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Micellar effect on the direct Fischer synthesis of alkyl glucosides
Applied Catalysis A, General 539 (2017) 13–18
Contents lists available at ScienceDirect
Applied Catalysis A, General journal homepage: www.elsevier.com/locate/apcata
Research Paper
Micellar effect on the direct Fischer synthesis of alkyl glucosides ⁎
J. Nowicki , J. Woch, M. Mościpan, E. Nowakowska-Bogdan
MARK
Institute of Heavy Organic Synthesis “Blachownia”, PL47-225 Kędzierzyn-Koźle, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords: Alkyl glucoside Fischer reaction Catalysis Micellar effect
This manuscript presents results from the investigation on the synthesis of alkyl glucosides by the novel, very efficient and environmentally friendly protocol of the Fischer-type synthesis from unprotected glucose and aliphatic alcohols. The use of the dual functionality catalysts (surfactant + acid catalyst) and micellar reaction system are the main novelty of described method. It has been found, that in developed method of synthesis the reaction of unprotected glucose with aliphatic alcohols carried out with significantly different route, than the normal (classical) route and leads to alkyl glucopyranoside derivatives with high yields. In progress analyses by DLS, HPLC and GC/MS confirm the general postulated pathway of developed method.
1. Introduction Alkyl glucosides (AGs) are considered as a very important class of biodegradable nonionic surfactants. AGs contain a carbohydrate head group (glucose, galactose, maltose, xylose etc.) and hydrocarbon tail (usually a linear alkyl chain of different length) [1–5]. AGs have numerous important applications as a main components of personal care products, cosmetics, in the preparation of microemulsions (especially in detergents and pesticide formulations) because of their excellent behavior at interfaces and also as agents for extraction of organic dyes and in the membrane protein research [6–8]. The common method of the synthesis of alkyl glucosides is Fischer glycosylation through direct or transacetalization route. In the direct synthesis (onestep protocol), fatty alcohol reacts with glucose or commercial glucose oligomers (e.g. glucose syrup, starch) producing a complex mixture of AGs with various degree of oligomerization (n ≥ 1, Scheme 1) [9,10]. The main disadvantage of this procedure, however, is poor solubility of carbohydrates in long chain aliphatic alcohols. One of the possible way to eliminate this inconvenience is to conduct this process in biphasic reaction system with phase transfer catalyst (ptc), such as ammonium salts or ammonium ionic liquids [11–15]. A special case of biphasic catalysis is the synthesis in micellar reaction systems [16–18]. Formation of micelles results in a great increase in interfacial surface of two immiscible phases, which makes the reaction proceeding at the interface more effective. Synthesis in micellar reaction systems, such as microemulsion is nowadays one of the most interesting and promising method applied in organic synthesis [19,20]. One of the most spectacular example described in literature of “micellar” effect is esterification reactions [21–23]. Water formed during esterification reaction is
⁎
Corresponding author. E-mail address: [email protected] (J. Nowicki).
http://dx.doi.org/10.1016/j.apcata.2017.04.002 Received 3 February 2017; Received in revised form 29 March 2017; Accepted 2 April 2017 Available online 04 April 2017 0926-860X/ © 2017 Published by Elsevier B.V.
not removed from the reaction mixture by conventional methods, but is captured inside the micelles. This phenomenon is the main essence of this method. Synthesis in biphasic systems has been also applied in carbohydrate chemistry, in particular in enzymatic methods of the synthesis of alkyl glucosides. Water in oil microemulsions with reverse micelles provide an interesting alternative to common organic solvents in enzyme catalysis with strong hydrophilic substrates, such as carbohydrates. Several examples of the synthesis of alkyl glycosides in reverse micelles using glycosidases as enzyme catalysts were reported with satisfying results compared to standard aqueous reaction media [24–29]. The use of micelles for separation of reactants was first described by Kobayashi in esterification of carboxylic acids and alcohols [22]. The surfactant-type acid catalyst (dodecylbenzenesulfonic acid, DBSA) and organic substrates in water form microdroplets with hydrophobic interior. For lipophilic substrates (carboxylic acids and alcohols), the equilibrium position between the substrates and the products (esters) lies at the ester side and water molecules would be moved out of the droplets due to hydrophobic nature of their interior. Application of interfacial phenomena in synthesis of alkyl glucosides is very limited. Only a few reports in this field described the use of micellar reaction systems, but not in the synthesis of alkyl glucosides directly from the unprotected glucose. Although Kobayashi et al. reported stereoselective C-glycosylation in water in microemulsion reaction system with the use of DBSA as a surfactant catalyst, but in this method only fully protected (acetylated) sugars were used [30]. Direct synthesis of alkyl glucosides from peracetylated 1-bromoglucose and aliphatic derivatives of phenols was also described by Kumar [31]. The reaction was carried out in the presence of surfactant ionic liquids,
