Vaulted Biaryls: Efficient Ligands for the Aluminum-Catalyzed Asymmetric Baeyer-Villiger Reaction

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

References

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• 14 To the best of our knowledge there is only a single report with a higher ee. Katsuki et al. found a zirconium catalyst bearing a complex salen ligand to give 87% ee in the conversion of 4 to 5. See: Watanabe A. Uchida T. Ito K. Katsuki T. Tetrahedron Lett.  2002,  43:  4481 

10

After 72 h at -40 °C the reaction was quenched to give the lactone with 84% ee in 40% yield.

11

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).

12

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.