Metal catalyzed, arylation

For recent reviews on transition metal-catalyzed aryl amine formation, see ... 

DoM-transition metal catalyzed aryl-aryl cross coupling symbiosis ... 

The DoM-Transition Metal Catalyzed Aryl-Aryl Cross-Coupling Symbiosis... 

Thus, the transition metal catalyzed arylation reactions of amines and alcohols would constitute powerful tools for synthetic chemists. We have been developing practical procedures for the palladium-catalyzed arylation of amines and alcohols with aryl halides or sulfonates. During the course of our investigations [9] as well as those of John Hartwig and co-workers [10] the substrate scope of these transformations has been incrementally expanded. With each cycle of this catalyst improvement process, advances in mechanistic understanding and ligand design have also been made. 

As a matter of fact the photochemically generation of the triplet phenyl cation is synthetically equivalent to activation by metal catalysis that likewise produces a sort of phenyl cation complexes to the metal by oxidative addition of the aryl halide and elimination of the halide anion (see above). In fact, a photochemical parallel has been found for most type of metal catalyzed arylations, via reactions that occur from the same or similar reagents. 

Over the past decade, significant advances have been made in the transition-metal-catalyzed aryl-aryl bond formation through C-H activation (for reviews see [23-32]). In this direct arylation reaction one of the preactivated reaction partners is replaced by a simple arene. This direct arylation process is not only just of academic interest but also attractive for industrial applications, since only one preactivated reaction partner is needed. Obviously, the cost of the reaction will be reduced by... 

With the success of transition-metal-catalyzed aryl-aryl cross-coupling reactions through C-H activation of one of the coupling partners, obviously, a more economic and attractive alternative is the coupling reaction via double C-H activation of two arenes, neither of which needs to be preactivated (Scheme 5). However, the development of such a process is very challenging because many difficulties remain to be solved. For instance, the control of selectivity when two coupling partners have more than one type of aromatic C-H bond is a challenge. 

Scheme 4 Transition-metal-catalyzed aryl-aryl bond formation through C-H activation...

An only partial execution of the aryl-aryl bridging approach at each second phenylene-phenylene linkage gives rise to PR PFs have been first synthesized by Yoshino [29,30] by the oxidative coupling of 9,9-dialkylfluorene monomers with FeCls- However, these initial products were characterized by an ill-defined chemical structure since the coupling did not proceed regioselectively at the 2- and 7-positions of the fluorene core. Later on, more effective transition metal-catalyzed aryl-aryl coupling methods have been also applied for the PF synthesis. 

Metal-Catalyzed Arylations of Nonactivated Alkyl (Pseudo) Halides via Cross-Coupling Reactions... 

The vast majority of methods are based on the use of palladium complexes as catalysts, although copper, ruthenium, rhodium and iridium catalysts have also been used. Progress in the understanding of the mechanisms of these reactions has only been made during the past few years. As comprehensive reviews have been recently published on aryl-aryl bond-formation reactions, covering both mechanistic and synthetic aspects of these reactions [3-7], in this chapter we wiU summarize only those mechanishc studies on metal-catalyzed arylation reactions that have been carried out in detail. 

Abstract This chapter focuses on recent developments in metal-free and metal-catalyzed arylations with diaryliodonium salts (diaryl-X -iodanes). Synthetic routes to diaryliodonium salts are briefly described, and chemoselectivity trends with unsymmetric iodonium salts are discussed. 

The arylation of aliphatic amines has been difficult under metal-free conditions, but intramolecular arylations using aminoalkyl-substituted iodonium salts to yield indolines were viable (Scheme 5b) [94]. The groups of Olofsson and Adolfsson recently accomplished a metal-free arylation of secondary amides at room temperature (Scheme 5c) [95], A chemoselectivity study revealed a substantial orthoeffect, allowing selective transfer of a TRIP moiety and providing access to sterically hindered tertiary amides that are inaccessible by metal-catalyzed arylations. 

Kang and coworkers developed a number of metal-catalyzed arylations with diaryliodonium salts under mUd reaction conditions at the turn of the century, including the Pd-catalyzed arylation of secondary amines at room temperature and the Cu-catalyzed arylation of amines, anilines, amides, imidazoles, and triazoles at room temperature to 50 °C [98, 99]. The copper-catalyzed N-arylation of indoles, on the other hand, required heating to 150 °C in DMF [100], and makes an interesting comparison with the recently developed C-arylation of the same substrates (see Sect. 4.2.2). 

Scheme 16 Products in metal-catalyzed arylations of arenes with directing groups...

Scheme 17 Metal-catalyzed arylations of heteroaromatic compounds (DMEDA = W, N -dlmethylethylenediamlne)...

Scheme 19 Metal-catalyzed arylations involving rearrangements or cyclic iodonium salts...

With the development of efficient synthetic routes to diaryliodonium salts, these hypervalent iodine compounds have become easily available and interesting alternatives to other arylation reagents. Their low toxicity, high stability, and high reactivity are attractive features which enable difficult transformations without the need for excess reagents or high temperatures. This chapter has summarized recent developments in metal-free and metal-catalyzed arylations of heteroatom nucleophiles and carbon-centered nucleophiles. The boundaries of the field have moved forward considerably in the last decade, and many more applications are foreseen based on the deepened mechanistic understanding of arylations under both metal-free and metal-catalyzed conditions. 

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