Metal-free hydroarylation of alkynes: A very convenient, simple procedure for substituted arylalkenes

Article abstract of DOI:10.1055/s-0028-1083635

Hydroarylation of aryl-substituted alkynes with simple and substituted arenes was conducted in the presence of trifluoroacetic acid in dichloromethane without any metal catalysts or additives. Electron-rich arenes coupled with aryl-substituted alkynes to give 1,1-diarylalkenes in good to high yields. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0028-1083635

PAPER
3755
Metal-Free Hydroarylation of Alkynes: A Very Convenient, Simple Procedure
for Substituted Arylalkenes
Metal-Free
H
ydroaryla
d
tion of
A
lky
.
nes Ataur Rahman, Osamu Ogawa, Juzo Oyamada, Tsugio Kitamura*
Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University,
Honjo-machi, Saga 840-8502, Japan
Fax +81(952)288550; E-mail: kitamura@cc.saga-u.ac.jp
Received 4 July 2008; revised 27 August 2008
alkynes with electron-rich arenes without any metal cata-
lysts.
Abstract: Hydroarylation of aryl-substituted alkynes with simple
and substituted arenes was conducted in the presence of trifluoro-
acetic acid in dichloromethane without any metal catalysts or addi-
tives. Electron-rich arenes coupled with aryl-substituted alkynes to
give 1,1-diarylalkenes in good to high yields.
In the present study, we confined our attention to optimiz-
ing the hydroarylation reaction of aryl-substituted alkynes
with various arenes in the presence of trifluoroacetic acid.
Initially, endeavors were mainly focused on the efficiency
of the hydroarylation reaction of phenylacetylene (1) with
arenes 2 in the presence of trifluoroacetic acid in dichlo-
romethane at 30 °C; the results are given in Table 1. The
reaction of electron-rich arenes 2 such as pentamethylben-
zene (2a), 1,2,4,5-tetramethylbenzene (2b), 1-bromo-
2,4,6-trimethylbenzene (2c), 1,3,5-trimethylbenzene (2d),
and 1,4-dimethylbenzene (2e) gave 1-aryl-1-phenyl-
ethenes 3 in good to high yields (entries 1–5). Sterically
bulky 1,4-di-tert-butylbenzene (2f) showed low reactivity
and gave 3f in 28% yield (entry 6). The reaction with tol-
uene (2g) gave an isomeric mixture of 3g in 31% yield
Key words: hydroarylation, alkynes, arenes, trifluoroacetic acid
Transition-metal-catalyzed hydroarylation reactions of
carbon–carbon multiple bonds are attractive methods for
carbon–carbon bond formation and provide a direct syn-
thesis of aromatic alkenes. Historically, Friedel–Crafts re-
actions of aromatic substrates have been employed for
direct functionalization of aromatic compounds, but these
reactions require more than an equimolar amount of a
Lewis acid such as aluminum(III) chloride.1
Recently, direct hydroarylation of alkynes or alkenes cat-
alyzed by transition metals or Lewis acid metals has at-
tracted considerable attention in organic synthesis.2 Such
metal-catalyzed hydroarylation reactions are, indeed, ex-
cellent and powerful methods, but in most cases they re-
quire high temperatures, strong acidic conditions, and
special precautions for handling metal catalysts under an
inert atmosphere. Furthermore, the contamination of
pharmaceutical materials with even a trace amount of a
metal in manufacturing processes causes serious prob-
lems. Thus, these drawbacks of metal-catalyzed hydro-
arylation reactions have attracted our attention to develop
a more convenient, simple synthetic method for hydroary-
lation reactions without metals. In addition, a very recent
paper3 reporting an efficient oxidative cleavage of double
bonds as a synthetic application of arylated alkenes en-
couraged us to study the hydroarylation of alkynes.
1
(entry 7). The H NMR spectrum showed the ratio of
ortho-, meta-, and para-isomers was 41:9:50, indicating
that this reaction was ortho,para-directing. The ortho/
para ratio (0.82) was similar to those in Friedel–Crafts
alkylations of toluene with the benzyl cation,5 but higher
than those in electrophilic aromatic substitution of trisub-
stituted arylvinyl cations (0.3–0.4).6 This higher ortho/
para ratio is attributable to the lower steric hindrance of
the phenylvinyl cation compared with the trisubstituted
arylvinyl cations. Benzene (2h) and naphthalene (2i)
showed a very low reactivity and afforded 1,1-diaryl-
ethenes 3h and 3i in 9% and 2% yields, respectively. Re-
action with a lower amounts of trifluoroacetic acid or
arene 2a resulted in a decrease in the yield of product 3a
(entry 10 or 11).
Next, we examined the reaction of para-substituted phe-
nylacetylenes 4, 5, and 6 with arenes 2 under the same
conditions as that of 1; the results are given in Table 2. In
the reaction of 4-methylphenylacetylene (4) with highly
electron-rich arenes 2a–d, excellent yields of products 7
were obtained (entries 1–4). However, the reaction with
xylene 2e resulted in a low yield of product 7e (entry 5).
