Hydroboration with NaBH

Although NaBH4 does not react on its own with unsaturated carbon— carbon bonds, alkenes and alkynes can now be hydroborated with NaBH4 (or LiBH4). A crown ether is needed with NaBH4, a cheaper but less soluble reagent than LiBH4. 

The process avoids the handling of diborane. The reaction is efficiently catalyzed by Ti derivatives such as Cp2TiCl2 [15] or TiCl3 [16]. 

The alkylborohydrides thus produced are converted to alcohols on oxidation, and alkenylboranes to alkanes by protonolysis. The hydroboration occurs preferentially in an anti-Markownikoff fashion. In contrast to what occurs with diborane, the Ti catalyzed hydroboration affords almost exclusively monohydroborated products (mixture of isomers) with alkynes or alkadienes, even in the presence of an excess of NaBH4. 

Another system, based on C0CI2—NaBH4 is also efficient for the hydroboration or hydrogenation of aUcenes [17]. It was possible to direct the reaction to give the hydroboration product by stirring the NaBH4 cobalt mixture in THF before addition of the olefin. Two moles of alkenes are hydroborated per mole of cobalt. Triply substituted double bonds do not react. 

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Acetylenes are reduced to cis-olefins by NaBHi, and Fe(II)/lipoamide and Fe(II)/dithiol catalysts in ethanol, and further to alkanes by NaBH, /-polyethyleneglycol/PdCl2 Allylic tosylates are selectively desulfonylated with LiBHEt3 in the presence of PdCl2(dppp) without isomerisation. RhCl(PPh3)3 changes the direction of the hydroboration of hexenone from C=0 to C C addition (Scheme 4). ... 

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