Enantioselective Michael addition to vinyl phosphonates <i>via</i> hydrogen bond-enhanced halogen bond catalysis
Mikk Kaasik, Jevgenija Martõnova, Kristin Erkman, Andrus Metsala, Ivar Järving, Tõnis Kanger
Abstract
An asymmetric Michael addition of malononitrile to vinyl phosphonates was accomplished by hydrogen bond-enhanced bifunctional halogen bond (XB) catalysis. NMR titration experiments were used to demonstrate that halogen bonding, with the support of hydrogen-bonding, played a key role in the activation of the Michael acceptors through the phosphonate group. This is the first example of the use of XBs for the activation of organophosphorus compounds in synthesis. In addition, the iodo-perfluorophenyl group proved to be a better directing unit than different iodo- and nitro-substituted phenyl groups. The developed approach afforded products with up to excellent yields and diastereoselectivities and up to good enantioselectivities.