Similarly, the reaction of 4-methoxyphenylacetylene (5)
with electron-rich arenes 2a–d gave 1,1-diarylethenes 8 in
high yields (entries 6–9). A similar result was obtained
even in the reaction of 4-fluorophenylacetylene (6) with
2a (entry 10). Furthermore, we checked the reaction of
diphenylacetylene (10) with arenes 2 under the above con-
ditions; as shown in Table 3, the desired products 11 were
obtained in quite good yields (entries 1–5).
Very recently, our group and Tunge et al. independently
reported the hydroarylation of arylalkenes, i.e., cinnamic
acid esters with phenols in trifluoroacetic acid affording
dihydrocoumarins, which were catalyzed by trifluoroace-
tic acid.4 To the best of our knowledge, however, there are
no reports on the hydroarylation of alkynes using trifluo-
roacetic acid as an acid catalyst. Here we wish to report a
highly convenient, simple hydroarylation reaction of
SYNTHESIS 2008, No. 23, pp 3755–3760
x
x.
x
x
.
2
0
0
8
Advanced online publication: 06.11.2008
DOI: 10.1055/s-0028-1083635; Art ID: F15508SS
© Georg Thieme Verlag Stuttgart · New York

3756
M. A. Rahman et al.
PAPER
Table 1 Reaction of Alkyne 1 with Arenes 2 in the Presence of Tri-
fluoroacetic Acid a
Table 2 Reaction of Alkyne 4, 5, or 6 with Arenes 2 in the Presence
of Trifluoroacetic Acida
Ph
R
TFA
Ar
+
H
Ph
CH2Cl2, 30 °C
Ar
1
2
3
TFA
Ar
+
H
Product Yieldb
(%)
R
Entry Ar-H 2
(mmol)
Time
(h)
CH2Cl2, 30 °C
Ar
2
4: R = Me
5: R = MeO
6: R = F
7: R = Me
8: R = MeO
9: R = F
Me
Me
H
1
2a (10)
48
24
3a
3b
93
75
Entry
Alkyne
Arene
2a
Time (h) Product
Yieldb (%)
Me
Me
1
2
4
4
4
4
4
5
5
5
5
6
48
24
60
24
24
48
24
60
24
48
7a
7b
7c
7d
7e
8a
8b
8c
8d
9
100
89
Me
Me
2b
2c
Me
H
3
85
2
2b (3)
4
2d
2e
100
40
Me
Me
Me
5
6
2a
94
H
3
2c (6)
2d (3)
2e (10)
60
24
24
3c
3d
3e
69
71
63
7
2b
2c
78
Me
Me
8
73
Br
Me
9
2d
2a
92
H
4
5
10
92
Me
Me
Me
a Reaction conditions: alkyne 4, 5, or 6 (1 mmol), arene 2 (10 mmol),
TFA (1 mL), CH2Cl2 (2 mL), 30 °C.
b Isolated yields based on alkyne 4, 5, or 6.
H
Table 3 Reaction of Alkyne 10 with Arenes 2 in the Presence of
Trifluoroacetic Acida
Me
tBu
Ph
Ar
H
TFA
+
Ph
C
C
Ph
Ar
H
H
CH2Cl2, 30 °C
6
2f (10)
60
3f
28
Ph
10
2
11
tBu
Me
Entry
Arene
2a
Time (h)
Product
11a
Yield (%)
1
2
3
4
5
48
24
60
24
24
68
65
73b
76
69
7c
8c
9
2g (100)
2h (100)
2i (5)
72
72
50
3g
3h
3i
31d
H
H
H
2b
11b
2c
11c
9
2d
11d
2e
11e
a Reaction conditions: 10 (1 mmol), 2 (10 mmol), TFA (1 mL),
CH2Cl2 (2 mL), 30 °C.
b A mixture of E- and Z-isomers.
2
10e
11
2a (10)
2a (1)
48
18
3a
3a
65
42
Finally, the hydroarylation reaction was applied to other
alkynyl systems without any metal catalysts. Using reac-
tive arenes 2a and 2d employed in the present study, the
reaction of but-3-yn-2-one (12), ethyl propynoate (13) and
ethyl phenylpropynoate (14) was conducted, respectively,
under the conditions similar to the above reactions
(Scheme 1, Table 4). Reaction of 12 with 2a and 2d effi-
a Reaction conditions: 1 (1 mmol), 2, TFA (1 mL), CH2Cl2 (2 mL), 30
°C.
b Isolated yields based on alkyne 1.
c No CH2Cl2 was added.
d A mixture of ortho-, meta-, and para-isomers were formed.
e TFA (0.15 mL) was used.
