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One-pot alkoxylation of benzene with alcohols occurs under photoirradiation of 3-cyano-1-methylquinolinium ion in an oxygen-saturated acetonitrile solution containing benzene and alcohols. The photocatalytic reaction mechanism was clarified by nanosecond laser flash photolysis. The photocataytic alkoxylation of benzene is initiated by photoinduced electron transfer from benzene to the singlet excited state of 3-cyano-1-methylquinolinium ion, followed by the nucleophilic addition of alcohols to benzene radical cation. In the case of photoethoxylation, the optimal product and quantum yields of ethoxybenzene were 20% and 10%, respectively.
©2012 Optical Society of America
1. Introduction
Alkoxybenzenes are used as an important precursor to pharmaceuticals, insect pheromones and perfumes [1–3]. They are prepared by the reaction of sodium phenoxide with alkyl halide or related alkylating reagents [4, 5]. Sodium phenoxide was synthesized from phenol with sodium hydroxide. Phenol is produced by a 3-step cumene process, which has serious problems as high temperature conditions and very low yield (~5%) due to the hazardous intermediate such as cumene hydroperoxide [6]. Thus, the synthesis of alkoxybenzene from benzene now requires a 4-step reaction sequence. There has so far been no report on one-pot synthesis of alkoxybenzene from benzene because of the poor reactivity of benzene. Obviously, an efficient way to activate benzene is electron removal from benzene. Benzene radical cation produced by electron-transfer oxidation can be a super electrophile. Thus, the electron-transfer oxidation of benzene is expected to easily give an efficient nucleophilic aromatic substitution. The nucleophilic substitution to aromatic radical cation has been investigated in the gas phase [7–14]. There has been no study on nucleophilic substitution of benzene such as aromatic alkoxylation in solution, because the electron-transfer oxidation of benzene in solution is relatively difficult due to the high one-electron oxidation potential of benzene, Eox = 2.48 V vs. SCE in acetonitrile (MeCN) [15]. We have recently found that 3-cyano-1-methylquinolinium ion (QuCN+) has an extremely strong oxidizing ability to easily oxidize benzene by electron transfer upon photoexcitation in MeCN [16]. We report herein photocatalytic alkoxylation of benzene with QuCN+ via formation of benzene radical cation, which reacts with alcohols to yield the corresponding alkoxybenzene.
2. Photocatalytic alkoxylation of benzene with alcohols
Photocatalytic alkoxylation of benzene (Fig. 1 ) occurs under photoirradiation of an oxygen-saturated acetonitrile (MeCN) solution containing 3-cyano-1-methyquinolinium perchlorate (QuCN+ ClO4–) (5.0 mM), benzene (30 mM) and methanol (1.0 M) with a xenon lamp (500 W, λ > 290 nm) to yield methoxybenzene and H2O2 (Fig. 1). The time course of the photocatalytic reaction is shown in Fig. 2(a) . The yield of methoxybenzene after 4h photoirradiation was 26%, which was determined by GC-MS and 1H NMR analyses. H2O2 was also detected by iodometry [17–19]. When methanol was replaced by ethanol, iso-propanol and tert-butanol, the photocatalytic alkoxylation of benzene also occurred to yield the corresponding alkoxybenzenes (Fig. 2(b)-2(d)). The quantum yield of formation of the corresponding alkoxybenzene (Φ) was determined from the initial rate of the product. The Φvalue increased with increasing of concentration of ethanol and reached a limiting value, Φ=10% as shown in Fig. 3 . The pertinent data of the photocatalytic alkoxylation of benzene are summarized in Table 1 .
Fig. 2 Time profiles of consumption of benzene (blue) and formation of alkoxybenzene (red) in the photocatalytic alkoxylation of benzene with (a) methanol, (b) ethanol, (c) iso-propanol and (d) tert-butanol catalyzed by QuCN+ in oxygen-saturated MeCN (1.0 mL) at 298 K.
Table 1. Conversions, selectivities, yields, and quantum yields (Φ) of photocatalytic alkoxylation of benzene with alcohols in MeCN[a]
3. Reaction mechanism of alkoxylation
Photoirradiation of the absorption band of QuCN+ results in fluorescence in MeCN as shown in Fig. 4(a) . The fluorescence of QuCN+ was quenched by benzene, and the quenching rate constants (kq) were determined from the slopes of the Stern-Volmer plots (Fig. 4(b)) and lifetimes of the singlet excited state of the QuCN+ (1QuCN+*, τ = 49 ns) [20]. The kq value thus obtained is 2.0 x 1010 M–1 s–1, which is close to the diffusion rate constant in MeCN. The free energy change of photoinduced electron transfer from benzene to 1QuCN+* is determined from the one-electron oxidation potential of benzene (Eox = 2.48 V vs. SCE) [15], the one-electron reduction potential of QuCN+ (Ered = –0.60 V vs. SCE) and the singlet excited energy of QuCN+ (1E* = 3.32 eV) [18] to be negative (ΔGet = e(Eox – Ered) – 1E* = –0.24 eV). Thus, the photocatalytic alkoxylation of benzene is made possible by the electron-transfer oxidation of benzene by 1QuCN+*.
