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Palladium-catalysed oxidative cross-esterification between two alcohols† Jianhui Xia,a Ailong Shaoa, Shan Tang,b Xinlong Gao,b Meng Gaoa,* and Aiwen Leia,b*
Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/
A simple palladium-catalysed oxidative cross-coupling between two different alcohols was developed. Various benzylic alcohols could couple with aliphatic alcohols in excellent yields. The use of benzyl chloride as the oxidant and the amount of aliphatic alcohol were both important for achieving the reaction selectivity. Oxidative cross-coupling between two nucleophiles has now become an important strategy for the construction of organic molecular.1 Over the past few years, direct utilization of readily available materials such as hydrocarbons, alcohols and amines in constructing complex moleculars through oxidative cross-coupling has become a hot topic.2 Esters are among the most important compounds in organic chemistry. Traditional esterification methods are based on the nucleophilic substitution of carboxylic acid derivatives with alcohols.3 As a result, multiple steps will be involved with production of toxic irremovable byproduct, which is incongruent with the current demand of environmentally-benign processes. Thus, direct oxidative esterification between readily available aldehydes and alcohols has been increasingly explored over the past decades.4 As aldehydes were generally synthesized from the oxidation of corresponding alcohols, oxidative cross-coupling between two alcohols represent the ideal way for the synthesis of esters.5 However, this oxidative crossesterification approach was challenging since both of the alcohols can be oxidized under oxidative conditions. In most cases, a poor selectivity was observed for the oxidative coupling between two different alcohols. Finding suitable catalytic systems for controlling the reaction selectivity is important for the oxidative crossesterification reactions. Over the past decade, increasing attention has been paid to this transformation and a few reaction systems has been developed.6 Until now, the reaction systems for oxidative crossesterification between two alcohols is still limited. Developing more reaction systems is still required for leading the esterification process becoming much more abundant.7 Palladium catalysis has been widely used in the oxidative transformation of alcohols.8 In 2011, Beller and our group reported
This journal is © The Royal Society of Chemistry 2012
the Palladium-catalysed aerobic oxidative esterification of benzylic alcohols with aliphatic alcohols.6c, 6d It is the first time that various aliphatic alcohols were able to be applied in oxidative crossesterification reactions. While in the work of Beller, a bulky monophosphine ligand was used for achieving this goal.6c In our previous work, the challenging oxidative esterification of benzylic alcohols with long-chain aliphatic alcohols were achieved by using a special Polefin ligand.6c All these reactions required the use of special ligands, which is of disadvantage for its practical application. Thus, oxidation systems without the use of special ligands need to be developed. Furthermore, understanding the factors that controlling the esterification selectivity is important for future improvement of the alcohol/alcohol oxidative cross-esterification protocol. Herein, we would like to communicate our progress on using simple PdCl2(PPh3)2 as the catalyst for the oxidative cross-esterification between benzylic alcohols and aliphatic alcohols (Scheme 1).
Scheme 1 PdCl2(PPh3)2-catalysed oxidative cross-esterification between benzylic alcohols and aliphatic alcohols.
It has been shown that electron-rich carbon ligands were quite efficient for promoting transition metal catalysed selective oxidative esterification reactions.9 Recently, our group have demonstrated that benzyl chloride could act both as a carbon ligand and an oxidant in the palladium catalysed selective oxidation of primary alcohols.10 With an interest in the special reactivity of benzyl chloride, we tried to apply it in the oxidative cross-esterification reactions. When equal amount of 4-methylbenzyl alcohol (1a) and methanol (2a) was used by using PdCl2(PPh3)2 as the catalyst and benzyl chloride as the oxidant, the cross-esterification product 3a and self-esterification product 5a were both observed and obtained in similar amount. Small amount of p-tolualdehyde (4a) could also be observed (eqn. (1)). When we raise the amount of methanol, the yield of methyl ester increased (Table 1, entries 1-4). An excellent selectivity was obtained when 10 equiv of methanol was used. After obtaining good results,
J. Name., 2012, 00, 1-3 | 1
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Journal Name be an effective alternative choice for the aerobic oxidative crossesterification systems. Table 2 Palladium-catalysed oxidative cross esterification between benzylic alcohols with methanol.a
Table 1 Effect of reaction parameters for the oxidative esterification between benzylic alcohol and methanol
a
Reaction conditions: 1a (0.5 mmol), 2, PdCl2(PPh3)2 (5 mol%), K2CO3 (0.5 mmol), BnCl (0.5 mmol) in THF (2 mL) at 60 oC for 20 h. bIsolated yield.
