Transition-metal-promoted hydroboration of alkenes: a unique reversal of regioselectivity

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Title: Transition-metal-promoted hydroboration of alkenes: a unique reversal of regioselectivity
Author: D'Souza, Christopher
Abstract: A unique reversal of regioselectivity was observed during the RhCb-nHbO catalyzed hydroboration/oxidation of 1-octene. While 4-octanol and 3-octanol were the major products, small amounts of octane and octanones were also obtained. Our experiments with isomeric alkenes revealed that the ratio of the isomeric alcohols was independent of the starting isomeric alkene. Under identical experimental conditions a similar reversal of regioselectivity was also observed with allylbenzene, but not with styrene and 3,3-dimethyl-1-butene. A highly efficient isomerization catalyst (Morrill's catalyst) generated in situ by the addition of catalytic amounts of a hydroborating reagent into a solution of RhCl3-nH20 in a suitable solvent like THF, has been found to be responsible for these novel results. Deuterium labeling experiments reveal very fast, reversible, addition-elimination steps, involving a rhodium-hydride species. It was found that ruthenium and iridium compounds also bring about a similar reversal of regioselectivity. Monitoring the isomerization of 1-heptene reveals the stepwise movement of the double bond towards the interior of the alkyl chain, accompanied by cis-trans transformations. A rhodium dihydride species has been implicated for the formation of alkanes; while the formation of ketones is a consequence of alkene insertion into the Rh-B bond, followed by p-hydride elimination. Two very valuable methodologies emerge from this research work. The first being the capability of converting a terminal alkene into internal organoboranes, while the second involves a very economical and highly efficient method for the isomerization of compounds containing double bonds.
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Date: 2001-06-01

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