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Original article
12 (
8
); 2476-2483
doi:
10.1016/j.arabjc.2015.04.006

Kinetic study of nucleophilic reactivity of heterocyclic amines with 4,6-dinitrobenzofuroxan in acetonitrile

 Department of Chemistry, Faculty of Applied Sciences, Umm Al-Qura University, P.O. Box: 9569, Makkah, Saudi Arabia
Received: , Accepted: ,
© The Author(s) 2015
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

The reaction kinetics of 4,6-dinitrobenzofuroxan with five heterocyclic amines was investigated spectrophotometrically (UV–Vis) in acetonitrile at 20 °C. It was observed that the rate constants of these reactions increased as follows: 2-aminopyrimidine > 2-aminothiazole > 2-aminobenzothiazole > 5-amino-3,4-dimethylisoxazole > 2-amino-5-trifluoromethyl-1,3,4-thiadiazole. Further, second-order rate constants (k1) pertaining to the C–N and C–C bond forming step of these complexation processes fit to the three parameter equation log k (20 °C) = sN (N + E), allowing the determination of the nucleophilicity parameters (N) of the five heterocyclic amines. The heterocyclic amines were subsequently ranked on the comprehensive nucleophilicity scale defined by Mayr et al. (2003), providing a direct comparison of n-, π-, and σ-nucleophiles.

Keywords

Heterocyclic amine
4,6-Dinitrobenzofuroxan
Nucleophilicity
Kinetics
σ-C–N bonded Meisenheimer complexes

Abbreviations

DNBF

4,6-dinitrobenzofuroxan

TNB

1,3,5-trinitrobenzene

1

1 Introduction

Recently, the design of aromatic and heteroaromatic electron-deficient structures exhibiting high reactivity in nucleophilic aromatic substitutions and related σ-complex formation processes has attracted much interest (Terrier, 1991a, 2013b; Terrier et al., 2012c; Buncel et al., 1984; Strauss, 1970; Crampton and Rabbitt, 2000a; Crampton et al., 2003; Asghar and Crampton, 2007a,b; Asghar et al., 2008c; Asghar, 2010d, 2012e). Annelation of a nitro-substituted phenyl ring by intrinsically electron-withdrawing five-membered rings such as a furazan, furoxan, or triazole N-oxide ring represents various types of electron-deficient heteroaromatic structures that are considerably more electrophilic than 1,3,5-trinitrobenzene (TNB), the conventional aromatic electrophile in σ-complex chemistry (Cottyn et al., 2009; Bernasconi, 1970; Lakhdar et al., 2006; Boga and Forlani, 2001a; Boga et al., 2005b; Buncel et al., 1987).

4,6-Dinitro-derivatives exhibit extremely high electrophilic reactivity, which is reflected in their ability to form Meisenheimer complexes even with very weak nucleophiles (Cormack et al., 1988; Terrier et al., 1981; Norris et al., 1983; Strauss et al., 1981). Thus, enols (Norris et al., 1983), aromatic amines (Strauss et al., 1981; Read et al., 1984), and π-excessive heterocycles (Strauss et al., 1983; Terrier et al., 1984; Niclas et al., 1991) add to 4,6-dinitrobenzofuroxan (DNBF) 1 to form Meisenheimer anionic σ-complexes 1a.

The unique stability (approximately 10 orders of magnitude higher than that of similar derivatives of trinitrobenzene for the respective equilibrium constants for formation of the Meisenheimer) of such Meisenheimer complexes is explained by the high electron deficiency of DNBF 1, which is usually considered a “superelectrophile” (Terrier, 1991a, 2013b; Terrier et al., 2012c; Drozd and Knyazev, 1995; Kurbatov et al., 1997).

Currently, there is a considerable interest in the quantitative measurement of nucleophilic and electrophilic reactivities, particularly in carbon–carbon and carbon–nitrogen bond forming reactions (Terrier et al., 1976a, 1992b, 1993c; Buncel et al., 1997; Crampton et al., 1999a). Mayr and coworkers have used the reactions involving a series of benzhydryl cations to assess the nucleophilicities of carbanions stabilized by acyl, ester, cyano, and nitro groups (Mayr et al., 2003; Mayr and Ofial, 2004, 2005; Herrlich et al., 2001; Gotta and Mayr, 1998; Corral-Bautista and Mayr, 2013a; Corral-Bautista et al., 2015b; Kaumanns et al., 2009; Appel et al., 2009; Appel and Mayr, 2010). Conversely, the electrophilicity of some superelectrophiles, including 4,6-dinitrobenzofuroxan, has been determined by measuring the rate constants of their reactions with standard nucleophiles, such as N-methylpyrrole, enamine, and indole in acetonitrile (Terrier et al., 2004).

