¹H NMR (400 MHz) and ¹³C NMR (100 MHz) spectra were taken with a Bruker AV 400M spectrometer for CDCl₃ solution using SiMe₄ as an internal standard. High-resolution mass spectra (HRMS) were measured on a Thermo Scientific Q Exactive mass spectrometer equipped with an electrospray ionization source. Almost HRMS spectra were measured at positive mode except for the cases of 4f and 4j which were measured in negative mode. Microscopic spectrophotometry was performed on a confocal laser scanning microscope (CLSM; Olympus FV? 300, Japan) equipped with a spectrophotometer (STFL 250, Seki Technotron, Japan) linked to the CLSM by an optical fiber. Using a 10 folds magnification lens, the measurable area was restricted to the inside of a circle with a 8.56 μm- diameter [18] . Microscopic absorption spectra were taken using a back-light as the light source.

General procedure of solid-phase nitration was performed as follows. A MeOH solution (10 mL) containing 1,4-dimethoxybenzene (1a; 3.62 mmol, 500 mg) and Mg(NO₃)₂・6H₂O (504.9 mg, 1.97 mmol, Wako Chemicals, Japan ) was added slowly to silica gel (2.92 g, 48.7 mmol, Fuji Silysia A type, the average diameter = 79 μm) in a flask. After standing for 30 min to adsorb 1a and Mg(NO₃)₂ on silica gel, the solvent was removed by evaporation. The resulting composite of 1a, Mg(NO₃)₂, and silica gel was heated at a given temperature under N₂ atmosphere for 4-10 h under vigorous magnetic steering. The reacted composite was set on a silica gel column (Fuji Silysia BW 300, 50 mL) and was subjected to chromatography. Starting material (1a) and the nitrated products (2a) were isolated by elution with hexane and CHCl₃, respectively. The products were listed in Scheme 1. The 2a-2c, 2e-2i, 3d, 4f, and 4j had the following spectral data.

Scheme 1. Products (2a-2i, 3d, and 4f, 4j) of solid-phase nitration. Reaction conditions: 1 (3.62 mmol), Mg(NO₃)₂ (1.97 mmol), silica gel (2.92 g) under heating at 150˚C.

1,4-Dimethoxy-2-nitrobenzene (2a). Yellow solid. M.p. 69˚C - 70˚C (lit. 72˚C - 73˚C [19] ). ¹H NMR δ = 3.82 (s, 3H), 3.92 (s, 3H), 7.04 (d, J = 9.2 Hz, 1H), 7.16 (dd, J = 9.2, 3.1 Hz, 1H), 7.39 (d, J = 3.08 Hz, 1H). ¹³C NMR δ = 55.03, 56.09, 108.96, 114.12, 119.91, 138.52, 146.37, 151.86. HRMS: m/z calcd for C₈H₁₀NO₈: ([M + H]⁺) 184.0604, found 184.0605.

1,2-Dimethoxy-4-nitrobenzene (2b). Yellow solid. M.p. 99˚C (lit 97.4˚C [20] ). ¹H NMR δ = 3.90 (3, 3H), 3.92 (s, 3H), 6.85 (d, J = 8.9 Hz, 1H), 7.69 (d, J = 2.6 Hz, 1H), 7.86 (dd, J = 8.9, 2.6 Hz, 1H). ¹³C NMR δ = 56.47, 56.33, 106.45, 109.86, 117.81, 141.50, 148.87, 154.52. HRMS: m/z calcd for C₈H₁₀NO₈: ([M + H]⁺) 184.0604, found 184.0602.

2,4-Dimethoxy-1-nitrobenzene (2c). Brown solid. M.p. 70˚C (lit 72˚C - 76˚C [21] ). ¹H NMR δ = 3.89 (s, 3H), 3.95 (s, 3H), 6.51 (d, J = 8.9, 2.5 Hz, 1H), 6.54 (d, J = 2.5 Hz, 1H), 8.01 (d, J = 8.9, 2.5 Hz, 1H). ¹³C NMR δ = 55.92, 56.48, 99.67, 104.70, 128.53, 129.87, 155.70, 164.80. HRMS: m/z calcd for C₈H₁₀NO₈: ([M + H]⁺) 184.0604, found 184.0602.

4-Methyl-2-nitroanisole (2e). M.p. 128˚C. ¹H NMR δ = 2.34 (s, 3H), 3.93 (s, 3H), 6.98 (d, J = 8.5 Hz, 1H), 7.34 (dd, J = 8.6, 2.2 Hz, 1H), 7.65 (d, J = 1.8 Hz, 1H). HRMS: m/z calcd for C₈H₉NO₃: ([M + H]⁺) 168.0655, found 168.0654.

