Characterization of tetraene intermediates formed in the [3+2]-photocycloaddition of 1,4-dicyano-6-methylnaphthalene with styrene
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The early stages of the [3+2]-photocycloaddition of 1,4-dicyano-6-methylnaphthalene (6) with styrene (7) were investigated by UV–visible absorption and 1H NMR spectroscopy. An intermediate species was detected and characterized as 8-methyl-2-phenyl-1,2,2a,8-tetrahydroacenaphthylene-2a,5-dicarbonitrile (9). Computational studies explained the regioselective [3+2]-photocycloaddition at the 4,5-position of 6 to form zwitterion 8, and subsequent thermal transformation to form 9.
Keywords[3+2]-Photocycloaddition 1,4-Dicyano-6-methylnaphthalene Tetraene intermediate Regioselectivity Computational studies
There has been much interest both synthetically and mechanistically in the photocycloaddition of arenes, for example benzene, naphthalene, anthracene, and phenanthrene derivatives, to alkenes and dienes [1, 2]. [2+2], [4+2], and [4+4]-photocycloadditions of naphthalene derivatives to alkenes and dienes have been extensively investigated and have provided important clues about the mechanism of photocycloadditions. In contrast with the numerous investigations on [2+2], [4+2], and [4+4]-photocycloadditions of arenes [3, 4, 5, 6, 7, 8], only a very limited number of [3+2]-cycloadditions forming five-membered rings have been reported. These include meta-cycloadditions of benzene  and naphthalene derivatives  and formal [3+2]-cycloadditions by way of rearrangement of initially formed 1,4-biradicals to carbenes  and nitrenes .
We report here that an intermediate species is formed in an early stage of the [3+2]-photocycloaddition of asymmetric 1,4-dicyano-6-methylnaphthalene (6) with an aromatic alkene, styrene (7), and determined its structure by UV–visible absorption and 1H NMR spectroscopy. Furthermore, regioselective [3+2]-photocycloaddition at the methyl side position of 6 was revealed by 1H NMR analysis and explained by computational studies.
All reagents and solvents were commercially available and used without further purification. All photoreactions were carried out under Ar atmosphere using an Eikosha EHB-WIF-500 high-pressure Hg lamp (500 W) as the light source. 1H and 13C NMR spectra were recorded on a Jeol AL-400 instrument (400 MHz for 1H, 100 MHz for 13C), and chemical shifts were reported in ppm relative to internal TMS (0.00 ppm, 1H) or the solvent peak (77.0 ppm, 13C). UV–visible absorption and steady-state fluorescence spectra were measured on a Jasco V-560 spectrophotometer and Hitachi 850 spectrofluorimeter, respectively. Computational studies were performed with the Gaussian 03 software package . Geometry optimizations were performed by DFT using the B3LYP density functional and 6-31+G(d) basis set [19, 20, 21], and zero-point energies were corrected. Calculations of excitation energies were carried out by TD-DFT (B3LYP/6-31+G(d)) using equilibrium geometries optimized as ground states [22, 23]. Geometry optimization of molecules in singlet excited states were carried out by CIS/6-31+G(d)//CIS/3-21G* level theory .
Photoreactions of 6 with 7 for UV–visible absorption spectroscopic experiments
To 3.0 mL of a benzene solution of 6 (0.10 mM) in a quartz cell (10 × 10 mm) was added 17.2 μL 7 (50 mM) and 2.3 μL trifluoroacetic acid (TFA, 10 mM), and the solution was deaerated by Ar-bubbling. Irradiation (313 nm) was performed at 25 °C, and the progress of the photoreaction was monitored by UV–visible absorption spectroscopy. Wavelength-selective irradiation at 313 nm was achieved by use of a 0.5 M aqueous solution of K2CrO4 and a Hoya U340 sharp cut filter.
Photoreactions of 6 with 7 for 1H NMR experiments
4.3 mg (22.4 μmol) 6 and 17.2 μL (150 μmol) 7 were dissolved in 1.5 mL benzene-d 6, and 1.1 mL (14.8 mmol) TFA was added, if required. 0.5 mL of the prepared solution was placed in a Pyrex NMR sample tube (5 mm ϕ × 200 mm) and deaerated by Ar-bubbling. Irradiation was essentially carried out in the same manner as the procedure for UV–visible absorption analysis.
