Stereoselective hydroarylation

A net stereoselective hydroarylation of 3-arylpropynamides is achieved by the Pd-catlayzed reaction with Arl in the presence of HCOOH and EtjNH. 

Ruthenium(II)-Catalyzed Regio- and Stereoselective Hydroarylation of Alkynes via Directed C-H Functionalization... 

Albrecht [133] and Sankararaman [174] have independently synthesized the cyclometallated phenyl-1,2,3-triazol-5-ylidenes complexes 77a-b (Scheme 11). Both groups have found that the cyclometallation is selective for aryl groups in the 1-position of the l,2,3-triazol-5-ylidenes units presumably because the direct attachment to the nitrogen atom increases the acidity of the 1-substituted aryl group, making the deprotonation more facile. It is noted that 77b is a catalyst for the stereoselective hydroarylation of alkynes [174]... 

Oligomerizations, polymerizations and telomerizations are covered in other parts of this section, since C-C/C-C bond forming additions (dicarborations) are involved. Metal-mediated hydroalkylations of olefins with CH acidic compounds and hydroarylations with formal addition of aromatic C—H bonds to olefins are also known2, however, only a few examples of stereoselective applications have been reported (Section 1.5.8.2.6.). 

Several other examples of palladium catalyzed asymmetric hydroarylations and hydroalkeny-lations of norbomene have been reported with up to 93% ee71,12. The same reaction type was used in stereoselective synthesis of epibatidine 1, a non-opiate analgesic alkaloid73. 

Palladium(II) catalysts can also be applied to stereo- and regioselective syn hydroarylation and hydrovinylation of symmetrically and unsymmetrically substituted alkynes8 9. Thus, alkyl 4-hydroxy-2-alkynoates in the presence of Pd(OAc)2(PPh3)2, formic acid, and tributylamine in dimethyl formamide or acetonitrile are converted with aryl iodides in a one-pot syn hydroary-lation/cyclization reaction to give functionalized substituted butenolides. The reactions occur with high syn stereoselectivities and with good regioselectivities8. 

Interestingly, none of the stereoselective reactions described previously assembles a new chiral center instead, the applied strategies rely on previously existing, special chiral characteristics of the substrates (prochiral moieties or fluxional axial chirahty). In contrast to these approaches, a recently published report relies solely on catalyst design to generate a new stereocenter in the product through chelate-assisted hydroarylation (Scheme 23.54) [166]. The most selective Rh catalyst 31 uses a biaryl-substituted Cp ring as the structural element to induce enantioselectivity. After the reaction, the hydroarylation product with a new quaternary stereocenter can be isolated in 87% yield and 91% ee. 

Various functional alkenes were similarly synthesized with high regio and E-stereoselective manner via [RuCl2(p-cymene)]2/AgSbF6 catalysed hydroarylation of phenyl or ester substituted alkynes with aromatic carbamates. The addition of 5 equiv. of pivalic acid improves the reaction in 1,4-dioxane. The alkenyl ester is converted into carboxylic acid by using 2 equiv. of LiOH, whereas phenol derivatives were deprotected by addition of 10 equiv. of LiOH [(Eq. 82)] [166]. 

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