The optimized conditions were then used for the synthesis of methyl esters from different substituted benzylic alcohols. Good to excellent results were obtained in most cases (Table 2). Simple benzyl alcohol showed a similar reactivity with the p-Methyl benzyl alcohol (Table 2, entries 1-2). It is worthy of noting that C-Cl bond was tolerated under the standard condition. The reaction chemoselectively afforded the desired methyl ester in excellent yield (Table 2, entry 3). In the cases of electron deficient benzyl alcohols, decreased yield was obtained but still with a good reaction selectivity (Table 2, entries 4-5). Some of the benzylic alcohol substrates were left unreacted. This might be due to the reason that electron-deficient benzylic alcohol were harder to accomplish β-hydride elimination in the oxidation step.6d As was expected, benzylic alcohol substituted with the strongly electron-donating group p-OMe furnished corresponding methyl ester in excellent yields (Table 2, entry 6). Benzylic alcohols with methyl and methoxy groups at the metal position all demonstrated good reactivity in this oxidative esterification reaction. More importantly, steric hindered benzylic alcohols such as o-tolylmethanol and 1naphthalenemethanol were still suitable in this transformation with a good efficiency. Despite benzylic alcohols, cinnamyl alcohol was also applied as the substrate in this transformation. The desired ester could be isolated in 78% yield (Table 2, entry 12). For all reactions in Table 2, no benzaldehydes were observed. Since good results were obtained in the synthesis of methyl esters, we decided to explore the oxidative esterification between benzylic alcohols with other aliphatic alcohols (Table 3). Preliminary results showed that this Pd-catalysed oxidation strategy could also applied to aliphatic alcohols such as npropanol and isobutanol (Table 3, entries 1-2). Good yields were obtained with both electron-deficient and electron-rich benzylic alcohols (Table 3, entries 3-4). Thus, this reaction strategy can
2 | J. Name., 2012, 00, 1-3
a
Reaction conditions: 1 (0.50 mmol), 2a (5.0 mmol), PdCl2(PPh3)2 (5.0 mol%), K2CO3 (1.0 mmol), BnCl (1.0 mmol) in THF (2 mL) at 65 OC for 20 h. b Isolated yield.
Based on the previous reports,4d, 10 a tentative mechanism was brought out to understand this oxidative esterification reaction. First, the oxidative addition of BnCl to Pd(0) generates intermediate I-1 (BnPdCl(PPh3)2). In the next step, both benzylic and aliphatic alcohol could undergo alcoholysis of I-1 leads to the formation of corresponding alkoxy palladium intermediates (I-2 and I-2ʹ). Since benzylic alcohol is easier to be oxidized, a β-hydride elimination step might take place to generate an aryl aldehyde for I-2. Then, in situ generated aldehyde can undergo insertion to Pd-O bond of both I-2 and I-2ʹ. In our reaction, the
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we tried to decrease the amount of palladium catalyst. A similar yield was still obtained with 5.0 mol% catalyst (Table 1, entry 5). However, a decrease of yield was observed with 2.0 mol% catalyst (Table 1, entry 6). Thus, 5 mol% PdCl2(PPh3)2, 10 equivalent methanol were the optimized combination for achieving this selective oxidative cross esterification reaction.
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Journal Name substrate ratio between benzylic alcohol and aliphatic alcohol is 10 to 1. Thus, the concentration of I-2ʹ should be higher than that of I-2. The aryl aldehyde would selectively inserts to I-2ʹ to generate intermediate I-3, which is a hemiacetal palladium complex. Intermediate I-3 occurs β-hydride elimination to furnish the final ester and a palladium hydride species I-4. The reductive elimination of I-4 releases simple toluene and regenerates Pd(0) species.
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Table 3 Palladium-catalysed oxidative cross esterification between benzylic alcohols with aliphatic alcohols.a
a
Reaction conditions: 1 (0.50 mmol), 2a (5.0 mmol), PdCl2(PPh3)2 (5.0 mol%), K2CO3 (1.0 mmol), BnCl (1.0 mmol) in THF (2 mL) at 65 oC for 20 h. b Isolated yield.
