Palladium-catalyzed arylations

Palladium-Catalyzed Arylation of Enolates. Very substantial progress has been made in the use of Pd-catalyzed cross coupling for arylation of enolates and enolate equivalents. This reaction provides an important method for arylation of enolates, which is normally a difficult transformation to accomplish.171 A number of phosphine ligands have been found to promote these reactions. Bulky trialkyl phosphines such as /n. v-(/-butyl)phosphinc with a catalytic amount of Pd(OAc)2 results in phenylation of the enolates of aromatic ketones and diethyl malonate.172... 

Intramolecular cyclization using palladium-catalyzed arylation of nitro compounds has been reported recently (Eq. 5.71).114... 

Palladium-catalyzed aromatic C—O bond formation is less developed than palladium-catalyzed aryl amination. Except when the aryl halide is strongly electron deficient,107-110 catalysts ligated by the conventional aryl phosphines such as DPPF and BINAP are ineffective for coupling of... 

The palladium-catalyzed arylation of alkenes by haloarenes, and applications in furan synthesis have been described earlier (see Eq. 14 in Section IV,B, 1). By employing o-aminoiodoarenes and appropriately substituted (Z)-alkenes it has proved possible to develop an efficient synthetic route to quinolin-2-ones (Scheme 140).213... 

Van Srijdonck GPF, Boete MDK, van Leeuwen PWN (1999) Fast palladium catalyzed arylation of alkenes using bulky monodentate phosphorus ligands. Eur J Inorg Chem 1073-1076... 

The palladium-catalyzed arylation of 2-phenylphenols and naphthols shows an interesting feature of arylation of C-H bonds, leading to the formation of an (aryl)(aryloxy)palladium(n) intermediate.65,65a,65b The phenolates are suitable as precoordinating groups. The reaction of 2-hydroxybiphenyl with an excess of iodobenzene occurs regioselectively at the two ortho-positions of phenyl group under palladium catalysis (Equation (57)). In the case of 1-naphthol, the peri-position is phenylated (Equation (58)). 

The palladium-catalyzed arylations of aromatic carbonyl compounds such as ketones,67,67a amides (Equation (60)),68 and aldehydes69 with aryl halides and triflates give the multiple arylation products similarly. 

Palladium-catalyzed arylation of olefins and the analogous alkenylation (Heck reaction) are the useful synthetic methods for carbon-carbon bond formation.60 Although these reactions have been known for over 20 years, it was only in 1989 that the asymmetric Heck reaction was pioneered in independent work by Sato et al.60d and Carpenter et al.61 These scientists demonstrated that intramolecular cyclization of an alkenyl iodide or triflate yielded chiral cyclic compounds with approximately 45% ee. The first example of the intermolecular asymmetric Heck reaction was reported by Ozawa et al.60c Under appropriate conditions, the major product was obtained in over 96% ee for a variety of aryl triflates.62... 

Palladium chemistry of heterocycles has its idiosyncrasies stemming from their different structural properties from the corresponding carbocyclic aryl compounds. Even activated chloroheterocycles are sufficiently reactive to undergo Pd-catalyzed reactions. As a consequence of a and y activation of heteroaryl halides, Pd-catalyzed chemistry may take place regioselectively at the activated positions, a phenomenon rarely seen in carbocyclic aryl halides. In addition, another salient peculiarity in palladium chemistry of heterocycles is the so-called heteroaryl Heck reaction . For instance, while intermolecular palladium-catalyzed arylations of carbocyclic arenes are rare, palladium-catalyzed arylations of azoles and many other heterocycles readily take place. Therefore, the principal aim of this book is to highlight important palladium-mediated reactions of heterocycles with emphasis on the unique characteristics of individual heterocycles. 

A. Greiner and W. Heitz, New synthetic approach to poly(l,4-phenylene vinylene) and its derivatives by palladium catalyzed arylation of ethylene, Macromol. Chem. Rapid Commun., 9 581-588, 1988 and references therein. 

An unusual palladium-catalyzed arylative fragmenation process of /3-hydroxy-substituted allenes was observed by Oh et al. [59]. Compounds such as 8 reacted with aryl bromides and iodides in the presence of Pd(PPh3)4 and K2C03 as base to give 1,3-dienes 120 and aldehydes 121 as second fragment (Scheme 14.28). The initially expected cyclization product, dihydropyran 122 (Scheme 14.29), was usually not formed. 

The palladium-catalyzed arylation and alkenylation of terminal alkynes with aryl or alkenyl hahdes in presence of a copper(l) co-catalyst is called Sonogashira reaction. In the same way as in the other cross-coupling reactions described before, it is possible to immobihze the alkyne or the aromatic bromides, iodides or triflates on sohd supports (Scheme 3.15). 

