Kinetic Study of the Reaction of Benzofuroxans with 2-Acetylthiophene: Effect of the Substituents on the Reaction Rate Using Hammett Equation

The present work reports kinetic study of the reaction of benzofuroxan and its derivatives with 2-acetylthiophene. Hammett equation was used to determine the rate of the reaction and substituent effect. Specifically, chloro, nitro and methyl substituted benzofuroxans react with α-carbonyl compounds to form a fungicidal product quinoxalinesdi-N-oxide (phenazine N5, N10-dioxides). The effect of the benzofuroxan substituents on the reactivity was performed and monitored by a UV/Visible spectrophotometer. The rate constants of the benzofuroxan, 5chlorobenzofuroxan, 4-nitrobenzofuroxan, 5-methylbenzofuroxan and 4,6-dinitrobenzofuroxan reactions were found to be 3.32x10, 4.24x10, 3.48x10, 8.03x10 and 9.41x10 min respectively. Moreover, Log k/k0 against the substituent constant σ gave a linear relationship, which indicates positive effect of electron withdrawing substituent on the reaction. Therefore, the substituents have substantial effect on the reaction of benzofuroxans with 2-acetylthiophene.


Introduction
Furoxans and benzofuroxans are well known compounds that continue to attract particular attention due to a broad spectrum of biological activity, including antibacterial, antifungal, antileukemic, acaricide and immunodepressive properties. These compounds are widely used in organic chemistry as intermediate compounds for the synthesis of numerous heterocycles due to their electronic behaviors and remarkable substituent effect [1]. Therefore, furoxans and benzofuroxans have been described as promising prototypes to design new systems which can be used for biological and/or pharmaceutical purposes. Consequently, intense works in the field of medicinal chemistry of these systems have produced hybrid compounds containing moieties of both furoxans/benzofuroxans and classical drug in a single molecule [2].
Quinoxalines-di-N-oxides are known as potent biologically active agents. Therefore, subtherapeutic levels of quinoxalines-di-N-oxides have been used as animal growth promoters. Quinoxaline derivatives show interesting biological properties. The quinoxaline ring has been described as a bioisoster of quinolein, naphthalene and some other heterocycles which are the base of many antimalarial, antibacterial or antitumor agents such as quinine, mefloquine, pyrazinamide or tirapazamine. Therefore, quinoxalines are receiving increasing interest in the field of medicinal chemistry [3].
Cercetto et al., studied the effects of benzofuroxan substituents in the outcome of their expansion reaction with phenolates and found that the electron donating substituents decrease the electrophilic characteristic of the benzofuroxans nitrogen at position 3 in the favored 5-substituted tautomers, whereas, the electron withdrawing substituents have been found to increase the electrophilic characteristic of benzofuroxans [15].
Although the reaction kinetic of 1 and substituted benzofuroxans with 2acetylthiophene is very important, however, it has not been reported in literature. Therefore, the present paper reports the reaction rate and the effect of substituent of precursor compound benzofuroxans in the formation of quinoxalines using Hammett equation.

Results and Discussion
The preparation of benzofuroxans (chlorobenzofuroxan 1a, methylbenzofuroxan 1b, nitrobenzofuroxan 1c and 4, 6dinitrobenzofuroxan 1d proceeded as described in literature. The melting points and physical characteristics of the synthesized compounds were identical to the reported literature [16][17][18][19]. The kinetic reactions of benzofuroxans (chlorobenzofuroxan, methylbenzofuroxan, nitrobenzofuroxan and 4, 6-dinitrobenzofuroxan) were conducted at room temperature under pseudo-first order conditions in DMSO to establish an appropriate rate law for the reaction. ΔC represents the change in concentration. Therefore, the reaction rate, R, R = ΔC/Δt M/s (2) The rate constant (k) for the reaction of benzofuroxan (BFO), 2-acetylthiophene and TEA in DMSO was determined by the integrated rate law for first order reaction. k = 1/t lnα (3) where α = [BFO]t/[BFO]0. Thus, for each reaction the average values of k (min -1 ) and R (M/s) were obtained as described above.
From the kinetic experiment, the average value of rate (R) M/s and the rate constants (k) were obtained. The rate constants (k) of reactions were found to be 3.32x10 -3 , 4.24x10 -3 , 3.48x10 -3 , 8.03x10 -3 and 9.41x10 -3 min -1 as calculated by the integrated rate law.  The effects of substituent on the reaction rate of benzofuroxan with 2-acetylthiophene as demonstrated by the kinetic measurements were correlated to Hammett constants of the chlorobenzofuroxan, methylbenzofuroxan, nitrobenzofuroxan or and 4,6dinitrobenzofuroxan.

