Research directed toward the design, synthesis, and screening of binaphthyl ketones for catalytic asymmetric epoxidation of alkenes is reported. To date, an easily accessible and recyclable catalyst that tolerates a wide structural variety of unfunctionalized alkenes has not been developed.
It has been discovered that fluorinated chiral binaphthyl ketones catalyze the asymmetric epoxidation of alkenes with a high degree of enantiocontrol. A series of five binaphthyl ketone catalysts with variable distribution of fluorine atoms alpha to the carbonyl were synthesized. These catalysts were screened in the asymmetric epoxidation of trans-beta-methyl styrene. A trend of epoxidation efficiency as a function of alpha fluorination was revealed. Although, several of these catalysts performed well in the screening process, the most accessible was an alpha-fluorinated binaphthyl ketone. This ketone epoxidized trans-beta-methylstyrene to trans-beta-methylstyrene oxide with 57% conversion in 80% enantioselectivity.
In order to improve conversion and enantioselectivity with this catalyst, a study of the various reaction parameters such as pH, organic cosolvent, and the amount of primary oxidant used was undertaken. During this study a significant dependence on these parameters was revealed, particularly, that of the organic cosolvent. Optimization of these conditions has led to dramatic improvements in the epoxidation efficiency of alpha-fluorinated binaphthyl ketone. This efficiency is highlighted in the epoxidation of trans -beta-methylstyrene to trans-beta-methylstyrene oxide with 100% conversion and 94% ee (from 57% conversion and 80% ee). Moreover, a substrate study suggests the optimized solvent allows for a pi-stack interaction which contributes to high enantioselectivity and conversions for both 1,1-disubstituted and terminal alkenes.