Chirality without a stereocentre template

Penta-2,3-diene enantiomers (the end groups lie in perpendicular planes) and BINOL-type biaryl atropisomers, with the conditions for axial chirality.

What the template shows

• Allenes and hindered biaryls are chiral about an axis: no carbon carries four different groups
• Axial chirality
• Non-superimposable mirror images; the central carbon is sp and the end CH(CH₃) groups lie in perpendicular planes
• BINOL (1,1′-bi-2-naphthol)
• drawn flat; the two rings are twisted out of plane
• In an allene R(H)C=C=C(H)R the two end CH groups lie in perpendicular planes; with two different groups on each end the molecule and its mirror image do not superimpose.
• Biaryls with bulky ortho groups cannot rotate about the aryl–aryl bond: the twisted forms are atropisomers, separable when the barrier is above about 93 kJ mol⁻¹ (half-life over 1000 s at 300 K).
• Such molecules are named with axial descriptors (Ra/Sa, or P/M for helicity) from the CIP rules applied along the axis.
• BINOL and BINAP are axially chiral ligands used in asymmetric catalysis.
• Wedge = toward you, hash = away from you · every part of this figure is editable

Drawn with 22 atoms, 63 shapes – every one editable in MolDraw.

Sources

• Eliel & Wilen, Stereochemistry of Organic Compounds, Wiley 1994, ch. 14 Textbook
• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 14 Textbook
• Favre & Powell, Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013, RSC 2014, P-9 IUPAC