Lewis acid-catalyzed tandem synthesis of 9-sulfonylamino- and 9-arylfluorenes

Abstract

A Lewis acid-catalyzed three-step tandem synthesis of 9-arylfluorene was developed by simply heating a mixture of 2-formyl biphenyl, TsNCO and an arene. In the absence of arene, a two-step tandem synthesis of 9-sulfonylaminofluorene was achieved. These advances were mainly attributed to the discovery of an anhydrous synthesis of N-tosyl arylaldimines from TsNCO and arylaldehydes.

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Introduction

When two benzene rings in diphenylmethane are coupled, the corresponding tricyclic fluorene is formed. Fluorene has a nearly planar structure and is easily substituted by aryl groups on C9. Thus, numerous 9-arylfluorenes 1 have been synthesized and used in various novel material applications.¹ As shown in Fig. 1, the structural unit of 9-mesitylfluorene (1a) in the molecules 2–4 has been proven to be the best precursor to generate a stable radical on C9.² The molecules containing two or more such unpaired electrons are called organic diradicals or polyradicals with open-shell structures, which have recently been the subject of intensive studies due to their fascinating photoelectric properties.

Fig. 1 Structures of 1 and 2.

A number of methods have been reported for the synthesis of 9-arylfluorenes 1 in the literature.^3–7 As shown in Scheme 1, three practical methods (a–c) starting from 2-carbonyl biphenyl 5 offered multiple choices for different applications.^2a,b,5–7 The method-c⁷ seems highly attractive due to its tandem process to convert 5 into 1 in one-pot. However, a major drawback to this method has been the requirements of highly electron-rich Ar² and Ar³ (such as indole, thiophene, multi-alkoxy substituted benzene, etc.).^7b–d Since the diarylmethanol 7 has been confirmed to be an intermediate, these drawbacks must be caused by two facts: (a) the aldehyde group of 5 has low electrophilicity; (b) the hydroxyl group of 7 is a poor leaving group but a good nucleophile. These hypotheses have been supported by some published references, in which the reaction efficiency was improved significantly by converting the hydroxyl group into the corresponding acetate.^6b,7a

Scheme 1 Four synthetic methods starting from 5.

Herein, we would like to report a new method-d for efficient synthesis of 9-arylfluorene 1 from 2-formyl biphenyl 5, tosyl isocyanate (TsNCO, 8) and an arene. In this method, the aldehyde was in situ converted into a highly electrophilic N-tosylimine and a low nucleophilic TsNH₂ was used as a good leaving group.

Results and discussion

Based on the analysis of method-c, two issues should be considered in developing the new method starting from 2-formyl biphenyl 5: (a) increasing the electrophilicity of the aldehyde; (b) using a leaving group with low nucleophilicity. It is well-known that N-tosylimine is significantly more electrophilic than the corresponding aldehyde (for example: E = −19.92 for PhCHO; E = −11.50 for PhCH=NTs).⁸ Therefore, N-tosylimines can efficiently carry out Lewis acid-catalyzed aza-Friedel–Crafts reactions.⁹ Thus, two conditional experiments catalyzed by FeCl₃ were made. As shown in Scheme 2, no reaction occurred between 2-phenyl-benzaldehyde (5a) and mesitylene (10a). However, the desired product 1a was obtained in 98% yield from N-tosyl-2-phenyl-benzaldimine (9a) and 10a, in which 12a was isolated as the intermediate instead of the expected 11a. These results indicated that the reaction initially carried out an intramolecular aza-Friedel–Crafts reaction followed by an intermolecular Friedel–Crafts reaction. But, the tosylimine 9a was required to be pre-made from 5a in this process.¹⁰

Scheme 2 Two conditional experiments from 5a and 9a.

Our investigation showed that N-tosylimines were most often prepared by dehydration between aldehydes and TsNH₂. Since some of these processes can be catalyzed by Lewis acids¹¹ including FeCl₃,^11b we realized that a FeCl₃-catalyzed three-step tandem synthesis of 1a starting from 5a may be achieved. Unfortunately, when the mixture of 5a, TsNH₂ and 10a was treated by FeCl₃, only N-tosylimine 9a was obtained in 36% yield. As shown in Scheme 3, the problem of FeCl₃ losing its catalytic activity after the dehydration (in step 1) may be caused by the fact that one molecule of H₂O formed and coordinated strongly with FeCl₃.

Scheme 3 A failed tandem synthesis and possible reasons.

Thus, our attention was focused on two processes that convert an aldehyde into the corresponding N-tosylimine without the formation of H₂O. As shown in Scheme 4, when TsNTeO^12a or TsNSO^12b reacted with an ArCHO, the desired ArCH=NTs was synthesized efficiently via a [2 + 2] cycloaddition followed by a [2 + 2] cycloreversion. Yet, these processes were rarely used in the literature possibly due to inaccessible reagents, high cost and unpleasant smell. To our surprise, the structurally similar TsNCO was never used for such purpose even though it is a very cheap commercially available product and CO₂ should be only by-product.

Scheme 4 Anhydrous syntheses of N-tosyl arylaldimines.

However, a very careful investigation showed that the reaction between TsNCO and 4-MeC₆H₄CHO (13) was reported by King¹³ in 1960, but it failed to give the desired 4-MeC₆H₄CH=NTs (14). King concluded that the failure was caused by the poor hydrolytic stability of 14 since only the hydrolyzed products (13 and TsNH₂) of 14 were isolated from the reaction. Since then, no further study was reported for the reaction between TsNCO and aryl aldehydes in the literature. Clearly, we now know that this conclusion is wrong because 14 has been prepared by many different methods and can be purified by chromatography or recrystallization.¹⁴ Thus, we were encouraged to retest the reaction described in Scheme 2, but in the presence of one equivalent of TsNCO. As shown in Scheme 5, when the mixture of 5a and TsNCO in mesitylene was treated by FeCl₃ at 80 °C for 1 h, the desired product 1a was obtained in 98% yield. As was expected, both 9a and 12a were isolated from the reaction as intermediates.

Scheme 5 Tandem synthesis of 1a from 5a in one-pot.

Therefore, a method for highly efficient synthesis of 1a was established. As shown in Scheme 6, this reaction is a Lewis acid-catalyzed and TsNCO-mediated three-step tandem reaction. FeCl₃ serves as a versatile Lewis acid to catalyze each of steps even though they have different mechanisms. Although no atom of TsNCO appears in the molecule of 1a, TsNCO plays three vital roles: (a) an anhydrous preparation of 9a was achieved in situ by using TsNCO as a substrate; (b) the electrophilicity of imine group in 9a is significantly promoted by the tosyl group; (c) neutral TsNH₂ was used as a leaving group with low nucleophilicity.

