Transmetallation pseudohalides

Oxidative addition of aryl and alkenyl halides, and pseudohalides, followed by transmetallation with various metal hydrides generates Ar—M—FI species, reductive elimination of which results in hydrogenolysis of halides. In the main, Pd is used as an efficient catalyst for the hydrogenolysis. 

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. 

Pd-catalyzed hydrogenolysis of aryl and alkenyl halides and pseudohalides such as triflates using various hydride sources is an established process. The reaction can be explained by the transmetallation with hydride to form palladium hydride, which undergoes reductive elimination. Among several hydride sources, HCOaH/EtsN, HSnBu3, and RsSiH are most extensively used. Typical recent examples of hydrogenolysis using these hydrides are briefly cited in this section. 

The oxidative addition step probably proceeds through the charge-transfer complex, Ar-X Pd(PPh3)4, to produce the relatively stable arylpalladium(II) halides (or pseudohalides), see Chapter 3. The formation of a stable ionic metal-salt, e.g. MgCl2, is the driving force for the further reaction step, transmetallation of an aryl-group from the arylmetallics to arylnickel(II) or arylpalladium(II) complex. Generally, all... 

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). 

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