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TEMPO oxidation cascade reaction
Journal of Photochemistry and Photobiology A: Chemistry 233 (2012) 46–49
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Conversion of aryl C O to C C bond through a UV light activation/TEMPO oxidation cascade reaction Baoquan Gou a , Dazhi Li a , Chao Yang a , Wujiong Xia a,∗ , Yali Li b , Xiaoming Chen b a b
State Key Lab of Urban Water Resource and Environment & the Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150080, China Zhejiang Provincial Key Laboratory of Medical Genetics, Wenzhou Medical College, Wenzhou 325035, China
a r t i c l e
i n f o
Article history: Received 7 November 2011 Received in revised form 8 January 2012 Accepted 1 February 2012 Available online 24 February 2012
a b s t r a c t Metal free conversion of aryl C O to C C bond through a photochemical rearrangement/oxidation cascade reaction is described. Irradiation of O-acetyl aryloxy benzene derivatives in benzene solution undergoes a unique photochemical rearrangement reaction to afford the ketal compounds, which are sequentially oxidized by 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) to yield the diketone compounds. © 2012 Elsevier B.V. All rights reserved.
Keywords: Photochemical rearrangement Oxidation Ketal Diketone
1. Introduction
2. Experimental
Conversion of C O to C C bond has received considerable interests from organic chemists because it provides a new pathway to broaden the diversity of the functional molecules and could be utilized in synthesis of biologically active compounds [1]. Recently, some elegant examples have been reported for direct transformations of aryl C OR (R = alkyl) to C C bond in the presence of metallic catalysts [2], such a research issue, however, still remains as a challenging task for organic chemists owing to the high energy of C O bond and requires to develop more environmentally friendly methods. In the past decades, light has always been regarded as a powerful and green energy source for chemists to perform reactions [3]. Nowadays, organic photochemical reactions have received a veritable revival of activity in both academics and industry, especially in the context of green chemistry or total synthesis [4]. Compared with the classical reactions in the ground state, one of the big advantages of the photochemical reaction is the generally mild conditions required for substrate activation, ideally light alone. With our continuous efforts in the field of organic photoreactions [5], recently, we discovered a new approach for direct conversions of aromatic C O to C C bonds through a unique photochemical rearrangement/oxidation cascade reaction. In this paper, we present what we have learned up to date about this unusual reaction.
All the starting materials and reagents were commercially available and used without further purification. The substrates for scope investigation were synthesized in straightforward pathways and described in Supporting Information. TLC analysis was performed on the pre-coated glass plates and the silica-gel (200–300 mesh) was used for column chromatography. General Procedure for the irradiation of 8 in benzene [6] solution. The solutions of 8 (60 mg) and TEMPO (1 equiv.) in benzene (30 mL) were purged with bubbling nitrogen gas for 30 min and then irradiated through Pyrex filter with a 450-W mediumpressure mercury lamp for 2 h. The solvent was removed in vacuo and the residues were purified by silica gel chromatography to give corresponding diketone product 9.
∗ Corresponding author. Tel.: +86 451 86403193; fax: +86 451 86403760. E-mail address: [email protected] (W. Xia). 1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jphotochem.2012.02.008
3. Results and discussion The reactant 4 chosen for preliminary investigations was synthesized from methyl 4-hydroxybenzoate 1 in a straightforward way. Treatment of 1 with Ac2 O/Et3 N in CH2 Cl2 afforded compound 2 in 78% yield, which was then converted to compound 3 through a Fries rearrangement reaction in the presence of AlCl3 in 73% yield [7]. The reactant 4 was obtained by protection of hydroxyl group of 3 with benzyl bromide in 91% yield. The photochemical behavior of compound 4 was conducted by irradiation of 4 with a 450 W medium-pressure mercury lamp through a Pyrex filter in the benzene and acetone solution, respectively. Irradiation of 4 in the
B. Gou et al. / Journal of Photochemistry and Photobiology A: Chemistry 233 (2012) 46–49
OH
OAc
OH O AlCl3, 155 C
CH2Cl2
MeOH,H2SO4
CO2Me
PhCH2Br
o
Ac2O, Et3N 78%
73%
CO2Me
1
K2CO3, DMF CO2Me
HO MeO2C
H O
O
OH H
MeO2C +
O
91%
Ph
hv 47%
4 Benzene , hv 59% O MeO2C
O
Ph
trans- 5
cis-5
Acetone
MeO2C
3
2
47
6
O
cis/trans=79:21 Scheme 1. Synthesis and photochemical studies of compound 4.
