Preparation, Structures, and Thermal Reactivity of Alkoxycarbonyl(cyano)palladium(II) Complexes trans-Pd(COOR)(CN)(PPh_3)_2 (R = Me, Et, ~nPr, ~iPr, ~nBu, ~tBu, and Bn) as Intermediates of the Palladium-Catalyzed Cyanoesterification of Norbornene Der

摘要

Alkoxycarbonyl(cyano)palladium(II) complexes trans-Pd(COOR)(CN)(PPh_3)_2 (1, R = Me; 2, R = Et; 3, R = ~nPr; 4, R = ~iPr; 5, R = ~nBu; 6, R = ~tBu; 7, R = Bn) are prepared via oxidative addition of the corresponding cyanoformates to Pd(PPh_3)_4 in toluene at room temperature or 50 °C and characterized by means of NMR (~1H, ~(13)C{~1H}, and ~(31)P{~1H}) and IR spectroscopy as well as elemental analyses. X-ray crystallography of 3, 4, and 6 showed a square-planar coordination around the Pd center that is bonded to alkoxycarbonyl and cyano ligands in the trans configuration. The Pd-CN and Pd-COOR bonds in 1, 3, 4, and 6 are similar to those of the cyanopalladium(II) and alkoxycarbonylpalladium(II) complexes reported to date. On reaction of Pd(PPh_3)_4 with phenyl cyanoformate a thermally induced ligand exchange takes place to afford the dicyanopalladium(II) complex, trans-Pd(CN)_2(PPh_3)_2 (8). Complex 2 is able to catalyze the reaction of norbornadiene with ethyl cyanoformate to produce (2R*,3S*)-ethyl 3-cyanobicyclo-[2.2.1]hept-5-ene-2-carboxylate (13) and is recovered after the reaction. This observation supports the identification of alkoxycarbonyl(cyano)palladium(II) complexes as intermediates in the catalytic cycle of cyanoesterification of norbornene derivatives.