Mechanisms of the CO₂ Insertion into (PCP) Palladium Allyl and Methyl σ-Bonds. A Kinetic and Computational Study

The reaction of the σ-bonded (PCP)Pd-Me complex (PCP = 2,6-bis[(di-tert-butylphosphino)methyl]phenyl) with CO2 is first-order in palladium and first-order in CO2 with a rate constant ks = 8.9 ± 0.8 M−1 s−1 at 353 K. Activation parameters are ΔH⧧ = 73 ± 7 kJ/mol and ΔS⧧ = −118 ± 19 J/K mol. Based on this and theoretical calculations we propose an SE2 mechanism where the coordinated methyl group attacks a completely noncoordinated carbon dioxide molecule in a bimolecular reaction. The PCPPd−crotyl complex was synthesized in an 65:35 E:Z mixture, and it was shown to react with CO2 to give the complex PCPPd−O(CO)CH(CH3)CHCH2 as a single isomer, where the former γ-carbon has been carboxylated. Theoretical calculations again suggest an SE2 mechanism with a noncoordinated carbon dioxide reacting with the terminal carbon on the allyl group, forming an η2-bonded olefin complex as an intermediate. The rearrangement of this intermediate to the O-bonded product is concluded to be rate determining. The crystal structure of PCPPd−O(CO)C(CH3)2CHCH2 is reported and as well as the solubility of carbon dioxide in benzene-d6 at different pressures and temperatures.