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Rapid and selective oxidation of benzyl alcohols to aldehydes and ketones with novel vanadium polyoxometalate under solvent-free conditions
Tetrahedron Letters 52 (2011) 2563–2565
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Rapid and selective oxidation of benzyl alcohols to aldehydes and ketones with novel vanadium polyoxometalate under solvent-free conditions Anindita Dewan a, Tridib Sarma a,b, Utpal Bora c,⇑, Dilip K. Kakati a,⇑ a
Dept. of Chemistry, Gauhati University, Guwahati 781014, Assam, India School of Chemistry, University of Hyderabad, Hyderabad 500046, Andhra Pradesh, India c Dept. of Chemistry, Dibrugarh University, Dibrugarh 786004, Assam, India b
a r t i c l e
i n f o
Article history: Received 26 January 2011 Revised 2 March 2011 Accepted 8 March 2011 Available online 15 March 2011
a b s t r a c t A novel vanadium polyoxometalate [(C6H5CH2)(CH3)3N]3[H3V10O28]3H2O works as a useful oxidant for selective and rapid oxidation of benzylic alcohols to the corresponding carbonyl compounds in the presence of PTSA under solvent-free, room temperature in excellent yield. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Polyoxometalate Oxidation Benzylic alcohol Solvent-free Vanadium
The selective oxidation of alcohols to aldehydes and ketones without forming over-oxidized product is one of the pivotal transformations in the contemporary organic synthesis.1 Traditionally, oxidation reactions are performed with stoichiometric amount of metal oxidants—notably permanganate,2 bromate3 or chromium4 based reagents. These processes usually generate abundant amount of environmentally unfriendly heavy-metal waste and therefore they are not always ideal. Moreover, several transition metals such as palladium,5 ruthenium,6 molybdenum,7 manganese,8 rhenium,9 copper,10 iron,11 cobalt,12 and platinum13 based aerobic and anaerobic systems have been developed for these transformations. However, the main drawbacks associated with these systems are expensive metals, toxic organic solvents, long reaction time, and in majority of cases they require an elevated temperature for effective catalytic activity. Additionally, some oxidation reactions have to be performed under high oxygen pressure, which cause significant inconvenience in handling. Thus development of oxidation systems which are readily accessible, air/moisture stable, inexpensive, environmentally acceptable that can promote selective oxidation under mild reaction conditions are still desirable. Among the various transition metals, vanadium exists on the surface of the earth more abundantly and vanadium based oxidants are effectively used for various oxidation reactions.14 Moreover, aqueous hydrogen peroxide is a well known oxidant because ⇑ Corresponding authors. Tel.: +91 373 2370210; fax: +91 373 2370323 (U.B.). E-mail addresses: [email protected], [email protected] (U. Bora). 0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2011.03.044
it is a cheap, mild and environmentally acceptable reagent with a high content of active oxygen and produces water as a by-product.15 Recently, Li et al. have developed a catalyst system based on H2O2, V2O5 and benzyltriethylammonium bromide for oxidation of benzyl alcohols and benzyl halides to aldehydes and ketones.16 Very recently, we have reported the synthesis and crystal structure of a novel vanadium polyoxometalate [(C6H5CH2)(CH3)3 N]3[H3V10O28]3H2O, obtained by treating V2O5 with H2O2 in presence of benzyltrimethylammonium chloride.17 Polyoxometalates are considered unique catalytic species due to their multifunctionality and structural mobility.18 Their acid and redox properties can be tuned by varying polyanion composition.18d Thus polyoxometalates have been found efficient bifunctional catalysts in various organic transformation such as alkane isomerization,19 Showa– Denko process for manufacturing acetic acid by oxidation of ethane,20 conversion of acetone to methyl isobutyl ketone21 etc. To the best of our knowledge there is no report for selective oxidation of benzylic alcohol with vanadium polyoxometalates under solvent-free, room temperature conditions. The objective of the present study is to establish the viability of the newly synthesized vanadium polyoxometalate in oxidation reactions under a green and mild reaction condition. To investigate the effectiveness of the vanadium polyoxometalate complex in oxidation reaction, 1-phenylethanol was chosen as a model substrate. Reactions were performed under different reaction conditions. Results are summarized in Table 1. Initially, we carried out the oxidation reaction by treating 1-phenylethanol (1 mmol) with vanadium polyoxometalate (0.05 mmol) using
