ACS Cent. Sci. (2019).

Trifluoromethylated organoborons are valuable synthetic building blocks as they provide access to a wide range of CF3-containing molecules via boron-mediated transformations. However, their asymmetric synthesis remains a challenge.

Credit: American Chemical Society, 2019

Frances H. Arnold’s group has pioneered the application of haem-containing enzymes for the catalysis of abiological carbene transfer reactions. Just one of the many impressive examples is the biocatalytic insertion of carbene reagents into B−H bonds to give chiral organoborane compounds — achieved by reprogramming the enzyme Rhodothermus marinus cytochrome c.

Now, Kendall N. Houk, Frances H. Arnold and co-workers report the expansion of the synthetic scope of this enzyme to accept trifluorodiazo alkanes in asymmetric carbene B−H bond insertion reactions for the synthesis of organofluorine compounds. For this purpose, directed evolution of the wild-type enzyme — that showed no considerable activity for the fluorinated substrates — was performed. Targeted residues for mutation were chosen in order to generate an enzyme active site that is surrounded by a more flexible loop, promoting substrate interactions with the carbene intermediate for increased enzyme activity, and also restricting the position and orientation of the carbene intermediate, whereby the CF3-group points towards the haem pocket and the alkyl substituent towards the solvent, for highly enantioselective C−B bond formation and a broad substrate scope.

Indeed, several steps of directed evolution afforded a mutant enzyme — designated as BOR-CF3 — which allowed the conversion of a variety of structurally diverse, fluorinated diazoalkane substrates with good turnovers and high enantioselectivities. Notably, several of the synthesized chiral α-CF3 organoborons appear to not be accessible by current alternative methods. Molecular dynamics simulations elucidated in detail the contributions of the mutations on the activity and stereospecificity of BOR-CF3.

Overall, this work is a valuable extension of an enzyme-catalysed abiological reaction, providing new opportunities for the synthetic chemist. The careful design of the study — taking structural and dynamical considerations into account — was the key to generating a broadly applicable and enantioselective biocatalyst.