acetone solution for 1 h led to the photocyclization products cis-5 and trans-5 with a ratio of 79:21 in 47% isolated yield, whereas in the benzene solution the irradiation of 4 for 3.5 h afforded compound 6 in 59% yield. The conversion of 4 to compound 5 provides a direct and efficient approach to the syntheses of benzofurans, which has been well documented in the past few years [8]. Noteworthy is the synthesis of 6 from 4, which is resulted from a novel photochemical transformation of ether to ketone group, establishing an easy access to the conversion of aromatic C O to C C bond in the absence of metallic catalysts. We are intrigued to explore this unique photochemical reaction in details (Scheme 1). Taking into account that the photochemical behavior of 4 is determined by the solvent, a variety of solvents, such as CH3 CN, CH2 Cl2 , CH3 CO2 CH2 CH3 , THF, MeOH and CHCl3 , therefore were employed as the reaction media. Except of the reaction in MeOH, in which irradiation of 4 led to the photocyclization products with a ratio of cis-5/trans-5 25:75, the reaction in other solvents gave the
complicated products in which no major product could be isolated for identification. We also speculated that diketone compound 6 might be generated from oxidation of the benzylic alcohol by air based upon initial estimate of the reaction mechanism and comparison of the molecular weight. To add credence to our hypothesis, the time-dependent 1 H NMR experiments were employed in the course of investigations on 4. Here, 10 mg of compound 4 dissolved in 0.75 mL C6 D6 in an NMR tube was purged with N2 for 30 min, which was then subjected to irradiation with a 450 W medium-pressure mercury lamp through a Pyrex filter and 1 H NMR analysis, the characteristic chemical shifts were depicted in Fig. 1. As shown in Fig. 1, after irradiation for 10 min, the reaction mixture contains both starting material 4 and photoproduct 7 in isomers with a 52% conversion of 4, and after 30 min, compound 4 was completely consumed and smoothly led to cis-7 and trans-7 with a ration of 43:57. To improve
Fig. 1. Time-dependent 1 H NMR experiments of irradiation of 4.
48
B. Gou et al. / Journal of Photochemistry and Photobiology A: Chemistry 233 (2012) 46–49
Table 1 A survey of the oxidation reagents on the conversion of 4–6. Entry
Reaction conditions
Conversion (%)
Yield (%)
1 2
Benzene, h, O2 Benzene, h, TEMPO (1 equiv.), without deoxygenation Benzene, h, H2 O2 (1 equiv.) Benzene, h, CrO3 (1 equiv.) Benzene, h, TEMPO (1 equiv.), N2
86 86
42 49
97 96 99
8 33 65
3 4 5
the chemoselectivity and yield of the product, the reaction condition was thus optimized by a survey on the influence of additives. A variety of oxidation reagents were employed in the reaction, and the results were summarized in Table 1.
As can be seen, compound 6 was obtained in low yield to moderate yield when O2 , H2 O2 , CrO3 or TEMPO was employed in the reaction as a terminal oxidant, among which the application of 1 equiv. of TEMPO and deoxygenation with N2 gave the highest yield (entry 5). Utilization of more TEMPO in the reaction, e.g. 2 equiv., had no efficiency on the improvement of the yield. We next applied the optimized reaction conditions with TEMPO (1 equiv.), benzene, N2 and UV light with a Pyrex filter to a series of substrates to explore the scope of the reaction [9]. The results were described in Table 2. As can be seen, irradiation of 8 in benzene solution led to diketone compound 9 as the major products in 34–68% yields [10]. The presence of electron-withdrawing or electron-donating substitution groups on phenol or benzyl groups were both tolerated for the reaction conditions. However, the presence of electron-withdrawing substitutes gave higher yields
Table 2 Substrate scope for the reaction.a .