Applied Catalysis A, General 539 (2017) 13–18
J. Nowicki et al.
min N2) and methanol/water was used as the mobile phase (0.5 ml/min flow rate). Data were collected and processed using CHROMELEON (version 6.80) software. Analyses were performed at 30 °C and chromatograms of synthesized alkyl glucosides are presented in Fig. S1–S3 (Supporting information). GC/MS analyses were carried out on the head-space system using HP 5890 Series II chromatograph equipped with flame ionization detector, capillary column, Ultra-high-2 (5HT), l = 15 m, d = 0,32 mm. All samples were transformed into silane derivatives with BSA (N,O-bis(trimethylsilyl)acetamide). Instrument settings: injector temp. −360 °C, detector temp. − 380 °C. Temperature programming: initial isotherm 100 °C – 1 min, 100–380 °C – gradient 15 °C/ min., final isotherm 380 °C − 3 min. Carrier gas: argon −1.8 ml/min. GC/MS chromatograms and corresponding MS spectra are presented in Figs. S4–S13 (Supporting information). Dynamic Light Scattering (DLS) analyses were performed with Zetasizer Nano ZS (Malvern) equipped with vertically polarized incident light of wavelength = 633 nm, supplied by a He-Ne laser. Measurements were carried out at angle 173° at 25 or 50 °C. The apparent hydrodynamic diameter of aggregates formed in studied materials, Dh, was calculated using the Stokes-Einstein equation from data derived by cumulants analysis and was given as average from 4 measurements. All samples were filtered before measurements through Pureland PVDF syringe filters of the nominal pore size 0.22 μm. Size distributions of micelles for synthesized alkyl glucosides are presented in Fig. S14 and in Table S1 (Supporting information).
Scheme 1. Synthesis of alkyl glucoside, “classic” route.
which play a dual role both as simple phase transfer catalyst and solvent. Fischer glycosidation may be considered as a very similar to esterification reaction (acidic catalyst and water as a main by-product). Our idea was based on the methodology successfully applied in the esterification reaction. In contrary to the above described esterification reaction, both reagents (glucose) and products (alkyl glucoside, water) have strong hydrophilic nature. This could suggest, that it will be difficult to remove water from the reaction mixture. Our study, however, clearly showed, that it could be done. We demonstrated, that Fischer glycosidation reaction in micellar system may be carried out according to similar methodology, after some modification. This paper presents investigation of the synthesis of alkyl glycosides direct from glucose and aliphatic alcohols in microemulsion reaction system. The use of micellar reaction system in the synthesis of alkyl glucosides from alcohols and unprotected glucose has been described for the first time. The proposed reaction pathway is presented below (Scheme 2). 2. Experimental
3. Results and discussion
2.1. Materials
3.1. Synthesis of alkyl glucosides
D-glucose (Sigma-Aldrich), 1-octanol, 1-decanol and 1-dodecanol (Alfa Aesar), dodecylbenzenesulfonic acid (DBSA) 95% (SigmaAldrich), methanesulfonic acid 99% (Sigma-Aldrich) and octyl-β-glucopyranoside (Sigma-Aldrich) were used as received. Glucopon 225 DK (C8-C10 alkyl polyglucoside, BASF) was used as commercial alkyl glucoside surfactant for comparative synthesis.
To demonstrate our assumptions reactions of D-glucose with selected aliphatic alcohols have been conducted under biphasic (micellar) reaction conditions in the presence of dodecybenzenesulfonic acid (DBSA) as surfactant catalyst (Table 1). It has been examined in terms of alcohol used, effect on additional amount of water and reaction temperature. The results clearly demonstrate, that DBSA effectively catalysed this reaction. Firstly, the reaction of glucose with 1-octanol (the molar ratio of glucose to alcohol = 1:10) has been investigated in the presence of dodecylbenzenesulfonic acid (5 mol% in relation to glucose) as surfactant-type Brønsted acid for 24 h at 60 °C, 70 °C and 80 °C, with addition of water (10 wt% in relation to glucose). In the presence of water the conversion of glucose (determined by HPLC analysis) decreased from 96.1% at 60 °C to 55.2% at 80 °C (Table 1, entries 1–3). It has been observed, that increasing reaction temperature had a negative impact on glucose conversion. In the absence of additional amounts of water the situation has significantly changed. At 60 °C conversion of glucose was 79.7%, but after increasing the temperature to 80 °C conversion was also increased and reached 99%.
2.2. Synthesis procedure A typical reaction was carried out by adding 3.6 g (0.02 mol) glucose to a 100 ml glass reactor containing 0.2 mol of corresponding alcohol and 0.33 g (0.01 mol) dodecylbenzenesulfonic acid. The resulting suspension was agitated intensively (700 rpm) at desired temperature for 24 h. After that the reaction mixture (clear liquid) was cooled down to room temperature and analysed (GPC and GC/MS). 2.3. Analytical methods High performance liquid chromatography (HPLC) analyses were carried out on Dionex UltiMate 3000 chromatograph (Thermo Fisher Sci.) and RP-18 Purospher HPLC column 250 × 4.6 mm, 5 μm. The system was fitted with ELSD 2000 UV detector (Alltech, 50 °C, 1.5 ml/
Table 1 Results of Fischer synthesis of alkyl glucosides in presence of surfactant- type Brønsted catalysts. Entry
Alcohol
Catalyst
Reaction temp. °C
Water wt%
Conversion %
1 2 3 4 5 6 7 8 9
1-octanol 1-octanol 1-octanol 1-octanol 1-octanol 1-octanol 1-decanol 1-dodecanol 1-octanol
DBSA DBSA DBSA DBSA DBSA DBSA DBSA DBSA APG + KMS
60 70 80 60 70 80 80 80 80
10 10 10 – – – – – –
96.1 66,3 55.2 79.7 80.5 99 98.2 97.5 45.7
(reaction conditions: glucose: alcohol molar ratio − 1:10; DBSA − 5 mol%; 24hr; 700 rpm).
