T-Selectride

Li(sec-Bu)3B H is accessible in an manner analogous to LiEtaB H, from Li H and sec-Bu3B . It has approximately the same reducing potential asLiEt3B H. However, since the stereoselectivity of hydride transfer from alkylboron hydrides increases with the size of 

Li-iec-BuaB H is also effective in the 1,4-reduction of a,/8-unsaturated esters. The label is incorporated cleanly into the chemically and metabolically stable position f3 to the carboxyl function. The reaction has to be performed at low temperature (—78 °C) in the presence of ferf-butanol as a proton donor in order to trap the enolate intermediate and prevent its undergoing Claisen condensation. Under these conditions even aromatic nitro groups are tolerated, as documented by the reduction of 258 to 259 °.  

Lithium [ H2]boratabicyclo[3.3.1]nonane (Li-BBNT, 262) is the tritiated isotopomer of a borohydride that has reductive characteristics intermediate between those of LiBH4 and the more powerful LiEtsBH therefore it is particularly useful for selective reductions. For 

Tritioborane of maximal isotopic purity is prepared by the treatment of a THF suspension 

This reaction gives a yield of about 60%. If cross-reactions between the LiBp4 byproduct and the reaction components are a concern, the B Hj THF solution can be vacuum transferred into a separate reaction flask. Tritio-borane can also be prepared by the reaction between NaB H4 and BF3 Et20 according to the stoichiometry 

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