O
O
R1
R1
hv, benzene O 8
R2
N2, TEMPO (1 equiv) R2
9
O
Entry
Substrate
Conversion (%)b
Yield (%)c
1 2 3 4 5 6 7 8
R1 = H, R2 = H R1 = H, R2 = CO2 Et R1 = H, R2 = [1,3]-dioxylmethane R1 = CO2 Et, R2 = H R1 = CO2 Et, R2 = CO2 Et R1 = CO2 Et, R2 = [1,3]-dioxylmethane R1 = H, R2 = F R1 = CO2 Et, R2 = F
48 81 97 99 94 >99 >99 >99
36 45 34 65 57 53 55 68
–
–
O
9 a b c
O
Reaction conditions: TEMPO (1 equiv.), benzene, N2 , a 450-W medium-pressure mercury lamp with a Pyrex filter, irradiation for 2 h. Determined by GC analysis. Isolated yield.
Fig. 2. Plausible mechanism of the photochemical reaction.
B. Gou et al. / Journal of Photochemistry and Photobiology A: Chemistry 233 (2012) 46–49
(entries 2, 4–5, 7–8). Compared with above examples, low conversion was obtained in the case of O-benzylacetophenone (entry 1). This might be owing to the fates of radical intermediate generated in the photochemical process that the reverse hydrogen transfer to regenerate the starting material is more favorable than others [11]. It should be pointed out that no reaction was observed when the alkyl group replaced the benzyl group even if prolonged time was employed, which might be ascribed to the stability of the radicals formed in the process of reaction (entry 9). To rationalize the results of the reaction, a possible mechanism was proposed and depicted in Fig. 2. Under irradiation with the UV light, ketone 8 was excited to 1,2-biradicals through an intersystem crossing, which was then followed by the ␦-hydrogen abstraction affording the 1,5-biradicals 11. As the reaction was conducted in acetone solution, 1,5-biradicals was cyclized to form the furan compound 12 via an intersystem crossing [12]. While the reaction was conducted in benzene, epoxy compound 13 was formed, containing an intramolecular hydrogen bonding between hydroxyl group and the epoxy ring, which then underwent an epoxide rearrangement to regenerate the aromatic ring and form the alcohol 14. Sequential oxidation of the alcohol by TEMPO led to the final product 9. 4. Conclusions In conclusion, a new pathway for direct conversion of aromatic C O to C C through a photochemical rearrangement /oxidation cascade reaction was developed. It is noteworthy that the reaction provides a unique access to the benzoylation of the aromatic rings substituted with electronic-withdrawing groups. For example, treatment of methyl 3-acetylbenzoate with the typical Friedal–Crafts reaction conditions, e.g. AlCl3 , benzoyl chloride, did not form the product 9 (Table 2, entry 4). In addition, the research presented in this paper would enrich the field of application of organic photoreactions in organic synthesis. We are continuing to explore other novel photochemical reactions as well as their mechanisms. Acknowledgments We are grateful for the financial supports from China NSFC (Nos. 20802013, 21002018 and 21072038), the Fundamental Research Funds for the Central Universities (No. HIT.BRET2.2010001), WZSTP (No. G20100056) and ZJSTP (No. 2011C23116).
