Unless specified, all of the chemicals used in the current study were purchased as the analytical grade from Merck, Aldrich, and Fluka Chemical Companies and used without further purification. The reaction products were characterized on the basis of their physical properties and by comparison with authentic samples. The reactions were monitored by thin-layer chromatography (TLC) analysis using silica gel plates (SIL G/UV 254 plates).

The purities of the products were determined by GC-MS on an Agilent 6890 GC-MS system (USA) under ionization conditions of 70 eV. Fourier transform infrared spectra (FT-IR) were recorded on a Perkin Elmer 781 spectrophotometer (USA) in the range of 4000-400 cm^-1. The solid samples were analyzed as KBr disks, whereas the liquid samples were analyzed neat. ¹H NMR spectra were recorded on 400 and 300 MHz Bruker Avance instruments (USA) using deuterated chloroform as the solvent. ¹³C NMR spectra were collected on Bruker Avance 100 and 75 MHz instruments. All of the chemical shift data were quoted relative to TMS, which was used as an internal reference standard. Microanalyses were performed on a Vario EL III CHNOS Elemental Analyzer (Germany). Melting points were recorded on a Büchi B-545 apparatus (Germany) in open capillary tubes. The thermal stability characteristics of the catalyst were evaluated by thermal gravimetric analysis (TGA) using a Shimadzu Thermo-gravimetric Analyzer-50 (Japan) in a temperature range of 20-700 °C. All of the experimental measurements were conducted under nitrogen.

Chlorosulfonic acid (2.0 mL, 30 mmol) was added in a drop- wise manner to a suspension of powdered poly(4- vinylpyridine) (5.0 g; cross-linked with 2% divinylbenzene (DVB), ~60 mesh, MW 60000; Fluka Chemicals) in dry CH₂Cl₂ (10 mL) at 0 °C over a period of 5 min, and the resulting mixture was stirred at room temperature for 6 h. The CH₂Cl₂ was then decanted from the mixture, and the residue was washed with dry CH₂Cl₂ (3 × 10 mL) before being dried under vacuum to afford N-sulfonic acid poly(4- vinylpyridinium) chloride (NSPVPC, 7.29 g) as a pale yellow powder ^[48]. Sulfuric acid (1.6 mL, 30 mmol, 100%) was then added in a drop-wise manner to a suspension of NSPVPC (7.29 g) in dry CH₂Cl₂ (10 mL) over a period of 3 min at room temperature, and the resulting mixture was vibrated and stirred for 2 h at 50 °C under a continuous flow of nitrogen to remove the hydrogen chloride gas. The mixture was then heated at the same temperature under high vacuum for 30 min to give N-sulfonic acid poly(4-vinylpyridinium) hydrogen sulfate (NSPVPHS, 8.52 g) as a yellow powder.

A mixture of the substrate (1 mmol), acetic anhydride (2-3 mmol), and NSPVPHS (5 mg) was stirred at room temperature under solvent-free conditions. Upon completion of the reaction, as determined by TLC, the mixture was diluted with ethyl acetate (15 mL). The solid acid catalyst was then collected by filtration and washed sequentially with ethyl acetate (5 mL) and acetone (5 mL) before being dried under vacuum at 40 °C. The recovered catalyst could then be used for the same reaction. The organic layer of the filtrate was washed sequentially with a saturated solution of NaHCO₃ (5 mL), brine (10 mL), and water (20 mL) and then dried over MgSO₄. The solvent was then removed under vacuum to give a residue, which was purified by column chromatography over silica gel to give the desired product in good to high yield.

The structure of several selected products is shown in Scheme 3.

Schemes 3. The structure of the selected products.

