Aluminum trisphenoxide polymer as a Lewis acidic, solid catalyst

Article abstract of DOI:10.1055/s-1999-2523

A newly introduced, solid polymer of an aluminum trisphenoxide has been demonstrated an efficient catalyst for promoting the Diels-Alder reaction of α,β-enals.

Full text of DOI:10.1055/s-1999-2523

LETTER
57
Aluminum Trisphenoxide Polymer as a Lewis Acidic, Solid Catalyst
Susumu Saito, Masaaki Murase, Hisashi Yamamoto*
Graduate School of Engineering, Nagoya University, CREST, Japan Science and Technology Corporation (JST), Furo-cho, Chikusa,
Nagoya 464-8603, Japan
Fax: +81 (52) 7893222; E-mail: j45988a@nucc.cc.nagoya-u.ac.jp
Received 17 September 1998
Abstract: A newly introduced, solid polymer of an aluminum
trisphenoxide has been demonstrated an efficient catalyst for pro-
moting the Diels–Alder reaction of a,b-enals.
Key words: aluminum trisphenoxide, solid catalyst, Lewis acidic
polymer, Diels–Alder reaction, ATPH
Homogeneous, Lewis acidic aluminum aryloxides have
shown stereochemical advantages in important carbon-
carbon bond-forming reactions; these transformations fre-
quently require at least a stoichiometric amount of the re-
agents.1 A catalytic application of these species is thus
limited to several cases especially where a bidentate
ligand is incorporated into the metal.2 However, turnover
frequencies observed are not always high. An entirely dif-
ferent strategy that enables a catalytic use of aluminum
aryloxides has yet to be explored. We report here a hither-
to unknown aluminum trisphenoxide solid polymer as an
efficient Lewis acid catalyst, a function of which is high-
lighted by the catalytic Diels–Alder reaction.
Ligand 1 for the aluminum polymer was readily accessi-
ble on a large scale as described in the literature.3 The alu-
minum polymer 2 was prepared as outlined in Scheme 1:
treatment of biphenol 1 (0.15 mmol) in toluene with a
hexane solution of Me3Al (0.10 mmol) at room tempera-
ture while maintaining a gentle stream of argon, followed
by exposure to ultrasonic irradiation4 for 15~20 min dur-
ing which time the reaction vessel was immersed in a wa-
ter bath to avoid a violent reflux. Insoluble pale yellow
precipitates appeared immediately following the treat-
ment with Me3Al, and after the prolonged irradiation time,
the color changed to a pale yellow-green.5 The mixture
was filtered and the filtrate was quenched with 1M HCl to
Scheme 1
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58
S. Saito et al.
LETTER
check the remainder in the organic layer, giving less than In summary, we developed an aluminum trisphenoxide
3% of 1. Furthermore, when the obtained solid portion polymer as an efficient solid catalyst which showed pro-
was exposed to methanol, we observed a negligible nounced Lewis acidity in the Diels–Alder reaction of a,b-
amount of gas evolution. An ultrasonic treatment is thus enals. An alternative strategy that further enhances the
assumed to be indispensable for complete consumption of catalytic activity of aluminum aryloxides is now under in-
1 on reaction with Me3Al. Under otherwise identical reac- vestigation.
tion and technical conditions, a significant amount of
starting biphenol 1 was recovered from the filtrate (20%).
References and Notes
We next investigated the reactivity profile of 2 in the
Diels–Alder reaction of a,b-enals, following the above
preparation procedure for 2 with exclusion of the filtration
(1) (a) Maruoka, K.; Ooi, T.; Yamamoto, H. J. Am. Chem. Soc.
1989, 111, 6431. (b) Maruoka, K.; Saito, S.; Yamamoto, H. J.
Am. Chem. Soc. 1992, 114, 1089. (c) Maruoka, K.; Saito, S.;
step. Table 1 shows several unique characteristics of this
catalytic process, and there are several key issues to be
discussed. (1) In general, a 1~2 mol% of the catalyst
proved sufficient to affect the Diels–Alder reactions ex-
amined. The enhanced catalytic activity was also con-
firmed by a reasonable yield obtained with 0.1 mol% of
