Diastereodivergent Asymmetric 1,3‐Dipolar Cycloaddition of Azomethine Ylides and β‐Fluoroalkyl Vinylsulfones: Low Copper(II) Catalyst Loading and Theoretical Studies

• Published in: Angewandte Chemie International Edition 2019-11, Vol.58 (46), p.16637-16643
• Main Authors: Cheng, Feng, Kalita, Subarna Jyoti, Zhao, Zhen‐Ni, Yang, Xing, Zhao, Yan, Schneider, Uwe, Shibata, Norio, Huang, Yi‐Yong
• Format: Article
• Language: English
• Subjects: Adducts; Alanine; Asymmetry; azomethine ylides; Catalysts; copper; Copper compounds; Cycloaddition; Dipoles; fluorine; Glycine; Pyrrolidine; Stereoselectivity; synthetic methods
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201908227

A CuII‐catalyzed asymmetric 1,3‐dipolar cycloaddition using β‐fluoroalkyl alkenyl arylsulfones as dipolarophiles and glycine/alanine iminoesters as azomethine ylide precursors has been developed. Remarkably, a catalyst loading as low as 0.5 mol % is highly efficient. Accordingly, a wide range of enantioenriched 3‐fluoroalkyl pyrrolidines, as well as Δ2‐pyrroline and pyrrole derivatives, are generated in good to excellent yields with high asymmetric induction. This synthetic approach is diastereodivergent in that exo‐adducts could be converted into the corresponding exo′‐adducts by 1,8‐diazabicyclo[5.4.0]undec‐7‐ene mediated epimerization at C2 of the pyrrolidine core. The free‐energy profiles from DFT calculations suggest the Michael addition of the 1,3‐dipole to be the rate‐ and enantiodetermining step, and the origin of stereoselectivity is studied by means of the noncovalent interaction (NCI) analysis. A familiar ring: The commercially available chiral Cu(OAc)2/(S)‐tol‐BINAP catalyst is effective in the asymmetric 1,3‐dipolar cycloaddition of fluoroalkylated building blocks and azomethine ylides. The reaction is enantio‐ and diastereoselective. Notably, both exo‐ and exo′‐pyrrolidine derivatives can be accessed.