Kinetic Resolution of 2-Cyclohexenones

The nature of the R Zn reagents has a profound influence on the selectivity in this process (Tab. 7.3). Contrary to expectations, the use of the bulkier i-Pr2Zn reagent in place of Et2Zn results in a lower selectivity, but with n-Bu2Zn the selectivity increases, providing unconverted 53 with an ee of 99% at 52-54% conversion (Fig. 7.9). High trans diastereoselectivity had previously been observed for 

In an ideal kinetic resolution (common in enzyme-catalyzed processes), one enantiomer of a racemic substrate is converted tvhile the other is unreactive [70]. In such a kinetic resolution of 5-methyl-2-cyclohexenone, even with 1 equivalent of Me2Zn, the reaction should virtually stop after 50% conversion. This near perfect situation is found with ligand 18 (Fig. 7.10) [71]. Kinetic resolutions of 4-methyl-2-cyclohexenone proceed less selectively (s = 10-27), as might be expected from the lower trans selectivity in 1,4-additions to 4-substituted 2-cyclohexenones [69]. 

---