4-Methyl-3-nitroanisole (2e'). ¹H NMR δ = 2.44 (s, 3H), 3.89 (s, 3H), 7.01 (d, J = 8.8 Hz, 1H), 7.84 (d, J = 8.8, 1H), 8.13 (s, 1H).

4-Methyl-2-nitrophenol (2f). Oil, ¹H NMR δ = 2.77 (s, 3H), 6.98 (d, J = 8.6 Hz, 1H), 7.32 (d, J = 8.8 and 2.0 Hz, 1H), 7.83 (d, J = 2.0 Hz, 1H).

1-Methoxy-4-nitronaphthalene (2g). Yellow solid. M.p. 80.0˚C (lit 81˚C - 83˚C [22] ), ¹H NMR δ = 4.11 (s, 3H), 6.82 (d, J = 8.7 Hz, 1H), 7.59 (dd, J = 8.2, 6.9 Hz, 1H), 7.74 (dd, J = 8.5, 6.9 Hz, 1H), 8.37 (d, J = 8.5 Hz, 1H), 8.40 (d, J = 8.7 Hz, 1H), 8.78 (d, J = 8.7 Hz, 1H). ¹³C NMR δ = 56.29, 101.90, 122.77, 123.50, 125.62, 126.58, 126.88, 127.20, 129.46, 130.07, 160.60. HRMS: m/z calcd for C₁₁H₉NO₃: ([M + H]⁺) 204.0655, found 204.0654.

2-Methoxy-1-nitronaphthalene (2h). Green solid. M.p. 126˚C, ¹H NMR δ = 4.00 (s, 3H), 7.31 (d, J = 9.2 Hz, 1H), 7.43 (dd, J = 8.2, 6.8 Hz, 1H), 7.58 (dd, J = 8.6, 6.8 Hz, 1H), 7.66 (d, J = 8.6 Hz, 1H), 7.82 (d, J = 8.2 Hz, 1H), 7.92 (d, J = 9.2 Hz, 1H). ¹³C NMR δ = 57.05, 113.06, 120.39, 125.15, 125.66, 128.07, 128.20 129.14, 132.21, 136.03, 148.62. HRMS: m/z calc. for [M + H], C₁₁H₉NO₃⁺: 204.0655; Found: 204.0655.

2,3-Dimethyl-1-nitronaphthalene (2i). Yellow solid. M.p. 86.0˚C - 88.0˚C, ¹H NMR δ = 2.33 (s, 3H), 2.43 (s, 3H), 7.45-7.60 (m, 2H), 7.58 (d, J = 7.6 Hz), 7.68 (s, 1H), 7.75 (d, J = 7.2 Hz, 1H). ¹³C NMR δ = 14.84, 20.47, 120.95, 126.66, 127.19, 127.50, 128.02, 129.76, 132.03, 133.70, 135.23, 148.51. HRMS: m/z calcd for C₁₂H₁₂NO₂: ([M + H]⁺) 202.0863, found 202.0862.

2,4,6,2',4',6'-Hexamethoxy-3-nitrobiphenyl (3d). Yellow solid. M.p. 69.0˚C - 70.0˚C, ¹H NMR δ = 3.46 (s, 3H), 3.72 (s, 6H), 3.76 (s, 3H), 3.92 (s, 3H), 3.86 (s, 3H), 6.21 (s, 2H), 6.34 (s, 1H). ¹³C NMR δ = 55.31, 55.95, 56.32, 56.36, 61.70, 90.87, 91.54, 102.73, 110.34, 130.87, 151.94, 152.52, 159.07, 160.24, 161.54. HRMS: m/z calcd for C₁₈H₂₂NO₈: ([M + H]⁺) 380.1340, found 380.1337.

4-Tolyl nitrate (4f). Oil, ¹H NMR δ = 2.24 (s, 3H), 6.72 (d, J = 8.4 Hz, 2H), 7.00 (dd, J = 8.4 Hz, 1H). ¹³C NMR δ = 19.46, 114.12, 128.93, 129.04, 152.08. HRMS: m/z calcd for C₇H₇NO₃: ([M − H]⁻) 152.0353, found 152.0353.

