Preparation, structures, and thermal reactivity of alkoxycarbonyl(cyano) palladium(II) complexes trans-Pd(COOR)(CN)(PPh₃)₂ (R = Me, Et, ⁿPr, ⁱPr, ⁿBu, ^tBu, and Bn) as intermediates of the palladium-catalyzed cyanoesterification of norbornene derivatives

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, ¹³C{¹H}), and ³¹P{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(PPh₃)₄ with phenyl cyanoformate a thermally induced ligand exchange takes place to afford the dicyanopalladium(II) complex, trans-Pd(CN)₂(PPh₃)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.