A12. FT-IR (neat) ν_max = 2952, 1716, 1450, 1365, 1248, 1153 cm^-1; [α]_D = +41.5°, neat, 96% e.e.; ¹H NMR (400 MHz, CDCl₃) δ = 0.82 (s, 3H), 0.88 (s, 3H), 0.92 (s, 3H), 1.26 (m, 4H), 1.72 (m, 2H), 2.08 (s, 3H), 2.36 (m, 1H), 4.89 (d, J = 9.8 Hz, 1H); ¹³C (100 MHz, CDCl₃) δ = 13.4, 18.7, 19.6, 21.2, 27.0, 28.0, 36.7, 44.8, 47.7, 48.6, 79.8, 171.3.

A16. FT-IR (neat) ν_max = 2953, 1770, 1476, 1374, 1186, 1039, 907, 815 cm^-1; ¹H NMR (400 MHz, CDCl₃) δ = 2.31 (s, 6H), 2.31 (s, 3H), 7.14 (d, J = 8.0 Hz, 2H), 7.28 (dd, J = 8.8, 7.6 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃) δ = 20.2, 20.7, 120.7, 125.9, 134.7, 143.6, 167.0, 167.9.

A19. FT-IR (neat) ν_max = 3070, 3075, 3025, 1765, 1679, 1577, 1492, 1445, 1370, 1337, 1238, 1214, 1194, 1091, 954, 821, 747, 705, 689, 612 cm^-1; ¹H NMR (400 MHz, CDCl₃) δ = 2.33 (s, 3H), 2.45 (s, 3H), 7.24 (d, J = 8.0 Hz, 1H), 7.33 (t, J = 7.6 Hz, 1H), 7.50 (t, J = 7.6 Hz, 1H) 7.53 (d, J = 7.6 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃) δ = 20.7, 26.2, 120.7, 125.9, 130.2, 134.7, 143.5, 167.0, 171.9.

A20. FT-IR (neat) ν_max = 3121, 2974, 2848, 2570, 1696, 1592, 1416, 1218, 1158, 1070, 1012, 927, 808, 741, 596 cm^-1; ¹H NMR (400 MHz, CDCl₃) δ = 2.36 (s, 3H), 4.29 (s, 2H), 6.01 (d, J = 2.8 Hz, 1H), 6.23 (dd, J = 2.8 and 0.8 Hz, 1H), 7.22 (d, J = 0.8 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃) δ = 22.4, 29.7, 107.2, 110.7, 141.7, 152.7, 194.3.

A25. FT-IR (neat) ν_max = 3295, 3051, 3018, 1635, 1554, 1505, 1341, 1284, 1021, 775, 726 cm^-1; ¹H NMR (400 MHz, CDCl₃) δ = 2.30 (s, 3H), 7.45-7.87 (m, 8H); ¹³C NMR (100 MHz, CDCl₃) δ = 22.8, 118.5, 122.3, 122.8, 125.2, 126.1, 127.3, 127.8, 128.4, 128.5, 133.1, 169.9.

A28. FT-IR (neat) ν_max = 3316, 2952, 2994, 1743, 1653, 1451, 1377, 1267, 1199, 1128, 715 cm^-1; ¹H NMR (400 MHz, CDCl₃): δ = 1.97 (s, 3H), 2.02 (s, 3H), 2.11 (s, 3H), 2.20 (s, 3H), 3.47 (t, J = 6.0 Hz, 2H), 3.61 (t, J = 5.6 Hz, 2H), 4.11 (t, J = 6.0 Hz, 2H), 4.19 (t, J = 5.6 Hz, 2H), 4.59 (s, 2H), 4.62 (s, 2H), 7.14-7.36 (m, 10H); ¹³C NMR (100 MHz, CDCl₃) δ = 20.7, 20.8, 21.5, 21.6, 45.0, 46.1, 48.2, 53.1, 61.2, 62.2, 126.2, 127.5, 127.7, 127.9, 128.6, 128.9, 136.6, 137.2, 170.5, 170.8, 171.1, 171.5.

