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 CO 2 is first-order in palladium and first-order in CO 2 with a rate constant k s = 8.9 ± 0.8 M −1 s −1 at 353 K. Activation parameters are Δ H ⧧ = 73 ± 7 kJ/mol and Δ S ⧧ = −118 ± 19 J/K mol. Based on this and theoretical calculations we propose an S E 2 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 CO 2 to give the complex PCPPd−O(CO)CH(CH 3 )CHCH 2 as a single isomer, where the former γ-carbon has been carboxylated. Theoretical calculations again suggest an S E 2 mechanism with a noncoordinated carbon dioxide reacting with the terminal carbon on the allyl group, forming an η 2 -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(CH 3 ) 2 CHCH 2 is reported and as well as the solubility of carbon dioxide in benzene- d 6 at different pressures and temperatures.