Enantioselective [2+2]-cycloadditions with triplet photoenzymes
Naturally evolved enzymes, despite astonishingly large variety and function diversity, operate predominantly through thermochemical activation. Integrating prominent photocatalysis modes into proteins such as triplet energy transfer could create artificial photoenzymes that expand the scope of natural biocatalysis.1-3 Here, we exploit genetically reprogrammed, chemically evolved photoenzymes embedded with a synthetic triplet photosensitizer that are capable of excited state enantioinduction4-6. Structural optimization through four rounds of directed evolution afforded proficient variants for the enantioselective intramolecular [2+2] photocycloaddition of indole derivatives with good substrate generality and excellent enantioselectivities (up to 99% enantiomeric excess). A crystal structure of the photoenzyme-substrate complex elucidated the noncovalent interactions which mediate reaction stereochemistry. This study expands the energy transfer reactivity7-10 of artificial triplet photoenzymes in a supramolecular protein cavity and unlocks an integrated approach to valuable enantioselective photochemical synthesis that is not accessible with either the synthetic or the biological world alone.