1, 2-Dihydro-1-arylnaphthalene derivatives are common structural units in many biologically active natural products and pharmaceuticals [1-6]. This skeleton contains an alkene C=C bond that can be easily derivatized; for example, podophyllotoxin with high anticancer activity can be constructed by using this skeleton as a precursor [7, 8]. Over the past decades, a variety of effective methods such as Friedel-Crafts reaction [9], Heck reaction [10], metal-catalyzed intramolecular epoxidation [11], and others [12, 13] have been developed for the construction of 1, 2-dihydro-1-arylnaphthalenes [14-21]. Nevertheless, these methods often require substrate pre-functionalization and high temperatures, and have limited product regioselectivity. In recent years, visible-light-induced chemical transformations have attracted increasing interest from organic chemists because of their green and sustainable features [22-27]. Despite their high oxidation potential, styrenes can undergo dimerization by acridinium-based organic dyes in the presence of a stoichiometric oxidant or hydrogen atom transfer (HAT) reagent [28, 29]. 9-Mesityl-10- acridinium-based organic dye shows a strong absorption band in the visible region (λ = 430 nm) and high oxidizing ability in the excited state, and has been successfully employed in the transformation of a diverse range of alkenes into cation radicals under irradiation [30-34]. Huang et al. [28] recently reported the [4+2] cycloaddition of styrenes by using 9-mesityl-10-phenylacridinium tetrafluoroborate (Ph-Acr⁺-Mes BF₄^–) as the photosensitizer and PhSSPh as an HAT reagent for the synthesis of 1, 2, 3, 4-tetrahydro-arylnaphthalenes (Scheme 1(a)). Liang and co-workers [29] took the lead in the oxidative synthesis of 1, 2-dihydro-1-arylnaphthalene derivatives from styrenes by using 9-mesityl-10-methyl acridinium perchlorate (Acr⁺-Mes ClO₄^–) as the photosensitizer, and 2-phenylmalononitrile (PMN) and Selectfluor as the oxidants (Scheme 1(b)). However, the use of an HAT reagent always leads to the reduced product (tetralins) rather than 1, 2-dihydronaphthalenes. The use of an oxidant, on the other hand, can result in overoxidized products, naphthalene derivatives. We contemplated whether we could combine catalytic amounts of the Acr⁺-Mes photosensitizer and cobalt catalyst [35-37] to synthesize 1, 2-dihydro-1-arylnaphthalene derivatives by a cross-coupling hydrogen-evolution (CCHE) reaction [38-50], which avoids the use of any sacrificial oxidant and yields hydrogen gas (H₂) as the sole byproduct. To our delight, the use of Acr⁺-Mes ClO₄^– and cobaloxime catalyst Co(dmgH)₂pyCl helped us execute our idea under visible-light irradiation at room temperature.

¹H NMR spectra were recorded using a Bruker Avance DPX 400MHz instrument with tetramethylsilane (TMS) as the internal standard. ¹³C NMR spectra were obtained at 100 MHz and referenced to the internal solvent signals. Hydrogen gas was analyzed by gas chromatography (7890-Ⅱ, Tianmei, China) using a thermal conductivity detector (TCD) with a 5 Å molecular sieve column and nitrogen as the carrier gas. Mass spectra were recorded using a Trio-2000 GC-MS spectrometer. Commercially available reagents and solvents were used without further purification. Blue LEDs (3 W, λ = 440 ± 10 nm, 145 lm @700mA) were used as the irradiation light sources.

A 10 mL Pyrex tube equipped with a magnetic stir bar was charged with styrene (73 μL, 0.7 mmol) Acr⁺-Mes (8.82 mg, 0.003 mmol), Co(dmgH)₂pyCl (19.6 mg, 0.007 mmol), NaH₂PO₄ (210 mg, 1.75 mmol), and dry MeCN (5 mL). Then, the system was degassed with a stream of argon for 30 min. The mixture was irradiated by blue LEDs under an argon atmosphere for 24 h. After removal of the solvent under vacuum, the residue was purified by silica gel column chromatography using petroleum ether/ethyl acetate as the eluent to obtain the products.