Fig. 4 (a) Fluorescence spectra of QuCN+ (20 μM) with various concentrations of benzene (0 – 4.0 mM) in deaerated MeCN excited at 330 nm. (b) Stern-Volmer plot for the fluorescence quenching of 1QuCN+* by benzene.
The occurrence of photoinduced electron transfer from benzene to 1QuCN+* is confirmed by nanosecond laser flash photolysis. Nanosecond laser excitation at 355 nm of QuCN+ in deaerated MeCN containing benzene (1.0 M) afforded transient absorption spectrum at 1 μs with appearance of new absorption bands at 520 and 850 nm as shown in Fig. 5(a) . The transient absorption band at 520 nm is assigned to the one-electron reduced QuCN+ (QuCN•). The near-IR absorption around 850 nm is due to benzene dimer radical cation [15, 16]. Theabsorption bands at 520 and 850 nm decay second-order kinetics due to the bimolecular back electron transfer from QuCN• to benzene dimer radical cation or benzene radical cation (Fig. 5(b)) [16]. When methanol is introduced to the QuCN+/benzene system, the broad absorption disappears as shown in Fig. 5(b), whereas the absorption band at 520 nm due to QuCN• remains [15, 16]. The decay time profile in the presence of methanol obeys first-order kinetics. The observed decay rate constant (kobs) increased linearly with increasing concentration of methanol as shown in Fig. 5(c). The rate constant of the reaction of benzene dimer radical cation or benzene radical cation with methanol (kMeOH) was determined to be 4.3 × 106 M–1 s–1 from the slope of Fig. 5(c). The rate constants of benzene radical cation with EtOH, i- PrOH, and t-BuOH were also determined to be 4.4 × 106, 1.5 × 107, and 1.9 × 106 M–1 s–1, respectively. Although the largest rate constant was obtained for i-PrOH, the yield of alkoxylation with i-PrOH was the smallest (Table 1). This suggests that the electron-transfer oxidation of i-PrOH may occur with benzene radical cation instead of the nucleophilic addition, in fact, small amount of acetone was formed as an oxygenated product of i-PrOH.
Fig. 5 (a) Transient absorption spectrum of an MeCN solution of QuCN+ and benzene (1.0 M) taken at 1 μs after nanosecond laser excitation (λex = 355 nm). (b) Decay time profiles of absorbance at 850 nm with various concentrations of MeOH (0 – 42 mM). (c) Plots of decay rate constants vs. concentration of alcohols.
The photocatalytic reaction is initiated by photoinduced electron transfer from benzene to 1QuCN+* as shown in Fig. 6 . Benzene radical cation, formed by the photoinduced electron transfer, exists in equilibrium with benzene dimer radical cation due to the large excess of benzene. It reacts with alcohol to yield the alkoxide-adduct radical. The radical QuCN•, formed by electron transfer to 1QuCN+*, can reduce O2 to O2•– and this is followed by protonation of O2•– to afford HO2•. The hydrogen abstraction of HO2• from OH-adduct radical affords alkoxybenzene and H2O2 (Fig. 6).
4. Cyclization of 3-phenyl-1-propanol catalyzed by QuCN+
Cyclization of 3-phenyl-1-propanol is one example of many of the nucleophilic capture of organic radical cations by tethered OH functions [21, 22]. Interestingly, photocatalytic cyclization occurred under photoirradiation of QuCN+ClO4– (5.0 mM) in an oxygen-saturated MeCN containing 3-phenyl-1-propanol to give a cyclization product, chroman (Fig. 7 ). The Φ value of formation of chroman was 3% and the yield of chroman after 15 min photoirradiation was 30%. The photocyclization is initiated by photoinduced electron transfer from 3-phenyl-1-propanol to 1QuCN+* to produce the radical cation. The cationic charge is localized on the phenyl group, to which the OH group attacks to yield chroman.
5. Conclusion
In summary, the use of QuCN+ as a photocatalyst has enabled to accomplish the one-pot photoalkoxylation from benzene to alkoxybenzene via photoinduced electron-transfer oxidation of benzene under homogeneous and ambient conditions. Benzene radical cation produced in the photoinduced electron transfer is a strong electrophile, being able to react with alcohols to afford alkoxybenzenes.
Acknowledgments
This work was supported by a Grant-in-Aid (Nos. 2370014 to K.O. and 20108010 to S.F.) and a Global COE program, “the Global Education and Research Center for Bio-Environmental Chemistry” from the Ministry of Education, Culture, Sports, Science and Technology, Japan,and KOSEF/MEST through WCU project (R31-2008-000-10010-0), Korea.
References and links
1. H. Fiege, H.-W. Voges, T. Hamamoto, S. Umemura, T. Iwata, H. Miki, Y. Fujita, H.-J. Buysch, D. Garbe, and W. Paulus, Phenol derivatives in Ullmann's encyclopedia of industrial chemistry (Wiley-VCH, Weinheim 2002).
