BIS(2-PYRIDYL)SILANE LIGANDS .2. STRUCTURAL CHARACTERIZATION AND CATALYTIC APPLICATIONS OF PALLADIUM COMPLEXES IN ORGANOSTANNANE CROSS-COUPLING REACTIONS

The synthesis and characterization of [R1R2Si(2-pyridyl)]PdCl2 (R1 = Me, R2 = phenyl; 3) are presented. Complex 3 crystallizes in the monoclinic space group P21/n (Z = 4) with β = 99.98 (1)° and a = 12.208 (2) Å, b = 12.651 (3) Å, c = 14.079 (2) Å. The single-crystal study showed the six-membered chelate ring to be in a boat conformation with the palladium and silicon atoms as the apexes out of plane. Cross-coupling reactions of organostannanes catalyzed by [Me2Si(2-pyridyl)]PdCl2 (2) were examined. Vinyltributylstannane undergoes cross coupling in high yield when treated with benzoyl chloride and 1 mol % of 2 in dichloromethane, whereas treatment with iodobenzene results in formation of [Me2Si(2-pyridyl)PdI2 (4) with only a minor amount of cross coupling observed. A solvent change to dimethylformamide, an increase in the reaction temperature, and the addition of lithium chloride or sodium iodide gave clean cross coupling with a relative rate 3 times that of (Ph3P)2PdCl2. Lithium chloride eliminated homocoupling of the vinylstannane partner, whereas addition of sodium iodide accelerated the reaction but still allowed homocoupling of the stannane partner (~ 10%). The addition of LiCl did not affect the cross-coupling reaction when (PPh3)2PdCl2 was used as the precatalyst. When trimethyl(phenylethynyl)stannane was used, the addition of lithium chloride did not decrease homocoupling of the organostannane reagent for precatalyst 2, whereas for (Ph3P)2PdCl2 homocoupling of the stannane partner was not observed with or without added lithium chloride. Attempts to employ (E)-1,2-[R1IC═CIR2] (R1 = R2 = H; R1 = H, R2 = Ph; R1 = R2 = Ph) in the cross-coupling reaction with trimethyl(phenylethynyl)stannane resulted in complete homocoupling of the stannane reagent and conversion of the (E)-diiodoalkenyl compounds to acetylenes (R1C≡CR2). © 1990, American Chemical Society. All rights reserved.