Acceptorless dehydrogenative condensation of o-aminobenzamides with aldehydes to quinazolinones in water catalyzed by a water-soluble iridium complex [Cp*Ir(H₂O)₃][OTf]₂

Abstract

A general and efficient method for the synthesis of quinazolinones via acceptorless dehydrogenative condensation of o-aminobenzamides with aldehydes in water has been accomplished. In the presence of [Cp*Ir(H₂O)₃][OTf]₂, a variety of desirable products were obtained in high yields with high atom economy under environmentally benign conditions. Notably, this research will facilitate the progress of acceptorless dehydrogenative reactions in water catalyzed by water-soluble organometallic complexes.

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Introduction

Quinazolinones represent a class of privileged scaffolds that occur in approximately 150 naturally occurring alkaloids, such as rutaecarpine, luotonin A, luotonin F, sildenafil, bouchardatine and raltitrexed (Scheme 1).¹ They also exhibit a wide range of biological and pharmacological activities, including antibacterial,² antifungal,³ antiviral,⁴ antiinflammatory,⁵ anticonvulsant,⁶ antimalarial⁷ and anticancer properties.⁸ Although numerous methods have been developed,⁹ the most classical and general protocols for the synthesis of quinazolinones are still through the condensation between o-aminobenzamides and aldehydes followed by the oxidation of the resulting aminal intermediates (Scheme 2).¹⁰ However, these procedures suffer from the use of stoichiometric or excess amounts of toxic and/or hazardous oxidants, such as KMnO₄,^10a MnO₂,^10b CuCl,^10c DDQ,^10d I₂,^10et-BuOOH^10f and PhI(OAc)₂,^10g and the generation of a large amount of harmful byproducts. In 2012, Mulakayala, Oruganti and co-workers reported the synthesis of quinazolinones from o-aminobenzamides with aldehydes in the presence of InCl₃ (10 mol%) without an additional oxidant and oxygen gas may work as the oxidant.¹¹ In addition, the above procedures were generally performed in organic solvents, which might cause environmental pollution.

Scheme 1 Selected examples of quinazolinones as scaffolds of naturally occurring alkaloids.

Scheme 2 Classical and general methods for the synthesis of quinazolinones.

In recent years, transition-metal-catalyzed acceptorless dehydrogenative oxidation with the liberation of hydrogen gas has attracted much attention because it represents a clean and atom economical strategy instead of traditional oxidation reactions.¹² The liberated hydrogen gas is also regarded as one of the most promising energies of the future.¹³ Significant advances include dehydrogenative oxidation of alcohols,¹⁴ nitrogen-containing heterocycles,¹⁵ amines to nitriles,¹⁶ C–C single bonds adjacent to functional groups to form α,β-unsaturated compounds.¹⁷ On the other hand, the development of organic synthesis in water has become increasingly important because water is a cheap, safe and environmentally benign solvent compared with organic solvents.¹⁸ Despite these advances, acceptorless dehydrogenative oxidation in water catalyzed by water-soluble organometallic complexes remains less explored. Recently, Fujita, Yamaguchi and co-workers demonstrated the dehydrogenative oxidation of alcohols to carbonyl compounds and dehydrogenative lactonization of diols in water catalyzed by a water-soluble bifunctional iridium complex [Cp*Ir(6,6′-(OH)₂bpy)(H₂O)][OTf]₂ (Cp* = η⁵-pentamethylcyclopentadienyl, bpy = 2,2′-bipyridine).¹⁹ From both synthetic and environmental points of view, the development of an efficient homogeneous catalytic system for the acceptorless dehydrogenative oxidation of nitrogen-containing heterocycles in water is apparently highly desirable.

We have reported a series of iridium-catalyzed C–N and C–C bond-forming reactions based on the activation of alcohols as electrophiles.²⁰ In 2014, we demonstrated the rearrangement of aldoximes to amides^21a and the N-alkylation of sulfonamides with alcohols to N-alkylated sulfonamides^21b in water catalyzed by water-soluble iridium complexes. As part of our continuing interest in the development of iridium-catalyzed reactions in water, we herein wish to report the acceptorless dehydrogenative condensation of o-aminobenzamides with aldehydes to quinazolinones in water catalyzed by a water-soluble iridium complex.

