Brönsted Acid of Keggin Type Polyoxometalate Catalyzed Pinacol Rearrangement

Keggin type polyoxometalates K4[a-SiW12O40] was synthesized and transformed to H4[a-SiW12O40]. Both catalysts have been used for pinacol rearrangement in toluene at 373 oK. The results showed that reaction of pinacol rearrangement did not proceed using K4[a-SiW12O40] as catalyst. The extent reaction time until 20 h also did not produce pinacolone as main product. By using H4[a-SiW12O40] as catalyst at 1 h reaction time gave conversion 100% with formation of pinacolone 72%. The reaction produce 27% of 2,3-dimethyl-1,3-butadiene as byproduct and 99% carbon balance for the reaction. This phenomenon indicated the Brönsted acid is a key role for catalytic reaction of pinacol rearrangement to pinacolone. © 2014 BCREC UNDIP. All rights reserved


Introduction
The pinacol rearrangement is a simple wellknown reaction under acid condition to give pinacolone as main product [1].The mechanisms of pinacol to pinacolone are elimination of water and skeletal rearrangement of 1,2diols in acid medium [2].The reaction is usually carried out in the presence of strong inorganic liquid acid such as hydrochloric acid, sulphuric acid, and nitric acid [3].The main problems using these catalysts are handling reagents and corrosive.Recently, the development of solid acid catalyst is sharply increased in order to obtain stable catalyst, non-corrosive, easy to handle, eco friendly, and reusability.Several solid acid catalysts have been tasted for pinacol to pinacolone rearrangement.Molecular sieve of aluminophosphate with different crystalline structures were successfully catalyzed pinacol rearrangement with two main products [4].Aluminosilicate material such as zeolite is also catalyzed pinacol to pinacolone rearrangement [5].Both molecular sieve and aluminosilicate materials are Lewis acid catalyzed pinacol rearrangement.Solid acid catalyst with metal-oxygen cluster that we called "polyoxometalate compounds" is investigated as acid catalyzed pinacol rearrangement [6].These catalysts have already been introduced into large scales industrial processes including oxidation, hydration, and polymerization [7].
The advantages of these catalysts are various structures, several oxidation states, and almost metals in periodic table can be exchanged to modify polyoxometalate structures.Polyoxometalate catalyzed pinacol rearrangement was carried out in both homogeneous and heterogeneous system, thus the typical acidity of catalyst is depending on medium reaction.Recently, Uchida et al. [8] reported ionic crystals based on polyoxometalate could catalyze pinacol rearrangement to pinacolone due to p-p stacking interaction in the crystal structure and act as an acid species.In the previous reports, study of acid catalyzed pinacol to pinacolone rearrangement is focuses only for acid can catalyzed the rearrangement without deeply search the kind of acid.Thus in this research we emphasized the topic of pinacol to pinacolone rearrangement deeply by evaluate the kind of acid.The aim of the present work was to study acid type in pinacol rearrangement using polyoxometalate catalysts.Polyoxometalates K4[a-SiW12O40] and H4[a-SiW12O40] as shown in Figure 1 are used in wide range area as catalyst but the reaction medium is play important rule to obtain high yield of product.In this research, toluene as medium to pinacol rearrangement using polyoxometalate catalyst was used.

Materials and Methods
All starting materials were used directly after purchased without further purification i.e. sodium tungsten, sodium metasilicate, hydrochloric acid, pinacol were from Kanto.Diethyl ether and naphthalene were from Merck.The polyoxometalate K4[a-SiW12O40] and H4[a-SiW12O40] were synthesized according to Téze and Herve [9].The KBr pellet was used for sample preparation and FTIR Shimadzu 8201PC was used to characterize the sample at wavenumber 400-4000 cm -1 . 1 H NMR Jeol 400 MHz was used to investigate the existence of proton in polyoxometalate and sample was dissolved in D2O as solvent.Monitoring reaction was performed using gas chromatography (GC) Perkin Elmer with FID detector and TC-Wax column.GC-MS was carried out using GC-MS Shimadzu 2010QP.

Synthesis and Characterization of K4 [a-SiW12O40]
Sodium metasilicate (11 g, 50 mmol) was dissolved into 100 ml of water (solution A).Sodium tungstate (182 g, 0.55 mol) was dissolved into 300 ml of boiling water (solution B).A solution of 4 M HCl (165 ml) was added drop by drop over 5 min with vigorous stirring in order to dissolve the local precipitate of tungstic acid.Then solution A was added quickly to solution B followed by adding 50 ml of 4 M HCl.The pH is around 5 to 6.The solution was kept at 373 K for 1 hour.A solution of 1 M sodium tungstate (50 ml, 50 mmol) and immediately thereafter, 80 ml of 4 M HCl were added.The solution was filtered after cooling to room temperature.The potassium salt was obtained by adjusting pH to around 2 with aqueous 1 M KOH, and then solid KCl (50 g) was added.White precipitate of K4[a-SiW12O40] was obtained.K4[a-SiW12O40] was characterized using FT-IR spectrophotometer.

Transformation and Characterization of K4[a-SiW12O40]to H4[a-SiW12O40][10].
Transformation of K4[a-SiW12O40] to H4[a-SiW12O40] was carried out by ion exchange concept.Solid material of K4[a-SiW12O40] (50 g) was dissolved in 100 ml water.Hydrochloric acid was added slowly in the solution of K4[a-SiW12O40] until volume of hydrochloric acid 50 ml.The mixture was dropped into separation funnel and 50 ml of diethyl ether was added.The mixture was extracted and the polyoxometalate layer was collected and evaporated.White crystal of H4[a-SiW12O40] was obtained after evaporation which was purified using water in several times.Polyoxometalate H4[a-SiW12O40] was characterized using FT-IR spectrophotometer and 1 H NMR in D2O as solvent.

