SYNTHESIS OF DIGERANYL ESTERS IN SOLVENT-FREE MEDIUM USING BUTYLSTANNOIC ACID AS CATALYST

 

Scigalski P.

Maria Curie-Sklodowska University,

Lublin, Poland.

second-year PhD student.

piotr.scigalski@gmail.com

Scientific adviser: Gawdzik B.

 

Esters are very important aroma compounds and due to their organoleptic properties they are also among important flavour and fragrance components [1]. They find a wide application in beverages, cosmetic and pharmaceutical products as a component of many deodorants, cosmetic formulations, care products, domestic and household products, flavours in food, etc. Terpene esters are a group of compounds especially important for commercial view. They occur naturally in many floral essential oils giving fruity and flowery odour [2]. Fatty acid esters of the acyclic terpene alcohols are used for obtaining citrus notes and rounding off other, unpleasant flavour types [3]. Among acyclic terpene alcohols, geraniol and its esters, e. g. formats, acetates, propionates, isobutyrates, isovalerates, etc. are the most important commercial products and are widely used in the reconstruction of such oils as well as in perfume and flavour composition.

Geranyl esters are in high demand and were historically obtained by extraction from plant sources and microbial fermentation, though these methods exhibited low yields of desired flavour component and imposed high processing cost for commercial production [4]. As a result of the development of large scale production processes of terpenes, the esters of acyclic terpene alcohols are usually made synthetically nowadays. Chemical methods include esterification of terpene alcohols with acids or acid anhydrides and transesterification processes. These methods usually require using strong acid catalyst, e. g. sulphuric acid, p -toluenesulphonic acid or hydrochloric acid, that often leads to structural rearrangements of substrates like isomerization and formation of undesirable byproducts. Thus it results in reduced yields of pure product and issues necessity of the use of considerable excess of terpene alcohol in esterification process [5]. In contrast, non-catalyzed esterification progresses slowly, also excess of alcoholic substrate and higher temperatures, which cause dehydratation of terpene, need to be employed.

For the purpose of my further research diesters of geraniol with several carboxylic acids and acid anhydrides were synthesized using butylstannoic acid as a catalyst. It is a solid, hydrolytically stable, neutral and non-corrosive catalyst that is active a low loadings and minimizes share of side reactions. In the industry it is used primarily as esterification and polycondensation catalyst.

Series of reactions with different amounts of bytulstannoic acid in temperature range from 110 to 150ºC were performed in order to investigate it’s influence on the esterification of geraniol and influence of temperature on the catalyst behaviour. Primarily examined reaction was an esterification of geraniol with succinic anhydride, there were also used other dicarboxylic anhydrides and acids, such as adypic acid, sebacic acid and glutaric anhydride.

Using aforementioned catalyst allowed for conversions of substrates that exceeds 90% of initial amount of functional groups. Analyses of samples drawn from the reaction mixture during the process allowed maintaining control over the reaction progress. Substrates conversions were calculated based on the ¹H NMR spectra of the samples in case of geraniol and the residual acid content for carboxylic groups.

Results confirm that butylstannoic acid is a very effective catalyst for geraniol esterification process, showing high yield of desired product within significantly short reaction times and in relatively low temperatures. It also doesn’t cause any rearrangement in the molecular structure. This shows that the butystannoic acid may be considered a valuable alternative for other esterification catalysts.

 

References:

[1] Keng P.S., Basri M., Zakaria, M.R.S., Abdul Rahman M.B., Ariff A.B., Abdul Rahman R.N.Z., Salleh A. B. Ind. Crop. Prod 29, 37-44 (2009)

[2] Chen W., Viljoen A.M. South African Journal of Botany 76, 643-651 (2010)

[3] Bauer K., Garbe D., Surburg H. Wiley-VCh, 43-45 (2001)

[4] Welsh W.W., Murray W.D. Williams R.E. Crit. Rev. Biotechnol. 9, 105-169 (1989)

[5] Gildemeister E., Hoffmann F.R. The volatile oils, John Wiley and Sons, Inc., New York, 1913