Preparation of Furan and Thiophene-Derived Fulvene Dialdehydes: Synthesis and Structural Characterization of a 22-Oxa-21-carbaporphyrin and a Related Palladium(II) Organometallic Complex

A series of fulvene monoaldehydes were prepared by reacting furan or thiophene carbaldehydes with an indene-derived enamine in the presence of di-n-butylboron triflate, but considerable difficulties were encountered in the preparation of fulvene dialdehydes needed for the synthesis of novel porphyrin analogues. These problems were overcome by reacting protected iodofulvenes with magnesium ate complexes at low temperatures, followed by addition of DMF and hydrolysis. The thiophene-containing fulvene gave good yields of the dialdehyde at −78 °C or −100 °C, but the furan system gave a major byproduct formally derived from valeraldehyde under the higher temperature conditions. This compound was fully characterized by NMR spectroscopy, mass spectrometry, and X-ray crystallography. However, this side reaction could be completely avoided at −100 °C, and the required furan-containing fulvene dialdehyde was isolated in 46% yield. The furan-derived dialdehyde reacted with a dipyrrylmethane in the presence of trifluoroacetic acid to give the 22-oxa-21-carbaporphyrin 19 in excellent yields (73−79%). However, the thiophene-containing fulvene dialdehyde failed to give any of the anticipated macrocyclic product. An unstable acyclic intermediate was isolated and partially characterized, but this species could not be induced to cyclize. Steric factors may play a role, but X-ray crystallography confirmed that the fulvene dialdehyde precursor does have the correct geometry to facilitate the formation of the porphyrinoid macrocycle. The new oxacarbaporphyrin was fully characterized and could easily be converted into the corresponding mono- and dicationic species. The second protonation involves addition onto the internal indene carbon and proton NMR spectroscopy for the sample in HCl−TFA demonstrates that it retains strongly diatropic characteristics. The free base oxacarbaporphyrin reacted with Pd(OAc)2 in DMF to give the corresponding palladium(II) organometallic derivative 27. The proton NMR spectrum for this complex also shows the retention of a strong, albeit slightly reduced, diatropic ring current. The free base oxacarbaporphyrin and the palladium derivative were both structurally characterized by X-ray crystallography. The bond lengths for 19 and 27 were consistent with the presence of significant 18π-electron delocalization pathways.