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Table 1 Optimization of reaction conditions for V-polyoxometalate mediated oxidationa
OH
O Oxidant Solvent/Additive
a
Entry
Oxidant
Solvent/additive
pH
Temperature (°C)
Time
Yield (%)
1 2 3 4 5 6 7 8 9 10 11
V-polyoxometalate V-polyoxometalate V-polyoxometalate V-polyoxometalate V-polyoxometalate V-polyoxometalate V-polyoxometalate V-polyoxometalate V-polyoxometalate V2O5/H2O2/benzyltrimethylammonium chloride V-polyoxometalate/hydroquinone
H2O H2O H2O/HCl H2O/HCl H2O/HCl H2O/HCl H2O/PTSA H2O/PTSA PTSA PTSA PTSA
— — 3.5 2 1.5 1 3 2 — — —
rt 60 60 60 60 60 60 60 rt rt rt
24 h 24 h 4h 4h 2h 2h 2h 2h 1 min 5 min 1h
Nil Nil 65 80 85 88 75 85 98 Nil 25
Reaction conditions: 1-phenylethanol (1 mmol); V-polyoxometalate (0.05 mmol); PTSA.H2O (1.0 mmol); all yields refer to isolated yields.
H2O (5 mL) as solvent under neutral condition. The reaction did not take place at room temperature or at 60 °C (Table 1, entries 1 and 2). On acidification to pH 3.5 with HCl, the oxidation of 1-phenylethanol took place with a moderate yield (Table 1, entry 3); further decreasing the pH up to 1 increases the yield of the product and reduces the reaction time (Table 1 entries 4–6). To examine the effectiveness of an organic acid in this oxidation process, we carried out the reaction in the presence of p-toluenesulphonic acid (PTSA) (1.0 mmol). It has been observed that at pH 2 the reaction proceeded with better yield of product compared to the reaction at pH 3 (Table 1, entry 7 vs 8). Surprisingly, there was an abrupt change in the rate of the reaction when the reaction was carried out in the absence of solvent. In this particular case the alcohol was converted to its carbonyl derivative in very high yield within a minute of time, just on grinding the alcohol and oxidant in the presence of PTSA at room temperature (Table 1, entry 9).22 However, no product formation was observed when the oxidant was replaced with V2O5, H2O2 and benzyltrimethylammonium chloride under the same reaction conditions (Table 1, entry 10). Although not confirmed, it is believed that in the presence of polyoxometalate complex the reaction probably proceeded via free radical pathway, as in the presence of free radical inhibitor hydroquinone the yield of the reaction was found to be very low (Table 1, entry 11). Several test reactions were carried out using different amounts of V-polyoxometalate and PTSA. The results are shown in Table 2. It was observed that 0.05 mmol of V-polyoxometalate is required for oxidation of 1.0 mmol of the substrate. Moreover 0.50 mmol of PTSA is found to be optimized amount for this reaction (Table 2, entry 10). However, the reaction did not proceed in the absence of either V-polyoxometalate or PTSA (Table 2, entries 12 and 13). To evaluate the scope and limitations of the current procedure, oxidation reactions with a wide array of electronically diverse benzylic alcohols were examined using polyoxometalate complex. The results are summarized in Table 3. It has been seen from Table 3 that in general, various hydroxyl compounds such as benzoin, benzyl alcohol, 1-phenylethynol and various phenyl ring substituted 1-phenylethanols underwent oxidation reactions in nearly quantitative yields (Table 3, entries 1–11). The electronic nature and the position of the substituents had little effect on the reaction process. Both electron withdrawing and electron donating substituents, such as p-NO2, m-NO2, o-Cl, p-Cl, p-Br, p-OMe in the benzene ring gave the desired products in 85–90% yield (Table 3, entries 4–9). However, the presence of bulky group at position-2 on the phenyl ring required relatively longer reaction time (Table 3, entry 7). The
Table 2 Reaction with different amounts of the oxidanta Entry
V-polyoxometalate (mmol)
PTSAH2O (mmol)
Yield (%)
1 2 3 4 5 6 7 8 9 10 11 12 13
0.001 0.005 0.01 0.03 0.05 0.08 0.10 0.20 0.05 0.05 0.05 0.05 —
2.00 2.00 2.00 2.00 2.00 2.00 2.00 1.50 1.00 0.50 0.25 — 0.50
10 15 30 85 97 97 95 97 98 97 75 — —
a Reaction conditions: 1-phenylethanol (1 mmol), time 3 min, room temperature, isolated yields.