Results and discussion

Our initial investigation focused on the reaction of o-aminobenzamide (1a) with benzaldehyde (2a) in water. As shown in Table 1, several different types of water-soluble Cp*Ir complexes were assayed for their catalytic ability towards this model reaction. In a typical procedure, the reaction of 1a and 2a was refluxed in water for 2 h and the refluxing of the reaction mixture was continued for 1 h after a catalyst (1 mol%) was added. In the presence of a cationic iridium complex [Cp*Ir(bpy)Cl]Cl (bpy = 2,2′-bipyridine) (Cat. 1), the reaction afforded the desired product 3aa in 83% yield (Table 1, entry 1). When Cp*Ir complexes bearing more aqua ligands [Cp*Ir(H₂O)₃][OTf]₂ (Cat. 2), [Cp*Ir(H₂O)₃][BF₄]₂ (Cat. 3) and [Cp*Ir(H₂O)₃][SO₄] (Cat. 4) were used as the catalyst, the product 3aa was obtained in 80–89% yields (Table 1, entries 2–4). Using [Cp*Ir(bpy)(H₂O)][OTf]₂ (Cat. 5) as an alternative catalyst, this reaction gave the product 3aa in only 30% yield (Table 1, entry 5). The Cp*Ir complex bearing three ammonia ligands [Cp*Ir(NH₃)₃][Cl]₂ (Cat. 6) was also screened, and the desired product 3aa could be obtained in 85% yield (Table 1, entry 6). Apparently, apart from Cat. 5, other tested catalysts exhibited high catalytic activities. Among them, [Cp*Ir(H₂O)₃][OTf]₂ is the most effective for this transformation.

Table 1 Dehydrogenative condensation of o-aminobenzamide (1a) with benzaldehyde (2a) under various conditions^a

Entry	Catalyst	Yield^b (%)
a Reaction conditions: (1) 1a (0.5 mmol), 2a (0.5 mmol), H2O (1 mL), reflux, 2 h; (2) catalyst (1 mol%), reflux, 1 h. b Isolated yield.
1	Cat. 1	83
2	Cat. 2	89
3	Cat. 3	80
4	Cat. 4	82
5	Cat. 5	30
6	Cat. 6	85

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Having established the optimal reaction conditions (Table 1, entry 2), the scope of reaction with respect to aldehydes was investigated and the results are shown in Table 2. Reactions with benzaldehydes bearing one or two electron-donating groups, such as methyl (2b–c), dimethyl (2d), isopropyl (2e), methoxy (2f) and dimethoxy (2g), gave the corresponding products 3ab–3ag in 82–91% yields (Table 2, entries 1–6). Similarly, benzaldehydes bearing a halogen atom, such as fluoro (2h), chloro (2i–j) and bromo (2k), were converted to the desired products 3ah–3ak in 78–87% yield (Table 2, entries 7–10). Benzaldehydes bearing a strong electron-withdrawing group, such as trifluoromethyl (2l) and trifluoromethoxy (2m), were also proven to be suitable substrates and the desired products 3al and 3am could be obtained in 81% and 78% yields, respectively (Table 2, entries 11 and 12). Furthermore, high catalytic activities were found in transformations of 1-naphthaldehyde (2n), 2-naphthaldehyde (2o) and 2-thiophenaldehyde (2p) to the corresponding products 3an–3ap (Table 2, entries 13–15). In the case of aliphatic aldehydes, such as phenylpropyl aldehyde (2q), butyraldehyde (2r) and cyclohexanecarbaldehyde (2s), the desired products 3aq–3as could be obtained in 81–84% yields as well (Table 2, entries 16–18).

Table 2 Dehydrogenative condensation of o-aminobenzamide (1a) with various aldehydes (2) in water^a

Entry	Aldehyde	Product	Yield^b (%)
a Reaction conditions: (1) 1a (0.5 mmol), 2 (0.5 mmol), H2O (1 mL), reflux, 2 h; (2) Cat. 2 (1 mol%), reflux, 1 h. b Isolated yield.
1			87
2			85
3			86
4			82
5			91
6			86
7			78
8			82
9			87
10			78
11			81
12			78
13			85
14			81
15			77
16			84
17			81
18			81

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To further expand the scope of reaction, transformations with respect to o-aminobenzamides were then examined. Reactions of o-aminobenzamides bearing one or two electron-donating groups, such as methyl (1b) and dimethoxy (1c), gave the corresponding products 3ba and 3ca in 81% and 78% yields, respectively (Table 3, entries 1 and 2). When o-aminobenzamides bearing an electron-withdrawing group, such as fluoro (1d–f), chloro (1g–h) and bromo (1i), were used as substrates, the desired products 3da–3ia were obtained in 75%–85% yields (Table 3, entries 3–8). The catalytic system was also applied to o-aminobenzenesulfonamide (1j), giving the corresponding product 3ja in only 26% NMR yield (Table 3, entry 9).

