Intramolecular hydroarylation of alkynes

In this suhehapter, I summarized the intramoleeular single-component reactions, which are able to construct benzenoid aromatic rings. Two types of cycloaromatization reactions (intramolecular hydroarylation of alkynes and cycUzation via transition metal vinyUdenes) are described. 

SCHEME 21.48 Transition metal-catalyzed intramolecular hydroarylation of alkynes. 

TRANSITION METAL-MEDIATED AROMATIC RING CONSTRUCTION 

60 mol% AuCl(SMe2) 90 mol% AgOTf 30 mol% (S)-BINAP (CH2C1)2, rt Then silica gel chromatography 

SCHEME 21.51 Enantio- and diastereoselective synthesis of S-shaped double azahelicene. 

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Similar intramolecular hydroarylations of alkynes and alkenes, which obviate the need for a halide or triflate group on the aryl ring, are now well established. Sames group screened over 60 potential catalysts and over 200 reaction conditions, and found that Ru(m) complexes and a silver salt were optimal. This process appears to tolerate steric hindrance and halogen substrates on the arene (Equations (175)—(177)). The reaction is thought to involve alkene-Ru coordination and an electrophilic pathway rather than a formal C-H activation of the arene followed by alkene hydrometallation, and advocates the necessary cautious approach to labeling this reaction as a C-H functionalization... 

Reetz and Sommer then studied the intramolecular hydroarylation of alkynes when they were looking for carbon triple bond coupling reactions. Depending on the substrate, the choice of gold(I) or gold(III) species was crucial [127]. 

An asymmetric version of the intramolecular hydroarylation of alkynes (286) to produce fused rings (287) has been developed. With the cationic Pd(II)/(5)-(285) catalyst, the resulting 4-aryl 2-quinolinones were obtained with <98% ee. Asymmetric addition of boronic acids R B(OH)2 to activated cumulenes R C=C=C=C(R )C02Et, catalysed by an NHC-palladium complex, has been shown to yield allenic products R C=C=C(R )CH(R )C02Et with moderate to good enantioselectivities. ... 

As the intermolecular multicomponent reactions, three-component cycloaddition reactions (21.2 [2+2-1-2] cycloaddition and 21.3 [3+2+1] cycloaddition) and two-component cycloaddition reactions (21.4 [4+2] cycloaddition) are described. As the intramolecnlar single-component reactions, cycloaromatization reactions (21.5 intramolecular hydroarylation of alkynes and cychzation via transition metal vinybdenes) are described. Aromatic ring constrnction reactions involving aryne reactions (Chapter 12), rearrangement reactions (Chapters 16 and 18), metathesis reactions (Chapter 17), and coupling reactions (Chapters 19 and 20) are described in these different chapters. 

The intramolecular hydroarylation of alkynes is a useful method for the synthesis of fused arenes. For example, biphenyl derivatives, bearing an alkyne unit at the ortho position, were converted into substituted phenanthrene derivatives in the presence of various jc-electrophihc transition metal catalysts such as platinum, palladium, gold, gaUium, indium, iron, and so on (Scheme 21.48) [54]. 

The sequential intramolecular hydroarylation of alkynes is applied to the synthesis of structurally complex extended x-systems. The gold-catalyzed sequential intramolecular hydroarylation of triynes followed by aromatization with DDQ proceeded to give triaryl-substituted diacenaphtho[l,2-y l, 2 -l] fluoranthenes, which can be used for organic light-emitting devices (Scheme 21.49) [55]. 

The sequential intramolecular hydroarylation of alkynes has been applied to the catalytic helicene synthesis. The platinum-catalyzed sequential intramolecular hydroarylation of diynes proceeded to give substituted [6]hehcenes (Scheme 21.50) [56]. 

This intramolecular hydroarylation of alkynes was also applied to the synthesis of imidazo[l,2-fl]pyridines by A -coupling of 2-aminopyridine, alkynes, and aldehydes [183]. Singh et al. reported that this reaction is successfully catalyzed by a heterogeneous catalyst composed by nano-FejO and KHSO SiOj, affording the desired imidazo[l,2-fl]pyri-dines in 55-89% yields. The catalyst could be recovered by applying an external magnetic field and reused six times without significant loss of activity. 

The transition-metal-catalyzed intramolecular hydroarylation of alkynes has emerged in recent years as a powerful tool for the atom-economical synthesis of carbocyclic and heterocyclic compotmds [101-104], including coiunarin derivatives [105-109]. In this context, Kitamura and coworkers have described the high-yield preparation of several angelicin derivatives 50 by intramolecular hydroarylation of 4-benzofuranyl alkynoates 49 using Pd(OAc)2 as catalyst (Scheme 23) [110]. The starting materials 49 are easily accessible substrates that can be generated by the condensation of... 

A catalytic enantioselective intramolecular hydroarylation of alkynes was conducted with a cationic palladium(II)/(5)-xyl-H8-BINAP complex (Scheme 18.33) [31], With electron-rich aryl groups such as 2-naphthyl on the nitrogen atom, the intramolecular hydroarylation proceeded efficiently to give chiral 4-aryl-2-quinolinone derivatives. This palladium-catalyzed reaction using (S )-xyl-Segphos... 

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