定义Definition
轴手性是分子没有手性中心、但取代基围绕一根轴的非平面排列使其与镜像不能重合而产生的手性，常见于取代丙二烯、阻转异构联芳烃和螺环化合物。
Axial chirality is chirality without a stereocentre, arising from a non-planar arrangement of substituents about an axis that makes the molecule non-superimposable on its mirror image — as in substituted allenes, atropisomeric biaryls and spiranes.
丙二烯Allenes
丙二烯两端的取代基位于互相垂直的平面内；当每一端的两个取代基不同（abC=C=Cab）时分子具有手性。范特霍夫 1875 年预言，1935 年首次拆分得到。
The substituents at the two ends of an allene lie in perpendicular planes; with two different groups at each end (abC=C=Cab) the molecule is chiral — predicted by van ’t Hoff in 1875 and first resolved in 1935.
构型标记Assigning configuration
沿手性轴观察，近端两个取代基优先于远端两个，按 CIP 规则排序后，1 → 2 → 3 顺时针为 Rₐ（或 M/P 螺旋描述）。
Viewed along the axis, the near pair outranks the far pair; ranked by CIP, a clockwise 1 → 2 → 3 is Ra (or use M/P helicity).
应用Applications
BINOL、BINAP、SEGPHOS 等轴手性配体是不对称催化的支柱；手性螺环骨架（如螺二氢茚，周其林等发展）也提供了高效配体。
Axially chiral ligands such as BINOL, BINAP and SEGPHOS underpin asymmetric catalysis; chiral spiro scaffolds (e.g. spirobiindanes developed by Zhou Qilin and others) give efficient ligands too.
年表Timeline
- 1875
范特霍夫预言丙二烯手性
van ’t Hoff predicts allene chirality
- 1935
梅特兰与米尔斯首次拆分手性丙二烯
Maitland and Mills first resolve a chiral allene