Role of basicity and the catalytic activity of KOH loaded MgO and hydrotalcite as catalysts for the efficient synthesis of 1-[(2-benzothiazolylamino)arylmethyl]-2-naphthalenols

Abstract

Novel KOH-loaded MgO and hydrotalcite as basic catalysts have been synthesized and were characterized by XRD, SEM and their basicity. The effect of the KOH wt%, reaction time, basicity and catalyst loading have been investigated in three-component reactions of aldehydes, 2-amino benzothiazole and 2-naphthol. The present studies showed the impact of KOH loading and reaction time. The best conversion and yield at 70 °C obtained were 93% and 88%, respectively. Hydrotalcite could be reused four times without a loss of the catalytic efficiency. Present catalytic system requires a short reaction time, is non toxic, easy to workup, has a good catalytic activity and gives a high efficiency.

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1. Introduction

The scaffold decoration of bioactive molecules represents one of the most exciting research areas in organic chemistry and has a rich history within the realm of fragment-based drug design. o-Quinone methides (o-QMs) are highly reactive, transient species that have been applied as intermediates in the synthesis of several natural compounds, including flavonoids, isoflavans, chromenes and benzopyranes.^1–4 Multicomponent reactions (MCRs) are of increasing significance in organic and medicinal chemistry. Multicomponent reactions have been refined in recent years into a powerful and useful tool in synthetic chemistry.⁵ Such type of processes facilitate the rapid expansion of complex structures with highly efficient, a single purification step, low expenditures, time and energy saving, higher yield than almost any sequential synthesis of the same target and easy adaptation to combinatorial synthesis.⁶ Literature survey revealed that 2-aminobezothiazolomethyl naphthol derivatives have been synthesized with many drawbacks such as long reaction time, higher catalyst loading, low yield, higher reaction temperature and no reusability of catalyst.^7,8

In recent years, due to increasing concerns about the environmental impact, tremendous efforts have been made towards the development of new processes that minimize pollution in chemical synthesis. For this reason and others (catalyst removal, recovery, and recycling), heterogeneous catalysis is clearly on the rise within industry.⁹ Heterogeneous catalysts can be separated easily from the reaction mixture by filtration and then reused. In recent years, hydrotalcite and metal-loaded catalysts have been investigated more for biodiesel production.^10,11 Hydrotalcites can be involved in the preparation of catalysts which are dedicated to the production of H₂,¹² and a wide range of organic compounds.¹³ Hydrotalcite has also been used as catalyst¹⁴ with good catalytic efficiency, recyclability, reaction time, yield etc.

However KOH-loaded MgO and hydrotalcite have not been used as catalysts in the synthesis of 2-aminobenzothiazolomethyl naphthols by multi-component reactions. So in the present study, we have used KOH-loaded MgO as a catalyst in the multi-component reactions of 2-amino benzothiazole, aldehydes and 2-naphthol and found a good yield. But due to low catalytic efficiency of recycled catalyst, this catalyst failed to produce a better yield. Further, for removing this problem, hydrotalcite was used as a catalyst and good catalytic activity and recyclability were found. Synthesis was tried with hydrotalcite, which is rapid, with low corrosion to the reactor, easy to separate, inexpensive and highly efficient.¹⁵ Herein, for the first time, we report the synthesis of 2-aminobenzothiazolomethyl naphthols by the three-component reaction of aldehydes, 2-amino benzothiazole and 2-naphthol using KOH-loaded MgO and hydrotalcite as novel catalysts under solvent free conditions (Scheme 1).

Scheme 1 Synthesis using hydrotalcite or KOH/MgO at 70 °C.

2. Experimental section

General

The ¹H NMR spectra were measured by a BRUKER AVANCE II 400 NMR spectrometer with tetramethylsilane as an internal standard at 20–25 °C; data for the ¹H NMR are reported as follows: chemical shift (ppm), integration, multiplicity (s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet and br, broad), and coupling constant (Hz). IR spectra were recorded by a SHIMADZU IR spectrometer with the sample dispersed in a KBr pellet and are reported in terms of frequency of absorption (cm⁻¹). E-Merck pre-coated TLC plates and RANKEM silica gel G were used for preparative thin-layer chromatography. Melting points were determined on electrical melting point apparatus in an open capillary and were uncorrected. 2-Amino benzothiazole was purchased from Sigma Aldrich and other chemicals were purchased from Himedia, Mumbai, India and used without any further purification.