1, 2-dihydro-1-arylnaphthalene (2): colorless oil, yield 60%, petroleum ether. ¹H NMR (400 MHz, CDCl₃) δ 7.33 (t, J = 8.0 Hz, 2H), 7.27–7.24 (m, 3H), 7.20 (t, J = 7.6 Hz, 1H), 7.13–7.08(m, 2H), 6.83 (d, J = 7.6 Hz, 1H), 6.56 (d, J = 9.6 Hz, 1H), 6.04–6.00 (m, 1H), 4.16 (t, J = 8.8 Hz, 1H), 2.69–2.64 (m, 2H). ¹³C NMR (101 MHz, CDCl₃) δ 144.5, 137.9, 134.1, 128.5, 128.4, 128.0, 127.9, 127.3, 127.2, 126.8, 126.5, 126.2, 43.9, 32.0. HRMS (EI) calcd. for C₁₆H₁₄ [M]: 206.1096, found: 206.1099.

7-Methyl-1-(p-tolyl)-1, 2-dihydronaphthalene (3): colorless oil, yield 66%, petroleum ether. ¹H NMR (400 MHz, CDCl₃) δ 7.04–7.00 (m, 4H), 6.94–6.87 (m, 2H), 6.57 (s, 1H), 6.41 (d, J = 9.6 Hz, 1H), 5.84–5.80 (m, 1H), 3.96 (t, J = 7.6 Hz, 1H), 2.54–2.50 (m, 1H), 2.49–2.46 (m, 1H), 2.24 (s, 3H), 2.12 (s, 3H). ¹³C NMR (101 MHz, CDCl₃) δ 141.6, 137.9, 137.0, 135.9, 131.5, 129.1, 128.6, 128.3, 127.8, 127.3, 126.1, 126.0, 43.5, 32.2, 21.3, 21.0. HRMS (EI) calcd. for C₁₈H₁₈ [M]: 234.1409, found: 234.1412.

5-Methyl-1-(o-tolyl)-1, 2-dihydronaphthalene (4): colorless oil, yield 71%, petroleum ether. ¹H NMR (400 MHz, CDCl₃) δ 7.21 (t, J = 3.2 Hz, 1H), 7.16–7.13 (m, 2H), 7.11 (t, J = 6.4 Hz, 1H), 7.01 (d, J = 7.6 Hz, 1H), 6.95 (t, J = 6.4 Hz, 1H), 6.78 (d, J = 9.6 Hz, 1H), 6.54 (d, J = 7.6 Hz, 1H), 6.10–6.05 (m, 1H), 4.38–4.34 (m, 1H), 2.59–2.54(m, 2H), 2.39 (s, 3H), 2.36 (s, 3H). ¹³C NMR (101 MHz, CDCl₃) δ 142.2, 138.1, 136.2, 133.2, 132.6, 130.4, 128.5, 128.4, 127.6, 126.9, 126.3, 126.1, 125.3, 124.7, 40.1, 30.2, 19.8, 19.3. HRMS (EI) calcd. for C₁₈H₁₈ [M]: 234.1409, found: 234.1409

4-(2, 5-Dimethylphenyl)-5, 8-dimethyl-1, 2-dihydronaphthalene (5): colorless oil, isolated yield 55%, petroleum ether. ¹H NMR (400 MHz, CDCl₃) δ 7.12 (d, J = 15.2 Hz, 1H), 7.08 (s, 1H), 7.03 (d, J = 8.0 Hz, 1H), 6.94 (d, J = 7.8 Hz, 1H), 6.90 (s, 1H), 6.47 (s, 1H), 5.91–5.71 (m, 1H), 4.39 (d, J = 8.9 Hz, 1H), 2.87 (ddd, J = 11.4, 6.0, 3.2 Hz, 1H), 2.48 (s, 3H), 2.44 (s, 3H), 2.41–2.37 (m, 1H), 2.14 (s, 3H), 2.00 (s, 3H). ¹³C NMR (101 MHz, CDCl₃) δ 141.2, 135.9, 135.1, 133.6, 132.8, 131.8, 131.0, 130.3, 129.3, 128.9, 128.4, 126.8, 125.3, 125.1, 34.6, 29.9, 21.3, 19.5, 18.9, 18.4. HRMS (EI) calcd. for C₂₀H₂₂ [M]: 262.1722, found: 262.1711

