Research on Chemical Intermediates

, Volume 39, Issue 1, pp 49–59 | Cite as

Characterization of tetraene intermediates formed in the [3+2]-photocycloaddition of 1,4-dicyano-6-methylnaphthalene with styrene



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.


[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 [9] and naphthalene derivatives [10] and formal [3+2]-cycloadditions by way of rearrangement of initially formed 1,4-biradicals to carbenes [11] and nitrenes [12].

We have discovered a novel 1,8-photoaddition of naphthalene derivatives, for example 1,4-dicyanonaphthalene (1), to alkenes such as isobutene (2), which constitutes a formal [3+2]-cycloaddition. The reaction proceeds from the singlet excited state of 1, possibly via formation of an exciplex 3 and almost synchronous two-bond formation in 3 to give zwitterion 4, followed by proton transfer (Scheme 1) [13, 14, 15, 16, 17].
Scheme 1

[3+2]-Photocycloaddition of 1 with 2

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 [18]. 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 [24].

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

The progress of the [3+2]-photocycloaddition of 6 with 7 was investigated by UV–visible absorption spectroscopy. Figure 1a shows UV–visible absorption spectra of a benzene solution of 6 (0.10 mM) and 7 (50 mM) upon irradiation (>320 nm) at 25 °C. A new broad band emerged at 370 nm in an early stage of irradiation (<2 min) accompanied by a rapid decay of the absorption band from 6 (310–360 nm). After a rapid rise, the absorbance of the 370-nm band became almost constant, and then slowly decreased with prolonged irradiation. The observed photoreaction profiles in this experiment were interpreted as the generation of an intermediate species immediately upon irradiation; this then became photostationary, and then decreased by secondary photochemical processes. Wavelength-selective irradiation (313 nm) of 6 with 7 to suppress photochemical decomposition of the intermediate(s) was conducted (Fig. 1b), and resulted in a large absorption band which was ca. 4 times larger but at the same position as that in Fig. 1a. The peak position of the absorption band of the intermediate(s) (ca. 370 nm) was significantly red-shifted (ca. 50 nm) compared with that of 6, indicating a conjugated π-system in the intermediate species. The thermal and photochemical reaction behavior of the intermediate(s) was investigated. Time-course absorption spectra of the intermediates, prepared in situ by irradiation (313 nm, 9 min) of 6 (0.10 mM) and 7 (50 mM) in benzene, were measured at 25 °C in the dark (Fig. 2a). Decay of the absorbance at 370 nm was well fit by a single-exponential function with a rate constant k = 1.09 × 10−5 s−1, and the 1st-order decay profile indicated that the 370-nm absorption band was that of one intermediate species only. Rate constants of thermal reactions of the intermediate were measured under several conditions, and are summarized in Table 1. Rate constants at 25 °C in the presence of 10 mM TFA and 0.9 mM pyridine were 1.79 × 10−6 and 3.3 × 10−3 s−1, respectively, and significant dependence of the rate constants on the concentration of TFA suggested that a deprotonation step should be included in the thermal reaction mechanism of the intermediate. Photochemical reactivity of the intermediate was confirmed by wavelength-selective irradiation experiments. Irradiation (>400 nm), selective for the intermediate only, caused rapid decay of the 370-nm band (Fig. 2b).
Fig. 1

Time-course of the absorption spectra of photoreaction of 6 (0.10 mM) with 7 (50 mM) in benzene at 25 °C irradiated a at >320 nm and b at 313 nm

Fig. 2

Time-course of the absorption spectra of an intermediate prepared in situ by irradiation (313 nm) of 6 (0.10 mM) with 7 (50 mM) in benzene at 25 °C under Ar atmosphere. a Thermal reaction behavior at 25 °C in the dark, and b photoreaction behavior on irradiation (>400 nm) at 25 °C

Table 1

Rate constants (k) of thermal reactions





1.09 × 10−5

TFA(10 mM)

1.79 × 10−6

Pyridine (0.9 mM)

3.3 × 10−3


TFA(10 mM)

1.95 × 10−6


TFA(10 mM)

3.00 × 10−6


TFA(10 mM)

5.65 × 10−6

The intermediates were prepared in situ by irradiation (313 nm) of 6 (0.10 mM) with 7 (50 mM) in benzene at 25 °C. Rate constants were obtained by single-exponential curve-fitting analysis of absorption decay at 370 nm

