It may be due to the following two reasons: 1) High steric hindrance caused by rigid structure of substituted fluorenone preventsthe approach of the chiral reductor;2) The prochiral center is, moreover, linked to two coplanar aromatic rings, which due to strong conjugation are highly electronically similar and thus it becomes extremely difficult to distinguish between them.

主要是以下两个原因:1)取代芴酮的刚性结构产生高的空间位阻,阻止了手性催化剂的接近;2)潜手性中心旁的两个芳香环不仅立体构型非常相似,而且高度的电子相似,很难被手性配体识别。