Arylations transition-metal catalysts

The general mechanism of coupling reactions of aryl-alkenyl halides with organometallic reagents and nucleophiles is shown in Fig. 9.4. It contains (a) oxidative addition of aryl-alkenyl halides to zero-valent transition metal catalysts such as Pd(0), (b) transmetallation of organometallic reagents to transition metal complexes, and (c) reductive elimination of coupled product with the regeneration of the zero-valent transition metal catalyst. 

Mikami M, Hatano M, Akiyama K (2005) Active Pd(II) Complexes as Either Lewis Acid Catalysts or Transition Metal Catalysts. 14 279-322 Minatti A, DOtz KH (2004) Chromium-Templated Benzannulation Reactions. 13 123-156 Miura M, Satoh T (2005) Catalytic Processes Involving b-Carbon Elimination. 14 1-20 Miura M, Satoh T (2005) Arylation Reactions via C-H Bond Cleavage. 14 55-84 Mizobe Y, see Hidai M (1999) 3 227-241... 

Direct aromatic substitution of unactivated aryl halides is slow and generally requires a catalyst to become a useful synthetic method. Copper reagents have been used in some cases in classical procedures for the formation of products from aromatic substitution. In many cases these copper-mediated reactions occur at high temperatures and are substrate dependent. Since the 1970s, transition metal catalysts have been developed for aromatic substitution. Most of the early effort toward developing metal-catalyzed aromatic substitution focused on the formation of... 

The reduction of organic halides is of practical importance for the treatment of effluents containing toxic organic halides and also for valuable synthetic applications. Direct electroreduction of alkyl and aryl halides is a kinetically slow process that requires high overpotentials. Their electrochemical activation is best achieved by use of electrochemically generated low-valent transition metal catalysts. Electrocatalytic coupling reactions of organic halides were reviewed in 1997.202... 

In hydrogenation, early transition-metal catalysts are mainly based on metallocene complexes, and particularly the Group IV metallocenes. Nonetheless, Group III, lanthanide and even actinide complexes as well as later metals (Groups V-VII) have also been used. The active species can be stabilized by other bulky ligands such as those derived from 2,6-disubstituted phenols (aryl-oxy) or silica (siloxy) (vide infra). Moreover, the catalytic activity of these systems is not limited to the hydrogenation of alkenes, but can be used for the hydrogenation of aromatics, alkynes and imines. These systems have also been developed very successfully into their enantioselective versions. 

A series of internal alkynes has been investigated, revealing that the presence of an alkyl or aryl group does not appear to change the course of the reaction. Internal alkynes, however, do not undergo germylformylation in this system. In the case of tin, two reports exist of the stannylformyla-tion of unsaturated carbon substrates, but both proceed by a free radical mechanism initiated by AIBN and do not require a transition metal catalyst.128... 

Arylations of weak organic nucleophiles are best achieved with iodonium salts possessing nucleofugic anions and, in some cases, can be facilitated with transition metal catalysts. Recent examples include Cu(II)-catalyzed S-phenyla-tions of 1-benzothiophenes with diphenyliodonium triflate [118], and Co(II)-catalyzed N-arylations of imidazoles with diaryliodonium tetrafluoroborates (Scheme 42) [119]. 

V-Ary I at ion of an amine generally consists in the condensation of the amine with an aryl substrate in the presence of a transition metal catalyst.250 Another strategy has been used, however, to obtain 3,3-dimethylbenzo r/1-1,3-azasilolines in moderate yields. It consists in the internal insertion of an N-H bond to the triple bond of a benzyne.251,252... 

The insight that zinc ester enolates can be prepared prior to the addition of the electrophile has largely expanded the scope of the Reformatsky reaction.1-3 Substrates such as azomethines that quaternize in the presence of a-halo-esters do react without incident under these two-step conditions.23 The same holds true for acyl halides which readily decompose on exposure to zinc dust, but react properly with preformed zinc ester enolates in the presence of catalytic amounts of Pd(0) complexes.24 Alkylations of Reformatsky reagents are usually difficult to achieve and proceed only with the most reactive agents such as methyl iodide or benzyl halides.25 However, zinc ester enolates can be cross-coupled with aryl- and alkenyl halides or -triflates, respectively, in the presence of transition metal catalysts in a Negishi-type reaction.26 Table 14.2 compiles a few selected examples of Reformatsky reactions with electrophiles other than aldehydes or ketones.27... 

Decarbonylation/carbonylation of alkyl and aryl halides, in presence of transition metal catalysts, is a reversible catalytic process that is useful in both directions for the formation of new C—X bonds (equation 162) ... 

Emphasis on the employment of transition metal catalysts to achieve useful synthetic transformations is illustrated by three procedures in this volume. The reaction of allylic alcohols with aryl halides in the presence of a palladium derived catalyst can be used to prepare various /3-arylaldehydes. This is illustrated by the preparation of 2-METHYL-... 

Vinylphosphonates are useful reagents but simple vinyl halides do not undergo the Michaelis-Arbuzov reaction except in the presence of a transition metal catalyst [Ni(II) or Cu(I), cf. Protocol 4] so vinylphosphonates are usually synthesized from other functionalized phosphonates or by the palladium-catalysed Michaelis-Becker reaction (cf. Protocol 8).38 Similarly, simple aryl halides undergo the Michaelis-Arbuzov reaction only under special conditions palladium or nickel species (Protocol 4) are suitable catalysts. Indeed these and other catalysts have been applied to the Michaelis-Arbuzov reaction of various substrates, though they are generally essential only with vinyl and aryl halides, as described herein.39... 

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