Expanding the Scope of Alcohol Dehydrogenases towards Bulkier Substrates: Stereo- and Enantiopreference for α,α-Dihalogenated Ketones

Alcohol dehydrogenases (ADHs) were identified as suitable enzymes for the reduction of the corresponding α,α‐dihalogenated ketones, obtaining optically pure β,β‐dichloro‐ or β,β‐dibromohydrins with excellent conversions and enantiomeric excess. Among the different biocatalysts tested, ADHs from Rhodococcus ruber (ADH‐A), Ralstonia sp. (RasADH), Lactobacillus brevis (LBADH), and PR2ADH proved to be the most efficient ones in terms of activity and stereoselectivity. In a further study, two racemic α‐substituted ketones, namely α‐bromo‐ α‐chloro‐ and α‐chloro‐α‐fluoroacetophenone were investigated to obtain one of the four possible diastereoisomers through a dynamic kinetic process. In the case of the brominated derivative, only the (1R)‐enantiomer was obtained by using ADH‐A, although with moderate diastereomeric excess (>99 % ee, 63 % de), whereas the fluorinated ketone exhibited a lower stereoselectivity (up to 45 % de). Bulking up: A series of β,β‐dihalohydrins are obtained through alcohol dehydrogenase (ADH) catalyzed bioreduction of the synthesized α,α‐dihalogenated ketones. Two racemic acetophenone derivatives are also subjected to this protocol to obtain stereoenriched alcohols through dynamic kinetic resolution (DKR).