Table 3 Solvent less oxidation of benzylic of alcohols at room temperaturea Entry
Substrate
Time
Yield (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
PhCOCH(OH)Ph PhCH2OH PhCH(OH)CH3 p-NO2-PhCH2OH m-NO2-PhCH2OH p-ClPhCH2OH o-ClPhCH2OH p-BrPhCH2OH p-MeOPhCH2OH PhCH(OH)Ph p-ClPhCH(OH)Ph Cyclohexanol Cholesterol Cinnamyl alcohol 2-Thiophenemethanol Menthol Benzyl halide
1 min 1 min 1 min 1 min 1 min 1 min 2 min 1 min 1 min 1 min 1 min 5 min 5 min 5 min 5 min 5 min 5 min
97 85 97 90 90 85 85 95 90 95 93 Nil Nil Nil Nil Nil Nil
a Reaction conditions: substrate (1 mmol); V-polyoxometalate (0.05 mmol); PTSA.H2O (0.5 mmol); room temperature; yields refer to isolated yields.
oxidant was found to be highly selective for benzyl alcohols only. Other hydroxyl groups such as cyclohexanol, steroidal, cinnamoyl, menthol (Table 3, entries 12–16) were not oxidized under the reaction conditions. The oxidant was also unable to oxidize benzyl
A. Dewan et al. / Tetrahedron Letters 52 (2011) 2563–2565
chloride to benzaldehyde (Table 3, entry 17). No over-oxidized product was observed in any of the alcohol studied. Moreover, the active oxidant could be regenerated by addition of appropriate amount of benzyltrimethylammonium chloride and 30% H2O2 and reused in further oxidation reaction.22b In summary, we have developed an efficient method for the selective and rapid oxidation of benzylic alcohols to the corresponding aldehydes and ketones under solvent less condition at room temperature. Synthesis of this oxidant is very simple and starting materials used are commercially available and cheap. The oxidant is also very stable hence, can be stored and handled very easily. The reaction time is very fast and no over-oxidized product was observed. After the reaction, the active oxidant can be regenerated and reused for further oxidation reaction hence minimizing the waste generation. References and notes 1. (a) Lenoir, D. Angew. Chem., Int. Ed. 2006, 45, 3206–3210; (b) Hudlick, M. Oxidations in Organic Chemistry; American Chemical Society: Washington DC, 1990; (c) Sheldon, R. A.; Kochi, J. A. Metal-Catalyzed Oxidations of Organic Compounds; Academic Press: New York, 1981; (d) Schultz, M. J.; Sigman, M. S. Tetrahedron 2006, 62, 8227–8241. 2. Menger, F. M.; Lee, C. Tetrahedron Lett. 1981, 22, 1655–1656. 3. Lee, K. C.; Koo, B. S.; Lee, Y. S.; Cho, H.; Lee, K. K-J. Bull. Korean Chem. Soc. 2002, 23, 1667–1670. 4. Cainelli, G.; Cardillo, G. Chromium Oxidations in Organic Chemistry; SpringerVerlag: Berlin, 1984. 5. (a) Seddon, K. R.; Stark, A. Green Chem. 2002, 4, 119–123; (b) Peterson, K. P.; Larock, R. C. J. Org. Chem. 1998, 63, 3185–3189. 6. (a) Shi, F.; Tse, M.; Beller, M. Chem. Asian J. 2007, 2, 411–415; (b) Wolfson, A.; Wuyts, S.; De, V. D.; Vankelecom, I. F. J.; Jacobs, P. A. Tetrahedron Lett. 2002, 43, 8107–8110. 7. Biradar, A. V.; Dongare, M. K.; Umbarkar, S. B. Tetrhedron Lett. 2009, 50, 2885– 2888. 8. Najafi, G. R. Chin. Chem. Lett. 2010, 21, 1162–1164. 9. Bianchini, G.; Crucianelli, M.; Angelis, F.; De Neri, V.; Saladino, R. Tetrahedron Lett. 2005, 46, 2427–2432. 10. Gamez, P.; Arends, I. W. C. E.; Sheldon, R. A.; Reedijik, J. Adv. Synth. Catal. 2004, 346, 805–811. 11. (a) Martin, S. E.; Suarez, D. F. Tetrahedron Lett. 2002, 43, 4475–4479; (b) Kumar, A.; Jain, N.; Chauhan, S. M. S. Synlett 2007, 411–414. 12. (a) Sharma, V. B.; Jain, S. L.; Sain, B. Tetrahedron Lett. 2003, 44, 383–386; (b) Jain, S. L.; Sain, B. J. Mol. Catal. A Chem. 2001, 176, 101–104.
