Oxidative arylation of ethylene with benzene to produce styrene

Article abstract of DOI:10.1246/cl.2000.1064

Rh complexes were found to work as catalysts for oxidative arylation of ethylene with benzene to produce styrene by addition of acetylacetone and O₂ without any oxidizing agent like Cu salt. Vinylacetate was not formed at all in spite of coexis

Full text of DOI:10.1246/cl.2000.1064

1064
Chemistry Letters 2000
Oxidative Arylation of Ethylene with Benzene to Produce Styrene
Takaya Matsumoto* and Hajime Yoshida
Central Technical Research Laboratory, Nippon Mitsubishi Oil Corporation, 8 Chidoricho, Naka-ku, Yokohama,
Kanagawa 231-0815
(Received June 26, 2000; CL-000620)
Rh complexes were found to work as catalysts for oxida-
10.6 M of benzene and 0.67 M of acacH. The reactor was
degassed with N2, pressurized with 1.55 MPa of ethylene,
sequentially pressurized with O2 up to 2.10 MPa and heated to
180 °C with stirring for 20 min. The liquid phase was analyzed
by GC (FID) at the end of the reaction. Styrene was obtained
(TOF = 188 × 10–4 s–1, TN = 23; Table 1, Entry 3). Although
the arylation of ethylene using Cu(OAc)2 as a co-catalyst also
resulted in styrene (TOF = 248 × 10–4 s–1) coupled with forma-
tion of vinyl acetate (VA) (TOF = 83 × 10–4 s–1; Entry 1), the
reaction was not catalyzed by 1 without Cu(OAc)2 (Entry 2).
The addition of acacH made oxidative arylation directly forced
by O2 possible. Furthermore, in spite of coexistence of acetic
acid in the reaction medium, no formation of vinyl acetate was
observed in the system of O2 / benzene / acetic acid / acacH.
tive arylation of ethylene with benzene to produce styrene by
addition of acetylacetone and O2 without any oxidizing agent
like Cu salt. Vinylacetate was not formed at all in spite of
coexistence of acetic acid.
Much work has been done at olefin oxidation catalyzed by
group VIII metal complexes. In presence of an oxidant, Pd cat-
alyzes partial oxidation of ethylene, namely well-known
Wacker reaction to form acetaldehyde1 and oxidative vinylation
of acetic acid to produce vinylacetate.2 Numerous efforts relat-
ed to homogeneous C–H bond activation in aromatic com-
pounds by discrete transition metal complexes through various
mechanisms have also been reported.3 Derivatives from aro-
matic compounds such as alkylbenzene, phenol, aniline and
naphthalene are the large-quantity chemicals manufactured by
chemical industries. Especially styrene holds a majority in the
derivatives from benzene. Fujiwara et al. have reported signifi-
cant work for direct formation of styrene by oxidative arylation
of ethylene catalyzed by Pd complexes combining arene activa-
tion with oxidation.4 Hong and Yamazaki have also reported
direct formation of styrene by another approach using Rh(0)
complex, whereas 3-pentanone is simultaneously produced, and
the mechanism is completely different from Fujiwara’s.5 Since
the current industrialized process to produce styrene consists of
two parts, which are the alkylation of benzene with ethylene to
form ethylbenzene and the dehydrogenation of ethylbenzene to
afford styrene,6 the attempt on direct formation of styrene from
benzene and ethylene through C–H bond activation is an attrac-
tive field, which has enough room to be developed from the
industrial point of view. Group VIII metal complexes can be
employed for the series of oxidation reactions such as Wacker
reaction, oxidative vinylation of acetic acid and oxidative aryla-
tion of ethylene. However, an oxidizing agent such as Cu salt,
Ag salt or heteropoly acid is required in order to proceed the
reactions catalytically. Namely, the oxidizing agent reoxidizes
the group VIII metal complex reduced in the reaction, and then
oxygen reoxidizes the oxidizing agent in reduced form.
Accordingly, the oxidizing agent works as a co-catalyst.
Herein we report oxidative arylation of ethylene to produce
styrene catalyzed by Rh complexes with no oxidizing agent like
Cu salt by addition of acetylacetone (acacH), in which oxygen
directly drive the catalytic cycle. The reaction, catalyzed by Rh
complex, is assumed to occur by the C–H activation of ben-
zene. The comparison study of reaction condition and catalysts
