Theoretical study on a new active species for the Pd(II)-catalyzed Mizoroki–Heck reaction

The formation process of an active species for the Pd(II)-catalyzed Mizoroki–Heck reaction between bromobenzene and ethylene was investigated theoretically using the DFT(B3PW91) method. In this mechanism, one need not consider the dissociation of the chloride ion from the PdCl2(dppe) complex. It was confirmed that the ethylene insertion into the Pd–Cl bond (Path A) occurs more easily than the C–Br oxidative addition (Path B) and the Cl− dissociation (Path Z) for the PdCl2(dppe) complex. In the next step, the C6H5Br approaches the Pd center of the complex with ethylene via the C–Br oxidative addition, followed by the ethylene insertion into the Pd–Cl bond (Path D) to form a six-coordinate complex, PdBrCl(C2H4Cl)(C6H5)(dppe). In the final step, a C–Cl elimination proceeds from the complex to give a four-coordinate complex, PdBr(C6H5)(dppe). The rate-determining step of Path D is the C–Cl reductive elimination, and the energy difference between reactant and TS was calculated to be 30.4 kcal/mol. Similar mechanisms releasing Cl–C4H8–Cl (Path E and F) are also acceptable, since those barrier heights were calculated to be 32.3 and 17.6 kcal/mol. Path D, E and F are plausible candidates to form the PdBr(C6H5)(dppe) complex as a starting complex of the catalytic cycle. The rate-determining step of the entire catalytic reaction using the PdBr(C6H5)(dppe) complex is the β–H abstraction about the electronic energy of solute in solution and the free energy in solution, and those barrier heights were calculated to be 26.4 and 24.9 kcal/mol, respectively. As a result, the first step of the catalytic reaction mechanism is not the Cl− dissociation from the PdCl2(dppe) complex. Instead, the ethylene insertion into the Pd–Cl bond first proceeds to produce an active species, and next the PdBr(C6H5)(dppe) complex forms through Path D, E or F. It is concluded that the catalytic cycle of the Mizoroki–Heck reaction starts from PdBr(C6H5)(dppe).

We investigated the starting complex of the Pd-catalyzed Mizoroki–Heck reaction between bromobenzene and ethylene using the DFT method. We examined the formation process of an active species without dissociation of the chloride ion from PdCl2(dppe) complex.Figure optionsDownload as PowerPoint slideHighlights
► New active species for the Mizoroki-Heck reaction was investigated using DFT method.
► We examined the formation of an active species without dissociation of a chloride ion.
► It is concluded that the catalytic cycle starts from PdBr(C6H5)(dppe).