Stereoelectronic control of the retro diels–alder reaction in 8‐(N‐pyrrolidyl)bicyclo[4.3.0]nona‐3,7‐diene isomers

Abstract

AbstractThe cis‐ and trans‐annulated isomers of 8‐(N‐pyrrolidyl)bicyclo[4.3.0]nona‐3,7‐diene show different propensities for the retro Diels–Alder fragmentation following electron impact ionization. Molecular ions of the cis‐annulated isomer decompose predominantly via the retro Diels–Alder reaction to give [C9H13N] +· fragments of the appearance energy (AE)=8.45±0.05eV and critical energy Ec=133±8kJ mol−1. The trans‐annulated isomer gives abundant [M–H]+ (AE=9.34±0.08eV) and [M–C6H6]+· fragments, in addition to [C9H13N]+· ions of AE=8.98±0.05eV and Ec=181±8kJ mol−1. The ionization energies (IE) were determined as IEcis=7.07±0.05 eV and IEtrans=7.10±0.06eV. The stereochemical information is much less pronounced in unimolecular decompositions of long‐lived (metastable) molecular ions which show very similar fragmentation patterns for both geometrical isomers. Nevertheless, the isomers exhibit different kinetic energy release values in the retro Diels–Alder fragmentation; T0.5=3.8±0.3 and 4.8±0.2 kJ mol−1 for the cis and trans isomer respectively. Topological molecular orbital calculations indicate that the retro Diels–Alder reaction prefers a two‐step path, with a subsequent cleavage of the C(5)C(6) and C(1)C(2) bonds. The open‐ring distonic intermediate represents the absolute minimum on the reaction energy hypersurface. The cleavage of the C(1)C(2) bond is the rate‐determining step in the decomposition of the cis isomer, with the critical energy calculated as 137 kJ mol−1. The cleavage of the C(5)C(6) bond becomes the rate‐determining step in the trans‐annulated isomer because of stereoelectronic control. The difference in the energy barriers to this cleavage in the isomers (ΔE=95k Jmol−1) provides a quantitative estimate of the magnitude of the stereoelectronic effect in cation radicals.