Palladium-catalyzed enantioselective and stereospecific assembly of axially chiral alkylidenecyclobutanes via N-tosylhydrazone-based carbene couplings

Synthesis of axially chiral alkylidenecyclobutanes exists sustained challenges by virtue of the compatibility of an efficient asymmetric catalytic system and the meticulous retention of its inherent strained ring structure. We herein disclose an enantioselective carbene cross-coupling reaction of cyclobutanecarbaldehyde-derived N‑tosylhydrazones with organohalides enabled by a combination of palladium catalysis and the modified sulfinamide phosphine ligand (Sadphos). This reaction proof the concept that axial chirality can be constructed on a strained metal carbene intermediate precisely through a sequential process of enantio-determined migratory insertion and β-H elimination. A variety of alkylidenecyclobutanes with heteroatom-substituted stereocenter, tertiary carbon stereocenter, and all-carbon quaternary stereocenter for particular, can be synthesized collectively with excellent yields and high enantioselectivities. Both the two enantiomers can be obtained via selecting the corresponding either cis or trans hydrazone substrates in a stereospecific manner. The synthetic applications of this asymmetric carbene coupling reaction are further demonstrated by the access of enantioenriched free amine by hydrolysis smoothly (99% ee) and the downstream transformations to reach versatile nitrogen-containing heterocycles without any enantiopurity erosion (99% ee).