Boronic acids, metal catalyzed halides

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. 

Reactions of organometallic nucleophiles are reviewed mainly under Reactivity of Substituents Metals and Metalloids - this is a change from the Handbook-II policy of considering these under the reactions of Reactivity of Substituents Halides. Transition metal-catalyzed reactions of halides are considered partly under Reactivity of Substituents Halides and partly in the metalloids sections. Transition metal-catalyzed reactions of stannanes, boronic acids, etc., are considered under Reactivity of Substituents Metals and Metalloids. These areas represent the largest proportion of the additional new material since Handbook-II and are certainly the most important. 

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. 

Figure1.36 Transition metal-catalyzed coupling of boronic acids (esters) with carbon halides/triflates (Suzuki cross-coupling...

In common with the synthesis of trifluoroisopropenyl-boronic acid, trifluoro-isopropenylsilane can be also prepared in 80% yield by treatment of 2-BrTFP with 1.2 equivalents of magnesium in the presence of 2.0 equivalents of dimethylphenylsilyl chloride in THF at — 10°C to room temperature (Scheme 26.45). This reagent can employ as CF2=C -CH2 synthon, but there have been no reports on the transition metal-catalyzed cross-coupling reaction of tri-fluorinated silane with halides. 

Esters. Acylation by aromatic esters,mentioned in a preceding section, can be catalyzed by aluminum chloride, boron trifluoride, metallic halides, and H-acids as catalysts. The possible mechanism is represented by the following equations ... 

The application of the ideas of Lewis on acids, which correlate a wide range of phenomena in qualitative fashion, has as yet led to very few quantitative studies of reaction velocity but has led to detailed speculations as to mechanisms (Luder and Zuffanti, 118). Friedel-Crafts reactions are considered to be acid-catalyzed, the formation of a car-bonium ion being the first step. The carbonium ion then acts as an acid relative to the base benzene which, upon loss of a proton, yields the alkylated product. Isomerizations of isoparaffins can be explained in similar fashion (Schneider and Kennedy, 119). An alkyl halide yields a carbonium ion on reaction with acids such as boron trifluoride, aluminum chloride, and other metal halides. 

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