Preparation of Dinuclear Phosphine-Bridged Palladium(II) Species and Their Silica-Bound Analogues as Catalysts for the Cyclization of Aminoalkynes

Abstract

Two new dinuclear palladium complexes, [Pd 2 Me 2 Cl 2 (dmpm) 2 ] ( 4 ) and [PdMe(Cl)(dmpm) 2 PdCl 2 ] ( 5 ), have been prepared and their structures determined by single-crystal X-ray analysis. 5 is an interesting example of a complex with both a cis -PdCl 2 center and a trans- MePdCl center, within the same molecule. The reactivity of 4, as well as the previously reported complexes [Pd 2 Me 2 Cl 2 (dppm) 2 ] ( 1 ), [Pd 2 ( μ -CH 2 )Cl 2 (dppm) 2 ] ( 2 ), and [Pd 2 Me 4 (dmpm) 2 ] ( 3 ), toward silica have been investigated. The complexes react readily with either dichloromethane or benzene suspensions of silica to give chemisorbed species, presumably by reaction of the Pd−Me group with the O−H bond of the surface silanol groups, in the case of 1, 3, and 4, and protonation of the methylene bridge, in the case of 2 . The complexes containing the more basic phosphine, dmpm, gave much higher palladium loadings (wt % Pd 1.37−2.47) on silica, compared to the dppm complexes (wt % Pd 0.55−0.80). The molecular and grafted materials were initially tested as catalysts for the cyclization of 6-aminohex-1-yne to give 2-methyl-1,2-dehydropiperidine. The silica/dmpm complexes were found to be the most active catalysts for this reaction and were also tested for the cyclization of 5-phenyl-4-pentyn-1-amine.