Scheme 6 A proposed mechanism.

Next, the reaction conditions for the synthesis of 1a were optimized. As shown in Table 1, four popular Lewis acids (entries 1–4) were tested and only FeCl₃ (entry 1) showed highly catalytic activity. When the reaction was catalyzed by 5 mol% of FeCl₃ (entry 5) or proceeded at 70 °C (entry 6), the yield of 1a was decreased significantly. Since 9-tosylamino-fluorene 12a is our key intermediate, we decided to develop a two-step tandem synthesis for it, too. However, when the mixture of 5a and TsNCO in DCE was treated by FeCl₃ (entry 7), the desired 12a was obtained in only 14% yield and the major product was lower polymers. Similar results were obtained also by using Bi(OTf)₃ as a catalyst (entry 8). Luckily, the reaction catalyzed by BF₃·Et₂O or Cu(OTf)₂ gave 12a as a single product (entries 9–10). These results indicated that both FeCl₃ and Bi(OTf)₃ are strong catalysts for the conversion of 12a into fluorene cation 15. Thus, the in situ formed highly active 15 may attack 12a to give lower polymers in the absence of a nucleophile (such as mesitylene). However, both BF₃·Et₂O and Cu(OTf)₂ are weak catalysts and they could not catalyze the conversion of 12a into 15 at all. We interestingly observed that product 12a was precipitated out of the reaction mixture when CCl₄ was used as a solvent (entry 11), by which the reaction equilibria were irreversibly shifted towards the products 12a. Entry 12 showed that 12a was obtained in 96% yield when the reaction time was extended to 4.5 h. Finally, the entries 1 and 12 were assigned as the standard conditions for the syntheses of the derivatives of 1a and 12a, respectively.

Table 1 The reaction conditions for 12a and 1a^a

Entry	Lewis acid (mol%)	Solvent	12a^b (%)	1a^b (%)
a A mixture of 5a (0.5 mmol) and TsNCO (0.6 mmol) in a solvent (2 mL) was heated at 80 °C for 1 h in the presence of a Lewis acid.b Isolated yields were obtained.c Reaction proceeded at 70 °C.d Large amounts of lower polymers were isolated.e 4.5 h were used.
1	FeCl3 (10)	Mesitylene	0	98
2	Bi(OTf)3 (10)	Mesitylene	82	15
3	BF3·Et2O (30)	Mesitylene	41	0
4	Cu(OTf)2 (10)	Mesitylene	52	0
5	FeCl3 (5)	Mesitylene	ND	79
6^c	FeCl3 (10)	Mesitylene	ND	73
7^d	FeCl3 (10)	DCE	14	NA
8^d	Bi(OTf)3 (10)	DCE	28	NA
9	BF3·Et2O (30)	DCE	33	NA
10	Cu(OTf)2 (10)	DCE	39	NA
11	Cu(OTf)2 (10)	CCl4	62	NA
12^e	Cu(OTf)2 (10)	CCl4	96	NA

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As shown in Scheme 7, the products 12a–12t were synthesized in satisfactory yields by simply heating the reactants and Cu(OTf)₂ together. The lower yields of 12f–12j may be caused by the electron effect of Ar², by which the nucleophilicity of Ar² was decreased. No significant influence was observed by using different Ar¹ (12k–12m). As was expected, the products 12q–12t were synthesized smoothly by using other commercial sulfonyl isocyanates (PhSO₂NCO or 4-Cl-C₆H₄SO₂NCO).

Scheme 7 The scope of 9-sulfonylaminofluorenes 12.

As shown in Scheme 8, all tested products were obtained in excellent yields except 1e–1i, in which the electron effect of substituents led to lower nucleophilicity of Ar². No significant electron effect was observed when Ar¹ was substituted by different groups. When higher electron-rich Ar³ were used as substrates, the reactions could proceed in DCE (1q–1t). For example, three equivalents of 1,3,5-trimethoxybenzene were good enough to give 1t in excellent yield. Under the standard conditions, 1a was prepared on a 10 gram scale in 92% yield. As shown in Fig. 2, the structure of product 1s was further confirmed by single crystal X-ray diffraction.

Scheme 8 The scope of 9-arylfluorenes 1.

Fig. 2 The structure of product 1s.

Conclusions

In conclusion, we have done four works in this article. (a) A strategy for the synthesis of 9-arylfluorenes was proposed by using N-tosyl arylaldimine as an alternative of benzaldehyde. (b) An anhydrous preparation of N-tosyl arylalimine in situ was achieved by reaction between TsNCO and an aryl aldehyde. (c) A Cu(OTf)₂-catalyzed two-step tandem synthesis of 9-sulfonyl-aminofluorenes was established by simply heating the mixture of 2-formyl biphenyl and TsNCO. (d) A FeCl₃-catalyzed three-step tandem synthesis of 9-arylfluorenes was achieved by simply heating the mixture of 2-formyl biphenyl, TsNCO and an arene.

Experimental

General information

All melting points were determined on a Yanaco melting point apparatus and are uncorrected. IR spectra were recorded as KBr pellets on a Nicolet FT-IR 5DX spectrometer. The ¹H NMR (300 or 400 MHz) and ¹³C NMR (75 or 100 MHz) spectra were recorded on a JEOL JNM-ECA spectrometers 300 or 400 in CDCl₃ or DMSO-d₆. TMS was used as an internal reference and J values are given in Hz. HRMS were obtained on a Bruker micrOTOF-Q II spectrometer.

A typical procedure for preparation of 9-[(4-methylphenyl)sulfonylamino]fluorene (12a)

To a stirred mixture of 2-formyl biphenyl (5a, 91 mg, 0.5 mmol) and Cu(OTf)₂ (18 mg, 0.05 mmol) in CCl₄ (3 mL) was added TsNCO (118 mg, 0.6 mmol). After the mixture was heated at 80 °C for 4.5 h, the solvent was then removed off in vacuum. The residue was purified by flash chromatography [silica gel, 20% EtOAc in petroleum ether (60–90 °C)] to give 161 mg (96%) of product 12a as a white solid, mp 200–201 °C (lit.^9b mp 199–201 °C); ¹H NMR (400 MHz, CDCl₃) δ 7.92 (d, J = 8.2 Hz, 2H), 7.58 (d, J = 7.8 Hz, 2H), 7.39 (d, J = 8.3 Hz, 2H), 7.35–7.31 (m, 2H), 7.20–7.19 (m, 4H), 5.36 (d, J = 9.6 Hz, 1H), 4.78 (d, J = 9.6 Hz, 1H), 2.50 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 143.7, 143.3 (2C), 140.0 (2C), 138.4, 129.9 (2C), 128.9 (2C), 127.8 (2C), 127.3 (2C), 125.2 (2C), 119.9 (2C), 58.3, 21.6.