Table 3 Dehydrogenative condensation of a series of o-aminobenzamides (1) with benzaldehyde (2a) in water^a

Entry	o-Aminobenzamides	Product	Yield^b (%)
a Reaction conditions: (1) 1 (0.5 mmol), 2a (0.5 mmol), H2O (1 mL), reflux, 2 h; (2) Cat. 2 (1 mol%), reflux, 1 h. b Isolated yield. c NMR yield.
1			81
2			78
3			84
4			81
5			75
6			84
7			85
8			75
9			26^c

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A possible mechanism was proposed to account for the acceptorless dehydrogenative condensation of o-aminobenzamides with aldehydes to quinazolinones in water (Scheme 3). The initial step of the reaction involved the formation of 2,3-dihydroquinazolinones via the condensation between o-aminobenzamides and aldehydes. Furthermore, the reaction of the resulting 2,3-dihydroquinazolinones with Cat. 2 afforded an amido iridium species A and/or B. Accompanied by the β-hydrogen elimination of species A and/or B, the hydrido iridium species C were generated and quinazolinones were released as products.²² Finally, hydrogen gas was liberated and catalytic active species A and/or B were regenerated through the reaction of iridium hydride species C and 2,3-dihydroquinazolinones.

Scheme 3 Proposed reaction mechanism.

To support the proposed mechanism, the confirmation of the liberation of hydrogen gas in the process of the acceptorless dehydrogenative oxidation of 2-phenyl-2,3-dihydroquinazolin-4(1H)-one (4), which is synthesized via the condensation between 1a and 2a, was first undertaken (Scheme 4). In the presence of Cat. 2 (1 mol%), the reaction was carried out for 2 h in water to give the product 3aa in 90% yield accompanied by the generation of 20.1 mL of gas by water displacement. The collected gas was confirmed to be hydrogen gas by GC analysis and the yield of gas is calculated to be 87%. In addition, a peak (δ −15.6) was observed in the ¹H NMR spectrum of the substoichiometric reaction of Cat. 2 with 4 (4 equiv.) in CDCl₃ at ambient temperature. It was speculated to be a characteristic signal of [Ir − H] (species C)²³ (see the ESI†). These experimental results supported the proposed mechanism shown in Scheme 3.

Scheme 4 Confirmation of the liberation of hydrogen gas.

Conclusion

We have demonstrated a general and efficient method for the synthesis of quinazolinones via the acceptorless dehydrogenative condensation of o-aminobenzamides with aldehydes in water. In the presence of [Cp*Ir(H₂O)₃][OTf]₂, a variety of desirable products were obtained in high yields with high atom economy under environmentally benign conditions. Notably, this research will facilitate the progress of acceptorless dehydrogenative reactions in water catalyzed by water-soluble organometallic complexes.

Experimental section

Experimental details

High-resolution mass spectra (HRMS) were obtained on a HPLC-Q-Tof MS(Micro) spectrometer and are reported as m/z (relative intensity). Accurate masses are reported for the deprotonated molecular ion [M − H]⁻. Proton nuclear magnetic resonance (¹H NMR) spectra were recorded at 500 MHz using a 500 spectrometer. Chemical shifts are reported in delta (δ) units, parts per million (ppm) downfield from tetramethylsilane or ppm relative to the center of the singlet at 7.26 ppm for CDCl₃ and 2.50 ppm for DMSO-d₆. Coupling constant J values are reported in hertz (Hz), and the splitting patterns were designated as follows: s, singlet; d, doublet; t, triplet; m, multiplet; b, broad. Carbon-13 nuclear magnetic resonance (¹³C NMR) spectra were recorded at 125 MHz using a 500 spectrometer. Chemical shifts are reported in delta (δ) units, ppm relative to the center of the triplet at 77.0 ppm for CDCl₃ and 39.5 ppm for DMSO-d₆. ¹³C NMR spectra were routinely run with broadband decoupling.

[Cp*Ir(bpy)Cl]Cl (Cat. 1),²⁴ [Cp*Ir(H₂O)₃][OTf]₂ (Cat. 2),²⁵ [Cp*Ir(H₂O)₃][BF₄]₂ (Cat. 3),²⁵ [Cp*Ir(H₂O)][SO₄] (Cat. 4),²⁵ [Cp*Ir(bpy)(H₂O)][OTf]₂ (Cat. 5)²⁶ and [Cp*Ir(NH₃)₃][Cl]₂ (Cat. 6)²⁷ were synthesized according to previous reports.