for arylation of ethylene is shown in Table 1. The typical reac-
tion procedure is as follows (Entry 3); a 10-mL stainless steel
autoclave equipped with a glass insert and a magnetic stir bar
was charged with 3 mL of catalyst / benzene / acetic acid /
acacH solution which contained 1.0 mM of Rh(acac)(CO)2 (1),
[Rh(NBD)(DPPB)]BF4 (NBD = bicyclo[2.2.1]hepta-2,5-
diene, DPPB = 1,4-bis(diphenylphosphino)butane) (2) also cat-
alyzed oxidative arylation of ethylene to afford styrene (TOF =
180 × 10–4 s–1, TN = 22; Entry 6) and no vinyl acetate by addi-
tion of acacH and O2. Though the TOF for styrene formation
with Pd(OAc)2 using Cu(OAc)2 was larger than that with 1,
vinyl acetate production was preferable to styrene formation
with Pd(OAc)2 (Entries 1 and 7). When acacaH and O2 were
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
1065
added, styrene was produced with smaller amount of vinyl
acetate (Entry 8) than O2/Cu(OAc)2 case. However, in contrast
to the reaction catalyzed by complex 1, vinyl acetate was still
afforded with Pd(OAc)2.
As a comparison study of additives, 2,2,6,6-tetramethyl-
3,5-heptanedione, 1,1,1,5,5,5-hexafluoro-2,4-pentanedione and
acetoacetic acid tert-butylester were respectively introduced
into the reaction catalyzed by 1 instead of acacH (Entries 9, 10
and 11). Among these additives, acacH and 2,2,6,6-tetra-
methyl-3,5-heptanedione showed much higher activity for this
reaction than others which have electron-withdrawing groups.
benzene was activated through electrophillic substitution.
Without acacH and O2, complex 1 didn’t show any activity for
H–D exchange at all. In contrast to this, by the combination of
complex 1, acacH and O2, deuterium incorporation was
observed in C6H6/CH3COOD (6.0 M of benzene) at 160 (TOF
= 1,486 × 10–4 s–1; Table 3, Entry 19), which corresponded with
the result that the combination was available for styrene forma-
tion.
In summary, we report that various Rh(I) complexes
worked as catalysts for oxidative arylation of ethylene to pro-
duce styrene without any oxidizing agent like Cu salt by addi-
tion of acacH and O2. Moreover, in this reaction environment,
namely the combination of Rh(I), acacH and O2, H–D
Exchange between C6H6 and AcOD was catalyzed by complex
1. Therefore, it is obvious that this catalysis occurs through
C–H bond activation of benzene.
References and Notes
1
a) C. Elschenbroich and A. Salzer, “Organometallics,” 2nd
ed., VCH Publishers Inc., New York (1992), Chap. 17, p.
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2
3
G. Roscher, in “Ullmann’s Encyclopedia of Industrial
Chemistry,” 5th ed., ed. by B. Elvers, and S. Hawkins,
VCH Verlagsgesellscaft, Weinheim (1994), Vol. A27, p.
419.
We also studied the reaction by other various Rhodium cat-
alysts in presence of acacH and O2. The results were shown in
Table 2. Rh complexes, Rh(acac)(ethylene)2, [Rh(COD)Cl]2
and RhCl(PPh3)3, which were introduced into the reaction as
catalysts in Rh(I) oxidation state at the beginning of the reac-
tion, showed almost same activities as those for 1 and 2 to pro-
duce styrene and no vinylacetate. On the other hand, The rates
with Rh(III) complexes, [Rh(Cp*)Cl2]2 and [Rh(ppy)2Cl]2,
were almost one order of magnitude slower than those with
Rh(I) complexes.
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We propose that the reaction occurs in five main steps: A)
aromatic CH bond activation by the Rh(III) center (possibly
formed by in-situ oxidation) to produce an Rh–phenyl interme-
diate species, B) olefin insertion to produce an Rh–alkyl, C)
product loss from the metal center with β-hydride elimination,
D) H+ release by reductive elimination and E) reoxidation of
reduced metal center. Although the mechanism has not been
fully elucidated yet, benzene seemed to be activated by Rh cen-
ter. To learn whether C–H bond activation of benzene was
occurring, we examined the effect of reaction environment in
catalyzing proton exchange (generally a good test for reversible
C–H bond activation) between C6H6 and CH3COOD in which
4
5
6
P. Hong and H. Yamazaki, Chem. Lett., 1335 (1979).
D. H. James and W. M. Castor, in “Ullmann’s
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