Disproportionation of dihydroarenes by sol-gel entrapped RhCl3-quaternary ammonium ion pair catalysts

Article abstract of DOI:10.1006/jcat.1996.0392

The ion pairs generated from RhCl₃ · 3H₂O and the quaternary ammonium salts [(C₈H₁₇)₃NMe]Cl (Aliquat 336) and [Me₃N(CH₂)₃Si(OMe)₃]Cl were physically and chemically entrapped, respectively, in SiO₂ sol-gel matrices under mild conditions. The resulting immobilized ion pairs proved to be stable, leach-proof, and recyclable catalysts for disproportionation of 1,3-cyclohexadiene and several other vic-dihydroarenes. In these reactions, equimolar quantities of the respective tetrahydro-and fully aromatic compounds were obtained. The entrapped catalysts, in most cases, proved to be more efficient and more selective than their homogeneous analogues. The reaction rates and conversions were shown to depend strongly on steric effects of substituents and on the bulkiness of the substrate skeleton. The recorded first order kinetics in the substrates (organic reactants) suggests that the mechanism involves stepwise addition of two molecules of the dihydroarenes to the rhodium nucleus, and that the addition of the first substrate molecule is rate limiting.

Full text of DOI:10.1006/jcat.1996.0392

JOURNAL OF CATALYSIS 164, 363–368 (1996)
ARTICLE NO. 0392
Disproportionation of Dihydroarenes by Sol–Gel Entrapped
RhCl -Quaternary Ammonium Ion Pair Catalysts
3
1
1
Ayelet Rosenfeld, Jochanan Blum, and David Avnir
Department of Organic Chemistry, The Hebrew University, Jerusalem 91904, Israel
Received February 5, 1996; revised June 5, 1996; accepted August 9, 1996
We now report the utilization of the SiO2 sol–gel
+
�
The ion pairs generated from RhCl3 � 3H2O and the quater- encapsulated ion pair [(C8H17)3NMe] [RhCl4 � n(H2O)]
nary ammonium salts [(C8H17)3NMe]Cl (Aliquat 336) and (10) (catalyst SG-1) as catalyst for the disproportiona-
[
Me3N(CH2)3Si(OMe)3]Cl were physically and chemically en-
tion of 1,3-cyclohexadiene and of various vic-dihydroarenes
to the corresponding fully aromatic and tetrahydroaro-
matic compounds. For comparison, we applied also an en-
trapped rhodium catalyst generated from RhCl3, [Me3N-
CH2)3Si(OMe)3]Cl and Si(OMe)4, in which the ion pair
had been bound chemically to the skeleton of the sol–gel
trapped, respectively, in SiO2 sol–gel matrices under mild con-
ditions. The resulting immobilized ion pairs proved to be stable,
leach-proof, and recyclable catalysts for disproportionation of 1,3-
cyclohexadiene and several other vic-dihydroarenes. In these reac-
tions, equimolar quantities of the respective tetrahydro- and fully
aromatic compounds were obtained. The entrapped catalysts, in
most cases, proved to be more efficient and more selective than
their homogeneous analogues. The reaction rates and conversions
were shown to depend strongly on steric effects of substituents and
on the bulkiness of the substrate skeleton. The recorded first or-
der kinetics in the substrates (organic reactants) suggests that the
mechanism involves stepwise addition of two molecules of the di-
hydroarenes to the rhodium nucleus, and that the addition of the
(
matrix (catalyst SG-2).
METHODS
Chemicals. 1,3-Cyclohexadiene, 1,4-cyclohexadiene,
tetramethoxysilane, methyltricaprylammonium chloride
Aliquat 336), and rhodium trichloride trihydrate were
(
first substrate molecule is rate limiting.
�c 1996 Academic Press, Inc.
purchased from Aldrich Chemical Co., Inc. N-Trimethoxy-
silylpropyl-N,N,N-trimethylammonium chloride (50%
in MeOH) was purchased from ABCR GmbH & Co.
