Aromatic nitro substitution reaction between 4-nitro

Research Article Received: 15 October 2008,

Revised: 17 November 2008,

Accepted: 19 November 2008,

Published online in Wiley InterScience: 24 December 2008

(www.interscience.wiley.com) DOI 10.1002/poc.1504

Aromatic nitro substitution reaction between 4-nitro-N-n-butyl-1,8-naphthalimide and n-heptanethiol in water–methanol binary mixtures Eduardo Rezende Tribonia *, Julio Cesar Arturb, Pedro Berci Filhob, Iolanda Midea Cuccovia a and Mario Jose´ Politia The second-order rate constants of thiolysis by n-heptanethiol on 4-nitro-N-n-butyl-1,8-naphthalimide (4NBN) are strongly affected by the water–methanol binary mixture composition reaching its maximum at around 50% mole fraction. In parallel solvent effects on 4NBN absorption molar extinction coefficient also shows a maximum at this composition region. From the spectroscopic study of reactant and product and the known H-bond capacity of the mixture a rationalization that involves specific solvent H-donor interaction with the nitro group is proposed to explain the kinetic data. Present findings also show a convenient methodology to obtain strongly fluorescent imides, valuable for peptide and analogs labeling as well as for thio-naphthalimide derivatives preparations. Copyright ß 2008 John Wiley & Sons, Ltd. Keywords: nitro-1,8-naphthalimide; aromatic nitro substitution; kinetic; solvatochromism; water–methanol binary mixtures

INTRODUCTION The displacement of aromatic nitro groups has been a subject of increasing interest for more than three decades.[1–7] Several approaches concerning the role of nitroaromatic substrates against numerous nucleophiles and reaction conditions have been established, constructing an impressive background for studies and applications. These data can be retrieved in many reports concerning the preparation of macromolecular structures,[8] hybrid organic–inorganic materials,[9] synthetic intermediates,[10] and kinetic studies.[11,12] Currently, research in this area is focusing on attaining a nitro group with the highest nucleofugacity available under mild and clear-cut reaction conditions. For example, it is well known that when electron withdrawing groups are ortho or para positioned to the aromatic nitro leaving group these substitutions take place by a polar addition–elimination mechanism via Mesenheimer or s complex intermediate;[8] although another hypothesis involving a SNAr radical chain mechanism has been proposed.[13] In this context, nitroaromatic imides are among the best compounds that yield smooth nucleophilic substitutions and accordingly several derivatives and convenient synthons for material preparations are found in the literature.[14–17] Solvents and solvent mixtures also play important roles in aromatic substitutions reactions including rates, mechanisms, and equilibria, and often also on molecular electronic spectra.[18] In general binary solvents mixtures give rise to greater reaction rate modifications ranging from a solvent-rich region to a poor one that underlie the solvation aspects of the reactants. When the solvent mixture contains the ‘‘best’’ or the ‘‘worst’’ solvation

condition for the reaction it will lead to a maximum or a minimum in the reaction profile, this behavior is characterized as a synergistic effect. In addition, more striking cases exist where optical and kinetic molecular behaviors maintain a close relationship, which permits studies involving the influence of the microenvironment.[19] In a previous work,[20] our group reported on the reactions between 4-nitro-N-n-butyl-1,8-naphthalimide (4NBN) and thiols (n-heptanethiol, mercaptoethanol, and thiobenzene) in micellar aqueous media and in water. The catalytic effect of micelles was simply that of concentrating the reactants, such that the second-order rate constants in water (k2w) and in micelles (k2m) presented similar values. However, analysis of the results showed that the second-order constants decreased according to the increase in pKa indicating that thiolate attack is not the rate-limiting step, as commonly reported in the literature.[8,13]

* Correspondence to: E. R. Triboni, Departamento de Bioquı´mica, Instituto de Quı´mica da Universidade de Saõ Paulo, Av. Prof. Lineu Prestes 748, CEP 05508900, Saõ Paulo, SP, Brazil. E-mail: [email protected] a E. R. Triboni, I. M. Cuccovia, M. J. Politi Departamento de Bioquı´mica, Instituto de Quı´mica da Universidade de Saõ Paulo, Av. Prof. Lineu Prestes 748, CEP 05508900, Saõ Paulo, SP, Brazil b J. C. Artur, P. Berci Filho Departamento de Fı´sico-Quı´mica, Instituto de Quı´mica de Saõ Carlos, Universidade de Saõ Paulo, Av. Trabalhador Saõ Carlense 400, CEP 13566590, Saõ Carlos, SP, Brazil

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E. R. TRIBONI ET AL.

