Novel Transformations Utilizing [1,2]-Phospha-Brook Rearrangement Under Bronsted Base Catalysis

The [1,2]-phospha-Brook rearrangement involves the migration of the phosphorus moiety of the alkoxide of an alpha-hydroxyphosphonate from carbon to oxygen to generate an alpha-oxygenated carbanion. We have been focusing on the [1,2]-phospha-Brook rearrangement as a useful tool for the development of novel synthetic reactions under Bronsted base catalysis. For instance, we utilized the rearrangement for the catalytic generation of carbanions of less acidic compounds through the formal umpolung process from the corresponding carbonyl compounds. We have developed catalytic addition reactions of carbanions, such as enolates derived from less acidic amides and esters, alpha-oxygenated allyl anions that can serve as homoenolate equivalents and benzyl anions, as nucleophiles. We have also developed new efficient syntheses of functionalized allenes involving the generation of propargyl anions followed by regioselective protonation. The allenes were further applied to cycloisomerization to afford heterocyclic compounds, such as 2-aminofuran derivatives and indolizine derivatives. On the other hand, we utilized the [1,2]-phospha-Brook rearrangement for facilitating catalyst turnover in Bronsted base-mediated processes. Specifically, the [2,3]-Wittig rearrangement and the alpha-oxygenation of carbonyl compounds, which generally require a stoichiometric amount of Bronsted bases, have been successfully combined with the [1,2]-phospha-Brook rearrangement to proceed in a catalytic fashion.