HPLC separations were carried out using an HPLC system (Water alliance e2695, separation module, USA) composed of a detector (2489 UV/Vis detector), vacuum degasser, quaternary pump, injector with a 50 μL sample loop, and a variable wavelength detector (VWD). The packing materials were dispersed in ethanol and packed in the column under 35 MPa with ethanol. The HPLC phase consisted of acetonitrile/10 mM ammonium acetate buffer pH 4.0 (30/70, v/v). The flow rate was set at 0.25 mL min⁻¹ and analysis was performed in an isocratic mode.

The surface area of the hydrotalcite samples was determined from the N₂ adsorption data measured at 77 K using Micromeritics, ASAP 2010. The samples were activated at 80 °C for 4 h under vacuum prior to the N₂ adsorption measurements. The specific surface area of the samples was calculated from the N₂ adsorption isotherms according to the BET method.

KOH-loaded catalyst preparation

The catalyst was prepared from magnesia with an aqueous solution of KOH. The catalysts were dried at 393 K for 16 h and calcined at 773 K for 5 h. X-ray powder diffraction patterns were obtained from calcined catalyst samples with a PC X-ray diffractometer using Cu Kα radiation at 40 kV and 40 mA and a scan speed of 2 °C min⁻¹. The surface morphology of the prepared catalysts was investigated by a scanning electron microscope (SEM).

Preparation of hydrotalcite

The catalyst hydrotalcite was synthesized using a literature procedure.¹⁶

Typical procedure. One-pot hydrothermal reactions at high temperature and autogenous pressure in aqueous media were carried out to obtain small and high surface area particles. In a typical reaction, Mg²⁺ and Al³⁺ hydroxide (metallic ratio of 3 : 1) were taken and the corresponding ratio of sodium bicarbonate was added to maintain the pH at 8.5. After aging the slurry, the precipitate was filtered, washed, and dried.

One-pot three component reaction

Typical procedure with KOH-loaded MgO. A mixture of the aldehydes (2 mmol), 2-naphthol (2 mmol) and 2-amino benzothiazole (2 mmol) was heated at 70 °C under solvent-free conditions using KOH-loaded MgO as a catalyst. The time taken by the different aldehydes for the reaction is mentioned in Table 2. After completion of the reaction (TLC analysis), the reaction mixture was cooled to room temperature and filtered. The solid cake was washed with ethyl acetate. The filtrate, having the product, was washed with water, extracted with ethanol and concentrated to crystallize the product. The recycled KOH/MgO catalyst was washed with ethyl acetate to remove organic impurities.

Table 1 Screening of catalysts^a

Entry	Catalyst (mole)	Time (h)	Yield% of 1a
a Reaction conditions: benzaldehyde (2 mmol), 2-naphthol (2 mmol) and 2-amino benzothiazole (2 mmol) in the presence of various catalysts under solvent-free conditions.
1	Without catalyst	24	15
2	KOH/MgO (40 mg KOH per gram of support)	3.0	88
3	AlCl3 (0.001)	4.5	71
4	MgCl2 (0.001)	5.0	49
5	Mg(OH)2 (0.001)	4.5	57
6	Al(OH)3 (0.001)	6.0	61
7	Ca(OH)2 (0.001)	5.0	52
8	MgO (0.001)	5.5	69
9	Al2O3 (0.001)	4.0	42
10	CaO (0.001)	4.5	43

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Table 2 Synthesis of 2-aminobenzothiazolomethyl naphthol derivatives^a

