Palladium carbocyclic aromatics

Intramolecular arylation of G-H bonds gives cyclic aromatic compounds. In this intramolecular arylation, the carbon-palladium cr-bond is first formed by the oxidative addition of Pd(0) species and then the resulting electrophilic Pd(n) species undergoes the intramolecular G-H bond activation leading to the formation of the palladacycle, which finally affords the cyclic aromatic compounds via reductive elimination.87 For example, the fluoroanthene derivative is formed by the palladium-catalyzed reaction of the binaphthyl triflate, as shown in Scheme 8.88 This type of intramolecular arylation is applied to the construction of five- and six-membered carbocyclic and heterocyclic systems.89 89 89 ... 

The catalytic dehydrogenation of dicyclohexeno-4-methyl-4f/-thiopyrans 340 (R = H, Me) on palladium-charcoal or with sulfur led to aromatization of both carbocycles and only dibenzo derivatives 426 (R = H, Me) were obtained.95,102... 

The carbocyclic ring in dihydrobenzodipyrazoles, benzodithiophenes, and benzodipyrroles may be aromatized by palladium on carbon <84IJC(B)918,87JOC1521,91H(32)4l>. 

Palladium chemistry involving heterocycles has many unique characteristics stemming from the inherently different structural and electronic properties of heterocyclic molecules in comparison to the corresponding aromatic carbocycles. One salient feature of heterocycles is the marked activation at positions a and y to the heteroatom. For N-containing heterocycles, the presence of the N-atom polarizes the aromatic ring, thereby activating the ex and y positions, making them more prone to nucleophilic attack. For example, the order of S Ar displacement of heteroaryl hahdes with EtO is [3] ... 

Internal alkynes will also readily undergo palladium-catalyzed annulation by functionally substituted aromatic or vinylic halides to afford a wide range of heterocycles and carbocycles. However, the mechanism here appears to be quite different from the mechanism for the annulation of terminal alkynes. In this case, it appears that the reaction usually involves (1) oxidative addition of the organic halide to Pd(0) to produce an organopalladium(II) intermediate, (2) subsequent insertion of the alkyne to produce a vinylic palladium intermediate, (3) cyclization to afford a palladacycle, and (4) reductive elimination to produce the cyclic product and regenerate the Pd(0) catalyst (Eq. 28). 

Based on a transformation described by Catellani and coworkers [80], Lautens s group [81] developed a series of syntheses of carbocycles and heterocycles from aryl iodide, alkyl halides and Mizoroki-Heck acceptors. In an early example, the authors described a three-component domino reaction catalysed by palladium for the synthesis of benzo-annulated oxacycles 144 (Scheme 8.37). To do so, they used an m-iodoaryl iodoalkyl ether 143, an alkene substimted with an electron-withdrawing group, such as t-butyl acrylate and an iodoalkane such as -BuI in the presence of norbomene. It is proposed that, after the oxidative addition of the aryliodide, a Mizoroki-Heck-type reaction with nor-bornene and a C—H activation first takes place to form a palladacycle PdCl, which is then alkylated with the iodoalkane (Scheme 8.37). A second C—H activation occurs and then, via the formation of the oxacycle OCl, norbomene is eliminated. Finally, the aryl-palladium species obtained reacts with the acrylate. The alkylation step of palladacycles of the type PdCl and PdCl was studied in more detail by Echavarren and coworkers [82] using computational methods. They concluded that, after a C—H activation, the formation of a C(sp )—C(sp ) bond between the palladacycle PdCl and an iodoalkane presumably proceeds by oxidative addition to form a palladium(IV) species to give PdC2. This stays, in contrast with the reaction between a C(sp )—X electrophile (vinyl or aromatic halide) and PdCl, to form a new C(sp )—C(sp ) bond which takes place through a transmetallation. 

Carbocyclization of aromatic iodides, bicychc aUcenes, and benzynes involving a palladium-catalyzed C-H bond activation as a key step. 

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