Z/E selectivity on the formation of 2,2 '-diacyl-9,9 '-bifluorenylidenes was controlled by the length of acyl side chains

Reaction of 2-acetyl-9-bromofluorene with base afforded Z- and E-2,2'-diace tyl-9,9'-bifluorenylidene in a ratio of 30/70. A similar treatment of 2-ste aroyl-9-bromofluorene gave the corresponding bifluorenylidene in a Z/E rati o of 90/10. The observed dependency of Z/E ratio on the chain length is att ributable not to steric repulsion, but to intra- and/or inter-molecular att ractive forces between the side chains.