7-Methyl-4-(m-tolyl)-1, 2-dihydronaphthalene (6) and 5-methyl-4-(m-tolyl)-1, 2-dihydro-naphthalene (6'): colorless oil, isolated yield 76%, petroleum ether. ¹H NMR (400 MHz, CDCl₃) δ 7.86–7.74 (m, 2H), 7.46 (m, 1H), 7.41–7.32 (m, 2H), 7.30 (m, 3H), 6.98–6.91 (m, 3H), 6.87 (m, 1H), 6.78 (m, 1H), 6.70 (m, 1H), 6.50 (t, 2H), 6.03–5.95 (m, 1H), 5.80–5.70 (m, 1H), 4.20 (t, J = 8.4 Hz, 1H), 4.05 (t, J = 8.9 Hz, 1H), 2.91 (m, 2H), 2.62–2.57 (m, 2H), 2.32 (s, 3H), 2.30 (s, 3H), 2.26 (s, 3H), 2.09 (s, 3H). ¹³C NMR (101 MHz, CDCl₃) δ 144.7, 137.9, 137.6, 137.0, 136.2, 136.1, 135.5, 134.1, 134.0, 130.8, 130.4, 129.6, 129.2, 128.5, 128.3, 128.0, 127.8, 127.7, 127.3, 127.2, 126.9, 126.7, 125.5, 125.1, 124.9, 124.5, 124.3, 43.4, 38.4, 32.4, 32.2, 21.6, 21.0. HRMS (EI) calcd. for C₁₈H₁₈ [M]: 234.1409, found: 234.1409

6-Methoxy-4-(4-methoxyphenyl)-1, 2-dihydronaphthalene (7): colorless oil, yield 47%, petroleum ether. ¹H NMR (400 MHz, CDCl₃) δ 7.14 (m, 3H), 7.02 (s, 1H), 6.84 (d, J = 1.8 Hz, 2H), 6.70 (m, 1H), 6.39 (m, 1H), 5.94–5.73 (m, 1H), 4.14–3.97 (m, 1H), 3.78 (s, 3H), 3.69 (s, 3H), 2.27 (d, J = 3.3 Hz, 1H), 2.06 (d, J = 4.8 Hz, 1H). ¹³C NMR (101 MHz, CDCl₃) δ 159.1, 157.9, 140.0, 137.0, 129.3, 127.5, 127.4, 127.2, 124.7, 114.5, 113.9, 113.7, 111.3, 111.0, 55.4, 54.9, 48.0, 26.0. HRMS (EI) calcd. for C₁₈H₁₈O₂ [M]: 266.1307, found: 266.1309

6-Ethoxy-4-(4-ethoxyphenyl)-1, 2-dihydronaphthalene (8): colorless oil, isolated yield 41%, petroleum ether. ¹H NMR (400 MHz, CDCl₃) δ 7.13 (s, 1H), 7.11 (s, 1H), 7.01 (d, J = 8.2 Hz, 1H), 6.80 (s, 1H), 6.68 (m, 1H), 6.48 (d, J = 9.6 Hz, 1H), 6.39 (d, J = 2.4 Hz, 1H), 5.94–5.78 (m, 1H), 4.01 (m, 2H), 3.90 (m, 2H), 3.44 (m, 1H), 2.58–2.53 (m, 1H), 2.27–2.25 (m, 1H), 2.07 (t, J = 5.0 Hz, 1H), 1.39 (s, 3H), 1.32 (t, J = 7.0 Hz, 3H). ¹³C NMR (101 MHz, CDCl₃) δ 158.2, 157.6, 157.4, 140.2, 136.4, 136.2, 129.5, 127.6, 127.0, 124.6, 115.0, 114.8, 114.3, 111.7, 63.3, 63.0, 43.5, 26.0, 15.0, 14.8. HRMS (EI) calcd. for C₂₀H₂₂O₂ [M]: 294.1620, found: 294.1620

