Metals-catalyzed a-arylation

LDA), resulted in the desired ring closure to form a pentacyclic core structure (Scheme 8.73) [18a, 124]. This was one of the first examples of metal-catalyzed a-arylation reactions. 

Brain and Brunton reported one of the early palladium-catalyzed cyclizations, which provided benzimidazoles in moderate to high yield. In this study, Pd(PPh3)4 was used to catalyze the intramolecular -arylation of o-bromoamidines to afford variously substituted 1-alkyl or 1-phenyl-benzimidazoles. This report set the stage for additional investigations of transition-metal mediated A -arylation as a tactic for benzimidazole synthesis. 

The scope of Pd-catalyzed a-arylation of enolates has been further expanded so as to include amides,malononitrile, a-cyanoesters, ° a-cyanophosphates, and a-cyanosulfones. " The metal countercations in these reactions are mostly alkali metals, such as Li, Na, K, and Cs. Some representative examples of these reactions are shown in Scheme 13. The Pd-catalyzed reaction of 2-methyl-l-propenyl chloride with NaCH(COOMe)2 in the presence of 2 mol% of Me2Pd(dcypb), where dcypb is Cy2P(CH2)3PCy2, led to 2-methylallylationf i (Scheme 14). 

Transition metal-catalyzed N-arylations of amidines and guanidines including those being part of a heteroaromatic system 12CSR2463. Transition metal-catalyzed C—C bond formation via C—S bond cleavage 13CSR5042. 

Rhodium-catalyzed direct arylations of 2-aryl pyridines were efficiently accomplished with arylstannanes through chelation-assistance (Scheme 9.3) [14]. This report constitutes an early example of a metal-catalyzed direct arylation with an... 

J70 I 77 Mechanistic Aspects of Transition Metal-Catalyzed Direct Arylation Reactions (a) assisted intramolecular... 

Finally, a number of efficient indole syntheses utilizing a transition metal catalyzed C-arylation of 1,3-dicarbonyl compounds with ortbo-iodoaniline derivatives followed... 

This section describes selected arylations of phosphorus, sulfur, and halide nucleophiles under metal-free and metal-catalyzed conditions. Arylations of other nucleophiles, e.g., selenium and tellurium, have been reviewed previously [4]. Aryl phosphonates [ArPO(OR)2] can be synthesized by arylation of phosphite anions with diaryliodonium salts and NaH in DMF at 70-80 °C [158]. A copper-catalyzed arylation of various phosphorous nucleophiles, e.g., diarylphosphine oxides and //-phosphonates, was recently reported to proceed at room temperature. The observed chemoselectivity with unsymmetric salts was opposite to the general trend in metal-catalyzed reactions (see Sect. 2.1), which was explained by a radical mechanism [159]. 

Ackermann, L. Vicente, R. Kapdi, A. R. Transition-metal-catalyzed direct arylation of (hetero)arenes by C-H bond cleavage. Angew. Chem., Int. Ed. 2009,48, 9792-9826. 

Transition-metal-catalyzed direct arylation of arenes with aryl halides via cleavage of aryl C—H bonds serves as a straightforward method for aryl-aryl bond formation [3], Hence, direct arylation of two aromatic rings tethered by aromatic/alkenyl moieties or heteroatoms (i.e. intramolecular variant of the direct arylation) constitute efficient syntheses of PAHs such as benzofurans, carbazoles, dibenzothiophenes, dibenzosiloles, and phenanthrenes. In this section we overview the progress with subsections categorized by product structures [5]. 

Metal-catalyzed N-arylation of amines with haloarenes (Buchwald-Hartwig coupling) has been developed since the 1990s and provided more efficient access to carbazoles. Nozaki et al. reported the palladium-catalyzed double N-arylation of primary amines with 2,2 -dihalobiphenyls [8], A variety of dihalobiphenyls 10 and anilines 11 with electron-donating or electron-withdrawing substituent(s) are applicable to give multisubstituted carbazoles 12 in moderate to high yields (Scheme 23.4). 

During the past several years the use of secondary phosphine oxides, chloro-phosphines, and related species as ligands for late-transition metal catalysts has been explored by several groups (reviews [81, 82]). Catalysts supported by these unusual ligands have shown some utihty in Pd-catalyzed A-arylation reactions. For example, treatment of 4-chlorobenzotrifluoride with piperidine and NaOfBu in the presence... 

In contrast to the asymmetric procedures discussed above, the metal-catalyzed oxidation of alkyl aryl sulphides by t-butylhydroperoxide carried out in a chiral alcohol gives rise to chiral sulphoxides of low optical purity290 (e.e. 0.6 9.8%). Similarly, a very low asymmetric induction was noted when prochiral sulphides were oxidized by sodium metaperiodate in chiral alcohols as solvents291. 

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