General procedure for acceptorless dehydrogenative condensation of o-aminobenzamides with aldehydes to quinazolinones in water catalyzed by a water-soluble iridium complex [Cp*Ir(H₂O)₃][OTf]₂

In a round-bottomed flask with a condenser tube, 2-aminobenzamide 1 (0.5 mmol), aldehyde 2 (0.5 mmol) and water (1 mL) were placed under an air atmosphere, and the reaction mixture was heated under reflux in an oil bath for 2 h. The reaction mixture was further heated under reflux for 1 h when Cat. 2 (0.005 mmol, 1 mol%) was added and the mixture was then cooled to ambient temperature, concentrated in vacuo and purified by flash column chromatography with hexanes/ethyl acetate to afford the corresponding products.

2-Phenylquinazolin-4(3H)-one (3aa)⁹ⁱ. White solid, 90% yield (100 mg); mp 237–238 °C; ¹H NMR (500 MHz, CDCl₃) δ 11.58 (br s, 1H), 8.34 (d, J = 7.8 Hz, 1H), 8.26 (dd, J = 3.1 Hz and 6.7 Hz, 2H), 7.80–7.85 (m, 2H), 7.59–7.60 (m, 3H), 7.51 (t, J = 7.3 Hz, 1H); ¹³C {¹H} NMR (125 MHz, CDCl₃) δ 163.9, 151.8, 149.5, 134.9, 132.8, 131.6, 129.0, 128.0, 127.4, 126.7, 126.3, 120.9.

2-(o-Tolyl)quinazolin-4(3H)-one (3ab)⁹ⁱ. White solid, 87% yield (102 mg); mp 222–224 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.44 (br s), 8.16 (d, J = 5.9 Hz), 7.84 (s, 1H), 7.69 (d, J = 6.2 Hz, 1H), 7.50–7.54 (m, 2H), 7.43 (s, 1H), 7.34 (s, 2H), 2.38 (s, 3H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 161.8, 154.3, 148.7, 136.1, 134.4, 134.2, 130.5, 129.8, 129.1, 127.3, 126.6, 125.8, 125.7, 121.0, 19.5.

2-(p-Tolyl)quinazolin-4(3H)-one (3ac)⁹ⁱ. White solid, 85% yield (100 mg); mp 242–244 °C; ¹H NMR (500 MHz, CDCl₃) δ 11.48 (br s, 1H), 8.33 (d, J = 7.8 Hz, 1H), 8.15 (d, J = 7.8 Hz, 2H), 7.78–7.83 (m, 2H), 7.49 (t, J = 7.0 Hz, 1H), 7.38 (d, J = 7.8 Hz, 2H), 2.46 (s, 3H); ¹³C {¹H} NMR (125 MHz, CDCl₃) δ 164.0, 151.8, 149.6, 142.1, 134.8, 130.0, 129.7, 127.9, 127.4, 126.5, 126.3, 120.8, 21.5.

2-(3,4-Dimethylphenyl)quinazolin-4(3H)-one (3ad)^9b. White solid, 86% yield (108 mg); mp 239–242 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.40 (br s, 1H), 8.14 (d, J = 7.5 Hz, 1H), 8.01 (s, 1H), 7.92 (d, J = 7.2 Hz, 1H), 7.82 (t, J = 6.9 Hz, 1H), 7.73 (d, J = 7.8 Hz, 1H), 7.50 (t, J = 7.0 Hz, 1H), 7.30 (d, J = 7.4 Hz, 1H), 2.31 (s, 3H), 2.30 (s, 3H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 162.2, 152.3, 148.8, 140.2, 136.6, 134.5, 130.1, 129.6, 128.6, 127.3, 126.3, 125.8, 125.1, 120.8, 19.4, 19.3.

2-(4-Isopropylphenyl)quinazolin-4(3H)-one (3ae)⁹ⁱ. White solid, 82% yield (108 mg); mp 210–212 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.48 (br s, 1H), 8.12–8.15 (m, 3H), 7.83 (t, J = 7.5 Hz, 1H), 7.73 (d, J = 7.7 Hz, 1H), 7.51 (t, J = 7.0 Hz, 1H), 7.42 (d, J = 7.3 Hz, 2H), 2.96–2.99 (m, 1H), 1.24 (d, J = 6.0 Hz, 6H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 162.3, 152.2, 152.1, 148.8, 134.5, 130.3, 127.8, 127.4, 126.5, 126.4, 125.8, 120.9, 33.4, 23.6.

2-(4-Methoxyphenyl)quinazolin-4(3H)-one (3af)^9d. White solid, 91% yield (115 mg); mp 246–248 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.41 (br s, 1H), 8.19 (d, J = 8.4 Hz, 2H), 8.13 (d, J = 7.6 Hz, 1H), 7.81 (t, J = 7.3 Hz, 1H), 7.70 (d, J = 8.0 Hz, 1H), 7.48 (t, J = 7.3 Hz, 1H), 7.09 (d, J = 8.3 Hz, 2H), 3.85 (s, 3H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 162.3, 161.8, 151.8, 148.9, 134.5, 129.4, 127.3, 126.1, 125.8, 124.8, 120.7, 113.9, 55.4.

