LETTER
Indium(III) Bromide-Catalyzed Chemioselective Dimerization of Vinylarenes
1725
ture was quenched with 10 mL H2O and extracted with CH2Cl2
(2 × 10 mL). The organic layer was dried (Na2SO4), filtered, and
evaporated to dryness. The residue was purified by column chroma-
tography on silica gel (hexanes) to produce the dimers 4, 5 and 7,
which were characterized by spectroscopic means as following:
2,4-Diphenyl-4-methyl-pent-1-en (4a): 1H NMR: d = 1.14 (s, 6 H),
2.75 (s, 2 H), 4.70 (d, J = 2 Hz, 1 H), 5.05 (d, J = 2 Hz, 1 H), 7.14
(m, 10 H). 13C NMR: d = 28.73, 38.59, 49.55, 116.81, 125.40,
125.85, 126.44, 126.76, 127.77, 127.95, 143.40, 146.64, 149.29.
DEPT-135 NMR: d = 28.73, 49.55, 116.81, 125.40, 125.85, 126.44,
126.76, 127.77, 127.95. MS (70 eV, EI): m/z (%) = 236 (4), 119
(100).
The production of terminal olefins, 4a–c without forma-
tion of higher oligomers 6 is a particularly useful finding.
However, besides a-methylstyrene, dimers 4 were ob-
tained only from styrene derivatives containing electron-
withdrawing groups at the aromatic portion of the mole-
cule. Since dimers 4 are obtained under kinetically con-
trolled conditions, we have examined the catalytic effect
of InBr3 on styrenes containing electron-donating substit-
uents. Reaction involving 1d and 1i in CH2Cl2 solutions at
0 °C produced, after 5 hours of continuous stirring, the
corresponding oligomers 6d (quantitative conversion) and
6i (50% conversion). In both cases, no sign of the corre-
sponding indans 5d and 5i was detected. These results
confirm our earlier finding (see the oligomerization of a-
methylstyrene above) that oligomerization is a kinetically
controlled process with dimers 4 acting as the key inter-
mediates.
1
1-Phenyl-1,3,3-trimethyl-indan (5a): H NMR: d = 0.93 (s, 3 H),
1.24 (s, 3 H), 1.58 (s, 3 H), 2.09 (d, J = 13 Hz, 1 H), 2.32 (d, J = 13
Hz, 1 H), 7.09 (m, 9 H). 13C NMR: d = 30.37, 30.68, 30.88, 42.84,
50.79, 59.25, 122.53, 124.98, 125.46, 126.61, 126.65, 127.18,
127.95, 148.70, 150.99, 152.17. DEPT-135 NMR: d = 30.37, 30.68,
30.88, 59.25, 122.53, 124.98, 125.46, 126.61, 126.65, 127.18,
127.95. MS (70 eV, EI): m/z (%) = 236 (6), 221 (100).
2,4-Di-(4¢-chlorophenyl)-4-methyl-pent-1-en (4b): 1H NMR:
d = 1.20 (s, 6 H), 2.75 (s, 2 H), 4.79 (d, J = 1.4 Hz, 1 H), 5.10 (d,
J = 1.4 Hz, 1 H), 7.14 (m, 8 H). 13C NMR: d = 28.76, 38.38, 49.72,
117.50, 127.43, 127.76, 127.79, 128.09, 131.32, 132.67, 141.54,
145.38, 147.29. DEPT-135 NMR: d = 28.76, 49.72, 117.50, 127.43,
127.76, 127.79, 128.09. MS (70 eV, EI): m/z for 35Cl (%) = 304 (1),
153 (100).
2,4-Di-(4¢-bromophenyl)-4-methyl-pent-1-en (4c): 1H NMR: d =
1.12 (s, 6 H), 2.66 (s, 2 H), 4.71 (d, J = 1.4 Hz, 1 H), 5.02 (d, J = 1.4
Hz, 1 H), 6.96 (m, 4 H), 7.21 (m, 4 H). 13C NMR: d = 28.68, 38.46,
49.66, 117.56, 119.45, 120.77, 127.86, 128.12, 130.76, 131.06,
141.97, 145.41, 147.76. DEPT-135 NMR: d = 28.68, 49.66, 117.56,
127.86, 128.12, 130.76, 131.06. MS (70 eV, EI): m/z for 79Br (%) =
392 (1), 197 (100).
1,3,3,6-Tetramethyl-1-p-toluyl-indan (5d): 1H NMR: d = 1.04 (s, 3
H), 1.32 (s, 3 H), 1.66 (s, 3 H), 2.16 (d, J = 13 Hz, 1 H), 2.14 (s, 3
H), 2.19 (s, 3 H), 2.22 (d, J = 13 Hz, 1 H), 7.06 (m, 7 H). 13C NMR:
d = 20.86, 20.87, 21.38, 30.51, 30.82, 42.46, 50.31, 59.51, 122.20,
125.41, 126.58, 128.00, 128.62, 134.81, 136.10, 148.11, 149.13,
149.31. DEPT-135 NMR: d = 20.86, 20.87, 21.38, 30.51, 30.82,
59.51, 122.20, 125.41, 126.58, 128.00, 128.62. MS (70 eV, EI):
m/z (%) = 264 (15), 249 (100), 157 (69).