Table 1. Thiolate molar extinction coefficient at 240 nm (eRS) in water–methanol mixtures. Aromatic nitro substitution observed rate constant (kobs) and derived second-order rate constant (k2)

XMeOH Figure 1. Aromatic nitro displacement; thiolysis reaction

To further exploit this reaction, in the present work, we investigate the nitro displacement reaction between 4NBN and n-heptanethiol in water–methanol binary mixtures. The second-order rate constants (k2) were determined as a function of the methanol mole fraction of the mixtures and revealed a synergistic effect, with a maximum rate constant at 50% methanol mole fraction (XMeOH). It was found a near lying correlation between the rate constants and the optical features of the 4NBN characterizing in such way the determinant-kinetic parameter with the changes in the solvation on the 4NBN’s nitro group caused by the binary mixture.

1 0.79 0.63 0.50 0.39 0.30 0.22 0.16 0.09 0.05 0

eRS 103 (M1 cm1)

[RS] 105 M

kobs (min1)

k2 (min1 M1)

1.8 4.2 4.7 4.3 4.6 5.4 5.1 5.1 5.3 5.2 5.0

2.8 1.9 2.6 3.2 5.3 5.9 7.5 9.9 8.3 8.4 8.7

0.020 0.041 0.080 0.130 0.187 0.173 0.174 0.168 0.134 0.080 0.074

714 2157 3077 4062 3528 2932 2320 1697 1614 952 851

Figure 1 illustrates the SNAr reaction between 4NBN and thiolate (RS) and in Fig. 2 the UV/Vis spectra characterization of the reagent and of the product. It is clear that nitro substitution gives rise to new bands with maximum absorption peaks at lmax abs ¼ 394, 258, and 232 nm, respectively. The thio product is highly fluorescent, in contrast with the nonemissive nitro reagent; thus, making this an excellent synthon for thiol probes and as well for thiol labeling processes. The observed pseudo first-order rate constants (kobs) are presented in Table 1. Since the thiolate [RS] is the nucleophile,[20] prior determination of its concentration in each mole fraction of the water–methanol mixtures was required for the

kinetic calculations. The concentrations determined by spectrophotometry are also included in Table 1. The extinction coefficients of the thiolate (eRS) at 240 nm showed higher values going from methanol to water and practically doubled their values, concomitantly the maximum absorption changed bathochromically from methanol lmax abs ¼ 230 nm to water is lmax ¼ 240 nm (data not shown). For convenience, the spectral abs data presented were measured at 240 nm. The higher electron transition probability in more polar media indicates a more polarized excited state in relation to the ground state; i.e., the energy gap decreased with the increase in solvent polarity. In parallel, the cross-section area also increased with the increase in polarity, showing greater exposure of the HOMO electrons what reflect a major polarization of the thiolate electron-cloud in more polar aqueous media. The k2 values versus the methanol mole fraction (XMeOH) are presented in Table 1 and Fig. 3. The reaction rate constants had

Figure 2. UV/Vis of 4NBN (---) and 4RSBN (—) in H2O–MeOH (XMeOH ¼ 0.39)