the catalyst (entry 2). Further evidence that a rate en-
hancement was apparently affected by 2 was provided by
the following studies: for comparison, the corresponding
monomeric, aluminum tris(2,6-diphenylphenoxide)
(ATPH)6 (1 mol%) was subjected to the Diels–Alder reac-
tion of 5 with 6 under similar conditions except a pro-
longed reaction time (0 °C, 22h) to give the cycloadduct
with a decline in isolated yield of 78%. (2) A rather low,
but nonnegligible level of exo (entries 1, 2, and 5) and
endo (entry 4) preferences can be ascribed to the ligand ef-
fect of the catalyst. In fact, the Diels–Alder reaction of 3
with 6 using ATPH (-78 °C, toluene) gives an endo/exo
ratio of 85:15.7 These comparable selectivities are sugges-
tive of a structural similarity between 2 and ATPH. (3)
Several Diels–Alder adducts underwent Tishchenko
reaction8 affording self-dimerization to give esters 9, 10
and 11 (entries 3, 6, and 8, respectively). These results
also support an increase in the catalytic activity.
Concepcion, A. B.; Yamamoto, H. J. Am. Chem. Soc. 1993,
115, 1183. (d) Maruoka, K.; Imoto, H.; Saito, S.; Yamamoto,
H. J. Am. Chem. Soc. 1994, 116, 4131. (e) Maruoka, K.; Saito,
S.; Yamamoto, H. J. Am. Chem. Soc. 1995, 117, 1165. (f)
Saito, S.; Ito, M.; Yamamoto, H. J. Am. Chem. Soc. 1997, 119,
611. (g) Saito, S.; Shiozawa, M.; Ito, M.; Yamamoto, H. J.
Am. Chem. Soc. 1998, 120, 813.
(2) (a) Maruoka, K.; Concepcion, A. B.; Yamamoto, H. Bull.
Chem. Soc. Jpn. 1992, 65, 3501. (b) Bao, J.; Wulff, W. D.;
Rheingold, A. L. J. Am. Chem. Soc. 1993, 115, 3814. (c) Hu,
Q.-S.; Zheng, X.-F.; Pu, L. J. Org. Chem. 1996, 61, 5200. (d)
Heller, D. P.; Goldberg, D. R.; Wulff, W. D. J. Am. Chem.
Soc. 1997, 119, 10551. There are few examples of catalysis
promoted by homogeneous aluminum reagents attached with
ligands of monodentate phenoxides activated by the bromo
group, see: (e) Ooi, T.; Maruoka, K.; Yamamoto, H. Org.
Synth. 1995, 72, 95. (f) Saito, S.; Shimada, K.; Yamamoto, H.
Synlett 1996, 720.
(3) (a) Hay, A. S. J. Org. Chem. 1971, 36, 218. (b) Weyland, H.
G.; Hoefs, C. A. M.; Yntema, K.; Mijs, W. J. Euro. Polym. J.
1970, 6, 1339.
(4) The internal irradiation system, ultrasonic disrupter UD-201
(TOMY TECH) equipped with a converter, horn connector,
and TP-040 (TOMY TECH) was used, and an output power of
150 W and 20 kHz applied for all the reactions tested. For the
review of ultrasonic irradiation in organic synthesis, see:
Einhorn, C.; Einhorn, J.; Luche, J.-L. Synthesis 1989, 787.
(5) The color of the precipitate is crucial and should be controlled
in order to retain and reproduce the catalytic activity of 2. For
example, the pale yellow color persists with insufficient
irradiation (<10 min) to cause a catalyst with a relatively weak
activating capability, whereas the pale yellow-green color
turned red with excess irradiation (>30 min), and the catalytic
activity decreased significantly, probably due to the decompo-
sition of highly active species. It is recommended that the
catalyst is prepared just before use, or stored at low tempera-
ture (<-40 °C) under argon atmosphere.
The polymer catalyst 2 could be recovered quantitatively
(>95%) by simple filtration and reused. The activity of the
recovered 2 did not decrease even after seven uses
(Scheme 2).
(6) Saito, S.; Yamamoto, H. Chem. Commun. 1997, 1585.
(7) The exo-selective Diels–Alder reaction of a,b-unsaturated ke-
tones using ATPH, see: Maruoka, K.; Imoto, H.; Yamamoto,
H. J. Am. Chem. Soc. 1994, 116, 12115.
(8) For Tishchenko reaction using aluminum alkoxides. (a)
Ogata, Y.; Kawasaki, A. Tetrahedron 1969, 25, 929. (b)
Saegusa, T.; Hirota, K.; Hirasawa, E.; Fujii, H. Bull. Chem.
Soc. Jpn. 1967, 40, 967. (c) Saegusa, T.; Kitagawa, S.;
Ueshima, T. Bull. Chem. Soc. Jpn. 1967, 40, 1960. (d)
Saegusa, T.; Ueshima, T.; Kauchi, K.; Kitagawa, S. J. Org.
Chem. 1968, 33, 3657.
Scheme 2
Synlett 1999, No. 1, 57–58 ISSN 0936-5214 © Thieme Stuttgart · New York