1,2-rearrangements of porphyrinato rhodium(III) alkyls: Mechanistic investigation

Alkyl 1,2-rearrangements in alkylmetal complexes play a crucial role in organometallic chemistry on both grounds of transition-metal catalysis and bio-inorganic chemistry. Reversible alkyl 1,2-rearrangements of porphyrinato rhodium(III) alkyl complexes have been successfully observed and the reaction mechanism of the phenethyl - alpha-methylbenzyl rearrangement was investigated. The 1,2-rearrangement was found to follow first order kinetics and proceed through Rh-C bond migration. Activation parameters (DeltaH obs‡ = 52.6 +/- 3.2 kJmol -1 and DeltaSobs‡ = -171 +/- 9 Jmol-1K-1 ) implied for a structurally highly organized transition state. The observed alkyl ligand exchanges with added olefins and the facile reaction of styrene with Rh-H complex to give rhodium(III) phenethyl complex established the intermediacy of Rh-H and olefin in the rearrangements. The observed retardation of rearrangement by added sigma-donor ligands supported that coordinative unsaturation is a primary requirement. An observed deuterium kinetic isotope effect (kH/kD) of 5.6 upon deuterating the beta (benzylic) position of the phenethyl ligand confirmed that beta C-H bond cleavage occurs before or at the rate determining step. Substituent effects on the alkyl rearrangement were investigated and the rates of rearrangement were found to increase with electron rich para-substituted phenethyl ligands. It implied that a positive charge develops at the benzylic (beta) position in the rate determining step. The mechanistic evidences obtained are consistent with a two step beta-hydride elimination/Rh-H olefin re-insertion mechanism. The mechanistic observations implied that cis-axial coordination is feasible in metalloporphyrin complexes.; The 1,2-rearrangement of porphyrinato rhodium(III) alkyl complexes was found to be a general reaction with several alkyl ligands, namely methoxyethyl, n-propyl and isopropyl. The isomeric preferences of the 1,2-rearrangement were determined to be predominantly secondary in both cases of phenethyl and methoxyethyl ligands and marginally primary with the n-propyl ligand. The observed isomeric preferences were attributed to the steric factor and the stabilization of the metal-carbon bond by alpha-electron withdrawing substituents.; In the attempted trapping of free radical intermediates in the 1,2-rearrangements with TEMPO, Rh(por)CH3 complexes were formed unexpectedly. This serendipitous formation of Rh(por)CH3 was further investigated and proposed to be resulted from the reaction of rhodium(II) porphyrin with TEMPO.