Organocobalt complexes. Part VIII. Specificity of the cyclopentenone synthesis from acetylenehexacarbonyldicobalt complexes and norbornene derivatives

Abstract

Numerous new examples of the annelation reactions leading to cyclopentenone derivatives are reported. They provide evidence of steric control resulting in preferred substitutions at positions 2 and 5 and placement of the larger substituent at position 2 if the acetylenic precursor is disubstituted. A pathway accounting for such preference is suggested and the bulky trimethylsilyl group is utilized as a removable direction-determining group to allow synthesis of the 3- instead of 2-substituted cyclopentenones. More remote substituents in the norbornene are shown to exert a significant but no longer decisive effect, so that mixtures of isomers result. A ¹H n.m.r. study has shown that the ferrocenyl substituent in 7a-ferrocenyl-3a,4,5,6,7,7a-hexahydro-4,7-methanoinden-1-one (IX) cannot rotate about the bond joining it to position 7a.