1. Introduction

The pyrazole is one of the most important heterocycles. It is a versatile fivemembered heterocycle and a lead molecule in pharmaceutical development, due to its wide range of biological activity (e.g., antimicrobial, anticancer, cytotoxic, analgesic, anti-inflammatory, antihypertensive, antiepileptic, and antidepressant activities) [1]. Some drugs containing pyrazoles (e.g., celecoxib, novalgin, ramifenazone, fipronil, rimonabant, and pyrazofurin) are already in the market. Because of this, research on the synthesis, structural characterization, and properties of pyrazoles is ever increasing. Currently, a search—using pyrazole as topic—on the Web of Science showed 14,000 results, 327 of which are reviews. Early works on the mass spectrum fragmentation pattern of pyrazoles date from the early 1970s. Over approximately one decade, a few scientists—including Thuijl et al. [2–6],

Bowie et al. [7, 8], and Finar and Millard [9]—investigated the mass fragmentation of substituted pyrazoles. In 2005, Santos et al. published a study focused on the mass fragmentation of substituted pyrazoles. Advances in the use, as an analytical tool, of gas chromatography coupled with mass spectrometry—including ionization modes such as electron spray and chemical ionization—and the use of highresolution mass spectrometry led to novel structural elucidation, fragmentation mechanisms, and other properties for pyrazoles. However, up until 2005, pyrazole fragmentation, as well as the substituent effect, had not been studied from the perspective of these advances. The aforementioned analytical tool brings new insights regarding structural and other properties of pyrazoles. Because of this, and considering that our objective in this chapter is to present a comprehensive study on the fragmentation pattern of substituted pyrazoles, contribute to the systematization of knowledge, and assist researchers in the characterization of pyrazoles via a comprehensive and versatile technique such as gas chromatography coupled with mass spectrometry, only the works published up until 2005 will be discussed here. Thus, in this chapter, we will present the studies on pyrazole fragmentation by gas chromatography coupled with mass spectrometry, in order to evaluate the substituent effect on pyrazole fragmentation and to present a comprehensive study on the fragmentation pattern of substituted pyrazoles.

phenyl-substituted pyrazoles. The authors included an unsubstituted pyrazole and a 1-methyl substituted pyrazole in the series, in order to evaluate the effect that this position has on the fragmentation trend of 4-substituted NH-pyrazoles. The simplest pyrazole analyzed was the unsubstituted pyrazole (1). The authors showed that the fragmentation follows two distinct routes. The predominant feature is expulsion of HCN (b, m/z 41, [C2H3N]) or H (a, m/z 67) from the [M]+• ion or

Trends for Pyrazole Fragmentation Determined by Gas Chromatography Coupled with Mass…

The second process involves the loss of a nitrogen from [M–H] (a, m/z 67),

the ions formed by the loss of HCN (b + c + d + f) and loss of N2 (e + g + [C3H]<sup>+</sup> +

z 39) and [C2H2N]<sup>+</sup> (c, m/z 40), respectively—this was not identified in the study of Thuijl et al. [2]. High-resolution measurements did not reveal the presence of

precursors: b and a. The transitions 39 ! 38 can be assigned to two processes: e ! g and d ! f—see Figure 2. The latter process was observed in the spectrum of 3(5),4-

4-Chloro- (2) and 4-bromopyrazole (3)—see Figure 3—have similar fragmentation. Although [M–H]+• is virtually absent, the loss of Br• (a, m/z 67) is followed by expulsion of N2 to give e, m/z 39. Two successive losses of HCN from [M]+• lead

<sup>+</sup>• (i, m/z 66), which formed [C2HN]<sup>+</sup>• (d, m/z 39) after elimination of

The mass spectrum of pyrazole 5 showed that the loss of HCN is still important

in unsubstituted pyrazoles. The expulsion of HCN and radical bromine was also observed in 3,4-dibromo (4). However, for pyrazole 4, the a ! e process (loss of N2) was absent. Instead of [C3H2N2Br]<sup>+</sup> being formed, a lost the second Br• to give

a bromine migration similar to that reported by Bowie et al. [7] for tribromoimidazole. Another example of a bromine migration is the formation of [CBr2]

also reported on the loss of N2 and HCN from [M–H], which leads to [C3H4]

) is 5:1. Elimination of acetylene also occurs but only to a very small extent. Apart from the loss of HCN and H• from the molecular ion, Khmel'nitskii et al. [10]

. Metastable defocusing indicated that [C2H2N]<sup>+</sup> (c, m/z 40) has only two

+

). The intensity ratio of

<sup>+</sup>• (h, m/z 14), which is also weak

<sup>+</sup>• (b, m/z 275–281) involves

+•

<sup>+</sup>• (e, m/

expulsion of HCN from [M–H] (c, m/z 40, [C2H2N])—see Figure 2.

dibromopyrazole 4 (see Figure 4), in which e, m/z 39 was absent.

(h, m/z 92, 94) in preference to [CH2]

for tribrominated pyrazoles. The formation of [C2NBr3]

which furnishes the cyclopropenyl ion (e, m/z 39, [C3H3]

DOI: http://dx.doi.org/10.5772/intechopen.81563

[C3] +•

[C3H4] +•

to [CH3Br]<sup>+</sup>•

[C3H2N2]

Figure 2.

79

Principal fragmentation of unsubstituted pyrazole 1.

HCN (Figure 4).