6-(tert-Butyl)-4-[4-(tert-butyl)phenyl]-1, 2-dihydronaphthalene (9): white solid, isolated yield 62%, petroleum ether. ¹H NMR (400 MHz, CDCl₃) δ 7.32 (d, J = 8.3 Hz, 2H), 7.26–7.23 (m, 1H), 7.14 (m, 2H), 7.07 (d, J = 8.0 Hz, 1H), 6.96 (s, 1H), 6.53 (d, J = 9.6 Hz 1H), 5.99–5.91 (m, 1H), 4.12 (t, J = 7.8 Hz, 1H), 2.75–2.69 (m, 1H), 2.66–2.61 (m, 1H), 1.34 (s, 9H), 1.25 (s, 9H). ¹³C NMR (101 MHz, CDCl₃) δ 150.2, 149.0, 141.6, 137.4, 131.6, 127.8, 127.7, 126.3, 125.7, 125.3, 125.1, 123.5, 43.3, 34.6, 34.4, 31.4, 31, 3. HRMS (EI) calcd. for C₂₄H₃₀ [M]: 318.2348, found: 318.2350

6-Fluoro-4-(4-fluorophenyl)-1, 2-dihydronaphthalene (10): colorless oil, yield 67%, petroleum ether/ethyl acetate = 100/1. ¹H NMR (400 MHz, CDCl₃) δ 7.19–7.15 (m, 2H), 7.06 (m, 1H), 7.04–6.98 (m, 2H), 6.88–6.84 (m, 1H), 6.52–6.48 (m, 2H), 5.98–5.93 (m, 1H), 4.10–4.07 (m, 1H), 2.64–2.58 (m, 1H), 2.57–2.49 (m, 1H). ¹³C NMR (101 MHz, CDCl₃) δ 163.0, 160.5, 140.3, 139.3, 130.3, 129.8, 127.6, 127.1, 126.2, 115.5, 115.0, 113.5, 43.3, 31.6. HRMS (EI) calcd. for C₁₆H₁₂F₂ [M]: 242.0907, found: 242.0909

6-Chloro-4-(4-chlorophenyl)-1, 2-dihydronaphthalene (11): colorless oil, isolated yield 62%, petroleum ether/ethyl acetate = 100/1. ¹H NMR (400 MHz, CDCl₃) δ 7.26 (d, J = 2.5 Hz, 2H), 7.12 (dd, J = 6.9, 4.9 Hz, 2H), 7.00 (dd, J = 10.1, 8.3 Hz, 2H), 6.80–6.74 (m, 1H), 6.49 (d, J = 9.6 Hz, 1H), 6.10–5.79 (m, 1H), 4.06–3.98 (m, 1H), 2.89–2.74 (m, 1H), 2.59–2.49 (m, 1H). ¹³C NMR (101 MHz, CDCl₃) δ 142.1, 139.2, 132.7, 132.6, 132.5, 129.7, 129.2, 128.8, 128.7, 128.0, 127.9, 127.4, 127.3, 127.2, 127.1, 43.2, 31.6. HRMS (EI) calcd. for C₁₆H₁₂Cl₂ [M]: 274.0316, found: 274.0353

5-Chloro-1-(o-chlorophenyl)-1, 2-dihydronaphthalene (12): colorless oil, isolated yield 50%, petroleum ether/ethyl acetate = 100/1. ¹H NMR (400 MHz, CDCl₃) δ 7.40 (dd, J = 7.4, 2.0 Hz, 1H), 7.25–7.20 (m, 2H), 7.17–7.16 (m, 1H), 7.15-7.13(m, 1H), 7.03–7.01 (m, 1H), 7.01–6.98 (m, 1H), 6.72 (d, J = 7.4 Hz, 1H), 6.10–6.05 (m, 1H), 4.69 (t, J = 8.0 Hz, 1H), 2.68-2.63 (m, 2H). ¹³C NMR (101 MHz, CDCl₃) δ 140.8, 138.8, 133.7, 131.9, 131.5, 129.9, 129.7, 128.7, 128.1, 127.9, 127.8, 126.9, 126.5, 125.2, 124.0, 39.9, 29.7. HRMS (EI) calcd. for C₁₆H₁₂Cl₂ [M]: 274.0316, found: 274.0266

