Comparative Reactivity of Zr– and Pd–Alkyl Complexes with Carbon Dioxide

Structure/reactivity
trends and DFT studies reveal mechanistic
differences and parallels for the carboxylation of Zr and Pd alkyls.
CO2 reacts with Cp2ZrMe­(C6D5Cl)+ >104 faster than with Cp2ZrMe2, yielding monoacetate products in both cases. These
reactions
proceed by insertion mechanisms in which Zr- - -O interactions
activate the CO2. In contrast, CO2 reacts readily
with [(PO-iPr)­PdMe2]− (PO-iPr– = 2-PiPr2-4-Me-C6H3SO3–) to yield [(PO-iPr)­PdMe­(OAc)]− but not with (PO-iPr)­PdMe­(L) species. Carboxylation of [(PO-iPr)­PdMe2]− occurs by direct SE2 attack
of CO2 at the Pd–Metrans‑to‑P group, and the nucleophilicity of the Pd–Me group controls
the reactivity. However, the SE2 process is accelerated
by a Li+- - -OCO interaction when Li+ is present.