Resolution cation stabilization

The use of ILs contributes many advantages in biocatalysis, including enhanced stability, activity, and stereoselectivity [41, 44, 45], When combined with SC-CO2, ILs showed a promising media for enhanced stereoselectivity and stabiUty of biocatalysts [9, 46-48]. The use of SC-CO2 and room temperature ILs as a combined bioreaction media has been widely demonstrated [5, 7, 8]. There has been much interest in research on an efficient enzymatic system in IL/CO2 media for race-mates kinetic resolution [49, 50]. Due to the wide spectmm of their physicochemical properties, several ILs, based on the N, N -dialkyUmidazoUum cations were used for testing the enantioselectivity of CALB [34—36]. 

The cationic complexes 24 and 25 were of low optical stability and racemization was observed after 6 hr. However, recent dynamic NMR studies have permitted the observation of chiral pentacoordinated species without resolution. The compounds are configurationally stable on the NMR time scale. The racemization of such species opens up the problem of permutational isomerization, which will be discussed in detail in Sect. V-C. 

This low resolution overview of the gas-phase stabilities of a variety of carbenium ions shows that they are by no means exotic data unrelated to solution reactivity. Quite on the contrary, they are the fundamental factor determining the relative stabilities of the same species in solution (as determined by the pA R and pAia scales, for example). Furthermore, the ranking of solvolysis rates of compounds R-X is largely dependent on the intrinsic stability of R" ". While this was already known in the case of bridgehead carbocations, we see now that this applies also to nonbridgehead tertiary cations as well as to a variety of secondary cations, even sometimes in cases wherein the access of solvent to the reaction center is not hindered. Intrinsic stability of carbocations thus appears as a very powerful unifying factor. 

---