TITANIUM-MEDIATED ADDITIONS OF BOROHYDRIDE TO ALKENES

This paper concerns additions of borohydride to alkenes promoted by ti tanium complexes. Isolated Cp(2)-Ti(mu-H)(2)BH2 was shown to be a cata lyst precursor for the hydroboration of phenylethene by borohydride. L ithium borohydride appears to be involved in the formation of the true catalytically active complex since borohydride/catalytic Cp(2)Ti(mu-H )(2)BH2 mixtures gave faster hydroboration than stoichiometric Cp(2)Ti (mu-H)(2)BH2 in the absence of borohydride. Furthermore, the catalytic and stoichiometric titanium-mediated hydroboration of phenylethene pr ovided different regioselectivities. Regio- and/or stereoselectivities for the hydroboration of phenylethene and beta-pinene also differ whe n mediated by BH4-/catalytic Cp(2)Ti(mu-H)(2)BH2, or by BH3 generated in situ. Extensive B-11 NMR experiments indicate the predominant produ cts in the hydroboration of phenylethene with borohydride are tetraalk ylborates; minor amounts of alkylborohydrides are formed (mostly trial kylborohydride) and little or no alkylboranes. Alcohols are formed in the transformations mediated by BH4-/catalytic Cp(2)Ti(mu-H)(2)BH2, af ter treatment with basic peroxide, but these result from oxidation of alkylborohydride intermediates since tetraalkylborates oxidize very sl owly under the conditions used. A mechanism is proposed for the hydrob oration of phenylethene mediated by Cp(2)Ti(mu-H)(2)BH2; this involves abstraction of BH3 from the complex by borohydride leading to a titan ium hydride species, conventional hydroboration of the alkene by BH3, insertion of phenylethene into the titanium-hydride bond, and then alk yl-transfer from titanium to boron giving tetraalkylborates. Differenc es for the hydroboration of other substrates are discussed also. 1-Dec ene and beta-pinene react slower than phenylethene in reactions promot ed by BH4-/catalytic Cp(2)Ti(mu-H)(2)BH2, and monoalkylborohydrides or boranes tend to be the predominant products. These alkenes gave littl e or no tetraalkylborates, possibly due to the relatively slow inserti on of these alkenes into titanium-hydride bonds or because of lack of titanium-to-boron alkyl transfer.