6-Bromo-4-(4-bromophenyl)-1, 2-dihydronaphthalene (13): colorless oil, isolated yield 62%, petroleum ether/ethyl acetate = 100/1. ¹H NMR (400 MHz, CDCl₃) δ 7.43 (d, J = 8.0 Hz, 2H), 7.31 (d, J = 10.0 Hz, 1H), 7.21 (dd, J = 8.4, 8.8 Hz, 1H), 7.07 (d, J = 8.4 Hz, 2H), 6.97(d, J = 4.0 Hz, 1H), 6.48 (d, J = 9.6 Hz, 1H), 6.02–5.99 (m, 1H), 4.08–4.03 (m, 1H), 2.63–2.60 (m, 1H), 2.57–2.53 (m, 1H). ¹³C NMR (101 MHz, CDCl₃) δ 142.6, 139.3, 132.9, 131.7, 131.6, 130.7, 130.0, 127.6, 127.8, 127.4, 127.2, 120.6, 43.1, 31.6. HRMS (EI) calcd. for C₁₆H₁₂Br₂ [M]: 361.9306, found: 361.9251

5-Bromo-1-(o-bromophenyl)-1, 2-dihydronaphthalene (14): colorless oil, isolated yield 48%, petroleum ether/ethyl acetate = 100/1. ¹H NMR (400 MHz, CDCl₃) δ 7.59 (dd, J = 7.9, 1.1 Hz, 1H), 7.42 (dd, J = 12.6, 4.7 Hz, 2H), 7.35–7.27 (m, 1H), 7.22–7.12 (m, 1H), 7.06 (dd, J = 7.7, 1.9 Hz, 1H), 6.97–6.90 (m, 1H), 6.75 (d, J = 7.8 Hz, 1H), 6.14–5.91 (m, 1H), 4.66 (t, J = 8.1 Hz, 1H), 2.85 (d, J = 6.2 Hz, 1H), 2.63 (m, 1H). ¹³C NMR (101 MHz, CDCl₃) δ 144.1, 142.4, 136.4, 133.0, 131.9, 130.1, 129.4, 128.4, 127.7, 127.0, 126.9, 126.6, 126.0, 123.6, 42.4, 30.0. HRMS (EI) calcd. for C₁₆H₁₂Br₂ [M]: 361.9306, found: 361.9183.

6-Ethoxy-4-(4-ethoxyphenyl)-1, 2-dihydronaphthalene (15): colorless oil, yield 53%, petroleum ether/ethyl acetate = 100/1. ¹H NMR (400 MHz, CDCl₃) δ 7.21 (d, J = 8.5 Hz, 2H), 7.15–7.03 (m, 2H), 7.01 (dd, J = 13.7, 8.4 Hz, 2H), 6.90 (dd, J = 8.1, 2.2 Hz, 1H), 6.51 (d, J = 11.4 Hz, 1H), 6.06–5.91 (m, 1H), 4.08 (dt, J = 50.1, 25.2 Hz, 1H), 2.58 (s, 1H), 2.56 (s, 1H), 2.31–2.24 (m, 3H), 2.18 (d, J = 6.8 Hz, 3H). ¹³C NMR (101 MHz, CDCl₃) δ 169.5, 169.5, 149.6, 149.4, 141.3, 139.2, 131.9, 129.4, 127.2, 126.9, 121.6, 121.0, 119.9, 43.5, 31.8, 21.2. HRMS (EI) calcd. for C₁₈H₁₈O₄ [M]: 322.1205, found: 322.1205

6-Benzyl-4-(4-benzylphenyl)-1, 2-dihydronaphthalene (16): colorless oil, isolated yield 60%, petroleum ether/ethyl acetate = 100/1. ¹H NMR (400 MHz, CDCl₃) δ 7.50 (s, 1H), 7.44 (d, J = 8.1 Hz, 3H), 7.39 (d, J = 7.6 Hz, 2H), 7.33 (d, J = 7.4 Hz, 3H), 7.25 (d, J = 8.1 Hz, 3H), 7.23–7.17 (m, 3H), 7.13–7.05 (m, 2H), 6.51 (d, J = 9.6 Hz, 1H), 6.05–5.84 (m, 1H), 4.14 (t, J = 8.2 Hz, 1H), 2.71–2.56 (m, 1H), 2.05 (s, 1H). ¹³C NMR (101 MHz, CDCl₃) δ 143.7, 143.6, 141.0, 141.1, 140.2, 139.4, 138.2, 133.4, 129.3, 128.9, 127.8, 128. 7, 127.8, 127.3, 127.2, 127.1, 127.1, 127.1, 127.0, 126.9, 126.7, 125.7, 43.8, 32.1. HRMS (EI) calcd. for C₂₈H₂₂ [M]: 358.1722, found: 358.1722

