In this review we focus on the widely used Suzuki–Miyaura reaction and show that Pd/C is an efficient catalyst for carbon–carbon bond formation. The advantages and limitations of Pd/C are presented through selected examples. Mechanistic as well as practical aspects are also discussed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

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Asymmetric hydrogenation by using chiral transitionmetal complexes represents one of the cleanest and most environmentally benign processes available for producing optically pure organic compounds.[1] Currently, a wide variety of chiral compounds with outstanding levels of enantioselectivity has been synthesized by reduction of C=C, [1] C= O, [2,3] C=N, [4][5][6] and, more recently, heteroaromatics compounds. [7][8][9][10][11][12][13][14][15][16][17][18] Among heteroaromatics, 2-substituted quinolines have been targeted [9][10][11][12][13][14][15][16][17][18] because optically active 2-substituted-1,2,3,4-tetrahydroquinoline derivatives are key components of many bioactive natural products and drugs. [19] In contrast with the successful asymmetric hydrogenation of 2-alkyl-substituted quinolines catalyzed by chiral iridium complexes with an iodide source or iodine, [9][10][11][12][13] which dramatically enhances both catalytic activity and enantioselectivity, [5, 9a] only limited success has been achieved in the catalytic hydrogenation of 2-aryl-substituted quinolines. So far, the only known examples used 2-phenylquinoline as a unique model substrate. The first example of catalytic hydrogenation of 2-phenylquinoline (72 % ee (enantiomeric excess)), described by Zhou and co-workers, [9a] is based on anMeO-biphep/I 2 (cod = 1,5-cyclooctadiene; MeO-biphep = 6,6'-dimethoxy-2,2'-bis(diphenylphosphino)-1,1'-biphenyl) catalytic system that was recently improved to 80 % ee with a moderate yield of 41 % by using benzyl chloroformate as an activating agent, although this required an additional deprotection step on the resulting carbamate. [18] In recent years, only a few examples of the catalytic hydrogenation of 2-phenylquinoline have been reported with ee values up to 88 %. [14] We describe herein a highly enantioselective hydrogenation of the HX salts (X = Cl, Br, and I) of various 2-aryl-substituted quinolines by using cationic dinuclear iridium complexes [6,17] with [(4,4'-bi-2,2-difluoro-1,3-benzodioxole)-5,5'-diyl]bis(diphenylphosphine) (difluorphos), [20] which demonstrates an unexpected halide effect in which iridium complexes with chloro and bromo ligands serve as better catalysts than an iodo-iridium complex. The present catalyst was also effective for the hydrogenation of 2-alkyl-substituted quinolinium salts, which shows the high versatility of this new catalyst system. Furthermore, this system was applied to a formal asymmetric synthesis of selective estrogen receptor modulator (SERM) 6-hydroxy-2-(4-hydroxyphenyl)-1-{4-[2-(pyrrolidin-1-yl)ethoxy]-benzyl}-1,2,3,4-tetrahydroquinoline (1) [21] by asymmetric hydrogenation of the HCl salt of 6-methoxy-2-(4-methoxyphenyl)quinoline (2·Cl) as a key step.We recently developed cationic dinuclear triply halogen- [3][4][5][6][7][8] X= Cl, Br, and I) (Figure 1), which were conveniently prepared by adding excess aqueous HX to a mixture of [{IrClA C H T U N G T R E N N U N G (coe) 2 } 2 ] (coe = cyclooctene) and the required chiral diphosphine ligand in toluene at room...