Hydrometallation and Hydro-Heteroatom Addition

Compared with hydrosilylation of alkenes, less extensive studies have been carried out on hydrosilylation of alkynes. Mono- and dihydrosilylation occur depending on conditions. Yamamoto found concomitant dimerization-hydrosilylation of 1-heptyne catalyzed by Pd-PPhs catalyst to give a mixture of products 77, 78, and 79. Their ratios depend on the hydrosilanes used. The head-tail dimer 78 was the main product when HSiCb was used. The expected monohydrosilylation 

Oshima found that hydrosilylation with less reactive triorganosilanes can be carried out by using electron-rich phosphines. Hydrosilylation of terminal alkyne 80 with HSiPhs was carried out using PCyj as a ligand at room temperature to provide 81 regioselectively in high yield [23], 

Preparation of chiral benzylic alcohols has been achieved by asymmetric dihy-drosilylation of terminal arylalkynes, followed by oxidation [24]. The chiral diol 85 with 95 % ee was obtained by direct Pd-catalyzed asymmetric dihydrosilylation of phenylacetylene (82) with HSiCl3 using MOP [(/ )-VI-18], followed by oxidation of 1,2-disilylated product 84, but the yield of 84 was 33 % (the main product was l-trichlorosilyl-2,4-diphenyl-l,3-butadiene). Better results were obtained by hydrosilylation using two catalysts. The first Pt-catalyzed monosilylation to give 83 was followed by the second hydrosilylation catalyzed by the chiral Pd catalyst. The diol 85 with 95 % ee was obtained in 87 % overall yield by this method. 

Hydrostannation of alkynes proceeds smoothly to give alkenylstaimanes, which are used for further transformafions. The reaction is induced by transition metal catalysts and free-radical sources, giving different regioisomers 86 and 87 [25]. 

Pd complexes are effective catalysts for cis addition. Usually a mixture of the regioisomers 86 and 87 is obtained, and the ratio depends on the substituents of the alkynes and ligands. Hydrostaimation of 82 afforded equal amounts of the regioisomers 88 and 89. 

---