Chiral Diene Ligands for Catalytic Asymmetric Reactions

开发催化不对称反应最重要的课题之一是设计和制备手性配体，使其在催化活性和对映选择性上与给定的反应有效匹配。许多含磷和/或氮原子的手性分子已被制备并用于过渡金属催化的不对称反应，但由于它们的催化活性或对映选择性较低，它们并不总是可用作手性配体。在本课题中，我们引入了一系列的手性二烯作为概念上的新手性配体，为不对称催化领域开辟了一个新的领域。手性二烯基于双环[2.2.1]七烯（nbd^*）、双环[2.2.2]八烯（bod^*）或双环[3.3.1]非二烯（bnd^*）骨架。手性二烯配体对铑催化的芳基硼试剂对缺电子烯烃的不对称1,4加成、对n -磺酰胺的1,2加成等反应非常有效。与其他类型的手性配体如手性双膦相比，用手性二烯配体观察到的对映选择性非常高。值得注意的是，铑催化剂与二烯配体的催化活性远高于与常规磷配体的催化活性，这使得用极少量（<0.005 mol %）的催化剂就可以实现催化不对称反应。

One of the most significant subjects for developing catalytic asymmetric reactions is the design and preparation of a chiral ligand which will fit in with a given reaction efficiently in catalytic activity and enantioselectivity. A number of chiral molecules containing phosphorus and/or nitrogen atoms have been prepared and used for transition metal-catalyzed asymmetric reactions, but they are not always useful as chiral ligands because of their low catalytic activity or enantioselectivity. In this research project, as conceptually new chiral ligands, we have introduced a series of chiral dienes which opened up a new dimension in the field of asymmetric catalysis. The chiral dienes are based on bicyclo[2.2.1]heptadiene (nbd^*), bicyclo[2.2.2]octadiene (bod^*), or bicyclo[3.3.1]nonadiene (bnd^*) skeleton. The chiral diene ligands were found to be very effective for rhodium-catalyzed asymmetric 1,4-addition of arylboron reagents to electron deficient olefins, 1,2-addition to N-sulfonylimines, and so on. The enantioselectivities observed with the chiral diene ligands are very high compared with those observed with other types of chiral ligands such as chiral bisphosphines. It is remarkable that the catalytic activity of the rhodium catalysts coordinated with the diene ligands is much higher than with conventional phosphorus ligands, which made it possible to realize the catalytic asymmetric reactions with a very small amount (<0.005 mol %) of the catalyst.