Reactivity of rhodium porphyrins toward alkenes and isocyanides

This dissertation has focused on rhodium(II) porphyrins as metalloradicals in reactions with alkenes and isocyanides.; Rhodium(II) porphyrins react with alkenes, including acrylates and methyl methacrylate in a manner that is formally similar to that of organic radicals. Octaethylporphyrinatorhodium(II) dimer, ((OEP)Rh) {dollar}sb2{dollar}, adds with acrylates and reduces the alkene unit by forming two-carbon-alkyl-bridged complexes (Equation 1). {dollar}{dollar}eqalign{lcub}&rmlbrack (OEP)Rhrbrack sb2 + CHsb2{lcub}={rcub}CH(COsb2X) rightleftharpoonscr &rm qquad(OEP)Rh{lcub}-{rcub}CHsb2CH(COsb2X){lcub}-{rcub}Rh(OEP)cr{rcub}leqno(1){dollar}{dollar}Steric demands of the tetramesitylporphyrin (TMP) ligand preclude dimerization of (TMP)Rh{dollar}cdot{dollar}, inhibit formation of the two-carbon-alkyl-bridged complexes for larger acrylates and promote alkene coupling in reactions of (TMP)Rh{dollar}cdot{dollar} with acrylates. The resulting four-carbon-alkyl-bridged complexes form exclusively with an alkyl fragment produced from head to head alkene dimerization (Equation 2). {dollar}{dollar}eqalign{lcub}&rm 2(TMP)Rh{lcub}cdot{rcub} + 2CHsb2{lcub}={rcub}CH(COsb2X) rightleftharpoonscr &rm qquad(TMP)Rh{lcub}-{rcub}CHsb2CH(COsb2X)CH(COsb2X)CHsb2 {lcub}-{rcub}Rh(TMP)cr{rcub}leqno(2){dollar}{dollar}Rhodium(II) porphyrin metalloradicals, (por)Rh{dollar}cdot{dollar}, are ineffective at initiating thermal polymerization of acrylates, but the four-carbon-bridged (TMP)Rh system serves as a self-assembling initiator in the photopromoted polymerization of acrylates that exhibits living character. A thermochemical model is used in unifying and discussing the observed reactions of acrylates with rhodium(II) porphyrins. Cobalt(II) porphyrins were also investigated as potential thermal and photolytic initiators for living radical polymerization but they function mainly as chain-transfer agents under the applied reaction conditions.; Reactions of alkyl and aryl isocyanides with (OEP)Rh-H, ((OEP)Rh) {dollar}sb2{dollar} and (TMP)Rh{dollar}cdot{dollar}, have been investigated for comparison with CO reactivity. Alkyl and aryl isocyanides interact with (OEP)Rh-H to form 1:1 adducts, (OEP)Rh(H)(CNR), which react further to produce formimidoyl complexes, (OEP)Rh-CH=NR, in analogy with the reaction of (OEP)Rh-H with CO that produces a formyl complex, (OEP)Rh-CHO (Equations 3, 4). {dollar}{dollar}leqalignno{lcub}&rm (OEP)Rh{lcub}-{rcub}H + CNR rightleftharpoons (OEP)Rh{lcub}-{rcub}CH{lcub}={rcub}NR&(3)cr&rm(OEP)Rh{lcub}-{rcub}H + CO rightleftharpoons (OEP)Rh{lcub}-{rcub}CHO&(4)cr{rcub}{dollar}{dollar}; ((OEP)Rh) {dollar}sb2{dollar} forms 1:1 complexes with isocyanides where the 2,6-dimethylphenyl isocyanide derivative persists at equilibrium, but alkyl isocyanides undergo CN-alkyl bond cleavage to form alkyl, (OEP)Rh-R, and cyanide, (OEP)Rh(CN)(CNR), complexes. Reactions of (TMP)Rh{dollar}cdot{dollar} with alkyl isocyanides gave similar results. No evidence was obtained for bridging isocyanide species analogous to the dimetalketone, RhC(O)Rh and dimetaldiketone RhC(O)C(O)Rh complexes observed in reactions of ((OEP)Rh) {dollar}sb2{dollar} and (TMP)Rh{dollar}cdot{dollar} with CO. Carbamoyl complexes, (OEP)Rh-C(O)NHR, are formed in reactions of ((OEP)Rh) {dollar}sb2{dollar} with CNR and H{dollar}sb2{dollar}O or CO and RNH{dollar}sb2{dollar}.