Synthesis 2008, No. 23, 3755–3760 © Thieme Stuttgart · New York

PAPER
Metal-Free Hydroarylation of Alkynes
3757
Table 4 Reaction of Alkynes 12–14 with Arenes 2a and 2da
are uncorrected. Column chromatographic separations were carried
out using silica gel as the stationary phase. Pre-coated plates (silica
gel 60 F254, Merck, on aluminum sheets) were used for TLC analy-
sis. Elemental analysis was performed by the Service Center of the
Elementary Analysis of Organic Compounds, Faculty of Science,
Kyushu University.
Entry Alkyne
(mmol)
Arene Time Product
(mmol) (h)
Yieldb
(%)
COMe
COMe
H
COMe
1
3
3
2a
2
3
3
96
97
Ar
12
15
Hydroarylation of Alkynes; General Procedure
2c
12
2d 2
A test tube with a stopper was charged with an appropriate arene
(10 mmol), an alkyne (1 mmol), and CH2Cl2 (2 mL). The mixture
was stirred for 5 min in an ice-water bath, and then TFA (1 mL) was
gradually added with constant stirring. The mixture was stirred for
~15 min in the ice-water bath and stirred again for ~15 min at r.t.
The mixture was then gradually increased to 30 °C and stirred. After
completion of the reaction, CH2Cl2 (20 mL) and H2O (20 mL) were
added. Solid NaHCO3 was gradually added to neutralize the TFA.
The neutral aqueous mixture was extracted with CH2Cl2 (4 × 10
mL) and dried (anhyd Na2SO4). Finally, CH2Cl2 was removed under
reduced pressure below 40 °C. Individual pure compounds were
isolated by column chromatography (silica gel).
Ar
16
e
3d
4f
1.5 2a 2.4
1.25 2a 2.4
4
5
–
–
–
H
CO2Et
CO2Et
13
e
Ph
–
14
a Reaction conditions: alkyne, arene, TFA (1 mL), r.t.
b Isolated yields based on alkyne.
c CH2Cl2 (0.1 mL) was added.
d TFA (1.5 mL) and CH2Cl2 (0.5 mL) were used.
e No hydroarylation products were formed.
f CH2Cl2 (0.25 mL) was added.
1-(Pentamethylphenyl)-1-phenylethene (3a)7
Yield: 0.2541 g (93%); mp 71.9–73.9 °C.
1H NMR (300 MHz, CDCl3): d = 7.24–7.31 (m, 5 HPh), 5.97 (d,
ciently proceeded to give hydroarylation products 15 and
16 in 96% and 97% yields, respectively. However, in the
J = 1.5 Hz, 1 Hvinyl), 5.07 (d, J = 1.5 Hz, 1 Hvinyl), 2.29 (s, 3 H, Me),
cases of 13 and 14, the reaction with 2a did not afford any 2.24 (s, 6 H, 2 Me), 2.10 (s, 6 H, 2 Me).
13C NMR (75 MHz, CDCl3): d = 148.65, 140.03, 138.71, 133.71,
132.33, 131.57, 128.35, 127.41, 126.01, 114.29, 17.84, 16.77,
16.56.
hydroarylation products at all. This result suggests that
ethyl propynoates are less reactive than arylalkynes and
but-3-yn-2-one, and their reactions require metal catalysts
such as palladium or platinum to activate them.2b,c
MS (EI): m/z = 250 (M+).
1-Phenyl-1-(2,3,5,6-tetramethylphenyl)ethene (3b)
Yield: 0.2241 g (75%); mp 68.2–69.4 °C.
R1
R2
TFA
r.t.
R1
R2
+
Ar
H
1H NMR (300 MHz, CDCl3): d = 7.22–7.29 (m, 5 HPh), 6.97 (s, 1
HAr), 5.98 (d, J = 1.5 Hz, 1 Hvinyl), 5.07 (d, J = 1.5 Hz, 1 Hvinyl), 2.25
(s, 6 H, 2 Me), 2.04 (s, 6 H, 2 Me).
Ar
H
12–14
2
15, 16
Scheme 1
13C NMR (75 MHz, CDCl3): d = 148.04, 141.08, 139.78, 133.53,
131.97, 130.29, 128.37, 127.48, 125.95, 114.19, 20.14, 16.65.
The present reactions are considered to proceed via a-
arylvinyl cations generated by protonation of arylalkynes.
The a-arylvinyl cations are stable enough to react with
electron-rich aromatics and undergo electrophilic aromat-
ic substitution.6 Since a-arylvinyl cations without b sub-
stituents predominantly undergo deprotonation, the
Anal. Calcd for C18H20: C, 91.47; H, 8.53. Found: C, 91.41; H, 8.59.
1-(3-Bromo-2,4,6-trimethylphenyl)-1-phenylethene (3c)2e
Liquid; yield: 0.2198 g (69%).
1H NMR (300 MHz, CDCl3): d = 7.24–7.31 (m, 5 HPh), 7.00 (s, 1
HAr), 5.98 (d, J = 1.4 Hz, 1 Hvinyl), 5.07 (d, J = 1.4 Hz, 1 Hvinyl), 2.42
corresponding vinyl derivatives are not a good choice for (s, 3 H, Me), 2.27 (s, 3 H, Me), 2.07 (s, 3 H, Me).