Entry	Aldehydes	Product	KOH/MgO		Hydrotalcite		MP (°C)
			Time (h)	Yield (%)	Time (h)	Yield (%)	Found	Reported^(ref.)
a Reaction conditions: 2-amino benzothiazole (2 mmol), 2-naphthol (2 mmol) and benzaldehyde (2 mmol), with hydrotalcite or KOH/MgO under solvent-free conditions.
1		1a	2.5	88	2.8	92	203–204	202–204^8c
2		1b	3.0	90	3.0	90	194–175	173–175^8f
3		1c	3.5	85	2.6	89	192–194	192–194^8g
4		1d	3.0	91	2.6	91	190–192	189–191^8a
5		1e	2.5	88	2.6	87	162–163	160–161^8f
6		1f	2.5	90	2.8	92	172–173	172–173^8c
7		1g	2.5	89	2.6	92	180–181	183–184^8c
8		1h	3.0	92	2.8	93	208–209	208–209^8f
9		1i	3.0	73	3.0	78	188–190	188–190^8g
10		1j	2.5	84	2.4	88	>250	242 (Dec.)^8f
11		1k	2.5	81	2.4	81	197–198	197–198^8c

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Typical procedure using hydrotalcite. A mixture of the aldehydes (2 mmol), 2-naphthol (2 mmol) and 2-amino benzothiazole (2 mmol) was heated at 70 °C under solvent-free conditions using hydrotalcite (Mg–Al–CO₃, 80 mg) as a catalyst. The time taken by the different aldehydes for the reaction is detailed in Table 2. After completion of the reaction (monitored by TLC), the reaction mixture was cooled to room temperature and poured into cold water. The solid mass was filtered and dissolved in ethanol, then filtered again so that the product was dissolved in the ethanol and the hydrotalcite was separated, and then the ethanol was concentrated to crystallize the product. The recycled hydrotalcite was washed with ethanol to remove organic impurities.

3. Results and discussion

The powder X-ray diffraction spectra of the prepared KOH/MgO catalyst are shown in the Fig. 1(A). From Fig. 1(A) it can be seen that there are typical characteristic peaks at 37.0°, 43.0°, and 62.3°, which can be assigned to the characteristic peaks of crystalline MgO.¹⁷ The XRD analysis of the 1 wt% KOH/MgO sample shows that the crystalline phase corresponds to magnesia. This interaction is the main difference between the homogeneous KOH catalyst and the KOH/MgO catalyst. Small crystallites of KOH were also observed at 4 wt% KOH/MgO catalyst (Fig. 2).

Fig. 1 Effect of different loading amounts on the catalyst: (a) 1%, (b) 2%, (c) 3%, and (d) 4%.

Fig. 2 SEM images of the catalysts.

The model reaction starting from benzaldehyde, 2-naphthol and 2-amino benzothiazole have proved to be a facile method for preparation of 2-aminobenzothiazolo-phenylmethyl-2-naphthol derivatives. To optimize the reaction conditions, first of all we investigated different catalysts for the synthesis of the 2-aminobenzothiazolo-phenylmethyl-2-naphthol derivatives. Table 1 suggests that without the catalyst, a longer reaction time was necessary (24 hours) with a poor yield. The best result was obtained when KOH-loaded MgO was used, with the maximum yield (Table 1, entry 2). Other metal catalysts gave a moderate to significant yield.

To optimize the reaction time for the yield of the target product, the model reaction was carried out at different times under solvent-free conditions using KOH-loaded MgO at 70 °C. A reaction time of 3.0 h was found to be the optimum time with the highest conversion (from 35–93%) and yield (from 31–88%) obtained (Fig. 3). The conversion of the starting materials to the product was measured by HPLC analysis. When the loading of the catalyst was not enough, the maximum yield could not be reached. To avoid this kind of problem, an optimum amount of the catalyst loading had to be investigated. For this study, model reaction has been carried out in the presence of different amount of prepared catalyst (10 mg, 20, mg, 50 mg, 80 mg and 100 mg) (Table 3). The highest yield was obtained with 50 mg of the KOH-loaded MgO catalyst. A catalyst loading of 50 mg was found to be the optimal quantity and was used for further studies.

Fig. 3 Effect of the reaction time on the yield of the target product.