We have also examined the catalyst activity in vinylarenes
with other substitution patterns around the double bond.
Polystyrene was slowly produced from the monomer after
a long stirring (48 h) in the presence of the catalyst and no
sign of dimer formation was detected. InBr3 does not in-
duce any reaction when in contact with b-methylstyrene,
tri- and tetraphenylethene. However, with 1,1-diphe-
nylethene (6) the cyclic dimer 3-methyl-1,3,3-triphenyl-
indane (7) was produced in 72% of yield (Equation 1).
The results indicate that the catalyst is not active with vi-
nyl arenes substituted at the b-position.
Ph
InBr3 (10 mol%)
CH2Cl2
Ph
24 h, 72%
Ph
6
Ph
Ph
7
Equation 1
1,3,3-Trimethyl-6-methoxy-1-p-anisolyl-indan (5e): 1H NMR:
d = 0.95 (s, 3 H), 1.22 (s, 3 H), 1.57 (s, 3 H), 2.08 (d, J = 13 Hz, 1
In summary, we have found a new catalyst for dimeriza-
tion of vinylarenes operating under mild neutral condi-
tions and producing good yields. The mild Lewis acid H), 2.27 (d, J = 13 Hz, 1 H), 3.66 (s, 3 H), 3.68 (s, 3 H), 6.54 (m, 1
H), 6.71 (m, 3 H), 7.01 (m, 3 H). 13C NMR: d = 30.52, 30.75, 30.91,
42.11, 50.09, 55.10, 55.36, 59.75, 109.86, 113.21, 113.24, 123.07,
127.61, 142.99, 144.43, 150.49, 157.35, 158.91. DEPT-135 NMR:
d = 30.52, 30.75, 30.91, 55.10, 55.36, 59.75, 109.86, 113.21,
113.24, 123.07, 127.61. MS (70 eV, EI): m/z (%) = 296 (20), 281
strength of InBr3 determines its activity only towards sub-
strates substituted at position a; other substitution patterns
(b; a,b; and a,b,b) remained inert under the experimental
conditions. The highlight of this protocol is the rigorous
chemoselective control of the products, depending on the
nature and the regiochemistry of the substituents at the
arylic portion of the vinylarene. The kinetic-controlled
production of terminal olefinic dimers 4 is a particular
useful finding, especially for those interested in the pro-
duction of polymers and co-polymers derived from viny-
larenes.
(100), 173 (46).
Unseparable mixture of 1,3,3-trimethyl-5-methoxy-1-m-anisolyl-
indan and 1,3,3-trimethyl-6-methoxy-3-m-anisolyl-indan (5f): H
1
NMR: d = 0.93 (s, 4 H), 1.23 (s, 4 H), 1.55 (s, 3 H), 1.69 (s, 1 H),
2.06 (d, J = 13 Hz, 0.3 H), 2.09 (d, J = 13 Hz, 1 H), 2.24 (d, J = 13
Hz, 0.3 H), 2.33 (d, J = 13 Hz, 1 H), 3.57 (s, 1 H), 3.63 (s, 4 H), 3.72
(s, 3 H), 6.64 (m, 6.5 H), 7.06 (m, 2.5 H). MS (70 eV, EI): m/z (%,
isomer # 1) = 296 (40), 281 (100), 189 (21), 174 (13), 173 (82), 158
(12), 121 (17), 91 (9). E/M: m/z (%, isomer # 2) = 296 (25), 281
(100), 189 (13), 173 (47), 158 (12), 121 (11), 91 (6).
General Procedure for the Synthesis of Dimers 4, 5 and 7.
The vinylarene, 1 or 6 (1 mmol) was dissolved in CH2Cl2 (2 mL) in
a Schlenk tube under N2 atmosphere, at 25 °C (except for 4a, pro-
duced at 0 °C). To this solution, kept under magnetic stirring, InBr3
(0.1 mmol) was added as a solid. The reaction was monitored by
TLC, for the time indicated at Table 1, after which the reaction mix-
2,4-Di(o-anisolyl)-4-methyl-pent-1-en (4g): 1H NMR: d = 1.21 (s, 6
H), 3.02 (s, 2 H), 3.55 (s, 3 H), 3.66 (s, 3 H), 4.86 (d, broad, 1 H),
4.77 (d, J = 1.2 Hz, 1 H).
1,3,3,4,7-Pentamethyl-1-(2¢,5¢-dimethylphenyl)-indan (5h): 1H
NMR: d = 1.90 (s, 3 H), 1.91 (s, 3 H), 2.04 (s, 3 H), 2.08 (s, 3 H),
Synlett 2004, No. 10, 1723–1726 © Thieme Stuttgart · New York