Measurement of nitrogen inversion barriers and invertomer ratios in constrained azabicyclic systems

Abstract

Both possible nitrogen invertomers of the N-chloro-derivatives of 2-azabicyclo[2.2.1]hept-5-ene (1b) and its 3,3-dideuterio-derivative (5), 2-azabicyclo[2.2.1]heptane (2b), and 2-azabicyclo[2.2.2]oct-5-ene have been observed directly at low temperatures using ¹H n.m.r. the signals due to the minor invertomers being identified with the aid of homonuclear INDOR spectroscopy. Inversion barriers increase as the size of the carbon bridges is decreased, reaching ca. 14.8 kcal mol^–1 in the case of endo-→exo-(1b). exo : endo Invertomer ratios in these and other azabicyclic systems are discussed.