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 ( E,E )-1,4-dihalo-1,2,3,4-tetrakis(carbomethoxy)-1,3-butadiene and PdX 2 (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- ( E,E )-2,3,4,5-tetrakis(carbomethoxy)palladacyclopentadiene dibromide(NN) ( 3av and 3bv, respectively), which were observed by low-temperature 1 H NMR. Above 243 K reductive elimination took place and the divalent compounds ( E,E )-{1,2,3,4-tetrakis(carbomethoxy)-4-halo-1,3-butadienyl}palladium(II) bromide(NN) ( 4av and 4bv ) were cleanly obtained. Similar (σ-1,3-dienyl)palladium halide complexes 4au − 4cw were prepared by reacting the palladacycles 1a − 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(Br 2 ) complex in the early stages of the reaction. Palladacycles 6 derived from hexafluorobutyne (E = CF 3 ) reacted analogously, to give ( E,Z )-{1,2,3,4-tetrakis(trifluoromethyl-4-halo-1,3-butadienyl}palladium(II) halide(NN) compounds 7u − w after E,Z isomerization of the Δ(3,4) alkene bond. When the (σ-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, ( E,E )-{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(σ 2 N, N ‘-2,2‘-bipyridyl)] ( 4cw ) and of [1,2,3,4-tetrakis(carbomethoxy)-1,3-pentadienylpalladium(II)( tert -butyl isocyanide)(σ 2 N, N ‘- p -tolyl-bian)] trifluoromethanesulfonate ( 11bz ) have been determined by X-ray diffraction.