Theoretical study on the nickel(0)-mediated coupling of carbon dioxide and benzylidenecyclopropane: Mechanism and selectivity

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Density functional theory (DFT) calculations have been conducted to gain insights into the mechanism and chemoselectivity of the Ni(0)-mediated carboxylation reactions of benzylidenecyclopropane with CO2. The experimentally observed selectivity of reaction products may be explained through the formation of a common π-complex Ni(L)2(η2-C2H4CCHPh), which can undergo a direct nucleophilic cyclic addition with CO2 or an isomerization to four-membered nickelacycle complex followed by the CO2 insertion. All possible pathways to afford the product precursor five/six-membered cyclic Ni–carboxylates species are examined, and their corresponding energetics are demonstrated. Among the various reaction pathways, we have found that the formation of five-membered cyclic Ni–carboxylate (2) via the bisligand route IA, leading to the target product cyclopropane derivative A, has a lower reaction barrier and is the most preferred in the polarity weaker solvent (toluene), which is very in good agreement with the experimental finding. As for the formation process of six-membered cyclic Ni–carboxylates (4 and 5), the rate-determining step is associated with the ring-opening of benzylidenecyclopropane to give a four-membered metallacyclic intermediate. In acetonitrile and using DBU as ligands, the monoligand route IIB is competitive with the bisligand route IIB because the activation barrier difference for the ring-opening benzylidenecyclopropane is small (1.34kcal/mol) and the energy barrier of CO2 insertion into the NiC(sp2) bond is lower. The monoligand cyclic Ni–carboxylate 4B′d, generated from the CO2 insertion into the NiC(sp2) bond of the proposed four-membered intermediate 2B′d, is predicted to be the most probable species leading to the branched α,β-unsaturated ester B.