1-(Naphthalen-1-yl)-1, 2-dihydronaphthalene (17): white solid, isolated yield 58%, petroleum ether/ethyl acetate = 100/1. ¹H NMR (400 MHz, CDCl₃) δ 8.21 (dd, J = 15.5, 8.6 Hz, 2H), 8.01–7.92 (m, 1H), 7.93–7.89 (m, 1H), 7.83–7.70 (m, 3H), 7.57–7.49 (m, 4H), 7.41–7.37 (m, 1H), 7.29 (d, J = 3.5 Hz, 1H), 7.04 (d, J = 6.4 Hz, 1H), 6.20–6.07 (m, 1H), 5.13 (t, J = 8.0 Hz, 1H), 3.05–2.78 (m, 2H). ¹³C NMR (101 MHz, CDCl₃) δ 139.3, 135.0, 134.4, 133.0, 131.4, 129.6, 129.2, 128.6, 127.8, 127.5, 127.2, 126.8, 126.2, 125.9, 125.6, 125.5, 125.4, 125.3, 123.1, 122.8, 39.8, 37.8. HRMS (EI) calcd. for C₂₄H₁₈ [M]:306.1409, found: 306.1407

Initially, styrene was chosen as the standard substrate for the desired product. As shown in Table 1, when using Acr⁺-Mes ClO₄^– (3 mol%) as the photocatalyst and Co(dmgH)₂pyCl (7 mol%) as the proton reduction catalyst, 35% yield of the desired product 2 was obtained after 24 h of irradiation (Table 1, entry 1). The produced hydrogen gas was also detected by GC–TCD. No other product was detected, except for a very small amount of tetralin derivative. We hypothesized that the generated hydrogen and the target product were further hydrogenated to give a hydrogenated compound. To enhance the reaction efficiency, various concentrations of 1 were screened (Table 1, entries 1–4). At a high concentration, the yield of product 2 was increased to 56% (Table 1, entry 3) due to the bimolecular reaction character. Further increasing the concentration led to a decrease in the product yield (Table 1, entry 4). Then, several cobaloxime catalysts were evaluated (Table 1, entries 3, 5–7), and Co(dmgH)₂pyCl was found to be the best (Table 1, entry 3), indicating that the structure of the cobalt complex has a greater impact on the reaction. In order to further optimize the yield, we added additives, including Na₂CO₃, NaHCO₃, NaH₂PO₄, DMAP, and K₂CO₃, to the system and found that 2.5 equiv. NaH₂PO₄ could increase the yield of the desired product 2 to 72% (Table 1, entry 8), where NaH₂PO₄ might aid in proton transfer. Other photocatalysts such as rose bengal, rhodamine B, [Ir(dtbbpy)(ppy)₂][PF₆], eosin Y, and Ru(bpy)₃Cl₂ did not afford the desired product at all. In the control experiments, no desired product was observed without the photosensitizer Acr⁺-Mes, cobalt catalyst, or visible light, indicating that Acr⁺-Mes and cobalt catalysts are essential for this transformation (Table 1, entries 9–11).

With the optimal conditions in hand, we attempted to explore the scope of the visible-light catalytic 1, 2-dihydro-1-arylnaphthalene formation. As summarized in Scheme 2, most of the styrenes containing electron-withdrawing groups or electron-donating groups could afford the corresponding 1, 2-dihydro-1-arylnaphthalenes in moderate yields under standard conditions. Meanwhile, styrenes with halo group substitution at the ortho or para-position were also smoothly converted into the 1, 2-dihydro-1-arylnaphthalene products in moderate yields, which have the potential for further transformation. When a meta-methyl styrene derivative was employed in the reaction, two region-isomers 6 and 6' were obtained. The presence of a methoxy group or ethyloxy group at the para-position of styrene afforded 1, 2-dihydro-1-arylnaphthalene (7, 8) in only 47% and 41% yields because of the formation of some unidentified byproducts. Styrene with a tertiary butyl substituent at the para-position also gave the desired product 9 in 62% yield. Interestingly, 2-vinyl-naphthalene also worked well for this protocol and afforded the product 17 in 58% yield. However, the electron-deficient 2-pyridylethylene failed to afford the corresponding annulation product, similar to the results observed for 4-nitrostyrene and 4-cyanogroup. A large gram-scale reaction (10 mmol 1 in 50 mL dry CH₃CN) was performed to obtain 1, 2-dihydro-1-arylnaphthalene 2 in 55% yield after 48 h of irradiation.