The products 12b–12t were prepared by the similar procedure.

2-Methyl-9-[(4-methylphenyl)sulfonylamino]fluorene (12b)

White solid, mp 203–204 °C (lit.^9b mp 209–210 °C); ¹H NMR (400 MHz, CDCl₃) δ 7.89 (d, J = 8.3 Hz, 2H), 7.49 (d, J = 7.3 Hz, 1H), 7.41–7.36 (m, 3H), 7.29–7.26 (m, 1H), 7.16–7.06 (m, 3H), 6.88 (s, 1H), 5.30 (d, J = 9.2 Hz, 1H), 4.71 (d, J = 9.2 Hz, 1H), 2.48 (s, 3H), 2.26 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 143.7, 143.6, 143.3, 140.1, 138.8, 137.8, 137.3, 129.9 (2C), 129.6, 128.8, 127.4 (2C), 127.3, 125.9, 125.1, 119.6, 119.5, 58.3, 21.5, 21.4.

4-Methyl-9-[(4-methylphenyl)sulfonylamino]fluorene (12c)

White solid, mp 192–193 °C (lit.^9b mp 184–186 °C); ¹H NMR (300 MHz, CDCl₃) δ 7.92 (d, J = 8.1 Hz, 2H), 7.72 (d, J = 8.1 Hz, 1H), 7.41–7.32 (m, 3H), 7.26–7.18 (m, 2H), 7.10–7.02 (m, 3H), 5.34 (d, J = 9.6 Hz, 1H), 4.74 (d, J = 9.3 Hz, 1H), 2.60 (s, 3H), 2.50 (s, 3H); ¹³C NMR (75 MHz, CDCl₃) δ 143.7 (2C), 143.5, 140.9, 138.5, 138.0, 133.1, 131.2, 129.8 (2C), 128.8, 127.5, 127.3, 127.2 (2C), 125.1, 123.0, 122.5, 58.2, 21.6, 20.7.

2-Isopropyl-9-[(4-methylphenyl)sulfonylamino]fluorene (12d)

White solid, mp 169–170 °C (lit.^9b mp 178–180 °C); ¹H NMR (400 MHz, CDCl₃) δ 7.92 (d, J = 7.8 Hz, 2H), 7.54–7.46 (m, 2H), 7.41 (d, J = 7.8 Hz, 2H), 7.33–7.20 (m, 4H), 6.97 (s, 1H), 5.35 (d, J = 9.2 Hz, 1H), 4.82 (d, J = 9.2 Hz, 1H), 3.29–3.23 (m, 1H), 2.51 (s, 3H), 1.27 (d, J = 4.1 Hz, 6H); ¹³C NMR (100 MHz, CDCl₃) δ 144.0, 143.9, 143.2, 139.4, 138.5, 138.4, 135.5, 132.1, 130.0 (2C), 129.0, 127.9, 127.8, 127.3 (2C), 125.2, 120.1, 119.8, 58.2, 38.0, 23.0 (2C), 21.6.

2-Phenyl-9-[(4-methylphenyl)sulfonylamino]fluorene (12e)

White solid, mp 217–219 °C (lit.^9b mp 188–190 °C); ¹H NMR (400 MHz, DMSO-d₆) δ 8.46 (d, J = 8.7 Hz, 1H), 7.93 (d, J = 7.8 Hz, 2H), 7.86–7.81 (m, 2H), 7.64 (d, J = 7.8 Hz, 1H), 7.51–7.32 (m, 9H), 6.75 (s, 1H), 5.44 (d, J = 8.2 Hz, 1H), 2.42 (s, 3H); ¹³C NMR (100 MHz, DMSO-d₆) δ 144.4, 144.2, 142.8, 139.5, 139.3, 139.1, 138.8, 138.7, 129.7 (2C), 128.7 (2C), 128.4, 127.5, 127.2, 126.9, 126.5 (2C), 126.1 (2C), 124.7, 122.6, 120.4, 120.0, 57.6, 20.8.

2-Methoxy-9-[(4-methylphenyl)sulfonylamino]fluorene (12f)

White solid, mp 194–195 °C (lit.^9b mp 188–190 °C); ¹H NMR (300 MHz, DMSO-d₆) δ 8.44 (d, J = 8.6 Hz, 1H), 7.92 (d, J = 8.3 Hz, 2H), 7.67 (d, J = 8.3 Hz, 2H), 7.51 (d, J = 7.9 Hz, 2H), 7.36–7.31 (m, 1H), 7.21–7.11 (m, 2H), 6.93–6.90 (m, 1H), 6.34 (d, J = 2.1 Hz, 1H), 5.29 (d, J = 8.3 Hz, 1H), 3.60 (s, 3H), 2.44 (s, 3H); ¹³C NMR (75 MHz, DMSO-d₆) δ 159.3, 145.8, 143.6, 143.0, 139.6, 139.5, 132.3, 129.9 (2C), 128.6, 126.7 (2C), 126.4, 124.7, 121.1, 119.3, 114.9, 109.8, 57.7, 55.2, 21.0.

2-Trifluoromethoxy-9-[(4-methylphenyl)sulfonylamino]-fluorene (12g)

White solid, mp 183–184 °C (lit.^9b mp 181–183 °C); ¹H NMR (400 MHz, DMSO-d₆) δ 8.54 (d, J = 8.7 Hz, 1H), 7.93–7.89 (m, 3H), 7.82 (d, J = 7.2 Hz, 1H), 7.50 (d, J = 8.2 Hz, 2H), 7.42–7.28 (m, 2H), 7.16 (d, J = 8.0 Hz, 1H), 6.60 (s, 1H), 5.41 (d, J = 8.7 Hz, 1H), 2.46 (s, 3H); ¹³C NMR (100 MHz, DMSO-d₆) δ 147.7, 146.2, 144.2, 143.1, 139.2, 138.8, 138.1, 129.9 (2C), 128.8, 128.0, 126.7 (2C), 124.9, 121.6, 120.5, 118.8, 117.5, 57.6, 20.9.

