Organic syntheses, borane catalysts

Scc Brown Heim Yoon J. Am. Chem. Soc. 1970,92. 1637 Cragg Orgnnoboranes in Organic Synthesis Marcel Dckkcr New York, 1973, pp. 319-371. For reviews of reductions with BH,. sec Wade J. Mol. Catal. 1983, IS, 273-297 (BH, and a catalyst) Lane Chem. Rev. 1976.76.773-799. Aldrichimica Acta 1977,10.41-51 Brown Krishnamurthy Aldrichimica Ada 1979, 12. 3-11. For reviews of reduction with boranc derivatives, sec Peltcr Smith Brown Borane Reagents-, Academic Press New York. 1988. pp. 125-164 Peltcr Chem. Ind. (London) 1976, 888-896. 

The hydroboration of olefins is a classic reaction in organic synthesis. - Dialkylbo-ranes add rapidly to alkenes in the absence of catalyst. However, dialkoxyboranes, such as catecholborane and pinacolborane, add more slowly to olefins and alkynes. Thus, transition metal complexes could catalyze the addition of dialkoxyboranes to olefins and alkynes without interference from the background reaction. The potential to alter chemoselectivity, regioselectivity, enantioselectivity, and diastereoselectivity has led a munber of groups to develop metal-catalyzed versions of hydroboration. " Enantioselective hydroboration would alleviate the need to use boranes containing stoichiometric amounts of chiral substituents to generate optically active alkylboranes. 

The reaction of certain palladium-heteroatom complexes to alkenes and alkynes is a versatile tool for the synthesis of alkanes and alkene having heteroatoms attached. In particular, the various B-B, B-Si, and B-Sn compounds can be used for palladium-catalyzed borylation of alkenes and alkynes (Scheme 5-2). Borostannylation takes place at ambient temperature, whereas silylboration " only proceeds at a temperature above 80 °C due to the slow oxidative addition of a B-Si bond to a palladium(O) catalyst. Both reactions selectively provide cz j-products via addition of silicone or tin to the internal carbon and boron. The reactions are compatible with various functional groups for both terminal and internal alkynes. Cross-coupling reaction of boranes with organic halides selectively occurs at the terminal C-B bonds to provide regiodefined and stereodefined alkenylboron, alkenylsilicon, and alkenyltin compounds. 

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