Catalytic enantioselective dearomatizing [2,3]-Wittig rearrangements allow divergent [2,3]-, [1,2]- and Sommelet–Hauser type products

The development of catalytic enantioselective dearomatizing [2,3]-rearrangement processes involving the C═C bond of a pendant (hetero)aromatic substituent remains a recognized significant challenge, primarily due to the energetic penalty associated with the loss of aromaticity in this process. The most common strategy in this area relies upon rearrangement from an onium ylide intermediate, followed by rearomatization, to generate Sommelet–Hauser-type products. Only limited enantioselective variants have been disclosed to date. In this paper, the enantioselective dearomatizing [2,3]-Wittig rearrangement of oxindole-substituted heteroaryl ethers employing a chiral bifunctional iminophosphorane (BIMP) catalyst is demonstrated. Solvent-dependent reactivity is observed, with 1,4-dioxane allowing preferential access to highly enantioenriched [2,3]-Wittig rearrangement products (up to 94:6 dr, 98:2 er). In mesitylene, the formation of enantioenriched [1,2]-Wittig products (up to 97:3 er) is favored via a cascade process consisting of initial [2,3]-rearrangement followed by an enantioretentive fragmentation/recombination event (equivalent to a [1,3]-rearrangement). Sommelet–Hauser-type products (up to 98:2 er) can be accessed by enantioselective [2,3]-Wittig rearrangement followed by selective tautomerization upon treatment with acid. Together, these processes deliver divergent access to heteroaryl-substituted 3-hydroxyoxindole derivatives in high yields and excellent stereoselectivities. Computational DFT studies provide insights into the origin of stereocontrol in the dearomatizing [2,3]-rearrangement event that underpins the observed stereoselectivity.