Vinylboronic acids, coupling with aryl

The Suznki reaction is an important C-C bond-forming reaction of an aryl- or vinylboronic acid (112) with an aryl- or vinylhaUde (111, 115), catalyzed by a pal-ladinm(0) complex. Recently, Akira Suzuki won the 2010 Nobel Prize in chanistry for his significant contribution and development of this reaction. The literature survey reveals several modifications to the reaction. The application of various ZnO-supported metal combinations such as Pd-Ag, Pd-Cu, Pd-Ni, and Pd (Kim and Choi 2009) were reported for the cross-coupling Snzuki reaction between aryl halides (111, 115) and phenyl boronic acid (112) (Schemes 9.34, 9.35). These metal combinations with ZnO resulted in excellent yields of the biaryl products. 

Traditionally, aryl bromides/iodides/triflates are employed for coupling with boronates. For example, a-trifluoromethyl vinylboronic acids 129 were utilized for coupling with wide variety of aryl halide substrates such as phenyl, naphthyl, and pyrazyl halides under standard conditions to provide trifluoromethyl-a-arylalkenes 130 (Scheme 28.35). Similarly, 5-vinylboronates 131 were also coupled with aryl halides to provide arylpiperidines 132 (Scheme 28.36). ... 

Through the use of arenediazonium salts, the straightforward transformation of amines into cross-coupling products can be realized. Whenever the diazonium salts do not tolerate bases and strong nucleophiles (e.g., phosphines), base- and phosphine-free protocols have to be used. Heterocyclic carbene ligands serve well in cross-coupling of Aryl- and vinylboronic acids, or alkylboronates with arenediazonium salts.369,370 Several convenient phosphine-free protocols have been developed for the same purpose.371-373... 

Kabalka and co-workers reported the direct cross-coupling of cinnamyl alcohols with aryl- and vinylboronic acids using simple rhodium salts in an environmentally benign ionic liquid medium (Eq. 6) [30]. The ability to utilize allylic alcohols, without activation, is significant from the viewpoint of atom economy, yet challenging due to the poor leaving group ability of hydroxide. 

The cross-coupling of allyl alcohols with aryl- and vinylboronic acids has been accomplished in [C4Ciim][BF4] and [C4Ciim][PF6] with rhodium catalysts.1"31 While no reaction takes place in polar solvents such as water or DMF, good results were obtained in [C4Ciim][PF6], see Scheme 6.12, and rhodium(I) as well as rhodium(III) compounds worked well in the latter solvent. The presence of copper salts as well as acids led to an increase of the reaction rate. 

Alkenylboron derivatives are readily available by stereo-defined hydroboration of alkynes and used for the preparation of styrene derivatives by alkenyl-aryl coupling. Vinylboronic acid (81), the most simple alkenylboronic acid, is difficult to handle, because it undergoes uncontrollable polymerization. Its anhydride, 2,4,6-trivinylcyclotriboroxane (82)-pyridine complex, is stable, and can be used conveniently for the coupling. Vinylation of 2-bromoanisole with 82 proceeds in refluxing DME using K2CO3 as a base fo provide 2-vinylanisole (83) [80]. 

Suzuki-Miyaura coupling that is the reaction of an aryl- or vinylboronic acid with an aryl or vinyl halide catalyzed by a palladium(O) complex (Figure 1.1) [63]. 

The Suzuki reaction was described in the presence of various [Pd(7 -allyl)Cl]2 (l)/ligand systems as well. The efficiency of the tetraphosphine ligand Tedicyp was demonstrated for various Suzuki couplings, and more recently for the reaction of alkenyl-boronic acids with arylbromides. 7i The preparation of various stryrene derivatives (eq 81) has been performed in the presence of vinylboronic acid and substituted alkenyl boronic acids. The catalyst system was tolerant of electronic variation in the aryl-bromide component. Tum-over numbers of 3100-8600 could be achieved for activated substrates such as 4-bromoacetophenone. The coupling of alkyl tosylates was described either in the presence of Pd(OAc)2 or dimer (1) as precursor. 7 a fiber-supported catalyst prepared from 1 was found to be efficient for Suzuki coupling between activated aryl bromides and phenylboronic acid. 

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