A29. FT-IR (neat) ν_max = 3320, 1730, 1682, 1540, 1502, 1354, 1310, 1240, 1196, 1020, 992 cm^-1; ¹H NMR (400 MHz, CDCl₃) δ = 2.16 (s, 3H), 2.36 (s, 3H), 7.14-7.17 (m, 2H), 7.22-7.25 (m, 1H), 7.39 (br s, 1H), 8.11 (d, J = 7.6 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃) δ = 20.6, 22.7, 117.8, 120.7, 123.9, 125.9, 130.7, 140.5, 167.0, 167.9.

B9. White solid, m.p. 65-67 °C; FT-IR (KBr) ν_max = 3010, 2910, 1760, 1610, 1590, 1490, 1430, 1370, 1245, 1200, 1160, 1060, 1010, 970, 940, 917, 790, 760, 700, 670, 600 cm^-1; ¹H NMR (300 MHz, DMSO-d₆) δ = 2.13 (s, 6H), 3.38 (s, 3H), 7.02 (d, J = 7.9 Hz, 1H), 7.50 (d, J = 7.9 Hz, 1H), 7.60-7.66 (m, 2H), 8.04 (s, 1H); ¹³C NMR (75 MHz, DMSO-d₆) δ = 21.00, 21.93, 87.83, 123.53, 125.12, 127.94, 133.82, 135.42, 147.74, 167.55.

B19. White solid, m.p. 96-98 °C; FT-IR (KBr) ν_max = 3002, 2920, 1752, 1430, 1362, 1340, 1240, 1200, 1158, 1106, 1060, 1000, 942, 910, 808, 710, 680, 650, 600, 530, 510 cm^-1; ¹H NMR (300 MHz, DMSO-d₆) δ = 2.10 (s, 12H), 7.52 (d, J = 7.8 Hz, 1H), 8.00 (d, J = 7.9 Hz, 1H), 8.17 (s, 1H), 8.43 (dd, J = 7.6, 8.2 Hz, 2H); ¹³C NMR (75 MHz, DMSO-d₆) δ = 22.55, 102.33, 126.32, 128.82, 129.62, 141.19, 167.00.

B20. White solid, m.p. 164-166 °C; FT-IR (KBr) ν_max = 3020, 2910, 1758, 1430, 1378, 1338, 1200, 1118, 1060, 1008, 960, 940, 910, 852, 818, 604, 580, 540 cm^-1; ¹H NMR (300 MHz, CDCl₃) δ = 2.37 (s, 12H), 7.41 (s, 4H), 8.26 (s, 2H); ¹³C NMR (75 MHz, CDCl₃) δ = 21.55, 104.33, 129.82, 141.19, 167.00.

The structure of the NSPVPHS synthesized in the current study was determined by FT-IR spectroscopy, elemental analysis, TGA and DTGA (differential thermogravimetry).

Figure 1 shows the FT-IR spectra of poly(4-vinylpyridine) (PVP) and NSPVPHS. The broad band between 3250 and 3750 cm^-1 in the spectrum of NSPVPHS was attributed to the stretching OH vibration of the sulfonic acid and hydrogen sulfate groups in the solid acid catalyst. The other characteristic absorption bands at 1288, 1165, 1035, and 808 cm^-1 were assigned to the asymmetric and symmetric stretching vibrations of the SO₂ group, as well as the bending and symmetric stretching vibrations of the S-OH and N-SO₂ groups, respectively ^{[51, 52, 53, 54, 55, 56]}. The characteristic bands of the pyridine rings at 1505 and 1412 cm^-1 disappeared from the spectrum, and a new band developed at 1637 cm^-1, which was attributed to the pyridinium ion. The in and out of plane bending of the pyridine C-H groups (994 cm^-1) is shown in Fig. 2 ^{[57, 58, 59]}. The spectra in Fig. 1 contained peaks that were characteristic of the pyridine ring vibrations at 1600 cm^-1 for free unsulfonated rings in PVP and at 1637 cm^-1 for the sulfonated rings in NSPVPHS at different loadings of the chlorosulfonic and sulfuric acids.

Fig. 1. FT-IR spectra of PVP and NSPVPHS in the range of 4000-400 cm-1.

Fig. 2. FT-IR spectra of PVP and NSPVPHS in the range of 2000-400 cm-1.

