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 organostannane s interrupted at the reductive elimination step has been applied to th e synthesis of oxa- and azapalladacycles with the general formula cis- [PdArR(L)(2)]. The synthesis of oxapalladacycles was achieved under mi ld conditions by reaction of 2-iodo- or 2-bromophenyloxymethylstannane s with [Pd(PPh(3))(4)]. The synthesis of an aza analogue was similarly carried out from the corresponding 2-iodoaniline derivative. One of t he substituted oxapalladacycles rearranged to release steric strain be tween the palladium and a chloride substituent on the aryl ring, an is omerization promoted by traces of water. In one case, the arylpalladiu m(II) intermediate of oxidative addition was isolated by using a palla dium(0) complex with a bidentate diphosphane. A variety of new pallada cycles, including complexes with weakly coordinating ligands, were pre pared by ligand substitution. Reaction of the palladacycles with dimet hyl acetylenedicarboxylate led to the formation of chromenes or dihydr oquinolines by insertion followed by reductive elimination.