Investigation and refinement of palladium-coupling conditions for the synthesis of diarylethyne-linked multiporphyrin arrays

The synthesis of diarylethyne-linked multiporphyrin arrays requires Pd-mediated coupling conditions that are performed in dilute solution in the absence of copper and that give reaction mixtures that are easily purified. We previously developed copper-free Pd-mediated coupling conditions (Pd-2(dba)(3) and AsPh3 in toluene/triethylamine (5:1) at, 35 degrees C) but extensive chromatography was required to remove unidentified porphyrinic byproducts. We have used laser-desorption mass' spectrometry (LD-MS) and analytical size-exclusion chromatography (SEC) to assay the products from the coupling reaction of a zinc(II) iodophenylporphyrin (ZnI) and a zinc(II) ethynylphenylporphyrin (ZnU'). Analysis of the crude reaction mixture revealed the diphenylethyne-linked dimer (Zn2U) as the major product, byproducts plausibly derived from phenyl-aryl exchange processes (a phenylated derivative of ZnU', a Ph2As-substituted derivative of ZnI), byproducts believed to originate from direct phenylation processes (a phenylated derivative of ZnI, the phenylated dimer Ph-Zn2U), and higher molecular weight material (HMWM) comprised of undefined porphyrin oligomers. The HMWM is readily removed but the phenylated dimer causes severe purification problems. Our observations concerning the phenylation processes are as follows: (1) The phenylated dimer was observed in significant quantity (similar to 7:1 ratio of Zn2U/Ph-Zn2U by LD-MS). (2) Control experiments showed that direct phenylation of free base (Fb) porphyrins or zinc porphyrins occurred with Pd-2(dba)(3) and AsPh3 in the absence of iodo- or ethynylporphyrins. (3) Phenylation occurred at the earliest stage of the reaction (3 min time point). (4) The inclusion of CuI (with Pd-2(dba)(3) and PPh3) accelerated formation of Zn2U but did not reduce the amount of phenylation. (5) The use of P(o-tol)(3) in conjunction with Pd-2(dba)(3) caused slower reaction, a slight increase in HMWM, and significantly diminished arylation (Zn2U/ Ar-Zn2U ratio similar to 65:1). (6) Replacement of Pd-2(dba)(3) with Pd(OAc)(2) gave the same arylation ratio (65:1) but slightly lower yield. Application of the refined conditions (Pd-2(dba)(3) and P(o-tol)(3) in toluene/triethylamine (5:1) at 35 degrees C) to the stepwise synthesis of porphyrin trimers having metalation states of Zn and Fb (ZnZnFbU) or Mg, Zn, and Fb (MgZnFbU) afforded coupling efficiencies of 45-55% and a substantially simpler chromatographic purification process.