Advances on Asymmetric Allylic Substitutions under Synergetic Catalysis System with Transition Metals and Organocatalysts

Transition metal catalysis is one of the most important tools to accurately forge chemical bonds in modern organic synthesis. Organocatalysis, a biomimetic catalysis usually with metal-free small organic molecules, is a relatively young research area that started to flourish at the beginning of this century. Catalytic allylic substitutions are a kind of versatile reactions in organic chemistry; the combination of transition metal catalysis and organocatalysis in these reactions not only significantly expands the scope of nucleophiles, but also helps to resolve the stereocontrol issues. This paper will summarize the advance in the field of catalytic asymmetric allylic substitutions through synergetic transition metal-and organocatalysis. According to the source of chirality, these advances will be classified to three types. The first type is the catalytic asymmetric allylic substitutions induced by chiral transition metal catalysts. For these reactions, chiral ligands, including phosphine ligands and hybrid P, N ligands, have been used to achieve the high enantioselectivity. The non-chiral organocatalysts, such as pyrrolidine, Bronsted acids and boron reagents, were only used to activate the nucleophile or assist the generation of p-allyl metal intermediates. The second type is the catalytic asymmetric allylic substitutions induced by chiral organocatalysts. For the reaction of this type, various chiral organocatalysts, including chiral amines, chiral ureas and others, not only activate the substrates, but also control the enantioselectivity of allylic substitutions well through covalent and non-covalent bonds. Non-chiral ligands were only used to improve the catalytic capacity of transition metals. The last type is the catalytic asymmetric allylic substitutions induced by both of chiral transition metal catalysts and chiral organocatalyst. This strategy can not only realize the excellent stereo-control, but also achieve the challenging diastereo-diversity, if there exist continuous chiral centers. Overall, the joint utilization of transition metals and organocatalysts can achieve many significant asymmetric allylic substitutions that were previously difficult to realize through single transition metal catalysis. Meanwhile, the mechanism of representative transformations will be briefly introduced and at last, the prospective in this area will be given, such as simpler allylic sources and greener catalyst system.