Substituent Effects on the Stereoelectronic and Chemical Properties of a Novel Phosphapalladacycle

A novel phosphapalladacycle was prepared by direct cyclopalladation of the ligand (diphenylmethyl)diphenylphosphane ( 2 ). The corresponding μ‐chloro five‐membered phosphapalladacycle 3 was obtained by heating a toluene solution of 2 and Pd(OAc) 2 followed by chloride ion metathesis. An optical resolution procedure was then employed by separation of the ( S )‐prolinato derivatives 4 by fractional crystallization followed by treatment with dilute hydrochloric acid to yield both enantiopodes of the dimers 3 in the optically active forms. The current phosphapalladacycle was noted to display different properties with respect to the analogous phosphapalladacycle prepared from (diphenylmethyl)di‐ tert ‐butylphosphane. For instance, the phosphapalladacycle based on the (diphenylmethyl)di‐ tert ‐butylphosphane was shown to be conformationally rigid. In contrast, the analogous five‐membered phosphapalladacycle of phosphane 2 was noted to exist in both the δ and λ conformations in the solid state, while a flattened conformation was observed in solution. In addition, the (diphenylmethyl)di‐ tert ‐butylphosphane‐based phosphapalladacycle was noted to exhibit a much more improved regioselectivity towards ancillary ligands such as ( S )‐prolinate and triphenylphosphane than 2 . Furthermore, the μ‐chloro dimer 3 of the current palladacycle was shown to be lacking in thermodynamic stability of the Pd–C bond that was noted for the t Bu analogue. In contrast, the Pd–C bonds of dimer 3 were immediately ruptured in the presence of concentrated HCl to give rise to the binuclear complex 5 , in which the phosphane was coordinated in the η 1 ‐P coordination mode.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)