2. J. J. Li and D. S. Johnson, Modern Drug Synthesis (Wiley, Hoboken, 2010).
3. S. Enthaler and A. Company, “Palladium-catalysed hydroxylation and alkoxylation,” Chem. Soc. Rev. 40(10), 4912–4924 (2011). [CrossRef] [PubMed]
4. G. S. Hiers and F. D. Hager, “Anisole,” Org. Syn. Coll. 1, 58 (1941).
5. J. M. Miller, K. H. So, and J. H. Clark, “Fluoride ion promoted synthesis of alkyl phenyl ethers,” Can. J. Chem. 57(14), 1887–1889 (1979). [CrossRef]
6. R. Molinari and T. Poerio, “Remarks on studies for direct production of phenol in conventional and membrane reactors,” Asia-Pac. J. Chem. Eng. 5(1), 191–206 (2010).
7. T. Maeyama and N. Mikami, “Nucleophilic substitution within the photoionized van der Waals complex: generation of C6H5NH3+ from C6H5Cl-NH3,” J. Am. Chem. Soc. 110(21), 7238–7239 (1988). [CrossRef]
8. T. Maeyama and N. Mikami, “Nucleophilic substitution within the photoionized van der Waals complex chlorobenzene-ammonia,” J. Phys. Chem. 94(18), 6973–6977 (1990). [CrossRef]
9. T. Maeyama and N. Mikami, “Intracluster ion molecule reactions within the photoionized van der Waals complexes of fluorobenzene with ammonia and with water,” J. Phys. Chem. 95(19), 7197–7204 (1991). [CrossRef]
10. H. Tachikawa, “Intramolecular SN2 reaction caused by photoionization of benzene chloride-NH3 complex: direct ab initio molecular dynamics study,” J. Phys. Chem. A 110(1), 153–159 (2006). [CrossRef] [PubMed]
11. D. Thölmann and H.-F. Grützmacher, “Aromatic substitution of halobenzenes in the gas phase: A kinetic study by FT-ICR spectrometry,” Chem. Phys. Lett. 163(2-3), 225–229 (1989). [CrossRef]
12. D. Thölmann and H.-F. Grützmacher, “Reactions of dihalobenzene radical cations with ammonia in the gas phase. Reactivity pattern for nucleophilic aromatic substitution,” J. Am. Chem. Soc. 113(9), 3281–3287 (1991). [CrossRef]
13. G. Bouchoux, R. Flammang, J. De Winter, and P. Gerbaux, “Reactions of ionized methyl benzoate with methyl isocyanide in the gas phase: nucleophilic aromatic substitutions vs hydrogen migrations,” J. Phys. Chem. A 113(41), 11075–11083 (2009). [CrossRef] [PubMed]
14. G. Bouchoux, J. De Winter, R. Flammang, and P. Gerbaux, “Aromatic substitution reactions between ionized benzene derivatives and neutral methyl isocyanide,” J. Phys. Chem. A 114(27), 7408–7416 (2010). [CrossRef] [PubMed]
15. E. Baciocchi, T. Del Giacco, O. Lanzalunga, P. Mencarelli, and B. Procacci, “Photosensitized oxidation of alkyl phenyl sulfoxides. C-S bond cleavage in alkyl phenyl sulfoxide radical cations,” J. Org. Chem. 73(15), 5675–5682 (2008). [CrossRef] [PubMed]
16. K. Ohkubo, T. Kobayashi, and S. Fukuzumi, “Direct oxygenation of benzene to phenol using quinolinium ions as homogeneous photocatalysts,” Angew. Chem. Int. Ed. Engl. 50(37), 8652–8655 (2011). [CrossRef] [PubMed]
17. R. D. Mair and A. J. Graupner, “Determination of organic peroxides by iodine liberation procedures,” Anal. Chem. 36(1), 194–204 (1964). [CrossRef]
18. S. Fukuzumi, S. Kuroda, and T. Tanaka, “Flavin analogue-metal ion complexes acting as efficient photocatalysts in the oxidation of p-methylbenzyl alcohol by oxygen under irradiation with visible light,” J. Am. Chem. Soc. 107(11), 3020–3027 (1985). [CrossRef]
19. S. Fukuzumi, M. Ishikawa, and T. Tanaka, “Mechanisms of photo-oxidation of NADH model compounds by oxygen,” J. Chem. Soc. Perkin Trans. 2(8), 1037–1045 (1989).
20. H. Kitaguchi, K. Ohkubo, S. Ogo, and S. Fukuzumi, “Electron-transfer oxidation properties of unsaturated fatty acids and mechanistic insight into lipoxygenases,” J. Phys. Chem. A 110(5), 1718–1725 (2006). [CrossRef] [PubMed]
21. H. Togo and M. Katohgi, “Synthetic uses of organohypervalent iodine compounds through radical pathways,” Synlett 2001(5), 565–581 (2001).
22. S. Furuyama and H. Togo, “An efficient preparation of chroman derivatives from 3-aryl-1-propanols and related compounds with 1,3-diiodo-5,5-dimethylhydantoin under irradiation conditions,” Synlett 2010(15), 2325–2329 (2010). [CrossRef]