Figure 3. Second-order rate constants, k2, versus methanol mole fraction, XMeOH

RESULTS AND DISCUSSION

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AROMATIC NITRO SUBSTITUTION REACTION their lowest values in pure solvents and increased approximately six-fold in XMeOH 0.5; thus, exhibiting a typical synergistic effect. In the region between XMeOH ¼ 0.16 and 0.39, precise determination of the rate constants was not possible due to a faster precipitation, so the rate constants presented are estimates calculated from the initial reaction rates. Interestingly the chemical analysis of the precipitated showed the formation of the 4HSBN product; whereas side products due to the attack of others nucleophiles, such as MeOH, OH, or H2O, did not occur and the imide group was not hydrolyzed. Despite the difficulty in determining the rate constant, this mole fraction region provided excellent condition for optimization of the 1,8-naphthalimide thio product synthetic preparation procedures. The two principal ways to rationalize the rate enhancement with the synergistic effect are: (i) a change in energetic parameters due to binary mixture application; and (ii) the occurrence of SNAr radical chain mechanism. The latter could be considered due to the fact that an excess of nucleophile, basic polar media, and electron donor RS/acceptor 4NBN reagents are a favorable condition for this mechanism.[13] This possibility was however dismissed when sulfur, a radical scavenger, was added to the reaction, since no variation was obtained in the substitution rates. Therefore, the SNAr sulfidation should be governed by the properties of the binary mixture and the solute/ solvent interactions and also by possible stabilization of the Mesenheimer intermediate. As first approach lets consider the optical evidences for the specific solvent/solute interactions that showed a closed relationship with the kinetic data. The spectrum of 4NBN as a function of methanol mole fraction is presented in Fig. 4. Besides the monotonic bathochromic maximum shift at the longer wavelength band (inset Fig. 4), an interesting change appears in the extinction coefficient maximum values of 4NBN (e4NBN) with XMeOH (Fig. 5). The curve profile shows a strong similarity to that for the rate constants (see Fig. 3). Since changes in absorption probabilities are attributed to variations in the molecular dipole moment in naphthalene chromophores,[21–23] that in electronic terms indicate a variation in 4NBN electron distribution by the binary mixture, an interesting correlation can be made between solvatochromism with the reaction driven force.

Figure 4. UV/Vis of 4NBN (2.0 105 M) in each methanol mole fraction of the water–methanol binary mixtures. Inset: monotonic bathochromic behavior of the lmax of the longest wavelength band (all other bands follow same pattern)

Figure 5. 4NBN molar extinction coefficient, e4NBN, values according to the methanol mole fraction, XMeOH

In addition, the solvatochromic property of the hydrogen-bond donor acidity (a) scale of water–methanol mixtures[24] showed a very close relation with both the rate constants and the change in e4NBN (Fig. 6). Maximum values for e4NBN and for k2 and minimum value for the solvatochromic property a occur at XMeOH 0.5. Further, it is reported that the nitro group conjugated with aromatic carbonyl undergoes specific H-bonding interactions in water–alcohol solvent that alter the electron spectrum of the chromophore.[25] Nevertheless, the effect of the water–methanol mixtures seems to follow specific solvent/solute interactions between the 4NBN nitro group and acidic hydrogen of the mixture expressed by the a property. Considering the nucleophilicity of the RS the observation of linear effects with the water fraction on its spectral parameters (lambda max, epsilon, and band width) would lead to a linear increase in k2. Therefore, the active group for the synergistic effect is assigned to the nitro group that is the solvation effect upon the nitro group dominates the kinetics. Thus, it is proposed a model depicted in Fig. 7, where in high H-bond mixture conditions the negative charge are stabilized on the oxygen atoms of the nitro group and the vicinal carbon atom becomes less susceptible to the thyolate attack (Fig. 7, resonance structure

Figure 6. Profile of the solvatochromic property a H-bond donor in each methanol mole fraction XMeOH

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Figure 7. Representative structures a and b with different electron distributions over the nitro group

b). In other hand in the low H-bond condition the charge spreads over the nitro group (as an usual zwitterion) favoring therefore the nucleophilic substitution (Fig. 7, resonance structure a). In other words, the electron density in the carbon vicinal to the nitro group present its lowest value at XMeOH 0.5 and summarizes the synergetic effect. In corroboration with the specific solvation of the nitro group, the reported small linear bathochromic shift with the increase in polarity for the nonsubstituted naphthalimide (NI, N-n-butyl-1,8-naphthalimide) follows a typical effect on p,p* transitions, and interactions over only the imide carbonyl groups.[26] Lets consider now the effects on the Mesenheimer intermediate. In light of a qualitative solvation model and considering only pure electrostatic interactions, according to the Hughes–Ingold rules,[18] the present reaction should have a small decrease in the rate with the solvent polarity increases because the thiolate negative charge would be dispersed in the mesomeric intermediate (Fig. 8). As can see in Table 1 the rate obtained in water has a small increase in relation to rate in methanol so that for a practical sense them can be considered identical. It is worth to note that this model takes no account on specific interactions, entropy changes, solvent reorientation rates, and changes in the solvent structure.[18] However, among these properties the H-bond specific interaction could be crucial for stabilization of the intermediate since that it is described a strong charge conjugation with naphthalimide carbonyl groups. But if this was occurring it would be expected an inverted kinetic behavior with fast rates in the region of more rich acidic solvent. Again, as exposed above, the specific solvation on 4NBN appears to be the kinetic driven force. Another aspect that should be considered is the spectral behavior of 4HSBN as a function of XMeOH (Fig. 9). Analysis of the spectrum showed that a synergistic effect similar to k2ap and to e4NBN values occurs at the longer wavelength and this also is

