Bromofluoroketene silyl acetals

The uncatalyzed aldol reaction of highly reactive enoxysilanes such as enoxysilacyclobutanes has previoudy been reported to proceed through six-membeied cyclic transition states (32,55), and the torsional structure of the difluoroketene acetal 5 is considered to be more suitable for the uncatalyzed aldol reaction tfian the planar geometry of 4 because the acetal 5 has a geometrical similarity to the ketene acetal in die cyclic transition states. As mentioned above (Scheme 1), a 60 40 mixture of the syn- and anri-aldols was obtained at 40°C from the bromofluoroketene silyl acetal 2 (E/Z =62/38), suggesting that the uncatalyzed aldol reaction of the fluorine-substituted ketene acetals 1 and 2 proceeds preferentially through boat-like cyclic transition states. Denmark et aL proposed diat the boat-like transition states are extremely predominant in the uncatalyzed aldol reaction of enoxysilacyclobutanes and trichlorosilyl enolates (55,54). 

Asymmetric Aldol Reaction of Bromofluoroketene Silyl Acetal 2 Catalyzed by Lewis Acids 6. We next examined the aldol reaction of the bromofluoroketene silyl acetal 2 mediated by the catalyst 6 (30). The reaction was carried out by the addition of an aldehyde in nitroethane to a solution of 1.2 equivalents of the acetal 2 and 20 mol% of the catalyst 6 in the same solvent over 3 h at -7S°C and stirring at that temperature for an additional hour prior to quenching. As shown in Table II, the reaction of benzaldehyde afforded a 69 31 mixture of (2S,31 )- and (2R,3/ )-2-bromo 2-fluoro-3-hydroxy-3-phenylpropanoates. The enantiomeric excess of fte syn-isomer is 98% ee and diat of die an/i-isomer is 90% ee (entry 1). Although die reactions are not diastereoselective in all cases (synlanti = 69/31 to 46/54), aU syn- and anri-aldol products were obtained with excellent-to good chemical and optical yields. 

Effects of Reaction Temperature on the Stereoselection during the Aldol Reaction with Bromofluoroketene Silyl Acetal 2. The enantioface selection for aldehydes during the aldol reaction of bromofluoroketene silyl acetal 2 using the... 

Figure 1 shows the NMR of a 1 1 mixture of the difluoroketene acetal 1 and the catalyst 6 in nitroethane-ds at -78 and -20 C (Iseki, K. Kuroki, Y. Kobayashi, Y., MEC Laboratory, Daikin Indushies, Ltd., unpublished data.). The appearance of new peaks at the higher temperature may suggest the formation of a boron enolate. A siniilar change was observ in the NMR of a 1 1 mixture of the bromofluoroketene acetal 2 and 6 (31). On the contrary, the NMR of a 1 1 mixture of c-labeled dimethylketene silyl acetal 3 in nitroethane-ds at -20 C provided almost the same spectrum as that at -78°C. 

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