Preparation of 8-methyl-2-phenyl-1,2,2a,8-tetrahydroacenaphthylene-2a,5-dicarbonitrile (9)
6 (15 mM), 7 (50 mM), and TFA (10 mM) were dissolved in 1,2-dichloroethane, and deaerated by Ar-bubbling for 5 min. Irradiation (313 nm) was performed at 25 °C for 4 h using the 0.5 M aqueous K2CrO4 filter to give 9 in 28 % yield (determined by 1H NMR). The reaction mixture was evaporated carefully to prevent complete drying. The residual solution was separated by silica gel column chromatography (Wakogel C-200) with benzene containing 5 % v/v TFA as mobile phase to give moderately pure 9 as a yellow solution. 9 was so unstable at high temperature or high pH that it was not purified further and was stored as a benzene solution containing TFA at 0 °C.
1H NMR (CDCl3): δ 7.3–7.1 (m, 5H, phenyl-H), 6.78 (dd, J = 9.7, 1.7 Hz, 1H, Hf), 6.54 (dd, J = 9.7, 4.0 Hz, 1H, Hg), 5.96 (d, J = 9.7 Hz, 1H, He), 5.37 (d, J = 9.7 Hz, 1H, Hd), 4.26 (d, J = 6.9 Hz, 1H, Hc), 3.46 (m, 1H, Hh), 3.35 (ddd, J = 18.3, 6.9, 3.4 Hz, 1H, Hb), 2.97 (d, J = 18.3 Hz, 1H, Ha), 1.44 (d, J = 7.4 Hz, 3H, CH3). 13C NMR (CDCl3): δ 158.01 (sp2, C–C=C), 153.12 (sp2, C–C=C), 147.87 (sp2, C–C=C), 143.57 (sp2, H–C=C), 139.45 (sp2, C–C=C), 137.38 (sp2, C–C=C), 128.89 (sp2, H–C=C), 128.81 (sp2, H–C=C), 127.80 (sp2, H–C=C), 126.27 (sp2, H–C=C), 125.99 (sp2, H–C=C), 122.19 (sp2, H–C=C), 121.85 (sp2, H–C=C), 119.85 (sp, CN), 117.22 (sp, CN), 97.21 (sp2, C–CN), 55.32 (sp3, CH–Ph), 48.95 (sp3, C–CN), 40.71 (sp3, CH2), 37.61 (sp3, CH–CH3), 17.72 (sp3, CH3).
Results and discussion
UV–visible absorption spectroscopic analysis of an early stage of [3+2]-photocycloaddition of 6 with 7
Rate constants (k) of thermal reactions
1.09 × 10−5
1.79 × 10−6
Pyridine (0.9 mM)
3.3 × 10−3
1.95 × 10−6
3.00 × 10−6
5.65 × 10−6
Preparation and characterization of an intermediate of [3+2]-photocycloaddition of 6 with 7
Another tetraene, 10, which would be characterized by a singlet CH3 peak, a singlet peak of Hi in the aromatic region, and geminally coupled Hj and Hk resonances in the 1H NMR spectrum, was also a probable product in the [3+2]-photocycloaddition of asymmetric 6 with 7. However, no evidence of formation of 10 was observed by 1H NMR analysis, indicating that the regioselective [3+2]-photocycloaddition proceeded not at the 1,8-position of 6 but at the 4,5-position.
Computational studies on the mechanism of formation of 9
The early stages of the [3+2]-photocycloaddition of 6 with 7 were investigated in detail by UV–visible absorption and 1H NMR spectroscopy, which provided clear evidence of intermediate formation. The intermediate was successfully separated and characterized as a cross-conjugated tetraene 9, which was photochemically and thermally reactive. Regioselective [3+2]-photocycloaddition at the 4,5-position of 6 was observed, and computational studies successfully provided a rationale to explain the regioselectivity. Finally, a thermal H-shift from zwitterion 8–9 was also supported by the results of computational studies.
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