Catalysis Science & Technology Blog

The oxidation of alcohols to carbonyl compounds holds an important place in the transformations of organic molecules, especially the selective oxidation to aldehydes or ketones without overoxidation. The importance of these transformations lies in the usefulness and reactivity of carbonyl compounds as synthons in organic synthesis. Many techniques and reagents are available for this reaction, but they tend to suffer from the need for stoichometric amounts of oxidants, as in the case of permanganate and chromate based oxidants.  Ideally, the most environmentally friendly and efficient process of transforming alchools in their respective carbonyl compounds would be by employing metal catalysts that can exploit molecular oxygen as the oxidant. Although several systems have been developed that achieve this goal, they generally require additives, ligands, sacrificial substrates or high pressures of oxygen.

With these limitations in mind, Ma and Lei have been focusing their efforts on the development of simpler systems to achieve selective oxidations of alcohols to aldehydes and ketons (a short selection of their recent publications can be found here and here) , and recently discovered a simple methodology that only requires AlBr₃ 6H₂O and athmospheric oxygen to efficiently convert primary benzylic alcohols in their corresponding aldehydes with high selectivity and yields.

Employing a catalyst loading of 30% in dioxane in a batch reactor at the mild temperature of 70 °C afforded conversions of primary benzylic alcohols up to 100% with total selectivity for aldehyde in several cases, with 89% being the minimum selectivity measured. Running the reaction in an argon athmosphere instead of air resulted in only trace amounts of benzaldehydes and the use of anhydrous conditions did not change the outcomes of the transformation, suggesting that moisture in solvents or gases does not affect the reaction; in addition, nitro and ether moyeties on the ring were well tolerated.

In order to gather insights into the mechanism of the reaction, a set of isotope-labelling experiments have been performed and suggested that the oxygen atom is derived from the oxidant and not from a rearrangement of the original molecule.

When the conditions were applied to secondary benzylic alcohols,  the conversion rate of several diphenylmethanols into the corrisponding benzophenones suroassed 94%  with selectivities up to 80%, with the main side-products being the brominated derivatives.

In summary, Ma and Lei developed a simple system that does not use chlorinated solvents, environmentally unfriendly metals or complex ligands and conditions but still provided good yields and conversions with the substrates tested. Further study on a  broader substrate scope, refinement of conditions and mechanisms are underway.

Read this interesting communication in Catalysis Science & Technology.

AlBr3·6H2O catalyzed oxidation of benzylic alcohols
Yun-Mei Zhong, Heng-Chang Ma, Jin-Xia Wang, Xiao-Jie Jia, Wen-Feng Li and Zi-Qiang Lei
Catal. Sci. Technol., 2011, Advance Article DOI: 10.1039/C1CY00165E, Communication

The beginning of the new century, in chemistry, has been often linked to the renaissance of organocatalysis as enthusiastically reported by articles appearing on Angewandte Chemie and Chemical Society Reviews in the past few years. Although presenting several advantages over traditional metal-based catalysts, such as high stereoselectivity, ease of synthesis and handling, and the general feeling of being more environmentally benign, they yet have to persuade industry into their large scale application.

Some of the reasons behind the fall-out of industry for organocatalysts lie in the lower efficiency they exhibit, the major difficulties of catalyst separation, recovery and recycling. Attempts to covalently bind organocatalysts on polymeric and other supports addressed the problems of separation but often resulted in a loss of activity triggered by modifications of structure of the catalyst.

In an interesting perspective appeared on Cat. Sci. Technol., Luo, Zhang and Chen presented an overview of alternative immobilisation techniques based on non-covalent bonds.

As stated by the authors, the loss of activity on covalently bound organocatalysts is likely to depend on the changes in the structure and chemical properties that these small organic molecules undergo when bound to the support. Non-covalent immobilisation, on the other hand, seems to circumvent this problem and, despite other shortcomings, might be a promising field of study.

Examples of acid-base immobilisation presented include the use of solid acid like polyoxometalates or polystyrene sulfonic acids as the support, used with chiral amines-based catalysts in the aldol reaction and Michael additions, where recovery of the catalyst could be achieved by precipitation with diethyl ether. Other immobilisation strategies presented are the incorporation of the organocatalyst into a phase transfer catalysts (PTC) and the immobilisation into clays. The latter is achieved on materials such as montmorillonite, which comprises negatively charged layers  alternated with layers of Na+ species.Using a cation exchange process, molecules such as proline and proline derived structures could be immobilized in the interlayers and the resulting material successfully used in aldol reactions. Another family of supports is represented by ionic liquids, where an organocatalysts is used as the anionic partner in the structure, creating chiral ionic liquid s that had been used to catalyse Mannich-type and aza-Diels-Alder reactions. Other supports mentioned are self-supported gel-type organocatalysts, biphasic immobilisation and techniques based on hydrophobic interactions, using cyclodextrines as the supports.

Several detailed examples and references for each category, inviting the interested reader to further reading.

In the authors` words  – the non-covalent immobilisation is anticipated to provide a viable solution to enhance the applications of organocatalysis with “practical” credentials.

Read the full article here

Perspective
Non-covalent immobilisation of asymmetric organocatalysts
Long Zhang, Sanzhong Luo and Jin-Pei Cheng
Catal. Sci. Technol., 2011, Advance Article
DOI: 10.1039/C1CY00029B

Dr Jamie Humphrey, Catalysis Science & Technology‘s Managing Editor, will be attending the North American Catalysis Society meeting next week in Detroit.

If you’d like to meet up with Jamie while he is there, please contact us.

To see which other conferences the team will be attending visit our earlier blog post…