13C NMR (75 MHz, CDCl3): d = 147.07, 139.97, 138.97, 136.83,
136.14, 134.91, 129.61, 128.50, 127.78, 125.81, 125.45, 114.81,
23.95, 21.42, 19.91.
the substrate. Therefore, the protonation tool using aryl-
acetylenes is suitable for the synthesis of arylalkenes with
hydroarylation.
In summary, we have demonstrated that metal-free hy-
droarylation of alkynes efficiently proceeds in the pres-
ence of trifluoroacetic acid when aryl-substituted alkynes
and electron-rich arenes are employed. The simplicity of
this procedure, along with the mildness, makes it practical
as a synthetic tool for arylalkenes. Furthermore, this
metal-free procedure is particularly attractive in pharma-
ceutical fields.
1-Phenyl-1-(2,4,6-trimethylphenyl)ethene (3d)2e
Liquid; yield: 0.1705 g (71%).
1H NMR (300 MHz, CDCl3): d = 7.24–7.28 (m, 5 HPh), 6.91 (s, 2
HAr), 2.11 (s, 6 H, 2 Me), 2.32 (s, 3 H, Me), 5.96 (d, J = 1.5 Hz, 1
Hvinyl), 5.10 (d, J = 1.5 Hz, 1 Hvinyl).
13C NMR (75 MHz, CDCl3): d = 146.89, 139.57, 138.17, 136.41,
136.12, 128.39, 128.10, 127.51, 125.82, 114.50, 21.02, 20.06.
1-(2,5-Dimethylphenyl)-1-phenylethene (3e)2e
Liquid; yield: 0.1376 g (63%).
All solvents and starting materials were used as received without
further purification. 1H NMR, 13C NMR and GC-MS were recorded
on a Jeol JNM-AL 300 spectrometer in CDCl3 soln (TMS as inter-
nal standard) and Shimadzu GCMS-QP5050, respectively. Melting
points were determined by a Yanaco melting point apparatus and
1H NMR (300 MHz, CDCl3): d = 7.23–7.28 (m, 5 HPh), 7.065–
7.068 (m, 2 HAr), 7.02–7.04 (m, 1 HAr), 5.18 (d, J = 1.5 Hz, 1 Hvinyl),
5.75 (d, J = 1.5 Hz, 1 Hvinyl), 2.34 (s, 3 H, Me), 2.01 (s, 3 H, Me).
Synthesis 2008, No. 23, 3755–3760 © Thieme Stuttgart · New York

3758
M. A. Rahman et al.
PAPER
13C NMR (75 MHz, CDCl3): d = 149.58, 141.47, 140.68, 135.03,
132.93, 130.65, 129.96, 128.29, 128.17, 127.49, 126.50, 114.62,
20.89, 19.58.
1-(3-Bromo-2,4,6-trimethyphenyl)-1-(4-methylphenyl)ethene
(7c)2e
Colorless viscous liquid; yield: 0.2710 g (85%).
1H NMR (300 MHz, CDCl3): d = 7.13 (d, J = 8.1 Hz, 2 HAr), 7.06
(d, J = 8.1 Hz, 2 HAr), 6.99 (s, 1 HAr), 5.93 (d, J = 1.2 Hz, 1 Hvinyl),
5.00 (d, J = 1.2 Hz, 1 Hvinyl), 2.42 (s, 3 H, Me), 2.32 (s, 3 H, Me),
2.26 (s, 3 H, Me), 2.06 (s, 3 H, Me).
1-(2,5-Di-tert-butylphenyl)-1-phenylethene (3f)
Yield: 0.1010 g (28%); mp 76.2–78.9 °C.
1H NMR (300 MHz, CDCl3): d = 1.22 (s, 9 H, t-Bu), 1.32 (s, 9 H, t-
Bu), 5.22 (d, J = 1.5 Hz, 1 Hvinyl), 5.90 (d, J = 1.5 Hz, 1 Hvinyl), 7.05
(d, J = 2.4 Hz, 1 HAr), 7.21–7.33 (m, 6 HAr), 7.43 (d, J = 8.4 Hz, 1
HAr).
13C NMR (75 MHz, CDCl3): d = 146.80, 140.12, 137.64, 136.69,
136.11, 136.09, 134.90, 129.55, 129.21 125.68, 125.40, 113.82,
23.96, 21.39, 21.12, 19.89.
13C NMR (75 MHz, CDCl3): d = 151.67, 147.80, 144.80, 141.58,
139.64, 130.24, 128.17, 127.32, 126.89, 126.48, 123.98, 114.93,
36.19, 34.05, 32.26, 31.31.
1-(4-Methylphenyl)-1-(2,4,6-trimethylphenyl)ethene (7d)2e
Colorless viscous liquid; yield: 0.2434 g (100%).
