Helical Chiral 2-Aminopyridinium Ions: A New Class of Hydrogen Bond Donor Catalysts

摘要

In recent years, electrophile activation by small-moleculenhydrogen bond (H-bond) donors has become an important paradigmnfor asymmetric catalysis.1 Chiral (thio)ureas are among the mostngenerally applicable classes of H-bonding catalysts. Their remarkablensuccess can be attributed to the ability to form two H-bondsnto a substrate. Such two-point binding is thought to further activatena substrate and constrain it to a well-defined orientation necessarynfor asymmetric induction. Other types of chiral catalysts that arencapable of donating two H-bonds, such as amidinium,2 aminophosphonium,n3 guanidinium,4 pyridinium5 species, quinoliniumnthio-amides,6 squaramides,7 and sulfonamides,8 have also beennreported.9