The substrates 1,2-dihydronaphthalene (IV, R = H)
INTRODUCTION
(
4
11), 1,2-dihydro-4-methylnaphthalene (IV, R = Me) (12),
The sol–gel methodology has so far been used in catalysis
in the context of inorganic catalysts, as part of the matrix
-ethyl-1,2-dihydronaphthalene (IV, R = Et) (13), 1,2-di-
hydro-4-phenylnaphthalene (IV, R = Ph) (13), 4-carbo
(
1, 2), as supports for dispersed metal particles (3) and for
copolymerization with suitable silicon-containing ligands
4), but, to the best of our knowledge, not for low tem-
ethoxy-1,2-dihydronaphthalene (IV, R = COOEt) (14),
1
,2-dihydro-8-methoxynaphthalene (VII) (12), 3,4-dihydro
(
phenanthrene (X) (15), 1,2-dihydrophenanthrene (XI)
15), 10,11-dihydrobenz[a]anthracene (XIV) (16), and
,10-dihydrobenzo[a]pyrene (XVII) (15) were prepared as
perature direct entrapment of metal catalysts that carry
organic moieties as reported briefly in our recent publi-
cations (5, 6). The success of this approach in enzymatic
catalysis and in photocatalysis, respectively, has also been
described (7, 8). According to this methodology, the dopant
molecules are simply added to tetraalkoxysilane during its
hydrolytic polymerization (9). The resulting immobilized
catalysts proved to exhibit either similar or superior cata-
lytic activity as compared with the nonentrapped metal
complexes (6). The immobilized catalysts were found to
be stable under ambient conditions, leach-proof, and recy-
clable in numerous runs.
(
9
described in the literature.
Instruments. 1H and 13C NMR spectra were recorded
on a Bruker AMX-400 instrument. MS measurements were
performed on a Hewlett–Packard Model 4989A mass spec-
trometer equipped with an HP gas chromatograph model
890 series II. Gas chromatographic separations and anal-
yses were carried out with the aid of a Hewlett–Packard
CG model 417 equipped with a 2 m long column packed
5
�
with 10% DEGS on Chromosorb W and operated at 120 C.
Electronic spectra were recorded on a Hewlett–Packard
spectrophotometer model 8454A. Nitrogen Langmuir sur-
face area and average pore diametersofthe sol–gelmatrices
1
To whom correspondence may be addressed.
363
0021-9517/96 $18.00
Copyright �c 1996 by Academic Press, Inc.
All rights of reproduction in any form reserved.

3
64
ROSENFELD, BLUM AND AVNIR
were measured using a Micromeritics ASAP 2000 instru-
ment. Atomic absorption spectrometry was carried out
on a Perkin–Elmer spectrophotometer model 403 using a
Juniper rhodium cathode lamp.
TABLE 1
Disproportionation of Some vic-Dihydroarenes by SG-1
�
a
and SG-2 at 107 C
Yields after 40 min (% )
Preparation of the entrapped ion pair catalysts SG-1
Expt. Substrate
Products
SG-1
SG-2
�
2
and SG-2. A mixture of 20 mg (7.59 � 10 mmol) of
RhCl3 � 3H2O and an equimolar amount of the appropri-
ate quaternary ammonium salt ([(C8H17)3NMe]Cl for SG-1
and [Me3N(CH2)3Si(OMe)3]Cl for SG-2) in 2.4 ml of triply
1
2
3
4
5
6
7
8
9
I
II; III
V, R = H; VI, R = H
97
83
58
16
52
28
20
8
96
99
41
17
87
73
50
23
—
IV, R = H
IV, R = Me V, R = Me; VI, R = Me
IV, R = Et
V, R = Et; VI, R = Et
VIII; IX (26)
XII; XIII (27)
XII; XIII (27)
XV; XVI (28)
no products
distilled water was stirred in a cylindrical vial (diameter
VII
�
X
XI
XIV
XVII
2
cm) at 25 C for 10 min. Methanol (3.5 ml) was added and
the stirring was continued for 30 min. Tetramethoxysilane
(
2.5 ml) was added (molar ratio silane : water = 1 :8) and
—
the stirring was continued for an additional 30 min. The
�
a
Reaction conditions as specified in the Experimental section.