2-Fluoro-9-[(4-methylphenyl)sulfonylamino]fluorene (12h)

White solid, mp 203–204 °C (lit.^9a mp 198–200 °C); ¹H NMR (400 MHz, DMSO-d₆) δ 8.50 (d, J = 8.2 Hz, 1H), 7.90 (d, J = 8.3 Hz, 2H), 7.84–7.77 (m, 2H), 7.50 (d, J = 7.8 Hz, 2H), 7.38 (t, J = 7.6 Hz, 1H), 7.25–7.19 (m, 2H), 7.07 (d, J = 7.6 Hz, 1H), 6.64 (d, J = 9.2 Hz, 1H), 5.36 (d, J = 8.3 Hz, 1H), 2.46 (s, 3H); ¹³C NMR (100 MHz, DMSO-d₆) δ 161.9 (d, J = 242.2 Hz), 146.5 (d, J = 7.6 Hz), 143.8, 143.0, 139.1, 138.6, 135.9, 129.9 (2C), 128.7, 127.3, 126.7 (2C), 124.8, 121.7 (d, J = 8.6 Hz), 120.0, 115.5 (d, J = 22.9 Hz), 111.9 (d, J = 23.9 Hz), 57.6, 21.0.

2-Chloro-9-[(4-methylphenyl)sulfonylamino]fluorene (12i)

White solid, mp 213–215 °C (lit.^9b mp 208–210 °C); ¹H NMR (300 MHz, DMSO-d₆) δ 8.48 (d, J = 8.6 Hz, 1H), 7.89 (d, J = 7.9 Hz, 2H), 7.79 (d, J = 7.9 Hz, 2H), 7.50 (d, J = 8.3 Hz, 2H), 7.42–7.36 (m, 1H), 7.27 (t, J = 7.2 Hz, 1H), 7.11 (d, J = 7.5 Hz, 1H), 6.68 (s, 1H), 5.36 (d, J = 8.6 Hz, 1H), 2.46 (s, 3H); ¹³C NMR (75 MHz, DMSO-d₆) δ 145.9, 143.6, 143.0, 139.0, 138.4, 138.3, 131.8, 129.8 (2C), 128.6, 128.4, 127.8, 126.7 (2C), 124.8, 124.7, 121.5, 120.2, 57.5, 20.9.

2-Chloro-9-[(4-methylphenyl)sulfonylamino]fluorene (12j)

White solid, mp 203–204 °C (lit.^9a mp 208–210 °C); ¹H NMR (400 MHz, DMSO-d₆) δ 8.53 (d, J = 8.2 Hz, 1H), 8.01–7.90 (m, 3H), 7.72–7.70 (m, 1H), 7.52–7.36 (m, 4H), 7.23 (d, J = 7.6 Hz, 1H), 6.86 (s, 1H), 5.46 (d, J = 8.7 Hz, 1H), 2.46 (s, 3H); ¹³C NMR (100 MHz, DMSO-d₆) δ 147.9, 147.6, 143.9 (2C), 139.3, 138.2, 130.3 (2C), 129.4 (2C), 128.1 (q, J = 31.5 Hz), 127.1 (2C), 126.2, 125.3, 124.6 (q, J = 270.8 Hz), 121.7, 121.5, 121.1, 58.0, 21.3.

3-Methyl-9-[(4-methylphenyl)sulfonylamino]fluorene (12k)

White solid, mp 192–193 °C (lit.^9b mp 189–191 °C); ¹H NMR (400 MHz, CDCl₃) δ 7.91 (d, J = 8.2 Hz, 2H), 7.54 (d, J = 7.8 Hz, 1H), 7.38 (d, J = 7.3 Hz, 3H), 7.32–7.30 (m, 1H), 7.17 (d, J = 4.1 Hz, 2H), 7.04–6.99 (m, 2H), 5.30 (d, J = 9.2 Hz, 1H), 4.73 (d, J = 9.6 Hz, 1H), 2.49 (s, 3H), 2.37 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 143.9, 143.7, 140.6, 140.1, 140.0, 138.9, 138.4, 129.9 (2C), 128.6, 128.7, 127.3, 127.5 (2C), 125.1, 124.8, 120.5, 119.7, 58.0, 21.6, 21.5.

3-Methoxy-9-[(4-methylphenyl)sulfonylamino]fluorene (12l)

White solid, mp 178–179 °C; IR (KBr) ν 3306, 2836, 1589, 1430, 1327, 1164 cm⁻¹; ¹H NMR (300 MHz, CDCl₃) δ 7.89 (d, J = 8.3 Hz, 2H), 7.43–7.36 (m, 4H), 7.28–7.22 (m, 1H), 7.09 (d, J = 4.4 Hz, 2H), 6.82–6.79 (m, 1H), 6.61 (s, 1H), 5.26 (d, J = 9.6 Hz, 1H), 4.82 (d, J = 9.3 Hz, 1H), 3.67 (s, 3H), 2.47 (s, 3H); ¹³C NMR (75 MHz, CDCl₃) δ 159.9, 145.3, 143.7, 142.9, 140.0, 138.6, 132.6, 129.9 (2C), 128.9, 127.3 (2C), 126.5, 124.9, 120.7, 119.0, 115.6, 110.0, 58.3, 55.4, 21.6. HRMS (ESI-TOF) (m/z): calcd for C₂₁H₁₉NO₃S, [M + Na]⁺ 388.0978; found 388.0971.

3-Chloro-9-[(4-methylphenyl)sulfonylamino]fluorene (12m)

White solid, mp 165–166 °C (lit.^9b mp 156–158 °C); ¹H NMR (400 MHz, DMSO-d₆) δ 8.47 (d, J = 8.2 Hz, 1H), 7.91 (d, J = 8.2 Hz, 2H), 7.85 (d, J = 7.8 Hz, 1H), 7.49 (d, J = 7.8 Hz, 2H), 7.38 (t, J = 7.4 Hz, 1H), 7.29–7.24 (m, 2H), 7.08–7.02 (m, 2H), 5.33 (d, J = 8.4 Hz, 1H), 2.46 (s, 3H); ¹³C NMR (100 MHz, DMSO-d₆) δ 144.3, 143.0, 142.7, 141.7, 139.2, 138.3, 133.5, 129.8 (2C), 128.7, 128.3, 127.2, 126.7 (2C), 126.3, 124.9, 120.7, 120.3, 57.3, 21.1.

