exo to endo Isomerisation of the nitrite group in 3-nitrobicyclo[2.2.1]hept-5-ene-2-carbonitrile

Abstract

The Diels–Alder reaction of (E)-3-nitropropenenitrile (1) and cyclopentadiene yields a 9 : 1 mixture of 3-endo-nitrobicyclo[2.2.1] hept-5-ene-2-exo-carbonitrile (2) and the 3-exo-nitro-2-endo-carbonitrile isomer (3). On treatment with aqueous sodium hydroxide followed by reacidification, the adduct is isomerised to a mixture of (2), (3), and 3-endo-nitrobicyclo[2.2.1]hept-5-ene-2-endo-carbonitrile (6) in a ratio of ca. 46 : 22 : 32. The stereochemistry of the nitrite group appears to be established in a rapidly attained equilibrium during the deprotonation phase. Deuteriation studies rule out an Isomerisation pathway involving skeletal rearrangement by way of a tricyclo[2.2.1.0^2,6]heptane intermediate; instead, the Isomerisation probably proceeds through a dianion intermediate. Compound (6) itself isomerises to the same mixture of products under identical conditions, but produces (3) after contact with silica gel.