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In the presence of Pd(OAc)2 and norbornene as catalysts, two molecules of an ortho-substituted aryl iodide undergo aryl–aryl coupling to a palladium-bonded biphenylyl intermediate, which is hydrogenolyzed by benzyl alcohol to give 2,3′-disubstituted biaryl derivatives in satisfactory yields. A curious behaviour was observed with o-chloroiodobenzene, which gave activation of the C—Cl bond with four-membered ring formation.Key words: biphenyls, aryl–aryl coupling, multistep reactions, palladium, palladacycles, C-H activation, catalysis.

In the presence of Pd(OAc) 2 and norbornene as catalysts, ortho-substituted aryl iodides and terminal olefins react to afford selectively substituted ortho-vinylbiphenyl derivatives in good yields. The reaction proceeds through a series of steps including norbornene insertion and C-H activation with formation of pallada-cycles able to promote arylation of their aromatic part. Norbornene deinsertion then leads to the selective formation of a palladium-bonded biphenylyl group able to react with terminal olefins.

The palladium/norbornene-catalyzed sequential one-pot-reaction of aryl iodides, aryl bromides and aryl boronic acids affords o-teraryls in high yields.

Pd(II) metallacyclic complexes are key intermediates in sequential reactions in which three new C−C bonds are formed in an aromatic ring. The Pd(II) metallacyclic complex 10 with phenanthroline as ligand undergoes oxidative addition to benzyl bromide or chloride in DMF to generate benzyl-Pd(IV) complexes. The rate constant of the oxidative addition is determined by means of electrochemical techniques, with the expected reactivity order PhCH2Br (k1Br = 3.6 M−1 s−1) > PhCH2Cl (k1Cl = 6 × 10−3 M−1 s−1) at 29 °C. When the benzyl-Pd(IV) complex is generated from PhCH2Br, the oxidative addition is followed by a slow C−C reductive elimination, which gives a Pd(II) complex. The rate constant of the reductive elimination has been determined in DMF (k2Br = 6 × 10−4 s−1) at 29 °C.

Alkylaromatic palladacycles, formed in situ by reaction of palladium acetate with a strained and rigid cycloolefin such as norbornene and an orthosubstituted aryl iodide, allow the selective arylation of the aromatic ring by a second molecule of aryl iodide with subsequent norbornene expulsion. Coupling the resulting complex with appropriate reagents liberates the organic product thus regenerating the catalyst.

A novel reaction sequence leading to the synthesis of substituted biphenyls containing a carbonyl group in an aliphatic chain has been achieved in one-pot reaction starting from iodoarenes and allylic alcohols under the catalytic action of palladium and norbornene. The latter is temporarily incorporated into a palladacycle, which directs the reaction towards the selective formation of an arylaryl bond. Norbornene spontaneously deinserts to allow the biphenylylpalladium bond thus formed to react in its turn with the allylic alcohol.

Selectively substituted o-teraryls are obtained by palladium/norbornene-catalyzed reaction of aryl iodides, aryl bromides and aryl boronic acids in ordered sequence; high selectivity is attained thanks to the addition of diethyl maleate acting as palladium ligand.