The Heck reaction for porphyrin functionalisation: synthesis of meso-alkenyl monoporphyrins and palladium-catalysed formation of unprecedented meso–β ethene-linked diporphyrins

Palladium-catalysed coupling of the vinyl derivatives methyl acrylate, styrene and acrylonitrile with 5-bromo-10,15,20-triphenylporphyrin (MTriPPBr; M = 2H, Ni, Zn) and 5,15-dibromo-10,20-bis(3,5-di-tert-butylphenyl)porphyrin (MDAPBr(2)) produced a series of mono- and disubstituted alkenylporphyrins, thus demonstrating the applicability of meso-haloporphyrins in Heck-type reactions. The same technique was also applied to meso-ethenylporphyrins and simple aryl halides, with mixed results. Only meso-vinyl nickel(ii) porphyrins showed any reactivity under our conditions. A mixture of 1,1- and 1,2-disubstitution across the alkene was observed for 5-vinyl-10,15,20-triphenylporphyrinatonickel(ii) (meso-vinylNiTriPP), whereas 5-vinyl-10,20-bis(3,5-di-t-butylphenyl)porphyrinatonickel(ii) (meso-vinylNiDAP) produced a mixture of meso-1,1-, meso-1,2- and, surprisingly, beta-1,2-disubstituted Heck products. Coupling meso-vinylNiDAP with MTriPPBr under similar Heck conditions led unexpectedly to transbeta-meso-NiDAP-ethene-MTriPP dyads, affording the first members of a new class of alkenyl-linked diporphyrins. A mechanism for the unusual meso to beta rearrangement is discussed. The electronic absorption spectra of the dyads have a red-shifted shoulder on the Soret (B) band, which is evidence of a moderate degree of electronic interaction between the porphyrins via the ethenyl bridge.