenediazonium tetrafluoroborate was initially examined as
the model system. Phosphine-free conditions were selected
on the basis of the known detrimental effect of phosphine
ligands in many palladium-catalyzed reactions of arenedi-
azonium salts12 and our previous experience with the
hydrovinylation of vinyl triflates wherein phosphines were
found to hamper the coordination of the alkyne to the
palladium atom of vinylpalladium intermediates (an event
which precedes the carbopalladation step) and favor the
reduction of vinyl triflates to alkenes via displacement of
Because of our ongoing interest in alkyne-based palladium-
catalyzed processes,10 we decided to investigate the pal-
ladium-catalyzed hydroarylation of internal alkynes with
arenediazonium salts (Scheme 1).
The palladium-catalyzed hydroarylation of alkynes with
aryl and vinyl halides or triflates10b,c,11 has been shown to
(1) Kikukawa, K.; Matsuda, T. Chem. Lett. 1977, 159.
10.1021/ol800266e CCC: $40.75
© 2008 American Chemical Society
Published on Web 03/20/2008

formation was obtained using diphenylacetylene and p-
methoxybenzenediazonium tetrafluoroborate with HCO2K
and Pd(OAc)2 in THF, EtOH, or DMF as the solvent at
temperatures ranging from rt to 60 °C. Surmising that in the
presence of formate anions the reduction of the alkyne and
the arylpalladium intermediate is faster than the carbopal-
ladation reaction, we decided to explore the utilization of
other, possibly more selective, reducing agents and focused
our attention on trialkylsilanes. After a number of experi-
ments, success was finally encountered under the conditions
reported in Table 1, entry 12, which allowed for the isolation
of 3a in 82% yield.14
Table 1. Reducing Agents, Solvents, and Temperature in the
Palladium-Catalyzed Hydroarylation of Diphenylacetylene with
p-Methoxybenzenediazonium Tetrafluoroboratea
reducing
agent
T
(°C)
time yieldb of
entry
solvent
THF
THF
THF/MeOHc
DMF
(h)
3a (%)
E/Z
1
2
3
4
5
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
rt 24
40 24
rt
rt 24
rt 24
rt 24
rt 24
rt 24
19
>99:1
traces
13
6
>99:1
MeCN
21
32
53
62
25
35
62
82d
71d
>99:1
>99:1
>99:1
>99:1
97:3
6
Oct3SiH THF
7
i-Pr3SiH THF
These “optimal” conditions were then applied to other
arenediazonium tetrafluoroborates (Table 2). Hydroarylation
8
9
10
11
12
13
Ph3SiH
Ph3SiH
Ph3SiH
Ph3SiH
Ph3SiH
THF
THF
MeCN
DMF
THF
40
rt
rt
rt
rt
2
2
0.75
3.5
1.5
97:3
>99:1
>99:1
>99:1
Table 2. Palladium-Catalyzed Hydroarylation of
Diphenylacetylene with Arenediazonium Tetrafluoroborates 2a
i-Pr3SiH THF
entry
R1
2
time (h)
yield of 3b,c (%)
a Unless otherwise stated, reactions of entries 7-19 were carried out on
a 0.5 mmol scale, under argon, using 1 equiv of diphenylacetylene, 1.5
equiv of p-methoxybenzenediazonium tetrafluoroborate, 2 equiv of R3SiH,
and 0.02 equiv of Pd(OAc)2 in 4 mL of solvent. b Yields are given for
isolated products. c THF/MeOH 1:1. d In the presence of 4 equiv of
p-methoxybenzenediazonium tetrafluoroborate.
1
2
3
4
5
6
7
8
9
p-MeO-
m-CF3-
p-Me-
p-MeCO-
m-MeO-
o-MeO-
3,4,5-(MeO)3-
2-Me,4-F-
p-CN-
2a
2b
2c
2d
2e
2f
2g
2h
2i
3.5
7
5
82, 3a
58, 3b
86, 3c
59, 3d
77, 3e
56,d 3f
51, 3g
66,d 3h
54, 3i
42, 3j
89, 3k
53, 3l
8
22
24
24
24
24
24
24
24
the triflate anion by formate.13 Assuming that some similarity
might exist between arylpalladium intermediates derived
from the reaction of arenediazonium salts with Pd(0) and
vinylpalladium intermediates derived from the oxidative
addition of vinyl triflates to Pd(0) (at least as far as the
intermediacy of cationic organopalladium complexes is
concerned), the utilization of phosphine-free conditions
appeared appropriate to our study. Representative optimiza-
tion experiments are summarized in Table 1.