Preparation and characterization of an intermediate of [3+2]-photocycloaddition of 6 with 7

The thermal and photochemical reaction behavior of the intermediate(s) were also investigated by 1H NMR spectroscopy. The 1H NMR spectrum of 6 in benzene-d 6 (15 mM) in the presence of 7 (50 mM) after irradiation (313 nm) at 25 °C for 4 h is shown in Fig. 3a. A characteristic doublet peak, not a pair of singlet peaks, was found at 0.71 ppm. The CH3–CH< substructure was suggested by the upfield (0.71 ppm) chemical shift, indicative of a CH3 group attached to an sp3 carbon, and the doublet peak (J = 7.3 Hz), indicative of a single proton neighbor (CH). This characteristic peak was completely diminished by irradiation (>400 nm), heating at 55 °C, or addition of pyridine (20 mM), as shown in Fig. 3b–d. This was the same photochemical and thermal reaction behavior observed in UV–visible analysis, thus this peak was assigned to the signals of the intermediate in the early stage of the [3+2]-photocycloaddition of 6 with 7.
Fig. 3

a 1H NMR spectrum of an intermediate prepared in situ by irradiation (313 nm) of 6 (15 mM) with 7 (50 mM) in benzene-d 6 at 25 °C. A characteristic doublet peak (0.71 ppm) of the intermediate was completely diminished by b irradiation (>400 nm) at 25 °C, c heating at 55 °C for 2 h, and d addition of pyridine (20 mM) at 25 °C

To fully elucidate the structure, we attempted to isolate the intermediate by column chromatography. A solution of 6 (15 mM), 7 (50 mM), and TFA (10 mM) in 1,2-dichloroethane was irradiated (313 nm) at 25 °C for 4 h, and the photoreaction mixture was separated by column chromatography on silica gel using 5 % TFA in benzene as mobile phase to give a yellow solution of the intermediate. Removal of solvent and TFA for further purification was unsuccessful, because treatment under acid and solvent-free conditions led to thermal decomposition of the intermediate. The 1H NMR spectrum of the separated intermediate, slightly contaminated with byproducts formed by thermal reaction, is shown in Fig. 4. A characteristic doublet peak from a methyl substituent attached to an sp3 carbon was also found at 1.44 ppm. The peaks of He (5.94 ppm) and Hd (5.37 ppm), coupled to each other with a relatively large coupling constant (J = 9.7 Hz), were found in the region where olefinic protons are observed, and interpreted as a cis He–C=C–Hf fragment. The peaks of Hf (6.78 ppm) and Hg (6.54 ppm) were also found in the olefinic-H region with J = 9.7 Hz, and a small coupling constant of Hf (1.7 Hz) was likely to be because of long-range coupling frequently found in allylic and aromatic π-systems, suggesting a cis Hf–C=C(–Hg)–C–H fragment. Ha (2.97 ppm) and Hb (3.35 ppm) had a typical large geminal coupling constant (J = 18.3 Hz), and were assigned to a Ha–C–Hb fragment. According to these results, the intermediate was determined to be 8-methyl-2-phenyl-1,2,2a,8-tetrahydroacenaphthylene-2a,5-dicarbonitrile (9) (Scheme 2), which has a cross-conjugated tetraene structure. The relative configuration of the phenyl substituent, illustrated as the S isomer in Scheme 2, was determined by from coupling of Hc (4.26 ppm) with Ha and Hb. According to the Karplus equation [25], the coupling constant between vicinal protons (H–C–C–H) is dependent on the dihedral angle of the two protons, ϕ(H–H), and is smallest when ϕ(H–H) is close to 90°. A 6.9 Hz coupling constant was obtained between Hb and Hc, and the peak of Ha appeared to be a simple doublet split by gem-Hb, indicating that ϕ(Ha–Hc) was close to 90°. Dihedral angles ϕ(Ha–Hc) = 98.3° and ϕ(Hb–Hc) = 21.6° were simulated by DFT calculations (B3LYP/6-31+G(d)), and were consistent with the configuration expected by 1H NMR analysis. Therefore, the configuration of the phenyl substituent of 9 was determined as shown in Scheme 2. 9 was expected to be formed via a thermal H-shift reaction of 8, and computational studies rationalized this hypothesis (discussed in the section “Computational studies on the mechanism of formation of 9 ”). UV–visible and fluorescence spectra of 9 measured in benzene (0.093 mM) are shown in Fig. 5. A broad absorption band (λ max = 371 nm) was consistent with the transient band observed in UV–visible analysis (discussed in the section “UV–visible absorption spectroscopic analysis of an early stage of [3+2]-photocycloaddition of 6 with 7 ”), and matched a spectrum simulated by TD-DFT calculation (TD-B3LYP/6-31+G(d); dashed line in Fig. 5). Fluorescence emission was observed at 501 nm, and fluorescence quantum yield was determined to be 0.12.
Fig. 4

