Aromatic Rings with Boron and Silicon Substituents

5 AROMATIC RINGS WITH BORON AND SILICON SUBSTITUENTS 

The intramolecular diyne intermediate A formed in situ from alkynyl-boronate 2.144 and propargyl alcohol 2.145 forms metallocycle 2.146 with the catalyst [Cp RuCl (cod)]. The regioselective addition of the ruthenacycle 2.146 to the terminal alkyne 2.147 results in formation of arylboronate B. The latter without isolation undergoes further functionalization to form substituted biphenyl 2.148 in a satisfactory yield. 

It is important that this two-step route is achieved using components of the [2-I-2-I-2] cycloaddition in equivalent amounts. A variety of terminal alkynes may be used. The heterobiaryls can also be synthesized using 2-iodopyridine or 2-iodothiophene as the aryl iodide component in the coupling reaction [98]. Further development of this methodology expanded the use of arylboronates as multipurpose intermediates in other organic transformations. 

The development of new routes for the synthesis of phosphines is crucial for the synthesis of organophosphorus ligands for use in transition metal catalysis. Chemical transformations involving triple C-C bonds, such as nucleophilic addition and cycloaddition, are widely used for the synthesis of a diverse range of phosphines, which cannot be synthesized by other means [100], On the other hand, the [2- -2- -2] cycloaddition of alkynes catalyzed by transition metals are attracting much attention due to their use in the synthesis of various substituted aryl phosphines. This methodology can also be applied with other methods to synthesize various aryl phosphines [100, 101]. 

The final step is desulfurization to form phosphines 2.155 either by a radical-mediated process initiated with AIBN (azoisobutironitrile) or by sulfur-lone pair exchange using tris(dimethylamino)phosphine. Dicyclohexyl(2,6-diphenylaryl)phosphine synthesized by this method was used as an effective ligand in the palladium-catalyzed amination of alkyl halides. 

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