2-(3,4-Dimethoxyphenyl)quinazolin-4(3H)-one (3ag)²⁸. White solid, 86% yield (121 mg); mp 242–243 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.44 (br s, 1H), 8.13 (d, J = 7.6 Hz, 1H), 7.87 (d, J = 8.1 Hz, 1H), 7.81–7.83 (m, 2H), 7.72 (d, J = 8.1 Hz, 1H), 7.49 (t, J = 7.4 Hz, 1H), 7.12 (d, J = 8.4 Hz, 1H), 3.88 (s, 3H), 3.85 (s, 3H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 162.3, 151.8, 151.6, 148.9, 148.5, 134.5, 127.3, 126.1, 125.8, 124.7, 121.1, 120.7, 111.3, 110.7, 55.7 (2C, overlap).

2-(2-Fluorophenyl)quinazolin-4(3H)-one (3ah)²⁹. White solid, 78% yield (94 mg); mp 162–164 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.58 (br s, 1H), 8.17 (d, J = 7.8 Hz, 1H), 7.86 (t, J = 7.4 Hz, 1H), 7.79 (t, J = 7.1 Hz, 1H), 7.73 (d, J = 8.1 Hz, 1H), 7.63 (q, J = 6.6 Hz, 1H), 7.57 (t, J = 7.5 Hz, 1H), 7.36–7.41 (m, 2H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 161.5, 159.6 (d, J_C–F = 249.1 Hz), 149.9, 148.7, 134.6, 132.8 (d, J_C–F = 8.3 Hz), 131.0, 127.5, 127.0, 125.8, 124.6, 122.3 (d, J_C–F = 12.7 Hz), 121.1, 116.1 (d, J_C–F = 21.0 Hz).

2-(2-Chlorophenyl)quinazolin-4(3H)-one (3ai)^9k. White solid, 82% yield (105 mg); mp 185–186 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.63 (br s, 1H), 8.18 (d, J = 7.4 Hz, 1H), 7.86 (t, J = 6.9 Hz, 1H), 7.71 (d, J = 7.8 Hz, 1H), 7.67 (d, J = 6.9 Hz, 1H), 7.62 (d, J = 7.6 Hz, 1H), 7.58 (t, J = 7.4 Hz, 2H), 7.50 (t, J = 7.4 Hz, 1H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 161.4, 152.2, 148.5, 134.5, 133.8, 131.6, 131.5, 130.8, 129.6, 127.4, 127.2, 127.0, 125.8, 121.2.

2-(4-Chlorophenyl)quinazolin-4(3H)-one (3aj)⁹ⁱ. White solid, 87% yield (112 mg); mp 300–302 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.59 (br s, 1H), 8.20 (d, J = 7.6 Hz, 2H), 8.15 (d, J = 7.1 Hz, 1H), 7.84 (t, J = 6.4 Hz, 1H), 7.74 (d, J = 7.7 Hz, 1H), 7.62 (d, J = 7.6 Hz, 2H), 7.55 (t, J = 6.6 Hz, 1H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 162.1, 151.3, 148.5, 136.3, 134.6, 131.5, 129.6, 128.6, 127.5, 126.7, 125.8, 121.0.

2-(4-Bromophenyl)quinazolin-4(3H)-one (3ak)³⁰. White solid, 78% yield (118 mg); mp 296–298 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.60 (s, br, 1H), 8.12–8.16 (m, 3H), 7.84 (t, J = 7.1 Hz, 1H), 7.74–7.77 (m, 3H), 7.53 (t, J = 7.1 Hz, 1H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 162.1, 151.5, 148.5, 134.6, 131.9, 131.6, 129.8, 127.5, 126.7, 125.8, 125.2, 121.0.

2-(4-(Trifluoromethyl)phenyl)quinazolin-4(3H)-one (3al)⁹ⁱ. White solid, 81% yield (117 mg); mp 308–310 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.75 (br s, 1H), 8.37 (d, J = 7.8 Hz, 2H), 8.18 (d, J = 7.5 Hz, 1H), 7.93 (d, J = 7.8 Hz, 2H), 7.87 (t, J = 7.4 Hz, 1H), 7.78 (d, J = 7.9 Hz, 1H), 7.57 (t, J = 7.5 Hz, 1H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 162.1, 151.1, 148.4, 136.6, 134.7, 131.1 (q, J_C–F = 31.9 Hz), 128.7, 127.6, 127.0, 125.8, 125.4 (q, J_C–F = 3.0 Hz), 123.9 (q, J_C–F = 270.9 Hz).

