Open Access Article
This Open Access Article is licensed under a
Creative Commons Attribution 3.0 Unported Licence

Oxidation of allylic and benzylic alcohols to aldehydes and carboxylic acids

Daniel Könning , Tobias Olbrisch , Fanni D. Sypaseuth , C. Christoph Tzschucke and Mathias Christmann *
Institute of Chemistry and Biochemistry, Freie Universität Berlin, Takustr. 3, 14195 Berlin, Germany. E-mail: m.christmann@fu-berlin.de; Fax: +49 30 838460182; Tel: +49 30 83860182

Received 19th February 2014 , Accepted 21st March 2014

First published on 26th March 2014


Abstract

An oxidation of allylic and benzylic alcohols to the corresponding carboxylic acids is effected by merging a Cu-catalyzed oxidation using O2 as a terminal oxidant with a subsequent chlorite oxidation (Lindgren oxidation). The protocol was optimized to obtain pure products without chromatography or crystallization. Interception at the aldehyde stage allowed for Z/E-isomerization, thus rendering the oxidation stereoconvergent with respect to the configuration of the starting material.


The direct oxidation of primary alcohols to their corresponding carboxylic acids is an important synthetic transformation that consists of two successive steps.1 While the first step (alcohol to aldehyde) is usually performed with an electrophilic oxidant, the second oxidation (aldehyde to carboxylic acid) often involves a nucleophilic attack of the oxidant. Alternatively, it is possible to intercept the aldehyde–hydrate equilibrium with an electrophilic oxidant (Scheme 1). The latter strategy requires a significant population of the hydrate,2 which is not very favorable in the case of aromatic aldehydes.3
image file: c4cc01305k-s1.tif
Scheme 1 Oxidation pathways of alcohols and aldehydes to carboxylic acids.

Alcohol-to-carboxylic-acid oxidations can be conducted either in a one-pot fashion or as a two-step procedure with isolation of the intermediate aldehyde. Classical one-pot methods involve chromium-,4 tungsten-5 or ruthenium-based6 oxidants as well as hypervalent iodine derivatives such as IBX.7 The Zhao-modification8 of Anelli's oxidation9 (TEMPO, NaClO2) constitutes another alternative but also has some drawbacks and, like the other above mentioned oxidants, may give rise to unwanted side reactions.

An elegant solution to these problems has been provided by using oxoammonium salts in combination with NaClO2.10 For sensitive substrates however, a two-step protocol is often preferred over the one-pot process using a mild oxidant (e.g. Dess–Martin periodinane11) for the initial oxidation to the aldehyde followed by Lindgren oxidation12 (NaClO2 as the oxidant) to give the desired carboxylic acid.

We recently investigated13 the stereoconvergent conversion of E- and Z-allylic alcohols into E-α,β-unsaturated aldehydes following in the footsteps of Semmelhack,14 Sheldon,15 Markó,16 Koskinen,17 and Stahl18 (Scheme 2). Our studies resulted in a protocol using 1 mol% CuIOTf/TEMPO/diMeObpy and DMAP (2 mol%) in acetonitrile as the solvent with oxygen as stoichiometric oxidant.


image file: c4cc01305k-s2.tif
Scheme 2 Development of Cu-catalyzed aerobic oxidation.

DMAP was shown to play an active role in both the oxidation reaction and the isomerization steps.19 Herein, we report a two-step one-pot conversion of E- and Z-allylic alcohols into E-α,β-unsaturated carboxylic acids by joining a further refined Cu/TEMPO-catalyzed aerobic oxidation protocol31,32 with Lindgren's oxidation.

