1,2-metallate rearrangements of a-alkoxyvinylborates

The rearrangement of borates derived from the addition of a trialkylborane to a lithiated enol ether was investigated, with a view to further functional-isation of the rearrangement product to afford a synthesis of tri-substituted double bonds.It was demonstrated that such borates may undergo rearrangement by one of two mechanisms, depending on the reaction conditions. In either case oxidation of the reaction mixture leads to formation of a keto alcohol, whilst treatment with a carboxylic acid prior to oxidation leads in one case to a diol and in the other to an alkenol.The former case rearranges by ^-addition onto the double bond, with concomitant 1,2-alkyl migration from boron onto the a-carbon. Oxidation then affords the keto alcohol, whilst treatment with a carboxylic acid induces a second, Matteson-type rearrangement, to afford a diol after peroxide oxidation. This mechanism, therefore, does not proceed through the intermediacy of a vinyl borane.The latter case rearranges, with assistance from chlorotrimethylsilane, via 1,2-alkyl migration from boron onto carbon, with opening of the dihydro-furanyl ring and hence does proceed through the intermediacy of a vinylbo-rane. Vinylborane chemistry may then be applied to this intermediate, such as halogenolysis or transmetallation followed by an electrophilic quench, to afford a synthesis of tri-substituted double bonds.The latter mechanism is also applicable to the rearrangement of borates derived from the addition of a trialkylborane to an ot-lithiated enol car-bamate, although chlorotrimethylsilane is not necessary for rearrangement to occur. Again, vinylborane chemistry may be applied to the reaction intermediate to afford a synthesis of tri-substituted double bonds.