Palladium-catalyzed coupling reactions comparing

At the same time, Schmidtchen et al. compared cationic phosphine ligands containing the hydrophilic guanidinium (4.3, 4.4) and the anionic phosphine ligand TPPTS for this palladium-catalyzed coupling reaction. They found that the cationic ligands were effective for the coupling reaction but less efficient than TPPTS 43... 

The use and the advantages of these catalysts are described in several papers especially for palladium-catalyzed coupling reactions [66,96,97]. Levels of selectivity have been compared with those achievable with conventional homogeneous catalysts in a variety of reactions. In general the FibreCat versions show comparable activities to the non-anchored versions. 

Aryl chlorides are important starting materials in palladium-catalyzed coupling reactions [7]. Compared with the corresponding aryl iodides or aryl bromides, the advantages of aryl chlorides are obviously that they are inexpensive, easy to prepare, stable, etc. The same is true if we compare aryl tosylates or aryl mesylates with their aryl triflates analogs. Even though aryl chlorides, aryl mesylates and aryl acetates have been studied and have succeeded in cross-coupling reactions, their... 

In addition, the reaction is very chemoselective and even a keto group is tolerated (Table 5). It should be underlined that this procedure compares advantageously to the corresponding palladium- or nickel-catalyzed coupling reactions. 

Relatively less acidic ketones compared to 1,3-dicarbonyl compounds are also suitable substrates for the palladium catalyzed coupling. a-Aryl ketones are obtained as products. In the early examples, masked ketone enolates such as silyl enol ethers [42] and enol acetates [43-45] were used in the presence of a tin source. These reactions involve tin enolates or acylmethyltins as intermediates and thus proceed by transmetalation (mechanism B in Scheme 1). 

Another topic often encountered in the literature on microwave-promoted reactions is the lower consumption of energy associated with the use of MW technology in small-scale chemistry. For the palladium-catalyzed Suzuki reaction there have been attempts to investigate this matter in more detail. Clark et al. have performed a comparative study of the energy efficiency of the different reaction techniques. The Suzuki reaction was analyzed and under the reaction conditions used the MW-assisted reaction was 85 times more energy-efficient than the corresponding oil-bath-heated reaction. As there are a multitude of reaction conditions for the Suzuki coupling, this value should be seen as an example, rather than a definite value [20]. 

Beller has demonstrated that a new bulky, electron-rich trialkylphosphine, di(l-adamantyl)-n-butylphosphine, w-BuPAd2, can afford excellent TONs in palladium-catalyzed Suzuki reactions of aryl chlorides [34]. For the coupling of 4-chlorotoluene, this phosphine achieves a TON of 17 400 (0.005 mol% Pd, 87% yield) (Equation 2.19), compared with a TON of 9200 with commercially available P(t-Bu)3 (0.01 mol% Pd, 92% yield). High TONs (>10000) can also be obtained with challenging aryl chlorides, such as 2-chloro-m-xylene and 4-chloroanisole. 

Chlorophenyl)-4-phenyl-l,2,5-thiadiazole 128 was prepared from 3-trifluoromethylsulfonyloxy-4-phenyl-1,2,5-thiadiazole 127 by palladium-catalyzed cross-coupling reaction with the tributyl(4-chlorophenyl)stannane (Equation 20) <1996H(43)2435>. The addition of lithium chloride improves the yield. The 3-chloro- and 3-bromo-l,2,5-thiadiazole derivatives were also reactive, but only the bromo compound gave the product in comparable yield (see Section 5.09.7.6). 

Additional examples of palladium-catalyzed cross-couplings, in particular with allenylzinc compounds, can be found elsewhere [11, 15, 36]. A systematic study comparing several chiral palladium phosphine catalysts in the reaction of 4,4-di-methyl-1,2-pentadienylzinc chloride and iodobenzene revealed that an enantiomeric excess of only 25% was obtained from the best catalyst combination PdCl2 and (R,R)-DIOP [15]. The synthetic value of these transformations of donor-substituted allenes as precursors is documented by the preparation of a/l-unsaturatcd carbonyl... 

The Hiyama Coupling is the palladium-catalyzed C-C bond formation between aryl, alkenyl, or alkyl halides or pseudohalides and organosilanes. This reaction is comparable to the Suzuki Coupling and also requires an activating agent such as fluoride ion or a base. 

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