The goal in the first oxidation step (alcohol to aldehyde) was to lower the catalyst loading as much as possible in order to minimize interference with the subsequent steps. Our starting point was built upon important findings from Sheldon, Koskinen and Stahl. Sheldon found the accelerating effect of 2,2′-bipyridine (bpy) ligands while Koskinen's careful kinetic studies17 established optimal ratios within the catalyst system. Recently, Stahl et al. demonstrated the importance of CuI salts18 supported by mechanistic investigations. As a key result in their subsequent mechanistic studies,20 aliphatic primary21 and secondary alcohols22 showed a clearly different behavior from allylic and benzylic primary alcohols.33

With our substrates limited to allylic and benzylic alcohols, we considered it necessary to re-evaluate CuI salts together with symmetrically (1) and newly synthesized23 unsymmetrically substituted bpy ligands (2–3). Using 4a as test substrate, we found that 0.5 mol% CuI or slightly more reactive CuBr and 1 mol% DMAP resulted in a quantitative conversion of the substrate within 3 h, whereas reactions using Cu(OTf) and CuCl were not complete within 5 h.

In order to compare the efficacies of different bpy derivatives, we further lowered the catalyst loading to 0.4 mol% CuBr·ligand·TEMPO and 0.8 mol% DMAP. The reaction using the parent 2,2′-bipyridine stopped at 70% conversion after 6 h. Ligand 1 afforded 90% conversion whereas the unsymmetrically substituted bipyridines 2 and 3 led to a quantitative conversion, with 3 being significantly faster than 2. Despite these encouraging results, we selected 0.75 mol% CuBr·bpy·TEMPO and 1.5 mol% DMAP in MeCN (0.75 M) as our standard protocol for practical reasons, since we consider these reagents to be inexpensive and in stock in most organic laboratories.

As shown in Table 1, the oxidation of alcohols relevant to our synthetic endeavoura proceeded smoothly with TBS (4a), Ac (4b) and Bn (4c) protecting groups. The difference in the rate of oxidation is negligible, while the rate of the Z/E-isomerization is strongly dependent on the substitution of the Michael acceptor aldehyde. For substrates 4d–4f, the catalyst loading was increased to 1 mol% in order to ensure complete oxidation. In order to accelerate the Z/E-isomerization (entries 4, 5 and 7) 9-azajulolidine,24 a more nucleophilic analogue of 4-DMAP was used. The low yield of volatile aldehyde 5e reflects the difficulties associated with its distillative purification. The oxidation of furfuryl alcohol 4h and the benzyl alcohols 4i–j afforded the corresponding aldehydes in moderate to excellent yields. With this protocol in hand, we next turned our attention to the development of a one-pot oxidation of the alcohols 4a–j to the corresponding carboxylic acids. The Lindgren oxidation of aldehydes to the corresponding carboxylic acids with sodium chlorite is a straightforward reaction. However, the formation of the stronger oxidant hypochlorite as the by-product is often a source of side-reactions. As a result, a variety of hypochlorite scavengers25 such as 2-methyl-2-butene26,27 have been in use. In order to avoid by-products that are soluble in organic solvents and allow for the isolation of the clean carboxylic acids by extraction, we selected H2O2 as a scavenger that was previously used by Dalcanale and Montanari.28 Since oxidation of 4a with NaClO2/H2O2 has been carried out in acetonitrile as the solvent by Sorensen et al. in their hirsutellone studies,29 we were confident of combining both subsequent oxidation steps in a one-pot procedure.

Table 1 Oxidation of alcohols to aldehydes

image file: c4cc01305k-u1.tif

Entry Alcohol Aldehyde t [h] [%]a
a Isolated yield. b 1 mol% catalyst loading. c 9-Azajulolidine was used instead of DMAP.
1 image file: c4cc01305k-u2.tif image file: c4cc01305k-u3.tif 2.5 99
2 image file: c4cc01305k-u4.tif image file: c4cc01305k-u5.tif 1.75 92
3 image file: c4cc01305k-u6.tif image file: c4cc01305k-u7.tif 4 98
4b,c image file: c4cc01305k-u8.tif image file: c4cc01305k-u9.tif 25 89
5b,c image file: c4cc01305k-u10.tif image file: c4cc01305k-u11.tif 18 63
6b image file: c4cc01305k-u12.tif image file: c4cc01305k-u13.tif 17 97
7c image file: c4cc01305k-u14.tif image file: c4cc01305k-u15.tif 16.5 98
8 image file: c4cc01305k-u16.tif image file: c4cc01305k-u17.tif 6 55
9 image file: c4cc01305k-u18.tif image file: c4cc01305k-u19.tif 1 84
10 image file: c4cc01305k-u20.tif image file: c4cc01305k-u21.tif 1 96