49
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jphotochem.2012.02.008. References [1] (a) S. Yonezawa, T. Komurasaki, K. Kawada, T. Tsuri, M. Fuji, A. Kugimiya, N. Haga, S. Mitsumori, M. Inagaki, T. Nakatani, Y. Tamura, S. Takechi, T. Taishi, M. Ohtani, J. Org. Chem. 63 (1998) 5831; (b) M.K. Gurjar, B. Karumudi, C.V. Ramana, J. Org. Chem. 70 (2005) 9658; (c) F. Diederich, de A. Meijere (Eds.), Metal-Catalyzed Cross-Coupling Reactions, Wiley-VCH, New York, 2004; (d) A.F. Littke, G.C. Fu, Angew. Chem. Int. Ed. 41 (2002) 4176. [2] (a) F. Kakiuchi, M. Usui, S. Ueno, N. Chatani, S. Murai, J. Am. Chem. Soc. 126 (2004) 2706; (b) S. Ueno, E. Mizushima, N. Chatani, F. Kakiuchi, J. Am. Chem. Soc. 128 (2006) 16516; (c) J.W. Dankwardt, Angew. Chem. Int. Ed. 43 (2004) 2428; (d) B.T. Guan, S.K. Xiang, T. Wu, Z.P. Sun, B.Q. Wang, K.Q. Zhao, Z.J. Shi, Chem. Commun. (2008) 1437. [3] (a) H.D. Roth, Angew. Chem. Int. Ed. 28 (1989) 1193; (b) N. Hoffmann, Chem. Rev. 108 (2008) 1052; (c) C.H. Tung, L.Z. Wu, L.P. Zhang, B. Chen, Acc. Chem. Res. 36 (2003) 39. [4] (a) C. Yang, W. Xia, Chem. Asian J. 4 (2009) 1774; (b) W. Xia, C. Yang, B.O. Patrick, J.R. Scheffer, C. Scott, J. Am. Chem. Soc. 127 (2005) 2725; (c) M. Fleck, T. Bach, Chem. Eur. J. 16 (2010) 6015; (d) R. Jahjah, A. Gassama, V. Bulach, C. Suzuki, M. Abe, N. Hoffmann, A. Martinez, J.M. Nuzillard, Chem. Eur. J. 16 (2010) 3341; (e) V. Dichiarante, M. Fagnoni, A. Albini, Green Chem. 11 (2009) 942; (f) C. Yang, W. Xia, J.R. Scheffer, Tetrahedron 63 (2007) 6791; (g) A.G. Griesbeck, J. Neudörfl, S. Specht, A. Raabe, J. Med. Chem. 52 (2009) 3420; (h) G. Zhao, C. Yang, Q. Chen, J. Jin, X. Zhang, L. Zhao, W. Xia, Tetrahedron 65 (2009) 9952; (i) J.L. Débieux, C.G. Bochet, J. Org. Chem. 74 (2009) 4519; (j) W. Xia, J.R. Scheffer, M. Botoshansky, M. Kaftory, Org. Lett. 7 (2005) 1315; (k) H.E. Zimmerman, S. Shorunov, J. Org. Chem. 74 (2009) 5411. [5] (a) W. Xia, Y. Shao, W. Gui, C. Yang, Chem. Commun. 47 (2011) 11098; (b) G. Zhao, C. Yang, L. Guo, H. Sun, C. Chen, W. Xia, Chem. Commun. 48 (2012) 2337. [6] Benzene is hazardous and toxic, please use it very carefully. [7] A.H. Blatt, Org. React. 1 (1942) 342. [8] (a) E.M. Sharshira, E. Okamura, E. Hasegawa, T. Horaguchi, J. Heterocycl. Chem. 34 (1997) 861; (b) P.J. Wagner, J.-S. Jang, J. Am. Chem. Soc. 115 (1993) 7914; (c) P.J. Wagner, M.A. Meador, B.-S. Park, J. Am. Chem. Soc. 112 (1990) 5199. [9] For the preparations of the substrates, see Supporting Information. [10] The cyclization products and other by-products could not be excluded in the reaction due to the low isolated yield. [11] D. Braga, S. Chen, H. Filson, L. Maini, M.R. Netherton, B.O. Patrick, J.R. Scheffer, C. Scott, W. Xia, J. Am. Chem. Soc. 126 (2004) 3511. [12] For a general review of the photocyclization reaction, see: P. Wagner, B.S. Park, in: A. Padwa (Ed.), Organic Photochemistry, vol. 11, Marcel Dekker, New York, 1991 (Chapter 4).