2,6-Dimethyl-9-[(4-methylphenyl)sulfonylamino]fluorene (12n)

White solid, mp 211–212 °C; IR (KBr) ν 3307, 1595, 1436, 1328, 1157 cm⁻¹; ¹H NMR (400 MHz, DMSO-d₆) δ 8.33 (d, J = 7.8 Hz, 1H), 7.89 (d, J = 7.8 Hz, 2H), 7.59 (d, J = 7.3 Hz, 1H), 7.52–7.49 (m, 2H), 7.15 (d, J = 7.4 Hz, 1H), 7.00 (d, J = 7.8 Hz, 1H), 6.89 (d, J = 7.3 Hz, 1H), 6.58 (s, 1H), 5.22 (d, J = 7.8 Hz, 1H), 2.47 (s, 3H), 2.34 (s, 3H), 2.21 (s, 3H); ¹³C NMR (100 MHz, DMSO-d₆) δ 144.5, 142.9, 140.9, 139.8, 139.6, 137.9, 137.0, 136.8, 129.8 (2C), 129.0, 127.8, 126.7 (2C), 125.5, 124.5, 120.2, 119.7, 57.4, 21.1 (2C), 21.0. HRMS (ESI-TOF) (m/z): calcd for C₂₂H₂₁NO₂S, [M + Na]⁺ 386.1185; found 386.1187.

2-Methyl-6-chloro-2-9-[(4-methylphenyl)sulfonylamino]-fluorene (12o)

White solid, mp 207–208 °C; IR (KBr) ν 3269, 1569, 1430, 1333, 1161 cm⁻¹; ¹H NMR (400 MHz, DMSO-d₆) δ 8.28 (d, J = 8.7 Hz, 1H), 7.77 (d, J = 8.2 Hz, 2H), 7.66 (d, J = 1.8 Hz, 1H), 7.54 (d, J = 7.8 Hz, 1H), 7.37 (d, J = 8.2 Hz, 2H), 7.12–7.09 (m, 1H), 7.02 (d, J = 7.8 Hz, 1H), 6.91 (d, J = 8.0 Hz, 1H), 6.39 (s, 1H), 5.08 (d, J = 8.2 Hz, 1H), 2.40 (s, 3H), 2.12 (s, 3H); ¹³C NMR (100 MHz, DMSO-d₆) δ 144.5, 143.0, 142.5, 141.8, 139.4, 137.7, 135.8, 133.5, 129.9 (2C), 129.3, 126.7 (2C), 126.6, 126.2, 125.5, 120.4, 119.9, 57.2, 21.1, 21.0. HRMS (ESI-TOF) (m/z): calcd for C₂₁H₁₈ClNO₂S, [M + Na]⁺ 406.0639; found 406.0634.

3-Chloro-6-methyl-9-[(4-methylphenyl)sulfonylamino]-fluorene (12p)

White solid, mp 197–198 °C; IR (KBr) ν 3290, 1579, 1424, 1338, 1163 cm⁻¹; ¹H NMR (400 MHz, DMSO-d₆) δ 8.42 (d, J = 8.3 Hz, 1H), 7.88 (d, J = 7.8 Hz, 2H), 7.72 (s, 1H), 7.49 (d, J = 8.4 Hz, 2H), 7.32–7.31 (m, 1H), 7.19–7.14 (m, 2H), 7.10 (d, J = 8.2 Hz, 1H), 6.87 (d, J = 6.4 Hz, 1H), 5.29 (d, J = 8.7 Hz, 1H), 2.58 (s, 3H), 2.46 (s, 3H); ¹³C NMR (100 MHz, DMSO-d₆) δ 144.8, 143.1, 142.9, 142.1, 139.3, 136.1, 133.2, 133.1, 130.8, 129.8 (2C), 128.0, 126.7 (3C), 126.1, 122.5, 122.4, 57.2, 21.1, 20.2. HRMS (ESI-TOF) (m/z): calcd for C₂₁H₁₈ClNO₂S, [M + Na]⁺ 406.0639; found 406.0642.

9-(Phenyl-sulfonylamino)fluorene (12q)¹⁵

White solid, mp 203–204 °C; ¹H NMR (400 MHz, DMSO-d₆) δ 8.55 (d, J = 7.8 Hz, 1H), 8.07 (d, J = 7.8 Hz, 2H), 7.80–7.70 (m, 4H), 7.40–7.36 (m, 2H), 7.25–7.21 (m, 2H), 7.03 (d, J = 7.3 Hz, 2H), 5.40 (d, J = 8.2 Hz, 1H); ¹³C NMR (400 MHz, DMSO-d₆) δ 143.9 (2C), 142.2, 139.5 (2C), 132.7, 129.5 (2C), 128.6 (2C), 127.6 (2C), 126.7 (2C), 124.8 (2C), 120.1 (2C), 57.8.

2-Methyl-9-(phenyl-sulfonylamino)fluorene (12r)

White solid, mp 191–192 °C (lit.^9b mp 200–201 °C); ¹H NMR (400 MHz, DMSO-d₆) δ 8.48 (d, J = 8.7 Hz, 1H), 8.04 (d, J = 7.3 Hz, 2H), 7.79–7.69 (m, 4H), 7.39 (t, J = 7.8 Hz, 1H), 7.23 (t, J = 7.4 Hz, 1H), 7.17–7.10 (m, 2H), 7.04 (d, J = 7.8 Hz, 1H), 6.87 (d, J = 6.9 Hz, 1H), 5.32 (d, J = 8.0 Hz, 1H), 2.58 (s, 3H); ¹³C NMR (100 MHz, DMSO-d₆) δ 144.3, 144.2, 142.3, 140.2, 137.3, 132.6 (2C), 130.7, 129.5 (2C), 128.6, 127.2, 126.9, 126.6 (2C), 124.7, 122.9, 122.2, 57.6, 20.4.

9-[(4-Chlorophenyl)sulfonylamino]fluorene (12s)¹⁵

White solid, mp 217–219 °C. ¹H NMR (400 MHz, DMSO-d₆) δ 8.66 (d, J = 8.3 Hz, 1H), 8.04 (d, J = 8.7 Hz, 2H), 7.81–7.77 (m, 3H), 7.39 (t, J = 7.4 Hz, 2H), 7.26 (t, J = 7.3 Hz, 2H), 7.10 (d, J = 7.3 Hz, 2H), 5.42 (d, J = 8.2 Hz, 1H); ¹³C NMR (400 MHz, DMSO-d₆) δ 143.7 (2C), 141.1, 139.5 (2C), 137.4, 129.6 (2C), 128.7 (2C), 128.6 (2C), 127.6 (2C), 124.8 (2C), 120.2 (2C), 57.8.

