Isolation of Transmetalation Intermediates in the Stille Cross‐Coupling Reaction of Stannanes: Synthesis of Palladacycles, Ligand Substitution, and Insertion Reactions

A strategy based on a Stille cross‐coupling reaction of organostannanes interrupted at the reductive elimination step has been applied to the synthesis of oxa‐ and azapalladacycles with the general formula cis‐[PdArR(L)2]. The synthesis of oxapalladacycles was achieved under mild conditions by reaction of 2‐iodo‐ or 2‐bromophenyloxymethylstannanes with [Pd(PPh3)4]. The synthesis of an aza analogue was similarly carried out from the corresponding 2‐iodoaniline derivative. One of the substituted oxapalladacycles rearranged to release steric strain between the palladium and a chloride substituent on the aryl ring, an isomerization promoted by traces of water. In one case, the arylpalladium(II) intermediate of oxidative addition was isolated by using a palladium(0) complex with a bidentate diphosphane. A variety of new palladacycles, including complexes with weakly coordinating ligands, were prepared by ligand substitution. Reaction of the palladacycles with dimethyl acetylene‐dicarboxylate led to the formation of chromenes or dihydroquinolines by insertion followed by reductive elimination.