Highly diastereoselective and enantioselective olefin cyclopropanation using engineered myoglobin-based catalysts

Abstract

Using rational design, an engineered myoglobin-based catalyst capable of catalyzing the cyclopropanation of aryl-substituted olefins with catalytic proficiency (up to 46,800 turnovers) and excellent diastereo- and enantioselectivity (98-99.9%) was developed. This transformation could be carried out in the presence of up to 20 g L(-1) olefin substrate with no loss in diastereo- and/or enantioselectivity. Mutagenesis and mechanistic studies support a cyclopropanation mechanism mediated by an electrophilic, heme-bound carbene species and a model is provided to rationalize the stereopreference of the protein catalyst. This work shows that myoglobin constitutes a promising and robust scaffold for the development of biocatalysts with carbene-transfer reactivity.

Keywords: biocatalysis; carbenes; iron; protein engineering; small-ring systems.

Figure 1

a) Active site of sperm whale myoglobin (pdb 1A6K). The residues targeted for mutagenesis are highlighted in orange. b) Proposed mechanism for myoglobin-catalyzed styrene cyclopropanation with diazo esters.

Figure 2

Proposed geometries for styrene approach to the putative heme-carbene leading to the (1S,2S)-ethyl 2-phenylcyclopropanecarboxylate stereoisomer. The orientation of the heme rings is the same as in Figure 1.