2-Methyl-9-[(4-chlorophenyl)sulfonylamino]fluorene (12t)

White solid, mp 204–206 °C; IR (KBr) ν 3279, 1577, 1403, 1329, 1164 cm⁻¹; ¹H NMR (400 MHz, DMSO-d₆) δ 8.60 (d, J = 8.2 Hz, 1H), 8.01 (d, J = 8.7 Hz, 2H), 7.81–7.76 (m, 3H), 7.40 (t, J = 7.3 Hz, 1H), 7.26 (t, J = 7.3 Hz, 1H), 7.19–7.10 (m, 2H), 6.93 (d, J = 6.9 Hz, 1H), 5.36 (d, J = 8.2 Hz, 1H), 2.60 (s, 3H); ¹³C NMR (100 MHz, DMSO-d₆) δ 144.5, 144.4, 141.6, 140.7, 137.8, 137.7, 133.1, 131.2, 129.9 (2C), 129.1, 129.0 (2C), 127.7, 127.4, 125.1, 123.4, 122.7, 58.0, 20.8. HRMS (ESI-TOF) (m/z): calcd for C₂₀H₁₆ClNO₂S, [M + Na]⁺ 392.0482; found 392.0480.

A typical procedure for preparation of 9-mesitylfluorene (1a)

To a stirred suspension of 2-formyl biphenyl (5a, 91 mg, 0.5 mmol) and FeCl₃ (8 mg, 10 mol%) in mesitylene (2 mL) was added TsNCO (118 mg, 0.6 mmol). After the resultant mixture was heated at 80 °C for 1 h, it was cooled down to room temperature. It was then subjected to pass a flash chromatography [silica gel, 1% EtOAc in petroleum ether (60–90 °C)] to give 139 mg (98%) of product 1a as a colorless oil;^{7a 1}H NMR (400 MHz, CDCl₃) δ 7.82 (d, J = 7.4 Hz, 2H), 7.40–7.37 (m, 2H), 7.26–7.20 (m, 4H), 7.01 (s, 1H), 6.64 (s, 1H), 5.48 (s, 1H), 2.67 (s, 3H), 2.27 (s, 3H), 1.08 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 147.2 (2C), 140.9 (2C), 137.8, 137.7, 136.2, 133.8, 130.5, 128.8, 127.1 (2C), 126.8 (2C), 124.1 (2C), 120.0 (2C), 49.7, 21.7, 20.8, 18.6.

The products 1b–1t were prepared by the similar procedure.

2-Methyl-9-mesitylfluorene (1b)^7a

White solid, mp 110–111 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.96 (d, J = 7.8 Hz, 1H), 7.40–7.36 (m, 1H), 7.23–7.22 (m, 2H), 7.14–7.13 (m, 2H), 7.06–7.04 (m, 1H), 7.00 (s, 1H), 6.64 (s, 1H), 5.45 (s, 1H), 2.76 (s, 3H), 2.65 (s, 3H), 2.26 (s, 3H), 1.07 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 147.6, 147.5, 141.8, 138.9, 137.8, 137.7, 136.1, 134.2, 133.1, 130.5, 129.0, 128.7, 126.8, 126.7, 126.4, 124.0, 123.1, 121.6, 49.7, 21.6, 21.1, 20.8, 18.7.

4-Methyl-9-mesitylfluorene (1c)^7a

White solid, mp 109–110 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.95 (d, J = 7.8 Hz, 1H), 7.39–7.35 (m, 1H), 7.22 (d, J = 4.4 Hz, 2H), 7.14–7.12 (m, 2H), 7.06–7.03 (m, 1H), 6.99 (s, 1H), 6.63 (s, 1H), 5.44 (s, 1H), 2.75 (s, 3H), 2.64 (s, 3H), 2.25 (s, 3H), 1.07 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 147.6, 147.5, 141.8, 138.9, 137.8, 137.6, 136.1, 134.2, 133.1, 130.5, 129.0, 128.7, 126.8, 126.7, 126.4, 124.0, 123.1, 121.5, 49.7, 21.7, 21.1, 20.8, 18.7.

2-Isopropyl-9-mesitylfluorene (1d)¹⁶

Colorless oil; ¹H NMR (400 MHz, CDCl₃) δ 7.78–7.71 (m, 2H), 7.44–7.34 (m, 2H), 7.26–7.18 (m, 3H), 7.00 (s, 1H), 6.64 (s, 1H), 5.45 (s, 1H), 3.35–3.26 (m, 1H), 2.65 (s, 3H), 2.27 (s, 3H), 1.23 (d, J = 6.4 Hz, 6H), 1.09 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 147.9, 147.1, 140.3, 139.8, 137.7, 137.6, 136.2, 134.1, 133.4, 130.9, 130.5, 128.9, 128.0, 127.2, 126.9, 124.1, 120.2, 119.9, 49.6, 38.6, 23.0, 22.9, 21.7, 20.8, 18.6.

2-Methoxy-9-mesitylfluorene (1e)^7a

White solid, mp 110–111 °C; ¹H NMR (400 MHz, CDCl₃) δ 8.10 (d, J = 7.8 Hz, 1H), 7.87–7.85 (m, 3H), 7.42–7.39 (m, 1H), 7.30 (t, J = 7.2 Hz, 1H), 7.24–7.22 (m, 1H), 7.03 (s, 1H), 6.64 (s, 1H), 5.50 (s, 1H), 3.86 (s, 3H), 2.68 (s, 3H), 2.27 (s, 3H), 1.05 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 167.3, 148.4, 147.2, 145.4, 139.7, 137.7, 137.6, 136.4, 133.0, 130.6, 128.9, 128.8, 128.4, 127.0, 125.3, 124.3, 120.8, 119.7, 52.0, 49.7, 21.7, 20.8, 18.6.

2-(Trifluoromethoxy)-9-mesitylfluorene (1f)

Colorless oil; IR (KBr) ν 3750, 2921, 1455, 1257 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ 7.79 (d, J = 7.8 Hz, 2H), 7.39–7.36 (m, 1H), 7.27–7.19 (m, 3H), 7.06 (s, 1H), 7.01 (s, 1H), 6.66 (s, 1H), 5.46 (s, 1H), 2.65 (s, 3H), 2.27 (s, 3H), 1.08 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 149.1, 148.7, 147.4, 139.6, 137.7, 136.6, 132.8, 130.6, 129.0, 127.5 (2C), 127.1 (2C), 124.2, 120.7, 120.1 (2C), 119.8, 117.3, 49.8, 21.7, 20.8, 18.6. HRMS (ESI-TOF) (m/z): calcd for C₂₃H₁₉FO, [M − H]⁻ 367.1309; found 367.1310.