2-Naphtyl nitrate (4j). Black solid. M.p. 150.0˚C, ¹H NMR δ = 7.09 (dd, J = 8.8, 2.5 Hz, 1H), 7.14 (d, J = 2.5 Hz, 1H), 7.31 (dd, J = 8.1, 6.9 Hz, 1H), 7.42 (dd, J = 8.1, 6.9 Hz, 1H), 7.66 (d, J = 8.1, 1H), 7.73 (d, J = 8.8 Hz, 1H), 7.75 (d, J = 8.1 Hz, 1H). ¹³C NMR δ = 109.48, 117.78, 123.59, 126.37, 126.52, 127.77, 129.84, 131.46, 134.60, 153.42. HRMS: m/z calcd for C₁₀H₇NO₃: ([M − H]⁻) 188.0353, found 188.0353.

As pH-indicator, meso-tetra(p-cyanophenyl)porphyrin (H₂tcp) was prepared according to the reported method as follows [23] . BF₃・OEt₂ (0.1 mL) were added to CHCl₃ solution (500 mL) of p-cyanobenzaldehyde (1048 mg; 8.0 mmol). A CHCl₃ solution (300 mL) of pyrrole (0.56 mL; 8.0 mmol) was then added. After the solution turned from pale orange to red-violet, it was confirmed that the Soret band appeared at 410 nm. NEt₃ (0.2 mL; 1.43 mmol) and chloranil (2.2 g; 9.0 mmol) were added to the solution and then heated at 60˚C for 1 h under dark conditions. After evaporation, the condensed solution was filtrated and washed with CHCl₃. The crude H₂tcp was purified by a column chromatography on silica gel (Fuji Silysia BW 300) using CHCl₃-MeOH (50:1) as eluent.

meso-Tetra(p-cyanophenyl)porphyrin. Yield 1.2%. ¹H NMR δ = 8.10 (d, J = 8.6 Hz, 8H), 8.33 (d, J = 8.3 Hz, 8H), 8.60 (s, 8H). HRMS: m/z calcd for C₄₈H₂₇N₈: ([M + H]⁺) 715.2359, found 715.2350.

According to the reported method [18] , the acidity of the Mg(NO₃)₂-SiO₂ composite was measured as follows. At first, absorption spectra of H₂tcp were measured in CHCl₃-MeOH (1:2, v/v) under different pHs which were adjusted by HClO₄. The absorptions were observed at 514 and 645 nm due to purple free base porphyrin (H₂tcp) and the greenish protonated porphyrin (H₄tcp²⁺), respectively. The absorbances (A_P and A_G) were measured at 514 nm and 645 nm, respectively. Fraction (F = A_P/(A_G + A_P)) was calculated at every pH and plotted against the pH to make the pH-profile of F values (Figure 1). The pH-profile was fitted by sigmoid curves (Equation (1)) which was presented by three parameters, F_max, pKa, and S, which denotes maximum F values, acid dissociation constant of H₄tcp²⁺, and slope of the fitting curve at pKa, respectively. The relative standard deviation (RSD) was 0.9996. Each value of F_max, pKa, and S for H₂tcp were determined to be 0.875, 1.376, and 6.220, respectively.

When the aromatic hydrocarbon was not added the Mg(NO₃)₂-SiO₂ composite, acidity of the Mg(NO₃)₂-SiO₂ composite was measured in a solid state using the pH-profile as follows. A CHCl₃ solution (10 mL) of H₂tcp (0.7 μmol) and an

aqueous solution (2 mL) of Mg(NO₃)₂ (173 mg) were mixed with SiO₂ (1.0 g, Fuji silysia A type, 1.8 - 5.0 mm). Here, large sizes of silica gel were used for CLSM analysis. After standing for 2 h until almost all of the H₂tcp had been adsorbed on Mg(NO₃)₂-SiO₂ composite, the solvent was evaporated and the H₂tcp-Mg(NO₃)₂-SiO₂ composite was dried under reduced pressure. Figure 2 shows the absorption spectra of H₂tcp adsorbed on a Mg(NO₃)₂-SiO₂ composite which was measured for five beads on CLSM. The F-values were determined to be 0.06 ± 0.04 by averaging five spectra. Using fitting curves, F-values were converted to pH, which was determined to be 0.96 for the Mg(NO₃)₂-SiO₂ composite under dry conditions.

