Stereospecific reaction of molecular halogens with palladacyclopentadienes containing bidentate nitrogen ligands to give 1,4-dihalo-1,3-dienes via palladium(IV) intermediates

A synthetic and computational study concerning the reactivity of palladacyclopentadienes containing bidentate nitrogen ligands toward dihalogens is described. The complexes 2,3,4,5-tetrakis(carbomethoxy)palladacyclopentadiene(NN) (1a-c; NN = 9,10-bis(phenylimino)-9,10-dihydrophenanthrene (phenyl-bip, a), bis(p-tolylimino)acenaphthene ((p-tolyl)-bian, b), 2,2'-bipyridine (bpy, c)) reacted with molecular dihalogens to give (EE)-1,4-dihalo-1,2,3,4-tetrakis(carbomethoxy)-1,3-butadiene and PdX2(NN) (X = Cl, Br, I). The palladacycles 1a,b react at 203-208 K with bromine to give the palladium(IV) species trans, cis, cis-(EE)-2,3,4,5-tetrakis(carbomethoxy)palladacyclopentadiene dibromide(NN) (3av and 3bv, respectively), which were observed by low-temperature H-1 NMR. Above 243 K reductive elimination took place and the divalent compounds (EE)-{1,2,3,4-tetrakis(carbomethoxy)-4-halo-1,3-butadienyl}-palladium(II) bromide(NN) (4av and 4bv) were cleanly obtained. Similar (sigma-1,3-dienyl)-palladium halide complexes 4au-4cw were prepared by reacting the palladacycles la-c with a stoichiometric amount of chlorine, bromine, or iodine. The energy profile obtained from DFT-B3LYP calculations, which have been carried out using [(HNCHCHNH)Pd(C-4-(CN)(4))] + Br-2 as a model system, show that this sequence of oxidative addition of molecular halogen to the palladacyclic compound, generating a Pd(IV) species, followed by reductive elimination with formation of a carbon-halogen bond is energetically feasible. The calculations also point to the possible involvement of a Pd(Br2) complex in the early stages of the reaction. Palladacycles 6 derived from hexafluorobutyne (E = CF3) reacted analogously, to give (EZ)-{1,2,3,4-tetrakis(trifluoromethyl-4-halo-1,3-butadienyl}palladium(II) halide(NN) compounds 7u-w after EZ isomerization of the Delta(3,4) alkene bond. When the (sigma-1,3-dienyl)-palladium halides 4au-4cw were reacted with an additional 1 equiv of another dihalogen, Y-2, it was found that an unsymmetric diene, (EE)-{1-X,4-Y}-1,2,3,4-tetrakis(carbomethoxy)-1,3-butadiene, was selectively formed, indicating that the intermediate Pd(IV) complex stereospecifically eliminates the (entering) apical halogen Y and the dienyl fragment. Finally, several ionic compounds 10bz, 11bz, and 11bz' were obtained by addition of silver triflate to solutions of 4bz in the presence of a noncoordinating solvent or isonitriles, respectively. The X-ray crystal structures of [1,2,3,4-tetrakis(carbomethoxy)-4-iodo-1,3-butadienylpalladium(II) iodide(sigma(2)N,N'-2,2'-bipyridyl)] (4cw) and of [1,2,3,4-tetrakis(carbomethoxy)-1,3-pentadienylpalladium(II)(tert-butyl isocyanide)((sigma(2)N,N-p-tolyl-bian)] trifluoromethanesulfonate (11bz) have been determined by X-ray diffraction.