Scheme 2 Proposed mechanism
In conclusion, we have achieved the oxidative crossesterification between benzylic alcohol and aliphatic alcohols by using PdCl2(PPh3)2 as the sole catalyst and benzyl chloride as the oxidant. Various benzylic alcohols can selectively couple with methanol in excellent yields. This reaction system is also suitable for the oxidative cross-esterification with other aliphatic alcohols. According to the proposed mechanism, the oxidant benzyl chloride and the use of excess amount of aliphatic alcohol is important for the selectivity of this reaction. This work was financially supported by the National Natural Science Foundation of China (Nos. 21262018 and 20862007) and the Natural Science Foundation of Jiangxi Province (2010GZH0070).
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National Research Center for Carbohydrate Synthesis, Jiangxi Normal University, Nanchang 330022, Jiangxi P. R. China. E-mail: [email protected]; [email protected]. b College of Chemistry and Molecular Sciences, the Institute for Advanced Studies (IAS), Wuhan University, Wuhan, Hubei 430072, P. R. China. † Electronic Supplementary Information (ESI) available: See DOI: 10.1039/c000000x/ 1. (a) C. Liu, H. Zhang, W. Shi and A. Lei, Chem. Rev., 2011, 111, 17801824; (b) C. Liu, L. Jin and A. Lei, Synlett, 2010, 527-2536. 2. (a) S. A. Girard, T. Knauber and C.-J. Li, Angew. Chem. Int. Ed., 2014, 53, 74-100; (b) J. Le Bras and J. Muzart, Chem. Rev., 2011, 111, 11701214; (c) C. S. Yeung and V. M. Dong, Chem. Rev., 2011, 111, 12151292; (d) A. N. Campbell and S. S. Stahl, Acc. Chem. Res., 2012, 45, 851863; (e) Y. Wu, J. Wang, F. Mao and F. Y. Kwong, Chem. Asian. J, 2014, 9, 26-47; (f) Zhang, Z.; Ouyang, L.; Wu, W.; Li, J.; Zhang, Z.; Jiang, H. J. Org. Chem. 2014, 79, 10734-10742; (g) Zhang, Z.; Wu, W.; Liao, J.; Li, J.; Jiang, H. Chem. Eur. J., 2015, 21, 6708-6712. 3. J. Otera and J. Nishikido, Esterification : methods, reactions, and applications, 2nd completely rev. and enl. edn., Wiley-VCH, Weinheim, 2010. 4. (a) E. G. Delany, C.-L. Fagan, S. Gundala, K. Zeitler and S. J. Connon, Chem. Commun., 2013, 49, 6513-6515; (b) A. M. Whittaker and V. M. Dong, Angew. Chem. Int. Ed., 2015, 54, 1312-1315; (c) B. A. Tschaen, J. R. Schmink and G. A. Molander, Org. Lett., 2013, 15, 500-503; (d) C. Liu, S. Tang, L. Zheng, D. Liu, H. Zhang and A. Lei, Angew. Chem. Int. Ed., 2012, 51, 5662-5666; (e) W.-J. Yoo and C.-J. Li, Tetrahedron Lett., 2007, 48, 1033-1035; (f) T. Yasukawa, H. Miyamura and S. Kobayashi, Chem. Asian. J, 2011, 6, 621-627; (g) X.-F. Wu and C. Darcel, Eur. J. Org. Chem., 2009, 1144-1147; (h) X.-F. Wu, Tetrahedron Lett., 2012, 53, 3397-3399; (i) R. Gopinath and B. K. Patel, Org. Lett., 2000, 2, 577-579; (j) B. E. Maki and K. A. Scheidt, Org. Lett., 2008, 10, 4331-4334; (k) S. D. Sarkar, S. Grimme and A. Studer, J. Am. Chem. Soc., 2010, 132, 11901191; (l) R. C. Samanta, S. De Sarkar, R. Frohlich, S. Grimme and A. Studer, Chem. Sci., 2013, 4, 2177-2184. 5. S. Tang, J. Yuan, C. Liu and A. Lei, Dalton. Trans., 2014, 43, 1346013470. 