A rapid approach to ferrocenophanes via ring-closing metathesis

The synthesis of [4]ferrocenophanes containing a (Z)-1,4-but-2-endiyl bridge is described. Starting from either achiral (R=H), meso (R=Me, Ph) or scalemic (R=Me, 4-MeOC6H4) 1,1′-di{CHR(OAc)}ferrocenes, vinylation with a mixture of vinyl magnesium chloride and zinc chloride proceeds readily and with significant retention of configuration to give allylferrocenes in an isomer ratio of >3:1. Ring-closing metathesis with Ru(CHPh)Cl2(PCy3)2 results in conversion of the achiral (R=H) and all the meso-diastereoisomers (R=Me, Ph, 4-MeOC6H4) to their corresponding cis-disubstituted [4]ferrocenophanes. Only a single trans-disubstituted [4]ferrocenophane was synthesised (R=Me), the larger substituents preventing cyclisiation by this method.