Chiral NHC-Catalyzed [4+2] Cycloadditions: Highly Diastereo/Enantioselective Access to Spiro[cyclohex-4-ene-1,3'-indoles] and DFT Calculations.

A highly efficient NHC-catalyzed cycloaddition of (E)-alkenylisatins and γ-chloroenals with a broad substrate scope has been developed to provide spiro[cyclohex-4-ene-1,3'-indole] in good yields (up to 99 % yield) with excellent diastereo- and enantioselectivities (up to >20 : 1 d.r., >99 % ee) under mild conditions without the use of metal and additives. Based on computational investigations, the role of the NHC on the diastereo- and enantioselectivity is discussed.