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13. Tonucci, L.; Nicastro, M.; Alessandro, N. d’.; Bressan, M.; Ambrosio, P. D’.; Morvillo, A. Green Chem. 2009, 11, 816–820. 14. (a) Velusamy, S.; Punniyamurthy, T. Org. Lett. 2004, 6, 217–219; (b) Csanyi, L. J.; Jaky, K.; Dombi, G.; Evanics, F.; Dezso, G.; bKota, Z. J. Mol. Catal. A Chem. 2003, 195, 101–111; (c) Maeda, Y.; Kakiuchi, N.; Matsumura, S.; Nishimura, T.; Kawamura, T.; Uemura, S. J. Org. Chem. 2002, 67, 6718–6724; (d) Kirihara, M.; Ochiai, Y.; Takizawa, S.; Takahata, H.; Nemoto, H. Chem. Commun. 1999, 1387– 1388; (e) Kobayashi, H.; Yamanaka, I. Chem. Lett. 2007, 36, 114–115. 15. Ishii, Y.; Yamawaki, K.; Ura, T.; Yamada, H.; Yoshida, T.; Ogawa, M. J. Org. Chem. 1988, 53, 3587–3593. 16. Li, C.; Zheng, P.; Li, J.; Zhang, H.; Cui, Y.; Shao, Q.; Ji, X.; Zhang, J.; Zhao, P.; Xu, Y. . Angew. Chem., Int. Ed. 2003, 42, 5063–5066. 17. Dewan, A.; Kakati, D. K.; Das, B. K. Indian J. Chem. 2010, 49A, 39–44. 18. (a) Maeda, Y.; Kakiuchi, N.; Mastsumura, S.; Nishimura, T.; Uemura, S. Tetrahedron Lett. 2001, 42, 8877–8879;; (b) Kodama, S.; Ueta, Y.; Yoshida, J.; Nomoto, A.; Yano, S.; Ueshima, M.; Ogawa, A. Dalton Trans. 2009, 9708–9711; (c) Radosevich, A. T.; Musich, C.; Toste, F. D. J. Am. Chem. Soc. 2005, 127, 1090– 1091; (d) Okuhara, T.; Mizuno, N.; Misono, M. Adv. Catal. 1996, 41, 113–252; (e) Kozhevnikov, I. V. Chem. Rev. 1998, 98, 171–198; (f) Misono, M. Chem. Commun. 2001, 1141–1152; (g) Kozhevnikov, I. V. Catalysts for Fine Chemicals. Vol. 2. Catalysis by Polyoxometalates; Wiley, Chichester: England, 2002. 19. (a) Baba, T.; Hasada, Y.; Nomura, M.; Ohno, Y.; Ono, Y. J. Mol. Catal. A Chem. 1996, 114, 247–255; (b) Na, K.; Okuhara, T.; Misono, M. Chem. Commun. 1993, 1422–1423; (c) Okuhara, T. Appl. Catal. A 2003, 256, 213–224. 20. Sano, K.; Uchida, H.; Wakabayashi, S. Catal. Surv. Jpn. 1999, 3, 55–60. 21. Hetterley, R. D.; Kozhevnikova, E. F.; Kozhevnikov, I. V. Chem. Commun. 2006, 782–784. 22. (a) General procedure for oxidation of benzylic alcohol: 1-phenylethanol (0.122 gm, 1.0 mmol), vanadium polyoxometalates [C7H7N(CH3)3]3V10O28H33H2O] (0.073 g, 0.05 mmol) and PTSAH2O (0.095 g, 0.5 mmol) were mixed at room temperature, ground thoroughly in a mortar until the mixture became homogeneous. The yellow colour of the mixture became green instantaneously on grinding, indicating progress of the reaction. The reaction mixture was diluted with ether (30 mL) and washed with water (20 mL). The organic layer was washed with brine (3 20 mL), dried over anhydrous Na2SO4, concentrated, and finally purified by column chromatography on silica, using ethyl acetate/petroleum ether (1:9) as the eluent to obtain acetophenone in 97% yield. 