Anal. Calcd for C22H28: C, 90.35; H, 9.65. Found: C, 90.47; H, 9.67.
1H NMR (300 MHz, CDCl3): d = 7.15 (d, J = 8.4 Hz, 2 HAr), 7.06
(d, J = 8.4 Hz, 2 HAr), 6.91 (s, 2 HAr), 5.91 (d, J = 1.5 Hz, 1 Hvinyl),
5.03 (d, J = 1.5 Hz, 1 Hvinyl), 2.32 (s, 6 H, 2 Me), 2.11 (s, 6 H, 2 Me).
1-Phenyl-1-tolylethenes 3g8
Liquid; yield: 0.0650 g (31%); mixture of ortho-, meta- and para-
isomers (41:9:50).
13C NMR (75 MHz, CDCl3): d = 146.58, 138.31, 137.33, 136.65,
136.30, 136.10, 129.10, 128.02, 125.69, 113.54, 21.11, 21.03,
20.04.
1H NMR (300 MHz, CDCl3): d = 7.13–7.34 (m, HAr), 5.20 (d,
J = 1.2 Hz, Hvinyl), 5.41 (d, J = 1.2 Hz, Hvinyl), 5.44 (d, J = 1.2 Hz,
Hvinyl), 5.77 (d, J = 1.2 Hz, Hvinyl), 2.05 (s, ortho-Me), 2.34 (s, meta-
Me), 2.37 (s, para-Me).
1-(2,5-Dimethylphenyl)-1-(4-methylphenyl)ethene (7e)2e
Colorless viscous liquid; yield: 0.0895 g (40%).
13C NMR (75 MHz, CDCl3): d = 149.92, 149.47, 141.69, 141.62,
140.58, 138.61, 137.44, 136.08, 130.05, 129.99, 128.92, 128.84,
128.44, 128.30, 128.27, 128.13, 128.09, 127.60, 127.53, 127.51,
126.46, 125.65, 125.43, 114.79, 114.07, 113.57, 21.39, 21.13,
20.07.
1H NMR (300 MHz, CDCl3): d = 7.14–7.20 (m, 2 HAr), 7.03–7.10
(m, 5 HAr), 5.72 (d, J = 1.5 Hz, 1 Hvinyl), 5.12 (d, J = 1.5 Hz, 1 Hvinyl),
2.33 (s, 6 H, 2 Me), 2.01 (s, 3 H, Me).
13C NMR (75 MHz, CDCl3): d = 149.30, 141.62, 137.79, 137.28,
134.96, 132.91, 130.60, 129.89, 128.99, 128.07, 126.36, 113.72,
21.11, 20.89, 19.57.
MS (EI): m/z = 194 (M+).
1,1-Diphenylethene (3h)9
Liquid; yield: 0.0208 g (9%).
1-(4-Methoxyphenyl)-1-(pentamethylphenyl)ethene (8a)10
Yield: 0.2687 g (94%); mp 102.2–103.7 °C.
1H NMR (300 MHz, CDCl3): d = 7.30–7.36 (m, 10 H, 2 Ph), 5.46
(s, 2 Hvinyl).
1H NMR (300 MHz, CDCl3): d = 7.25–7.20 (m, 2 HAr,), 6.82–6.77
(m, 2 HAr,), 5.85 (d, J = 1.2 Hz, 1 Hvinyl), 4.95 (d, J = 1.2 Hz,
1 Hvinyl), 3.78 (s, 3 H, OMe), 2.28 (s, 3 H, Me), 2.23 (s, 6 H, 2 Me),
2.10 (s, 6 H, 2 Me).
1-(1-Naphthyl)-1-phenylethene (3i)8c
Yield: 0.0067 g (2%); mp 46.5–48.2 °C.
13C NMR (75 MHz, CDCl3): d = 159.09, 147.93, 138.91, 133.58,
132.67, 132.29, 131.51, 127.20, 113.66, 112.23, 55.19, 17.78,
16.75, 16.55.
1H NMR (300 MHz, CDCl3): d = 7.21–7.91 (m, 12 HAr), 5.98 (d,
J = 1.5 Hz, 1 Hvinyl), 5.39 (d, J = 1.5 Hz, 1 Hvinyl).
1-(4-Methylphenyl)-1-(pentamethylphenyl)ethene (7a)
Yield: 0.2696 g (100%); mp 86.4–86.9 °C.
1-(4-Methoxyphenyl)-1-(2,3,5,6-tetramethylphenyl)ethene (8b)
Yield: 0.2149 g (78%); mp 124.3–126.3 °C.
