Mechanisms of the CO2 Insertion into (PCP) Palladium Allyl and Methyl sigma-Bonds. A Kinetic and Computational Study
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The reaction of the sigma-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 CO, with a rate constant k(s) = 8.9 +/- 0.8 M-1 s(-1) at 353 K. Activation parameters are Delta H double dagger = 73 7 kJ/mol and Delta S double dagger = 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, to give the complex PCPPd-O(CO)CH(CH3)CHCH2 as a single isomer, where the former gamma-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 eta(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(CH3)(2)CHCH2 is reported and as well as the solubility of carbon dioxide in benzene-d(6) at different pressures and temperatures.