1H NMR (CDCl3, 300 MHz): d 7.9– 8.2 (m, 2H), 7.4–7.7 (m, 3H), 2.6 (s, 3H); 13C NMR (CDCl3, 75.5 MHz): d 197.4, 136.9, 134.2, 129.3, 129.9, 26.5; IR (KBr): 1688 cm 1. (b) Regeneration of the oxidant: The volume of the aqueous extract (20 mL) was reduced to one fourth under reduced pressure and 30% H2O2 (2 mL) and benzyltrimethylammonium chloride (0.046 mg, 0.25 mmol) was added and stirred at room temperature for one hour. A pale yellowish coloured precipitate appeared. The precipitate was recovered by filtration under suction and dried under vacuum over fused CaCl2. The regenerated oxidant was used for oxidation of 1-phenylethanol following the above mentioned procedure and found to be effective for three cycles, yields: 92% (cycle-1), 87% (cycle-2), (82% cycle-3).
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Rapid and selective oxidation of benzyl alcohols to aldehydes and ketones with novel vanadium polyoxometalate under solvent-free conditions Anindita Dewan a, Tridib Sarma a,b, Utpal Bora c,⇑, Dilip K. Kakati a,⇑ a
Dept. of Chemistry, Gauhati University, Guwahati 781014, Assam, India School of Chemistry, University of Hyderabad, Hyderabad 500046, Andhra Pradesh, India c Dept. of Chemistry, Dibrugarh University, Dibrugarh 786004, Assam, India b
a r t i c l e
i n f o
Article history: Received 26 January 2011 Revised 2 March 2011 Accepted 8 March 2011 Available online 15 March 2011
a b s t r a c t A novel vanadium polyoxometalate [(C6H5CH2)(CH3)3N]3[H3V10O28]3H2O works as a useful oxidant for selective and rapid oxidation of benzylic alcohols to the corresponding carbonyl compounds in the presence of PTSA under solvent-free, room temperature in excellent yield. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Polyoxometalate Oxidation Benzylic alcohol Solvent-free Vanadium
The selective oxidation of alcohols to aldehydes and ketones without forming over-oxidized product is one of the pivotal transformations in the contemporary organic synthesis.1 Traditionally, oxidation reactions are performed with stoichiometric amount of metal oxidants—notably permanganate,2 bromate3 or chromium4 based reagents. These processes usually generate abundant amount of environmentally unfriendly heavy-metal waste and therefore they are not always ideal. Moreover, several transition metals such as palladium,5 ruthenium,6 molybdenum,7 manganese,8 rhenium,9 copper,10 iron,11 cobalt,12 and platinum13 based aerobic and anaerobic systems have been developed for these transformations. However, the main drawbacks associated with these systems are expensive metals, toxic organic solvents, long reaction time, and in majority of cases they require an elevated temperature for effective catalytic activity. Additionally, some oxidation reactions have to be performed under high oxygen pressure, which cause significant inconvenience in handling. Thus development of oxidation systems which are readily accessible, air/moisture stable, inexpensive, environmentally acceptable that can promote selective oxidation under mild reaction conditions are still desirable. Among the various transition metals, vanadium exists on the surface of the earth more abundantly and vanadium based oxidants are effectively used for various oxidation reactions.14 Moreover, aqueous hydrogen peroxide is a well known oxidant because ⇑ Corresponding authors. Tel.: +91 373 2370210; fax: +91 373 2370323 (U.B.). E-mail addresses: [email protected], [email protected] (U. Bora). 