6. (a) N. A. Owston, A. J. Parker and J. M. J. Williams, Chem. Commun., 2008, 624-625; (b) N. A. Owston, T. D. Nixon, A. J. Parker, M. K. Whittlesey and J. M. J. Williams, Synthesis, 2009, 1578-1581; (c) S. Gowrisankar, H. Neumann and M. Beller, Angew. Chem. Int. Ed., 2011, 50, 5139-5143; (d) C. Liu, J. Wang, L. Meng, Y. Deng, Y. Li and A. Lei, Angew. Chem. Int. Ed., 2011, 50, 5144-5148; (e) D. Srimani, E. Balaraman, B. Gnanaprakasam, Y. Ben-David and D. Milstein, Adv. Synth. Catal., 2012, 354, 2403-2406; (f) N. Yamamoto, Y. Obora and Y. Ishii, J. Org. Chem., 2011, 76, 2937-2941; (g) Y. Kita, Y. Nishii, A. Onoue and K. Mashima, Adv. Synth. Catal., 2013, 355, 3391-3395; (h) A. B. Powell and S. S. Stahl, Org. Lett., 2013, 15, 5072-5075; (i) R. V. Jagadeesh, H. Junge, M.-M. Pohl, J. Radnik, A. Brückner and M. Beller, J. Am. Chem. Soc., 2013, 135, 10776-10782; (j) R. Ray, R. D. Jana, M. Bhadra, D. Maiti and G. K. Lahiri, Chem. Eur. J., 2014, 20, 15618-15624; (k) X.-F. Wu, Chem. Eur. J., 2012, 18, 8912-8915. 7. B. Liu, F. Hu and B.-F. Shi, ACS Catal., 2015, 1863-1881. 8. (a) M. S. Sigman and D. R. Jensen, Acc. Chem. Res., 2006, 39, 221-229; (b) R. A. Sheldon, I. W. C. E. Arends, G.-J. ten Brink and A. Dijksman, Acc. Chem. Res., 2002, 35, 774-781. 9. (a) M. Zhang, S. Zhang, G. Zhang, F. Chen and J. Cheng, Tetrahedron Lett., 2011, 52, 2480-2483; (b) D. Zhang and C. Pan, Catal. Commun., 2012, 20, 41-45; (c) F. Luo, C. Pan, J. Cheng and F. Chen, Tetrahedron, 2011, 67, 5878-5882. 10. C. Liu, S. Tang and A. Lei, Chem. Commun., 2013, 49, 1324-1326.
Notes and references This journal is © The Royal Society of Chemistry 2012
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This article can be cited before page numbers have been issued, to do this please use: J. Xia, A. Shao, S. Tang, X. Gao, M. Gao and A. Lei, Org. Biomol. Chem., 2015, DOI: 10.1039/C5OB00677E.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Cite this: DOI: 10.1039/x0xx00000x
Palladium-catalysed oxidative cross-esterification between two alcohols† Jianhui Xia,a Ailong Shaoa, Shan Tang,b Xinlong Gao,b Meng Gaoa,* and Aiwen Leia,b*
Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/
A simple palladium-catalysed oxidative cross-coupling between two different alcohols was developed. Various benzylic alcohols could couple with aliphatic alcohols in excellent yields. The use of benzyl chloride as the oxidant and the amount of aliphatic alcohol were both important for achieving the reaction selectivity. Oxidative cross-coupling between two nucleophiles has now become an important strategy for the construction of organic molecular.1 Over the past few years, direct utilization of readily available materials such as hydrocarbons, alcohols and amines in constructing complex moleculars through oxidative cross-coupling has become a hot topic.2 Esters are among the most important compounds in organic chemistry. Traditional esterification methods are based on the nucleophilic substitution of carboxylic acid derivatives with alcohols.3 As a result, multiple steps will be involved with production of toxic irremovable byproduct, which is incongruent with the current demand of environmentally-benign processes. Thus, direct oxidative esterification between readily available aldehydes and alcohols has been increasingly explored over the past decades.4 As aldehydes were generally synthesized from the oxidation of corresponding alcohols, oxidative cross-coupling between two alcohols represent the ideal way for the synthesis of esters.5 However, this oxidative crossesterification approach was challenging since both of the alcohols can be oxidized under oxidative conditions. In most cases, a poor selectivity was observed for the oxidative coupling between two different alcohols. Finding suitable catalytic