1H NMR (300 MHz, CDCl3): d = 7.17 (d, J = 8.4 Hz, 2 HAr), 7.06
(d, J = 8.4 Hz, 2 HAr), 5.93 (d, J = 1.5 Hz, 1 Hvinyl), 5.00 (d, J = 1.5
Hz, 1 Hvinyl), 2.32 (s, 3 H, Me), 2.29 (s, 3 H, Me), 2.23 (s, 6 H,
2 Me), 2.10 (s, 6 H, 2 Me).
1H NMR (300 MHz, CDCl3): d = 7.23–7.18 (m, 2 HAr), 6.96 (br s,
1 HAr), 6.82–6.77 (m, 2 HAr), 5.86 (d, J = 1.2 Hz, 1 Hvinyl), 4.94 (d,
J = 1.2 Hz, 1 Hvinyl), 3.78 (s, 3 H, OMe), 2.24 (s, 6 H, 2 Me), 2.04
(s, 6 H, 2 Me).
13C NMR (75 MHz, CDCl3): d = 148.38, 138.85, 137.22, 137.16,
133.60, 132.29, 131.55, 129.07, 125.89, 113.33, 21.11, 17.83,
16.76, 16.55.
13C NMR (75 MHz, CDCl3): d = 159.15, 147.35, 141.29, 133.48,
132.43, 131.91, 130.16, 127.14, 113.70, 112.10, 55.21, 20.13,
16.58.
Anal. Calcd for C20H24: C, 90.85; H, 9.15. Found: C, 90.78; H, 9.25.
Anal. Calcd for C19H22O: C, 85.67; H, 8.32. Found: C, 85.57; H,
8.43.
1-(4-Methylphenyl)-1-(2,3,5,6-tetramethylphenyl)ethene (7b)
Yield: 0.2272 g (89%); mp 117.4–117.9 °C.
1-(2-Bromo-2,4,6-trimethylphenyl)-1-(4-methoxyphe-
nyl)ethene (8c)2e
Colorless viscous liquid; yield: 0.2434 g (73%).
1H NMR (300 MHz, CDCl3): d = 7.16 (d, J = 8.1 Hz, 2 HAr), 7.06
(d, J = 8.1 Hz, 2 HAr), 6.96 (s, 1 HAr), 5.94 (d, J = 1.5 Hz, 1 Hvinyl),
5.00 (d, J = 1.5 Hz, 1 Hvinyl), 2.32 (s, 3 H, Me), 2.24 (s, 6 H, 2 Me),
2.04 (s, 6 H, 2 Me).
1H NMR (300 MHz, CDCl3): d = 7.21–7.16 (m, 2 HAr), 6.99 (br s,
1 HAr), 6.82–6.77 (m, 2 HAr), 2.41 (s, 3 H, Me), 2.27 (s, 3 H, Me),
2.07 (s, 3 H, Me), 5.85 (d, J = 1.5 Hz, 1 Hvinyl), 4.95 (d, J = 1.5 Hz,
1 Hvinyl), 3.78 (s, 3 H, OMe).
13C NMR (75 MHz, CDCl3): d = 147.77, 141.21, 137.30, 136.90,
133.47, 131.96, 130.18, 129.09, 125.82, 113.22, 21.11, 20.15,
16.64.
13C NMR (75 MHz, CDCl3): d = 159.36, 146.39, 140.22, 136.68,
136.08, 134.87, 131.59, 129.57, 127.03, 125.41, 113.83, 112.69,
55.23, 23.93, 21.34, 19.84.
Anal. Calcd for C19H22: C, 91.14; H, 8.86. Found: C, 91.08; H, 8.90.
Synthesis 2008, No. 23, 3755–3760 © Thieme Stuttgart · New York

PAPER
Metal-Free Hydroarylation of Alkynes
3759
1-(4-Methoxyphenyl)-1-(2,4,6-trimethylphenyl)ethene (8d)2e
Colorless viscous liquid; yield: 0.2424 g (92%).
(Z)-1-(2,5-Dimethylphenyl)-1,2-diphenylethene (11e)2i
Colorless viscous liquid; yield: 0.1967 g (69%).
1H NMR (300 MHz, CDCl3): d = 7.22–7.17 (m, 2 HAr), 6.90 (br s,
2 HAr), 6.82–6.77 (m, 2 HAr), 5.84 (d, J = 1.2 Hz, 1 Hvinyl), 4.98 (d,
J = 1.2 Hz, 1 Hvinyl), 3.78 (s, 3 H, OMe), 2.31 (s, 3 H, Me), 2.11 (s,
6 H, 2 Me).
1H NMR (300 MHz, CDCl3): d = 6.94–7.34 (m, 14 HAr and Hvinyl),
2.27 (s, 3 H, Me), 1.99 (s, 3 H, Me).
13C NMR (75 MHz, CDCl3): d = 142.45, 141.36, 139.45, 137.35,
135.76, 133.27, 130.46, 130.36, 128.96, 128.36, 128.30, 128.08,
127.97, 127.28, 126.83, 126.60, 20.97, 19.09.
13C NMR (75 MHz, CDCl3): d = 159.17, 146.16, 138.38, 136.28,
136.06, 132.17, 128.03, 127.00, 113.71, 112.42, 55.21, 21.02,
19.99.