As shown in Table 2, after formation of the E-α,β-unsaturated aldehydes, a Lindgren oxidation was conducted without prior isolation of the aldehydes. The corresponding carboxylic acids were obtained in high purity and good to excellent yields without further chromatographic purification.30 Furfuryl alcohol was the only problematic substrate in both oxidations. Skipping the isolation of the intermediate volatile aldehyde 5e resulted in a clean conversion of 4e into carboxylic acid 6e and a higher yield (compared to 5e).

Table 2 Oxidation of alcohols to carboxylic acids

image file: c4cc01305k-u22.tif

Entry Alcohol Carboxylic acid t 1 [h] t 2 [h] [%]a
a Isolated yield. b 1 mol% catalyst loading. c 9-Azajulolidine was used instead of DMAP.
1 image file: c4cc01305k-u23.tif image file: c4cc01305k-u24.tif 3 6.25 93
2 image file: c4cc01305k-u25.tif image file: c4cc01305k-u26.tif 3 6.75 86
3 image file: c4cc01305k-u27.tif image file: c4cc01305k-u28.tif 5 6 97
4b,c image file: c4cc01305k-u29.tif image file: c4cc01305k-u30.tif 15 19 94
5b,c image file: c4cc01305k-u31.tif image file: c4cc01305k-u32.tif 16 12 82
6b image file: c4cc01305k-u33.tif image file: c4cc01305k-u34.tif 15 12 95
7c image file: c4cc01305k-u35.tif image file: c4cc01305k-u36.tif 13 9 95
8 image file: c4cc01305k-u37.tif image file: c4cc01305k-u38.tif 5 8.5 53
9 image file: c4cc01305k-u39.tif image file: c4cc01305k-u40.tif 1 1.25 95
10 image file: c4cc01305k-u41.tif image file: c4cc01305k-u42.tif 1 9 96


In conclusion, we have developed an inexpensive system for the aerobic oxidation of allylic and benzylic alcohols to the corresponding aldehydes and carboxylic acids. For the first time, submol% quantities of a CuI catalyst were sufficient for converting alcohols into the corresponding aldehydes. The subsequent oxidation to the corresponding carboxylic acids was performed in the same reaction vessel, thereby avoiding isolation of the labile aldehydes. The carboxylic acids were isolated by extraction with sufficient purity without the need for further chromatographic purification thus rendering this protocol both cost and time efficient.

