Propeller-like Molecules Several Chirality Axes

Propeller like Molecules (Several Chirality Axes) 

Structures of symmetry Dn can be viewed as n-bladed propellers where, in a way, n screw fragments are grouped around a real or fictitious central unit. This category includes above all triaryl-element and comparable polyaryl-derivatives, doubly bridged biphenyls, and a number of twisted molecules. Here again the apparent helicities can easily be determined by evaluating the 4-point figures imprinted into the structural formulae. 

This helicity descriptor should actually read (P,P,P) because in principle (though never observed as yet) one phenyl group could also be rotated in the contrary sense, thus leading to a (P,P,M) diastereomer. 

Tris-chelated compounds with three symmetrical bidendate ligands (in inorganic chemistry these are the prototypical tris-chelate complexes of transition metals) can also be conceived as three-bladed propellers with their helicities specified accordingly. 

Inorganic chemists prefer here the symbols A, A (lamda) and A, S (delta). 

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