Determination of the Absolute Configuration by 3D ED to Elucidate the Atroposelectivity in Aromatic Ring-Opening Metathesis.

The determination of the absolute configuration of molecules bearing different stereogenic elements represents a fundamental and indispensable task in the field of stereochemistry. Whereas X-ray crystallographic analysis has been established as a broadly utilized and reliable technique to achieve this goal, limitations remain arising from the demanding requirements for the size and diffraction quality of the analyzed crystals. As an emerging technique, 3D microcrystal electron diffraction (3D ED) has increasingly been recognized to complement single crystal X-ray diffraction (SC-XRD). Having encountered challenges in determining the absolute configuration for the products obtained during the development of atroposelective aromatic ring-opening metathesis (AArROM), we herein describe in detail, how 3D ED allowed an assignment, which proved unfeasible using the conventional approach with X-ray crystallography.