0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2011.03.044
it is a cheap, mild and environmentally acceptable reagent with a high content of active oxygen and produces water as a by-product.15 Recently, Li et al. have developed a catalyst system based on H2O2, V2O5 and benzyltriethylammonium bromide for oxidation of benzyl alcohols and benzyl halides to aldehydes and ketones.16 Very recently, we have reported the synthesis and crystal structure of a novel vanadium polyoxometalate [(C6H5CH2)(CH3)3 N]3[H3V10O28]3H2O, obtained by treating V2O5 with H2O2 in presence of benzyltrimethylammonium chloride.17 Polyoxometalates are considered unique catalytic species due to their multifunctionality and structural mobility.18 Their acid and redox properties can be tuned by varying polyanion composition.18d Thus polyoxometalates have been found efficient bifunctional catalysts in various organic transformation such as alkane isomerization,19 Showa– Denko process for manufacturing acetic acid by oxidation of ethane,20 conversion of acetone to methyl isobutyl ketone21 etc. To the best of our knowledge there is no report for selective oxidation of benzylic alcohol with vanadium polyoxometalates under solvent-free, room temperature conditions. The objective of the present study is to establish the viability of the newly synthesized vanadium polyoxometalate in oxidation reactions under a green and mild reaction condition. To investigate the effectiveness of the vanadium polyoxometalate complex in oxidation reaction, 1-phenylethanol was chosen as a model substrate. Reactions were performed under different reaction conditions. Results are summarized in Table 1. Initially, we carried out the oxidation reaction by treating 1-phenylethanol (1 mmol) with vanadium polyoxometalate (0.05 mmol) using
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Table 1 Optimization of reaction conditions for V-polyoxometalate mediated oxidationa
OH
O Oxidant Solvent/Additive
a
Entry
Oxidant
Solvent/additive
pH
Temperature (°C)
Time
Yield (%)
1 2 3 4 5 6 7 8 9 10 11
V-polyoxometalate V-polyoxometalate V-polyoxometalate V-polyoxometalate V-polyoxometalate V-polyoxometalate V-polyoxometalate V-polyoxometalate V-polyoxometalate V2O5/H2O2/benzyltrimethylammonium chloride V-polyoxometalate/hydroquinone
H2O H2O H2O/HCl H2O/HCl H2O/HCl H2O/HCl H2O/PTSA H2O/PTSA PTSA PTSA PTSA
— — 3.5 2 1.5 1 3 2 — — —
rt 60 60 60 60 60 60 60 rt rt rt
24 h 24 h 4h 4h 2h 2h 2h 2h 1 min 5 min 1h
Nil Nil 65 80 85 88 75 85 98 Nil 25
Reaction conditions: 1-phenylethanol (1 mmol); V-polyoxometalate (0.05 mmol); PTSA.H2O (1.0 mmol); all yields refer to isolated yields.
