Book Description

Hydrogen bonding has been widely observed in biosynthesis and enzyme catalysis. It plays an important role in such fields as molecular recognition, supramolecular chemistry and small molecule catalysis. A new concept of organocatalysis emerged in recent two decades. Hydrogen bonding is the keystone for thiourea catalysis or phosphoric acid catalysis. With high turnover numbers and excellent enantioselectivity, transition metal catalysis not only is the arts in academia, but also find its merit in industrial application. Homogeneous hydrogenation, among many successful transition metal catalyzed reactions, has been serving the synthetic communities for many years, both in academia or in industry. The success of secondary interaction and hydrogen bonding offer us an alternative: the combination of hydrogen bonding and steric hindrance help to create a chiral environment in which substrates could be reduced efficiently. Guided by this rationale, a ferrocene-based bisphosphine/thiourea ligand, ZhaoPhos was synthesized in our group. It was applied in the asymmetric hydrogenation of nitroolefins with hydrogen bonding between the ligand and substrates. Thiourea-carbonyl hydrogen bonding is another model of non-covalent interaction. Therefore, the asymmetric hydrogenation of unsaturated carbonyl compounds is also practical. Besides neutral compounds, ionic unsaturated substrates could form non-covalent interactions with thiourea through anion binding as well. It makes the asymmetric reduction of iminium ions and N-heteroaromatics (quinolines, isoquinolines and indoles) possible. These type of unsaturated compounds were challenging for traditional transition metal catalyzed hydrogenation. They could now be achieved with high enantioselectivity with a rhodium/ZhaoPhos complex. An outer-sphere mechanism involving hydride transfer was proposed since these substrates were thought to lack of coordinating ability to the metal center. As a traditional catalytic system, rhodium/bisphosphine complexes have been applied to catalyze hydrogenation of C=C bonds in a long time. The application in hydrogenation of non- or weak coordinating unsaturated compounds changed our view towards of this class of complexes. In the meanwhile, the role of hydrogen bonding is still a mechanistically unexplored area: what is the geometry of the ligand-substrate complex? How does the hydrogen bonding influence the activation energy in the transition state? Many in-depth studies are needed in the future.