OsO₄⋅Streptavidin: A Tunable Hybrid Catalyst for the Enantioselective cis‐Dihydroxylation of Olefins

Abstract

Enzymatic and homogeneous catalysis have evolved independently to address the challenges in the synthesis of enantiopure products. With the aim of complementing these fields, artificial metalloenzymes, which combine the structural diversity of biocatalysts with the wealth of metal-catalyzed reactions, have attracted increasing attention.[1] In homogeneous catalysis the cis-selective, OsO 4 -dependent asymmetric dihydroxylation (AD) of olefins ranks among the most powerful methods for the synthesis of vicinal diols. Ligands for homogeneous catalysis have been largely developed by Sharpless and co-workers, and are, with few exceptions, almost exclusively based on quinidine or quinine derivatives.[2] Although most classes of prochiral olefins are dihydroxylated with good activity and selectivity, the cissubstituted olefins are problematic. Nature relies on nonheme iron dioxygenases such as naphthalene dioxygenase (NDO) to perform a related reaction. These enzymes display broad substrate scope.[3] It is believed that both the OsO 4 -and NDO-catalyzed dihydroxylations proceed by an outer sphere [3+2] mechanism in which the substrate is not bound to the metal in the transition state (Figure 1). [2a,f, 3c] Considering a biomimetic approach, we hypothesized that anchoring a catalytically competent Os VIII center within a protein might afford an artificial metalloenzyme for the AD of olefins. Encouraged by a report by Kokubo et al., [2d] we set out to screen various proteins and to test whether the resulting dihydroxylases could be optimized by genetic means.Five proteins were evaluated as hosts for the AD of amethylstyrene: Wild-type streptavidin (SAV) clearly performed best. In contrast, bovine serum albumin (BSA) yielded the opposite enantiomer, albeit with a low turnover number (Table 1). In view of the size of the proteins (66 kDa for BSA and 16 kDa for the SAV monomer), the difficult recombinant production of BSA, [4] and the number of Figure 1. Postulated transition-state structure for the dihydroxylation of olefins: a) for the naphthalene dioxygenase; b) for the osmium-catalyzed AD of prochiral olefins; and c) for an artificial cis-dihydroxylase resulting from anchoring of OsO 4 within a host protein. TON [c]

Protein Design: Toward Functional Metalloenzymes

“…By anchoring an Os(VIII) center directly within Sav, Ward and coworkers were able to develop a catalyst specific for this substrate type (Figure 20). 561 While crystal structures of the metalloprotein did not clearly reveal the specific binding site for Os(VIII), mutations to the biotin-binding site resulted in diminished selectivity. Further mutations were used to fine-tune the protein for dihydroxylation of seven different olefins, resulting in the highest ee values ever reported for allyl phenyl sulfide and cis - β -methylstyrene.…”

Coordination complexes and biomolecules: A wise wedding for catalysis upgrade

“…A very simple strategy to form hybrid catalyst was introduced by Ward and co-workers, who incorporated vanadium oxide [78] or osmium oxide [79] into streptavidin. The resulting systems displayed selectivities up to 95% ee for olefins dihydroxylation.…”

The Evolution of New Catalytic Mechanisms for Xenobiotic Hydrolysis in Bacterial Metalloenzymes

“…[51,171] However, extensive rational design, computational enzyme design, and rounds of directed evolution are routinely employed to improve the substrate and metal ion affinity and overall turnover rate in artificial metalloenzymes. [172][173][174][175] Just as in natural metalloenzyme evolution towards xenobiotic hydrolysis, structural optimisation will lead to improved specificity and turnover rates. To improve the activity of a transfer hydrogenase, it was computationally optimised.…”