Long-range coupling in strained derivatives of the 2,3-diazabicyclo[2.2.1]hept-2-ene radical anion: An EPR, ENDOR and TRIPLE-resonance study

Abstract

Radical anions of 2,3-diazabicyclo[2.2.1]hept-2-ene 1 and its 18 substituted and tricyclic derivatives (2a–11c) have been characterized by their hyperfine data with the use of EPR and ENDOR spectroscopy. The structural modifications of 1, yielding 2a–11c, are methyl and/or phenyl substitutions in the 1,4-positions, introduction of methyl groups or a spirocyclopentane ring in the 7,7-positions and annelation by a cyclopropane, cyclobutane, cyclobutene, cyclopentane or cyclopentene ring in the 5,6-endo-positions. Although these modifications do not markedly alter the π-spin and charge distribution in 1^˙-, they have a strong effect on the long-range coupling constant of the γ-protons in the 5,6-exo-positions: the pertinent a_H(γ) value varies from +0.26 to +0.64 mT. The effect is particularly impressive for the 5,6-endo-annelation by a cycloalkane or a cycloalkene ring and it goes along with only slight changes in the geometry of the carbon framework in 1^˙-. Thus, owing to the through-bond mechanism of π,σ-spin transfer (homohyperconjugation), the coupling constant, a_H(γ), of the 5,6-exo-protons responds sensitively to such changes in the geometry.