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

Vera Silva,José Elguero,Rosa Claramunt,Felipe Reviriego,Ibon Alkorta,Lourdes Infantes,Artur Silva,Ángela Martínez-López,Dionisia Sanz

doi:10.3390/molecules24030568

Abstract

The reaction in phase-transfer catalyzed conditions of 3(5)-methyl-1H-pyrazole with chloroform affords four isomers 333, 335, 355 and 555 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 (u) and down (d) conformation of the pyrazolyl rings with regard to the Csp3–H atom was established by X-ray crystallography and by 1H-, 13C- and 15N-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.

Highlights

N-unsubstituted pyrazoles, and in general N-unsubstituted azoles, react with chloroform in phase-transfer catalysis (PTC) conditions to afford trispyrazolylmethanes [1], the neutral equivalents of anionic scorpionates [2]
We have shown that in the reaction of polyhaloalkanes, such as dichloromethane, and chloroform with non-symmetrical 1H-pyrazoles, the proportion of isomers in the crude always follows a binomial expansion, in this case (a + b)3, where a + b = 1 [11,12]
The synthesis of N,N’,N”-3(5)-trimethylpyrazolylmethanes was performed following the protocol described by Juliá et al for the synthesis of N,N’,N”-triazolylmethanes [3]

Summary

Introduction

N-unsubstituted pyrazoles, and in general N-unsubstituted azoles, react with chloroform in phase-transfer catalysis (PTC) conditions to afford trispyrazolylmethanes [1], the neutral equivalents of anionic scorpionates [2]. The reaction of 3(5)-methyl-1H-pyrazole (1) with chloroform was reported in 1984 and the only compound isolated in a pure state was the tris(3-methylpyrazol-1-yl)methane. 333 derivative (1 H, CDCl3 , Csp3 -H: 8.11 ppm) (Figure 1) [3]. The reaction was reported again in 1999 and, surprisingly, the only isolated isomer (17% yield) was the 335 isomer (1 H, CDCl3 , Csp3 -H: 8.21 ppm) [7]. In 2012 the more hindered 555 derivative was prepared from tri(pyrazol-1-yl)methane (tpzm) by alkylation of the lithium derivative (1 H, CDCl3 , Csp3 -H: 8.31 ppm) [8].

Methods

Results

Conclusion