Scheme 2. Synthesis of alkyl glucosides, “microemulsion” route.
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Applied Catalysis A, General 539 (2017) 13–18
J. Nowicki et al.
Table 2 Effect of amount of added catalyst on glucose conversion. Entry
Alcohol
Reaction temp °C
DBAS mol%
Conversion %
9 10 11
1-octanol 1-octanol 1-octanol
80 80 80
5 2.5 1
99.0 96.4 67.1
(reaction conditions: glucose: alcohol molar ratio − 1:10.
Fig. 3. HPLC chromatogram of Oct-G sample (entry 6).
3.2. Micellar effect in Fischer synthesis of alkyl glucosides—general assumption As shown before, the satisfactory conversions of glucose (96–99%) were achieved in experiments with the amounts of catalyst (DBSA) at the level 2.5–5 mol%. Because of the hydrophobicity of continuous phase (octanol) and hydrophilicity of cluster interior in dispersed phase, water molecules could be gradually trapped in the cluster interior while the product (alkyl glucoside) enters the continuous phase. With the progress of the reaction, surfactant clusters are enlarged to form inverse micelles with the aid of alcohol as a cosurfactant (Figs. 1 and 2). This mechanism was also observed in similar solvent-less esterification carried out in micellar system [23]. Synthesis of alkyl glucosides in micellar reaction system in the presence of surfactant catalysts is more complex than conventional method. In the reaction mixture are present both surfactant catalyst (DBSA) and alkyl glucoside. Alkyl glucosides
Fig. 1. Cluster effect of water capture.
(Table 1, entries 4–6). The same reaction conditions, using 1-dodecanol instead 1-decanol resulted with similar conversions (Table 1, entries 7–8). The effect of the amount of the added catalyst (DBSA) on the conversion of D-glucose has been also investigated. Table 2 shows the lower conversion of glucose for low load of DBAS (only 67% for 1 mol %) and the best result for 5 mol% of added catalyst.
Fig. 2. Proposed micelle structure.
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J. Nowicki et al.
Fig. 4. GC chromatogram of Oct-G sample (a, entry 6) and octyl-β-glucopyranoside (b).
acid, 0.2 g) were used (Table 1, enter 9). As can be seen the presence of APG and homogeneous sulfonic acid were not sufficient to obtain high yield of alkyl glucoside. Conversion of glucose was only 67%. One of the possible explanations of this phenomenon is the different solubility of used reagents and formed products. Water is fully miscible with APG and does not form micelles able to capture water molecules.
3.3. Micellar effects on Fischer synthesis of alkyl glucosides—effect on selectivity Fig. 3 presents HPLC chromatogram of reaction product of glucose and 1-octanol (Oct-G). As can be observed in Fig. 3, the reaction of glucose with 1-octanol in micellar reaction system leads to product contains octyl glucoside and small amount of octyl diglucoside. The presence of main product was confirmed by HPLC chromatography and CG/MS spectroscopy using octyl-β-D-glucopyranoside and D-glucose as analytical standards. GC/MS chromatograms of Oct-G sample (a) and octyl-β-D-glucopyranoside (b) are presented in Fig. 4. Surprisingly, it has been found, that on the GC chromatogram only two main compounds were seen (no α and β anomers of octylglucofuranoside present). The results of GC/MS analysis confirmed, that the product was a mixture of α and β anomers of octylglucopyranoside 1. GC/MS analysis also shows the specific selectivity of developed method. In the reaction mixture α and β anomers of octylglucofuranoside 2 (generally observed in post-synthesis mixtures obtained in Fischer glycosidation conducted in conventional reaction conditions) were not seen (Scheme 3) [32]. Table 3 presents α and β anomers ratios for selected alkyl glucosides. As can be seen from Table 3, the dominant anomer is octyl-α-
Scheme 3. Possible change of selectivity in “micellar” Fischer reaction. Table 3 α and β anomer ratios for selected alkyl glucosides. alkyl glucoside Oct-G Dec-G Laur-G
Yield, %
anomer α, wt%
anomer β, wt%
99.0 98.6 97.4
60.4 63.8 67.7
39.6 36.2 32.3
are characterized by very good surfactant properties and may affect the course of the reaction. However, medium chain alkyl glucosides (e.g. C8) are characterized rather by foaming than emulsifier properties. In order to verify the possible impact of APG, the simulated experiment has been done. In place of surfactant catalyst commercial APG (Glucopon, 0.4 g) and homogeneous acid catalyst (methanesulfonic
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Applied Catalysis A, General 539 (2017) 13–18
J. Nowicki et al.
Fig. 5. Micelle hydrodynamic diameters.