The solid acid catalyst was subjected to elemental analysis (Table 1). The amount of chloride was determined to be 3.98 mmol Cl per gram of polymer using gravimetric and potentiometric titration methods ^[60]. Using a previously reported method ^[61], the chloride content of NSPVPHS was found to be less 260 ppm. The densities of the SO₃H and SO₄H groups were measured by acid-base potentiometric titration using NaOH (0.01 mol/L) as a titrant. In a separate experiment, a suspension of NSPVPHS in benzene was titrated using n-butylamine as an indicator. This method allowed for the amount of Brnsted acid to be determined, but is very rarely applied to highly colored or dark samples because the color change can be difficult to observe under these conditions. The acid loading of the NSPVPHS was found to be in the range of 3.91-3.94 mmol/g using these parallel experiments.

Table 1 Elemental analysis data of the initial polymer and modified poly(4-vinylpyridine). Entry Compound Calcd (%) Found (%) Conversion (%) C H N Cl C H N Cl 1 PVP 80.29 6.69 13.01 — 80.14 6.64 13.22 — — 2 NSPVPC 38.55 3.66 6.24 15.84 43.08 3.96 6.95 14.16 ~80 3 NSPVPHS 30.25 3.22 4.90 — 34.53 3.50 5.62 — ~80

Table 1 Elemental analysis data of the initial polymer and modified poly(4-vinylpyridine).

TGA was used to develop a deeper understanding of the degradation kinetics of NSPVPHS. The TGA experiments were conducted over a temperature range of 30 to 700 °C, with the samples being heated at a rate of 10 °C/min. The derivative weight loss with respect to temperature (DTGA) revealed that the decomposition of the NSPVPHS involved five distinct stages (Fig. 3). The first decomposition stage was observed around 100 °C, whereas the second, third, fourth, and fifth weight loss events occurred at 320, 390, 510, and 630 °C, respectively, with the second and third events representing significant weight losses. The first, second, third, and fourth weight loss stages were attributed to the expulsion of water molecules from the catalyst matrix, the decomposition of the sulfonic acid and hydrogen sulfate groups of the catalyst, and the splitting of the main DVB chain followed by the decomposition of the polymer backbone at temperatures above 550 °C, respectively. These data therefore demonstrate that the NSPVPHS was thermally stable up to 320 °C. In a separate control experiment, all of the bulk PVP was lost at 410 °C under an atmosphere of nitrogen ^{[62, 63]}.

Fig. 3. TGA-DTGA curves of the NSPVPHS.

The acidity of the solid acid was evaluated using the Hammett acidity functions, which were derived by UV-visible spectroscopy (Hammett method) using a basic indicator to trap the dissociative proton ^{[64, 65, 66]}. In the presence of the solid acid, the absorbance of the unprotonated form of the indicator, which could be observed at 349 nm, decreased (Fig. 4). These results clearly demonstrated that a dichloromethane solution of NSPVPHS was more acidic than a dichloromethane solution of poly(4-vinylpyridinium) hydrogen sulfate (PVPHS) or N-sulfonic acid poly(4-vinylpyridinium) chloride (NSPVPC). Based on this result, the acidities of the catalysts under our experimental conditions were determined to be of the order NSPVPHS > PVPHS > NSPVPC (Table 2).

Fig. 4. Absorption spectra of 4-nitroaniline (1), NSPVPC (2), PVPHS (3), and NSPVPHS (4) in CH2Cl2.

Table 2 Calculated Hammett acidity functions (H0) of NSPVPC, PVPHS, and NSPVPHS. Catalyst Amax [I]s (%) [IH+]s (%) H0 — 2.9839 100 0 — NSPVPC 1.4387 48.2 51.8 0.96 PVPHS 1.0942 36.7 63.3 0.75 NSPVPHS 0.6831 22.9 77.1 0.46 UV-vis spectroscopy conditions: CH2Cl2 solvent with 4-nitroaniline pK(I)aq = 0.99) as the indicator at 1.44 x 10-4 mol/L. PVPHS or NSPVPHS (2 mg) was used as the catalyst at 20.9 °C. Calibration curves with a linearity of greater than 0.999 for the indicator in the unprotonated form were constructed using a CH2Cl2 solution of the indicator.

