Aryl iodides, oxidative addition palladium complexes

The oxidative addition of an aryl iodide to a zerovalent complex such as [Pd(PPh3)4] gives tra s-[Pd(Ar)I(PPh3)2] having a palladium-aryl and a palladium-iodide bond indeed, this is one of the oldest examples of oxidative addition of aryl iodide to Pd(0) complex (Eq. 1.1) [10]. 

The first step of the catalytic cycle proposed for this complex reaction consists of the oxidative addition of the aryl iodide reactant to palladium(O) (Scheme 19.11). Subsequent reversible insertion of norbornene into the Pd-Ar bond gives rise to complex 9, which undergoes base-mediated intramolecular C-H activation to give palladacycle 10. Oxidative addition of the primary alkyl iodide furnishes palladium(IV) complex 11, which, after reductive elimination and deinsertion of norbornene, generates a paUadium(II) species (12) able to undergo cross-coupling reactions such as the Mizorold-Heck coupling, as illustrated in Scheme 19.10. 

Substantially more work has been done on reactions of square-planar nickel, palladium, and platinum alkyl and aryl complexes with isocyanides. A communication by Otsuka et al. (108) described the initial work in this area. These workers carried out oxidative addition reactions with Ni(CNBu )4 and with [Pd(CNBu )2] (. In a reaction of the latter compound with methyl iodide the complex, Pd(CNBu )2(CH3)I, stable as a solid but unstable in solution, was obtained. This complex when dissolved in toluene proceeds through an intermediate believed to be dimeric, which then reacts with an additional ligand L (CNBu or PPh3) to give PdL(CNBu )- C(CH3)=NBu I [Eq. (7)]. 

Recently, Larock and coworkers used a domino Heck/Suzuki process for the synthesis of a multitude of tamoxifen analogues [48] (Scheme 6/1.20). In their approach, these authors used a three-component coupling reaction of readily available aryl iodides, internal alkynes and aryl boronic acids to give the expected tetrasubsti-tuted olefins in good yields. As an example, treatment of a mixture of phenyliodide, the alkyne 6/1-78 and phenylboronic acid with catalytic amounts of PdCl2(PhCN)2 gave 6/1-79 in 90% yield. In this process, substituted aryl iodides and heteroaromatic boronic acids may also be employed. It can be assumed that, after Pd°-cata-lyzed oxidative addition of the aryl iodide, a ds-carbopalladation of the internal alkyne takes place to form a vinylic palladium intermediate. This then reacts with the ate complex of the aryl boronic acid in a transmetalation, followed by a reductive elimination. 

Normally, the most practical vinyl substitutions are achieved by use of the oxidative additions of organic bromides, iodides, diazonium salts or triflates to palladium(0)-phosphine complexes in situ. The organic halide, diazonium salt or triflate, an alkene, a base to neutralize the acid formed and a catalytic amount of a palladium(II) salt, usually in conjunction with a triarylphosphine, are the usual reactants at about 25-100 C. This method is useful for reactions of aryl, heterocyclic and vinyl derviatives. Acid chlorides also react, usually yielding decarbonylated products, although there are a few exceptions. Likewise, arylsulfonyl chlorides lose sulfur dioxide and form arylated alkenes. Aryl chlorides have been reacted successfully in a few instances but only with the most reactive alkenes and usually under more vigorous conditions. Benzyl iodide, bromide and chloride will benzylate alkenes but other alkyl halides generally do not alkylate alkenes by this procedure. 

The first detailed study of the individual steps of the cationic pathway of the intramolecular Heck reaction was recently described by Brown (Scheme 8G.21) [46], Oxidative addition of aryl iodide 21.1 to [l,l -bis(diphenylphosphino)ferrocene](cyclooctatetraene)palladium generated 21,2. Complex 21.2 was stable at room temperature and was characterized by X-ray crystallography no interaction between the palladium center and the tethered alkene was observed in this intermediate. Treatment of 21.2 with AgOTf at -78°C removed iodide from the palladium coordination sphere, which facilitated a rapid alkene coordination and subsequent... 

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... 

Carbon—carbon coupling reactions of aryl halides are commonly catalyzed by palladium triarylphosphine complexes and proceed well for aryl bromides and iodides while aryl chlorides are generally unreactive. More basic chelating trialkylphosphines, however, render palladium sufficiently electrophilic to undergo rapid oxidative addition with chlorobenzene ... 

Brown and coworkers reported studies of the addition of aryl iodides to a series of palladium complexes of trialkylphosphines with the general formula Pd[(P(Cy) (Bu-f)3 )]2 (n = 0-3)181. They concluded that the complexes of the smaller phosphines (n = 2, 3) react with Phi through an associative mechanism and undergo oxidative addition directly to the PdL2 complex. They also concluded that oxidative addition of Phi to complexes of the bulkier phosphines (n = 0-1) proceed after dissociation of ligand to generate PdL. In unpublished work, Barrios-Landeros and Hartwig have found that the kinetic behavior of the reactions of these complexes is complex and that these reactions occur with profiles that are characteristic of autocatalysis. 

Cooper et al. reported that the cascade reaction of the palladium-catalyzed cyclization and the Barbier-type allylation of the 1,3-diene-aryl iodide 514, the aldehydes 515, and indium gave the heterocycles 516 in good yields (Scheme 154).220b The reaction proceeds through oxidative addition of a C—I bond of 514 to Pd(0) and subsequent insertion of a double bond of 517 to give the jr-allylpalladium intermediate 518. Transmetalation of the jr-allylpalladium 518 with indium leads to the allylindium complex 519, and the following reaction with the aldehydes 515 gives 516. 

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