aliphatic alcohols. Dodecylbenzenesulfonic acid has shown dual capabilities, both as surfactant and Brønsted catalyst in Fischer glycosidation reaction. Dodecylbenzenesulfonic acid is also considered as environmentally acceptable catalyst. Sodium salt of DBSA formed after alkali working out is fast and fully biodegradable. The main originality and novelty of the described protocol is related to creating microemulsion reaction system for effective course of reaction. The described method has additional advantages compared to the classical method of Fischer. First of all at the lower reaction temperature high carbohydrate conversion is achieved. Furthermore the reaction does not result in oligomers of alkyl glucoside formation. The process also does not require the removal of water using distillation methods, which generally requires a vacuum equipment. The described method may have significant application potential, especially in synthesis of series of important carbohydrate surfactants—alkyl glucosides.
glucopyranoside, which is the main component in reaction product of 1octanol and glucose according well described Fischer glycosylation mechanism [9]. For C10 and C12 alcohols in post-synthesis mixtures only single products: corresponding alkyl glucosides were observed. The results of HPLC and GC/MS analysis of all glucosides have been shown in Supporting Information file. 3.4. Micellar effect in Fischer synthesis of alkyl glucosides—formation of micelles The key pathway assumption for the research described in this paper was to create a micellar system for the Fischer reaction. The reaction products were clear liquids. It is well known that the solubility of water and carbohydrates (e.g. glucose) in C8–C12 alcohols is very low. HPLC analyses and GC/MS showed a virtual absence of glucose in post-synthesis mixtures. Described in Scheme 2 path of the reaction assumed, that the water molecules, a main byproduct of the Fischer reaction were captured inside the surfactant micelles. To confirm this assumption, the micelle size was measured by DLS method (Fig. 5). For C8–C12 alkanols micelle hydrodynamic diameter was in the range of 6.5-40.9 nm and could be classified as micromicelles. Analyses of dynamic light scattering results indicated Dh = 6.5 nm (C8 alcohol), Dh = 6.7 nm (C10 alcohol) and Dh = 40.9 nm (C12 alcohol). 1Dodecanol is solid in 25 °C, so size measurements must be carried out in 50 °C. Hydrodynamic diameter of microemulsion droplets, where the continuous phase is 1-dodecanol was, therefore, significantly larger (40.9 nm) than for the 1-octanol and 1-decanol (6.5 and 6.7 nm, respectively). This is consistent with the published literature data. Microemulsion droplet size grows with increasing temperature is common [33,34]. Calculated polydispersity indexes (Pdi) were 0.3 for C8 − C10 glucosides, which are characteristic for moderate type polydispersity. For C12 glucoside Pdi = 0.6, which is characteristic for broad type polydispersity.
Acknowledgements The research was conducted with financial support from the Polish Ministry of Science and Higher Education for statutory research. This work has been also done within the framework of COST Actions MP1106 “Smart and green interfaces − from single bubbles and drops to industrial, environmental and biomedical applications (SGI)” References [1] D. Svensson, S. Ulvenlund, P. Adlercreutz, Green Chem. 11 (2009) 1222–1226. [2] S. Izawa, Y. Sakai-Tomito, K. Kinomura, S. Kitazawa, M. Tsuda, T. Tsuchiya, J. Biochem. 113 (1993) 573–576. [3] P. Drouet, M. Zhang, M.D. Legoy, Biotechnol. Bioeng. 43 (1994) 1075–1080. [4] G. Milkereit, V.M. Garamus, K. Veermans, R. Willumert, V. Vill, J. Colloid Interf. Sci. 284 (2005) 704–713. [5] O.D. Yakimchuk, A.A. Kotomin, M.B. Petelskij, V.N. Naumov, J. Appl. Chem. 77 (2004) 2001–2005. [6] J.T. Lin, S. Riedel, R. Kinne, Biophys. Acta 557 (1979) 179–187. [7] T. Vanaken, S. Foxall-Vanaken, S. Castleman, S. Ferguson-Miller, Enzymol 125 (1986) 27–35. [8] W. Von Rybinski, K. Hill, Angew. Chem. Int. Ed. 37 (1998) 1328–1345. [9] H. Tesmann, J. Kahre, H. Hensen, B.A. Salka, K. Hill, W. von Rybinski, G. Stoll (Eds.), Salka in Alkyl Polyglycosides: Technology, Properties and Applications, Verlag Chem Ch. 5, 1997, pp. 71–98. [10] Alkyl Polyglucosides From Natural-origin Surfactants to Prospective Delivery Systems, [Ed.: I. Pantelic], Woodhead Publ., 2014. [11] R. Juang, Sh. Liu, Ind. Eng. Chem. Res. 37 (1998) 4625–4630.