(E)-4-(Pentamethylphenyl)but-3-en-2-one (15)7
Yield: 0.4153 g (96%); mp 79.4–81.7 °C.
1-(4-Fluorophenyl)-1-(pentamethylphenyl)ethene (9)
Yield: 0.2905 g (92%); mp 77.6–78.8 °C.
1H NMR (300 MHz, CDCl3): d = 7.73 (d, J = 16.5 Hz, 1 Hvinyl), 6.15
(d, J = 16.5 Hz, 1 Hvinyl), 2.40 (s, 3 H, Me), 2.26 (s, 3 H, Me), 2.23
(s, 6 H, 2 Me), 2.22 (s, 6 H, 2 Me).
1H NMR (300 MHz, CDCl3): d = 7.25 [dd, J = 5.1 (F-Hmeta) and 9.0
Hz, 2 HAr], 6.93 [dd, J = 9.0 (F-Hortho) and 9.0 Hz, 2 HAr], 5.89 (s, 1
Hvinyl), 5.03 (s, 1 Hvinyl), 2.28 (s, 3 H, Me), 2.23 (s, 6 H, 2 Me), 2.09
(s, 6 H, 2 Me).
13C NMR (75 MHz, CDCl3): d = 198.31, 145.07, 135.30, 133.82,
132.81, 132.57, 131.05, 27.34, 17.90, 16.87, 16.44.
13C NMR (75 MHz, CDCl3): d = 162.36 (d, 1JC-F = 246 Hz), 147.61,
138.48, 136.15 (d, 4JC-F = 3 Hz), 133.86, 132.43, 131.44, 127.65 (d,
3JC-F = 8 Hz), 115.16 (d, 2JC-F = 21 Hz), 113.95, 17.77, 16.75, 16.54.
(E)-4-(2,4,6-Trimethylphenyl)but-3-en-2-one (16)7
Yield: 0.3652 g (97%); mp 65.3–66.0 °C.
1H NMR (300 MHz, CDCl3): d = 7.67 (d, J = 16.8 Hz, 1 Hvinyl), 6.90
(s, 2 HAr), 6.33 (d, J = 16.8 Hz, 1 Hvinyl), 2.38 (s, 3 H, Me), 2.32 (s,
6 H, 2 Me), 2.28 (s, 3 H, Me).
Anal. Calcd for C19H21F: C, 85.03; H, 7.89. Found: C, 85.29; H,
7.97.
13C NMR (75 MHz, CDCl3): d = 198.41, 141.95, 138.50, 136.75,
132.38, 130.88, 129.22, 27.40, 21.03, 21.02.
(Z)-1-(Pentamethylphenyl)-1,2-diphenylethene (11a)7
Yield: 0.2234 g (68%); mp 112.8–115.5 °C.
1H NMR (300 MHz, CDCl3): d = 6.91–7.35 (m, 11 HAr and Hvinyl),
2.31 (s, 3 H, Me), 2.21 (s, 6 H, 2 Me), 2.01 (s, 6 H, 2 Me).
References
13C NMR (75 MHz, CDCl3): d = 142.18, 141.67, 137.60, 136.24,
133.90, 132.72, 131.08, 128.59, 128.35, 128.13, 127.82, 127.09,
126.71, 126.18, 17.20, 16.89, 16.65.
(1) (a) Carey, F. A.; Sundberg, R. J. Advanced Organic
Chemistry, Part B, 4th ed.; Kluwer Academic/Plenum Press:
New York, 2001, Chap. 11. (b) Smith, M. B.; March, J.
March’s Advanced Organic Chemistry, 6th ed.; Wiley-
Interscience: New York, 2007, Chap. 11.
(2) (a) Fujiwara, Y.; Kitamura, T. In Handbook of C-H
Transformations; Dyker, G., Ed.; Wiley-VCH: Weinheim,
2005, 194. (b) Jia, C.; Kitamura, T.; Fujiwara, Y. Acc.
Chem. Res. 2001, 34, 633. (c) Oyamada, J.; Kitamura, T.
Chem. Lett. 2005, 34, 1430. (d) Oyamada, J.; Kitamura, T.
Tetrahedron 2007, 63, 12754. (e) Reetz, M. T.; Sommer, K.
Eur. J. Org. Chem. 2003, 3485. (f) Shi, Z.; He, C. J. Org.
Chem. 2004, 69, 3669. (g) Nevado, C.; Echavarren, A. M.
Synthesis 2005, 167. (h) Nakao, Y.; Kanyiva, K. S.;
Hiyama, T. J. Am. Chem. Soc. 2008, 130, 2448.
(Z)-1,2-Diphenyl-1-(2,3,5,6-tetramethylphenyl)ethene (11b)
Yield: 0.2023 g (65%); mp 87.9–90.1 °C.
1H NMR (300 MHz, CDCl3): d = 6.90–7.34 (m, 12 HAr and Hvinyl),
2.23 (s, 6 H, 2 Me), 1.95 (s, 6 H, 2 Me).
