An Example of Polynomial Expansion: The Reaction of 3(5)-Methyl-1-Pyrazole with Chloroform and Characterization of the Four Isomers.

The reaction in phase-transfer catalyzed conditions of 3(5)-methyl-1-pyrazole with chloroform affords four isomers , , and in proportions corresponding to the polynomial expansion (a + b)³, with a = 0.6 and b = 0.4, a and b being 3-methyl and 5-methyl proportions. The up () and down () conformation of the pyrazolyl rings with regard to the Csp³⁻H atom was established by X-ray crystallography and by ¹H-, C- and N-NMR in solution combined with gauge-including atomic orbitals (GIAO)/B3LYP/6-311++G(d,p) calculations. A comparison with other X-ray structures of tris-pyrazolylmethanes was carried out.