PREPARATION AND REACTIONS OF PALLADIUM(II) COMPLEXES WITH C2 BONDED HETEROAROMATIC LIGANDS trans-N)(PPh3)2> (RN = 2-PYRYDYL, 2-PYRAZYL, 2-PYRYMIDYL GROUP). A NEW REACTION PATHWAY IN THE INSERTION OF ISOCYANIDES INTO THE Pd-C BOND OF trans- COMPOUNDS

Article abstract of DOI:10.1016/0022-328X(86)80139-5

The complexes trans-N)(PPh3)2> (I) N = 2-pyridyl (2-py), 2-pyrazyl (2-pyz), 2-pyrimidyl (2-pym) group> have been prepared in high yield by deprotonation with NEt3 of the corresponding cationic compounds trans-NH)(PPh3)2>⁺ (R_NH = N-protonated C²-heteroaromatic ligand) in the presence of an excess of PPh3.In chlorinated solvents, complexes I undergo a slow reversible dimerization into the binuclear derivatives N)(PPh3)>2 (II) (μ-R_N = C², N¹-bridging ligand).From the ³¹P NMR spectra in 1,2-dichloroethane the following dissociation constants were obtained: 1.9 mol l^-1 (R_N = 2-py), 5.1 x 1E-2 (2-pym), 6.6 x 1E-3 (2-pyz).The dimerization becomes fast and quantitative if the PPh3, involved in the equilibrium is removed by oxidation or by reaction with 3-2-MeC3H4)>2.Only the pyridyl complex Ia reacts (slowly) with CO yielding the migratory insertion product trans-(PPh3)2>, together with dimer IIa.All the complexes I undergo migratory insertion of t-butylisocyanide with formation of trans-N)=NCMe3>(PPh3)2> at rates which depend on the heterocyclic group (R_N = 2-py > 2-pyz >> 2-pym).The reaction of the 2-pyrazyl complex Ib with CNCMe3 has been studied in detail by conductivity measurements and by IR and ³¹P NMR spectroscopy.The data suggest a complex mechanism in which insertion occurs through rearrangement of a four-coordinate intermediate , and through interaction of a cationic intermediate trans-⁺ (Vb) with Cl^- and with the free isocyanide of the initial equilibria.The occurence of the latter reactions is confirmed by independent experiments in which the cationic complex Vb (isolated as perchlorate salt) is treated with an equimolar amount of Cl or CNCMe3.The isocyanide-promoted insertion step represents a new mechanistic pathway for isocyanide insertion into the Pd-C bond of trans- complexes.