The ability to carry out the above reactions has led to the conclusion that DNBF can be a suitable probe to assess the reactivity of weakly nucleophilic carbon or nitrogen centres in general. Thus far, however, no quantitative assessment of the nucleophilic character of heteroaromatic amines has been made. In the present paper, we report the results of a kinetic study on the reactions of DNBF with five heterocyclic amines: 2-aminothiazole 2a, 2-aminobenzothiazole 2b, 2-amino-5-trifluoromethyl-1,3,4-thiadiazole 2c, 5-amino-3,4-dimethylisoxazole 2d, and 2-aminopyrimidine 2e.

2

2 Experimental

2.1

2.1 Materials

4,6-Dinitrobenzofuroxan 1 was prepared according to the procedure by Drost (Drost, 1899) with mp 172 °C. Heterocyclic amines 2a–e were of the highest purity available and were recrystallized or distilled before use whenever necessary. They were commercial specimens and were obtained from Sigma–Aldrich. Acetonitrile was distilled over P2O5 and stored under nitrogen.

2.2

2.2 Kinetics

Kinetic determinations were carried out on a conventional Shimadzu (model 3600 PC) UV–Vis spectrophotometer, whose cell compartments were maintained at 20 ± 0.1 °C. All kinetic runs were carried out three times under pseudo-first-order conditions with the 4,6-dinitrobenzofuroxan 1 concentration of ∼5 × 10−5 mol dm−3 and a concentration in the range 5 × 10−4–3 × 10−1 mol dm−3 for anilines. In a given experiment, the rates were found to be reproducible to 2–3%.

3

3 Results and discussion

The kinetic study was performed under pseudo-first-order conditions with the concentration of heterocyclic amines 2a–e in excess over the substrate DNBF 1 concentration (∼5 × 10−4). The reactions were monitored spectrophotometrically at wavelengths corresponding to the absorption maxima of the adducts. The formation of 3a (λmax = 475 nm), 3b (λmax = 472 nm), 3c (λmax = 482 nm), 3d (λmax = 474 nm), and 3e (λmax = 478 nm), was studied by mixing an acetonitrile solution of DNBF 1 (λmax = 418 nm) with heterocyclic amine solutions, i.e. 2-aminothiazole 2a, 2-aminobenzothiazole 2b, 2-amino-5-trifluoromethyl-1,3,4-thiadiazole 2c, 5-amino-3,4-dimethylisoxazole 2d, and 2-aminopyrimidine 2e in the concentration range 5 × 10−4–3 × 10−1 mol dm−3. Fig. 1 shows, as a representative example, the UV–Vis spectra for the progressive formation of σ-adduct 3b. Under these conditions, only one single relaxation time was observed. Fig. 2 shows the oscilloscope traces illustrating the unique relaxation process corresponding to the formation of σ-adduct 3b at various concentrations of 2-aminobenzothiazole 2b.

UV–Vis absorption spectra illustrating the progressive decomposition of 4,6-dinitrobenzofuroxan 1 (λmax = 418 nm) into the formation of adduct 3b (λmax = 472 nm) at T = 20 °C in acetonitrile.
Figure 1 UV–Vis absorption spectra illustrating the progressive decomposition of 4,6-dinitrobenzofuroxan 1 (λmax = 418 nm) into the formation of adduct 3b (λmax = 472 nm) at T = 20 °C in acetonitrile.
Oscilloscope traces illustrating the unique relaxation process corresponding to the formation of σ-adduct 3b (λmax = 472 nm) resulting from the reaction of 4,6-dinitrobenzofuroxan 1 with excess 2-aminobenzothiazole 2b ((a) [2b] = 0.01 mol dm−3, (b) [2b] = 0.02 mol dm−3, (c) [2b] = 0.04 mol dm−3) at T = 20 °C in acetonitrile.
Figure 2 Oscilloscope traces illustrating the unique relaxation process corresponding to the formation of σ-adduct 3b (λmax = 472 nm) resulting from the reaction of 4,6-dinitrobenzofuroxan 1 with excess 2-aminobenzothiazole 2b ((a) [2b] = 0.01 mol dm−3, (b) [2b] = 0.02 mol dm−3, (c) [2b] = 0.04 mol dm−3) at T = 20 °C in acetonitrile.

