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DOI: 10.1055/s-2004-829080
Vaulted Biaryls: Efficient Ligands for the Aluminum-Catalyzed Asymmetric Baeyer-Villiger Reaction
Publication History
Publication Date:
29 June 2004 (online)
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Abstract
Vaulted biaryls (VANOL and VAPOL) have been applied in the aluminum-catalyzed asymmetric Baeyer-Villiger reaction of prochiral 3-substituted cyclobutanones. Optically active γ-butyrolactones are obtained in high yields with enantioselectivities of up to 84% ee.
Key words
aluminium - asymmetric catalysis - Baeyer-Villiger reaction - biaryls - lactones
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References
After 72 h at -40 °C the reaction was quenched to give the lactone with 84% ee in 40% yield.
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Experimental Procedure for the Al-Catalyzed Asymmetric Baeyer-Villiger Reaction.
To a solution of (R)-VANOL (26.4 mg, 0.06 mmol) in toluene (2.5 mL) was added dropwise Me2AlCl (0.06 mmol, 60 µL, 1 M solution in hexanes) at r.t. The mixture was stirred for 30 min and then cooled at -30 °C. After 15 min at this temperature, a solution of the ketone (0.3 mmol) in toluene (0.5 mL) was added. Stirring was continued for 15 min, cumene hydroperoxide (technical grade 80%, 1.2 equiv, 75 µL) was then added in one portion. Subsequently, the reaction mixture was stirred at -30 °C until full conversion of the ketone (as monitored by GC). The solvent was then evaporated, and the crude product absorbed onto silica gel. Extraction with Et2O afforded the crude product, which was analyzed by GC or HPLC using chiral columns (for details of the enantiomeric ratio determinations and assignments of absolute configurations, see ref. 4 and references therein).
Slightly better results can be obtained for benzyl-substituted substrates when 50 mol% of the BINOL/Me2AlCl catalyst system are applied (comp. ref. 4a). Thus, conversions of 6c and 6d give lactones with 73% and 58% ee, respectively.