systems for controlling the reaction selectivity is important for the oxidative crossesterification reactions. Over the past decade, increasing attention has been paid to this transformation and a few reaction systems has been developed.6 Until now, the reaction systems for oxidative crossesterification between two alcohols is still limited. Developing more reaction systems is still required for leading the esterification process becoming much more abundant.7 Palladium catalysis has been widely used in the oxidative transformation of alcohols.8 In 2011, Beller and our group reported
This journal is © The Royal Society of Chemistry 2012
the Palladium-catalysed aerobic oxidative esterification of benzylic alcohols with aliphatic alcohols.6c, 6d It is the first time that various aliphatic alcohols were able to be applied in oxidative crossesterification reactions. While in the work of Beller, a bulky monophosphine ligand was used for achieving this goal.6c In our previous work, the challenging oxidative esterification of benzylic alcohols with long-chain aliphatic alcohols were achieved by using a special Polefin ligand.6c All these reactions required the use of special ligands, which is of disadvantage for its practical application. Thus, oxidation systems without the use of special ligands need to be developed. Furthermore, understanding the factors that controlling the esterification selectivity is important for future improvement of the alcohol/alcohol oxidative cross-esterification protocol. Herein, we would like to communicate our progress on using simple PdCl2(PPh3)2 as the catalyst for the oxidative cross-esterification between benzylic alcohols and aliphatic alcohols (Scheme 1).
Scheme 1 PdCl2(PPh3)2-catalysed oxidative cross-esterification between benzylic alcohols and aliphatic alcohols.
It has been shown that electron-rich carbon ligands were quite efficient for promoting transition metal catalysed selective oxidative esterification reactions.9 Recently, our group have demonstrated that benzyl chloride could act both as a carbon ligand and an oxidant in the palladium catalysed selective oxidation of primary alcohols.10 With an interest in the special reactivity of benzyl chloride, we tried to apply it in the oxidative cross-esterification reactions. When equal amount of 4-methylbenzyl alcohol (1a) and methanol (2a) was used by using PdCl2(PPh3)2 as the catalyst and benzyl chloride as the oxidant, the cross-esterification product 3a and self-esterification product 5a were both observed and obtained in similar amount. Small amount of p-tolualdehyde (4a) could also be observed (eqn. (1)). When we raise the amount of methanol, the yield of methyl ester increased (Table 1, entries 1-4). An excellent selectivity was obtained when 10 equiv of methanol was used. After obtaining good results,
J. Name., 2012, 00, 1-3 | 1
Organic & Biomolecular Chemistry Accepted Manuscript
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Journal Name be an effective alternative choice for the aerobic oxidative crossesterification systems. Table 2 Palladium-catalysed oxidative cross esterification between benzylic alcohols with methanol.a
Table 1 Effect of reaction parameters for the oxidative esterification between benzylic alcohol and methanol
a
Reaction conditions: 1a (0.5 mmol), 2, PdCl2(PPh3)2 (5 mol%), K2CO3 (0.5 mmol), BnCl (0.5 mmol) in THF (2 mL) at 60 oC for 20 h. bIsolated yield.
