Carbon transition-metal-catalyzed cross-coupling

Recent trend in the synthesis of olefinic pheromones is the use of transition metal-catalyzed cross coupling reaction for carbon-carbon bond formation. Scheme 8 summarizes a synthesis of the termite trail marker pheromone, (3Z,6Z)-3,6-dodecadien- l-ol (2) by Oehlschlager [19]. The key-step is the palladium-catalyzed cross-coupling of allylic chloride A and alkenylalane B. 

Transition-metal-catalyzed cross-coupling reactions also represent a powerful approach for the construction of carbon-carbon bonds. As a result, these processes have been widely studied in the past few decades. Among the transition metals... 

To make the transformation even more useful, different carbon electrophiles should be connected sequentially in a stepwise manner. For this purpose, a transition-metal-catalyzed cross-coupling reaction opened the way. As shown in Scheme 22, cinnamyl chloride is treated with bis(iodozincio)methane (3) in the presence of palladium catalyst with various phosphine ligands. Phosphine ligands, having an electron-withdrawing group, such as tris[3,5-bis(trifluoromethyl)phenyl]phosphine and tris(2-furanyl)phosphine, show excellent results47. 

Transition metal-catalyzed cross-coupling is now recognized to be one of the most powerful carbon-carbon bond-formation reactions [1], The palladium-catalyzed coupling of aryl halides or their synthetic equivalents, for example aryl triflates, with arylmetals is very often employed in the synthesis of biaryl molecules, whose skeletons are found in a wide range of important compounds including natural products and organic functional materials [1-3]. 

Selective cross-coupling reactions between C(sp ) and C(5p ) centers had been one of the most difficult tasks in carbon-carbon bond synthesis until the early 1970s, when it was first reported that iron, nickel, palladium and copper - catalysts are extremely effective for cross-coupling of Grignard reagents with organic halides. Now, nearly 20 years later, transition metal catalyz cross-coupling has become the reaction of first choice for this purpose. 

The 1,3-disposition of the heteroatoms also imparts differential acidities to the ring positions, enabling sequential deprotonation. This rich acid-base chemistry permits selective functionalization of each ring-carbon atom. The synthesis of other organometaUic oxazoles and transition-metal-catalyzed cross-coupling reactions are also described. Applications of new cross-coupling methods for preparing polyoxazoles in the context of the syntheses of natural products are described in Section 1.5. 

Transition metal-catalyzed cross-coupling and C-H activation have become an important method for the formation of carbon-carbon bonds in the synthesis of furan and benzofuran containing pharmaceutical compounds. The reaction conditions required for these transformations are generally mild and tolerant of a number of different functional groups including ethers, esters, and amines to name a few. 

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. 

Transition metal-catalyzed cross-coupling methodologies have become privileged approaches and are recognized as powerful, practical, and versatile methods for carbon-carbon and carbon-heteroatom bond formation (see Chapters 19 and 20) [168-170]. They play an important role in both modem drag... 

Transition-metal catalyzed cross-coupling reactions have been demonstrated to be an extremely powerful tool for the construction of carbon-carbon bonds in regio- and stereoselective manners. These metal catalysts enable the transmetallation of carbon-silicon bonds into other carbon-metal bonds from which further reactions can proceed, leading to carbon-carbon bond formation. The use of palladium catalyst in the activation of the carbon-silicon bond is particularly noteworthy various important transformations have thus been developed. The presence of fluoride ions will make the reaction more facile, pentacoordinate organofluorosiUcates being presumably formed under these conditions . ... 

Preparations of all these organic materials involve the constmction of new carbon-carbon bonds and have prompted the development of many catalytic cross-coupling reactions. One of the most reliable synthetic methods to form carbon-carbon bonds is transition metal-catalyzed cross-coupling between organo-metallic nucleophiles and electrophilic organic halides or pseudohalides, respectively (Scheme 2a). The mechanisms of common cross-coupling reactions such as the Suzuki, Negishi, or Stille catalysis can be described by a catalytic cycle, differ in detail, but all include three main steps in the order oxidative addition, transmetallation, and reductive elimination (Scheme 1). 

Hydrocarbons containing main-group elements such as boron, silicon, and tin are ubiquitous building blocks for the synthesis of complex molecules. Transition-metal-catalyzed cross-coupling reactions of carbon-main-group element linkages as well as their transformation into more valuable functional groups are powerful tools of modem synthetic chemistiy. 

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