I think your notation of "Changing one atropisomer to another requires bond breaking (in some cases the removal and reattachement of steric groups, according to my understanding)" is not correct. To my knowledge, the free rotation of one single bond of an atropisomer is not restricted by connecting bond(s). I think you get confused by some literature explaining breaking a bond selectively directed to pair of atropisomers (e.g., Ref.1):
According to your post, IUPAC definition of atropisomers:
Atropisomers - A subclass of conformers which can be isolated as separate chemical species and which arise from restricted rotation about a single bond.
A variety of sources confirm this definition. For instance, Ref.2 and 3 described atropisomers as:
Atropisomers are stereoisomers (rotamers) resulting from hindered rotation about single bonds (having an energy barrier to rotation about a single $\sigma$ bond) usually due to steric hindrance. This energy barrier to rotation is high enough to allow the isolation of the conformers (rotamers).
Nonetheless, the name, coined by German biochemist Richard Kuhn in application to a theoretical concept 1n 1933 (Wikipedia) is derived from Greek (a = not and tropos = turn; thus, atropos meaning "without turn"), which supported the definition. However, the atropisomerism was first detected in 6,6'-dinitro-2,2'-diphenic acid by Cristie, et al. in 1922 (Ref.4):
It is also important to know the nomenclature of atropisomers (Wikipedia):
Determining the axial stereochemistry of biaryl atropisomers can be accomplished through the use of a Newman projection along the axis of hindered rotation. The ortho, and in some cases meta substituents are first assigned priority based on Cahn–Ingold–Prelog priority rules (Ref.5 & Ref.6). One scheme of nomenclature in based on envisioning the helicity defined by these groups (Ref.7). Starting with the substituent of highest priority in the closest ring and moving along the shortest path to the substituent of highest priority in the other ring, the absolute configuration is assigned $P$ or $\Delta$ for clockwise and $M$ or $\Lambda$ for counterclockwise. Alternately, all four groups can be ranked by Cahn–Ingold–Prelog priority rules, with overall priority given to the groups on the "front" atom of the Newman projection. The two configurations are termed $R_\mathrm{a}$ and $S_\mathrm{a}$ in analogy to the traditional $R/S$ for a traditional tetrahedral stereocenter (Ref.8}:
Atropisomers are also found in nature. The main pigment of cotton seeds, gossypol is an atropisomer, which exists in both form of $(R_\mathrm{a})$- and $(S_\mathrm{a})$-conformations. Most commercial Upland (Gossypium hirsutum) cottonseeds have an $(R_\mathrm{a})$- to $(S_\mathrm{a})$-gossypol ratio of ~$2:3$, but some Pima (Gossypium barbadense) seeds have an excess of $(R_\mathrm{a})$-gossypol. Between two isomers, $(R_\mathrm{a})$-gossypol is more toxic and exhibits significantly greater anticancer activity than its $(S_\mathrm{a})$-atropisomer:
Two fun facts about atropisomers are (KU.edu):
- Atropisomers are detectable by $\mathrm{NMR}$ if half lives exceed $\pu{10^{-2} s}$.
- Atropisomers are isolatable if the half-life is above $\pu{10^3 s}$.
References:
- Gerhard Bringmann, Thomas Hartung, “Atropo-enantioselective biaryl synthesis by stereocontrolled cleavage of configuratively labile lactone-bridged precursors using chiral H-nucleophiles,” Tetrahedron 1993, 49(36), 7891-7902 (https://doi.org/10.1016/S0040-4020(01)88014-5).
- Alan R. Katritzky, Christopher A. Ramsden, John A. Joule, Viktor V. Zhdankin, In Handbook of Heterocyclic Chemistry, Third Edition; Elsevier Limited: Amsterdam, The Netherlands, 2010 (ISBN: 978-0-08-095843-9).
- Jonathan Clayden, “Atropisomerism,” Tetrahedron 2004, 60(20), 4335 (https://doi.org/10.1016/j.tet.2004.03.002).
- George Hallatt Christie, James Kenner, "LXXI.—The molecular configurations of polynuclear aromatic compounds. Part I. The resolution of $\gamma$-6 : 6′-dinitro- and 4 : 6 : 4′ : 6′-tetranitro-diphenic acids into optically active components," Journal of the Chemical Society, Transactions 1922, 121, 614–620 (https://doi.org/10.1039/CT9222100614).
- R. S. Cahn, Christopher Ingold, V. Prelog, “Specification of Molecular Chirality,” Angew. Chem. Internat. Ed. Engl. 1966, 5(4), 385-415 (https://doi.org/10.1002/anie.196603851) and Corrigendum: Angew. Chem. Internat. Ed. Engl. 1966, 5(5), 511-511 (https://doi.org/10.1002/anie.196605111).
- G. P. Moss, "Basic terminology of stereochemistry: IUPAC Recomendations 1996," Pure and Applied Chemistry 1996, 68(12), 2193-2222 ()(PDF).
- http://goldbook.iupac.org/terms/view/H02763: IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Online version (2019-) created by S. J. Chalk. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook.
- http://goldbook.iupac.org/terms/view/A00547: IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Online version (2019-) created by S. J. Chalk. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook.
