Efficient rhodium-catalyzed installation of unsaturated ester functions onto porphyrins: site-specific Heck-type addition versus conjugate addition

A facile introduction of alpha,beta- or alpha,beta,gamma,delta-unsaturated ester functions onto porphyrins was achieved through rhodium-catalyzed addition of beta-borylporphyrins to acrylate or 2,4-pentadienoate esters. The reaction of meso-borylporphyrins with acrylates exclusively afforded saturated esters by 1,4-conjugate addition under the same reaction conditions. Thus, the reaction mode (Heck-type versus conjugate addition) is highly dependent on the reaction site (beta versus meso). This functionalization has a significant impact on the electronic properties of the pi system of porphyrins, which induces a substantial redshift and broadening in the absorption spectra by effective conjugation through the beta positions. The coplanar structure of the unsaturated ester moieties with respect to the porphyrin core has been unambiguously elucidated by X-ray crystallographic analysis.