  The study of cycloaddition mechanisms is central to the fabrication of
extended sp2 carbon nanostructures. Reaction modeling in this context has
focused mostly on putative, energetically preferred, exothermic products with
limited consideration for symmetry allowed or forbidden mechanistic effects.
Here, we introduce a scheme for classifying symmetry-forbidden reaction
coordinates in Woodward-Hoffmann correlation diagrams. Topological classifiers
grant access to the study of reaction pathways and correlation diagrams in the
same footing, for the purpose of elucidating mechanisms and products of
polycyclic aromatic azomethine ylide (PAMY) cycloadditions with
pentacene-yielding polycyclic aromatic hydrocarbons with an isoindole core in
the solid-state and on surfaces as characterized by mass spectrometry and
scanning tunneling microscopy, respectively. By means of a tight-binding
reaction model and density functional theory (DFT) we find
topologically-allowed pathways if a product is endothermic, and
topologically-forbidden if a product is exothermic. Our work unveils
topological classification as a crucial element for reaction modeling for
nanographene engineering, and highlights its fundamental role in the design of
cycloadditions in on-surface and solid-state chemical reactions, while
underscoring that exothermic pathways can be topologically-forbidden.