Table 2 Calculated Hammett acidity functions (H0) of NSPVPC, PVPHS, and NSPVPHS.

The reaction of benzyl alcohol with acetic anhydride to give benzyl acetate was selected as a model reaction for the optimization of the catalytic conditions using NSPVPHS. The reaction was initially evaluated using different amounts of NSPVPHS at room temperature under solvent free conditions. The results showed that the reaction could be carried out in the presence of as little as 5 mg of NSPVPHS under solvent-free conditions, with the desired product being formed in high yield. The reaction was also conducted in a variety of different solvents (i.e., CH₂Cl₂, CH₃CN, MeOH, EtOH, and H₂O), but the reaction time became longer. Furthermore, when the reaction was conducted in the absence of the catalyst, none of the desired product was formed.

To confirm the importance of the NSPVPHS catalyst to the success of the acetylation reaction, we also conducted two control experiments, which involved the protection of 1- adamantanol and 4-methoxy benzyl alcohol with Ac₂O under similar reaction conditions in the presence of 5 mg of the chlorosulfonic acid and sulfuric acid used in the preparation of the solid acid catalyst. The results of these experiments are shown in Table 3. The results clearly show that when the acetylation reactions were carried out in the presence of ClSO₃H or H₂SO₄, they gave rise to a mixture of products that were observed by GC-MS. Interestingly, however, when the same substrates were acetylated in the presence of NSPVPHS, the desired products were formed in high yields without the formation of any side products.

Table 3 Acetylation of 1-adamantanol and 4-methoxy benzyl alcohol with Ac2O using chlorosulfonic acid, sulfuric acid, or NSPVPHS as the catalyst. Catalyst 1-Adamantanol 4-Methoxy benzyl alcohol Time (min) Yield a (%) Time (min) Yield a (%) Chlorosulfonic acid (neat) 10 mix. b 5 mix. b Sulfuric acid (100%) 10 mix. b 5 mix. b NSPVPHS 10 89 5 97 Reaction conditions: Substrate (1.0 mmol) and Ac2O (1.0 mmol) in the presence of catalyst (5 mg). a GC-MS yield of the corresponding acetylated product. b Mixture of compounds.

Table 3 Acetylation of 1-adamantanol and 4-methoxy benzyl alcohol with Ac2O using chlorosulfonic acid, sulfuric acid, or NSPVPHS as the catalyst.

Following on from these preliminary experiments, we proceeded to investigate the use of a catalytic amount of NSPVPHS (5 mg, 0.018 mmol) for the acetylation of a variety of different functional groups, including functionalized alcohols, phenols, thiols, and amines with acetic anhydride at room temperature under solvent-free conditions (Table 4). As shown in Table 4, a variety of different benzylic alcohols (including those bearing electron-donating or -withdrawing groups) and aliphatic alcohols underwent the acetylation with 1.0 equivalent of Ac₂O in the presence of a catalytic amount of NSPVPHS under solvent-free conditions at room temperature. The mild nature of the current reaction conditions meant that they did not have an adverse impact on acid sensitive functional groups such as the methoxy group (Table 4, entry 8). Furthermore, no competitive dehydration reactions were observed under the current conditions (Table 4, entries 9-13). It is noteworthy that optically active alcohols reacted enantioselectively in the presence of NSPVPHS with complete retention of configuration, with the corresponding products being formed as single enantiomers in high yield (Table 4, entries 11 and 12). The optical rotations of the products were determined and compared with those reported by Aldrich.