A cross-dimerization reaction using two styrenes with different substituents on the phenyl ring (e.g., a 1:1 mixture of 4-methylstyrene and 4-fluorostyrene) was also conducted (Scheme 3). The corresponding homodimerized and cross-dimerized products were obtained as a mixture (3:21:10 = 2.2:2.1:1.0) in 57% total isolated yield, which further broadened the scope of our substrates.

Mechanistic studies were conducted to gain insights into the aforementioned transformation. When 2, 2, 6, 6-tetramethyl-1-piperidinyloxy (TEMPO) was used as the radical scavenger, no desired products were formed (Scheme 4), implying that radicals are involved in our reaction process. To shed more light on the process of the reaction, we examined the interaction between styrene 1 and the excited Acr⁺-Mes. The luminescence of the excited Acr⁺-Mes could be significantly quenched by 1 with a rate constant of 9.6 × 10⁸ (mol/L)^–1 s^–1. On the other hand, Co(dmgH)₂pyCl could also quench the luminescence of Acr⁺-Mes with a quenching rate constant of 1.6 × 10⁷ (mol/L)^–1 s^–1, which is two orders of magnitude smaller than that of 1. The concentration of styrene was also much higher than that of Co(dmgH)₂pyCl. Thus, we considered that the excited Acr⁺-Mes preferentially reacted with styrene than with Co(dmgH)₂pyCl (Fig. 1(a)). Furthermore, upon irradiation, the absorption spectrum (Fig. 1(b)) of the system containing styrene, Acr⁺-Mes, and Co(dmgH)₂pyCl in degassed dry MeCN immediately exhibited absorption bands at 440–500 nm and 550–650 nm, which indicated the formation of Co^Ⅱ and Co^Ⅰ species, respectively, based on reports in the literature [49].

Based on the above results, we proposed a plausible mechanism for this reaction (Scheme 5). Photosensitizer Acr⁺-Mes is irradiated by visible light to reach its excited state Acr^•-Mes^•+ (E_1/2^red = + 2.06 V vs. SCE) [30-34], which undergoes single-electron transfer (SET) with styrene to generate styrene radical cation 1^+• and Acr^•-Mes radical anion. Electrophilic trapping of intermediate 1^+• by a neutral alkene molecule gives distal 1, 4-radical cation A, which readily undergoes Friedel–Crafts annulation to form intermediate B via deprotonation and aromatization. The generated Acr^•-Mes radical anion donates an electron to Co(dmgH)₂pyCl (Co^Ⅲ) to produce Co^Ⅱ species and regenerate the photosensitizer Acr⁺-Mes. The carbon- centered radical B is further oxidized by Co^Ⅱ to afford Co^Ⅰ species and product 2. Protonation of the Co^Ⅰ species provides Co^Ⅲ-H [52, 53], which can react with a proton to release H₂ eventually and regenerate the generate Co^Ⅲ catalyst.

In summary, we have developed a hydrogen-evolution dimerization methodology for the synthesis of 1, 2-dihydro-1-arylnaphthalenes by combining photocatalysis and proton reduction catalysis under visible-light irradiation. By using the dual catalyst system of photosensitizer Acr⁺-Mes and cobaloxime catalyst Co(dmgH)₂pyCl, various styrenes can be transformed into the desired products in moderate to good yields, and H₂ is the only byproduct. As opposed to previous methods, this reaction proceeds smoothly under extremely mild conditions, and no oxidized products (naphthalene derivatives) are produced. We believe that this methodology would be significant for application to a series of organic reactions.