1H NMR spectrum of 9 in CDCl3. The values in parentheses are chemical shifts in ppm, peak shape, and coupling constants of peaks, respectively. Signals of phenyl protons were found at 7.3–7.1 ppm (not shown)

Scheme 2

Tetraene 9 formed by [3+2]-photocycloaddition of 6 with 7 via H-shift of 8. Isomer 10 was not detected

Fig. 5

a Experimental (straight line) and simulated (dashed line) UV–visible absorption spectra of 9 in benzene. b Fluorescence spectrum of 9 measured by excitation at 370 nm in benzene

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

Recently, computational methods have become powerful tools for study of photochemical reactions, especially those involving short-lived transient species, for example photo-excited molecules and biradicals, to determine complete reaction behavior [22, 23]. Thus, we conducted computational studies to explain the regioselectivity of the [3+2]-photocycloaddition of 6 with 7. The mechanism of formation of tetraenes was expected to be a 2-step path which included photochemical [3+2]-cycloaddition to form zwitterions 8 and 13, followed by thermal H-shift reactions of 8 and 13 to yield tetraenes 9 and 10 (Scheme 3). Geometry optimizations of 11 and 12, which are electronically excited transition states in the paths forming zwitterions 8 and 13, respectively, were performed by the CI-single method (CIS/6-31+G(d)//CIS/3-21G*). It was found that both transition states had only one imaginary vibration associated with simultaneous two-bond formation (Fig. 6). The total energy of 12 was 1.29 kcal mol−1 higher than that of 11, strongly indicating that the reaction path to form 8 was energetically favorable compared with the path to 12. The different energies of the two transition states can be explained successfully by the stability of the methyl-substituted allyl cations. 11 and 12 were expected to have zwitterion-like structures, which were similar to those of the resulting zwitterions 8 and 13, i.e., 1-methyl- and 2-methylallyl cationic moieties, respectively (Scheme 3). The electron-donating methyl substituent attached to the 1-position stablizes the allyl cation by hyperconjugation, which was supported by experimental [26, 27] and computational studies [28] that showed the energy of the allyl cation was stabilized more by methyl substitution at the 1-position than at the 2-position.
Scheme 3

Reaction scheme in the early stage of the [3+2]-photocycloaddition of 6 with 7. Regioselective [3+2]-photocycloaddition followed by thermal H-shift to form 9

Fig. 6

Optimized geometries of the singlet excited states of 11 and 12, calculated by CI-single theory (CIS/6-31+G(d)//CIS/3-21G*). Values in parentheses are relative energies in kcal mol−1

The H-shift reaction of 8 to form 9 was also investigated. 8, 9, and a transition state between 8 and 9, i.e., 14, were optimized by DFT (B3LYP/6-31+G(d)); computational results are illustrated in Fig. 7. The relative energies of 14 and 9 compared with that of 8 were +13.50 and −39.45 kcal mol−1, respectively, indicating that the thermal H-shift reaction was favorable.
Fig. 7

Optimized geometries of 8, 9, and a transition state 14, calculated by DFT (B3LYP/6-31+G(d)). Values in parentheses are relative energies in kcal mol−1


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 89 was also supported by the results of computational studies.


