Suzuki-type reactions bromides

The pincer-type palladacycle (120) (R = 1Pr), which is actually a derivative of a dialkylphos-phinous acid (themselves excellent ligands see Section 9.6.3.4.6) was shown to allow the crosscoupling of aryl chlorides with terminal acetylenes ((120), ZnCl2, Cs2C03, dioxane, 160 °C). However the high reaction temperature may be prohibitive for the actual application of this catalytic system, as acetylenes are known to be thermally sensitive.433 The same palladacycle (R = Ph) is effective in the Suzuki-Miyaura reaction with aryl bromides and activated aryl chlorides (K2C03, toluene, 130 °C). 

Chen, H. Deng, M.-Z. A novel Suzuki-type cross-coupling reaction of cyclopropylboronic esters with benzyl bromides./. Chem. Soc. Perkin Trans. 1 2000, 1609-1613. 

One of the challenges in the Suzuki-type cross-coupling is to extend this reaction from electron-rich aryl iodides, bromides, and triflates to less reactive aryl sulfonates and aryl chlorides, which show poor reactivity in terms of oxidative addition in the catalytic cycle. Aryl mesylates, benzenesulfonates, and tosylates are much less expensive than triflates, and are unreactive toward palladium catalysts. The Ni(0)-catalyzed Suzuki-type cross-coupling reaction of aryl sulfonates, including mesylates, with arylboronic acids in the presence of K3P04 has been reported [123]. 

Wei et al. synthesized a new class of tridentate, monoanionic, meridionally coordinating pincer-type NHC ligand precursors and amido-NHC palladium(II) complexes 45-47 (Figure 4.17) [53]. They showed to be effective in the Suzuki-Miyaura reactions of a variety of aryl bromides, even with less active aryl bromides. 

Phosphine 23 proved in general to be an excellent ligand for Suzuki coupling reactions to form stericaUy hindered tetra-ortho-substituted biaryls in good yields. Ortho substituents such as methyl, primary alkyl, phenyl, and alkoxy groups are accommodated. It was necessary to use 2.0 equiv of the boronic acid to effect complete consumption of the aryl bromide in some cases, presumably due to competitive pro-todeboronation. Crystallographic analysis of 23/Pd(dba) revealed an unusual rc-coor-dination of the phenanthrene moiety - the first of its type for a Pd(0) complex (Equation 50) [61]. 

Firooznia has reported the synthesis of 4-substituted phenylalanine derivatives via cross-coupling of protected (4-pinacolylboron)phenylalanine derivatives such as 61 with aryl and alkenyl iodides, bromides and triflates [44]. They have further shown that BOC derivatives of (4-pinacolylboron)phenylalanine ethyl ester 61 or the corresponding boronic acids undergo Suzuki-Miyaura reactions with a number of aryl chlorides in the presence of PdCl2(PCy)3 or NiCl2(dppf), respectively providing diverse sets of 4-substituted phenylalanine derivatives of type 62 [45]. This strategy has also been used for the synthesis of enantiomerically enriched 4-substituted phenylalanine derivatives (Scheme 3.28) [46]. 

Bedford and coworkers disclosed iron-catalyzed Suzuki-Miyaura-type coupling reactions of benzyl bromides with sodium tetraphenylborate in the presence of 5 mol% 15 as the catalyst and 10 mol% of dianisylzinc as a promoter. No reaction occurred in its absence. The coupling furnished 38-88% yield of 3 (entry 26) [66]. The transformation proceeds probably by initial B-Zn transmetalation. The resulting arylzinc transfers the aryl group to the iron catalyst as in the Negishi couplings above. An aryliron(I) complex was proposed to be formed initially. 

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