Heck reaction between free base 2-Br-porphyrin and vinyl-ferrocene derivatives. Electrochemical and spectroscopic characterisation of β-functionalised alpha and trans-vinyl-ferrocene porphyrin derivatives. A comparative study

Abstract

Porphyrins belong to a peculiar class of natural molecules involved in many different biological processes of paramount importance, from photosynthesis in green plants, to the oxygen binding and transportation in breathing and to the oxidation reactions catalyzed by the enzymatic family of cytochromes P450. This versatility is due to the tight structure/properties relationship of these macrocycles. Many different synthetic strategies have been used to modify the porphyrin core by connecting electro- and photo-active moieties in both meso- and beta-pyrrole positions. Among them all, C–C bond formation still remains one of the most useful approaches to ensure fine control over the chemical–physics properties of porphyrins. In this work we successfully tested the catalytic system Pd(dba)₂/AsPh₃ in the Heck cross-coupling reaction between β-monobrominated tetraphenylporphyrin and ferrocenyl-alkene derivatives, avoiding the use of excess alkene and Ni(II) or Zn(II) porphyrinate complexes. The preferential formation of the alpha- versus trans-isomer was observed when vinylferrocene was used as the starting alkene, compared to styrene and 4-ferrocenylstyrene, either at lower (60 °C) or higher (90 °C) reaction temperature. Subsequent electrochemical and photochemical studies on ferrocene–porphyrin dyads pointed out the importance of the type of β-C–C bond between the two chromophores in modulating/affecting the electron-transfer processes. Such studies can be useful for the realization of highly sensible fluorescent-redox switches or even in the development of highly efficient ferrocene/porphyrin-based chemodynamic therapy agents.