Structure reactivity relationships
The study of the effect of substituents on the reaction rate was carried out by the reaction of 1a, 1b, 1c and 1d with 2-acetylthiophene and compared with that of 1. Logk/ko = σ ρ (4) where ko, is the specific rate constant for a given reaction with an unsubstituted reactant, k is the rate constant of the reaction using a substituted reactant, ρ and σ are constants determined by reaction and substituent respectively 20]. Specifically, the rate constants of benzofuroxans reactions were related to the structure and composition of the parent molecule. Notably, the electron density of the ring influenced the reaction reactivity of both 1 and its substituents.
As seen in Table 1, the electron donating group (CH3) has a negative σ value. The negative σ value was anticipated as 1b has small rate constant. Alternatively, the electron withdrawing groups (Cl, NO2) have positive σ values. The reactivities of 1a, 1c and 1d followed their high rate constants. Thus, σ values reveal specific substituent effects. As shown in Table 1 and Figure 3, the plotting of Log k/k0 of electron withdrawing substituents versus σ gives straight line. While the electron donating group gives negative value. Thus, the positive value indicates that the substituents enhance the reaction. Alternatively, the negative value affects the reaction in the opposite way. Obviously, these relationships describe the susceptibility of the reaction to substituents, characteristic of Hammett plot [20]. Moreover, these results resemble that of Cercetto et al., [15].

Chemicals
Triethylamine (TEA) and o-nitroaniline were purchased from Koch-light laboratories Ltd. Colnbrook Bucks, England. Sodium nitrite, sodium azide and dimethyl sulphoxide (DMSO) were obtained from Hopkin and Williams Ltd. Chadwell Health Essex, England. Hydrochloric acid, nitric acid, sulphuric acid, glacial acetic acid, 2-acetylthiophene, ethanol and ether were purchased from Lancaster Synthesis, Eastgate, White Lundmore Camb, England. The chemicals were of analytical reagent grade and used without further purification.

Synthesis of benzofuroxan (BFO)
It was prepared by the method of diazotization of o-nitroaniline followed by thermal decomposition of o-nitrophenylazide. A dark yellow cluster was formed, recrystallized from 75% ethanol, melting point: 71-72 °C [16].

Kinetic measurements
Kinetic measurements were performed on 6505 UV/Vis. spectrophotometer, at room temperature. All kinetic runs were carried out in triplicate under pseudo-first order conditions with a mixture of benzofuroxan 1 x10 -4 M and 2-acetylthiophene 0.004 M and TEA 0.1 mL in DMSO through suitable interval time at  max 365 nm of the parent benzofuroxan as a function of time. The same procedure was repeated for the rate measurements of substituted benzofuroxans. The initial concentrations of 2acetylthiophene were 40 times concentration of benzofuroxan. TEA was used as a catalyst for the reaction.
The pseudo first order rate constants (k) were calculated by the integrated rate law for first order reaction (Equation 6).

Conclusions
The reactions of 1 and its derivatives (Cl, NO2, CH3) with 2-acetylthiophene to form quinoxaline-di-N-oxide proceed by the first order kinetics and the reaction rate depends on the reactivity of substituent, so that the reaction occurred more rapidly in the presence of an electron withdrawing group while the opposite occurs with an electron donating group.