13C NMR (75 MHz, CDCl3): d = 141.78, 140.95, 138.85, 137.48,
134.00, 131.72, 130.69, 128.55, 128.40, 128.17, 127.83, 127.20,
126.83, 126.11, 20.22, 16.09.
Anal. Calcd for C24H24: C, 92.26; H, 7.74. Found: C, 92.19; H, 7.80.
1-(3-Bromo-2,4,6-trimethylphenyl)-1,2-diphenylethene (11c)
Colorless semi-solid; yield: 0.2784 g (73%); mixture of E- and Z-
isomers (44:56).
(i) Kakiuchi, F.; Chatani, N. Adv. Synth. Catal. 2003, 345,
1077. (j) Yoon, M. Y.; Kim, J. H.; Choi, D. S.; Shin, U. S.;
Lee, J. Y.; Song, C. E. Adv. Synth. Catal. 2007, 349, 1725.
(k) Li, R.; Wang, S. R.; Lu, W. Org. Lett. 2007, 9, 2219.
(l) Biffis, A.; Tubaro, C.; Buscemi, G.; Basato, M. Adv.
Synth. Catal. 2008, 350, 189.
1H NMR (300 MHz, CDCl3): d = 6.92–7.30 (m, 23 HAr and Hvinyl),
6.48 (s, 1 Hvinyl), 2.45 (s, 3 H, Me), 2.43 (s, 3 H, Me), 2.40 (s, 3 H,
Me), 2.20 (s, 6 H, 2 Me), 1.95 (s, 3 H, Me).
13C NMR (75 MHz, CDCl3): d = 142.43, 140.99, 140.81, 139.55,
138.66, 137.87, 137.25, 137.00, 136.87, 136.19, 135.00, 134.89,
130.65, 130.28, 129.76, 129.54, 129.21, 128.71, 128.54, 128.44,
128.31, 128.16, 128.05, 127.47, 127.23, 127.21, 126.96, 125.96,
125.84, 125.63, 24.10, 23.97, 21.72, 20.90, 20.22, 19.61.
(3) Xing, D.; Guan, B.; Cai, G.; Fang, Z.; Yang, L.; Shi, Z. Org.
Lett. 2006, 8, 693.
(4) (a) Li, K.; Foresee, L. N.; Tunge, J. A. J. Org. Chem. 2005,
70, 2881. (b) Aoki, S.; Amamoto, C.; Oyamada, J.;
Kitamura, T. Tetrahedron 2005, 61, 9291.
MS (EI): m/z = 376 (M+)/378 (M+ + 2).
(5) Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry,
Part A, 4th ed.; Kluwer Academic/Plenum Press: New York,
2000, Chap. 10.
(6) (a) Stang, P. J.; Anderson, A. G. Tetrahedron Lett. 1977, 18,
1485. (b) Stang, P. J.; Anderson, A. G. J. Am. Chem. Soc.
1978, 100, 1520. (c) Kitamura, T.; Kobayashi, S.;
Taniguchi, H.; Rappoport, Z. J. Org. Chem. 1982, 47, 5003.
(7) Jia, C.; Lu, W.; Oyamada, J.; Kitamura, T.; Matsuda, K.;
Irie, M.; Fujiwara, Y. J. Am. Chem. Soc. 2000, 122, 7252.
Anal. Calcd for C23H21Br: C, 73.21; H, 5.61. Found: C, 73.09; H,
5.57.
(Z)-1,2-Diphenyl-1-(2,4,6-trimethylphenyl)ethene (11d)2j
Yield: 0.2300 g (76%); mp 138.8–140.9 °C.
1H NMR (300 MHz, CDCl3): d = 6.92–7.34 (m, 13 HAr and Hvinyl),
2.35 (s, 3 H, Me), 2.00 (s, 6 H, 2 Me).
13C NMR (75 MHz, CDCl3): d = 141.51, 139.79, 137.47, 136.86,
136.03, 135.89, 128.70, 128.44, 128.42, 128.21, 128.05, 127.27,
126.94, 125.99, 21.19, 19.80.
Synthesis 2008, No. 23, 3755–3760 © Thieme Stuttgart · New York

3760
M. A. Rahman et al.
PAPER
(9) Identified by comparison with a commercial sample from
Tokyo Chemical Industry Co.
(10) Sun, H.; Hua, R.; Chen, S.; Yin, Y. Adv. Synth. Catal. 2006,
348, 1919.
(8) (a) Barluenga, J.; Moriel, P.; Valdés, C.; Aznar, F. Angew.
Chem. Int. Ed. 2007, 46, 5587. (b) Stavber, S.; Sotler-
Pecan, T.; Zupan, M. Bull. Chem. Soc. Jpn. 1996, 69, 169.
(c) Berthiol, F.; Doucet, H.; Santelli, M. Eur. J. Org. Chem.
2003, 1091.
Synthesis 2008, No. 23, 3755–3760 © Thieme Stuttgart · New York