Table 3 Effect of the catalyst loading on the yield^a

Entry	Catalyst loading (mg)	Yield
a Reaction conditions: benzaldehyde (2 mmol), 2-naphthol (2 mmol) and 2-amino benzothiazole (2 mmol) in the presence of the KOH/MgO catalyst under solvent-free conditions.
1	10	60
2	20	76
3	50	88
4	80	88
5	100	88

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The reaction temperature can influence the reaction rate and the yield because the intrinsic rate constants are strong functions of the temperature.¹⁸ In this study, the effect of the reaction temperature was investigated at four different reaction temperatures, such as room temperature, 50, 70 and 100 °C. The catalyst used in this experiment was 4 wt% KOH/MgO. The results are shown in Fig. 4. The best conversion and yield at 70 °C were obtained as 93% and 88%, respectively.

Fig. 4 Effect of the temperature on the conversion and yield of the product.

To understand the effect of the KOH loading of the catalyst on the catalytic activity, different loadings of KOH ranging from 1 to 5 wt% were studied with the model reaction. The results are demonstrated in Fig. 5. As the loading of KOH was raised from 1 to 4 wt%, the conversion of the starting material was increased. However, by increasing the amount of KOH from 4 to 5 wt%, significant changes in the conversion and yield were not observed. The highest yield was obtained with a loading of KOH of 4 wt% on MgO. It is obvious that change in the catalytic activity was well correlated with the change in the basicity, as shown in Table 4. These results are also in agreement with the results obtained by D’Cruz et al.¹⁹

Fig. 5 Effect of the KOH loading on the conversion and yield.

Table 4 Effect of the basicity on the yield

Entry	Samples	Time (h)	Yield (%)	Basicity (mmol g^−1)	BET area (m^2 g^−1)
1	1% wt KOH/MgO	6.0	57	1.065	22.52
2	2% wt KOH/MgO	4.5	74	1.096	20.01
3	3% wt KOH/MgO	3.5	81	1.117	18.21
4	4% wt KOH/MgO	3.0	88	1.187	17.13
5	5% wt KOH/MgO	3.0	88	1.010	15.50

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The basicity of the catalyst was determined using Hammett indicators and the benzoic acid titration method.^20–22 The model reaction was carried out in the presence of the KOH-loaded MgO catalyst. Excellent catalytic activity was observed using the different substituted aromatic aldehydes. We have studied different amounts of KOH loading on the metal and correlated it with the basicity of catalyst. As evident from Table 4, the KOH loading on the surface of MgO could induce the basicity of the catalyst. As the loading of KOH was raised from 1 to 4 wt%, the yield of the target product was increased and the yield was 88%. Moreover, as seen in Table 4, the basicity plays an important role in the catalytic activity of the catalyst prepared. The BET surface area is given in Table 4 which shows that as the KOH loading increases, the surface area decreases.

The dependence of the activity of the KOH/MgO catalysts on the loading amount of KOH has been investigated with the effect of the basicity. The most basic properties of catalysts was found with 4% alkali base is loaded onto a support. The presence of alkali metal enhances the electron density of the framework oxygen, resultant increases basic sites. The data clearly demonstrate the involvement of potassium in the activity and basicity. The incorporation of potassium increased the basic strength of the pure MgO, resulting in an increase of the yield. The results in Table 4 show that the activity increased by increasing the KOH content (4%) due to presence of the higher number of basic sites or suggesting that basic sites have a very specific environment. But basic sites may be covered by excess KOH when the amount of loaded KOH is from 4–5 wt% as the resultant basic strength and the basic sites of the catalyst were not raised, and surface areas of active components and the catalytic activity were lower. At a low loading of KOH, the active sites are more dispersed on the pure MgO surface and cannot disperse properly with too much KOH.

The recyclability of the catalyst was also investigated for the 4 wt% KOH/MgO catalyst, which gives the highest yield of 88%. After the reaction, the catalyst was separated by filtration and then reused in the model reaction. It was shown that the reaction catalyzed by a used catalyst provided a 70% yield, which was lower than the yield over the fresh catalyst. This indicated that the spent catalyst was slightly deactivated. Thus, the decrease in catalytic activity probably resulted from the leaching of some potassium content. This low activity may be explained by the dissolution of K species in the heterogeneous KOH/MgO catalyst.²³