10
11
12
p-NO2-
2,4-Me2-
p-MeO2C-
2j
2k
2l
a Reactions were carried out under argon on a 0.5 mmol scale using 1
equiv of diphenylacetylene, 4 equiv of 2, 2 equiv of Ph3SiH, 0.02 equiv of
Pd(OAc)2 in 4 mL of anhydrous THF at rt. b Yields are given for isolated
products. c E/Z ratios were calculated by NMR analyses and were usually
found to be higher than 99:1. d E/Z ) 96:4.
Potassium formate is commonly used in hydroarylation
reactions with aryl halides, but no evidence of alkene
(2) For some recent selected references, see: (a) Brunner, H.; Le
Cousturier de Courcy, N.; Geneˆt, J.-P. Synlett 2000, 201. (b) Cai, M. Z.;
Hu, R. H.; Zhou, J. Chin. Chem. Lett. 2001, 12, 861. (c) Hu, R.-H.; Liu,
X.-L.; Cai, M.-Z. Jiangxi Shifan Daxue Xuebao, Ziran Kexueban 2001,
25, 246; Chem. Abstr. 2001, 136, 355024. (d) Darses, S.; Pucheault, M.;
Geneˆt, J.-P. Eur. J. Org. Chem. 2001, 1121. (e) Sengupta, S.; Bhattacharyya,
S. Tetrahedron Lett. 2001, 42, 2035. (f) Andrus, M. B.; Song, C.; Zhang,
J. Org. Lett. 2002, 4, 2079. (g) Selvakumar, K.; Zapf, A.; Spannenberg,
A.; Beller, M. Chem. Eur. J. 2002, 8, 3901. (h) Andrus, M. B.; Ma, Y.;
Zang, Y.; Song, C. Tetrahedron Lett. 2002, 43, 9137. (i) Ma, Y.; Song, C.;
Chai, Q.; Ma, C.; Andrus, M. B. Synthesis 2003, 2886. (j) Wang, C.; Tan,
L.-S.; He, J.-P.; Hu, H.-W.; Xu, J.-H. Synth. Commun. 2003, 33, 773. (k)
Masllorens, J.; Moreno-Man˜as, M.; Pla-Quintana, A.; Roglans, A. Org. Lett.
2003, 5, 1559. (l) Schmidt, B. Chem. Commun. 2003, 1656. (m) Nelson,
M. L.; Ismail, M. Y.; McIntyre, L.; Bhatia, B.; Viski, P.; Hawkins, P.;
Rennie, G.; Andorsky, D.; Messersmith, D.; Stapleton, K.; Dumornay, J.;
Sheahan, P.; Verma, A. K.; Warchol, T.; Levy, S. B. J. Org. Chem. 2003,
68, 5838. (n) Dai, M.; Liang, B.; Wang, C.; Chen, J.; Yang, Z. Org. Lett.
2004, 6, 221. (o) Kabalka, G. W.; Dong, G.; Venkataiah, B. Tetrahedron
Lett. 2004, 45, 2775. (p) Xu, L.-H.; Zhang, Y.-Y.; Wang, X.-L.; Chou,
J.-Y. Dyes Pigm. 2004, 62, 283. (q) Sabino, A. A.; Machado, A. H. L.;
Correia, C. R. D.; Eberlin, M. N. Angew. Chem., Int. Ed. 2004, 43, 2514.
(r) Sabino, A. A.; Machado, A. H. L.; Correia, C. R. D.; Eberlin, M. N.
Angew. Chem., Int. Ed. 2004, 43, 4389. (s) Masllorens, J.; Bouquillon, S.;
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M. A. G.; Santana, C. C.; Correia, C. R. D. J. Org. Chem. 2005, 70, 1050.
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1657. (v) Perez, R.; Veronese, D.; Coelho, F.; Antunes, O. A. C. Tetrahedron
Lett. 2006, 47, 1325.
products were isolated in moderate to high yields. The
reaction tolerates a variety of substituents including keto,
ester, cyano, and nitro groups (palladium-catalyzed reactions
with the latter frequently fail because of their tendency to
(3) For some recent references, see: (a) Willis, D. M.; Strongin, R. M.
