Studies in nitrosopyrazoles. Part 2.1 Solution and solid-state NMR studies of 3,5-disubstituted and 1,3,5-trisubstituted-4-nitrosopyrazoles. Crystal structures of 1,3,5-trimethyl-4-nitrosopyrazole and 3,5-di-tert-butyl-4-nitrosopyrazole

Abstract

Variable temperature NMR studies of a range of 3,5-disubstituted (R = R′ = Me, Bu^t) and 1,3,5-trisubstituted (R, R′, R″ = Me, CF₃, Ph or Buⁱ)-4-nitrosopyrazoles have led to the identification of individual rotational isomers at low solution temperatures arising from the slowing down of the rotation of the nitroso function with respect to the pyrazole ring. Rotational energy barriers [ΔG^‡ (298.15 K)], calculated by total NMR bandshape analysis, were in the range 38–65 kJ mol^-1. The –NO rotation process also occurs in the solid state and leads to disorder in the crystal structures of 1,3,5-trimethyl-4-nitrosopyrazole and 3,5-di-tert-butyl-4-nitrosopyrazole. The 1-H-3,5-disubstituted-4-nitrosopyrazoles exhibit rapid annular tautomerism in solution.