H2O (5 mL) as solvent under neutral condition. The reaction did not take place at room temperature or at 60 °C (Table 1, entries 1 and 2). On acidification to pH 3.5 with HCl, the oxidation of 1-phenylethanol took place with a moderate yield (Table 1, entry 3); further decreasing the pH up to 1 increases the yield of the product and reduces the reaction time (Table 1 entries 4–6). To examine the effectiveness of an organic acid in this oxidation process, we carried out the reaction in the presence of p-toluenesulphonic acid (PTSA) (1.0 mmol). It has been observed that at pH 2 the reaction proceeded with better yield of product compared to the reaction at pH 3 (Table 1, entry 7 vs 8). Surprisingly, there was an abrupt change in the rate of the reaction when the reaction was carried out in the absence of solvent. In this particular case the alcohol was converted to its carbonyl derivative in very high yield within a minute of time, just on grinding the alcohol and oxidant in the presence of PTSA at room temperature (Table 1, entry 9).22 However, no product formation was observed when the oxidant was replaced with V2O5, H2O2 and benzyltrimethylammonium chloride under the same reaction conditions (Table 1, entry 10). Although not confirmed, it is believed that in the presence of polyoxometalate complex the reaction probably proceeded via free radical pathway, as in the presence of free radical inhibitor hydroquinone the yield of the reaction was found to be very low (Table 1, entry 11). Several test reactions were carried out using different amounts of V-polyoxometalate and PTSA. The results are shown in Table 2. It was observed that 0.05 mmol of V-polyoxometalate is required for oxidation of 1.0 mmol of the substrate. Moreover 0.50 mmol of PTSA is found to be optimized amount for this reaction (Table 2, entry 10). However, the reaction did not proceed in the absence of either V-polyoxometalate or PTSA (Table 2, entries 12 and 13). To evaluate the scope and limitations of the current procedure, oxidation reactions with a wide array of electronically diverse benzylic alcohols were examined using polyoxometalate complex. The results are summarized in Table 3. It has been seen from Table 3 that in general, various hydroxyl compounds such as benzoin, benzyl alcohol, 1-phenylethynol and various phenyl ring substituted 1-phenylethanols underwent oxidation reactions in nearly quantitative yields (Table 3, entries 1–11). The electronic nature and the position of the substituents had little effect on the reaction process. Both electron withdrawing and electron donating substituents, such as p-NO2, m-NO2, o-Cl, p-Cl, p-Br, p-OMe in the benzene ring gave the desired products in 85–90% yield (Table 3, entries 4–9). However, the presence of bulky group at position-2 on the phenyl ring required relatively longer reaction time (Table 3, entry 7). The
Table 2 Reaction with different amounts of the oxidanta Entry
V-polyoxometalate (mmol)
PTSAH2O (mmol)
Yield (%)
1 2 3 4 5 6 7 8 9 10 11 12 13
0.001 0.005 0.01 0.03 0.05 0.08 0.10 0.20 0.05 0.05 0.05 0.05 —
2.00 2.00 2.00 2.00 2.00 2.00 2.00 1.50 1.00 0.50 0.25 — 0.50
10 15 30 85 97 97 95 97 98 97 75 — —
a Reaction conditions: 1-phenylethanol (1 mmol), time 3 min, room temperature, isolated yields.
Table 3 Solvent less oxidation of benzylic of alcohols at room temperaturea Entry
Substrate
Time
Yield (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
PhCOCH(OH)Ph PhCH2OH PhCH(OH)CH3 p-NO2-PhCH2OH m-NO2-PhCH2OH p-ClPhCH2OH o-ClPhCH2OH p-BrPhCH2OH p-MeOPhCH2OH PhCH(OH)Ph p-ClPhCH(OH)Ph Cyclohexanol Cholesterol Cinnamyl alcohol 2-Thiophenemethanol Menthol Benzyl halide
1 min 1 min 1 min 1 min 1 min 1 min 2 min 1 min 1 min 1 min 1 min 5 min 5 min 5 min 5 min 5 min 5 min
97 85 97 90 90 85 85 95 90 95 93 Nil Nil Nil Nil Nil Nil
a Reaction conditions: substrate (1 mmol); V-polyoxometalate (0.05 mmol); PTSA.H2O (0.5 mmol); room temperature; yields refer to isolated yields.
