Theoretical studies of the oxidative addition of PhBr to Pd(PX3)2 and Pd(X2PCH2CH2PX2) (X = Me, H, Cl)

The density functional theory calculations were used to study the influence of the substituent at P on the oxidative addition of PhBr to Pd(PX3)(2) and Pd(X2PCH2CH2PX2) where X = Me, H, Cl. It was shown that the C-ipso-Br activation energy by Pd(PX3)(2) correlates well with the rigidity of the X3P-Pd-PX3 angle and increases via the trend X = Cl < H < Me. The more rigid the X3P-Pd-PX3 angle is, the higher the oxidative addition barrier is. The exothermicity of this reaction also increases via the same sequence X = Cl < H < Me. The trend in the exothermicity is a result of the Pd(II)-PX3 bond strength increasing faster than the Pd(0)-PX3 bond strength upon going from X = Cl to Me. Contrary to the trend in the barrier to the oxidative addition of PhBr to Pd(PX3)(2), the C-ipso-Br activation energy by Pd(X2PCH2CH2PX2) decreases in the following order X = Cl > H > Me. This trend correlates well with the filled d(pi) orbital energy of the metal center. For a given X, the oxidative addition reaction energy was found to be more exothermic for the case of X2PCH2CH2PX2 than for the case of PX3. This effect is especially more important for the strong electron donating phosphine ligands (X = Me) than for the weak electron donating phosphine ligands (X = Cl). (c) 2007 Elsevier B.V. All rights reserved.