Cross-coupling reactions copper effect

Most of the work on the C-N bond-forming crosscoupling reactions has concentrated on the formation of aromatic C-N bonds. Recent studies show that the application of cross-coupling reactions to alkenyl halides or triflates furnished enamines (Scheme 19) (for palladium-catalyzed reaction, see 28,28a-28d, and for copper-catalyzed reaction, see 28e-28g). Brookhart et al. studied the palladium-catalyzed amination of 2-triflatotropone 109 for the synthesis of 2-anilinotropone 110.28 It was found that the reaction of 109 proceeded effectively in the presence of racemic BINAP and a base. As a simple method for the synthesis of enamines, the palladium-catalyzed reactions of alkenyl bromide 111 with secondary amine were achieved under similar conditions.2841 The water-sensitive enamine 112 was isolated as pure compound after dilution with hexane and filtration through Celite. The intramolecular cyclization of /3-lactam 113, having a vinyl bromide moiety, was investigated by Mori s... 

Copper-catalyzed Suzuki cross-coupling reactions using mixed nanocluster catalysts have been studied recently. Copper-based catalysts were shown to be effective as reagents that can present an inexpensive and environmentally friendly alternative to noble metal catalysts. In the hydrogenation of cinnamic acid to corresponding alcohol, the selectivity can be varied by doping Sn with Rh colloid catalysts. A selectivity of 86% was achieved using a colloidal Rh/Sn (Rh/Sn = 1.5 1) catalyst on... 

Synthetic reactions involving activation of alkenyl- and arylsilanes by copper salts have also been reported. A copper-mediated system is effective in the cross-coupling reaction of aryl- and heteroarylsilanes with aryl halides under Pd-free conditions (Equation (13)).66 Alkenyl- and arylsilanes as well as alkynylsilanes undergo homocoupling in the presence of Cul and air.65,65a Functionalized alkenyl- and arylsilanes bearing a coordination site in the vicinity of silicon (e.g., 7) are easily activated by copper salts even at room temperature (Scheme 5).67,67l 67e 68 q-jle organocoppers such as 8 are useful for carbon-carbon bond-forming reaction with carbon electrophiles. 

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. 

We have found that alkynylsilanes are smoothly converted into alkynylcopper compounds by treatment with CuCl in l,3-dimethyl-2-imidazohdinone (DMI) the copper reagents can be isolated in good yields [563]. This study was the first example of preparation and isolation of organocopper compounds by use of organosilicon reagents. The Si-Cu transmetalation is applicable to the synthesis of alkynyl ketones by Cu-catalyzed alkynylation of acid chlorides (Scheme 10.217). We have also shown that a Cu-mediated system is effective in the cross-coupling reaction between arylsilanes or heteroarylsilanes and aryl hahdes [564]. 

Various transition metals (Pd, Rh, Ru, Cu, Fe, etc.) have been shown to be effective for cross-coupling reactions involving C-H bond activation in recent years (for copper, see [33-38] for iron, see [39, 40]). Pd(II) salts have emerged as the preferred catalysts to promote the C-H bond cleavage in catalytic direct arylation reactions. This process can be classified into two parts, the organometallic reagent is replaced by a simple arene in one case, and the aryl halide is replaced by a simple arene in another (Scheme 4). 

The copper-catalyzed cross-coupling of vinyl bromides with secondary phosphites has been reported (Scheme 4.334) [497]. Copper iodide was an effective catalyst for this reaction however, three equivalents of Cul were needed for each vinyl bromide. The authors were pleased to find that the stereochemistry of the vinylphosphonate mirrored the starting vinyl bromide. Although HMPA was a superior solvent for the reaction chemistry, the authors successfully used other solvents snch as DMF and tetramethylurea due to the potential carcinogenicity of HMPA. The overall reaction was carried out in two stages with the phosphonate anion generated in the first step, while the cross-coupling reaction was carried out in the second step. 

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