Aryl chlorides Suzuki coupling reactions

Palladium catalysts have been found which are effective in the Suzuki coupling reaction of arylboronic acids with aryl chlorides carrying electron-withdrawing groups.73 Biaryls may also be synthesized by cross-coupling of arylboronic acids with arenediazonium salts.74,75 There has been a report of the polymer-bound palladium-catalysed Suzuki coupling of aryl triflates with organoboron compounds.76 Arylbor-onates may themselves be synthesized by the palladium-catalysed reactions of... 

As Figure 16.7 reveals, an aryl iodide reacts more rapidly with an alkynylcopper compound than an aryl bromide. The palladium-catalyzed C,C coupling reactions, which will be discussed later in the present chapter, also proceed more rapidly with an aryl iodide than with an aryl bromide (example Suzuki coupling in Figure 16.22) or an aryl chloride (example Stille reaction in Figure 16.27). There are still some details that are not fully understood one is inclined to assume that in accordance with the Hammond postulate the weaker C-I bond (dissociation energy DE = 51 keal/mol) breaks more rapidly with the initial oxidative addition... 

Phenylazo-1,2,4-triazole were prepared from benzenediazonium chloride and 1,2,4-triazole <07SC1977>. 5-Aryltriazole acyclonucleosides 134 with various aromatic groups on the triazole ring were synthesized from precursor 133 via the Suzuki coupling reaction in aqueous solution and promoted by microwave irradiation <07TL2389>. 1-Methyl-1,2,4-triazole 135 participated in a palladium-catalyzed C-H arylation reaction with 3,5-dimethoxychlorobenzene 136 to give coupled product 137 <07OL1449>. 

Bulky electron rich phosphino-amines as ligands for the Suzuki coupling reaction of aryl chlorides... 

BULKY ELECTRON RICH PHOSPHINO-AMINES AS LIGANDS FOR THE SUZUKI COUPLING REACTION OF ARYL CHLORIDES... 

The procedures described in Chapters 4-7 all relate to a set of broadly similar transformations that are becoming exceptionally important and well used in laboratories worldwide. In Chapter 4, eight examples of the Suzuki coupling reaction are described four accounts describe the use on activated alkene (vinyl bromide, triflate or tosylate) as the coupling partner for boronic acid derivatives. The other examples of Suzuki couplings involve aryl bromides and aryl chlorides. It is noteworthy that the methodology introduced by Nolan has been extended to include amination reactions. 

In contrast, coupling reactions with aryl chlorides and electron rich aryl bromides are not as well optimized in the fixed-bed system. For exanple, with 4-bromoanisole only 85 % conversion was observed at 80 °C after 52 h. Unfortunately, we were unable to achieve reasonable reactivity (25 % conv., 125 h, 80 °C) with an aryl chloride substrate (4-chlorobenzotrifluoride) using conditions that we previously developed for single-cycle batch reactions. In both cases, the catalyst activity seemed to erode over time and reaction stalled before conqilete conversion. The difference in reactivity conqjared to the batch studies may be attributed to the low Pd loading of the catalyst (i.e., 0.8 w/w% vs. a typical 5 w/w %) and the pre-reduced nature of the catalyst which may not be optimized for the Suzuki coupling reaction. 

Calo and coworkers reported the use of Pd nanoparticles in ionic liquids (quaternary ammonium salts) as catalysts for the Suzuki coupling reaction of aryl bromides and chlorides with phenylboronic acid. The catalytic system was found... 

Demir, S., Ozdemir, I. and Cetinkaya, B. (2006) Use of bis(benzimidazolium)-paUadium system as a convenient catalyst for Heck and Suzuki coupling reactions of aryl bromides and chlorides. Appl. Organomet. Chem., 20, 254-9. 

The utility of these reactions were dramatically improved by the crosscoupling reactions involving transition metals as catalysts. The most prominent among these is the Suzuki coupling (Figure 17) (60-62). These reactions allow for aryl-aryl couplings also (62). Recently Buchwald (65) and Fu (64) developed modified phosphines which will allow the inclusion of aryl chlorides in Suzuki coupling reactions. 

A variety of complexes of benzimidazole-based NHCs were also investigated in Suzuki coupling reactions. Huynh and co-workers studied dimeric complexes 37 as pre-catalysts in Suzuki coupling reactions (Figure 9.8). Good activities were obtained for the cross-coupling of 4-bromobenzaldehyde with phe-nylboronic acid, in water as solvent. However, the catalysts were less effective for coupling aryl chlorides. 

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