Figure 8. Electron distribution on the naphthalimide’s Mesenheimer intermediate

Figure 9. UV/Vis of 4HSBN (1.0 105 M) for each methanol mole fraction, XMeOH. Inset: lmax of the longest wavelength band versus XMeOH

achieved at XMeOH 0.5. Thus, the energy gap for the electronic transition appears to follow the same pattern according to the solvatochromic property of the hydrogen-bond donor acidity (a) scale of water–methanol mixtures. The naphthalimide derivatives with electron-releasing groups (such as amino, alkoxides, and thiolates) at position 4- of the naphthalene framework have a photophysical process denominated as intramolecular photoinduced charge transfer (PICT). The tendency for this process to take place is straightforward related to the solvent/solute interaction.[27] To the 4HSBN is clear that the changing on H-bonding donor property of the water–methanol mixture has a fundamental interference in the PICT process due to the specific H-bonding interaction between sulfur unpaired electron at position 4-. A review of the literature regarding naphthalimide derivatives revealed that these compounds are excellent medium reporters.[27] Such as, both 4NBN and 4HSBN showed a peculiar response to medium proticity in water–methanol binary mixtures; the former by altering electron cloud exposure and susceptibility to attack by the thiolate nucleophile and the latter by altering the energy gap due to H-bond formation with the unpaired sulfur electrons. Thus substitution at C4 in 1,8 naphthalimides leads to special chemical and physical properties.

CONCLUSION In conclusion, we demonstrated straight correlation behavior between kinetic and solvatochromism data on SNAr sulfidation on nitronaphthalimide. This reaction is affected by the water– methanol binary mixture, achieving great acceleration, in the region of lower hydrogen-bond donor acidity (a) scale of the mixtures. Based on UV/Vis changing in both thionucleophile and 4NBN we propose that H-bond specific solvation over 4NBN nitro group is the kinetic driven force for the observed synergism effect. Furthermore, the experimental protocol can be easily scaled up and optimized for preparation of thiol-naphthalene imide as well with others aromatics and heterocycles groups given the good clean up and the mild condition. This methodology also constitutes an excellent protocol for protein thiol-labeling processes.

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AROMATIC NITRO SUBSTITUTION REACTION

EXPERIMENTAL Materials All solvents were reagent grade. All chemicals were purchased from Aldrich. Acros Chemical Co. silica gel 60 (0.04–0.06 mm) was used for column chromatography. 1H-NMR spectra were recorded on a Bruker AC-200 in CDCl3. IR spectra were recorded with a BOMEM-MB 102-FT-IR Spectrometer, using KBr plates. UV/Vis spectra and Kinetic measurements were performed with a Cary 3E spectrophotometer. Capillary GC analyses were performed on a HP-5890 coupled to an MSD-5970 mass selective detector. The Bransonic ultrasonic cleaner bath 150 W/25 kHz was used to prepare the nitroimide. The melting points were determined in a conventional electrothermal melting point apparatus. Syntheses The imides 4NBN and NI were prepared following procedures previously described in the literature:[28] the respective anhydride (1 mmol) was placed in an eight-fold excess of n-butylamine in an aqueous suspension (40 mL) under sonication at room temperature; 2 h for 4NBN and 4 h for NI. Next, diluted HCl solution was added to the reaction vessel until neutralization and precipitation occurred. The precipitates were filtered, washed with water, and dried in an oven at 808C. For further purification, the products were flash-chromatographed in a silica gel column using chloroform as the eluent.