All the reactions obeyed first-order kinetics. Pseudo-first-order rate constants (kobs) were calculated from the equation ln(A∞ − At) = – kobst + ln(A∞ − Ao). Kinetic data were collected by using an integration method by plotting ln(A∞ − At) vs. time, where A∞ stands for absorbance at infinite dilution, and Ao and At represent the initial absorbance and absorbance at time t (s), respectively. The plots obtained for a constant concentration of DNBF 1 and a varying concentration of heterocyclic amines 2a–e produced a straight line. These plots indicate that the reaction is first-order (Fig. 3), which was developed based on the first-order rate constant integral equation. The kobs values and reaction conditions are summarized in Table 1.

Natural log of the absorbance of σ-adduct 3b plotted as a function of time for reactions at various concentrations of 2-aminobenzothiazole 2b ((a) [2b] = 0.01 mol dm−3, (b) [2b] = 0.02 mol dm−3, and (c) [2b] = 0.04 mol dm−3) at T = 20 °C in acetonitrile. Each reaction shows a linear relationship, indicating a first-order reaction. The slope of each straight line is the value of the observed rate constant (kobs).
Figure 3 Natural log of the absorbance of σ-adduct 3b plotted as a function of time for reactions at various concentrations of 2-aminobenzothiazole 2b ((a) [2b] = 0.01 mol dm−3, (b) [2b] = 0.02 mol dm−3, and (c) [2b] = 0.04 mol dm−3) at T = 20 °C in acetonitrile. Each reaction shows a linear relationship, indicating a first-order reaction. The slope of each straight line is the value of the observed rate constant (kobs).
Table 1 Kinetic results for the reaction of heterocyclic amines 2a–e with 4,6-dinitrobenzofuroxan 1 in acetonitrile at 20 °C.
[2a] (mol dm−3) kobs (s−1) [2b] (mol dm−3) kobs (s−1) [2c] (mol dm−3) kobs (s−1) [2d] (mol dm−3) kobs (s−1) [2e] (mol dm−3) kobs (s−1)
1 × 10−3 9.30 × 10−4 1 × 10−2 1.86 × 10−3 5 × 10−2 6.36 × 10−5 1 × 10−3 5.80 × 10−5 5 × 10−4 1.78 × 10−2
2 × 10−3 2.71 × 10−3 2 × 10−2 3.60 × 10−3 1 × 10−1 1.46 × 10−4 2 × 10−3 1.16 × 10−4 7.5 × 10−4 2.38 × 10−2
4 × 10−3 5.62 × 10−3 4 × 10−2 8.04 × 10−3 1.5 × 10−1 2.09 × 10−4 3 × 10−3 1.41 × 10−4 1 × 10−3 3.48 × 10−2
6 × 10−3 6.76 × 10−3 6 × 10−2 1.23 × 10−2 2 × 10−1 2.81 × 10−4 4 × 10−3 1.99 × 10−4 1.25 × 10−3 4.27 × 10−2
8 × 10−3 9.19 × 10−3 8 × 10−2 1.95 × 10−2 2.5 × 10−1 3.85 × 10−4 5 × 10−3 2.33 × 10−4 1.5 × 10−3 5.17 × 10−2
1 × 10−2 1.18 × 10−2 1 × 10−1 2.26 × 10−2 3 × 10−1 4.82 × 10−4 6 × 10−3 2.93 × 10−4 2 × 10−3 7.19 × 10−2

The pseudo-first-order rate constants observed (kobs) for all reactions obey Eq. (1) with negligible ko (≈0) in CH3CN (Fig. 4). The second-order rate constants, k1, were determined from Eq. (1) using the slopes of the kobs vs. [amine] plots. No third-order or higher-order terms were detected, and no complications were found in the determination of kobs or in the linear plot of Eq. (1).

(1)
k obs = k o + k 1 [ amines ]
Effect of the concentration of 2-aminothiazole 2a, 2-aminobenzothiazole 2b, 2-amino-5-trifluoromethyl-1,3,4-thiadiazole 2c, 5-amino-3,4-dimethylisoxazole 2d, and 2-aminopyrimidine 2e on the observed rate of formation of the DNBF adduct at T = 20 °C in acetonitrile.
Figure 4 Effect of the concentration of 2-aminothiazole 2a, 2-aminobenzothiazole 2b, 2-amino-5-trifluoromethyl-1,3,4-thiadiazole 2c, 5-amino-3,4-dimethylisoxazole 2d, and 2-aminopyrimidine 2e on the observed rate of formation of the DNBF adduct at T = 20 °C in acetonitrile.

