Applications of Chiral Aluminum and Boron Catalysts in Asymmetric Synthesis

Download or read book Applications of Chiral Aluminum and Boron Catalysts in Asymmetric Synthesis written by Li Zheng and published by . This book was released on 2020 with total page 271 pages. Available in PDF, EPUB and Kindle. Book excerpt: A potent chiral aluminum catalyst has been developed for asymmetric MPV reduction of ketones with broad substrate scope and excellent yields and enantiomeric inductions. The catalyst consists an aluminum core, a VANOL-derived chiral ligand and an isopropoxy group. Different ligands have been screened and reaction parameters have been optimized. A variety of aromatic (both electron-poor and electron-rich) and aliphatic ketones were converted to chiral alcohols in good yields with high enantioselectivities (26 examples, 70-98% yield and 82-99% ee). This method operates under mild conditions (-10 °C) and low catalyst loading (1-10 mol%). Furthermore, this process is catalyzed by the earth-abundant main group element aluminum and employs inexpensive and environmentally benign 2-propanol as hydride source. This catalyst has also been employed in resolution of racemic alcohols. The kinetic resolution of alcohols by Oppenauer oxidation has been achieved with moderate results. The formal dynamic kinetic resolution via Oppenauer oxidation/ MPV reduction sequence has also been examined and discussed, which avoided acylation and the use of enzymes.A highly efficient asymmetric heteroatom Diels-Alder reaction between diene and aldehydes for the construction of 6-membered heterocycles catalyzed by chiral boron catalysts has been developed. A BINOL-derived propeller borate is found to be effective catalyzing the reaction of aromatic aldehydes. A VANOL-derived meso-borate is found to be able to catalyze the reaction of both aromatic and aliphatic aldehydes with high asymmetric inductions. Excellent yields and enantioselectivities have been achieved after optimization. Furthermore, the skeleton of 6-carbon saccharides is synthesized in the reaction of 2-hydoxyacetaldehyde with different protecting groups, which can be derivatized into many saccharide analogs. The mechanism of this reaction is proposed to be concerted based on experiments involving different methods for the reaction quench. A reversal of direction of the asymmetric induction by switching boron to aluminum has been observed. Computational studies show that catalysts derived from boron and aluminum have different geometries at the Lewis acid center.