oxidant was found to be highly selective for benzyl alcohols only. Other hydroxyl groups such as cyclohexanol, steroidal, cinnamoyl, menthol (Table 3, entries 12–16) were not oxidized under the reaction conditions. The oxidant was also unable to oxidize benzyl
A. Dewan et al. / Tetrahedron Letters 52 (2011) 2563–2565
chloride to benzaldehyde (Table 3, entry 17). No over-oxidized product was observed in any of the alcohol studied. Moreover, the active oxidant could be regenerated by addition of appropriate amount of benzyltrimethylammonium chloride and 30% H2O2 and reused in further oxidation reaction.22b In summary, we have developed an efficient method for the selective and rapid oxidation of benzylic alcohols to the corresponding aldehydes and ketones under solvent less condition at room temperature. Synthesis of this oxidant is very simple and starting materials used are commercially available and cheap. The oxidant is also very stable hence, can be stored and handled very easily. The reaction time is very fast and no over-oxidized product was observed. After the reaction, the active oxidant can be regenerated and reused for further oxidation reaction hence minimizing the waste generation. References and notes 1. (a) Lenoir, D. Angew. Chem., Int. Ed. 2006, 45, 3206–3210; (b) Hudlick, M. Oxidations in Organic Chemistry; American Chemical Society: Washington DC, 1990; (c) Sheldon, R. A.; Kochi, J. A. Metal-Catalyzed Oxidations of Organic Compounds; Academic Press: New York, 1981; (d) Schultz, M. J.; Sigman, M. S. Tetrahedron 2006, 62, 8227–8241. 2. Menger, F. M.; Lee, C. Tetrahedron Lett. 1981, 22, 1655–1656. 3. Lee, K. C.; Koo, B. S.; Lee, Y. S.; Cho, H.; Lee, K. K-J. Bull. Korean Chem. Soc. 2002, 23, 1667–1670. 4. Cainelli, G.; Cardillo, G. Chromium Oxidations in Organic Chemistry; SpringerVerlag: Berlin, 1984. 5. (a) Seddon, K. R.; Stark, A. Green Chem. 2002, 4, 119–123; (b) Peterson, K. P.; Larock, R. C. J. Org. Chem. 1998, 63, 3185–3189. 6. (a) Shi, F.; Tse, M.; Beller, M. Chem. Asian J. 2007, 2, 411–415; (b) Wolfson, A.; Wuyts, S.; De, V. D.; Vankelecom, I. F. J.; Jacobs, P. A. Tetrahedron Lett. 2002, 43, 8107–8110. 7. Biradar, A. V.; Dongare, M. K.; Umbarkar, S. B. Tetrhedron Lett. 2009, 50, 2885– 2888. 8. Najafi, G. R. Chin. Chem. Lett. 2010, 21, 1162–1164. 9. Bianchini, G.; Crucianelli, M.; Angelis, F.; De Neri, V.; Saladino, R. Tetrahedron Lett. 2005, 46, 2427–2432. 10. Gamez, P.; Arends, I. W. C. E.; Sheldon, R. A.; Reedijik, J. Adv. Synth. Catal. 2004, 346, 805–811. 11. (a) Martin, S. E.; Suarez, D. F. Tetrahedron Lett. 2002, 43, 4475–4479; (b) Kumar, A.; Jain, N.; Chauhan, S. M. S. Synlett 2007, 411–414. 12. (a) Sharma, V. B.; Jain, S. L.; Sain, B. Tetrahedron Lett. 2003, 44, 383–386; (b) Jain, S. L.; Sain, B. J. Mol. Catal. A Chem. 2001, 176, 101–104.
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