4. Conclusions In conclusions, there has been demonstrated for the first time the possibility of effective application of micellar reaction system in acidcatalysed direct Fischer glycosidation unprotected glucose and selected 17
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[24] P.L. Luisi, M. Giomini, M.P. Pileni, B.H. Robinson, Biochem Biophys. Acta 947 (1998) 209–246. [25] S.D. Chen Wei, Z. Hu, J. Mol. Catal. B Enzym. 16 (2001) 109–114. [26] C. Chen, C. Ou-Yang, C. Yeh, Enzyme Microbiol Technol. 33 (2003) 497–507. [27] O.S. Kouptsova, N.L. Klyachko, A.V. Levashov, J. Bioorg Chem. 27 (2001) 380–384. [28] S. Papanikolaou, Bioresour. Technol. 77 (2001) 157–161. [29] C. Shah, S. Sellappan, D. Madamwar, Process Biochem. 35 (2000) 971–975. [30] S. Shirakawa, Sh. Kobayashi, Org. Lett. 9 (2007) 311–314. [31] V. Kumar, I.J. Talisman, O. Bukhari, J. Razzaghy, S.V. Malhotra, RSC Adv. 1 (2011) 1721–1727. [32] A. Nowacki, J. Błażejowski, A. Wiśniewski, J. Mol struct. (Teochem) 664-665 (2003) 217–228. [33] B. Dong, X. Xing, R. Wang, B. Wang, X. Zhou, C. Wang, L. Yu, Z. Wu, Y. Gao, Chem. Commun. 51 (2015) 11119–11122. [34] J. Lang, N. Zana, L. Lalem, The Structure Dynamics and Equilibrium Properties of Colloidal Systems, Academic Publisher., Kluwer, 1990, pp. 253–278.
[12] N.M.T. Lourenco, C.A.M. Afonso, Tetrahedron 59 (2003) 789–794. [13] R.T. Dere, R.R. Pal, P.S. Patil, M.M. Salunkhe, Tetrahedron Lett. 44 (2003) 5351–5353. [14] G.V. Kryshtal, G.M. Zhdankina, S.G. Zlotin, Eur. J. Org. Chem. (2005) 2822–2827. [15] J. Bender, D. Jepkens, H. Hüsken, Org. Process Res. Dev. 14 (2010) 716–721. [16] S. Eriksson, U. Nylen, S. Rojas, M. Boutonnet, Appl. Catalysis. A: General 265 (2004) 207–219. [17] M. Vashishtha, M. Mishra, D.O. Shah, Appl. Catalysis. A: General 466 (2013) 38–44. [18] M. Vashishtha, M. Mishra, S. Undre, M. Singh, D.O. Shah, J. Mol. Catalysis A: Chemical 396 (2015) 143–154. [19] M. Shiri, M.A. Zolfigol, Tetrahedron 65 (2009) 587–598. [20] G. La Sorella, G. Strukul, A. Scarso, Green Chem. 17 (2015) 644–683. [21] H. Stamatis, A. Xenakis, U. Menge, F.N. Kolisis, Biotechnol. Bioeng. 42 (1993) 931–937. [22] K. Manabe, X. Sun, Sh Kobayashi, J. Am. Chem. Soc. 123 (2001) 10101–10102. [23] G. Li, X. Li, W. Eli, New J. Chem. 31 (2007) 348–351.
18
Contents lists available at ScienceDirect
Applied Catalysis A, General journal homepage: www.elsevier.com/locate/apcata
Research Paper
Micellar effect on the direct Fischer synthesis of alkyl glucosides ⁎
J. Nowicki , J. Woch, M. Mościpan, E. Nowakowska-Bogdan
MARK
Institute of Heavy Organic Synthesis “Blachownia”, PL47-225 Kędzierzyn-Koźle, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords: Alkyl glucoside Fischer reaction Catalysis Micellar effect
This manuscript presents results from the investigation on the synthesis of alkyl glucosides by the novel, very efficient and environmentally friendly protocol of the Fischer-type synthesis from unprotected glucose and aliphatic alcohols. The use of the dual functionality catalysts (surfactant + acid catalyst) and micellar reaction system are the main novelty of described method. It has been found, that in developed method of synthesis the reaction of unprotected glucose with aliphatic alcohols carried out with significantly different route, than the normal (classical) route and leads to alkyl glucopyranoside derivatives with high yields. In progress analyses by DLS, HPLC and GC/MS confirm the general postulated pathway of developed method.
1. Introduction Alkyl glucosides (AGs) are considered as a very important class of biodegradable nonionic surfactants. AGs contain a carbohydrate head group (glucose, galactose, maltose, xylose etc.) and hydrocarbon tail (usually a linear alkyl chain of different length) [1–5]. AGs have numerous important applications as a main components of personal care products, cosmetics, in the preparation of microemulsions (especially in detergents and pesticide formulations) because of their excellent behavior at interfaces and also as agents for extraction of organic dyes and in the membrane protein research [6–8]. The common method of the synthesis of alkyl glucosides is Fischer glycosylation through direct or transacetalization route. In the direct synthesis (onestep protocol), fatty alcohol reacts with glucose or commercial glucose oligomers (e.g. glucose syrup, starch) producing a complex mixture of AGs with various degree of oligomerization (n ≥ 1, Scheme 1) [9,10]. The main disadvantage of this procedure, however, is poor solubility of carbohydrates in long chain aliphatic alcohols. One of the possible way to eliminate this inconvenience is to conduct this process in biphasic reaction system with phase transfer catalyst (ptc), such as ammonium salts or ammonium ionic liquids [11–15]. A special case of biphasic catalysis is the synthesis in micellar reaction systems [16–18]. Formation of micelles results in a great increase in interfacial surface of two immiscible phases, which makes the reaction proceeding at the interface more effective. Synthesis in micellar reaction systems, such as microemulsion is nowadays one of the most interesting and promising method applied in organic synthesis [19,20]. One of the most spectacular example described in literature of “micellar” effect is esterification reactions [21–23]. Water formed during esterification reaction is
⁎
Corresponding author. E-mail address: [email protected] (J. Nowicki).