Table 4 NSPVPHS catalyzed acetylation of alcohols, phenols, thiols and amines. Reaction conditions: Substrate (1.0 mmol) and Ac2O (1.0 mmol) in the presence of catalyst (5 mg). a Isolated yield of the corresponding acetylated product. b Results obtained using recycled catalyst. c Substrate: [α]D = -49.0 °, c = 10 in 95% ethanol, 99% e.e.; product: [α]D = -73.0 °, neat, 98% e.e. d Substrate: [α]D = +37.9 °, c = 5 in ethanol, 99% e.e.; product: [α]D = +41.5 °, neat, 96% e.e. e Molar ratio of Ac2O: substrate was 2.0:1.0 and isolated yield of the corresponding di-acetate. f Molar ratio of Ac2O: substrate was 3.0:1.0 and isolated yield of the corresponding tri-acetate.

Table 4 NSPVPHS catalyzed acetylation of alcohols, phenols, thiols and amines.

The characteristic spectroscopic properties of 2-(N- benzylacetamido)ethyl acetate are worthy of additional comment (Table 4, entry 28). Shirini et al. ^[67] recently conducted a study aimed at developing a deeper understanding of the unusual ¹H and ¹³C NMR spectra of 2-(N- benzylacetamido)ethyl acetate. In the current study, conformers I and II were proposed as possible structures and optimized computationally with the Gaussian 98 set of programs at the Hartree Fock level with analytical gradients using the 6-31G* basis sets (Scheme 4). These results suggested that the two conformers could become trapped in the shown conformation until enough energy was available to overcome the high interconversion barrier (data not shown).

Schemes 4. Conformational heterogeneity of 2-(N-benzylacetamido) ethyl acetate.

The results of our study have also shown that NSPVPHS can be used as a catalyst to catalyze the acetylation of phenols, thiols, and amines in good to high yields under the same reaction conditions (Table 4, entries 14-29). In some cases, substrates bearing two or even three nucleophilic groups were successfully acetylated under the optimized reaction conditions (Table 4, entries 19 and 26-29). Furyl mercaptan, which is particularly acid sensitive, was also smoothly converted to the corresponding acetate derivative under the standard conditions without any decomposition (Table 4, entry 20). One of the principle objectives of the current study was to develop a recyclable solid acid catalyst. With this in mind, we investigated the recyclability of the NSPVPHS catalyst. Pleasingly, the catalytic activity of the recycled NSPVPHS towards the acetylation of benzyl alcohol with acetic anhydride was found to be almost the same as that of the fresh catalyst after five runs (Table 4, entry 1). This result showed that there was no significant reduction in the activity of catalyst, with the reactions using the recycled catalyst providing yields in the range of 96% to 98% in an average reaction time of approximately 4 min. It is noteworthy that the FT-IR spectra of the recovered catalyst was the same as that of the freshly prepared catalyst and that pH analysis of the reused NSPVPHS catalyst showed nearly the same loading of H⁺ as the fresh catalyst. The leakage of sulfuric acid from the catalyst matrix was studied under the optimized conditions, and an activity loss of 10% was observed following the tenth use of the catalyst (in this case, a small amount of sulfuric acid leakage was detected in the reaction medium and wash solutions). These observations suggested that the catalyst remained largely intact after each run with only minor leaching of the acid species during the course of the reaction.

To highlight the efficiency of the anion exchange process observed in the current reaction, we have compared our results for the acetylation of pyrocatechol, pyrogallol, and 2-nitro- benzylalcohol with acetic anhydride in the presence of NSPVPHS with those conducted in the presence of NSPVPC (Table 5). This comparison clearly shows that the replacement of the chloride anion in NSPVPC with a hydrogen sulfate anion led to a significant increase in the acidity of the catalyst, and considerable improvements in the reaction times and product yields. These results therefore clearly demonstrate that the hydrogen sulfate anion plays a critical role in promoting the acetylation reaction.

