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

The orientation of the orthopalladation of iminophosphoranes R3P=NCH2Aryl (R=Ph, Aryl=Ph (1a), C6H(4)-2-Br (1b), C6H4-Me-2 (1e), C6H3-(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(micro-Cl){C,N-C6H4(PPh2=NCH2Ph)-2}]2 (3a), while exo-[Pd(micro-Br){C,N-C6H4(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(micro-OAc){C,N-C6H4(CH2N=PPh3)-2}]2 (2b) into endo-[Pd(micro-OAc){C,N-C6H4(PPh2=NCH2Ph)-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(micro-Cl){C6H4(CH2N=PPh2Me)-2}]2 (exo-3g) is obtained in refluxing CH2Cl2, while endo-[Pd(micro-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(micro-OAc){C6H4(CH2N=PPh2Me)-2}]2 (exo-2g) into [Pd(micro-OAc){C,N-C6H4(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.