mixture was then left to stand at 25 C for 3–4 days until
�
gelation was completed. The gel was heated to 45 C un-
til a constant weight was achieved. The resulting catalyst
granules were washed with 20 ml of boiling CH2Cl2 and
sonicated for 30 min in the same solvent in order to re-
move any rhodium compound that adhered onto the outer
RESULTS AND DISCUSSION
+
Encapsulation of the ion pair [(C H ) NMe] �
8
17 3
�
surface of the matrix. The catalyst was then dried for 3 h [RhCl � nH O] in a SiO sol–gel matrix has been accom-
4
2
2
at 1 mm, washed with 20 ml of boiling triply distilled wa- plished by addition of equimolar quantities of RhCl � 3H O
3
2
�
ter and dried again at 25 C at 1 mm for 20 h before use. and Aliquat 336 at room temperature to the reaction mix-
The rhodium content within SG-1 and SG-2 was shown to ture of Si(OMe) , MeOH, and water. After the gel was
4
�
be practically the same as in the RhCl3 � 3H2O employed. dried at 45 C the doped xerogel was activated by reflux-
Atomic absorption measurements (17) revealed that the ing in CH Cl , sonication, washing with boiling water, and
2
2
rhodium content in the washings was usually <1 ppm drying under reduced pressure. The electronic spectrum of
and in no case exceeded 5 ppm. To ensure the existence the immobilized catalyst which consisted of a strong ab-
of the entrapped catalysts in the state of ion pairs, elec- sorption band at 510 nm was found to resemble that of
+
�
tronic spectra of the sol–gel glasses were recorded [cf., the soluble ion pair [(C H ) NMe] [RhCl � nH O] (19)
8
17 3
4
2
e.g., (18)].
(� = 507 nm), but shifted slightly to a higher wavelength.
max
A similar bathochromic shift (from 474 to 480 nm) (18) was
General procedure for the disproportionation experi- observed during the entrapment of the ammonium ion-free
ments. A reaction flask equipped with a reflux condenser, RhCl3 � 3H2O in a xerogel matrix.
a magnetic bar, and a rubber sealed side arm was charged,
When the Aliquat 336 in the above process had been re-
under argon atmosphere, with the catalyst material and 2 ml placed by [Me3N(CH2)3Si(OMe)3]Cl the resulting homoge-
oftoluene. The vesselwasplaced in an oilbath thermostated neous ion pair copolymerized with the Si(OMe)4 forming
at the desired temperature. After temperature equilibra- SG-2, in which the ligand is bound chemically to the sol–gel
tion, 2 mmol of the substrate was either injected or intro- matrix.
duced with the aid of a Tygon tube into the flask. Samples
Both the physically encapsulated and the chemically
of ca. 2 �l were withdrawn periodically. The products were bound catalysts were shown to be microporous materials,
analyzed either by direct comparison with authentic sam- with a narrow pore size distribution, centered around 15
1
ples or by comparison of their spectral data (UV, H NMR, and 20 A˚ for SG-1 and SG-2, respectively. Their corre-
1
3
C NMR, GC-MS, or LC-MS) with those reported in the spondingN2-Langmuir surface area wasfound to be 722and
2
literature. Typical results are summarized in Table 1. Upon 832 m /g.
achievement of the desired conversion, the liquid reaction
The entrapped ion pairs, SG-1 and SG-2, catalyzed the
mixture was decanted and analyzed for leached rhodium disproportionation of 1,3-cyclohexadiene (I) equally well.
by atomic absorption. The catalyst was refluxed in 20 ml of They promoted the formation of equal molar quantities of
CH2Cl2, sonicated for 30 min, washed again with the same benzene (II) and cyclohexene (III) in 96–97% yield within
amount of solvent, and dried for 3 h at 1 mm. The dried 40 min.
catalyst was used in a subsequent run.
