| Title
| - Oxybenziporphyrins, Oxypyriporphyrins, Benzocarbaporphyrins,and Their 23-Oxa and 23-Thia Analogues: Synthesis,Spectroscopic Characterization, Metalation, and StructuralCharacterization of a Palladium(II) Organometallic Derivative
|
| Abstract
| - A series of nine porphyrin analogues have been synthesized using the “3 + 1” variant on theMacDonald condensation. Tripyrrane-type systems with a centrally unsubstituted pyrrole, furan,or thiophene ring were prepared using conventional methods, and these were condensed with indene-1,3-dicarbaldehyde, 5-formylsalicylaldehyde, or 3-hydroxy-2,6-pyridinedicarbaldehyde in the presence of TFA to generate benzocarba-, oxybenzi-, and oxypyriporphyrins, respectively. The furan-containing analogues proved to be highly basic and could only be isolated as the correspondinghydrochloride salts. All nine analogue systems showed porphyrin-like UV−vis spectra with one ortwo Soret absorptions near 400 nm and a series of weaker bands at longer wavelengths. Thesesystems also showed large diatropic ring currents by proton NMR spectroscopy that were comparableto true porphyrins. In the presence of trace amounts of TFA, benzocarbaporphyrin 12 formed amonocation, and in 50% TFA a C-protonated dication was generated. The 23-oxacarbaporphyrin14 gave a monocation in chloroform, although the free base was generated in 5% Et3N−chloroform.In 50% TFA−CHCl3, 14 afforded a mixture of mono- and diprotonated species. Thiacarbaporphyrin15 also formed a monocation in the presence of TFA, but C-protonation was relatively disfavoredfor this system. Nonetheless, in the presence of TFA-d, 12, 14, and 15 all showed rapid exchangeof the internal NH and CH protons. Carbaporphyrin 12 also showed slow exchange at the meso-positions, but this process was not observed for its heteroanalogues 14 and 15. Protonation studieswere also conducted for oxybenziporphyrins and oxypyriporphyrins 16−21. Oxacarbaporphyrin 14was shown to be a superior organometallic ligand and afforded good yields of the related nickel(II)and palladium(II) derivatives under mild conditions. A low yield of the platinum(II) complex couldalso be isolated. All three complexes retained their aromatic character, although the Pd(II) derivativeappeared to possess a slightly larger diatropic ring current. The palladium(II) complex 27 wasfurther characterized by X-ray crystallography. The macrocyclic core was shown to be highly planarwhere the dihedral angles of the component pyrrole, furan and indene rings relative to the mean[18]annulene plane were all ≤2.1°.
|
