SYNTHESIS OF ETHYNE-LINKED OR BUTADIYNE-LINKED PORPHYRIN ARRAYS USING MILD, COPPER-FREE, PD-MEDIATED COUPLING REACTIONS

The synthesis of multi-porphyrin light-harvesting arrays requires copper-free coupling reactions. Two studies of copper-free Pd-catalyzed coupling reactions are presented. First, we investigate the effects of different ligands in the Pd-catalyzed cross-coupling of a porphyrin bearing two aryl iodides with a porphyrin bearing an aryl ethyne. Triphenylarsine affords faster rates than triphenylphosphine or tri-2-furylphosphine when experiments are performed in conjunction with tris(dibenzylideneacetone)dipalladium(0) under anaerobic conditions. Reactions performed at 35 degrees C are relatively clean, fast (complete in 1-2 h), and afford the ethyne-linked porphyrin array in similar to 70% yield with a minimal amount of higher molecular weight byproducts. This reaction is ideal for preparing multi-porphyrin arrays. Second, treatment-of a free base porphyrin-ethyne with a stoichiometric amount of Pd(PPh(3))(2)Cl-2, or a catalytic amount of tris(dibenzylideneacetone)-dipalladium(0) and tri-2-furylphosphine, in toluene/triethylamine (5:1) in air at 50-60 degrees C affords the butadiyne-linked porphyrin dimer in similar to 70% yield. With Pd(PPh(3))(4) in toluene under anaerobic conditions at 100 degrees C, the porphyrin-ethyne reacts to give the head-to-tail enyne-linked dimer (29% yield). Size exclusion chromatography is effective in analyzing reaction mixtures and purifying the porphyrin arrays. These copper-free coupling reactions afford direct access to ethyne-, butadiyne-, or enyne-linked arrays of free base and/or zinc porphyrins from readily available porphyrin building blocks.