Spontaneous Multiple Insertion of a Bulky Aromatic Isocyanide into the Palladium−Hydride Bond of trans-[Pd(H)Cl(PPh3)2], Leading to Formation of Heterobicyclic and Pyrrole Compounds

The reaction of [PdCl2(PPh3)2] (1) with an excess of 2,6-xylyl isocyanide (XylNC) and H2SiMePh in refluxing toluene gave a mononuclear palladium complex of a polyimino ligand, [Pd{C(NR)C(NR)C(NR)C(NR)CH(NR)}(C⋮NR)Cl]· 1/2C6H6 (3·1/2C6H6: R = Xyl, 30%), and a novel heterobicyclic compound, 2,6-(2‘,6‘-xylyl)2-3,7-(N-2‘,6‘-xylylimino)2-4,8-(N-2‘,6‘-xylylamino)2-2,6-diazabicyclo[3.3.0]octa-4,8-diene (4a, 15%), structures of which were determined by X-ray analyses. When the reaction was carried out in the presence of triethylamine, the yield of 4a increased up to 44%. The complex 3 involves a pentaimino moiety [−C(NR){C(NR)}3CH(NR)] derived from a successive insertion of isocyanide into the palladium−hydride bond of trans-[Pd(H)Cl(PPh3)2] (2), which was generated from the reaction of 1 and H2SiMePh in situ. The pentaimino ligand attaches to the metal through the carbon atom of the α-imino group and the lone pair electron of the γ-imino nitrogen atom, resulting in a five-membered chelate ring. The reaction of 3 with XylNC in the presence of H2SiMePh afforded the heterobicyclic compound 4a. Similar treatments of 3 with 2,4,6-mesityl isocyanide (MesNC) and carbon monoxide (∼80 kg cm-2) in the presence of H2SiMePh gave 2,6-(2‘,6‘-xylyl)2-3-(2‘,4‘,6‘-mesitylimino)-7-(2‘,6‘-xylylimino)-4,8-(2‘,6‘-xylylamino)2-2,6-diazabicyclo[3.3.0]octa-4,8-diene (5) and 2,6-(2‘,6‘-xylyl)2-7-(2‘,6‘-xylylimino)-4,8-(2‘,6‘-xylylamino)2-2,6-diazabicyclo[3.3.0]octa-4,8-dien-3-one (6), respectively. The structure of 6 was confirmed by X-ray crystallography. When complex 3 was treated with HC⋮CPh, a pyrrole compound, 1-(2‘,6‘-xylylamino)-2-phenyl-4-R‘-pyrrole (R‘ = C(NR){C(NR)}2CH(NR), R = Xyl) (7), was obtained. The structure of 7·C6H6 was confirmed by X-ray crystallography. Possible mechanisms for 4−7 were proposed.