Comparative Study of Pd-Mediated Carbon−Carbon, Carbon−Heteroatom, and Heteroatom−Heteroatom Bond Formation/Breakage (C = Csp3, Csp2, Csp; X = B, N, O, Si, P, S, Se, Te)
Carbon–carbon and carbon–heteroatom bond formations via direct reductive elimination as one of the possible mechanisms of reductive elimination in Pd(II) complexes are the key stages of catalytic processes in fine organic synthesis. For the (R)2Pd(L)2, (X)2Pd(L)2 and (R)(X)Pd(L)2 complexes (where R = Me, Vin, Ph, or Eth; X = B, N, O, Si, P, S, Se, or Te; L = PPh3), the R–R, R–X, and X–X bond formation barriers and reaction energies were calculated. The reaction barriers for C–C and C–X coupling decrease in the series Csp3 > Csp > Csp2. The activity of coupling groups X containing a heteroatom decreases in the series of heteroatoms P, S, Se ? N ? O (for Csp2 and Csp types of carbon centers) and P > S, Se ? N ? O (for Csp3 type of carbon center). The relationship between the structural lability of the (R)2Pd(L)2 complexes and the probability of reductive elimination was determined by DFT molecular dynamics. An analysis of the calculated bond formation barriers and reaction energy showed that, in most cases, their values for unsymmetrical RX coupling are intermediate between the values for the reactions of symmetrical RR and XX coupling. The influence of the electronic properties of the coupling groups on the stabilization of the cis form of the complexes, which are suitable pre-reaction complexes for reductive elimination, was shown. The additivity of the energy difference between the cis and trans isomers was established: the cis–trans isomerization energies for the (R)(X)Pd(L)2 complexes are intermediate between the corresponding energies for the (R)2Pd(L)2 and (X)2Pd(L)2 complexes. A high degree of additivity of the QTAIM charge of the palladium atom in all of the considered complexes was analyzed. In the present detailed study, we establish a hierarchy in bond formation barriers, emphasizing the influence of carbon center types, and discern the impact of coupling groups containing heteroatoms, revealing distinct trends based on carbon center types.
Reference: Organometallics, 2024, 43, 1-13.
