E. Farnetti, N. Marsich / Journal of Organometallic Chemistry 689 (2004) 14–17
15
formed, together with traces of the other isomer 1,3,5-
triphenylbenzene, whereas polymerization and linear
oligomerization products (enynes [11] and linear trimers
[12]) were not present in the final mixture. The reaction
was repeated employing the analogue compounds with
dppp and dppb, respectively: again 1,2,4-triphenylben-
zene was the main product, but both the conversion and
the regioselectivity were lower than with the dppe
derivative. In contrast, the reaction promoted by
HIr(cod)(dppm) [13] was very fast, as phenylacetylene
was completely consumed after 15 min, and 1,2,4-tri-
phenylbenzene was now the only reaction product, apart
from a hardly detectable amount of the symmetric iso-
mer. The latter catalytic reaction was repeated at 40 and
20 °C in order to observe the effect of temperature on
both catalytic activity and selectivity: the expected de-
crease in activity was observed, yet with a conversion
above 80% in 8 h at 20 °C. On the other hand, the lower
temperatures had a slightly negative effect on the regi-
oselectivity, which decreased to 96% at 20 °C. Other
solvents were also tested for the catalytic reactions: good
results were obtained with toluene and methanol, even if
the iridium derivative is poorly soluble in the latter, thus
giving lower conversion; at variance, chloroform and
dichloromethane could not be employed, because the
iridium precursors undergo partial decomposition in
these solvents.
In order to study the substrate dependent influence
on the regioselectivity, as well as the generality of the
reaction, cyclotrimerization of the substituted phenyl-
acetylenes p-MeOC6H4CBCH and p-CF3C6H4CBCH
catalyzed by HIr(cod)(dppm) was investigated. The re-
actions performed at 60 °C with the substituted mono-
mers were very fast, and comparable with that of
unsubstituted phenylacetylene (see Table 2, runs 1–3);
all these reactions yield the 1,2,4-trisubstituted benzene
with excellent regioselectivity. The effect of the sub-
stituent on the alkyne aryl ring could be better appre-
ciated at a lower reaction temperature: at 40 °C,
substrate consumption became faster when changing
from p-CF3C6H4CBCH to C6H5CBCH to p-
MeOC6H4CBCH (see Table 2 runs 4–6). Therefore,
cyclotrimerization appears to be accelerated when more
electron-rich acetylenes are employed; on the contrary,
Ar
HIr(cod)(P-P)
THF, 60˚C
Ar
3
H
Scheme 1.
recorded in Nujol mull on a Perkin–Elmer System 2000
FT-IR spectrometer.
Chemical yields of the catalytic mixtures were deter-
mined by GC on a Hewlett Packard 6890 instrument
equipped with a HP-1 column, using naphthalene as
internal standard.
2.1. Experimental procedure for the catalytic reactions
A typical procedure is described in the following: A
solution of HIr(cod)(P-P) (0.017 mmol) and of the GC
standard naphthalene (100 mg) in 5.0 ml of THF (or
other solvent of choice) was heated to the desired reac-
tion temperature under inert atmosphere. Then, 173 mg
of phenylacetylene (1.7 mmol, [sub]/[Ir] ¼ 100) was ad-
ded. Samples were withdrawn from the reaction mixture
at time intervals and composition of the mixture was
followed with time by GC. The final reaction mixture
1
was concentrated to dryness and analyzed by H and
13C NMR. In all the reactions reported, neither po-
lyphenylacetylenes nor linear oligomers were detected.
3. Results and discussion
The results obtained in the cyclotrimerization of
phenylacetylene catalyzed by HIr(cod)(P-P) are sum-
marized in Table 1. When HIr(cod)(dppe) [9] was trea-
ted in tetrahydrofuran (THF) at 60 °C with 100
equivalents of the monomer, the pale yellow solution
turned dark orange immediately. After 1 h the resulting
brown solution according to GC analysis contained less
than 30% of the monomer and after 6 h the conversion
was 98%. H and 13C NMR spectra of the crude prod-
ucts [10] revealed that only 1,2,4-triphenylbenzene was
1
Table 1
Cyclotrimerization of phenylacetylene catalyzed by HIr(cod)(P-P)
P-P
T (°C)
Time (h)
Conv. (%)
1,2,4-Ph3C6H3 (%)
1,3,5-Ph3C6H3 (%)
Dppb
Dppp
Dppe
Dppm
Dppm
Dppm
60
60
60
60
40
20
6
31
47
28
45
97
99
95
78
3
2
1
1
3
3
6
6
98
0.25
1.5
8
100
98
81
Experimental conditions: [Ir] ¼ 3.4 ꢀ 10ꢁ3 mol lꢁ1; [sub] ¼ 0.34 mol lꢁ1; [sub]/[Ir] ¼ 100; solvent THF. dppb ¼ 1,4-bis(diphenylphosphino)butane;
dppp ¼ 1,3-bis(diphenylphosphino)propane; dppe ¼ 1,2-bis(diphenylphosphino)ethane; dppm ¼ bis(diphenylphosphino)methane.