The Formation of π-Allylpalladium Complexes from 3-Methylenecyclohexenes (s-trans Conjugated Dienes )

Abstract

π-Allylpalladium complexes were prepared from s-trans conjugated dienes, such as 3-methylenecyclohexene [7 a], and 6-methyl [7 b]1 6-isopropyl [7 c] and 6-t-butyl [7 d]analogs. The structures of the complexes were examined by ¹H and ¹³C NMR spectroscopy. Reactions of dienes with sodium tetrachloropalladate(II) in methanol at 0°C gave the exo-cyclic complexes [10], in which a methoxyl uoup is incorporated onto the endo-cyclic terminal carbon of the conjugated system. Endo-cyclic complexes, [11 c] and [11 d], in which a hydrogen is incorporated onto the exo-cyclic terminal of the diene were formed from [7 c] and [7 d], respectively, while [10 a] and [10 b] from [7 a] and [7 b] respectively, when the reaction was carried out in methanol at 50∼60°C. In tetrahydrofuran [7 b] was converted to [11 b]. The results indicate that π-allylpalladium complexes are formed directly from s-trans conjugated dienes. The hydrogen atom, which is incorporated onto the diene, is proved to come from the excess diene, which converts to the aromatic compound by double bond migration and dehydrogenation. The mechanism of complex formation is discussed.