Directed Evolution of Metalloenzymes for Organic Synthesis
Carl Denard, Lars Martin Jarenmark, Ramesh Giri, John F. Hartwig and Huimin Zhao, UIUC
Catalytic asymmetric tandem reactions have been reported extensively in synthetic chemistry literature. The use in tandem of catalysts from different disciplines has been reported. However, there are no reports on combining two metal catalysts from completely different fields, i.e., an organometallic catalyst and a metalloenzyme. In the Zhao and Hartwig labs, we have developed a novel catalytic asymmetric tandem reaction to achieve a successive olefin isomerization-enantioselective epoxidation, a reaction that is difficult to envision occurring with a single catalytic system. This process can potentially provide a practical and efficient strategy for the enantioselective preparation of linear terminal epoxides in high ee from a mixture of simple substrates. To address the issue of catalyst activity in the biocompatible solvent hexadecane, ruthenium hydride complexes were synthesized that can isomerize several olefins of interest with high activity and interesting selectivities, at room temperature and at enzymatic conditions. Sulfur activation further enhances the isomerization reaction rates. By combining the isomerization catalyst with cell lysates or purified P450 BM3 mutants coupled with a cofactor regeneration system, we report for the first time that the two reactions work in tandem. Starting with a terminal olefin, we were able to detect the corresponding internal epoxide at a ratio of 1.5:1 to 2:1 compared to the terminal epoxide concentration. In addition, we tackled the thermodynamically challenging internal to terminal isomerization in tandem with the epoxidation reaction and were able to detect the terminal epoxide. In the case of 4-methyl-cis- and 4-methyl-trans-2-pentene, we obtained the most promising result of detecting the terminal epoxide in an equimolar amount to the internal epoxides. This represents the first report that shows that two metal catalysts from very different fields, namely a metalloenzyme and an organometallic catalyst, are capable of working in tandem to tackle an important synthetic challenge. We have further expanded the substrate scope of the reaction system to a number of straight and branched C5-C7 aliphatic olefins. This tandem process should be generally applicable to other high-impact and challenging transformations that require excellent selectivities.