http://dx.doi.org/10.1016/j.apcata.2017.04.002 Received 3 February 2017; Received in revised form 29 March 2017; Accepted 2 April 2017 Available online 04 April 2017 0926-860X/ © 2017 Published by Elsevier B.V.
not removed from the reaction mixture by conventional methods, but is captured inside the micelles. This phenomenon is the main essence of this method. Synthesis in biphasic systems has been also applied in carbohydrate chemistry, in particular in enzymatic methods of the synthesis of alkyl glucosides. Water in oil microemulsions with reverse micelles provide an interesting alternative to common organic solvents in enzyme catalysis with strong hydrophilic substrates, such as carbohydrates. Several examples of the synthesis of alkyl glycosides in reverse micelles using glycosidases as enzyme catalysts were reported with satisfying results compared to standard aqueous reaction media [24–29]. The use of micelles for separation of reactants was first described by Kobayashi in esterification of carboxylic acids and alcohols [22]. The surfactant-type acid catalyst (dodecylbenzenesulfonic acid, DBSA) and organic substrates in water form microdroplets with hydrophobic interior. For lipophilic substrates (carboxylic acids and alcohols), the equilibrium position between the substrates and the products (esters) lies at the ester side and water molecules would be moved out of the droplets due to hydrophobic nature of their interior. Application of interfacial phenomena in synthesis of alkyl glucosides is very limited. Only a few reports in this field described the use of micellar reaction systems, but not in the synthesis of alkyl glucosides directly from the unprotected glucose. Although Kobayashi et al. reported stereoselective C-glycosylation in water in microemulsion reaction system with the use of DBSA as a surfactant catalyst, but in this method only fully protected (acetylated) sugars were used [30]. Direct synthesis of alkyl glucosides from peracetylated 1-bromoglucose and aliphatic derivatives of phenols was also described by Kumar [31]. The reaction was carried out in the presence of surfactant ionic liquids,
Applied Catalysis A, General 539 (2017) 13–18
J. Nowicki et al.
min N2) and methanol/water was used as the mobile phase (0.5 ml/min flow rate). Data were collected and processed using CHROMELEON (version 6.80) software. Analyses were performed at 30 °C and chromatograms of synthesized alkyl glucosides are presented in Fig. S1–S3 (Supporting information). GC/MS analyses were carried out on the head-space system using HP 5890 Series II chromatograph equipped with flame ionization detector, capillary column, Ultra-high-2 (5HT), l = 15 m, d = 0,32 mm. All samples were transformed into silane derivatives with BSA (N,O-bis(trimethylsilyl)acetamide). Instrument settings: injector temp. −360 °C, detector temp. − 380 °C. Temperature programming: initial isotherm 100 °C – 1 min, 100–380 °C – gradient 15 °C/ min., final isotherm 380 °C − 3 min. Carrier gas: argon −1.8 ml/min. GC/MS chromatograms and corresponding MS spectra are presented in Figs. S4–S13 (Supporting information). Dynamic Light Scattering (DLS) analyses were performed with Zetasizer Nano ZS (Malvern) equipped with vertically polarized incident light of wavelength = 633 nm, supplied by a He-Ne laser. Measurements were carried out at angle 173° at 25 or 50 °C. The apparent hydrodynamic diameter of aggregates formed in studied materials, Dh, was calculated using the Stokes-Einstein equation from data derived by cumulants analysis and was given as average from 4 measurements. All samples were filtered before measurements through Pureland PVDF syringe filters of the nominal pore size 0.22 μm. Size distributions of micelles for synthesized alkyl glucosides are presented in Fig. S14 and in Table S1 (Supporting information).
Scheme 1. Synthesis of alkyl glucoside, “classic” route.
which play a dual role both as simple phase transfer catalyst and solvent. Fischer glycosidation may be considered as a very similar to esterification reaction (acidic catalyst and water as a main by-product). Our idea was based on the methodology successfully applied in the esterification reaction. In contrary to the above described esterification reaction, both reagents (glucose) and products (alkyl glucoside, water) have strong hydrophilic nature. This could suggest, that it will be difficult to remove water from the reaction mixture. Our study, however, clearly showed, that it could be done. We demonstrated, that Fischer glycosidation reaction in micellar system may be carried out according to similar methodology, after some modification. This paper presents investigation of the synthesis of alkyl glycosides direct from glucose and aliphatic alcohols in microemulsion reaction system. The use of micellar reaction system in the synthesis of alkyl glucosides from alcohols and unprotected glucose has been described for the first time. The proposed reaction pathway is presented below (Scheme 2). 2. Experimental
3. Results and discussion
2.1. Materials
3.1. Synthesis of alkyl glucosides
D-glucose (Sigma-Aldrich), 1-octanol, 1-decanol and 1-dodecanol (Alfa Aesar), dodecylbenzenesulfonic acid (DBSA) 95% (SigmaAldrich), methanesulfonic acid 99% (Sigma-Aldrich) and octyl-β-glucopyranoside (Sigma-Aldrich) were used as received. Glucopon 225 DK (C8-C10 alkyl polyglucoside, BASF) was used as commercial alkyl glucoside surfactant for comparative synthesis.
