By Matt Gradiski, Member-at-Large for the GCI
Bananas. They’re a fantastic healthy snack, delicious to bake into bread or flavour medicine, and even the choice speak-and-spell for singer Gwen Stefani. Now, thanks to two excellent reports in 2015, an efficient medium for two sophisticated organic transformations can be added to its list of uses.
Published in Green Chemistry in January 2015, researchers were able to perform Suzuki-Miyaura (SM) cross-coupling in a neat solution of water extract of banana (WEB).1 WEB is made by simply drying the peel of a banana, burning the dried remains, and extracting the ashes with water (Figure 1). What results is a brown-orange liquid holding tremendous catalytic capability.Typically, SM coupling requires the addition of external ligands, base, or other reaction promoters that can often be very expensive. The reaction is known to be able to take place in aqueous media; however, organic solvents are usually the more common choice. While the SM reaction still requires a noble-metal palladium catalyst, using a WEB medium for this reaction completely replaces the use of external additives and organic solvents (Figure 2). The only thing better than being able to do your reaction in water, is to do your reaction in water quickly! The longest reported reaction time using this system was 20 minutes, with times as a low as 5 minutes, and yields as high as 99%, all being carried out at room temperature for 12 different products.
Extending the scope of WEB’s usefulness, a report in July of the same year in Green Chemistry showed that the medium can also be used effectively for the catalytic Dakin reaction.2 This reaction converts an ortho- or para-hydroxy aromatic aldehyde or ketone into its corresponding benzenediol through reaction with hydrogen peroxide in base (Figure 3).Similar to SM coupling, the Dakin reaction requires addition of an external base, typically sodium or potassium hydroxide. However, it was found in the study that no external base was required when the reaction was carried out in WEB. The WEB solution was effective enough to initiate the reaction via deprotonation of hydrogen peroxide, generating the nucleophilic hydroperoxide anion that is required for the reaction to take place. All 16 reactions screened in the study were carried out at room temperature with the use of no external additives or organic solvent. Reaction times were as long as 60 minutes, and isolated yields ranged from 90-98%!
But what makes WEB such an efficient medium for green chemistry? Although the exact identity of the active species is currently unknown, the two aforementioned studies gathered valuable information about what could be promoting their reactions from a report in 2007.3 It was identified that banana peels contain a large amount of potassium and sodium carbonate as well as sodium chloride and other trace elements. It was speculated that the high concentration of alkali metal carbonates in WEB was responsible for the acceleration of these organic transformations.
So, the next time you are finished having a banana, don’t monkey around and throw it away! Give it to a chemist in need, it may help them out more than you think!
1) P. R. Boruah, A. A. Ali, B. Saikia and D. Sarma, Green Chem., 2015, 17, 1442–1445. DOI:10.1039/C4GC02522A
2) B. Saikia, P. Borah and N. Chandra Barua, Green Chem., 2015, 17, 4533–4536. DOI:10.1039/C5GC01404B
3) D. C. Deka and N. N. Talukdar, IJTK, 2007, 6 (1), 72-78.
Figures from Boruah et al. 2015 and Saikia et al. 2015 reproduced with the permission of the Royal Society of Chemistry.