Stereoselective cis-addition of aromatic C - H bonds to alkynes catalyzed by dinuclear palladium complexes

Article abstract of DOI:10.1021/ja0375075

Reactions of alkynes with arenes proceeded in the presence of dinuclear palladium complexes and trialkylboranes to yield alkyne hydroarylation products with high stereoselectivity. In the reactions of monosubstituted benzenes, meta and para products were

Full text of DOI:10.1021/ja0375075

Published on Web 09/13/2003
Stereoselective cis-Addition of Aromatic C-H Bonds to Alkynes Catalyzed by
Dinuclear Palladium Complexes
Naofumi Tsukada,* Tomohiro Mitsuboshi, Hiroyuki Setoguchi, and Yoshio Inoue*
Department of Materials Chemistry, Graduate School of Engineering, Tohoku UniVersity, Sendai 980-8579, Japan
Received July 24, 2003; E-mail: tsukada@aporg.che.tohoku.ac.jp
Table 1. Pd-Catalyzed Hydrophenylation of Alkynesa
The catalytic C-C bond forming reaction via C-H bond
c
activation by transition metal complexes is one of the most attractive
subjects in synthetic organic chemistry,1 especially the addition of
C-H to unsaturated bonds,1e which is an ideal atom economical
process.2 Although many addition reactions of ortho and peri C-H
bonds of arenes with directing substituents to alkenes and alkynes
via chelation-assisted C-H activation have recently been reported
by several groups,3,4 the addition of simple arenes via direct C-H
activation remains rare. Rhodium-catalyzed photochemical reactions
of benzene with alkenes afford alkylbenzenes along with alkenyl-
benzenes.5 Recently, iridium complexes were reported to catalyze
anti-Markovnikov hydroarylation of unactivated alkenes with
unactivated arenes with good selectivity.6 Palladium acetate also
provides efficient hydroarylation of activated alkenes in trifluoro-
acetic acid.7 This system can be applied to the hydroarylation of
alkynes in which addition of C-H bonds proceeds in an exclusive
trans manner. cis-Hydroarylation of unactivated alkynes is promoted
by rhodium catalysts such as Rh4(CO)128 and RhCl(CO)(PMe3)2.9,10
However, these reactions require extreme conditions (220 °C, 25
kg/cm2 pressure of carbon monoxide) or irradiation. During the
course of our study on the reactivity of novel bimetallic com-
plexes,11 we found that dinuclear palladium complexes Pd2R2(µ-
OH)(µ-dpfam) [dpfam ) N,N′-bis[2-(diphenylphosphino)phenyl]-
formamidinate, R ) p-Tol (1a), Me (1b)] catalyzed cis-hydroarylation
of alkynes with unactivated arenes at lower temperatures in the
presence of trialkylborane.
catalyst
(mol %)
yield
entry
alkyne
boraneb
product
(%)
1
2
3
4
2a
2a
2a
2a
2a
2b
2c
2d
2e
1a (2)
1b (2)
1a (2)
1a (2)
1a (2)
1a (5)
1a (5)
1a (5)
1a (5)
B(n-Bu)3
B(n-Bu)3
B(sec-Bu)3
BEt3
3a
3a
3a
3a
3a
3b
3c
3d
100 (86)d
100
100
31
5
BPh3
5
6e
7e
8e
9e
B(n-Bu)3
B(n-Bu)3
B(n-Bu)3
B(n-Bu)3
74d
91d
64d
4 + 5 (68:32)
73d
a A mixture of 2 (0.5 mmol), 1 (0.01-0.025 mmol), and borane (0.15
mmol) in benzene (2 mL) was heated at 100 °C for 4 h. b THF solutions.
c GC yields based on alkynes. d Isolated yields based on alkynes. e Carried
out at 120 °C for 17 h.
diphenyl-1-butene under the same reaction conditions did not afford
4, indicating that cis-addition is kinetically favored. No product
was obtained using terminal alkynes such as 1-octyne and ethy-
nylbenzene.
Treatment of 3-hexyne (2a) with benzene in the presence of 1a
(2 mol %) and tri(n-butyl)borane (30 mol %) at 100 °C for 4 h
afforded (E)-3-phenyl-3-hexene (3a) in 86% isolated yield (100%
GC yield) (Table 1, entry 1). The Z-isomer of 3a was not detected
by 1H NMR or GC. Although the dinuclear methylpalladium
complex 1b was also effective (entry 2), no product was obtained
using the corresponding mononuclear methylpalladium complex
PdMe(dpfam)11 or the dinuclear complex [PdPh(PPh3)(µ-OH)]2,12
without the binucleating ligand dpfam. Other typical palladium
catalysts, such as Pd(OAc)2 and Pd2(dba)3‚CHCl3, also were
ineffective for the hydroarylation. Using triethylborane and triphen-
ylborane as additives decreased the yield of 3a remarkably (entries
4 and 5), and no reaction occurred in the absence of boranes. The
hydrophenylation of several symmetrical internal alkynes (2b-d)
proceeded under similar conditions, giving phenyl-substituted
alkenes (3b-d) in satisfactory yields (entries 6-8). Although the
reaction of 1-phenyl-1-butyne (2e) yielded both regioisomers, (E)-
1,2-diphenyl-1-butene (4) and 1,1-diphenyl-1-butene (5), no other
isomers were detected (entry 9 and eq 2). Treatment of (Z)-1,2-
Table 2. Pd-Catalyzed Hydroarylation of 3-Hexyne (2a) with
Monosubstituted Benzenesa
c
entry
arene
product
a:bb
yield (%)
1
2
3
4
toluene
anisole
methyl benzoate
chlorobenzene
6a + 6b
7a + 7b
8a + 8b
9a + 9b
73:27
80:20
61:39
79:21
64
63
71
32 (75)d
a A mixture of 2a (0.5 mmol), arene (2 mL), 1a (0.01 mmol), and B(n-
Bu)3 (0.15 mmol) was heated at 100 °C for 4 h. b Determined by GC. c Total
GC yields based on 2a. d The reaction was performed for 42 h in the
presence of 5 mol % of 1a.
Table 2 summarizes results of hydroarylation of 2a with several
monosubstituted benzenes (eq 3). Both electron-rich and electron-
poor arenes were reactive and afforded (E)-3-aryl-3-hexenes 6-9,
although the reaction of chlorobenzene required a higher catalyst
loading and longer reaction time. The meta and para isomers were
obtained in a ratio of ∼2:1; no ortho isomer was detected from
any of the reactions. This selectivity is similar to that observed in
9
12102
J. AM. CHEM. SOC. 2003, 125, 12102-12103
10.1021/ja0375075 CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Acknowledgment. This work was supported by a Grant-in-Aid
for Scientific Research (No. 14550816) from the Ministry of
Education, Science, Sports, and Culture of Japan.
Supporting Information Available: Experimental procedures and
spectral data for all of the new compounds (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
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Scheme 1
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In summary, we have found dinuclear palladium complexes 1a-c
catalyzed cis-hydroarylation of alkyne with monosubstituted arenes,
in which meta and para products were formed in statistical ratios.
Further studies of reaction mechanisms and the improvement of
catalyst activity are currently under investigation.
JA0375075
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