Tetrahedron Lett. 2000, 41, 6271. (b) Babudri, F.; Farinola, G. M.; Naso,
F.; Panessa, D. J. Org. Chem. 2000, 65, 1554. (c) Andrus, M. B.; Song, C.
Org. Lett. 2001, 3, 3761. (d) Frohn, H.-J.; Adonin, N. Y.; Bardin, V. V.;
Starichenko, V. F. J. Fluorine Chem. 2002, 117, 115. (e) Peyroux, E.;
Berthiol, F.; Doucet, H.; Santelli, M. Eur. J. Org. Chem. 2004, 1075. (f)
Jo, J.; Chi, C.; Ho¨ger, S.; Wegner, G.; Yoon, D. Y. Chem. Eur. J. 2004,
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N. Eur. J. Inorg. Chem. 2005, 582. See also: (h) Darses, S.; Genet, J. P.
Eur. J. Org. Chem. 2003, 4313.
(4) (a) Kikukawa, K.; Kono, K.; Wada, F.; Matsuda, T. Chem. Lett. 1982,
35. (b) Kikukawa, K.; Kono, K.; Wada, F.; Matsuda, T. J. Org. Chem.
1983, 48, 1333. (c) Kikukawa, K.; Idemoto, T.; Katayama, A.; Kono, K.;
Wada, F.; Matsuda, T. J. Chem. Soc., Perkin Trans. 1 1987, 1511. (d)
Dughera, S. Synthesis 2006, 1117.
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Tetrahedron Lett. 1980, 21, 2877. (b) Nagira, K.; Kikukawa, K.; Wada, F.;
Matsuda, T. J. Org. Chem. 1980, 45, 2365. (c) Kikukawa, K.; Kono, K.;
Nagira, K.; Wada, F.; Matsuda, T. J. Org. Chem. 1981, 46, 4413. (d)
Sengupta, S.; Sadhukhan, S. K.; Bhattacharyya, S.; Guha, J. J. Chem. Soc.,
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1598
Org. Lett., Vol. 10, No. 8, 2008

decompose12). It is also highly stereoselective: E/Z ratios
usually exceed 99:1. Only with o-methoxy- and 2-methyl-
4-fluorobenzenediazonium tetrafluoroborates a lower ste-
reoselectivity was attained (Table 2, entries 6 and 8). The
(E) stereochemistry of 3a was assigned by NOESY experi-
ments which showed a strong cross-peak between the vinylic
proton and the ortho hydrogens of the added aryl unit. That
of the other hydroarylation derivatives was assigned based
on this data and all our previous work on hydroarylation
reactions in which the cis adduct has usually been the main
product.
preparation of p-methoxybenzenediazonium tetrafluoroborate
concentrated under reduced pressure.
The hydroarylation of diarylacetylenes asymmetrically
substituted with electron-donating and electron-withdrawing
groups was then explored to investigate the role of electronic
effects on the regioselectivity of this hydroarylation process
(Table 3). According to previous hydroarylations of aryl
Table 3
It is worth noting that arenediazonium salts containing
ortho substituents can give hydroarylation products in good
to high yields (Table 2, entries 6, 8, and 11), while ortho-
substituted aryl iodides are reluctant to give the same
derivatives. A remarkable example of this behavior is
provided by the reaction of o-iodoanisole with diphenyl-
acetylene. Under conditions that are known to afford the
desired products with a variety of meta- and para-substituted
aryl iodides (HCOOK, HCOOH/Et3N, with and without
PPh3, in DMF or THF at rt -40 °C), the corresponding
hydroarylation derivative was isolated in 7-17% yields.15
time 5 + 5′ yield yield of yield of
R1, 2
p-MeO, 2a p-MeO, 4a
p-Me, 2c p-MeO, 4a
p-MeO, 2a p-MeCO, 4b
R2, 4
(h)
(%) (5/5′)
6 (%)
7 (%)
The hydroarylation reaction was also performed generating
the arenediazonium salt in situ.16 This protocol was attempted
with p-anisidine and diphenylacetylene (Scheme 2). The best
7
6
7
71 (56/44)
86 (53/47)
20a (56/44)
15
7
31
a 4b was recovered in 36% yield.