Kinetic measurements were carried out in modified pres- to promote double bond migration in allylbenzene (6),
sure tubes with sampling devices. they failed, in contrast to other disproportionation catalysts
Although both immobilized catalysts have been shown

DISPROPORTIONATION BY SOL–GEL ENTRAPPED CATALYSTS
365
limited space available within the narrow pores that permits
the presence of only few solvent molecules, which forces
the catalyst to exist as contact ion pairs. Under homoge-
neous conditions the catalyst may exist as heavily solvated
solvent-separated ion pairs in which the metal nucleus is
less accessible by the substrate than in the entrapped cat-
alysts. The porous nature of the sol–gel structure seems to
be responsible also for the strong dependence of the rate
on the bulkiness and steric effects of the substrates. Table 1
indicates that the tricyclic dihydrophenanthrenes, X and
XI react more slowly than the small 1,3-cyclohexadiene (I)
and the unsubstituted 1,2-dihydronaphthalene (IV, R = H).
The tetracyclic 10,11-dihydrobenz[a]anthracene (XIV) dis-
proportionates even more slowly, and the pentacyclic 9,10-
dihydrobenzo[a]pyrene (XVII) seems not to be able to
penetrate the matrix pores, and does not react at all. The
rate was also strongly affected by substituents that extend
steric effects on the reaction site. Table 1 reveals that in-
troduction of a methyl group in position 1 of IV reduces
the rate considerably. An ethyl group inhibits the dispro-
portionation to an even larger extent, and 4-carboethoxy-
1
1
,2-dihydronaphthalene (IV, R = COOEt) as well as
,2-dihydro-4-phenylnaphthalene (IV, R = Ph) fail to dis-
proportionate at all under our experimental conditions.
Substituents at a more remote position from the reaction
site, such as in 1,2-dihydro-8-methoxynaphthalene, proved
to hardly affect the reaction rate. Because of steric rea-
sons 3,4-dihydrophenanthrene (X) reacts faster than the
1
,2-dihydro-isomer, XI, in which the reacting double bond
SCHEME 1. Disproportionation of dihydroarenes.
(
20–22), to convert 1,4-cyclohexadiene into the 1,3-isomer,
i.e., the nonconjugated diene could not be converted into
II and III under the above reaction conditions. The vic-
dihydroarenes IV (R = H, Me, Et), VII, X, XI and XIV,
which can formally be regarded as higher benzologues of I,
were found to undergo catalytic disproportionation to form
the corresponding tetrahydro-, and fully aromatic com-
pounds (see Scheme 1). Dihydronaphthalenes IV (R = H,
Me, and Et) yielded the respective naphthalene and tetralin
derivatives V and VI, and the methoxy-compound VII gave
VIII and IX. Both 3,4- and 1,2-dihydrophenanthrene (X
and XI, respectively) were transformed to phenanthrene
(
1
XII) and 1,2,3,4-tetrahydrophenanthrene (XIII), and
0,11-dihydrobenz[a]anthracene (XIV) yielded the aro-
matic benz[a]anthracene XV and the 8,9,10,11-tetrahydro-
derivative XVI. Representative results are shown in
Table 1. The disproportionation of I and IV (R = H) by
SG-1 was found to proceed faster than that by the ho-
mogeneous ion pair (23). Typical reaction profiles for the
disproportionation of IV (R = H) by SG-1 and SG-2 are
FIG. 1. Concentration-time profiles for the reactant and products
in SG-1 (—) and SG-2 (– – –) catalyzed disproportionation of 1,2-
presented in Fig. 1. We assume that the enhanced rates ob- dihydronaphthalene (᭡) to naphthalene (� ) and tetralin (᭜ ) (obtained
tained by the immobilized catalysts are associated with the always in a 1 : 1 ratio).

3
66
ROSENFELD, BLUM AND AVNIR
by the difference between the inter-matrix construction of
the physically encapsulated and chemically bound catalysts.
The structural differences and particularly the larger pore
diameter and surface area of SG-2 (20 A˚ as compared to
1
5 A˚ for SG-1), may explain also why both entrapped cata-
lysts promote almost equally well the disproportionation of
small substrates, but that SG-2 is superior to SG-1 for the
disproportionation of the large polycyclic dihydroarenes
(
see Table 1).