References

  1. J.-E. Bäckvall, Modern oxidation methods, Wiley-VCH, Weinheim, Germany, 2nd edn, 2010 Search PubMed.
  2. A.-K. C. Schmidt and C. B. W. Stark, Org. Lett., 2011, 13, 4164 CrossRef CAS PubMed.
  3. J. C. Qiu, P. P. Pradhan, N. B. Blanck, J. M. Bobbitt and W. F. Bailey, Org. Lett., 2012, 14, 350 CrossRef CAS PubMed.
  4. K. Bowden, I. M. Heilbron, E. R. H. Jones and B. C. L. Weedon, J. Chem. Soc., 1946, 39 RSC.
  5. R. Noyori, M. Aoki and K. Sato, Chem. Commun., 2003, 1977 RSC.
  6. M. Schröder and W. P. Griffith, J. Chem. Soc., Chem. Commun., 1979, 58 RSC.
  7. A. Schulze and A. Giannis, Synthesis, 2006, 257 CAS.
  8. M. Zhao, J. Li, E. Mano, Z. Song, D. M. Tschaen, E. J. J. Grabowski and P. J. Reider, J. Org. Chem., 1999, 64, 2564 CrossRef CAS.
  9. P. L. Anelli, C. Biffi, F. Montanari and S. Quici, J. Org. Chem., 1987, 52, 2559 CrossRef.
  10. M. Shibuya, T. Sato, M. Tomizawa and Y. Iwabuchi, Chem. Commun., 2009, 1739 RSC.
  11. D. B. Dess and J. C. Martin, J. Org. Chem., 1983, 48, 4155 CrossRef CAS.
  12. B. O. Lindgren and T. Nilsson, Acta Chem. Scand., 1973, 27, 888 CrossRef CAS PubMed.
  13. D. Könning, W. Hiller and M. Christmann, Org. Lett., 2012, 14, 5258 CrossRef PubMed.
  14. M. F. Semmelhack, C. R. Schmid, D. A. Cortes and C. S. Chou, J. Am. Chem. Soc., 1984, 106, 3374 CrossRef CAS.
  15. R. A. Sheldon, I. W. C. E. Arends, G.-J. ten Brink and A. Dijksman, Acc. Chem. Res., 2002, 35, 774 CrossRef CAS PubMed.
  16. I. E. Markó, A. Gautier, R. Dumeunier, K. Doda, F. Philippart, S. M. Brown and C. J. Urch, Angew. Chem., Int. Ed., 2004, 43, 1588 CrossRef PubMed.
  17. E. T. T. Kumpulainen and A. M. P. Koskinen, Chem. – Eur. J., 2009, 15, 10901 CrossRef CAS PubMed.
  18. J. M. Hoover and S. S. Stahl, J. Am. Chem. Soc., 2011, 133, 16901 CrossRef CAS PubMed.
  19. G. E. Keck, E. P. Boden and S. A. Mabury, J. Org. Chem., 1985, 50, 709 CrossRef CAS.
  20. J. M. Hoover, B. L. Ryland and S. S. Stahl, J. Am. Chem. Soc., 2013, 135, 2357 CrossRef CAS PubMed.
  21. J. E. Steves and S. S. Stahl, J. Am. Chem. Soc., 2013, 135, 15742 CrossRef CAS PubMed.
  22. M. B. Lauber and S. S. Stahl, ACS Catal., 2013, 2612 CrossRef CAS.
  23. S. Duric and C. C. Tzschucke, Org. Lett., 2011, 13, 2310 CrossRef CAS PubMed.
  24. I. Held, S. Xu and H. Zipse, Synthesis, 2007, 1185 CAS.
  25. A. Raach and O. Reiser, J. Prakt. Chem., 2000, 342, 605 CrossRef CAS.
  26. G. A. Kraus and M. J. Taschner, J. Org. Chem., 1980, 45, 1175 CrossRef CAS.
  27. G. A. Kraus and B. Roth, J. Org. Chem., 1980, 45, 4825 CrossRef CAS.
  28. E. Dalcanale and F. Montanari, J. Org. Chem., 1986, 51, 567 CrossRef CAS.
  29. S. D. Tilley, K. P. Reber and E. J. Sorensen, Org. Lett., 2009, 11, 701 CrossRef CAS PubMed.
  30. The NMR spectra of the carboxylic acids shown in the ESI were recorded on the crude material obtained after extractive workup.
  31. For a review on aerobic oxidations with stable radicals, see: Q. Cao, L. M. Dornan, L. Rogan, N. L. Hughes and M. J. Muldoon, Chem. Commun., 2014, 50 10.1039/C3CC47081D.
  32. For a review on aerobic Cu-catalyzed reactions, see: S. E. Allen, R. R. Walvoord, R. Padilla-Salinas and M. C. Kozlowski, Chem. Rev., 2013, 113, 6234 CrossRef CAS PubMed.
  33. Recently, the substrate scope was further extended to amino alcohols: Y. Sasano, S. Nagasawa, M. Yamazaki, M. Shibuya, J. Park and Y. Iwabuchi, Angew. Chem., Int. Ed., 2014, 53, 3236 CrossRef CAS PubMed.

Footnote

Electronic supplementary information (ESI) available: Experimental details and spectroscopic data for all compounds. See DOI: 10.1039/c4cc01305k

This journal is © The Royal Society of Chemistry 2014