Titanium-Mediated Additions of Borohydride to Alkenes
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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. © 1994, American Chemical Society. All rights reserved.
author list (cited authors)
Burgess, K., & van der Donk, W. A.