Porphyrins with (un)saturated side–chains containing boron residues were developed as synthons for porphyrin functionalization. Porphyrins with mono and bis-substituted unsaturated boronyl residues were prepared in good yields (52–66 %) using a cross–metathesis approach in the presence of Grubbs I-generation catalysts. In all cases complete E–stereoselectivity (100 %) was observed. Furthermore, formal cross–metathesis products with α,β–unsaturated chains smoothly underwent addition with bis(pinacolato)diboron [(Me4C2O2)B–B(O2C2Me4)] to yield the corresponding saturated boron compounds in 6070 % yields. ; PUBLISHED ; This work was generously supported by a Science Foundation Ireland Research Professorship grant (SFI 04/RP1/B482).