Rearrangement of Pinacol to Pinacolone
Rearrangement pinacol to pinacolone was carried out in reaction tube equipped with seal and magnetic stirring.Detail procedure as follows: Into a reaction tube were successively placed pinacol (46.5 mg, 0.394 mmol), naphthalene (0.023 g, 0.18 mmol) as internal standard, polyoxometalate as catalyst (0.0183 mmol), and toluene (3 ml).Then nitrogen gas was introduced into the system.The products and yields

Characterization of Catalyst
Polyoxometalates K4[a-SiW12O40] is white solid material and water soluble.These potassium polyoxometalate can be easily transformed to H4[a-SiW12O40] by simple extraction to form also white solid acid [11].Polyoxometalate H4[a-SiW12O40] is soluble in water and organic solvent such as methanol and ethanol.Polyoxometalate K4[a-SiW12O40] and H4[a-SiW12O40] unable be distinguished by physical observation, thus spectroscopy such as FTIR Characteristics vibration of polyoxometalates K4[a-SiW12O40] and H4[a-SiW12O40] are appeared at wavelength 980 cm -1 for W=O, 926 cm -1 for Si-O, 881 cm -1 for W-Oc-W, and 786 cm -1 for W-Oe-W.Both polyoxometalates K4[a-SiW12O40] and H4[a-SiW12O40] have similar absorption bands in FTIR spectrum [12].Although we unable to distinguish the characteristic bands in polyoxometalates K4[a-SiW12O40] and H4[a-SiW12O40] but from their specific vibration in Figure 2 we can conclude the availability of [a-SiW12O40] 4-species in the white solid material of polyoxometalates.In order to know the potassium salt or acid properties in polyoxometalates K4[a-SiW12O40] and H4[a-SiW12O40], the measurement using 1 H NMR is vital.Spectrum 1 H NMR of polyoxometalates H4[a-SiW12O40] is shown in Figure 3.
Spectrum 1 H NMR polyoxometalates H4[a-SiW12O40] in D2O as a solvent indicated single peak at 6.38 ppm which has contribution from proton in polyoxometalate H4[a-SiW12O40].These results showed that four proton in polyoxometalate H4[a-SiW12O40] are equivalent in the structure of [a-SiW12O40] 4-and act as Brönsted acid [13], while no 1 H NMR spectrum was observed for polyoxometalate K4[a-SiW12O40] due to no proton peak in the chemical shift from 1-15 ppm as shown in Figure 4. Brönsted acid can be distinguished clearly using both 1 H NMR spectrum in Figures 3 and 4.

Pinacol rearrangement to pinacolon using polyoxometalate as catalyst
Polyoxometalates K4[a-SiW12O40] and H4[a-SiW12O40] were used as catalysts in pinacol rearrangement to pinacolone in toluene as medium.As described in the introduction, catalysis using polyoxometalate is depending on the reaction medium [14].Medium can be selected from inorganic to organic solvent with polarity wide range area.The results of pinacol rearrangement using polyoxometalate K4[a-SiW12O40] and H4[a-SiW12O40] is shown in Ta-

Bulletin of Chemical Reaction Engineering & Catalysis, 9 (2), 2014, 138
Copyright © 2014, BCREC, ISSN 1978-2993      butadiene with m/z 82. Figure 6 shows the fragmentation pattern of these two products.Calculation of peak area of two products resulted 72% yield for pinacolone and 27% for 2,3 -dimethyl-1,3-butadiene.It was found that carbon balance for these catalytic reactions is 99%, which indicated the equilibrium mass of reactant to product.All results showed that pinacol rearrangement to pinacolone was carried out using Brönsted acid catalyst from polyoxometalate H4[a-SiW12O40] and summarized in Scheme 2 with total reaction in Scheme 3.

Conclusions
The Polyoxometalate K4[a-SiW12O40] can be transformed to H4[a-SiW12O40] by ion exchange method.Polyoxometalate K4[a-SiW12O40] did not active for pinacol rearrangement.Polyoxometalate H4[a-SiW12O40] showed its catalytic activity for pinacol rearrangement to pinacolone with formation of 2,3-dimethyl-1,3butadiene as byproduct.The pinacolone was 72% yield and 27% byproduct.The reaction system is stable due to stability of carbon balance in the reaction until 99%.

Figure 4 . 1 HFigure 6 .
Figure 4. 1 H NMR spectrum of K4[a-SiW12O40] chromatogram in Figure 5.The results shows polyoxometalate K4[a-SiW12O40] did not have catalytically active properties for pinacol rearrangement.The extent reaction time until 20 h also did not give pinacolone as main product.By the same condition with polyoxometalate K4[a-SiW12O40] we change the catalyst using polyoxometalate H4[a-SiW12O40].The results showed that at 1 h conversion of pinacol is reached 100%.Percentage of conversion and selectivity were calculated base on equation in scheme 1. Identification using GC in Figure 5 indicated of pinacolone and butadiene were found as products in the reaction system.The main product with m/z 100 is pinacolone and another is 2,3-dimethyl-1,3-Bulletin of Chemical Reaction Engineering & Catalysis, 9 (2), 2014, 139 Copyright © 2014, BCREC, ISSN 1978-2993