  1. 1.
    J.J. McCullough, Chem. Rev. 87, 811–860 (1987)CrossRefGoogle Scholar
  2. 2.
    J. Malkin, Photophysical and Photochemical Properties of Aromatic Compounds (CRC Press, Boca Raton, 1992)Google Scholar
  3. 3.
    I.A. Akhtar, J.J. McCullough, J. Org. Chem. 46, 1447–1450 (1981)CrossRefGoogle Scholar
  4. 4.
    J.J. McCullough, W.K. MacInnis, C.J.L. Lock, R. Faggiani, J. Am. Chem. Soc. 104, 4644 (1982)CrossRefGoogle Scholar
  5. 5.
    H.D. Scharf, H. Leismann, W. Erb, H.W. Gaindetzka, J. Aretz, Pure Appl. Chem. 41, 581–600 (1975)CrossRefGoogle Scholar
  6. 6.
    D. Dopp, C. Kruger, H.R. Memarian, Y.-H. Tsay, Angew. Chem. Int. Ed. Engl. 24, 1048–1049 (1985)CrossRefGoogle Scholar
  7. 7.
    K. Mizuno, C. Pac, H. Sakurai, J. Chem. Soc., Chem. Commun. 648–649 (1974)Google Scholar
  8. 8.
    K. Kan, Y. Kai, N. Yasuoka, N. Kasai, Bull. Chem. Soc. Jpn. 52, 1634–1636 (1979)CrossRefGoogle Scholar
  9. 9.
    J. Cornelisse, Chem. Rev. 93, 615–669 (1993)CrossRefGoogle Scholar
  10. 10.
    H. Mukae, H. Maeda, K. Mizuno, Angew. Chem. Int. Ed. Engl. 45, 6558–6560 (2006)CrossRefGoogle Scholar
  11. 11.
    W.C. Agosta, P. Margaretha, Acc. Chem. Res. 29, 179–182 (1996)CrossRefGoogle Scholar
  12. 12.
    K. Nakatani, K. Tanabe, I. Saito, Tetrahedron Lett. 38, 1207–1210 (1997)CrossRefGoogle Scholar
  13. 13.
    Y. Kubo, T. Inoue, H. Sakai, J. Am. Chem. Soc. 114, 7660–7663 (1992)CrossRefGoogle Scholar
  14. 14.
    Y. Kubo, T. Noguchi, T. Inoue, Chem. Lett. 2027–2030 (1992)Google Scholar
  15. 15.
    Y. Kubo, M. Yoshioka, K. Kiuchi, S. Nakajima, I. Inamura, Tetrahedron Lett. 40, 527–530 (1999)CrossRefGoogle Scholar
  16. 16.
    Y. Kubo, K. Kusumoto, S. Nakajima, I. Inamura, Chem. Lett. 113–114 (1999)Google Scholar
  17. 17.
    Y. Kubo, K. Kiuchi, I. Inamura, Bull. Chem. Soc. Jpn. 72, 1101–1108 (1999)CrossRefGoogle Scholar
  18. 18.
    M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery, Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Revision D.02, Gaussian, Inc. Wallingford, CT (2004)Google Scholar
  19. 19.
    P.C. Hariharah, J.A. Pople, Theor. Chim. Acta 28, 213–222 (1973)CrossRefGoogle Scholar
  20. 20.
    A.D. Becke, J. Chem. Phys. 98, 5648–5652 (1993)CrossRefGoogle Scholar
  21. 21.
    C. Lee, W. Yang, R.G. Paar, Phys. Rev. B 37, 785–789 (1988)CrossRefGoogle Scholar
  22. 22.
    D.P. Chong, ed. M.E. Casisa, Recent Advances in Density Functional Methods, Part I, World Scientific Publishing Co. Pte. Inc. Singapore (1995)Google Scholar
  23. 23.
    B.O. Roos, Computational Photochemistry (Elsevier B. V, Amsterdam, 2005)Google Scholar
  24. 24.
    J.B. Foresman, M. Head-Gordon, J.A. Pople, M.J. Frisch, J. Phys. Chem. 96, 135–149 (1992)CrossRefGoogle Scholar
  25. 25.
    M. Karplus, J. Am. Chem. Soc. 85, 2870–2871 (1963)CrossRefGoogle Scholar
  26. 26.
    C. A. Vernon, J. Chem. Soc. 423–428 (1954)Google Scholar
  27. 27.
    H.C. Brown, C.G. Rao, M. Ravindranathan, J. Org. Chem. 43, 4939–4943 (1978)CrossRefGoogle Scholar
  28. 28.
    H. Mayr, W. Foerner, P.v.R. Schleyer, J. Am. Chem. Soc. 101, 6032–6040 (1979)CrossRefGoogle Scholar

Copyright information

© Springer Science+Business Media B.V. 2012

Authors and Affiliations

  1. 1.Department of Materials Science, Faculty of Science and EngineeringShimane UniversityMatsueJapan

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