Aryl-Csp³ bond rotation barriers of 2-aryl perhydropyrrolo[3,4- c]pyrrole-1,3-diones

(Chemical Equation Presented) The static and dynamic features of 2-aryl perhydropyrrolo[3,4-c]pyrrole-1,3-diones bearing N-acyl substituents have been assessed with the aid of crystal structures and VT NMR spectra. Rotation barriers for the aryl-Csp³ bonds in these molecules show surprising variation. Amide-substituted derivatives and fused piperazinediones (six-membered fusion) exhibit very substantial barriers of 14-15 kcal/mol. Fused benzodiazepinediones (seven-membered fusion) have lower but still significant barriers (10 kcal/mol), while fused hydantoins (five-membered fusion) have barriers that are too low to measure by VT NMR (≤10 kcal/mol). A rationale for the origin of the barriers is presented.