Stereospecific Reaction of Molecular Halogens with Palladacyclopentadienes Containing Bidentate Nitrogen Ligands To Give 1,4-Dihalo-1,3-dienes via Palladium(IV) Intermediates

Abstract

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 ∆(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.

Oxidative Addition of Phenylacetylene through CH Bond Cleavage To Form the MgII–dpp‐bian Complex: Molecular Structure of [Mg{dpp‐bian(H)}(CCPh)(thf)2] and Its Diphenylketone Insertion Product [Mg(dpp‐bian).−{OC(Ph2)CCPh}(thf)]

“…In conclusion, we have found that the magnesium complex with dianionic dpp-bian ligand [Mg(dpp-bian)] readily reacts with phenylacetylene in an oxidative addition manner through C À H bond cleavage to form alkynylmagnesium derivative [Mg{dpp-bian(H)}(C CPh)(thf) 2 ]. This for nontransition-metal complexes unprecedented reaction pathway became possible due to the high proton affinity of the dppbian ligand and its rigid bidentate bonding character.…”

Easy Entry into Reduced Ar‐BIANH2 Compounds: A New Class of Quinone/Hydroquinone‐Type Redox‐Active Couples with an Easily Tunable Potential

“…Compounds of the family bis(aryl)acenaphthenequinonediimine (Ar‐BIAN) ( 1 ; see Scheme ) have been known for some time,1–3 but have been brought to the general attention only in the 90s by Elsevier and his group 4. Since then, they have found widespread use mostly as ligands for transition metals and the corresponding complexes have been employed as catalysts for a wide variety of reactions, such as alkene hydrogenation,5 polymerization,6–11 copolymerization,12–18 aziridination,19 cyclopropanation,19 and epoxidation20 alkene‐CO copolymerization,21–24 alkyne coupling in the presence of halogens or organic halides and tin compounds25–27 or just tin compounds,28 selective semihydrogenation of alkynes29 and allenes,30 allylic aminations of olefins by nitroarenes in the presence of CO,31–33 the synthesis of pyrroles and oxazines from dienes, nitroarenes and CO,34 the reduction of nitroarenes to anilines by CO/H 2 O,35–37 the hydroamination of alkynes,38 the homo‐coupling of organic halides39 and their cross‐coupling reaction with organomagnesium, ‐zinc, and ‐tin reagents,40–44 the Suzuki–Miyaura cross coupling,45 the synthesis of 4‐quinolones and 2,3‐dihydroquinolones from 2′‐nitrochalcones and CO,46, 47 the synthesis of indoles from o ‐nitrostyrenes and CO,47, 48 the Heck arylation of olefins,49, 50 and oxidative Heck reaction,51, 52 the synthesis of dihydrofurans and dihydrobenzoxepines53 and N ‐acyl vinilogous carbamic acids,54 the cyclotrimerization of alkynes,55 the click reaction,56 the dimerization of vinyl ethers to acetals,57 and the diboration of α,β‐unsaturated carbonyl compounds 58. For some of these syntheses, the use of the Ar‐BIAN ligands was instrumental in achieving a high performance of the catalytic system.…”

Study on the electronic effects on stereoconservativity of Suzuki coupling in chiral groove of binaphthyl

“…Pt(IV)-complexes containing a biphenyl moiety similar to that in complex 3, were characterized [14]. Also, structurally relative Pd,Pd-dibromopallada(IV)cyclopentadiene species stabilized with an N,N-ligand were observed as intermediates [15].…”