Since the disproportionation of 1,3-cyclohexadiene and
1
,2-dihydronaphthalene derivatives by soluble complexes
(
20, 21), by palladium on carbon (22) and by silica-
supported catalysts (24) is generally accompanied by de-
hydrogenation processes, it is remarkable that SG-1 and
SG-2-catalyzed disproportionations give in all cases only
equimolar quantities of the aromatic and tetrahydroaro-
matic products. Previous experiments revealed that un-
der phase transfer conditions, in a toluene/H2O system,
+
�
[
(C8H17)3NMe] [RhCl4 � nH2O] (23) converts the dihy-
droaromatic substrates IV (R = H), X, XI, and XIV into
mixturesofproductsin which the fullyaromaticcompounds
prevail. Similar results have now been obtained under ho-
mogeneous conditions in water-free toluene. The results
of some representative experiments are summarized in
Table 2. It is notable that none of the 1,2-dihydronaphtha-
lene derivatives IV that have a substitutent at the reaction
site (i.e., on C4) could be disproportionated by the homo-
geneous ion-pair catalyst.
FIG. 2. Conversion-time profiles for SG-1-catalyzed disproportiona-
tion of I (� ), IV, R = H (᭡), IV, R = Me (� ), IV, R = Et (4), VII (� ), X
� ), XI (᭹), and XIV (+), under the conditions of Table 1.
(
is shielded by the “bay-region” hydrogen atoms H4 and
H5. The relative activities of the various substrates can
be deduced from the conversion-time profiles shown in
Fig. 2 and Fig. 3. These figures reveal some differences
in the order of the activities of SG-1 and SG-2-catalyzed
disproportionations. This observation can be rationalized
The most important feature ofthe sol–gelentrapped cata-
lysts is obviously their ability to be recycled in numerous
runs. We recall that in 1,3-cyclohexadiene disproportiona-
tion, the soluble RhCl3-Aliquat 336 catalyst could be recy-
cled for a few runs, but the rates dropped after each run
by more than 30% (23). Both the physically encapsulated
catalyst, SG-1, and the chemically bound SG-2 have been
shown to be perfectly leachproof and their catalytic ac-
tivity has hardly changed upon recycling. It is necessary,
TABLE 2
Disproportionation and Dehydrogenation of some Dihydro-
+
�
arenes by [(C8H17)3NMe] [RhCl4 � nH2O] under Homogeneous
�
a
Conditions at 104 C
Reaction
time (h)
Conversion
(% )
Tetrahydroaromatic: aromatic
product ratio
Substrate
I
2
1
0.5
1
70
90
100
81
1 :1b
1 : 1.9
1 : 4.3
1 :5
IV, R = H
X
XI
XIV
1
100
1 : 4
a
� 2
Reaction conditions: ion pair generated from 7.59 � 10 mmol of
�
2
FIG. 3. Conversion-time profiles for SG-2-catalyzed disproportiona- RhCl3 � 3H2O and 7.59 � 10 mmol of Aliquat 336, 2 ml H2O, and 2 ml
tion of I (� ), IV, R = H (᭡), IV, R = Me (� ), IV, R = Et (4), VII (� ), toluene; aqueous layer removal; 2 mmol substrate; Ar atmosphere.
b
X (� ), XI (᭹), and XIV (+), under the conditions of Table 1.
In (CH2Cl)2 the ratio was 1 : 2 [cf. (22)].

DISPROPORTIONATION BY SOL–GEL ENTRAPPED CATALYSTS
367
mixtures showed that together with the rhodium, equiva-
lent amounts of the quaternary ammonium salt had been
eluted.
+
The disproportionation of I by [(C8H17)3NMe] �
�
[
RhCl4 � nH2O] (23) as well as by some silica-supported
dirhodium catalysts (24) could be explained best in terms
of the mechanism of Moseby and Maitlis (20) in which
the rate determining step involves a species with two un-
saturated cyclohexane residues, leading to second order
in the substrate. However, the data recorded for SG-1-
and SG-2-catalyzed disproportionation of I and of IV,
R = H, proved incompatible with such a mechanism. Ki-
�
netic measurements at 107 C for 0.25–0.75 M solutions of
1
,2-dihydronaphthalene (IV, R = H) in toluene revealed
that under the conditions given in the Experimental sec-
tion, the disproportionation is pseudo zero order in the
rhodium and apparent first order in the substrate [kobs
�
4
� 1
(
1
SG-1) = 4.34 � 0.08 � 10 s ; kobs (SG-2) = 2.73 � 0.07 �
�
3
� 1
0
s ].
