Boronic acids reaction with aryl triflates

Thus formed, organometallics are trapped with tiialkyl borates to produce whether the desired arylboronic [149-151,153] or diarylboronic esters [152], depending on the reaction conditions discussed above. The diarylboronic acids have also been successfully used in the Pd(PPh3)4-catalysed SM reactions with aryl triflates, under the standard conditions [152]. Beside trialkyl borates, A,A-dimethylaminoboron bromide has been trapped with organolithiums to give an unstable bis-aminoboronate 349, which, upon esterification with pinacol, gave the 1,4-phenylene-bis-pinacolato boronate (266) in 21% overall yield [154], Scheme 50. 

Suzuki and co-workers found that aryl triflates react with arylboronic acids [ArB(OH)2], in the presence of a palladium catalyst, to give biaryls in a reaction that is now known as Suzuki coupling.269 Even hindered boronic acids give good yields of the coupled product.2 0 Organoboranes, react with aryl triflates under these conditions as well. In a simple example, boronic acid 426 reacted with 3-bromopyridine in the presence of Pd(PPh3)4 to give a 90% yield of the biaryl 427.2 2... 

The overall mechanism is closely related to that of the other cross-coupling methods. The aryl halide or triflate reacts with the Pd(0) catalyst by oxidative addition. The organoboron compound serves as the source of the second organic group by transmetala-tion. The disubstituted Pd(II) intermediate then undergoes reductive elimination. It appears that either the oxidative addition or the transmetalation can be rate-determining, depending on reaction conditions.134 With boronic acids as reactants, base catalysis is normally required and is believed to involve the formation of the more reactive boronate anion.135... 

The Suzuki coupling reaction is a powerful tool for carbon-carbon bond formation in combinatorial library production.23 Many different reaction conditions and catalyst systems have been reported for the cross-coupling of aryl triflates and aromatic halides with boronic acids in solution. After some experimentation, we found that the Suzuki cleavage of the resin-bound perfluoroalkylsulfonates proceeded smoothly by using [l,l -bis (diphenylphosphino)ferrocene]dichloropalladium(II), triethylamine, and boronic acids in dimethylformamide at 80° within 8 h afforded the desired biaryl compounds in good yields.24 The desired products are easily isolated by a simple two-phase extraction process and purified by preparative TLC to give the biaryl compounds in high purity, as determined by HPLC, GC-MS, and LC-MS analysis. 

The original procedure involved the coupling of aryl bromides with secondary amines. However, since then the scope has been expanded to include substrates such as aryl iodides, chlorides, fluorides, triflates, tosylates, nonaflates, iodonium salts, and even boronic acids. While the reaction has not been extensively utilized with vinyl or alkynyl substrates, it can be performed with various heteroaryl halides. Similarly, numerous types of nitrogen-containing coupling partners, including primary amines, imines, various azoles, lactams, and simple amides, can now be used in this reaction. 

Different conditions (including additives and solvent) for the reaction have been reported,often focusing on the palladium catalyst itself," or the ligand." Catalysts have been developed for deactivated aryl chlorides," and nickel catalysts have been used." Modifications to the basic procedure include tethering the aryl triflate or the boronic acid to a polymer, allowing a polymer-supported Suzuki reaction. Polymer-bound palladium complexes have also been used." " The reaction has been done neat on alumina," and on alumina with microwave irradiation." Suzuki coupling has also been done in ionic liquids," in supercritical... 

The palladium-catalysed reactions with alkenyl, aryl, and heteroaryl halides or triflates as one partner ( electrophilic ) and alkenes (Heck), aryl or vinyl stannanes (Stille) or aryl, vinyl and even alkyl boronic acids (Suzuki) as the other ( nucleophilic ) partner provide a synthetic method of astonishing power and versatility. These reactions are only just starting to be explored and great things are expected of them. 

The reaction is not restricted to forming biaryls. Aryl boronic acids can also react with vinyl halides and vinyl triflates in the presence of palladium (0). Thallium (I) ethoxide is known to promote Suzuki cross coupling reactions for vinyl or arylboronic acids with vinyl and aryl halide partners. 3 synthetic example is... 

The Suzuki-Miyaura cross-coupling reaction is a standard method for carbon-carbon bond formation between an aryl halide or triflate and a boronic acid derivative, catalyzed by a palladium-metal complex. As with the Mizoroki-Heck reaction, this cross-coupling reaction has been developed in ionic liquids in order to recycle and reuse the catalyst. In 2000, the first cross-coupling of a halide derivative with phenylboronic acid in [bmim] [BF4] was described. As expected, the reaction proceeded much faster with bromobenzene and iodobenzene, whereas almost no biphenyl 91 was obtained using the chloride derivative (Scheme 36). The ionic liquid allowed the reactivity to be increased, with a turnover number between 72 and 78. Furthermore, the catalyst could be reused repeatedly without loss of activity, even when the reaction was performed under air. Cross-coupling with chlorobenzene was later achieved - although with only a moderate yield (42%) - using ultrasound activation. 

Under noncompetitive conditions, Greaney functionalized C2-chlorides or C4-triflates with a variety of aryl and heteroaryl boronic acids. The authors attempted Suzuki reactions with the C2-sulfonates but found facile thermal decomposition with both triflates and nonaflates. ... 

The Pd-catalyzed cross-coupling reaction of the pinacol ester of diboronic acid with aryl halides and triflates gives a direct procedure for various functionalized arylboronic esters (Table 6), which are useful for highly efficient and selective carbon-carbon bond formation by Pd catalysts (Sect. III.2.2). The combination of PdCl2(dppf) with KOAc (and dppf) is effective in the carbon-boron bond formation. KOAc is essential not only to accelerate the reaction but also to prevent the formation of biaryl by-products. 

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