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Conversion of aryl C O to C C bond through a UV light activation/TEMPO oxidation cascade reaction Baoquan Gou a , Dazhi Li a , Chao Yang a , Wujiong Xia a,∗ , Yali Li b , Xiaoming Chen b a b
State Key Lab of Urban Water Resource and Environment & the Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150080, China Zhejiang Provincial Key Laboratory of Medical Genetics, Wenzhou Medical College, Wenzhou 325035, China
a r t i c l e
i n f o
Article history: Received 7 November 2011 Received in revised form 8 January 2012 Accepted 1 February 2012 Available online 24 February 2012
a b s t r a c t Metal free conversion of aryl C O to C C bond through a photochemical rearrangement/oxidation cascade reaction is described. Irradiation of O-acetyl aryloxy benzene derivatives in benzene solution undergoes a unique photochemical rearrangement reaction to afford the ketal compounds, which are sequentially oxidized by 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) to yield the diketone compounds. © 2012 Elsevier B.V. All rights reserved.
Keywords: Photochemical rearrangement Oxidation Ketal Diketone
1. Introduction
2. Experimental
Conversion of C O to C C bond has received considerable interests from organic chemists because it provides a new pathway to broaden the diversity of the functional molecules and could be utilized in synthesis of biologically active compounds [1]. Recently, some elegant examples have been reported for direct transformations of aryl C OR (R = alkyl) to C C bond in the presence of metallic catalysts [2], such a research issue, however, still remains as a challenging task for organic chemists owing to the high energy of C O bond and requires to develop more environmentally friendly methods. In the past decades, light has always been regarded as a powerful and green energy source for chemists to perform reactions [3]. Nowadays, organic photochemical reactions have received a veritable revival of activity in both academics and industry, especially in the context of green chemistry or total synthesis [4]. Compared with the classical reactions in the ground state, one of the big advantages of the photochemical reaction is the generally mild conditions required for substrate activation, ideally light alone. With our continuous efforts in the field of organic photoreactions [5], recently, we discovered a new approach for direct conversions of aromatic C O to C C bonds through a unique photochemical rearrangement/oxidation cascade reaction. In this paper, we present what we have learned up to date about this unusual reaction.
All the starting materials and reagents were commercially available and used without further purification. The substrates for scope investigation were synthesized in straightforward pathways and described in Supporting Information. TLC analysis was performed on the pre-coated glass plates and the silica-gel (200–300 mesh) was used for column chromatography. General Procedure for the irradiation of 8 in benzene [6] solution. The solutions of 8 (60 mg) and TEMPO (1 equiv.) in benzene (30 mL) were purged with bubbling nitrogen gas for 30 min and then irradiated through Pyrex filter with a 450-W mediumpressure mercury lamp for 2 h. The solvent was removed in vacuo and the residues were purified by silica gel chromatography to give corresponding diketone product 9.
