One-pot reactions with opposing reagents: Sol-gel entrapped catalyst and base [3]

Full text of DOI:10.1021/ja003029b

J. Am. Chem. Soc. 2000, 122, 11999-12000
11999
Scheme 1
One-Pot Reactions with Opposing Reagents:
Sol-Gel Entrapped Catalyst and Base
Faina Gelman, Jochanan Blum,* and David Avnir*
Institute of Chemistry, The Hebrew UniVersity of Jerusalem
Jerusalem 91904, Israel
ReceiVed August 15, 2000
Standard multistep synthetic procedures utilizing reagents,
which are destructive to each other, require consecutive product
separation and isolation. Configurations which offer total avoid-
ance of contact of reactants by placing them in different locations
in the reacting vessel must be practiced in such cases, as, e.g., in
electrochemical cells which require separation of the mutually
annihilating redox reagents. The potential ability to carry out
simultaneously in one-pot reactions with hostile reagents is
therefore of great practical use. Here we offer a methodology
designed to solve this inherent problem in chemical synthesis.
The key to this has been the utilization of organically doped
porous sol-gel oxide matrices, which have the remarkable
property of being reactive practically only in their inner, diffu-
sionally dopant-accessible regions, thus protecting the dopant from
interferences which cannot penetrate the pores. This is an
advantage which does not exist in most other heterogenization
methods such as surface derivations of inorganic supports or the
use of organic-polymer supports: In these configurations the
heterogenized reagents are much more exposed, and can affect
each other. Intensive activity in the field of reactive doped sol-
gel materials demonstrated the generality of this heterogenization
methodology, which by now has covered practically all major
types of chemical reactions. These include catalysis, redox, acid/
base, and complexation reactions, carried out either thermally,
photochemically, electrochemically, or biochemically.1 When it
comes to multiple reagent reactions, the approach has been to
isolate within the same matrix the various reactive species.2 Also
relevant in this context is the elegant work of Soumillion et al.,3
who demonstrated chemical communication between derivatized
silica beads in a photoredox reaction. The application developed
here is tailored to the needs of chemical synthesis in the following
way: First, having at hand such heterogenized, protected reagents,
one has the freedom to create combinations of different reagents
as required by the specific synthetic target, allowing these reagents
to perform only their useful tasks without affecting each other;
and second, as emphasized above, it allows one to simultaneously
carry out consecutive synthetic steps.
and its substrate was synthesized with a base from the corre-
sponding halide in the same pot. The catalyst was physically
entrapped in an SiO2 sol-gel matrix (SG-cat) as previously
described.5 The base, H2N(CH2)2NH(CH2)3-, was covalently
heterogenized by copolymerizing the base-Si(OCH3)3 monomer
with Si(OCH3)4 (SG-base).6,7 Using (SG-cat) and (SG-base) in
one pot as a slurry in benzene at 80 °C, â-iodoethylbenzene was
smoothly converted to ethylbenzene (18% after 4 h, 52% after
17 h) according to Scheme 1.8 In benzene, the bound SG-base
and SG-cat do not leach (and indeed the benzene filtrates were
not reactive8) so that both reactions 1 and 2 are confirmed to be
heterogeneous, and to take place simultaneously without interfer-
ing with each other. The catalyst was completely deactiVated
(either in its homogeneous or immobilized form) in the presence
of 2 equiV of the free diamines H2N(CH2)2NH2 or H2N(CH2)2NH-
(CH2)3Si(OCH3)3.9
The rate determining step is the dehydroiodination, as is evident
from the fact that styrene is, during the entire process, below the
detection limit. Blank experiments confirmed that â-iodoethyl-
benzene is not converted to ethylbenzene by the catalyst in the
absence of the base, but that the catalyst is capable of hydrogenat-
ing styrene.9
Finally, â-bromoethylbenzene could be used similarly in
Scheme 1, however, along with competing polymerization of
styrene. Polystyrene formation in this case may be attributed to
the lower rate of dehydrobromination, compared to the dehy-
(5) Sertchook, H.; Avnir, D.; Blum, J.; Joo, F.; Kortho, A.; Schumann, H.;
Weimann, R.; Wernik, S. J. Mol. Catal. A: Chem. 1996, 108, 153. Rosenfeld,
A.; Blum, J.; Arnir, D. 1996, 164, 363.
(6) Hu¨sing, M.; Schubert, U.; Mezei, R.; Fratzl, P.; Riegel, B.; Kiefer, W.;
Kohler, D.; Mader, W. Chem. Mater. 1999, 11, 451.
(7) A mixture of 3.0 mL of MeOH, 1.4 mL of water, and 1.0 mL of
tetramethoxyorthosilicate (TMOS) was stirred at room temperature for 10 min.
