A new fundamental type of conformational isomerism

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 parvoand 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. Isolable compounds displaying new fundamental forms of conformational isomerism were last discovered in 1914 (atropisomerism) and 1961 (hindered pyramidal inversion). Now, a new form—referred to as akamptisomerism—has been described through four resolved stereoisomers of a transoid (BF)O(BF)-quinoxalinoporphyrin compound. The stereodescriptors parvoand amplo, necessary for their classification, have also been introduced.