[118724-91-7]  · C57H43AlO2Si2  · (R)-3,3-Bis(triphenylsilyl)binaphthomethylaluminum  · (MW 843.16)

(chiral Lewis acid for asymmetric hetero-Diels-Alder,2,3 Diels-Alder,6 and ene reactions,7 Claisen rearrangement,8,9 and polymerization10)

Solubility: sol toluene and CH2Cl2; slightly sol hexane.

Form Supplied in: prepared and used in situ.

Preparative Methods: can be prepared by treatment of (R)-3,3-bis(triphenylsilyl)binaphthol1 in CH2Cl2 or toluene with a 1 M hexane solution of Trimethylaluminum room temperature for 1-2 h.2 This reagent can be also generated in situ by discrimination of its racemates with optically active (-)-bromocamphor.3

Handling, Storage, and Precautions: the dry solid and solutions are highly flammable and must be handled in the absence of oxygen and moisture. The solution should be used as prepared for best results. Use in a fume hood.

Asymmetric Hetero-Diels-Alder Reaction.

This chiral organoaluminum reagent has been developed for effecting the hetero-Diels-Alder reaction of aldehydes and siloxydienes with high enantioselectivity.2,4 Thus reaction of benzaldehyde with 1-methoxy-2-methyl-3-trimethylsiloxy-1,3-pentadiene under the influence of 10 mol % of the chiral aluminum reagent affords cis-dihydropyrone as a major product in 95% ee (eq 1). The enantioselectivity is highly dependent on the bulk of the triarylsilyl moiety of the aluminum reagent. Thus replacement of the triphenylsilyl group by the tris(3,5-xylyl)silyl group enhances the enantio as well as cis selectivity, but replacement by trimethylsilyl groups lowers the enantioselectivity. This chiral aluminum reagent is generated in situ by discrimination of racemic organoaluminum reagents with (-)-bromocamphor by diastereoselective complexation, and utilized as a chiral Lewis acid for the asymmetric hetero-Diels-Alder reaction.3

Asymmetric Diels-Alder Reaction.

Although the asymmetric Diels-Alder reaction of cyclopentadiene with methacrolein is quite disappointing (17-23% ee) with the chiral aluminum reagent,5 the use of a,b-unsaturated esters as dienophiles gives good enantioselectivity (eq 2).6

Asymmetric Ene Reaction.

The enantioselective activation of carbonyl groups with the chiral aluminum reagent also enabled the asymmetric ene reaction of electron-deficient aldehydes with various alkenes (eq 3).7 In the presence of powdered 4 &AAring; molecular sieves, the chiral aluminum reagent can be utilized as a catalyst without loss of enantioselectivity.

Asymmetric Claisen Rearrangement.

The enantioselective activation of an ether oxygen with the chiral organoaluminum reagent allows for the first example of the asymmetric Claisen rearrangement of allylic vinyl ethers.8 This method provides a facile asymmetric synthesis of various acylsilanes and acylgermanes from allylic a-(trimethylsilyl)vinyl ethers and allylic a-(trimethylgermyl)vinyl ethers, respectively (eq 4). Among various trialkylsilyl substituents of the chiral aluminum reagent, use of the bulkier t-butyldiphenylsilyl group results in the highest enantioselectivity.

Notably, the asymmetric Claisen rearrangement of cis-allylic a-(trimethylsilyl)vinyl ethers with the chiral aluminum reagent produced optically active acylsilanes with the same absolute configuration as those from trans-allylic a-(trimethylsilyl)vinyl ethers (eq 5).9

Asymmetric Polymerization.

The chiral organoaluminum catalyst is utilized for asymmetric polymerization of racemic a-methyl and b-methyl b-lactones.10 Optically active polymers possessing negative optical rotation values are produced, suggesting that (S) enantiomers of racemic b-lactones are preferentially activated by the aluminum catalyst.

1. Maruoka, K.; Itoh, T.; Araki, Y.; Shirasaka, T.; Yamamoto, H. BCJ 1988, 61, 2975.
2. Maruoka, K.; Itoh, T.; Shirasaka, T.; Yamamoto, H. JACS 1988, 110, 310.
3. Maruoka, K.; Yamamoto, H. JACS 1989, 111, 789.
4. Burns, C.; Sharpless, K. B. Chemtracts: Org. Chem. 1988, 1, 123.
5. Bao, J.; Wulff, W. D. JACS 1993, 115, 3814.
6. Maruoka, K.; Concepcion, A. B.; Yamamoto, H. BCJ 1992, 65, 3501.
7. Maruoka, K.; Hoshino, Y.; Shirasaka, T.; Yamamoto, H. TL 1988, 29, 3967.
8. (a) Maruoka, K.; Banno, H.; Yamamoto, H. JACS 1990, 112, 7791. (b) Maruoka, K.; Banno, H.; Yamamoto, H. TA 1991, 2, 647.
9. Maruoka, K.; Yamamoto, H. SL 1991, 793.
10. Sato, R.; Miyaki, N.; Takeishu, M. Polymer Reprints, Jpn. 1992, 41, 331.

Keiji Maruoka & Hisashi Yamamoto

Nagoya University, Japan

Copyright 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.