Experimental Support for Planar Pseudopericyclic Transition States in Thermal Cheletropic Decarbonylations

Low-temperature crystal structures of three pyrrolediones (3−5) and a furandione (9) were obtained and compared to structurally related compounds that cannot undergo decarbonylation. Systematic trends in bond lengths and angles are consistent with distortions along the reaction coordinate, in accord with the structure correlation principle of Dunitz. Since the pyrroledione and furandione rings in 3−5 and 9 are planar, these ground-state geometries prefigure the calculated planar, pseudopericyclic transition states.