Solid-phase nitration of aromatic hydrocarbons (1) was performed by heating a mixture of 1 (3.62 mmol), Mg(NO₃)₂・6H₂O (504.9 mg, 1.97 mmol), and silica gel (2.92 g, 48.7 mmol) at a given temperature under N₂ atmosphere under magnetic steering. The solid-phase nitration of 1,4-dimethoxybenzene (1a) with Mg(NO₃)₂ on silica gel produced 1,4-dimethoxy-2-nitrobenzene (2a). Figure 3 showed the time- conversion plots of 2a at various temperatures. From the plots, the optimized temperature was determined to be 150˚C. Therefore, the reaction temperature was fixed at 150˚C in the solid-phase nitration of other aromatics (1b-1j). The results are summarized in Table 1.

The solid-phase nitration of 1,2- and 1,3-dimethoxybenzenes (1b and 1c) and 4-methylanisole (1e) gave the nitroaromatics (2b and 2c, 2e and 2e’) but yields were low. In the cases of naphthalene derivatives (1g-1i), the α-nitrated compounds (2g-2j) were obtained. Moreover, it was noteworthy that the nitration of 1,3,5-trimethoxybenzene (1d) produced 2,4,6,2’,4’,6’-hexamethoxy-3-nitrobi-

phenyl (3d) which was the nitrated dimer of 1d. However, 1,3,5-trimethoxy-2- nitrobenzene was not formed, though it was reported that the reaction of 1d with HNO₃ in the presence of HClO₄ gave 1,3,5-trimethoxy-2-nitrobenzene [24] . The structure of 3d was undoubtedly confirmed by MS and NMR spectra. In the cases of p-cresol (1f) and 2-naphthol (1j), the nitration occurred at the hydroxyl group to give 4-tolyl nitrate (4f) and 2-naphthyl nitrate (4j), respectively. Usual nitration of 4f and 4j in solution occurred at aromatic ring [25] . Recently it was reported that the efficient nitration of phenol and phenol derivatives with Al(NO₃)₃・9H₂O [12] [13] and Bi(NO₃)₃・5H₂O [26] on silica gel occurred at aromatic ring. Therefore, it was suggested that the nitration ability of SiO₂-Mg(NO₃)₂ was not so strong, because the nitration was restricted to the electron rich substrates. In the case of phenol derivatives, silica gel supported the dehydration between OH group and HNO₃ to give Ar-ONO₂. Since aromatic nitrate had been recognized to be unstable [27] , the present synthesis had synthetic worthy.

The silica gel is constructed by Si-O bonds such as Si-O-Si, Si=O, and Si-OH. It is well-known that the Si-OH group remained on the surface under heating below 300˚C [7] . We previously elucidated that the Si-OH on silica gel could react with MgCl₂ and MgSO₄ under dry conditions to release proton [18] . Their acidities were determined by the colorimeter analysis using meso-tetraarylporphyrin as a pH-indicator. In the present study, the reaction of Mg(NO₃)₂ with silica gel generated HNO₃ along with the adsorption of Mg²⁺ ion on silica gel (Equation (2)). The acidity (pH) of Mg(NO₃)₂-SiO₂ composite was determined to be 0.96, which was more acidic compared with MgCl₂-SiO₂ and MgSO₄-SiO₂ composites whose pH were 1.73 and 1.61, respectively. The presence of excess water made the pH of Mg(NO₃)₂-SiO₂ neutral. Aromatic nitration usually occurs under acidic conditions. It was suggested that HNO₃ reacted with the proton to form the (Equation (3)). More electron-rich aromatic hydrocarbons (1a and 1d) were allowed the efficient nitration. Therefore, the nitration proceeded through electrophilic attack of to the aromatic ring (Equation (4)). Mg(NO₃)₂ and silica gel played as nitration reagent and desiccant, respectively.

Generation of H⁺

Generation of

Nitration

In order to elucidate the mechanism of the formation of 3d, acid-catalyzed dimerization of 1d was attempted by the heating of 1d on silica gel at 150˚C for 7 h with MgCl₂ which can release proton but has no electrophilicity. The dimerization of 1d did not occur. It is well known that the dimerization of aromatic hydrocarbons in the presence of electrophiles has been reported [28] . The electrophilic attack of NO₂⁺ to 1d gave intermediates 5A and/or 5B. The 5A is less stable and more reactive than 5B, leading to the reaction of 5Awith another 1d at ortho and/or para positions (Friedel-Crafts acylation) to give the dimer which allowed the nitration to form 3d (Scheme 2).

Scheme 2. Possible mechanism of the formation of 3d.

a) The solid-phase nitration was performed by heating 1a-1j (3.62 mmol) with Mg(NO₃)₂ (1.97 mmol) on silica gel (2.92 g) at 70˚C - 170˚C; b) Conversion of 1; c) Isolated yields based on the 1 used.