Cross-coupling reactions carbon-heteroatom bonds

Recently, interest in copper-catalyzed carbon-heteroatom bond-forming reactions has shifted to the use of boronic acids as reactive coupling partners [133], One example of carbon-sulfur bond formation is displayed in Scheme 6.65. Lengar and Kappe have reported that, in contrast to the palladium(0)/copper(l)-mediated process described in Scheme 6.55, which leads to carbon-carbon bond formation, reaction of the same starting materials in the presence of 1 equivalent of copper(II) acetate and 2 equivalents of phenanthroline ligand furnishes the corresponding carbon-sulfur cross-coupled product [113]. Whereas the reaction at room temperature needed 4 days to reach completion, microwave irradiation at 85 °C for 45 min in 1,2-dichloroethane provided a 72% isolated yield of the product. 

The transition metal catalyzed synthesis of seven membered and larger heterocycles attracted considerably less attention than the preparation of their five and six membered analogues. Typical examples in this chapter include the formation of heterocycles in insertion reactions, or through carbon-heteroatom bond formation. Although the formation of some macrocyclic natural products was also achieved in cross-coupling reactions they will not be discussed in detail. 

From reading the other sections of this book, one readily sees that palladium complexes serve as catalysts for a variety of C—C bond-forming cross-coupling processes. This cross-coupling chemistry involves the metal-catalyzed reactions between nucleophiles and electrophiles that display a wide variety of steric and electronic properties. It is, therefore, surprising that carbon-heteroatom bond formation by cross-coupling processes lay close to dormant until roughly five years ago. 

As illustrated by other chapters of this book, a diverse array of pincer and pincer-type ligands are known in the literature and can be incorporated to make new nickel complexes. Their catalytic activity in various cross-coupling reactions is likely to be the focus of future research. Another research direction that deserves more attention is carbon-heteroatom bond-forming reactions very few nickel pincer complexes have been studied for these catalytic applications. 

Although not a direct N—H bond arylation, a related new reductive protocol for the amination of arylboronic acids has been developed which utilizes nitroso arenes. This reaction is certainly worthy of inclusion here, as it broadens our understanding of the cross-coupling processes of heteroatom-carbon bonds via N—O bonded species [61]. These new protocols rely on either stoichiometric amounts of CuCl (Scheme 4.23) or on catalytic amounts of copper(II) methylsalicy-late (CuMeSal) (Scheme 4.24). The need for stoichiometric amounts of a copper(I)... 

CROSS-COUPLING REACTIONS THAT FORM CARBON-HETEROATOM BONDS... 

Cross-Coupling Reactions that Form Carbon-Heteroatom Bonds... 

Recent renaissance in Ulmann chemistry has opened new opportunities for successful implementation of novel cross-coupling approaches, especially useful for carbon-heteroatom bond formation under mild conditions with copper compounds. Numerous publications have appeared dealing with thiolation and selenation of aryl-and alkenyl halides using various Cu complexes as catalysts. A high reaction temperature of 200-300 °C [55] was significantly reduced, to 100 °C and less. Therefore, a simplified reaction technique and cheaper solvents may be utilized in synthetic procedures. Lower temperature was also of much importance to avoid side reactions. 

D.v. Carbon-Heteroatom Bond-Formation Cross-Coupling Reactions... 

During the past few decades, transition metal-catalyzed cross-coupling reactions have become a powerful tool for the construction of C—C and C-heteroatom bonds [1]. This strategy allows the conceptually simple and yet powerful and reliable approach for synthesizing structurally complex pharmaceuticals and biologically active molecules. The two most vastly used transition metal catalysts in carbon-heteroatom bond formation are palladium (mainly depends on its ancillary ligands) and copper (depends on the optimization of the catalytic system as a whole copper source, solvent, base, concentrations, etc.). Besides, considerable developments have also been made with other transition metal catalysts such as nickel, iron, etc. 

Some alkenes with heteroatoms can be used in cross-coupling reactions. Vinyl boranes are an important group, as the carbon-boron bond can subsequently be converted into so many other functional groups, such as halides (Scheme 8.97). ° An example of cross-metathesis of a vinyl borane, followed by Suzuki coupling, can be found in Scheme 11.40. An example of a vinyl silane metathesis can be found in Scheme 2.110. 

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