Selective cyclopalladation of R3P=NCH2Aryl iminophosphoranes. Experimental and computational study

Article abstract of DOI:10.1021/ic701144y

The orientation of the orthopalladation of iminophosphoranes R ₃P=NCH₂Aryl (R = Ph, Aryl = Ph (1a), C₆H ₄-2-Br (1b), C₆H₄-Me-2 (1e), C ₆H₃-(Me)₂-2,5 (1f); R = p-tolyl, Aryl = Ph (1c); R = m-tolyl, Aryl = Ph (1d); R₃P = MePh₂P, and Aryl = Ph (1g)) has been studied. 1a reacts with Pd(OAc)₂ (OAc = acetate) giving endo-[Pd(μ-Cl){C,N-C₆H₄(PPh₂=NCH ₂Ph)-2}]₂ (3a), while exo-[Pd(μ-Br){C,N-C ₆H₄(CH₂N=PPh₃)-2}]₂ (3b) could only be obtained by the oxidative addition of 1b to Pd₂(dba) ₃. The endo form of the metalated ligand is favored kinetically and thermodynamically, as shown by the conversion of exo-[Pd(μ-OAc){C,N-C ₆H₄(CH₂N=PPh₃)-2}]₂ (2b) into endo-[Pd(μ-OAc){C,N-C₆H₄(PPh₂=NCH ₂Ph)-2}]₂ (2a) in refluxing toluene. The orientation of the reaction is not affected by the introduction of electron-releasing substituents at the Ph rings of the PR₃ (1c and 1d) or the benzyl units (1e and 1f), and endo complexes (3c-3f) were obtained in all cases. The palladation of MePh₂P=NCH₂Ph (1g) can be regioselectively oriented as a function of the solvent. The exo isomer [Pd(μ-Cl){C ₆H₄(CH₂N=PPh₂Me)-2}]₂ (exo-3g) is obtained in refluxing CH₂Cl₂, while endo-[Pd(μ-Cl){C,N-C₆H₄(PPh(Me)=NCH₂Ph)-2}] ₂ (endo-3g) can be isolated as a single isomer in refluxing toluene. In this case, the exo metalation is kinetically favored while an endo process occurs under thermodynamic control, as shown through the rearrangement of [Pd(μ-OAc)-{C₆H₄(CH₂N=PPh ₂Me)-2}]₂ (exo-2g) into [Pd(μ-OAc){C,N-C ₆H₄(P(Ph)Me=NCH₂Ph)-2}]₂ (endo-2g) in refluxing toluene. The preference for the endo palladation of 1a and the kinetic versus thermodynamic control in 1g has been explained through DFT studies of the reaction mechanism.