Oxidative addition arylpalladium complexes

Only palladium- and platinum-containing heterocycles were described between 1995 and 2006. Mateo et a/, reported that iodoaryl stannane 70 reacts with Pd(PPh3)4 to afford palladacycle 71, as a result of an intramolecular Pd/Sn transmetallation of the intermediate oxidative addition arylpalladium(ll) complex (Equation 10) <1996CEJ1596>. 

Aromatic acyl halides and sulfonyl halides undergo oxidative addition, followed by facile elimination of CO and SO2 to form arylpalladium complexes. Benzenediazonium salts are the most reactive source of arylpalladium complexes. 

Oxidative addition of the sulfonyl chlorides 144 is followed by facile generation of SO2 to form arylpalladium complexes which undergo alkene inser-tion[112,113]. 

The mechanism of action of the cyanation reaction is considered to progress as follows an oxidative addition reaction occurs between the aryl halide and a palladium(O) species to form an arylpalladium halide complex which then undergoes a ligand exchange reaction with CuCN thus transforming to an arylpalladium cyanide. Reductive elimination of the arylpalladium cyanide then gives the aryl cyanide. 

Presumably, the oxidative cyclization of 3 commences with direct palladation at the a position, forming o-arylpalladium(II) complex 5 in a fashion analogous to a typical electrophilic aromatic substitution (this statement will be useful in predicting the regiochemistry of oxidative additions). Subsequently, in a manner akin to an intramolecular Heck reaction, intermediate 5 undergoes an intramolecular insertion onto the other benzene ring, furnishing 6. (i-Hydride elimination of 6 then results in carbazole 4. 

The reaction is initiated by oxidative addition of the halide to a palladium(O) species generated in situ from the Pd(II) catalyst. The arylpalladium(II) intermediate then forms a complex with the alkene, which rearranges to a a complex with carbon-carbon bond formation. The er-complex decomposes with regeneration of Pd(0) by /i-climination. 

The common feature of the first set of examples discussed is the coupling of an arylpalladium complex, formed in oxidative addition, with a five membered heterocyclic ring via the formal displacement of a hydrogen atom. This reaction, formally a Heck coupling, is often called the heteroaryl Heck reaction . 

Innovations in the chemistry of aromatic compounds have occurred by recent development of many novel reactions of aryl halides or pseudohalides catalysed or promoted by transition metal complexes. Pd-catalysed reactions are the most important [2,29], The first reaction step is generation of the arylpalladium halide by oxidative addition of halide to Pd(0). Formation of phenylpalladium complex 1 as an intermediate from various benzene derivatives is summarized in Scheme 3.1. 

Scheme 2 shows the mechanism generally accepted for the catalytic arylation of olefins with aryl iodides in the presence of a tertiary phosphine-coordinated palladium catalyst and a base (4). Oxidative addition of aryl iodide (Arl) to a Pd(0) species (A), which is most commonly generated from palladium diacetate and a tertiary phosphine ligand, forms an arylpalladium iodide complex (B). Coordination of olefin on B followed by insertion of the coordinated olefin into the Pd-Ar bond forms a a-alkylpalladium species (C), which undergoes p-hydrogen elimination reaction to give the arylation... 

Mechanistically, this new insertion-CI-Diels-Alder hetero domino sequence can be rationalized as follows (Scheme 64) After the oxidative addition of the aryl halide 115 or 118 to the in situ generated Pd(0) species the arylpalladium halide 120 intramolecularly coordinates and inserts into the tethered triple bond via a syn-carbopaUadation to furnish cyclized vinylpalladium species 121 with a p-acceptor substitution in a stereospecific fashion. Transmetallation of the in situ generated copper acetylide 122 gives rise to the diorganylpalladium complex 123 that readily undergoes a reductive elimination and liberates the electron poor vinylpropargylallyl ether 124. The triethylamine catalyzed propargyl-allene isomerization furnishes the... 

The synthesis of unsymmetrical biaryls 8 from two monoaryl species involves the coupling of a metallated aromatic molecule 6 with an aryl halide or triflate 4 under the action of palladium(O) catalysis. The reaction involves a catalytic cycle in which palladium(O) inserts into the C-halogen bond via an oxidative addition to generate an arylpalladium(II) species 5 (Scheme 10.18). This undergoes a trans-metallation with the metallated component, producing a biarylpalladi-um(II) complex 7. The biaryl product is formed by reductive elimination. In the process, Pd(0) is regenerated and this can then react with a second molecule of aryl halide. Pd(0) is therefore a catalyst for the reaction. 

The basic mechanism of the Heck reaction (as shown below) of aryl or alkenyl halides or triflates involves initial oxidative addition of a pal-ladium(O) species to afford a a-arylpalladium(II) complex III. The order of reactivity for the oxidative addition step is I > OTf > Br > Cl. Coordination of an alkene IV and subsequent carbon-carbon bond formation by syn addition provide a a-alkylpalladium(II) intermediate VI, which readily undergoes 3-hydride elimination to release the product VIII. A base is required for conversion of the hydridopalla-dium(II) complex IX to the active palladium(O) catalyst I to complete the catalytic cycle. 

The monoligated [PdL] species has been proposed many times as an intermediate in coupling reactions catalyzed by complexes of hindered monodentate ligand. Oxidative addition to this species would produce an arylpalladium halide complex with only... 

A plausible mechanism for the above reaction involves oxidative addition of the heteroaryl chloride to Pd(0) to provide Pd(II) intermediate 75, which subsequently inserts into benzoxazole to form the arylpalladium(II) complex 76. P-Hydride elimination of 76 would afford 11 with concomitant regeneration of Pd(0) for the next catalytic cycle. 

The syntheses of trans and cis isomers of unsymmetrical diorganopalladium(II) complexes have been reported [104]. For example, the reaction of arylpalladium(II) halides, which have been prepared either by the oxidative addition of aryl halides to palladium(O) phosphine complexes or by the treatment of diarylpalladium(II) with HCl, with 1 equivalent of methyllithium provides /ra x-PdAr(Me)L2 complexes (eq (75)). On the other hand, treatment of arylpalladium(II) halides with excess methyllithium followed by methanolysis of the resulting dimethyl(aryl)palladate intermediates leads to c/.v-PdAr(Me)L2 complexes (eq (76)). 

The palladation products exhibit reactivity similar to that of the arylpalladium complexes formed by oxidative addition of aryl halides to Pd(0) species, although the reactions are stoichiometric with respect to palladium. Representative examples include vinylation via an olefin insertion process (eq (88)) [119], double and single carbonylation (eq (89) and (90)) [120,121], and alkylation via a transmetallation process (eq (91)) [122]. 

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