Enantioselective [2+2] Cycloaddition of Allenyl Imide with Mono- or Disubstituted Alkenes.

ABSTRACT

An efficient catalytic asymmetric [2+2] cycloaddition of allenyl imide and mono- or disubstituted alkenes is disclosed. The key feature of this method is the use of bidentate allenyl imide and weakly activated and less steric hindered alkene pair by utilizing chiral magnesium(II) complex of N,N'-dioxide, which could provide through-space dispersion interactions to orientate the arrangement of the alkene. This protocol allows the generation of a series of axially chiral cyclobutenes and four-membered ring-containing spirocycles (80 examples) in high yield (up to 99 %) with excellent enantioselectivity (up to >99 % ee), and the late-stage modification of biologically active molecules as well. Experimental studies and DFT calculations revealed that this [2+2] cycloaddition proceeded via a stepwise mechanism involving a short-lived zwitterionic intermediate. The π-π interaction between the alkenes and the amide moiety in the ligand was crucial for the enantiocontrol.