The stereospecific sigmatropic [1,5] carbon shift of C3 ammonium enolates is discovered. According to mechanistic, kinetic and computational experiments, this new rearrangement proceeds via the catalytic generation of a transient C3 ammonium enolate by intramolecular aza-Michael addition. This intermediate rapidly undergoes [1,5] sigmatropic carbon migration to furnish the respective tetrahydroquinoline-4-ones with excellent diastereoselectivities of d.r. > 99 : 1 and in 61-98 % yield.Rearrangement reactions are one of the most useful tools for construction of organic scaffolds. [1] In fact, [1,2] [2] and [3,3] [3] sigmatropic rearrangements are typical reactions used to achieve molecular complexity with high stereocontrol. Observations of sigmatropic shifts of hydrogen, silicon or acyl groups across extended π-systems, such as cyclopentadiene, indenyl [4] or quinone [4d, 5] structures, have provided a basis for in-depth understanding of organic chemistry [6] in addition to the synthetic benefits. Electrophilic nitrogenylide rearrangements, like the [1,2] and [2,3] Stevens [7] and Sommelet-Hauser [8] rearrangements, offer stereoselective access to synthetically useful amino building blocks through nitrogen to carbon chirality transfer. [9] The [1,2] Stevens progresses by homolytic CÀ N bond dissociation with formation of a caged radical pair, while the [2,3] Stevens and the Sommelet-Hauser rearrangements proceed by a concerted mechanism (Scheme 1a and b).Both rearrangements require, first, quaternization of the nitrogen atom and, as a second step, formation of the ylide by deprotonation. There have been only a few reported examples of catalytic creation of the ylide intermediate. [10] And although C3 ammonium enolates [11] are well-known intermediates in aza-Michael and Morita-Baylis-Hillman reactions, [12] cyclic C3 ammonium enolates have yet to be considerd as precursors for sigmatropic [1,5] rearrangements (Scheme 1c).
