Small-molecule H-bond donors in asymmetric catalysis

In the recent years, a remarkable number of new enantioselective reactions subject to H-bond donor catalysis have been identified, providing solutions to challenging transformations of importance to asymmetric synthesis. This can be attributed both the discovery of diverse H-bond donor motifs for catalysis and to the design of novel catalyst framework to encompass those motifs. Given the range of different acidities and structures for the various H-bond donor catalysts and the different classes of electrophile amenable to asymmetric catalysis by these donors. The fact that both phosphoric acids and thiourea derivatives, which reside on opposite ends of the spectrum of the pΚ_a scale of known H-bond donor catalysts, are capable of mediating enantioselective transformations of prochiral imminium and Ν-acyliminium ion intermediates is unexpected. While most Brønsted-acid catalysts are thought to promote reactions by electrophile activation via direct H-bond donation, the basic mechanisms of activaiton and stereoinduction in reactions of iminium and Ν-acyliminium ions are most certainly more complicated.