Hydroarylation, of alkynes

Fig. 2.18 PaUadium and platinum NHC complexes as catalysts in hydroarylations of alkynes...

Hydroarylations of alkynes are catalyzed by gold complexes and these bear some resemblance to the Fujiwara Pd-catalyzed reaction. In general, when using gold chemistry, better Z/E selectivities are observed compared with palladium, lower catalyst loadings and milder conditions (neutral not TFA) are used. The mechanism involves the attack of ArH on the Au-coordinated alkyne. Flowever, electron-poor acetylenes only appear to work with palladium chemistry (Equations (75) and (76)).72... 

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... 

Dinuclear palladium complexes catalyze m-hydroarylation of alkynes with arenes.56 The reaction of 3-hexyne with benzene in the presence of a dinulear palladium complex Pd2R2(M-OH)(//-dpfam) [dpfam = j/V,Ar -bis[2-(diphenyl-phosphino)phenyl]formamidinate, R=/>-Tol] and tri(/z-butyl)borane at 100 °C for 4h affords ( )-3-phenyl-3-hexene quantitatively (Equation (53)). The hydroarylation of 3-hexyne with monosubstituted benzenes ( )-3-aryl-3-hexenes with a 2 1 ratio of the meta- and ra -isomers. This regioselectivity is different from that of the hydroarylation of diphenylacetylene catalyzed by Rh4(GO)12.57... 

As an extension of the Heck reaction, Pd-catalyzed hydroarylation of alkynes and alkenes continnes to attract high level of research interest in simple couphng processes and in cyclization reactions. The use of this type of transformation as part of a domino reaction will be of increasing interest. The research in the field of domino reactions is attracting considerable attention in synthetic organic chemistry since it enables the rapid assembly of complex molecirles in one-pot processes. Very elegant examples of palladium-catalyzed cascade processes where a single catalytic cycle entails several sequential bond transformations have been recently reported [la, b, 2a, b, c]. 

Ahlquist, M. Fabrizi, G. Cacchi, S. Norrby, P.-O. The Mechanism of the Phosphine-Free Palladium-Catalyzed Hydroarylation of Alkynes. J. Am. Chem. Soc. 2006, 128, 12785-12793. 

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]. 

From an atom economy perspective, He and Shi achieved efficient solventless hydroarylation of alkynes. Reaction took place efficiently under air atmosphere at ambient temperature and different functional groups could be tolerated. The catalytic system employed was AuCl3/AgTfO (1 3) and both inter-molecular and intra-molec-ular processes were described [128]. 

Scheme 1. Conventional (top) and direct (bottom) hydroarylation of alkynes.

General Procedure for Hydroarylation of Alkynes 4 and 3-Butyn-2-one 26 by Different Arenes with Ph3PAuCI under Optimized Conditions (Small-scale Experiments)... 

Fujiwara Reaction Palladium-catalyzed Hydroarylations of Alkynes and Alkenes... 

Scheme 1. General equations for hydroarylation of alkynes or alkenes.

Scheme 6. Possible mechanism for Pd-catalyzed hydroarylation of alkynes.

Very recently, it was found that montmorillonite-enwrapped Pd(II) catalyst had high activity in hydroarylations of alkynes or alkenes [8], The results are summarized in Table 4. The catalyst could be reused without any loss of activity. 

Gold-catalyzed Hydroarylation of Alkynes 157 Manfred T. Reetz and Knut Sommer... 

Ionic liquids (Coni.) cations used for, 253 characteristics of, 252-253 chemical and physical properties, 251 green credentials, 253 hydroarylations of alkynes in, 273 Kamlet/Taft parameters for, 255 oxidation, 278-279... 

The gold-catalyzed intermolecular hydroarylation of alkynes (or alkenylation of arenes) leads to 1,1-disubstituted alkenes (equation 91) 240-243 1 2-Disubstituted derivatives are obtained from alkynes with electron-withdrawing groups (equation 92). ... 

Hayashi. T.. Inoue. K.. Taniguchi, N., and Ogasawara, M., Rhodium-catalyzed hydroarylation of alkynes with arylboronic acids. 1,4-Shift of rhodium from 2-ai yl-l-alkcnylrhodium to 2-alkenylaryl-rhodium intermediate. 7. Am. Chem. Soc., 123, 9918, 2001. 

Scheme 20 Rhodium-catalyzed hydroarylation of alkynes with vinylic to aryl rhodium migration...

Metal-catalyzed hydroarylation of alkynes catalyzed by electrophilic transition metal complexes has received much attention as a valuable synthetic alternative to the Heck and cross-coupling processes for the synthesis of alkenyl arenes (384). Metal trifluoromethanesulfonates (metal triflates) [M(OTQn M = Sc, Zr, In] catalyze the hydroarylation of alkynes via 71 complexation to give 1,1-diarylalkenes in very good yields (Scheme 32) (385). The reaction likely proceeds by a Friedel-Crafts mechanism via the alkenyl cation intermediate where the aryl starting material also serves as the solvent. 

Scheme 33. Catalytic hydroarylation of alkynes by gold complexes.

The hydroarylation of alkynes can be performed in a catalytic manner at room temperature when a combination of HOOCCF3 and CH2CI2 is used as reaction medium [94]. The nature of the NHC ligand has little influence on the rate of the reaction. Arenes bearing alkyl, alkoxy, or halide substituents are competent substrates as well as various internal and terminal alkynes. 

Formation of heterocycles by transition metal-catalyzed hydroarylation of alkynes 05S167. 

Mono-substituted anilines bearing an internal alkyne can be cyclized to quinolines either by metal-mediated hydroarylation of alkynes or through an eleetrophile-mediated Friedel-Craft type reaction. ... 

As a typical intermolecular carbopalladation and termination, hydroarylation of alkynes are carried out extensively in the presence of HCO2H as a hydride source. Formation of regioisomers is observed in the reaction of asymmetric alkynes, and ratios depend on the nature of the substituents. High regioselectivity was observed in the reaction of the tertiary propargylic alcohol 14 to give 15 as a major product [5]. The (Z)-2-arylcinnamates 17, rather than 3-arylcinnamate 18, was obtained by the hydroarylation of methyl phenylpropiolate (16) [6]. 3-Substituled quinoline 21 was prepared by the regioselective hydroarylation of 19, followed by treatment of 20 with an acid without isolation [6]. 

---