In-Cage Recombination Facilitates the Enantioselective Organocatalytic [1,2]-Rearrangement of Allylic Ammonium Ylides.

The [1,2]-rearrangement of allylic ammonium ylides is traditionally observed as a competitive minor pathway alongside the thermally allowed [2,3]-sigmatropic rearrangement. Concerted [1,2]-rearrangements are formally forbidden, with these processes believed to proceed through homolytic C-N bond fission of the ylide, followed by radical-radical recombination. The challenges associated with developing a catalytic enantioselective [1,2]-rearrangement of allylic ammonium ylides therefore lie in biasing the reaction pathway to favor the [1,2]-reaction product, alongside controlling a stereoselective radical-radical recombination event. Herein, a Lewis basic chiral isothiourea facilitates catalytic [1,2]-rearrangement of prochiral aryl ester ammonium salts to generate unnatural α-amino acid derivatives with up to complete selectivity over the [2,3]-rearrangement and with good to excellent enantiocontrol. Key factors in favoring the [1,2]-rearrangement include exploitation of disubstituted terminal allylic substituents, cyclic N-substituted ammonium salts, and elevated reaction temperatures. Mechanistic studies involving