To demonstrate our assumptions reactions of D-glucose with selected aliphatic alcohols have been conducted under biphasic (micellar) reaction conditions in the presence of dodecybenzenesulfonic acid (DBSA) as surfactant catalyst (Table 1). It has been examined in terms of alcohol used, effect on additional amount of water and reaction temperature. The results clearly demonstrate, that DBSA effectively catalysed this reaction. Firstly, the reaction of glucose with 1-octanol (the molar ratio of glucose to alcohol = 1:10) has been investigated in the presence of dodecylbenzenesulfonic acid (5 mol% in relation to glucose) as surfactant-type Brønsted acid for 24 h at 60 °C, 70 °C and 80 °C, with addition of water (10 wt% in relation to glucose). In the presence of water the conversion of glucose (determined by HPLC analysis) decreased from 96.1% at 60 °C to 55.2% at 80 °C (Table 1, entries 1–3). It has been observed, that increasing reaction temperature had a negative impact on glucose conversion. In the absence of additional amounts of water the situation has significantly changed. At 60 °C conversion of glucose was 79.7%, but after increasing the temperature to 80 °C conversion was also increased and reached 99%.
2.2. Synthesis procedure A typical reaction was carried out by adding 3.6 g (0.02 mol) glucose to a 100 ml glass reactor containing 0.2 mol of corresponding alcohol and 0.33 g (0.01 mol) dodecylbenzenesulfonic acid. The resulting suspension was agitated intensively (700 rpm) at desired temperature for 24 h. After that the reaction mixture (clear liquid) was cooled down to room temperature and analysed (GPC and GC/MS). 2.3. Analytical methods High performance liquid chromatography (HPLC) analyses were carried out on Dionex UltiMate 3000 chromatograph (Thermo Fisher Sci.) and RP-18 Purospher HPLC column 250 × 4.6 mm, 5 μm. The system was fitted with ELSD 2000 UV detector (Alltech, 50 °C, 1.5 ml/
Table 1 Results of Fischer synthesis of alkyl glucosides in presence of surfactant- type Brønsted catalysts. Entry
Alcohol
Catalyst
Reaction temp. °C
Water wt%
Conversion %
1 2 3 4 5 6 7 8 9
1-octanol 1-octanol 1-octanol 1-octanol 1-octanol 1-octanol 1-decanol 1-dodecanol 1-octanol
DBSA DBSA DBSA DBSA DBSA DBSA DBSA DBSA APG + KMS
60 70 80 60 70 80 80 80 80
10 10 10 – – – – – –
96.1 66,3 55.2 79.7 80.5 99 98.2 97.5 45.7
(reaction conditions: glucose: alcohol molar ratio − 1:10; DBSA − 5 mol%; 24hr; 700 rpm).
Scheme 2. Synthesis of alkyl glucosides, “microemulsion” route.
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Table 2 Effect of amount of added catalyst on glucose conversion. Entry
Alcohol
Reaction temp °C
DBAS mol%
Conversion %
9 10 11
1-octanol 1-octanol 1-octanol
80 80 80
5 2.5 1
99.0 96.4 67.1
(reaction conditions: glucose: alcohol molar ratio − 1:10.
Fig. 3. HPLC chromatogram of Oct-G sample (entry 6).
3.2. Micellar effect in Fischer synthesis of alkyl glucosides—general assumption As shown before, the satisfactory conversions of glucose (96–99%) were achieved in experiments with the amounts of catalyst (DBSA) at the level 2.5–5 mol%. Because of the hydrophobicity of continuous phase (octanol) and hydrophilicity of cluster interior in dispersed phase, water molecules could be gradually trapped in the cluster interior while the product (alkyl glucoside) enters the continuous phase. With the progress of the reaction, surfactant clusters are enlarged to form inverse micelles with the aid of alcohol as a cosurfactant (Figs. 1 and 2). This mechanism was also observed in similar solvent-less esterification carried out in micellar system [23]. Synthesis of alkyl glucosides in micellar reaction system in the presence of surfactant catalysts is more complex than conventional method. In the reaction mixture are present both surfactant catalyst (DBSA) and alkyl glucoside. Alkyl glucosides
Fig. 1. Cluster effect of water capture.
(Table 1, entries 4–6). The same reaction conditions, using 1-dodecanol instead 1-decanol resulted with similar conversions (Table 1, entries 7–8). The effect of the amount of the added catalyst (DBSA) on the conversion of D-glucose has been also investigated. Table 2 shows the lower conversion of glucose for low load of DBAS (only 67% for 1 mol %) and the best result for 5 mol% of added catalyst.
Fig. 2. Proposed micelle structure.