2-(4-(Trifluoromethoxy)phenyl)quinazolin-4(3H)-one (3am)^10f. White solid, 76% yield (120 mg); mp 268–271 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.65 (br s, 1H), 8.30 (d, J = 8.7 Hz, 2H), 8.16 (d, J = 7.8 Hz, 1H), 7.85 (t, J = 7.6 Hz, 1H), 7.75 (d, J = 8.1 Hz, 1H), 7.53–7.56 (m, 3H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 162.1, 151.1, 148.4, 136.6, 134.6, 131.1 (q, J_C–F = 31.9 Hz), 128.7, 127.6, 127.0, 125.8, 125.4, 123.9 (d, J_C–F = 255.8 Hz), 121.1.

2-(Naphthalen-1-yl)quinazolin-4(3H)-one (3an)^9d. White solid, 85% yield (102 mg); mp 287–288 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.67 (br s, 1H), 8.22 (d, J = 7.7 Hz, 1H), 8.17 (d, J = 7.6 Hz, 1H), 8.12 (d, J = 8.1 Hz, 1H), 8.05 (d, J = 7.3 Hz, 1H), 7.87 (t, J = 7.2 Hz, 1H), 7.80 (d, J = 6.7 Hz, 1H), 7.74 (d, J = 7.9 Hz, 1H), 7.65 (t, J = 7.6 Hz, 1H), 7.57–7.62 (m, 3H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 161.8, 153.6, 148.7, 134.5, 133.1, 131.7, 130.3, 130.2, 128.3, 127.6, 127.4, 127.0, 126.7, 126.3, 125.8, 125.2, 125.0, 121.2.

2-(Naphthalen-2-yl)quinazolin-4(3H)-one (3ao)⁹ⁱ. White solid, 81% yield (110 mg); mp 276–278 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.67 (br s, 1H), 8.82 (s, 1H), 8.31 (d, J = 8.4 Hz, 1H), 8.19 (d, J = 7.6 Hz, 1H), 8.07 (t, J = 7.1 Hz, 1H), 8.02 (d, J = 7.2 Hz, 1H), 7.86 (t, J = 7.2 Hz, 1H), 7.80 (d, J = 7.9 Hz, 1H), 7.61–7.67 (m, 2H), 7.55 (t, J = 7.2 Hz, 1H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 162.2, 152.2, 148.8, 134.59, 134.1, 132.3, 129.9, 128.9, 128.1, 128.1, 127.9, 127.6, 127.5, 126.9, 126.6, 125.9, 124.5, 121.0.

2-(Thiophen-2-yl)quinazolin-4(3H)-one (3ap)^9d. White solid, 77% yield (88 mg); mp 276–277 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.66 (br s, 1H), 8.82 (s, 1H), 8.12 (d, J = 7.1 Hz, 1H), 7.87 (d, J = 3.2 Hz, 1H), 7.80 (t, J = 7.3 Hz, 1H), 7.65 (d, J = 7.3 Hz, 1H), 7.49 (t, J = 7.2 Hz, 1H), 7.24 (s, 1H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 161.8, 148.6, 147.8, 137.4, 134.6, 132.1, 129.4, 128.5, 126.9, 126.3, 126.0, 120.9.

2-Phenethylquinazolin-4(3H)-one (3aq)³¹. White solid, 84% yield (105 mg); mp 209–211 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.26 (br s, 1H), 8.08 (d, J = 7.1 Hz, 1H), 7.78 (t, J = 6.9 Hz, 1H), 7.62 (d, J = 7.8 Hz, 1H), 7.46 (t, J = 7.1 Hz, 1H), 7.28 (s, 4H), 7.19 (s, 1H), 3.05 (t, J = 7.5 Hz, 2H), 2.89 (t, J = 7.6 Hz, 2H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 161.7, 156.5, 148.8, 140.7, 134.3, 128.3, 126.8, 126.0, 126.0, 125.7, 120.8, 36.3, 32.4.

2-Propylquinazolin-4(3H)-one (3ar)^9a. White solid, 81% yield (76 mg); mp 198–199 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.16 (br s, 1H), 8.07 (d, J = 7.8 Hz, 1H), 7.76 (t, J = 7.4 Hz, 1H), 7.59 (d, J = 8.1 Hz, 1H), 7.45 (t, J = 7.4 Hz, 1H), 2.57 (t, J = 7.5 Hz, 2H), 1.71–1.78 (m, 2H), 0.93 (t, J = 7.3 Hz, 3H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 161.8, 157.3, 148.9, 134.2, 126.8, 125.9, 125.6, 120.8, 36.3, 20.2, 13.4.

