1202
GRISHA, DE VEKKI
Table 1. Acetoxylation of octa-1,3-dienylbenzene in the presence of various platinum-group catalysts 150°C, 9.1 MPa
Selectivity of formation of isomers, %
M–Te/C
catalysta
Total
Total yield,
wt %
selectivity, %
Z-(I)
E-(I)
Z-(II)
E-(II)
Z-(III)
E-(III)
М:
Pd
Pt
8.1
7.6
18.8
25.6
7.8
7.4
18.1
25.8
0.7
4.3
6.5
2.0
60.0
72.7
59.9
17.8
Rh
Ir
30.1
35.9
5.4
3.4
29.8
36.1
5.6
3.3
6.2
3.0
80.1
78.7
80.0
10.1
Traces
Traces
a As the second element of the catalytic formulation was chosen Te, which exhibits good performance in acetoxylation of 1,3-butadiene
_0.001 _0.003
0.013 0.014
catalyst–substrate chemisorbates. For example, the band
appearing in IR spectra of octa-1,3-dienylbenzene–Pt–
Sb and octa-1,3-dienylbenzene–Ir–Bi sorbates in the
spectral range 2800–3700 cm–1 at 3015 cm–1 has low
intensity, compared with the spectrum of the substrate
or with similar spectra for Pd and Rh, which may be
indicative of the presence of both π- and σ-adsorbed
forms. It is known [4] that the σ-binding leads to a firmer
adsorption of the substrate on active catalytic centers,
which impairs the activity of the catalysts.
_0.030
0.026
0.007
0.000
0.026
_0.030
0.001
0.016
Theresultsobtaineddemonstratethatthedistributions
of effective charges on carbon atoms at double bonds
are fundamentally different in the molecules under
consideration. For example, in octa-1,3-dienylbenzene,
the electron densities on the first and third, second
and fourth carbon atoms nearly coincide. In addition,
a certain difference in signs is observed for the first
and second, third and fourth atoms. All the described
features are also characteristic of 1,3-butadiene, only
being more pronounced. By contrast, a different situation
was observed for 2,4-nonadiene, in which the electron
density on the respective carbon atoms is lowered. The
data obtained suggest that 1,3-butadiene is more active
in complexation with the catalyst (in chemisorption of
reagents), and then follows octa-1,3-dienylbenzene and,
further, 2,4-nonadiene. This suggestion is in agreement
with the data previously obtained for 1,3-buta- and
2,4-octadienes[3], withtheyieldofdiacetoxyderivatives
and the rate of the chemical reaction being very low
in the last case. By contrast, 1,3-butadiene was found
to be the most reactive among conjugated alkadienes.
Presumably, steric factors exert a considerably weaker
influence in the given cases, compared with the
electronic characteristics of the molecules.
In the next stage of the study, it was necessary to
choose the most efficient acetoxylation catalyst among
the known set of catalysts previously synthesized
for similar purposes. Of stronger interest were binary
heterogeneousintermetalliccatalystsbasedonpalladium
and rhodium, which contained As, Te, Ge, Sb, Ga, Sn,
Bi, In, or Tl as a second element (Table 2).
As can be seen in Table 2, the formation selectivity
of isomers I and II varies rather widely, from 50 to 70%,
and that of isomer III, from 5 to 12%. It can be clearly
seen that this is due to the nature of the second atom
in the platinum-group component of the catalyst, with
the highest selectivity with respect to isomers I and II
observed in the presence of Pd–Sb and Rh–Bi systems;
the yield of isomer III is the lowest for palladium and
rhodium catalysts.
The phase compositions were reported in [5] for
some systems of this kind and it was demonstrated that
just these phases are active in acetoxylation. To these
belong Pd3Sb, Pd4Te, PdSn (PdSn3), RhBi4, Rh2Te
(Rh10Te).
It was of indubitable interest to relate the efficient
operation of catalysts to a physical parameter of the
second component of the catalytic systems. Previously,
It became possible to understand the substantially
lower yields of diacetoxy derivatives in the presence
of Pt and Ir catalysts on measuring IR spectra of
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 7 2011