NHC-Catalyzed Enantioselective Dearomatizing Hydroacylation of Benzofurans and Benzothiophenes for the Synthesis of Spirocycles

Benzofurans and benzothiophenes have been efficiently employed as substrates in an enantioselective intramolecular hydroacylation. Breaking aromaticity in a 5-exo-trig cyclization of easily accessible heteroarenes by NHC-catalyzed hydroacylation gives access to a simple class of mainly unexplored spirocycles with up to 99% enantiomeric excess (ee). The formed product class bears interesting three-dimensional pseudoaxial chirality and shows typical ketone reactivity. Density functional theory (DFT) calculations reveal erosion of the ee via a stepwise mechanism bearing a hetero Wheland intermediate. Theoretical data are in agreement with deuterium-labeling studies and a linear correlation between electronics (σ) and the ee.