Thus, to removing the drawback of recyclability or leaching of catalyst (KOH/MgO catalyst), further synthesized hydrotalcite (Mg : Al; 3 : 1) and used as new heterogeneous catalyst. The Mg : Al atomic ratio was measured using X-ray microanalysis and 3.16 was found, which is in good agreement with the metallic ratio (3.0) taken in solution. The value of x [x = M^III/M^II + M^III] was found to be 0.24, which suggested the purity of the hydrotalcite.²⁴ The powder X-ray diffraction (P-XRD) pattern for the sample Mg–Al–CO₃ is shown in Fig. 6. The presence of the CO₃²⁻ anion in the interlayer gallery of the hydrotalcite is confirmed by the characteristic basal spacing d₀₀₃ = 7.76 Å. This indicates a gallery height of 2.96 Å (assuming a thickness of 4.8 Å for the cationic sheets). The material is reasonably crystalline and suggests a relatively well-ordered sheet arrangement.²⁵ Diffraction peaks of the (003) basal plane, which gives the distance between the layers, became sharper which indicates a higher crystallinity and order. This fact is supported by the increase in the ratio of the intensities of the diffraction from the (006) basal plane to that of the (003) one. The crystallite size of this sample was found to be 24.87 nm, as calculated using Scherrer formula.²⁶ More intense and sharper reflections of the (003) and (006) planes were found at low 2θ values (11–23°).

Fig. 6 P-XRD pattern and SEM image of hydrotalcite.

A typical SEM image of the Mg–Al–CO₃ hydrotalcite is shown in Fig. 6. This figure indicates the existence of lamellar particles which have a rounded hexagonal shape and are typical of a hydrotalcite-like material. The material was found to be mesoporous with a surface area of 90 m² g⁻¹.

The basicity of the hydrotalcite is a key for the preparation of the material with a high performance.²⁷ It can be achieved by changing the nature of the M²⁺/M³⁺ metals.²⁸ As far as the Mg–Al mixed oxides of hydrotalcite are concerned, a correlation can be established between the composition and the basicity; when the amount of Al increases, the total number of basic sites decreases.^15,29 The decrease in basic site density observed in Mg–Al mixed oxides derived from hydrotalcites when increasing the Al content is reported to be the reason for the decreasing activity in the Knoevenagel condensation reaction between glyceraldehyde acetonide and ethyl acetoacetate.³⁰ The performance of the Mg–Al mixed oxide in the methanolysis of soyabean oil was shown to be dependent on the Mg/Al ratio.^11g Numerous authors have tried to identify the best composition of Mg–Al mixed oxide catalysts for various reactions. A set of Mg–Al hydrotalcite-like precursors with different Mg/Al atomic ratios were studied by Diez et al.³¹ The optimum Mg/Al molar ratio is dependent on the target reaction and, more precisely, on the basic strength needed to activate the reactant. It is clear from previous results³² that among the all metallic ratio of hydrotalcites, 3 : 1 ratio of hydrotalcite (Mg–Al–CO₃) gives better results. In this communication we have thus synthesized a hydrotalcite catalyst (Mg–Al–CO₃, 3 : 1) and used it in the present reaction.

In order to evaluate the appropriate catalyst loading, the reaction of benzaldehyde (2 mmol), 2-amino benzothiazole (2 mmol) and 2-naphthol (2 mmol) was carried out using 20 mg, 50 mg, 80 mg, 100 mg and 120 mg of hydrotalcite as the catalyst at 70 °C under solvent-free conditions. The catalyst loading of 80 mg was found to be the optimal quantity (Table 5). The catalyst was reused and the results show that the hydrotalcite (Mg–Al–CO₃) can be reused as such without a significant loss in yield (Table 6). A variety of electron donating and electron withdrawing groups on the aromatic aldehydes have been studied (Table 2). It was found that meta-substituted aldehydes give lower yields as compared to ortho and para substituents (Table 2, entry 9, 11).

