Borane-Catalyed Syntheses Of Tetrahydroquinolines

Because of the prevalence of tetrahydroquinoline scaffords in natural products and drugs,development of methods for their syntheses and transformation is an important area of research.With the emergence of frustrated lewis pairs chemistry（FLPs）,organoborane have been found to be able to activate not only hydrogen with help of Lewis bases（eg.quinoline）,but alsoα-C–H bonds adjacent to a nitrogen atom.Using these reactivities,we have developed borane-catalyzed asymmetric hydrogenation reactions of quinolines and borane-catalyzed intramolecular cyclization reactions of vinyl-substituted arylamines,both to give tetrahydroquino-lines as the reaction products.The asymmetric hydrogenation of quinolines is a research subject of significant interest to synthetic community.However,the substrates investigated by most of the existing methods are limited to those with relatively inert substitutents which are inconvenient for further elaboration.To overcome this limitation,we developed a series of new chiral spiro bisborane catalysts generated by hydroboration of spiro[4,4]diene with HB（C₆F₅）₂（or HB（p-C₆F₄H）₂）.These catalysts gave excellent yields（TON up to 460）and enantiomeric excesses（up to 99%ee）with unprecedented broad functional group tolerance in asymmetric hydrogenation reactions of quinolines.Based on the reactivity that B（C₆F₅）₃ can abstract a hydride fromα-carbons of an amine,a catalytic amount of B（C₆F₅）₃,that functions as a transient hydride acceptor,is used to achieve intramolecular cyclizations of vinyl-substituted N,N-dialkyl arylamines.The mechanistic studies suggested a three-step mechanism including borane-mediated hydride abstraction,cyclization,and rebound of hydride from B（C₆F₅）₃.This protocol features broad substrate scope and provides easy acess to various synthetically useful tetrahydroquinolines.