METALLORADICAL REACTIONS OF RHODIUM(II) PORPHYRINS WITH ACRYLATES - REDUCTION, COUPLING, AND PHOTOPROMOTED POLYMERIZATION

(Octaethylporphyrinato)rhodium(II) dimer, [(OEP)Rh]2, adds with acrylates and reduces the alkene unit in forming two-carbon-alkyl-bridged complexes (OEP)RhCH2CH(CO2X)Rh(OEP) (X = H, CH3, CH2CH3). Stereoisomers resulting from inhibition of rotation around the C-CO2X bond wedged between two porphyrin rings are observed and discussed. Steric demands of tetramesitylpophyrin (TMP) preclude dimerization of (TMP)Rh., inhibit formation of two-carbon-alkyl-bridged complexes, and promote alkene coupling in reactions of (TMP)Rh. with acrylates. The resulting four-carbon-alkyl-bridged complexes (TMP)RhCH2CH(CO2X)CH(CO2X)CH2Rh(TMP) form exclusively with an alkyl fragment produced from head to head alkene dimerization. The presence of two chiral centers results in diastereomers observed by H-1 NMR. Rhodium(II) porphyrins react with methyl methacrylate to form alkenyl derivatives (por)RhCH2C(CO2CH3)=CH2 and an equal quantity of either a hydride or alkyl complex by a process related to alkyl radical disproportionation. Rhodium(II) porphyrin metalloradicals, (por)Rh., are ineffective at initiating thermal polymerization of acrylates, but the (TMP)Rh system catalyzes a photopromoted polymerization of acrylates that has living character. A thermochemical model is used in unifying and discussing the observed reactions of acrylates with rhodium(II) porphyrins.