∗ Corresponding author. Tel.: +86 451 86403193; fax: +86 451 86403760. E-mail address: [email protected] (W. Xia). 1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jphotochem.2012.02.008
3. Results and discussion The reactant 4 chosen for preliminary investigations was synthesized from methyl 4-hydroxybenzoate 1 in a straightforward way. Treatment of 1 with Ac2 O/Et3 N in CH2 Cl2 afforded compound 2 in 78% yield, which was then converted to compound 3 through a Fries rearrangement reaction in the presence of AlCl3 in 73% yield [7]. The reactant 4 was obtained by protection of hydroxyl group of 3 with benzyl bromide in 91% yield. The photochemical behavior of compound 4 was conducted by irradiation of 4 with a 450 W medium-pressure mercury lamp through a Pyrex filter in the benzene and acetone solution, respectively. Irradiation of 4 in the
B. Gou et al. / Journal of Photochemistry and Photobiology A: Chemistry 233 (2012) 46–49
OH
OAc
OH O AlCl3, 155 C
CH2Cl2
MeOH,H2SO4
CO2Me
PhCH2Br
o
Ac2O, Et3N 78%
73%
CO2Me
1
K2CO3, DMF CO2Me
HO MeO2C
H O
O
OH H
MeO2C +
O
91%
Ph
hv 47%
4 Benzene , hv 59% O MeO2C
O
Ph
trans- 5
cis-5
Acetone
MeO2C
3
2
47
6
O
cis/trans=79:21 Scheme 1. Synthesis and photochemical studies of compound 4.
acetone solution for 1 h led to the photocyclization products cis-5 and trans-5 with a ratio of 79:21 in 47% isolated yield, whereas in the benzene solution the irradiation of 4 for 3.5 h afforded compound 6 in 59% yield. The conversion of 4 to compound 5 provides a direct and efficient approach to the syntheses of benzofurans, which has been well documented in the past few years [8]. Noteworthy is the synthesis of 6 from 4, which is resulted from a novel photochemical transformation of ether to ketone group, establishing an easy access to the conversion of aromatic C O to C C bond in the absence of metallic catalysts. We are intrigued to explore this unique photochemical reaction in details (Scheme 1). Taking into account that the photochemical behavior of 4 is determined by the solvent, a variety of solvents, such as CH3 CN, CH2 Cl2 , CH3 CO2 CH2 CH3 , THF, MeOH and CHCl3 , therefore were employed as the reaction media. Except of the reaction in MeOH, in which irradiation of 4 led to the photocyclization products with a ratio of cis-5/trans-5 25:75, the reaction in other solvents gave the
complicated products in which no major product could be isolated for identification. We also speculated that diketone compound 6 might be generated from oxidation of the benzylic alcohol by air based upon initial estimate of the reaction mechanism and comparison of the molecular weight. To add credence to our hypothesis, the time-dependent 1 H NMR experiments were employed in the course of investigations on 4. Here, 10 mg of compound 4 dissolved in 0.75 mL C6 D6 in an NMR tube was purged with N2 for 30 min, which was then subjected to irradiation with a 450 W medium-pressure mercury lamp through a Pyrex filter and 1 H NMR analysis, the characteristic chemical shifts were depicted in Fig. 1. As shown in Fig. 1, after irradiation for 10 min, the reaction mixture contains both starting material 4 and photoproduct 7 in isomers with a 52% conversion of 4, and after 30 min, compound 4 was completely consumed and smoothly led to cis-7 and trans-7 with a ration of 43:57. To improve
Fig. 1. Time-dependent 1 H NMR experiments of irradiation of 4.
48
B. Gou et al. / Journal of Photochemistry and Photobiology A: Chemistry 233 (2012) 46–49
Table 1 A survey of the oxidation reagents on the conversion of 4–6. Entry
Reaction conditions
Conversion (%)
Yield (%)
1 2
Benzene, h, O2 Benzene, h, TEMPO (1 equiv.), without deoxygenation Benzene, h, H2 O2 (1 equiv.) Benzene, h, CrO3 (1 equiv.) Benzene, h, TEMPO (1 equiv.), N2
86 86
42 49
97 96 99
8 33 65
3 4 5
the chemoselectivity and yield of the product, the reaction condition was thus optimized by a survey on the influence of additives. A variety of oxidation reagents were employed in the reaction, and the results were summarized in Table 1.
