Isomerism is a fundamental chemical concept, reflecting the fact that the arrangement of atoms in a molecular entity has a profound influence on its chemical and physical properties. Here we describe a previously unclassified fundamental form of conformational isomerism through four resolved stereoisomers of a transoid (BF)O(BF)-quinoxalinoporphyrin. These comprise two pairs of enantiomers that manifest structural relationships not describable within existing IUPAC nomenclature and terminology. They undergo thermal diastereomeric interconversion over a barrier of 104 ± 2 kJ mol−1, which we term ‘akamptisomerization’. Feasible interconversion processes between conceivable synthesis products and reaction intermediates were mapped out by density functional theory calculations, identifying bond-angle inversion (BAI) at a singly bonded atom as the reaction mechanism. We also introduce the necessary BAI stereodescriptors parvo and amplo. Based on an extended polytope formalism of molecular structure and stereoisomerization, BAI-driven akamptisomerization is shown to be the final fundamental type of conformational isomerization.
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The authors thank the University of Sydney for a Gritton Scholarship, the Australian Research Council (grants DP0666378, DP0773847 and DP150103137), the National Natural Science Foundation of China (NSFC; grant no. 11674212) and the Shanghai High-End Foreign Experts Grant for funding this research, as well as National Computational Infrastructure (NCI, d63) and INTERSECT (r88) for the provision of computing resources. The authors also give special thanks to G. Price for his help with the Latin terms. This work is dedicated to the stereochemistry pioneers James Kenner FRS 1885-1974 and Kurt Mislow 1923–2017.
The authors declare no competing interests.
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Definitions of isomerization and polytope formalism; Synthesis and characterization; DFT calculations; Nomenclature considerations
Compounds 2a, 2b, 3a, 3b
Compounds 4a, 4b, 5a and 5b
BAI associated vibrational mode
BAI reaction coordinate
Cartesian coordinates of all optimized molecular structures
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Canfield, P.J., Blake, I.M., Cai, ZL. et al. A new fundamental type of conformational isomerism. Nature Chem 10, 615–624 (2018). https://doi.org/10.1038/s41557-018-0043-6
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