2-Cyclohexylquinazolin-4(3H)-one (3as)^9b. White solid, 81% yield (92 mg), mp 229–230 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.08 (br s, 1H), 8.07 (d, J = 7.9 Hz, 1H), 7.76 (t, J = 7.6 Hz, 1H), 7.59 (d, J = 8.2 Hz, 1H), 7.44 (t, J = 7.5 Hz, 1H), 2.57 (tt, J = 11.8 Hz and J = 3.3 Hz, 1H, CH), 1.78–1.91 (m, 4H), 1.54–1.69 (m, 3H), 1.18–1.34 (m, 3H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 161.9, 160.7, 148.9, 134.2, 126.9, 125.8, 125.6, 120.9, 42.8, 30.2, 25.5, 25.3.

7-Methyl-2-phenylquinazolin-4(3H)-one (3ba)³². White solid, 81% yield (96 mg), mp 239–240 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.45 (br s, 1H), 8.17 (d, J = 7.3 Hz, 2H), 8.04 (d, J = 8.0 Hz, 1H), 7.53–7.60 (m, 4H), 7.34 (d, J = 8.0 Hz, 1H), 2.47 (s, 3H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 162.1, 152.3, 148.8, 145.0, 132.8, 131.3, 128.5, 128.0, 127.7, 127.0, 125.7, 118.6, 21.3.

6,7-Dimethoxy-2-phenylquinazolin-4(3H)-one (3ca)³². White solid, 78% yield (111 mg); mp 281–284 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.40 (br s, 1H), 8.16 (d, J = 7.3 Hz, 2H), 7.48–7.55 (m, 4H), 7.21 (s, 1H), 3.93 (s, 3H), 3.89 (s, 3H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 161.5, 154.7, 150.8, 148.6, 144.8, 132.8, 131.0, 128.5, 127.4, 114.0, 108.2, 105.0, 55.9, 55.7.

5-Fluoro-2-phenylquinazolin-4(3H)-one (3da)³². ¹H NMR (500 MHz, DMSO-d₆) δ 12.55 (s, br, 1H), 8.17 (d, J = 6.8 Hz, 2H), 7.82–7.78 (m, 1H), 7.62–7.55 (m, 4H), 7.26 (d, J = 9.1 Hz, 1H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 161.6, 159.5 (d, J_C–F = 9.1 Hz), 153.3, 150.8, 135.1 (d, J_C–F = 9.1 Hz), 132.2, 131.6, 128.6, 127.8, 123.5, 112.9 (d, J_C–F = 20.2 Hz), 110.4.

6-Fluoro-2-phenylquinazolin-4(3H)-one (3ea)³³. Gray solid, 81% yield (97 mg); mp 277–278 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.69 (s, br, 1H), 8.25–8.23 (m, 2H), 7.76–7.69 (m, 2H), 7.58 (dt, J = 8.7 Hz and 2.8 Hz, 1H), 7.50–7.47 (m, 3H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 165.0, 159.2 (d, J_C–F = 241.9 Hz), 155.2, 146.7, 135.5, 130.3, 129.5 (d, J_C–F = 7.7 Hz), 128.2, 127.7, 122.3 (d, J_C–F = 7.5 Hz), 121.6, 121.4

7-Fluoro-2-phenylquinazolin-4(3H)-one (3fa)⁹ⁱ. Gray solid, 75% yield (113 mg); mp 252–253 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.60 (s, br, 1H), 8.21–8.14 (m, 3H), 8.15 (d, J = 7.30 Hz, 2H), 7.60 (t, J = 6.85 Hz, 1H), 7.55 (t, J = 7.13 Hz, 2H), 7.49 (d, J = 9.60 Hz, 1H), 7.37 (t, J = 8.03 Hz, 1H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 165.9 (d, J_C–F = 250.0 Hz), 161.7, 153.9, 151.0 (d, J_C–F = 12.5 Hz), 132.5, 131.7, 129.0 (d, J_C–F = 10.0 Hz), 128.7, 128.0, 118.0, 115.2 (d, J_C–F = 23.8 Hz), 112.4 (d, J_C–F = 21.2 Hz).

6-Chloro-2-phenylquinazolin-4(3H)-one (3ga)^9a. White solid, 84% yield (107 mg); mp 287–289 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.71 (br s, 1H), 8.17 (d, J = 7.1 Hz, 2H), 8.09 (s, 1H), 7.86 (d, J = 8.0 Hz, 1H), 7.76 (d, J = 8.6 Hz, 1H), 7.54–7.62 (m, 3H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 161.3, 152.8, 147.4, 134.6, 132.4, 131.5, 130.7, 129.7, 128.6, 127.8, 124.8, 122.2.