General procedures for the synthesis of 10f/-[l]benzothieno[3,2- ]indoles 359a-c <2003T3737> and 2,4-dimethylpyrrolo[3,2-, ]indole 360 <1997J(P1)2843> using palladium-catalyzed intramolecular cyclization of the appropriate o-bromodiarylamines and palladium-catalyzed arylation of a cyano group, respectively, were described. 

Palladium-catalyzed arylation of the electron-rich olefin bntyl vinyl ether has been accomplished in the ionic liquid l-butyl-3-methylimidazolium tetrafluoroborate using as the arylating agents aryl iodides and bromides instead of the commonly used, but commercially unavailable and expensive, aryl triflates. The reaction proceeds with high efficiency and remarkable regioselectivity, leading almost exclnsively to substitution by various aryl groups at the olefinic carbon a to the heteroatom of butyl vinyl ether (Xu et ak, 2001). 

Palladium-catalyzed aryl-aryl coupling, 66, 70 PALLADIUM-CATALYZED COUPLING OF ARYL HALIDES, 66, 67 Palladium sponge, 66, 54... 

A number of methods are now available for the palladium-catalyzed arylation of alkenes. Mizoroki et al. showed that, in the presence of PdCl2, iodobenzene reacted with ethylene to give styrene.623 These reactions require a stoichiometric amount of base to neutralize the hydriodic acid formed (equation 172). 

A third method for the palladium-catalyzed arylation of ethylene involves aroyl chlorides as the source of the aryl group and is accompanied by decarbonylation (equation 178).629 [Pd(OAc)2]... 

Analogous to the palladium-catalyzed arylation of alkenes with aryl halides (Section 61.2.8.4), vinyl halides can be used to prepare dienes according to equation (191).649 A base is again needed in stoichiometric quantities. It should be noted that the dienes can be isolated as such only in certain cases. When vinyl iodide itself was used, only Diels-Alder products were obtained (Scheme 74). 

He, H.S., Zhang, C., Ng, C.K.-W. and Toy, P.H. (2005a) Polystyrene-supported triphenylarsines useful ligands in palladium-catalyzed aryl halide homocoupling reactions and a catalyst for alkene epoxidation using hydrogen peroxide. Tetrahedron, 61(51), 12053-57. 

Similar to alkenyliodonium salts (see Sect. 6.3), aryliodonium salts are highly reactive substrates in Heck-type olefination and other palladium-catalyzed coupling reactions. Aryliodonium salts can serve as very efficient reagents in the palladium-catalyzed arylations of acrylic acid 101 [75], organotin compounds 102 [76], sodium tetraphenylborate 103 [77], and copper acetylide 104 [78] (Scheme 45). 

Mukhopadhyay, S., Rothenberg, G., Wiener, H. and Sasson, Y. (1999) Palladium-catalyzed aryl-aryl coupling in water using molecular hydrogen kinetics and process optimization of a solid-liquid-gas system. Tetrahedron, 55, 14, 763. 

Kiji, J., Okano, T. and Hasegawa, T. (1995) Palladium-catalyzed arylation of ethylene (the Heck reaction) under aqueous conditions. J. Mol. Catal. A Chem.,... 

Radhakrishnan, U. Stang, P. J. Palladium-catalyzed arylation of enynes and electron-deficient alkynes using diaryliodonium salts. Org. Lett. 2001, 3, 859-860. 

Kashin, A. N. Mitin, A. V. Beletskaya, I. P. Wife, R. Palladium-catalyzed arylation of sulfo-nyl CH-acids. Tetrahedron Lett. 2002, 43, 2539— 2542. 

Beare, N. A. Hartwig, J. F. Palladium-catalyzed arylation of malonates and cyanoesters using sterically hindered trialkyl- and ferro-cenyldialkylphosphine ligands. J. Org. Chem. 2002, 67, 541-555. 

N. Aoyagi, Y. Palladium-catalyzed arylation of furan, thiophene, benzo[b]furan and ben-zo[b]thiophene. Heterocycles 1990, 33, 1951-1958. 

Pivsa-Art, S. Satoh, T. Kawamura, Y. Miura, M. Nomura, M. Palladium-catalyzed arylation of azole compounds with aryl halides in the presence of alkali metal carbonates and the use of Cul in the reaction. Bull. Chem. Soc. 

Scheme 19 Palladium-catalyzed aryl cyanation using a triaryl phosphine on polymer-support...

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