Scheme 2
iodides17 and related palladium-catalyzed additions of “arylpal-
ladium halides” to alkynes,18 the results obtained suggest that
electronic effects play a minor role in controlling the regio-
chemistry of the reaction. Regioisomeric hydroarylation prod-
ucts (their regiochemistry was not assigned) were indeed iso-
lated in almost equimolar amounts. The nature of the sub-
stituents on the alkyne partner, however, can influence the
carbopalladation/reduction (of the C-C triple bond) ratio
(Table 3).
result was obtained by adding the reagents required for the
hydroarylation step to the crude mixture resulting from the
The same minor role of electronic effects on the regio-
chemistry of the reaction was observed with ethyl phenyl-
propynoate (Table 4). Apparently, the phenyl group exerts
more influence on the direction of the addition than the ester
groups does and regioisomeric derivatives 9′, which would
be expected to be favored by an electronic biased process,
were isolated in small amounts, the main products being ethyl
(Z)-2-arylcinnamates 9 (E/Z > 99:1). The latter were isolated
in yields comparable to those obtained with aryl iodides,16
providing a simple straightforward route to this class of
compounds.
(7) (a) Willis, D. M.; Strongin, R. M. Tetrahedron Lett. 2000, 41, 8683.
(b) Ma, Y.; Song, C.; Jiang, W.; Xue, G.; Cannon, J. F.; Wang, X.; Andrus,
M. B. Org. Lett. 2003, 5, 4635.
(8) For an excellent recent review on the palladium chemistry of
arenediazonium salts, see: Roglans, A.; Pla-Quintana, A.; Moreno-Man˜as,
M. Chem. ReV. 2006, 106, 4622.
(9) Darses, S.; Michaud, G.; Geneˆt, J. P. Eur. J. Org. Chem. 1999, 1875.
(10) (a) Cacchi, S.; Fabrizi, G.; Goggiamani, A. AdV. Synth. Catal. 2006,
348, 1301. (b) Ahlquist, M.; Fabrizi, G.; Cacchi, S.; Norrby, P.-O. J. Am.
Chem. Soc. 2006, 128, 12785. (c) Arcadi, A.; Cacchi, S.; Fabrizi, G.;
Marinelli, F.; Verdecchia, M. Synlett 2006, 909. (d) Ambrogio, I.; Cacchi,
S.; Fabrizi, G. Tetrahedron Lett. 2007, 48, 7721. (e) Soler, R.; Cacchi, S.;
Fabrizi, G.; Forte, G.; Mart´ın, L.; Mart´ınez, S.; Molins, E.; Moreno-Man˜as,
M.; Petrucci, F.; Roig, A.; Sebastia´n, R. M.; Vallribera, A. Synlett 2007,
3068.
(11) (a) Cacchi, S.; Fabrizi, G. In Handbook of Organopalladium
Chemistry for Organic Synthesis; Negishi, E., Ed.; John Wiley & Sons:
New York, 2002; Vol. 1, pp 1335-1360. (b) Cacchi, S.; Fabrizi, G.;
Goggiamani, A. J. Mol. Catal. A: Chem. 2004, 214, 57.
(12) (a) Kikukawa, K.; Nagira, K.; Wada, F.; Matsuda, T. Tetrahedron
1981, 37, 31. (b) Sengupta, S.; Bhattacharyya, S. J. Chem. Soc., Perkin
Trans. 1 1993, 1943.
(13) Arcadi, A.; Cacchi, S.; Fabrizi, G.; Marinelli, F.; Pace, P. Eur. J.
Org. Chem. 1999, 3305.
(14) The utilization of a large excess of the arenediazonium salt is due
to its tendency to undergo several competitive side reactions such as the
formation of the corresponding arene and fluoroarene.
(15) The reaction of p-iodoanisole with diphenylacetylene in the presence
of Ph3SiH, Pd(OAc)2 at 40 °C using THF and DMF as solvents was also
attempted, but no evidence of formation of the corresponding hydroarylation
product was attained.
(16) Doyle, M. P.; Bryker, W. J. J. Org. Chem. 1979, 44, 1572.