2-Fluoro-9-mesitylfluorene (1g)

Colorless oil; IR (KBr) ν 3662, 3621, 3074, 1460 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ 7.74–7.69 (m, 2H), 7.36–7.32 (m, 1H), 7.20–7.16 (m, 2H), 7.07–7.02 (m, 1H), 7.00 (s, 1H), 6.91–6.89 (m, 1H), 6.65 (s, 1H), 5.43 (s, 1H), 2.63 (s, 3H), 2.26 (s, 3H), 1.09 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 162.7 (d, J = 244.1 Hz), 149.4 (d, J = 7.6 Hz), 147.0, 140.0, 137.6 (d, J = 12.4 Hz, 2C), 136.8, 136.4, 133.2, 130.6, 128.9, 126.8 (d, J = 11.5 Hz, 2C), 124.1, 120.9 (d, J = 8.6 Hz), 119.6, 113.9 (d, J = 22.9 Hz), 111.4 (d, J = 22.9 Hz), 49.8, 21.6, 20.8, 18.6. HRMS (ESI-TOF) (m/z): calcd for C₂₂H₁₉F, [M + H]⁺ 303.1544; found 303.1542.

2-Chloro-9-mesitylfluorene (1h)^7a

Colorless oil; ¹H NMR (400 MHz, CDCl₃) δ 7.76 (d, J = 7.8 Hz, 1H), 7.70 (d, J = 8.2 Hz, 1H), 7.38–7.32 (m, 2H), 7.26–7.18 (m, 3H), 7.01 (s, 1H), 6.65 (s, 1H), 5.43 (s, 1H), 2.63 (s, 3H), 2.27 (s, 3H), 1.09 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 148.9, 147.0, 139.8, 139.4, 137.7, 137.6, 136.5, 133.0, 132.8, 130.6, 128.9, 127.4, 127.1, 127.0, 124.4, 124.1, 120.9, 120.0, 49.6, 21.7, 20.8, 18.7.

2-(Trifluoromethyl)-9-mesitylfluorene (1i)^7a

White solid, mp 104–105 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.89–7.84 (m, 2H), 7.64 (d, J = 8.4 Hz, 1H), 7.46 (s, 1H), 7.40 (t, J = 7.6 Hz, 1H), 7.30 (t, J = 7.2 Hz, 1H), 7.23 (d, J = 7.6 Hz, 1H), 7.03 (s, 1H), 6.66 (s, 1H), 5.49 (s, 1H), 2.66 (s, 3H), 2.27 (s, 3H), 1.05 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 147.9, 147.6, 144.2, 139.4, 137.7, 137.6, 136.7, 132.7, 130.7, 129.1 (q, J = 31.5 Hz), 129.0, 128.4, 127.1, 124.5 (q, J = 270.3 Hz), 124.3, 124.2 (q, J = 4.3 Hz), 121.0 (q, J = 3.6 Hz), 120.7, 120.1, 49.8, 21.7, 20.8, 18.7.

3-Methoxy-9-mesitylfluorene (1j)^7a

White solid, 105–106 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.70–7.66 (m, 2H), 7.33–7.29 (m, 1H), 7.14–7.12 (m, 2H), 6.99 (s, 1H), 6.92–6.89 (m, 1H), 6.75 (s, 1H), 6.64 (s, 1H), 5.42 (s, 1H), 3.73 (s, 3H), 2.63 (s, 3H), 2.25 (s, 3H), 1.13 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 167.8, 159.7, 149.1, 146.6, 140.8, 137.9, 137.5, 136.1, 133.9, 130.5, 128.7, 126.7, 126.0, 123.9, 120.7, 119.1, 113.0, 109.5, 55.4, 49.8, 21.7, 20.8, 18.7.

2,6-Dimethyl-9-mesitylfluorene (1k)

White solid, mp 107–108 °C; IR (KBr) ν 3004, 2916, 2859, 2732, 1448 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ 7.67 (d, J = 7.8 Hz, 1H), 7.59 (s, 1H), 7.17 (d, J = 7.4 Hz, 1H), 7.06 (d, J = 7.8 Hz, 1H), 7.03–7.01 (m, 3H), 6.65 (s, 1H), 5.40 (s, 1H), 2.65 (s, 3H), 2.44 (s, 3H), 2.33 (s, 3H), 2.27 (s, 3H), 1.11 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 147.8, 144.1, 141.1, 138.3, 137.9, 137.6, 136.9, 136.3, 136.0, 134.3, 130.4, 128.7, 127.6 (2C), 124.7, 123.7, 120.2, 119.6, 49.2, 21.7, 21.6 (2C), 20.8, 18.7. HRMS (ESI-TOF) (m/z): calcd for C₂₄H₂₄, [M + Na]⁺ 335.1770; found 335.1774.

3,5-Dimethyl-9-mesitylfluorene (1l)

Colorless oil; IR (KBr) ν 3002, 2914, 2859, 1447 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ 7.77 (s, 1H), 7.13–7.09 (m, 3H), 7.06–7.02 (m, 2H), 6.99 (s, 1H), 6.63 (s, 1H), 5.40 (s, 1H), 2.76 (s, 3H), 2.64 (s, 3H), 2.47 (s, 3H), 2.26 (s, 3H), 1.08 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 148.0, 144.7, 142.0, 138.9, 137.8, 137.6, 136.1, 136.0, 134.4, 133.0, 130.4, 129.0, 128.7, 127.3, 126.6, 123.9, 123.6, 121.6, 49.3, 21.8, 21.7, 21.1, 20.8, 18.7. HRMS (ESI-TOF) (m/z): calcd for C₂₄H₂₄, [M + Na]⁺ 335.1770; found 335.1774.

3-Methyl-9-mesitylfluorene (1m)

Colorless oil; IR (KBr) ν 3697, 3012, 2858, 1444 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ 7.76 (d, J = 7.8 Hz, 1H), 7.60 (s, 1H), 7.34–7.30 (m, 1H), 7.19–7.17 (m, 2H), 7.09–7.01 (m, 2H), 6.98 (s, 1H), 6.62 (s, 1H), 5.42 (s, 1H), 2.63 (s, 3H), 2.42 (s, 3H), 2.24 (s, 3H), 1.09 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 147.5, 144.3, 141.0, 140.9, 137.8, 137.6, 136.3, 136.0, 134.0, 130.5, 128.7, 128.1, 127.0, 126.7, 124.1, 123.8, 120.5, 119.8, 49.3, 21.6, 21.7, 20.8, 18.6. HRMS (ESI-TOF) (m/z): calcd for C₂₃H₂₂, [M − H]⁻ 297.1649; found 297.1642.