This suggests that there is no base catalysis or noticeable side reactions, and that the overall reaction follows the mechanism described in Scheme 1. Treatment of DNBF 1 with excess 2 in acetonitrile solution resulted in the precipitation of an orange solid corresponding to the heterocyclic ammonium salt of adduct 3 (Scheme 1). Because of the strong acidifying effect exerted by a negatively charged DNBF moiety, the deprotonation of the NH2+ group of the initially formed zwitterions ZH± by 2, which acts as a base reagent, is a facile process and accounts for the formation of adduct 3; 4 is the thermodynamically stable product of the interaction and therefore 2 mol of 2 is needed to drive the overall equilibrium process to completion in acetonitrile solution. It is noteworthy that Forlani et al. have recently demonstrated the reactions of 2-aminothiazole 2a, 4-methyl-2-aminothiazole 2a′, and 4,5-dimethyl-2-aminothiazole 2a″ with superelectrophilic 4,6-dinitrobenzofuroxan (DNBF) 1, while exhibiting a somewhat higher nitrogen basicity than that of anilines, and 2a and 2a′ do not react as nitrogen nucleophiles, affording exclusively anionic C-bonded σ-adducts through electrophilic SEAr substitution of the thiazole ring by DNBF. Only in the case of the 4,5-dimethyl derivative 2a″ a N-adduct, was obtained (Forlani et al., 2006).

Scheme 1

Regarding the proton transfer step corresponding to the conversion of zwitterions ZH± into conjugate bases 3, it is noteworthy that Crampton and Rabbitt have reported that the deprotonation of the intermediate σ-adducts of type ZH± that occur during the addition of various amines to 4,6-dinitrobenzofuroxan is a both thermodynamically and kinetically favourable process (Crampton et al., 1999b). Based on the finding that the acidifying effect exerted by a negatively charged 4,6-dinitrobenzofuroxanyl structure on the ammonium site directly bonded to the sp3 carbon amounts to 2.5–3 pKa units, the pKa values for the deprotonation of adducts ZH± should be in the range 0–3, as compared with the pKa values in the range 2.37–5.91 for the parent heterocyclic ammonium cations. This is consistent with the essentially complete and fast formation of adduct 3 from the initially formed ammonium complexes ZH±.

All information is in perfect agreement with the mechanism described in Scheme 1, the reaction between 1 and 2a follows the pathway (A) with formation of anionic C-bonded σ-adducts, while the reaction between 1 and 2b–e follows the pathway (B) with formation of anionic N-bonded σ-adducts.

The second-order rate constants, k1, of the complexation reactions of DNBF 1 with 2-aminothiazole 2a, 2-aminobenzothiazole 2b, 2-amino-5-trifluoromethyl-1,3,4-thiadiazole 2c, 5-amino-3,4-dimethylisoxazole 2d, and 2-aminopyrimidine 2e at 20 °C in MeCN are 1.14, 2.40 × 10−1, 1.65 × 10−3, 4.53 × 10−2, and 36.43 dm3 mol−1 s−1, respectively. The addition of a benzo moiety to a thiazole group decreases the rate of reaction with DNBF by fivefold. The rate obtained for the reaction of DNBF 1 with 2c is the lowest because of the presence of the CF3 group, which is a very strong electron-withdrawing group and it decreases the reactivity of the nucleophile of the corresponding amine. Interestingly, the reaction of DNBF 1 with 2e yielded the largest rate constant, and thus amine 2e showed the highest reactivity of this series towards DNBF. This can be due to the size of the heteroaromatic ring, which leads us to conclude that heterocycles containing six atoms have a stronger nucleophilic reactivity than those containing five atoms. This behaviour was observed by Mayr and coworkers in the study of a series of 2-imidazolines and the related N-heterocyclic compounds with differently substituted benzhydrylium ions in dichloromethane at 20 °C. They showed that 2-phenyl-1,4,5,6-tetrahydropyrimidine has a higher nucleophilic reactivity than 2-phenyl-4,5-dihydro-1H-imidazole (Maji et al., 2013).

3.1

3.1 Determination of the estimated nucleophilicity parameters N of heterocyclic amines 2a–e

According to Mayr and coworkers, it is possible to describe the rates of a large variety of electrophile–nucleophile combinations by the three parameters as shown in Eq. (2) Mayr et al., 2003; Mayr and Ofial, 2004, 2005. From this equation, it is clear that the estimated E parameter expresses the strength of the electrophile, while the N and s parameters express the sensitivity of the nucleophile. According to Eq. (2), general electrophilicity (E) and nucleophilicity (N) scales have been identified. These scales have been found to be useful for predicting the reactivity (Mayr et al., 2001; Bug and Mayr, 2003; Bug et al., 2014; Phan and Mayr, 2006).