7-Chloro-2-phenylquinazolin-4(3H)-one (3ha)⁹ⁱ. White solid, 85% yield (109 mg); mp 276–288 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.67 (br s, 1H), 8.17 (d, J = 7.5 Hz, 2H), 8.14 (d, J = 8.5 Hz, 1H), 7.79 (m, 1H), 7.61 (t, J = 7.1 Hz, 1H), 7.54–7.57 (m, 3H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 161.3, 152.8, 147.4, 134.6, 132.4, 131.5, 130.7, 129.7, 128.6, 127.8, 124.8, 122.2.

5-Bromo-2-phenylquinazolin-4(3H)-one (3ia). White solid, 75% yield (113 mg); mp 283–284 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.56 (s, br, 1H), 8.18–8.16 (m, 2H), 7.73 (m, 1H), 7.71 (dd, J = 3.5 Hz and 1.2 Hz, 1H), 7.66–7.59 (m, 2H), 7.55 (tt, J = 7.5 Hz and 1.5 Hz, 2H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 160.5, 152.7, 151.1, 134.6, 132.6, 132.1, 131.6, 128.5, 127.8, 127.7, 120.1, 118.8. HRMS-EI (70 eV) m/z calcd for C₁₄H₈BrN₂O [M–H]⁻ 298.9820, found 298.9827.

3-Phenyl-2H-benzo[e][1,2,4]thiadiazine 1,1-dioxide (3ja)³⁴. White solid, 26% yield (NMR); mp > 300 °C; ¹H NMR (500 MHz, DMSO-d₆) δ 12.18 (s, br, 1H), 8.05 (d, J = 7.9 Hz, 2H), 7.86 (d, J = 8.0 Hz, 1H), 7.76–7.69 (m, 2H), 7.65–7.62 (m, 3H), 7.51 (t, J = 7.6 Hz, 1H); ¹³C {¹H} NMR (125 MHz, DMSO-d₆) δ 154.8, 135.5, 133.1, 132.8, 131.8, 128.8, 128.2, 126.7, 123.3, 121.5, 118.4.

Procedure for the hydrogen evolution experiment (Scheme 4)³⁵

4 (1 mmol), Cat. 2 (1 mol%) and H₂O (1 mL) were added into a 5 ml thick walled glass vessel fitted with a side arm and a rubber septum. The vessel was previously degassed three times and placed under a N₂ atmosphere. The vessel was connected to the gas collection apparatus (standard water displacement apparatus, using a graduated cylinder to determine volume), and the entire system was flushed with N₂ for 5 min and allowed to equilibrate for 5 min. The reaction was stirred vigorously at a constant temperature until gas evolution ceased (2 h). The presence of hydrogen in the collected gas was confirmed by GC analysis.

The GC analysis was performed on a gas chromatograph with a TCD detector. Injector temperature = 100 °C, column temperature = 50 °C, detector temperature (TCD) = 80 °C, carrier gas = N₂, column flow = 20 mL min⁻¹, t = 0.558 min.

The volume of 1 mol of H₂ at 283.15 K, 101 810 Pa was calculated according to the van der Waals equation as shown below

where R = 8.3145 m³ Pa mol⁻¹ K⁻¹; T = 283.15 K; p = 101 810 Pa; a = 0.002476 m⁶ Pa mol⁻¹; b = 0.02661 × 10⁻³ m³ mol⁻¹; thus, V (H₂, 283.15 K, 101 810 Pa) = 23.15 L mol⁻¹.

The collected volume of gas in the experiment above was 20.1 mL, which corresponds to 0.87 mmol of H₂.

Procedure for the substoichiometric reaction of [Cp*Ir(H₂O)₃][OTf]₂ with 2-phenyl-2,3-dihydroquinazolin-4(1H)-one

Under an atmosphere of nitrogen, Cat. 2 (6.8 mg, 0.01 mmol), 4 (8.9 mg, 0.04 mmol) and CDCl₃ (1.0 mL) were placed in a NMR tube. The NMR tube was put into the NMR probe. After 30 min, ¹H NMR analysis was performed. The result is shown in the ESI.†

Acknowledgements

Financial support by the National Natural Science Foundation of China (21272115) and the State Key Laboratory of Fine Chemicals (KF1401) is greatly appreciated.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Copies of the ¹H NMR and ¹³C NMR spectra for all products. See DOI: 10.1039/c5qo00255a

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