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 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-(E,E)-2,3,4,5-tetrakis(carbomethoxy)palladacyclopentadiene dibromide(NN) (3av and 3bv, respectively),
which were observed by low-temperature 1H 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(C4(CN)4)] + Br2 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 (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,
Y2, 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(σ2N,N‘-2,2‘-bipyridyl)] (4cw) and of [1,2,3,4-tetrakis(carbomethoxy)-1,3-pentadienylpalladium(II)(tert-butyl isocyanide)(σ2N,N‘-p-tolyl-bian)] trifluoromethanesulfonate (11bz) have been determined by X-ray diffraction.