Table 5 The impact of the exchange of the chloride anion with a hydrogen sulfate anion on the reaction time and yields. Substrate Product NSPVPC NSPVPHS Time (min) Yield a (%) Time (min) Yield a (%) Pyrocatechol 1,2-(OAc)2-C6H4 50 74 5 93 Pyrogallol 1,2,3-(OAc)3-C6H3 55 82 8 92 2-Nitro- benzylalcohol 2-NO2-C6H4-CH2OAc 65 78 12 86 Reaction conditions: Substrate (1.0 mmol) and Ac2O (1.0 mmol) in the presence of the NSPVPC or NSPVPHS catalyst (5 mg). a Isolated yield of the corresponding mono-, di-, and tri-acetate.

Table 5 The impact of the exchange of the chloride anion with a hydrogen sulfate anion on the reaction time and yields.

Our result for the acetylation of benzyl alcohol with acetic anhydride in the presence of NSPVPHS was compared with the results of several other methods reported in the literature (Table 6). In the most cases, our newly developed method was found to be superior to the existing methods in terms of the reaction time, catalyst loading, or product yield.

Table 6 Comparison of NSPVPHS as a catalyst for the acetylation of benzyl alcohol with several other recently reported catalysts at room temperature under solvent-free conditions. Entry Catalyst Amount of catalyst/substrate/acetic anhydride Time (min) Yield a (%) Ref. 1 Montmorillonit KSF 100 mg/5 mmol/10 mmol 60 90 [21, 22] 2 Zeolite HSZ-360 100 mg/5 mmol/10 mmol 60 84 [23] 3 Sulfuric acid ([3-(3-silicapropyl)sulfanyl]propyl)ester 2 mg/1mmol/1 mL (10.5 mmol) 6 96 [24] 4 Alumina supported MoO3 0.05 mmol/5 mmol/10 mmol 20 99 [25] 5 Silica-bonded N-propyl sulfamic acid 100 mg/1 mmol/1 mL (10.5 mmol) 20 98 [26] 6 Silica sulfuric acid 50 mg/1mmol/1.5 mmol 10 85 b [27] 7 HClO4-SiO2 0.01 mmol/1mmol/1 mmol 60 96 [28] 8 H14[NaP5W30O110] 74 mg/1 mmol/1.5 mmol 75 95 [29] 9 NSPVPC 5 mg/1 mmol/2 mmol 25 92 this work this work 10 NSPVPHS 5 mg/1 mmol/2 mmol 2 98

Table 6 Comparison of NSPVPHS as a catalyst for the acetylation of benzyl alcohol with several other recently reported catalysts at room temperature under solvent-free conditions.

A plausible mechanism for the current acetylation reaction is shown in Scheme 5. According to this mechanism, acetic anhydride would be activated by the solid acid NSPVPHS cation and acidic hydrogen sulfate anion. The activated acetic anhydride would then be attacked by the substrate, which would ultimately be converted to the final product with the release of NSPVPHS for the next catalytic cycle.

Scheme 5. Proposed mechanism for the acetylation of the substrates.

The selectivity of this method was evaluated using a competitive acetylation reaction involving the simultaneous acetylation of benzyl alcohol, phenol, aniline, and thiophenol (Table 7). The results showed that the current procedure could be used as a practical method for the selective acetylation of amines in the presence of alcohols, phenols, and thiols.

Table 7 Competitive acetylation of benzyl alcohol, phenol, thiophenol, and aniline catalyzed by NSPVPHS. Entry Substrate Product Time (min) Yield a (%) 1 C6H5-NH2+C6H5-SH C6H5-NHAc+C6H5-SAc 5 100+0 2 C6H5-NH2+C6H5-OH C6H5-NHAc+C6H5-OAc 5 100+0 3 C6H5-CH2-OH+C6H5-OH C6H5-CH2-OAc+C6H5-OAc 5 88+12 4 C6H5-CH2-OH+C6H5-SH C6H5-CH2-OAc+C6H5-SAc 10 85.6+14.4 5 C6H5-OH+C6H5-SH C6H5-OAc+C6H5-SAc 10 64+36 Reaction conditions: Substrate (1.0 mmol) and Ac2O (1.0 mmol) in the presence of catalyst (5 mg). a Determined by GC-MS of the corresponding acetylated products.