In contrast to the homogeneous processes (23, 24) in
FIG. 4. Conversion-time profiles for SG-1-catalyzed disproportiona-
which deuteriated solvents caused deuterium labeling of
the tetrahydroaromatics, the application of toluene-d8 to
our systems did not lead to incorporation of deuterium in
the products. Likewise, sol–gel matrices that had been pre-
pared by hydrolysis of Si(OMe)4 in D2O and methanol-d4
did not promote deuterium transfer to V or VI, nor to the
unreacted starting material IV. These observations can be
interpreted in the light of a mechanism similar to that of
Fischer et al. (21), shown in Scheme 2, in which the metal
hydride is generated by abstraction of a proton from the co-
ordinated substrate rather than from the medium, and the
rate limiting step is the initial coordination of the starting
dihydro compound to the rhodium nucleus.
tion of 1,2-dihydronaphthalene under the conditions of Table 1. First run
(
᭡) (catalyst was not activated with boiling water); second run (4); third
run (᭹); fourth run (� ); fifth run (� ).
however, to prevent any pore clogging between the cycles
by washing the doped material with an organic solvent (fol-
lowed by sonication) and eventually with boiling water. The
latter breaks the Si–O–Si bonds and opens somewhat the
pore network. Figure 4 presents a typical set of conversion-
time plots for SG-1-catalyzed disproportionation of 1,2-
dihydronaphthalene (IV, R = H) in the first five runs (SG-2
gave similar profiles). In this set of experiments the catalyst
had not been treated with boiling water prior to the first
run. Consequently, the rate of the first run was lower than
those of the subsequent cycles for which the ceramic mate-
rial had been reactivated. The rates for all subsequent runs
were almost identical (Fig. 4).
Under phase transfer conditions the nature of the quater-
nary ammonium salt was shown to have only a small effect
on the rate of disproportionation of dihydroarenes (23).
The encapsulated catalyst proved however to be strongly
affected by the size of the ammonium ion. When ions with
relative short alkyl chains were employed, the resulting
ion-pairs leached gradually from the sol–gel matrix. For
example, replacement of the Aliquat 336 in SG-1 with
The catalytic disproportionation of IV, R = H, proved to
�
have the same reaction patterns between 100 and 120 C.
Therefore, we could express the initial rates measured
�
at 101, 110, 113, 115, and 120 C in terms of Arrhenius
plots (Fig. 5). It should be noted that it was not at all
obvious that the Arrhenius equation could be applied to
[Bu4N]Br resulted in the formation of a catalyst which, in
contrast to SG-1, lost part of its activity due to leaching.
In a typical set of experiments of disproportionations of I
by the RhCl3–[Bu4N]Br catalyst (initial rhodium content,
7
.8 mg) metal leaching extended during each of the first
�
four runs (of 90 min at 107 C) to 0.8 mg. Varying additional
amounts of rhodium were eluted during the washings be-
tween the cycles. H NMR measurements of the reaction portionation of 1,3-cyclohexadiene and similar compounds.
SCHEME 2. Possible mechanism of SG-1- and SG-2-catalyzed dispro-
1

3
68
ROSENFELD, BLUM AND AVNIR
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�
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1
�
1
#
�
� 1
Ea = 15.0 kcal mol and 1H (115 C) = 14.0 kcal mol ,
(
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1
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2
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controlled reactions (25), suggest, as one could expect,
that both processes contribute to the progress of the
catalysis.
2
2
2
ACKNOWLEDGMENTS
25. See e.g., Smith, J., “Chemical Engineering Kinetics,” p. 297. McGraw–
We acknowledge the support of this study by the Volkswagen-Stiftung,
Hill, New York, 1975.
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