2-Methyl-6-chloro-9-mesitylfluorene (1n)

Colorless oil; IR (KBr) ν 3516, 3450, 2914, 1445 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ 7.69 (s, 1H), 7.61 (d, J = 7.8 Hz, 1H), 7.17–7.12 (m, 2H), 7.07–7.04 (m, 1H), 7.0 (s, 2H), 6.64 (s, 1H), 5.36 (s, 1H), 2.63 (s, 3H), 2.31 (s, 3H), 2.25 (s, 3H), 1.09 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 147.8, 145.2, 142.7, 137.9, 137.8, 137.6, 137.1, 136.3, 133.4, 132.7, 130.5, 128.8, 127.9, 126.5, 125.0, 124.8, 120.0, 119.9, 49.1, 21.7, 21.6, 20.8, 18.7. HRMS (ESI-TOF) (m/z): calcd for C₂₃H₂₁Cl, [M − H]⁻ 331.1253; found 331.1254.

3-Chloro-5-methyl-9-mesitylfluorene (1o)

Colorless oil; IR (KBr) ν 3521, 2961, 2915, 1445 cm⁻¹; ¹H NMR (300 MHz, CDCl₃) δ 7.90 (s, 1H), 7.20–7.09 (m, 4H), 7.04–7.01 (m, 1H), 6.99 (s, 1H), 6.63 (s, 1H), 5.37 (s, 1H), 2.72 (s, 3H), 2.62 (s, 3H), 2.25 (s, 3H), 1.06 (s, 3H); ¹³C NMR (75 MHz, CDCl₃) δ 148.1, 145.8, 143.4, 137.8, 137.7, 137.6, 136.3, 133.5, 133.4, 132.5, 130.5, 129.2, 128.8, 127.5, 126.3, 124.8, 123.2, 121.7, 49.3, 21.7, 21.0, 20.8, 18.7. HRMS (ESI-TOF) (m/z): calcd for C₂₂H₁₉F, [M − H]⁻ 331.1253; found 331.1254.

3-Chloro-9-mesitylfluorene (1p)

White solid, 91–92 °C; IR (KBr) ν 3494, 3449, 2912, 1610, 1442 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ 7.78–7.76 (m, 2H), 7.38 (t, J = 7.8 Hz, 1H), 7.28–7.24 (m, 1H), 7.21–7.18 (m, 2H), 7.11 (d, J = 7.8 Hz, 1H), 7.00 (s, 1H), 6.64 (s, 1H), 5.42 (s, 1H), 2.64 (s, 3H), 2.26 (s, 3H), 1.07 (s, 3H); ¹³C NMR (75 MHz, CDCl₃) δ 147.6, 145.4, 142.6, 139.7, 137.7, 137.6, 136.4, 133.2, 132.8, 130.6, 128.9, 127.8, 127.0 (2C), 125.1, 124.2, 120.2 (2C), 49.3, 21.6, 20.8, 18.6. HRMS (ESI-TOF) (m/z): calcd for C₂₂H₁₉Cl, [M − H]⁻ 317.1103; found 317.1106.

9-(2,3,5,6-Tetramethylphenyl)fluorene (1q)

White solid, mp 120–122 °C; IR (KBr) ν 3507, 3449, 2914, 1443 cm⁻¹; ¹H NMR (300 MHz, CDCl₃) δ 7.82 (d, J = 6.0 Hz, 2H), 7.37–7.33 (m, 2H), 7.24–7.20 (m, 4H), 6.92 (s, 1H), 5.56 (s, 1H), 2.55 (s, 3H), 2.37 (s, 3H), 2.01 (s, 3H), 0.99 (s, 3H); ¹³C NMR (75 MHz, CDCl₃) δ 147.9 (2C), 140.5 (2C), 136.6, 134.6, 134.0, 133.9, 133.3, 130.9, 127.1 (2C), 126.6 (2C), 124.0 (2C), 120.1 (2C), 50.6, 21.3, 20.2, 16.9, 14.9. HRMS (ESI-TOF) (m/z): calcd for C₂₃H₂₂, [M − H]⁻297.1649; found 297.1644.

9-(2,3,4,5,6-Pentamethylphenyl)fluorene (1r)¹⁷

White solid, mp 120–122 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.84 (d, J = 7.6 Hz, 2H), 7.39–7.35 (m, 2H), 7.25–7.20 (m, 4H), 5.57 (s, 1H), 2.62 (s, 3H), 2.37 (s, 3H), 2.26 (s, 3H), 2.01 (s, 3H), 1.04 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 148.3 (2C), 140.4 (2C), 134.0, 133.8, 133.7, 133.5, 132.2, 127.0 (2C), 126.5 (2C), 123.9 (2C), 120.1 (3C), 51.0, 17.9, 17.6, 17.1, 16.4, 16.3.

9-(2,4-Dimethoxyphenyl)fluorene (1s)

White solid, mp 143–144 °C; IR (KBr) ν 3426, 2945, 2835, 1610, 1448, 1328 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ 7.77 (d, J = 7.8 Hz, 2H), 7.36–7.32 (m, 4H), 7.25–7.21 (m, 2H), 6.57 (s, 1H), 6.47 (s, 1H), 6.24 (d, J = 6.4 Hz, 1H), 5.58 (s, 1H), 3.92 (s, 3H), 3.74 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 159.3, 158.7, 148.3 (2C), 141.1 (2C), 128.6, 127.1 (2C), 126.9 (2C), 125.2 (2C), 122.4, 119.7 (2C), 104.4 (2C), 98.7, 55.6, 55.3. HRMS (ESI-TOF) (m/z): calcd for C₂₁H₁₈O₂, [M + H]⁺ 303.1380; found 303.1384.

9-(2,4,6-Trimethoxyphenyl)fluorene (1t)^7a

White solid, mp 116–117 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.78 (d, J = 8.0 Hz, 2H), 7.31–7.28 (m, 2H), 7.22–7.15 (m, 4H), 6.29 (d, J = 2.4 Hz, 1H), 5.92 (d, J = 2.4 Hz, 1H), 5.60 (s, 1H), 3.94 (s, 3H), 3.76 (s, 3H), 2.94 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 160.1, 159.7, 159.6, 148.7 (2C), 141.0 (2C), 126.4 (2C), 126.0 (2C), 123.6 (2C), 119.3 (2C), 110.6, 92.5, 91.0, 56.2, 55.7, 55.2, 43.6.

Acknowledgements

This work was supported by NNSFC (No. 21372142 and 21472107).

Notes and references

1. W.-Y. Wong, Coord. Chem. Rev., 2005, 249, 971–997.
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Footnote

† Electronic supplementary information (ESI) available: ¹H and ¹³C NMR spectra for products 12a–12t and 1a–1t. CIF file for the single crystal X-ray diffraction analysis of 1s (CCDC 1452989). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra03889a

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