Substituent effects on the stereoelectronic and chemical properties of a novel phosphapalladacycle

Article abstract of DOI:10.1002/ejic.200700171

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 tBu 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 η¹-P coordination mode. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.