Table 5 Effect of the catalyst [Mg–Al–HT (Mg/Al = 3 : 1)] loading^a

Entry	Catalyst loading (mg)	Time (min)	Yield (%)
a Reaction conditions: benzaldehyde (0.0025), 2-amino benzothiazole (0.0025 mol) and 2-naphthol (0.0025 mol), hydrotalcite (80 mg), temp. at 70 °C.
1	20	180	40
2	50	170	76
3	80	160	92
4	100	160	92
5	120	160	92

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Table 6 Recyclability of hydrotalcite [Mg–Al–HT (Mg/Al = 3 : 1)]^a

Product	Fresh HT	Reuse (I)	Reuse (II)	Reuse (III)	Reuse (IV)
a Reaction conditions: benzaldehyde (0.0025 mol), 2-amino benzothiazole (0.0025 mol) and 2-naphthol (0.0025 mol), hydrotalcite (80 mg), temp. at 70 °C.
1a	92	92	91	90	90

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A model reaction has been carried out using of benzaldehyde (0.0025 mol), 2-amino benzothiazole (0.0025 mol) and 2-naphthol (0.0025 mol) for the recyclability and reusability of the hydrotalcite as a catalyst, incorporating 80 mg of catalyst. After completion of reaction, the contents were filtered to recycle the hydrotalcite catalyst through a Whatman filter 42. The recycled hydrotalcite was washed with 5 mL ethanol to remove organic impurities. The hydrotalcite catalyst could be readily recovered and reused for at least four runs without any significant loss of activity.

XRD data of the recovered hydrotalcite (Fig. 7) which showed a similar profile as the fresh catalyst, which confirmed that layered structure of hydrotalcite was maintained after the reaction.

Fig. 7 XRD pattern of the recovered hydrotalcite.

4. Conclusion

In conclusion, 4 wt% KOH/MgO and hydrotalcite catalysts have the highest yields of target products and gave the best catalytic activities for the multi-component reaction of substituted aldehydes, 2-amino benzothiazole and 2-naphthol at 70 °C under solvent-free conditions. Loading the KOH with 4 wt% gave the highest basicity and the best catalytic activity. The highest conversion and yield of the product were obtained at 3.0 h of reaction time. After recycling, KOH/MgO could not perform better than the fresh catalyst due to a leaching of the basic sites, but this drawback was removed by using hydrotalcite as the catalyst. The morphology and characterization of catalysts were achieved by XRD and SEM. Hydrotalcite and KOH-loaded MgO have shown better results when compared to simple metal catalysts. The present methodology involves operational simplicity, short reaction times, a simple catalyst system, higher yield and ease in the work-up procedure.

Characterization data

1-[(2-Benzothiazolylamino)phenylmethyl]-2-naphthalenol (1a). White powder, mp 203–204 °C; IR (KBr) (ν_max, cm⁻¹): 3381 (OH_str), 1599 (C=C_str), 1542 (N–H_ben), 1515 (C–H_ben), 1449 (C–H_ben); ¹H NMR (400 MHz, DMSO): δ_H 6.69–7.93 (16H, m, 15H–arom and 1H–CH), 8.67 (1H, s, NH), 10.11 (1H, s, OH); ¹³C NMR (100 MHz, DMSO): 53.36, 118.02, 118.57, 118.81, 120.49, 120.84, 122.27, 123.58, 125.23, 126.04, 127.83, 128.34, 128.56, 129.28, 130.59, 132.12, 142.25, 151.96, 153.19, 166.33; ESI-MS: m/z calculated for C₂₄H₁₈N₂OS 382; found [M + H]⁺ 283.3.

1-[(2-Benzothiazolylamino)(4-nitrophenyl)methyl]-2-naphthalenol (1d). White powder, mp 120–122 °C; IR (KBr) (ν_max, cm⁻¹): 3348 (OH_str), 2933 (C–H_str), 1625 (C=C_str), 1510 (N–H_ben), 1267 (C–N_str), 962–812 (C–H_def); ¹H NMR (400 MHz, DMSO): δ_H 6.88–7.75 (15H, m, 14H–arom and –CH), 8.65 (1H, s, NH), 10.00 (1H, s, OH); ¹³C NMR (100 MHz, DMSO): 53.43, 117.81, 118.31, 118.58, 119.59, 120.41, 121.46, 122.54, 122.91, 123.24, 122.98, 125.20, 125.33, 126.65, 127.32, 128.57, 128.98, 129.89, 145.99, 150.68, 153.38, 166.69; ESI-MS: m/z calculated for C₂₄H₁₇N₃O₃S 427.2; found [M + H]⁺ 428.1.

