Asymmetric Carbohydroxylation of Alkenes Using Photoenzymatic Catalysis
Yao Ouyang, Joshua Turek-Herman, Tianzhang Qiao, Todd K. Hyster
Abstract
Alkene difunctionalizations enable the synthesis of structurally elaborated products from simple and ubiquitous starting materials in a single chemical step. Carbohydroxylations of olefins represent a family of reactivity that furnish structurally complex alcohols. While examples of this type of three-component coupling have been reported, catalytic asymmetric examples remain elusive. Here, we report an enzyme-catalyzed asymmetric carbohydroxylation of alkenes catalyzed by flavin-dependent "ene"-reductases to produce enantioenriched tertiary alcohols. Seven rounds of protein engineering reshape the enzyme's active site to increase activity and enantioselectivity. Mechanistic studies suggest that C-O bond formation occurs via a 5-endo-trig cyclization with the pendant ketone to afford an α-oxy radical which is oxidized and hydrolyzed to form the product. This work demonstrates photoenzymatic reactions involving "ene"-reductases can terminate radicals via mechanisms other than hydrogen atom transfer, expanding their utility in chemical synthesis.