- are a kind of atropisomerism.
- called molecular propeller because of their analogy with the propellers of airplanes or boat molecules of this type – consist of two or more subunits. – radiating from the axis of rotation – which my be single or combination of atoms.
- if the blades or subunits are identical in structure this may leads to symmetry as high as Dn.
- Molecular propeller is not confined to cases where the subunits are identical.
The examples of the three-bladed propeller are tri-substituted triaryl born. if the three aryl rings are not co-planar – the molecule is chiral.
if we assume that rings are perpendicular to the plane defined by the borane and the three attached carbon atoms of the aryl rings, there are four diastereomeric arrangements.
each arrangement exists in two enantiomeric forms, there are four racemic pairs.
The interconversion of stereoisomers has been considered in terms of the Flip Mechanism.