1-[(2-Benzothiazolylamino)(2-hydroxyphenyl)methyl]-2-naphthalenol (1e). Off-white powder, mp 162–163 °C; IR (KBr) (ν_max, cm⁻¹): 3504 (OH_str), 2941 (C–H_str), 1627 (C=C_str), 1508 (N–H_ben), 1280 (C–N_str), 962–813 (C–H_def); ¹H NMR (400 MHz, DMSO): δ_H 6.68–7.49 (15H, m, 14H–arom and –CH), 8.68 (1H, s, NH), 10.04 (1H, s, OH), 10.34 (1H, s, OH); ¹³C NMR (100 MHz, DMSO): 51.82, 116.74, 116.95, 117.76, 118.84, 119.40, 120.60, 121.11, 121.24, 125.54, 122.52, 126.31, 128.20, 128.98, 129.80, 131.73, 132.12, 136.31, 150.86, 152.65, 154.47, 160.83, 166.54; ESI-MS: m/z calculated for C₂₄H₁₈N₂O₂S 398.2; found [M + H]⁺ 399.1.

1-[(2-Benzothiazolylamino)(4-methoxyphenyl)methyl]-2-naphthalenol (1f). Off-white powder, mp 172–173 °C; IR (KBr) (ν_max, cm⁻¹): 3510 (OH_str), 3012 (C–H_str), 2922 (C–H_str), 1625 (C=C_str), 1500 (N–H_ben), 1278 (C–N_str); ¹H NMR (400 MHz, DMSO): δ_H 3.69 (3H, s, OCH₃), 6.69–7.87 (15H, m, 14H–arom and –CH), 8.51 (1H, s, NH), 10.53 (1H, s, OH); ¹³C NMR (100 MHz, DMSO): 54.81, 59.12, 113.33, 118.11, 119.03, 119.12, 120.58, 122.53, 125.52, 126.22, 127.42, 128.47, 128.79, 129.38, 130.22, 132.16, 133.56, 151.69, 153.35, 158.00, 166.84; ESI-MS: m/z calculated for C₂₅H₂₀N₂O₂S 412.2; found [M + H]⁺ 413.1.

1-[(2-Benzothiazolylamino)(4-methylphenyl)methyl]-2-naphthalenol (1g). White powder, mp 180–181 °C; IR (KBr) (ν_max, cm⁻¹): 3007 (C–H_str), 2922 (C–H_str), 1625 (C=C_str), 1510 (N–H_ben), 1267 (C–N_str); ¹H NMR (400 MHz, DMSO): δ_H 2.23 (3H, s, CH₃), 6.75–7.80 (15H, m, 14H–arom and –CH), 8.66 (1H, s, NH), 10.14 (1H, s, OH); ¹³C NMR (100 MHz, DMSO): 20.60, 53.33, 118.00, 118.66, 118.92, 120.51, 120.85, 122.27, 125.26, 126.03, 128.34, 128.50, 128.60, 129.22, 130.55, 132.13, 135.17, 139.13, 151.97, 153.20, 166.39; ESI-MS: m/z calculated for C₂₅H₂₀N₂OS 396.2; found [M]⁺ 396.1.

1-[(2-Benzothiazolylamino)(4-chlorophenyl)methyl]-2-naphthalenol (1h). White powder, mp 208–209 °C; IR (KBr) (ν_max, cm⁻¹): 3504 (OH_str), 1627 (C=C_str), 1267 (C–N_str), 1122 (C–H_ben); ¹H NMR (400 MHz, DMSO): δ_H 6.97–7.98 (15H, m, 14H–arom and –CH), 8.57 (1H, s, NH); ¹³C NMR (100 MHz, DMSO): 53.48, 117.71, 118.13, 118.44, 118.96, 120.95, 121.12, 122.43, 122.47, 125.81, 126.82, 127.30, 127.78, 128.22, 129.54, 130.32, 131.39, 134.52, 138.16, 151.61, 153.28, 155.11, 166.89; ESI-MS: m/z calculated for C₂₄H₁₇N₂OClS 416.4; found [M + H]⁺ 417.2.

Acknowledgements

We are grateful to Punjab University, Chandigarh for the spectral analytical data.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11857c

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