using samples of n-heptanethiol (ca. 0.01 mL of a 0.01 M stock solution in acetonitrile) added to a 1 cm optical path length quartz cell containing 2.0 mL of 0.1 M NaOH. Under these conditions, only the deprotonated thiol species are present and e at 240 nm was determined. All the reactions were carried out in a 1.0 cm quartz cell containing 2.0 mL of the desired binary mixture and 0.001 M of NaOH at 30 8C. Kinetics were also followed using tetraethylammonium hydroxide. The presence of Naþ or tetraethylammonium did not influence the reaction rates. 4NBN thiolysis reaction rates were conveniently followed by the appearance of the products at 394 nm (see Fig. 1). In all runs, the thiol concentrations were at least ten times greater than that of 4NBN (2.5 105 M) to ensure pseudo first-order kinetics (kobs). All the reactions were followed for at least ten half-lives and the rate constants were calculated from linear regression plots. The second-order rate constants were calculated by the standard relationship: k2 ¼ kobs/[RS]. The sulfur was used with radical scavenger in 1 and 5 equivalent in relation to 4NBN and the runs were made in the regions of XMeOH ¼ 0, 0.16, 0.50, and 1.

Acknowledgements The authors acknowledge the support of Brazilian grant agencies FAPESP, Proc. 04/15069-7, CNPQ and CAPES.

4-Nitro-N-n-butyl-1,8-naphthalimide (4NBN)

REFERENCES

Yield: 70%; yellow needles; mp 103.5–104 8C (literature[29,30] ¼ 103.5–104.5 8C); IR (KBr) y (cm1) ¼ 3074, 2961, 2872, 1706, 1656, 1624, 1530, 1347, 1231, 1082; 1H-NMR (CDCl3) d (ppm) ¼ 0.9 (t, 3H, CH3), 1.4 (sextet, 2H, CH2), 1.7 (quintet, 2H, CH2), 4.2 (t, 2H, CH2), 7.9 (t, J ¼ 8.0 Hz, 1H, Ar), 8.4 (d, J ¼ 8.0 Hz, 1H, Ar), 8.7–8.8 (m, 3H, Ar); CG-MS: m/z (%) ¼ 298 Mþ (100), 256, 243, 225, 209, 179, 151, 126, 75; Anal. Calcd for C16H14N2O4: N, 9.39; C, 64.42; H, 4.69. Found: N, 9.33; C, 64.38; H, 4.72. N-n-butyl-1,8-naphthalimide (NI) Yield 40%; white crystals; mp 95–97 8C (literature[31,32] ¼ 97– 98 8C); IR (KBr) 3075, 2948, 1697, 1658, 1623, 1589, 1352, 1266, 1073 cm1, 1H-NMR (CDCl3) d 0.9 (t, 3H, CH3), 1.5 (sextet, 2H, CH2), 1.7 (quintet, 2H, CH2), 4.2 (t, 2H, CH2), 7.7 (t, J ¼ 7.5 Hz, 2H, Ar), 8.2 (d, J ¼ 8.3 Hz, 2H, Ar), 8.6 (d, J ¼ 7.2 Hz, 2H, Ar); CG-MS (m/z) 253 (Mþ), 211, 197 (100), 180, 152, 126, 77; Anal. Calcd for C16H14NO2: N, 5.55; C, 76.20; H, 5.55. Found: N, 5.47; C, 76.32; H, 5.58. 4-Heptanethio-N-n-butyl-1,8-naphthalimide (4HSBN) 4HSBN was characterized after extraction of the reaction media with chloroform (30 mL) and solvent evaporation. A yellow precipitate was obtained: mp > 300 8C; IR (KBr) y (cm1) ¼ 3048, 2953, 1698, 1654, 1590, 1363, 1273, 1073; 1H-NMR (CDCl3) d (ppm) ¼ 0.9 (m, 6H, CH3), 1.3–1.5 (m, 8H, CH2), 1.6–1.8 (m, 6H, CH2), 3.1 (t, 2H, CH2), 4.2 (t, 2H, CH2), 7.5 (d, 1H, Ar), 7.7 (t, 1H, Ar), 8.4–8.6 (m, 3H, Ar); CG-MS: m/z (%) ¼ 383 Mþ (100), 327, 284, 229, 212. Kinetics and extinction coefficients Determination of the thiol molar extinction coefficients (eRS) for each mole fraction of the water–methanol mixtures were realized

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