The optimized conditions were then used for the synthesis of methyl esters from different substituted benzylic alcohols. Good to excellent results were obtained in most cases (Table 2). Simple benzyl alcohol showed a similar reactivity with the p-Methyl benzyl alcohol (Table 2, entries 1-2). It is worthy of noting that C-Cl bond was tolerated under the standard condition. The reaction chemoselectively afforded the desired methyl ester in excellent yield (Table 2, entry 3). In the cases of electron deficient benzyl alcohols, decreased yield was obtained but still with a good reaction selectivity (Table 2, entries 4-5). Some of the benzylic alcohol substrates were left unreacted. This might be due to the reason that electron-deficient benzylic alcohol were harder to accomplish β-hydride elimination in the oxidation step.6d As was expected, benzylic alcohol substituted with the strongly electron-donating group p-OMe furnished corresponding methyl ester in excellent yields (Table 2, entry 6). Benzylic alcohols with methyl and methoxy groups at the metal position all demonstrated good reactivity in this oxidative esterification reaction. More importantly, steric hindered benzylic alcohols such as o-tolylmethanol and 1naphthalenemethanol were still suitable in this transformation with a good efficiency. Despite benzylic alcohols, cinnamyl alcohol was also applied as the substrate in this transformation. The desired ester could be isolated in 78% yield (Table 2, entry 12). For all reactions in Table 2, no benzaldehydes were observed. Since good results were obtained in the synthesis of methyl esters, we decided to explore the oxidative esterification between benzylic alcohols with other aliphatic alcohols (Table 3). Preliminary results showed that this Pd-catalysed oxidation strategy could also applied to aliphatic alcohols such as npropanol and isobutanol (Table 3, entries 1-2). Good yields were obtained with both electron-deficient and electron-rich benzylic alcohols (Table 3, entries 3-4). Thus, this reaction strategy can
2 | J. Name., 2012, 00, 1-3
a
Reaction conditions: 1 (0.50 mmol), 2a (5.0 mmol), PdCl2(PPh3)2 (5.0 mol%), K2CO3 (1.0 mmol), BnCl (1.0 mmol) in THF (2 mL) at 65 OC for 20 h. b Isolated yield.
Based on the previous reports,4d, 10 a tentative mechanism was brought out to understand this oxidative esterification reaction. First, the oxidative addition of BnCl to Pd(0) generates intermediate I-1 (BnPdCl(PPh3)2). In the next step, both benzylic and aliphatic alcohol could undergo alcoholysis of I-1 leads to the formation of corresponding alkoxy palladium intermediates (I-2 and I-2ʹ). Since benzylic alcohol is easier to be oxidized, a β-hydride elimination step might take place to generate an aryl aldehyde for I-2. Then, in situ generated aldehyde can undergo insertion to Pd-O bond of both I-2 and I-2ʹ. In our reaction, the
This journal is © The Royal Society of Chemistry 2012
Organic & Biomolecular Chemistry Accepted Manuscript
Published on 23 April 2015. Downloaded by East Carolina University on 24/04/2015 03:53:57.
we tried to decrease the amount of palladium catalyst. A similar yield was still obtained with 5.0 mol% catalyst (Table 1, entry 5). However, a decrease of yield was observed with 2.0 mol% catalyst (Table 1, entry 6). Thus, 5 mol% PdCl2(PPh3)2, 10 equivalent methanol were the optimized combination for achieving this selective oxidative cross esterification reaction.
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DOI: 10.1039/C5OB00677E
Organic & Biomolecular Chemistry
Journal Name substrate ratio between benzylic alcohol and aliphatic alcohol is 10 to 1. Thus, the concentration of I-2ʹ should be higher than that of I-2. The aryl aldehyde would selectively inserts to I-2ʹ to generate intermediate I-3, which is a hemiacetal palladium complex. Intermediate I-3 occurs β-hydride elimination to furnish the final ester and a palladium hydride species I-4. The reductive elimination of I-4 releases simple toluene and regenerates Pd(0) species.
Published on 23 April 2015. Downloaded by East Carolina University on 24/04/2015 03:53:57.
Table 3 Palladium-catalysed oxidative cross esterification between benzylic alcohols with aliphatic alcohols.a
a
Reaction conditions: 1 (0.50 mmol), 2a (5.0 mmol), PdCl2(PPh3)2 (5.0 mol%), K2CO3 (1.0 mmol), BnCl (1.0 mmol) in THF (2 mL) at 65 oC for 20 h. b Isolated yield.
Scheme 2 Proposed mechanism
In conclusion, we have achieved the oxidative crossesterification between benzylic alcohol and aliphatic alcohols by using PdCl2(PPh3)2 as the sole catalyst and benzyl chloride as the oxidant. Various benzylic alcohols can selectively couple with methanol in excellent yields. This reaction system is also suitable for the oxidative cross-esterification with other aliphatic alcohols. According to the proposed mechanism, the oxidant benzyl chloride and the use of excess amount of aliphatic alcohol is important for the selectivity of this reaction. This work was financially supported by the National Natural Science Foundation of China (Nos. 21262018 and 20862007) and the Natural Science Foundation of Jiangxi Province (2010GZH0070).
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DOI: 10.1039/C5OB00677E
COMMUNICATION a
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