To this solution was added 2.0 mL of H2N(CH2)2NH(CH2)3Si(OCH3)3, which
caused rapid gelling. The gel was dried at 1 mm for 24 h at room temperature,
sonicated twice in benzene, and finally dried at 1 mm for 3 h, to give 2.3 g
of the sol-gel material.
(8) A miniautoclave was charged with â-iodoethylbenzene (1.6 mmol), the
sol-gel base (3 mmol, 1.15 g of the material), the sol-gel encapsulated RhCl-
We have decided to demonstrate this concept in the field of
organometallic catalysis because of the known sensitivity of these
compounds to be poisoned by various chemical species. The
complex RhCl[P(C6H5)3]3 (Wilkinson catalyst), which is a com-
mon alkene-hydrogenation catalyst,4 was taken for this purpose,
5
[P(C6H5)3]3 (0.018 mmol), and 15 mL of dry benzene. The reaction vessel
was purged with hydrogen and the mixture was stirred magnetically at 80 °C
under 200 psi of H2. The solid material was filtered off and the products and
starting material in the filtrate were analyzed by GC, 1H NMR, and MS. The
filtrate was also checked for its content of leached base and metallic
compounds by elemental analysis and atomic absorption spectroscopy
(detection limit of less than 1 ppm), and by its ability to dehydrohalogenate
fresh samples of haloethylbenzene or to catalyze hydrogenation of styrene.
The result of this blank test was negative.
(1) Some recent examples: (a) Fennough, S.; Guyon, S.; Jourdat, C.;
Livage, J.; Roux, C. C. R. Acad. Sci. Paris, t.2. Ser. II 1999, 625. (b) Lan, E.
H.; Dave, B. C.; Fukuto, J. M.; Dunn, B.; Zink, J. I.; Valentine, J. S. J. Mater.
Chem. 1999, 9, 45. (c) Kanti Das, T.; Khan I.; Rousseau, D. L.; Friedman, J.
M. J. Am. Chem. Soc. 1998, 120, 10268. (d) Chen, Q.; Kenausis, G. L.; Heller,
A. J. Am. Chem. Soc. 1998, 120, 4582. (e) Rao, M. S.; Dave, B. C. J. Am.
Chem. Soc. 1998, 120, 13270. (f) Rottman, C.; Grader, G.; De Hazan, Y.;
Melchior, S.; Avnir, D. J. Am. Chem. Soc. 1999, 121, 8533. (g) Blum, J.;
Avnir, D.; Schumann, H. Chemtech 1999, 29, 32.
(2) (a) Rabinovich, L.; Lev, O.; Tsirlina, G. A. J. Electroanal. Chem. 1999,
466, 45. (b) Gill, I.; Pastor, E.; Ballesteros, A. J. Am. Chem. Soc. 1999, 121,
9487. (c) Slama-Schwok, A.; Ottolenghi, M.; Avnir, D. Nature 1992, 355,
240.
(3) Ayadim, M.; Habib Jiwan, J. L.; Soumillion, J. Ph. J. Am. Chem. Soc.
1999, 121, 10436.
(9) While a benzene solution of 1.6 mmol of styrene is reduced to
ethylbenzene in quantitative yield by heating the mixture at 80 °C for 2.5 h
under 200 psi of H2 with 0.018 mmol of sol-gel encapsulated RhCl[P(C6H5)3]3,
in the presence of 4.5 mmol of either free H2N(CH2)2NH(CH2)3Si(OCH3)3 or
of free H2N(CH2)2NH2, no reaction took place.
(10) Triton B was entrapped by first stirring a mixture of 2.0 mL of TMOS,
1.0 mL of water, and 2.0 mL of MeOH for 15 min at room temperature. To
this solution was added 4.0 mL of 40% Triton B in MeOH. The solution
gelled rapidly, and the gel was dried at 1 mm for 24 h, sonicated twice in
benzene, and dried at 1 mm for 3 h. A reaction mixture of 1.6 mmol of
â-bromoethylbenzene, sol-gel containing 4.5 mmol of entrapped Triton B,
and 0.018 mmol of sol-gel encapsulated RhCl[P(C6H5)3]3 in benzene yielded
after 17 h under 150 psi of H2 at 60 °C a mixture of 18% of ethylbenzene,
29% of â-phenylethanol, and 52% of unreacted starting material. The filtrate
was checked for possible ability to dehydrohalogenate the bromoethylben-
zene: No reaction was detected in this blank test.
(4) E.g., see: Burgess, K.; Van der Donk, W. A. In Encyclopedia of
Reagents for Organic Synthesis; Paquette, L. A.. Ed.; Wiley: Chichester, 1995;
Vol. 2, pp 1253-1261.
10.1021/ja003029b CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/15/2000

12000 J. Am. Chem. Soc., Vol. 122, No. 48, 2000
Communications to the Editor
droiodination (e.g., 6% vs 18% after 4 h). Typical reaction yield
was therefore only 12% after 6.5 h. Better yields were obtained
by replacing the covalently bound diamine with physically
entrapped Triton B ([C6H5CH2N(CH3)3]+OH-) which does not
leach in the benzene slurry and which also proved to be friendly
to the catalyst in this form.10 With this base, nucleophilic
substitution with OH- took place along with the dehydrobromi-
nation, yielding ethylbenzene and â-phenylethanol at a ratio of
3:2.10
In conclusion we comment that the methodology of changing
“chemical hostility” into “chemical friendliness” and compat-
ibility, which was demonstrated here on an idea-feasibility level,
is general. Other examples are indeed in preparation.
Acknowledgment. We gratefully acknowledge support from the Israel
Science Foundation and from the Infrastructure (Tashtiot) Project of the
Israel Ministry for Science, Arts and Sports.
JA003029B