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Fig. 4. GC chromatogram of Oct-G sample (a, entry 6) and octyl-β-glucopyranoside (b).
acid, 0.2 g) were used (Table 1, enter 9). As can be seen the presence of APG and homogeneous sulfonic acid were not sufficient to obtain high yield of alkyl glucoside. Conversion of glucose was only 67%. One of the possible explanations of this phenomenon is the different solubility of used reagents and formed products. Water is fully miscible with APG and does not form micelles able to capture water molecules.
3.3. Micellar effects on Fischer synthesis of alkyl glucosides—effect on selectivity Fig. 3 presents HPLC chromatogram of reaction product of glucose and 1-octanol (Oct-G). As can be observed in Fig. 3, the reaction of glucose with 1-octanol in micellar reaction system leads to product contains octyl glucoside and small amount of octyl diglucoside. The presence of main product was confirmed by HPLC chromatography and CG/MS spectroscopy using octyl-β-D-glucopyranoside and D-glucose as analytical standards. GC/MS chromatograms of Oct-G sample (a) and octyl-β-D-glucopyranoside (b) are presented in Fig. 4. Surprisingly, it has been found, that on the GC chromatogram only two main compounds were seen (no α and β anomers of octylglucofuranoside present). The results of GC/MS analysis confirmed, that the product was a mixture of α and β anomers of octylglucopyranoside 1. GC/MS analysis also shows the specific selectivity of developed method. In the reaction mixture α and β anomers of octylglucofuranoside 2 (generally observed in post-synthesis mixtures obtained in Fischer glycosidation conducted in conventional reaction conditions) were not seen (Scheme 3) [32]. Table 3 presents α and β anomers ratios for selected alkyl glucosides. As can be seen from Table 3, the dominant anomer is octyl-α-
Scheme 3. Possible change of selectivity in “micellar” Fischer reaction. Table 3 α and β anomer ratios for selected alkyl glucosides. alkyl glucoside Oct-G Dec-G Laur-G
Yield, %
anomer α, wt%
anomer β, wt%
99.0 98.6 97.4
60.4 63.8 67.7
39.6 36.2 32.3
are characterized by very good surfactant properties and may affect the course of the reaction. However, medium chain alkyl glucosides (e.g. C8) are characterized rather by foaming than emulsifier properties. In order to verify the possible impact of APG, the simulated experiment has been done. In place of surfactant catalyst commercial APG (Glucopon, 0.4 g) and homogeneous acid catalyst (methanesulfonic
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Fig. 5. Micelle hydrodynamic diameters.
aliphatic alcohols. Dodecylbenzenesulfonic acid has shown dual capabilities, both as surfactant and Brønsted catalyst in Fischer glycosidation reaction. Dodecylbenzenesulfonic acid is also considered as environmentally acceptable catalyst. Sodium salt of DBSA formed after alkali working out is fast and fully biodegradable. The main originality and novelty of the described protocol is related to creating microemulsion reaction system for effective course of reaction. The described method has additional advantages compared to the classical method of Fischer. First of all at the lower reaction temperature high carbohydrate conversion is achieved. Furthermore the reaction does not result in oligomers of alkyl glucoside formation. The process also does not require the removal of water using distillation methods, which generally requires a vacuum equipment. The described method may have significant application potential, especially in synthesis of series of important carbohydrate surfactants—alkyl glucosides.
glucopyranoside, which is the main component in reaction product of 1octanol and glucose according well described Fischer glycosylation mechanism [9]. For C10 and C12 alcohols in post-synthesis mixtures only single products: corresponding alkyl glucosides were observed. The results of HPLC and GC/MS analysis of all glucosides have been shown in Supporting Information file. 3.4. Micellar effect in Fischer synthesis of alkyl glucosides—formation of micelles The key pathway assumption for the research described in this paper was to create a micellar system for the Fischer reaction. The reaction products were clear liquids. It is well known that the solubility of water and carbohydrates (e.g. glucose) in C8–C12 alcohols is very low. HPLC analyses and GC/MS showed a virtual absence of glucose in post-synthesis mixtures. Described in Scheme 2 path of the reaction assumed, that the water molecules, a main byproduct of the Fischer reaction were captured inside the surfactant micelles. To confirm this assumption, the micelle size was measured by DLS method (Fig. 5). For C8–C12 alkanols micelle hydrodynamic diameter was in the range of 6.5-40.9 nm and could be classified as micromicelles. Analyses of dynamic light scattering results indicated Dh = 6.5 nm (C8 alcohol), Dh = 6.7 nm (C10 alcohol) and Dh = 40.9 nm (C12 alcohol). 1Dodecanol is solid in 25 °C, so size measurements must be carried out in 50 °C. Hydrodynamic diameter of microemulsion droplets, where the continuous phase is 1-dodecanol was, therefore, significantly larger (40.9 nm) than for the 1-octanol and 1-decanol (6.5 and 6.7 nm, respectively). This is consistent with the published literature data. Microemulsion droplet size grows with increasing temperature is common [33,34]. Calculated polydispersity indexes (Pdi) were 0.3 for C8 − C10 glucosides, which are characteristic for moderate type polydispersity. For C12 glucoside Pdi = 0.6, which is characteristic for broad type polydispersity.
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