Organometallic dehydro[14]annulenes containing Vollhardt's cyclobutadiene:Are CpCo-complexed cyclobutadienes more aromatic than benzene?

Pd-catalyzed coupling of 1,2-diethynyl-3,4-(bistrimethylsilyl)cyclobutadien ecyclopentadienylcobalt to a series of 1-iodo-2-(trimethylsilylethynyl)benz enes and 1-chloro-4-trimethylsilylbut-1-ene-3-yne is followed by desilylati on with potassium carbonate. Cu(OAc)(2)-promoted oxidative ring closure lea ds to dehydro[14]annulenes and dehydro[14]benzoannulenes fused to a cyclobu tadiene(cyclopentadienylcobalt) complex. Five of these fused dehydroannulen es were structurally characterized. H-1 NMR spectroscopy of the organometal lic dehydro[14]annulenes incorporating the (bistrimethylsilyl)cyclobutadien e(cyclopentadienylcobalt) unit suggested that the aromaticity of the fused cyclobutadiene complex might be stronger than that of benzene according to the ring-current criterion.