Titanium-Mediated Additions of Borohydride to Alkenes

This paper concerns additions of borohydride to alkenes promoted by titanium complexes. Isolated Cp₂-Ti(μ-H)₂BH₂ was shown to be a catalyst precursor for the hydroboration of phenylethene by borohydride. Lithium borohydride appears to be involved in the formation of the true catalytically active complex since borohydride/catalytic Cp₂Ti(μ-H)₂BH₂ mixtures gave faster hydroboration than stoichiometric Cp₂Ti(μ-H)₂BH₂ in the absence of borohydride. Furthermore, the catalytic and stoichiometric titanium-mediated hydroboration of phenylethene provided different regioselectivities. Regio- and/or stereoselectivities for the hydroboration of phenylethene and β-pinene also differ when mediated by BH₄^-/catalytic Cp₂Ti(μ-H)₂BH₂, or by BH3 generated in situ. Extensive ¹¹B NMR experiments indicate the predominant products in the hydroboration of phenylethene with borohydride are tetraalkylborates; minor amounts of alkylborohydrides are formed (mostly trialkylborohydride) and little or no alkylboranes. Alcohols are formed in the transformations mediated by BH₄^-/catalytic Cp₂Ti(μ-H)₂BH₂, after treatment with basic peroxide, but these result from oxidation of alkylborohydride intermediates since tetraalkylborates oxidize very slowly under the conditions used. A mechanism is proposed for the hydroboration of phenylethene mediated by Cp₂Ti(μ-H)₂BH₂; this involves abstraction of BH3 from the complex by borohydride leading to a titanium hydride species, conventional hydroboration of the alkene by BH3, insertion of phenylethene into the titanium-hydride bond, and then alkyl-transfer from titanium to boron giving tetraalkylborates. Differences for the hydroboration of other substrates are discussed also. 1-Decene and β-pinene react slower than phenylethene in reactions promoted by BH₄^-/catalytic Cp₂Ti(μ-H)₂BH₂, and monoalkylborohydrides or boranes tend to be the predominant products. These alkenes gave little or no tetraalkylborates, possibly due to the relatively slow insertion of these alkenes into titanium-hydride bonds or because of lack of titanium-to-boron alkyl transfer.