Formation of 2,2′-Diacyl-9,9′-bifluorenylidene Isomers from 2-Acyl-9-bromofluorene and Base-Catalyzed Isomerization of the Formed Alkenes

Abstract

Abstract We studied the formation of 2,2′-diacyl-9,9′-bifluorenylidene from 2-acyl-9-bromofluorene. The E-isomer was obtained mainly when the acyl group was a short alkyl chain, such as an acetyl group, and Z-isomer was isolated predominantly when a long acyl chain such as stearoyl group was attached. 9,9′-Bifluorenylidene was formed via anti elimination of hydrogen bromide from the intermediate 9-bromo-9,9′-bifluorenyl. This indicated that the stereochemistry of 2,2′-diacyl-9,9′-bifluorenylidene was determined by the configuration of 9-bromo-9,9′-bifluorenyl isomers, giving Z- and E-isomers in a ratio of 1:1. Facile base-catalyzed Z/E isomerization of 9,9′-bifluorenylidenes followed the formation of the alkenes, which showed different stereoselectivity depending upon the length of the acyl side-chain. On the other hand, thermal treatment of 2,2′-diacyl-9,9′-bifluorenylidene afforded a ca. 1:1 mixture of Z- and E-isomers, suggesting that thermal isomerization took place through a different pathway than base-catalyzed isomerization.