Development of enzyme -based screening methodology for combinatorial catalysis - Identification of novel chiral salens for the hydrolytic kinetic resolution of epoxides

Abstract

This dissertation describes the development of a sensitive, information-rich, parallel screening method for combinatorial catalysis, in which enzymes serve as catalytic reporting agents for an organic reaction of interest. The organic reaction (organic phase) and the enzymatic reporting reaction (aqueous phase) are run in parallel. Hence, this method is termed ISES (In Situ Enzymatic Screening). The variants of ISES developed here are the first to predict both sense and magnitude of enantioselectivity (“double-cuvette ISES”) and substrate specificity (“cassette-ISES”). The principal reaction under study in this dissertation is the Co(III)-salen mediated hydrolytic kinetic resolution (HKR) of epoxides. Two model epoxide substrates are screened, (±)-propylene oxide and (±)-hexene oxide. In the former case, alcohol dehydrogenases from horse liver (HLADH) and from Thermoanaerobium brockii (TBADH), serve as reporting enzymes. HLADH is selective for the (S)-antipode of the 1,2-propanediol HKR product, whereas TBADH prefers the (R)-antipode. For the latter case, Lactobacillus kefir (LK) ADH shows a large enantiopreference for the (S)-antipode (v_S/v _R ≈ 20), while HLADH displays a modest preference for the same antipode (v_S/v_R ≈ 2). This set of reporting enzymes gives rise to a “cassette” ISES screen, allowing for the simultaneous monitoring of the HKR of both (±)-propylene oxide and (±)-hexene oxide, for each catalyst in a combinatorial array. The enantiomeric excesses predicted by the “cassette” ISES show good correlations with those measured by chiral HPLC. Selected catalysts from “cassette”-ISES showing promise in the HKR of both test substrates, were evaluated more extensively, across a battery of structurally and functionally diverse terminal epoxides.

The screen identified new synergistic combinations of both D-fructopyranose- and β-pinene-derived chiral 1,2-diamines and sterically unencumbered “salicylaldehydes.” These salens yield Co(III)-complexes exhibiting notable enantioselectivity, though lacking C₂-symmetry, and the steric constraints (i.e. 3’,5’-di-tert-butyl substitution in the salicylaldehyde sector) usually built into such catalysts. An unusual salicylaldehyde-dependent “enantioswitch” was also observed for the β-D-fructopyranose-derived diamine. That is, when paired with 3’,5’-di-tert-butylsalicylaldehyde, this diamine yields a modestly (R)-selective Co(III)-salen catalyst (E _(R:S) ∼ 5 for propylene oxide). However, the same diamine, when paired with 3’,5’-diiodosalicylaldehyde, gives a highly (S)-selective catalyst (E_(S:R) ∼ 11-13). To unambiguously rule out anomerization as the underlying basis of this “enantioswitch,” the carbacyclic analogue of the β-D-fructopyranose-derived 1,2-diamine was synthesized. Not only was the “enantioswitch” retained, but the catalyst prepared from the β-D-carbafructopyranose-derived diamine and 3’,5’-diiodosalicylaldehyde showed improved enantioselectivity across a range of structurally diverse substrates.