(17) Cacchi, S.; Fabrizi, G.; Moro, L.; Pace, P. Synlett 1997, 1367.
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6, 2029. (b) Maassarani, F.; Pfeffer, M.; Le BOrgne, G. Organometallics
1987, 6, 2043. (c) Beydoun, N.; Pfeffer, M. Synthesis 1990, 729. (d) Larock,
R. C.; Yum, E. K.; Doty, M. J.; Sham, K. K. C. J. Org. Chem. 1995, 60,
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Tetrahedron 1996, 52, 10225.
Org. Lett., Vol. 10, No. 8, 2008
1599

likely in view of the known19 reluctancy of simple vinyltri-
alkylsilanes to enter transmetalation processes. This is due
to the extremely low polarity of the C-Si bond. Vinylfluoro-
or vinylalkoxysilanes in the presence of fluoride anions are
usually required to perform palladium-catalyzed cross-
coupling reactions.20 Even an addition-elimination mech-
anism on vinyltrialkylsilane intermediates, as proposed for
the palladium-catalyzed arylation of substituted vinylsilanes
with arenediazonium tetrafluoroborates,3b,21 seems to be
flawed by the stereo- and regioselectivity of the hydroary-
lation reaction. Indeed, a lack of stereo- and regioselectivity
would be expected if an addition-elimination mechanism
was operating.
In conclusion, we have described the first alkyne-based
palladium-catalyzed synthetic application of arenediazonium
salts. The reaction tolerates a variety of substituents including
keto, ester, cyano, and nitro groups, is highly stereoselective,
and can be performed as a one-pot process generating the
arenediazonium salt in situ; and with ethyl phenylpropynoate
a satisfactory regioselectivity is observed, comparable to that
obtained with aryl iodides. Further studies on this chemistry
are currently underway.
Table 4. Pd-Catalyzed Hydroarylation of 8 with 2
R1, 2
time (h) yield of 9 (%) yield of 9′ (%)
9/9′
p-MeO, 2a
m-CF3, 2b
p-Me, 2c
7
9
8
9
55, 9a
55, 9b
44, 9c
55, 9d
9, 9a′
5, 9b′
6, 9c′
86:14
91:9
88:12
81:19
m-MeO, 2e
13, 9d′
As to the mechanism, we believe that the reaction proceeds
through the basic steps outlined in Scheme 3: (a) stereose-
Scheme 3
Acknowledgment. We gratefully acknowledge Glaxo-
SmithKline for their generous financial support and a
fellowship position and Dr. Antonella Carangio and Dr.
Alcide Perboni of GlaxoSmithKline for valuable discussions.
We are also indebted to MURST and to the University “La
Sapienza”.
Supporting Information Available: A complete descrip-
tion of experimental details and product characterization. This
material is available free of charge via the Internet at
http://pubs.acs.org.
lective addition of the initially formed σ-arylpalladium
complex A to the C-C triple bond, (b) reaction of the
resultant carbopalladation adduct B with trialkylsilane to give
the vinylpalladiumhydride intermediate C (along with tri-
alkylsilylfluoride, the presence of which has been evidenced
in almost all the reactions investigated), (c) reductive
elimination.
OL800266E
(19) Andreini, B. P.; Carpita, A.; Rossi, R.; Scamuzzi, B. Tetrahedron
1989, 45, 5621.
(20) (a) Hatanaka, Y.; Hiyama, T. Synlett 1991, 845. (b) Hiyama, T.;
Hatanaka, Y. Pure Appl. Chem. 1994, 66, 1471. (c) Hagiwara, E.; Gouda,
K.; Hiyama, T. Tetrahedron Lett. 1997, 38, 439.
(21) (a) Kikukawa, K.; Ikenaga, K.; Wada, F.; Matsuda, T. Chem. Lett.
1983, 1337. (b) Ikenaga, K.; Kikukawa, K.; Matsuda, T. J. Chem. Soc.,
Perkin Trans. 1 1986, 1959. (c) Ikenaga, K.; Matsumoto, S.; Kikukawa,
K.; Matsuda, T. Chem. Lett. 1988, 873.
The alternative mechanism involving an hydrosilylation
step to give vinyltrialkylsilane intermediates followed by their
cross-coupling with arylpalladium complexes appears less
1600
Org. Lett., Vol. 10, No. 8, 2008