Table 7 Competitive acetylation of benzyl alcohol, phenol, thiophenol, and aniline catalyzed by NSPVPHS.

In a separate study, NSPVPHS was used to efficiently catalyze the conversion of aldehydes to the corresponding 1,1- diacetates with acetic anhydride (Scheme 2). As shown in Table 8, NSPVPHS was used to catalyze the 1,1-diacetylation of a variety of different aromatic aldehydes bearing a range of different substituents such as NO₂, Cl, Br, CN, Me, and MeO in good to high yields (Table 8, entries 1-12). Aromatic aldehydes bearing a phenolic functionality were also acetylated o the phenol under the current conditions (Table 8, entries 13 and 14). The acid sensitive substrate furfural was also successfully acetylated under the current conditions in high yield without the formation of any side products, which are normally observed under strongly acidic conditions (Table 8, entry 15). The current method could also be used for the protection of aliphatic and α,β-unsaturated aldehydes (Table 8, entries 16-18). No isomerization reactions were observed when conjugated aldehydes were used in the presence of NSPVPHS, and the application of these conditions to iso-phthalaldehyde and terephthalaldehyde gave the tetra-acetylated products in high yields (Table 8, entries 19 and 20).

Table 8 NSPVPHS catalyzed 1,1-diacylation of aldehydes a. a Products were characterized by their physical properties and IR and NMR spectra. b Isolated yield. c Results obtained using recycled catalyst. d 3 mmol of acetic anhydride was used. e Starting material recovered intact. f Conversion.

Table 8 NSPVPHS catalyzed 1,1-diacylation of aldehydes a.

To evaluate the intra- and intermolecular chemoselectivity of the current method, we conducted two competitive reactions involving the 1,1-diacetylation of aldehydes in the presence of ketones using NSPVPHS. The results of these experiments revealed a high level selectivity towards the conversion of aldehydes to the corresponding 1,1-diacetates in the presence of ketones (Table 8, entries 21 and 22).

The NSPVPHS catalyst could be effectively recovered from the reaction mixture during the work-up procedure (Table 8, entry 5). The percentage recovery of the catalysts was greater than 98% in most cases, and subsequent reuse of the catalyst revealed that there was no significant loss in its catalytic activity.

To highlight the merits of our current 1,1-diacetylation method, we have compared our results for the 1,1-diacetylation of benzaldehyde and cinnamaldehyde with other results reported in the literature using different catalysts (Table 9). These results of this comparison clearly show that NSPVPHS is superior to all of the other reported catalysts in terms of the yield or reaction time.

Table 9 Comparison of the effect of different catalysts in the synthesis of the 1,1-diacetates of benzaldehyde and cinnamaldehyde. Entry Catalyst Benzaldehyde Cinnamaldehyde Ref. Time (min) Yield (%) Time (min) Yield (%) 1 Saccharin sulfonic acid 48 90 — — [40] 2 NBS 360 95 1200 48 [41] 3 p-TSA 25 96 35 92 [42] 4 Silica sulfuric acid 30 84 15 81 [43] 5 Copper methane- sulfonate-HOAc 78 94 120 67 [44] 6 1,3-Dibromo-5,5-dimethyl hydantoin 90 98 48 92 [45] 7 Zirconium sulfophenyl phosphonate 12 84 180 72 [47] 8 NSPVPC 32 92 25 83 [48] 9 SuSA 12 98 5 93 [67] 10 NSPVPHS 5 98 5 94 this work

Table 9 Comparison of the effect of different catalysts in the synthesis of the 1,1-diacetates of benzaldehyde and cinnamaldehyde.

A possible mechanism for this reaction could involve either the intermolecular or intramolecular transfer of the second acetate group following the initial attack of the activated aldehyde by acetic anhydride (Scheme 6). The role of NSPVPHS in the reaction could be to activate the carbonyl group towards these nucleophilic attacks.

Scheme 6. Proposed mechanism of the 1,1-diacetylation of aldehydes.