(2)
log k = s ( N + E ) On the basis of Eq. (2), E and N scales, each covering a reactivity range of about 30 orders of magnitude, have been defined and successfully used to assess the reactivity of many families of electrophilic and nucleophilic substrates. Among the variety of nucleophiles studied, a number of aromatic and aliphatic primary amines have been classified by Mayr on the N scale with the finding that the corresponding s parameter does not vary much with the aromatic and aliphatic primary amine structure (0.60 < s < 0.75) Kanzian et al., 2009; Nigst et al., 2012; Brotzel et al., 2007. Thus, combining an average value of s = 0.7 for N-Nucleophile and s = 0.9 for C-Nucleophile (Forlani et al., 2006) with the E value recently determined for DNBF (E = −5.06), along with the log k1 values from our kinetic measurements (k1) in acetonitrile, makes it possible to approximate the N values for 2a–e through Eq. (2). The obtained N values are given in Table 2.
Table 2 Positioning of heterocyclic amines structures on the nucleophilicity scale.
Heterocyclic amines k1 dm3 mol−1 s−1 N
1.14 (2.06)b 5.12a (5.56)b
2.40 × 10−1 4.17c
1.65 × 10−3 1.08c
4.53 × 10−2 3.14c
36.43 7.29c

The k1 values for the coupling of heterocyclic amines structures with DNBF (E = −5.06) in acetonitrile; T = 20 °C.

a Approximate N values through rearrangement of Eq. (2) into N = (log (k1)/s) – E with s = 0.9 for C-Nucleophile.
c Approximate N values through rearrangement of Eq. (2) into N = (log (k1)/s) – E with s = 0.7 for N-Nucleophile.

The resulting nucleophilicity parameters (N) for each of the heterocyclic amines are: N = 5.12 for 2a, N = 4.17 for 2b, N = 1.08 for 2c, N = 3.14 for 2d, and N = 7.29 for 2e.

The quantitative ranking of nucleophiles 2a–e on the nucleophilicity scale (Fig. 5) allows a comparison of their nucleophilic reactivity with that of related compounds previously classified on this scale. The experimental N values of 2a and 2b show that the addition of one benzo group of 2b to give 2a decreases the nucleophilicity strength by approximately one order of magnitude. Interestingly, the observed difference between the nucleophilicity values of 2a and 2b is comparable to those of other structurally related compounds such as 1-methyl-benzimidazole (N = 10.37) and 1-methyl-imidazole (N = 11.90), with ΔN = 1.53 (Baidya et al., 2010). On the other side, the addition of the methyl group on position 4 of 2-aminothiazole 2a increases the nucleophilicity strength by approximately two orders of magnitude.

Ranking of heterocyclic amines 2a–e on the N scale, as defined by Mayr et al. (2003).
Figure 5 Ranking of heterocyclic amines 2a–e on the N scale, as defined by Mayr et al. (2003).

The nucleophilicity of 2e compares well with that of 2-methylindole (N = 6.91) and was found to approach that of benzotriazole (N = 7.69) but is lower than that of other aromatic amines such as 4-amino-3,5-dibromopyridine and substituted anilines or aliphatic amines such as butylamine and allylamine (Fig. 5).

4

4 Conclusions

The above results provide a quantitative demonstration that DNBF 1, the standard superelectrophilic reference in σ-complexation and nucleophilic aromatic substitution processes, may be a suitable probe to assess the reactivity of weakly nucleophilic small molecules such as heterocyclic amines 2a–e.

Applying the general approach to nucleophilicity/electrophilicity developed by Mayr et al. (2003), the N parameters that quantify the nucleophilic reactivity of the heterocyclic amines 2a–e, have been determined in acetonitrile through kinetic investigations of σ-complexation reactions involving 4,6-dinitrobenzofuroxan 1 as the reference electrophile. It has been shown that these reactions form an integral part of electrophile–nucleophile interactions through the application of E and N scales. Overall, this relationship appears to be a useful probe for predicting the feasibility of the reactions of σ-complexation processes, which will be highly beneficial for broadening the range of synthetic and analytical applications in this field.

Acknowledgement

I would like to extend my gratitude to all my colleagues in the Chemistry Department, Umm Al-Qura University, for their helpful advice and constant support.

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