Chiral guanidines and their derivatives in asymmetric synthesis
Over the past two decades, chiral guanidines and their derivatives have emerged as one of the most powerful organocatalysts mainly on the basis of their strong basicity and/or hydrogen-bond donor ability. Structurally diverse guanidine catalysts (bicyclic, monocyclic and acyclic types) sprang up and enabled numerous fundamental organic transformations to be realized in high efficiency and stereoselectivity. Moreover, chiral guanidinium salts were successfully employed in H-bond donor catalysis, phase-transfer catalysis and others. Recently, several novel chiral guanidine derivatives [e.g., guanidinium salt, pentanidium, bis(guanidino)iminophosphorane] have been designed and used for the synthesis of valuable molecules. In addition, the combination of chiral guanidines and their derivatives with cationic or anionic metal species dramatically expanded their utility and provided solutions to challenging transformations of importance which cannot be achieved with conventional chiral catalysts. This article reviews current achievements of chiral guanidines and their derivatives in organocatalysis, and updates versatile guanidine–metal salt combinations in asymmetric catalytic reactions. In addition, representative examples of achiral guanidine-related catalysis are also covered.