As can be seen, compound 6 was obtained in low yield to moderate yield when O2 , H2 O2 , CrO3 or TEMPO was employed in the reaction as a terminal oxidant, among which the application of 1 equiv. of TEMPO and deoxygenation with N2 gave the highest yield (entry 5). Utilization of more TEMPO in the reaction, e.g. 2 equiv., had no efficiency on the improvement of the yield. We next applied the optimized reaction conditions with TEMPO (1 equiv.), benzene, N2 and UV light with a Pyrex filter to a series of substrates to explore the scope of the reaction [9]. The results were described in Table 2. As can be seen, irradiation of 8 in benzene solution led to diketone compound 9 as the major products in 34–68% yields [10]. The presence of electron-withdrawing or electron-donating substitution groups on phenol or benzyl groups were both tolerated for the reaction conditions. However, the presence of electron-withdrawing substitutes gave higher yields
Table 2 Substrate scope for the reaction.a .
O
O
R1
R1
hv, benzene O 8
R2
N2, TEMPO (1 equiv) R2
9
O
Entry
Substrate
Conversion (%)b
Yield (%)c
1 2 3 4 5 6 7 8
R1 = H, R2 = H R1 = H, R2 = CO2 Et R1 = H, R2 = [1,3]-dioxylmethane R1 = CO2 Et, R2 = H R1 = CO2 Et, R2 = CO2 Et R1 = CO2 Et, R2 = [1,3]-dioxylmethane R1 = H, R2 = F R1 = CO2 Et, R2 = F
48 81 97 99 94 >99 >99 >99
36 45 34 65 57 53 55 68
–
–
O
9 a b c
O
Reaction conditions: TEMPO (1 equiv.), benzene, N2 , a 450-W medium-pressure mercury lamp with a Pyrex filter, irradiation for 2 h. Determined by GC analysis. Isolated yield.
Fig. 2. Plausible mechanism of the photochemical reaction.
B. Gou et al. / Journal of Photochemistry and Photobiology A: Chemistry 233 (2012) 46–49
(entries 2, 4–5, 7–8). Compared with above examples, low conversion was obtained in the case of O-benzylacetophenone (entry 1). This might be owing to the fates of radical intermediate generated in the photochemical process that the reverse hydrogen transfer to regenerate the starting material is more favorable than others [11]. It should be pointed out that no reaction was observed when the alkyl group replaced the benzyl group even if prolonged time was employed, which might be ascribed to the stability of the radicals formed in the process of reaction (entry 9). To rationalize the results of the reaction, a possible mechanism was proposed and depicted in Fig. 2. Under irradiation with the UV light, ketone 8 was excited to 1,2-biradicals through an intersystem crossing, which was then followed by the ␦-hydrogen abstraction affording the 1,5-biradicals 11. As the reaction was conducted in acetone solution, 1,5-biradicals was cyclized to form the furan compound 12 via an intersystem crossing [12]. While the reaction was conducted in benzene, epoxy compound 13 was formed, containing an intramolecular hydrogen bonding between hydroxyl group and the epoxy ring, which then underwent an epoxide rearrangement to regenerate the aromatic ring and form the alcohol 14. Sequential oxidation of the alcohol by TEMPO led to the final product 9. 4. Conclusions In conclusion, a new pathway for direct conversion of aromatic C O to C C through a photochemical rearrangement /oxidation cascade reaction was developed. It is noteworthy that the reaction provides a unique access to the benzoylation of the aromatic rings substituted with electronic-withdrawing groups. For example, treatment of methyl 3-acetylbenzoate with the typical Friedal–Crafts reaction conditions, e.g. AlCl3 , benzoyl chloride, did not form the product 9 (Table 2, entry 4). In addition, the research presented in this paper would enrich the field of application of organic photoreactions in organic synthesis. We are continuing to explore other novel photochemical reactions as well as their mechanisms. Acknowledgments We are grateful for the financial supports from China NSFC (Nos. 20802013, 21002018 and 21072038), the Fundamental Research Funds for the Central Universities (No. HIT.BRET2.2010001), WZSTP (No. G20100056) and ZJSTP (No. 2011C23116).
49
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