Phosphine-Catalyzed Tandem Reaction of Allenoates with Nitroalkenes

Application of a new chiral phosphepine to the catalytic asymmetric synthesis of highly functionalized cyclopentenes that bear an array of heteroatom-substituted quaternary stereocenters

Through the design and synthesis of a new chiral phosphepine, the first catalytic asymmetric method for the [3 + 2] cycloaddition of allenes with olefins has been developed that generates cyclopentenes that bear nitrogen-, phosphorus-, oxygen-, and sulfur-substituted quaternary stereocenters. A wide array of racemic γ-substituted allenes can be employed in this stereoconvergent process, which occurs with good enantioselectivity, diastereoselectivity, regioselectivity, and yield. Mechanistic studies, including a unique observation of a (modest) kinetic resolution of a racemic allene, are consistent with addition of the phosphepine to the allene being the turnover-limiting step of the catalytic cycle.

Phosphine-catalyzed β'-umpolung addition of nucleophiles to activated α-alkyl allenes

Highly functionalized alkenes can be prepared through phosphine-catalyzed β'-umpolung additions of nucleophiles (carbon-, oxygen-, nitrogen-, and sulfur-centered) to activated α-disubstituted allenes, providing many potentially useful synthetic intermediates in good to excellent yields, often with high levels of stereoselectivity for the product olefin geometry. Various substitution patterns around the allene are compatible with the process, showcasing the synthetic utility of allenes under the conditions of nucleophilic phosphine catalysis.

Unusual formal [4 + 2] cycloaddition of ethyl allenoate with arylidenoxindoles: synthesis of dihydropyran-fused indoles

An unusual DABCO-catalyzed formal [4 + 2] cycloaddition of ethyl allenoate, as a surrogate of a "1,2-dipole", with various arylidenoxindoles has been developed for the synthesis of dihydropyran-fused indoles. The DFT mechanistic study indicates that the cycloaddition takes place stepwise and the essential role of the catalyst is to raise the HOMO of allenoate.