Suzuki pseudohalides

Diazonium salts are important intermediates for the preparation of halides, and azo compounds. Diazonium salts can react as pseudohalide-type electrophiles, and can therefore be used in specific protocols for the Heck Reaction or Suzuki Coupling. 

The Hiyama Coupling is the palladium-catalyzed C-C bond formation between aryl, alkenyl, or alkyl halides or pseudohalides and organosilanes. This reaction is comparable to the Suzuki Coupling and also requires an activating agent such as fluoride ion or a base. 

The Stille Coupling is a versatile C-C bond forming reaction between stannanes and halides or pseudohalides, with very few limitations on the R-groups. Well-elaborated methods allow the preparation of differen, products from all of the combinations of halides and stannanes depicted below. The main drawback is the toxicity of the tin compounds used, and their low polarity, which makes them poorly soluble in water. Stannanes are stable, but boronic acids and their derivatives undergo much the same chemistry in what is known as the Suzuki Coupling. Improvements in the Suzuki Coupling may soon lead to the same versatility without the drawbacks of using tin compounds. 

The Suzuki Coupling, which is the palladium-catalysed cross coupling between organoboronic acid and halides. Recent catalyst and methods developments have broadened the possible applications enormously, so that the scope of the reaction partners is not restricted to aryls, but includes alkyls, alkenyls and alkynyls. Potassium trifluoroborates and organoboranes or boronate esters may be used in place of boronic acids. Some pseudohalides (for example triflates) may also be used as coupling partners. 

The palladium-catalyzed coupling of boronic acids (as well as other boron derivatives) with aryl and vinyl halides and pseudohalides is known as the Suzuki or Suzuki-Miyaura reaction. Because boron is nontoxic, this reaction has been used in pharmaceutical syntheses. In addition, hydroboration or borate substitution allows for the synthesis of virtually any desired coupling partner. For these reasons, as well as the high yields and functional group compatibility, the Suzuki reaction is the first reaction to consider for carrying out a cross coupling. Representative substrates and catalysts are shown in Scheme 17. The various bases are used to generate four-coordinate boron ate complexes that are more reactive in transmetalation. 

Traditionally, Suzuki-Miyaura cross-couplings have been performed mainly with aryl iodides, triflates, and bromides as electrophiles because of the lower activation barriers for the oxidative addition step (I OTf > Br > Cl 3> F) [2d, 32). However, because of the cost associated with relatively expensive organoiodides, bromides, and triflates, the development of other cheaper and more readily accessible electrophiles became necessary. In this section, recent advances in the Suzuki-Miyaura reaction employing various organohahdes and pseudohalides will be discussed in detail. 

Preparations of all these organic materials involve the constmction of new carbon-carbon bonds and have prompted the development of many catalytic cross-coupling reactions. One of the most reliable synthetic methods to form carbon-carbon bonds is transition metal-catalyzed cross-coupling between organo-metallic nucleophiles and electrophilic organic halides or pseudohalides, respectively (Scheme 2a). The mechanisms of common cross-coupling reactions such as the Suzuki, Negishi, or Stille catalysis can be described by a catalytic cycle, differ in detail, but all include three main steps in the order oxidative addition, transmetallation, and reductive elimination (Scheme 1). 

The Suzuki reaction (also referred to as the Suzuki-Miyaura reaction) is the palladium-catalyzed cross-coupling of an alkenyl-, aryl-, or heteroaryl halide or pseudohalide (R-X) with an alkyl, alkenyl, aryl, or heteroaryl boron reagent (R -BY2). This transformation leads to formation of a carbon-carbon... 

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