Kinetic Resolution When the Chiral Auxiliary Is Not Enantiomerically Pure:  Normal and Abnormal Behavior

Abstract

Kinetic resolutions with enantioimpure chiral auxiliaries (reagents or catalysts) are considered, and a kinetic treatment for various rate laws is described. A useful parameter, the apparent stereoselectivity factor, is defined and correlated to the enantiomeric excess of the chiral auxiliary. Deviations from the regular laws are possible (asymmetric amplification or depletion). These anomalies have their origins in the same phenomena that cause nonlinear effects in enantioselective catalysis. Asymmetric amplifications have been experimentally observed.

Kinetic Resolution of Chiral Secondary Alcohols by Dehydrogenative Coupling with Recyclable Silicon‐Stereogenic Silanes

“…The optical purity of (R)-6 (25 % ee) as well as of ( Si R)-1 (48 % ee) increased as the reaction proceeded until 50 % conversion was reached. [1,20] An investigation of the substrate scope demonstrated some generality for the class of 2-pyridyl-substituted secondary alcohols (Table 2, entries 2-7). Replacement of a phenyl group (rac-6) by 1-naphthyl (rac-10) only had a marginal [h] dppm = 1,1-bis(diphenylphosphanyl)methane, dppe = 1,2-bis(diphenylphosphanyl)ethane, dppp = 1,3-bis(diphenylphosphanyl)propane, dppb = 1,4-bis(diphenylphosphanyl)butane.…”

Mononuclear diastereopure non-heme Fe(II) complexes of pentadentate ligands with pyrrolidinyl moieties: Structural studies, and alkene and sulfide oxidation

“…The ee values of the recovered sulfoxides show kinetic resolution to some extent. The oxidation of S-sulfoxide (k S > k R ) occurs predominantly with stereoselective factors (s) [47] of 4.8 without an additive, 2.03 for acetic acid and 7.2 for p-anisic acid. The ee of the recovered sulfoxides remained constant in time; only in the presence of p-anisic acid was a slight decrease observed.…”

Asymmetric Amplification by Kinetic Resolution Using A Racemic Reagent: Example in Amine Acetylation

“…[17] Usually the reagent or the catalyst are fully resolved. The use of enantio-impure reagents [18,19] or catalysts [20][21][22][23] in kinetic resolution has been discussed. The key factor for a high ee for the recovered starting material (ee sm ) is the size of the stereoselectivity factors.…”

“…The special case of a partially resolved compound versus a racemic reagent has also been treated, clearly showing an increase of ee sm with conversion, with a correlation to the size of s. The influence of the initial ee of a reagent on the kinetic resolution of a racemic mixture was briefly revisited by us in 1999. [20] Plotted in Figure 1 are some curves computed as in reference [20] with s = 30 (initial ee substrate = 10 %) and two substrate/reagent initial ratios. The general trends are seen on these curves: the ee of the substrate is amplified by the racemic reagent, simultaneously the racemic reagent is progressively resolved by the substrate.…”

“…The results of Tables 1 and 2 confirm that a racemic reagent can easily enhance the ee of its partner if the stereoselectivity factor is not too small. [20], for reactions that are first order with respect to substrate and reagent.…”