Theory, Experiment and Unusual Features of Potential Energy Surfaces of Pericyclic and Pseudopericyclic Reactions with Sequential Transition Structures

Abstract

Recent computational and experimental studies of organic reactions that show sequential transition structures on the potential energy surfaces are reviewed. The specific focus is on pericyclic and pseudopericyclic reactions. A distinction is made between symmetric and unsymmetric systems; in the former, the intrinsic reaction coordinate connects the two transition structures. The importance of the change in the reaction coordinate for establishing the existence of the second transition structure and the influence of the energy of the second transition structure on the barrier height of the first and emphasized. In this latter context, corners on a reaction pathway are also significant. Lastly, the merging of two sequential transition structures is shown to give an "effective monkey saddle" transition structure.

Stereodynamics of E/Z isomerization in rotaxanes through mechanical shuttling and covalent bond rotation

“…This theorem has been challenged theoretically, 65 and exceptions consisting of transformations involving small molecules with particular symmetry features have been identified. 66 , 67 The realization of a rotaxane like that shown in Figure 5 B has yet to come, but the results described here, together with the broad knowledge available on molecular recognition in MIMs, suggest that it is not an impossible task.…”

A Shapeshifting Roadmap for Polycyclic Skeletal Evolution

“…In both steps, the carbonyl π-bond (C4 = O) is first weakened and then slowly recovers, indicating that participation of the adjacent π C–O * orbital facilitates these anionic migrations. , Population analysis using the CHELPG (CHarges from ELectrostatic Potentials using a Grid-based method) scheme also indicates negative charge accumulation and release about the carbonyl oxygen during both reactions. Thus, these rearrangements circumvent the Woodward–Hoffmann rules by exploiting the interaction of orthogonal orbitals at the migrating carbon, thereby enabling a pseudopericyclic or coarctate reaction. , Importantly, the compact structure of the polycyclic hydrocarbons studied here, which position multiple π-systems in proximity (homoconjugation), appears to facilitate these processes. These calculations further expand the mechanistic set of known reactions amenable to rapid shapeshifting dynamics …”

“…Thus, these rearrangements circumvent the Woodward−Hoffmann rules by exploiting the interaction of orthogonal orbitals at the migrating carbon, thereby enabling a pseudopericyclic or coarctate reaction. 57,58 Importantly, the compact structure of the polycyclic hydrocarbons studied here, which position multiple π-systems in proximity (homoconjugation), appears to facilitate these processes. These calculations further expand the mechanistic set of known reactions amenable to rapid shapeshifting dynamics.…”

Highlighting the Underrated Side of the High Profile Nitroso Entity as a Nucleophile

“…o ‐Quinone β‐Nitrosomethides (N‐ o QMs) of type VII or VIII encompass both the nitrosoalkene and the o ‐quinone methide ( o ‐QM) V or VI motifs (Figure 2b). [63–67,72–78] …”

“…[49,[69][70][71][72] o-Quinone β-Nitrosomethides (N-oQMs) of type VII or VIII encompass both the nitrosoalkene and the o-quinone methide (o-QM) V or VI motifs (Figure 2b). [63][64][65][66][67][72][73][74][75][76][77][78] The NO group (in its E/Z conformations) (Figure 2) exhibits its electrophilic or nucleophilic profile, [65][66][67] driven by rearomatization of the structure. Intriguing outcomes for VII or VIII, unlike their V or VI congeners, have been unveiled [65][66][67] or are in progress: [79][80][81][82] The intramolecular peri 1,5-(NÀ ) or 1,6-(OÀ ) driven cyclisations to 63 or 64 expectedly succumb to geometry and energy constraints.…”