Cyclopentadienones benzene ring

The ring system can be generated by D-A addition of a substituted cyclopentadienone and an alkyne. A reaction sequence involving addition followed by CO elimination can be used for the synthesis of highly substituted benzene rings.308... 

Benzene ring from cyclopentadienones. Allyl phenyl sulfone and tetraphenylcyclo-pentadienone heated at 180° in polyethyleneglycol 200 under N2 for 15 min 1,2,3,4-tetraphenylbenzene. Y 62%. F.e. and with cinnamyl p-tolyl sulfone as dienophile s. P. Raja Kumar, J. Chem. Soc. Chem. Commun. 1989, 509-10. 

In the case of tt complexes of substituted cyclopentadienones, such as the iron tricarbonyl derivatives prepared by Weiss and H libel (30), qualitative molecular-orbital theory (20) predicted a considerable reduction of the ketonic carbonyl bond order. It was observed that the ketonic carbonyl frequency dropped by as much as 65 cm-1, in agreement with theory. A similar explanation can also be provided in terms of valence bond theory (Fig. 14). It has been suggested that n complexing of arenes such as benzene results in loss of aromaticity of the ring in contrast to the dicyclopentadienyl... 

The reaction between acetylenes and ruthenium carbonyls produces a series of n complexes with cyclic ligands which, as in the iron system, have either the metal or a CO group incorporated into the ring. Accordingly, 3-hexyne 536) and hexafluoro-2-butyne 90) react with Ru3(CO)i2 to give the (substituted cyclopentadienone)tricarbonylruthenium complexes with structures presumably comparable to those of the iron complexes (93-95). Although diphenylacetylene will not react directly with Ru3(CO)i2 to produce this type of complex 536), it can be prepared 90) by treating Ru3(CO)i2 with tetracyclone in benzene under reflux. 

The intramolecular [2+2+1] cycloaddition promoted by pentacarbonyliron can also be extended to silyl protected yne-ynamides (Scheme 4-7) and yne-ynol ethers (Scheme 4—8) ° to give after demetalation cyclopentadienones with fused N- or 0-heterocycles of different ring size. Subsequent Diels-Alder reaction with dimethyl acetylenedicarboxylate (DMAD) or ethyl propiolate transforms the cyclopentadienone moiety into the corresponding benzene derivative. 

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