Research on hydration of phenylacetylene assisted with additives in near-critical water

Available online at www.sciencedirect.comCHINESEScienceDirectC HEMICALLETTERSEL SEVIERChinese Chemical Letters 22 (2011) 393 -396www.clsevier.com/locate/ccletResearch on hydration of phenylacetylene assistedwith additives in near-critical waterShuang Li, Yong Juan Chang, Yu Wang, Li Yi Dai *Shanghai Key Laboratory of Green Chemistry and Chemical Process, Deparment of Chemistry,East China Normal University, Shanghai 200062 ChinaReceived 22 June 2010Available online 15 January 201 IAbstractThe hydration of phenylacetylene in near-critical water, assisted with additives (NaHSO4, ZnCl2, FeCl3), has been sucessfullyconducted with temperature and residence time ranges of 220- -300 °C and 60-180 min, respectively. The results showed thecatalytic ability is FeCl3 > ZnClz > NaHSO4. The maximum yield of product acetophenone was 96.68% at 260 °C, 120 min. Basedon the results found, a possible mechanism of hydration of phenylacetylene in near-critical water was proposed.C 2010 Li Yi Dai. Publisbed by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.Keywords: Hydration; Phenylacetylene; Additives; Near-critical water, MechanismNear-critical water (NCW,T= 150 -374。C, P=0.4 -21.83 MPa) is a promising green medium in which to conductorganic reactions without additional acid/base catalyst due to its special properties, such as good solubility, high ionproduct and low dielectric constant [1]. The self-dissociation of NCW can offer sufficiently high concentration of H*and OHT to catalyze some reactions [2- -5]. Moreover, NCW is inexpensive, nontoxic, neither inflammable norexplosive and ecologically safe. So it has the potential application in green chemistry, chemical process and organicreaction areas.The hydration of alkynes is one of the most useful approaches for group transformation in organic synthesis, as theaddition of water to alkynes generates valuable carbonyl compounds from unsaturated hydrocarbon precursors {6].Traditionally, the reaction is carried out in sulfuric aqueous with excess HgO as catalyst. Recent years, some Bronstedacid catalysts were developed [7-11]. Nearly all of these catalysts play their roles in organic solvents [12,13]. Andsome less toxic metals, such as gold, platinum, palladium and ruthenium were studied to replace mercury [6].Obviously, these metal catalysts are not economically available.Katritzky et al. [14] heated phenylacetylene in NCW up to 5 days, and their best result showed acetophenone in51% conversion. Jerome and Parsons [15] heated alkynes in supercritical water without catalyst up to 2 h, and theyobtained no ketone but some low molecular weight oligomers. Alternatively, Kremsner and Kappe reported thecomplete conversion of phenylacetylene to acetophenone in pure water under microwave conditions at least 150 min at295 °C, with dimeric and trimeric self-condensation products [16]. During our study, effects of reaction conditions* Corresponding author.中国煤化工E-mail address: lydai@cbem.ecnu.cdu.cn (L.Y. Dai),MHCNMHG1001-8417/S - see front matter◎2010 Li Yi Dai. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All nights reserved.doi:10.1016j.cet.2010.11.003394S. Li et al./Chinese Chemical Letters 22 (2011) 393 -396such as temperature, residence time and additives (NaHSO4, ZnCl2, FeCl3) have been investigated. High yield ofacetophenone was obtained in NCW under optimal conditions. A possible reaction mechanism for catalytic hydrationof phenylacetylene in NCW system was proposed.1. ExperimentalThe principal item we used is a high-pressure bath reactor consisted of stainless steel. The same concentration ofadditive was 5.00 mol%. Solutions were prepared by dissolving the additives in the double-distilled water. The totalamount of stock solution was calculated to drive the pressure inside the reactor to the saturated water vapor pressure atany given temperature [17]. The solution and phenylacetylene (the volume ratio is 25:1) were fed via a microlitersyringes. The loaded reactor was put into the salt bath. When the desired reaction time had elapsed, it was immersedinto an ice water bath to terminate the reaction. And then the reactor was rinsed with acetone at least three times torecover the product via a microliter syringes. Five replicate experiments were conducted and the points given in theplots with error bars were all the average values of the experimental data.The liquid sample was quantitatively analyzed via an Agilent 6890GC by comparing the peak area to that of theinternal standard toluene. Product identifcation was achieved by matching the retention time to those of the authenticcompounds and by inspecting mass spectra.2. Results and discussionThe hydration of alkynes is acid-catalyzed. Traditionally, the reaction was carried out by mercuric salt catalysts inexcess sulfuric aqueous [13], which could provide rich H+ to initiate the reaction. NCW could offer sufficiently highconcentration of H* resulting from its self-dissociation. Therefore, NCW had a potential ability to promote thehydration of alkynes. Moreover, fewer additives were introduced to NCW to modify the microenvironment, which isadvantageous to the hydration of alkynes.Fig. 1 demonstrates the effect of temperature on the yield of acetophenone. As we can see, there were the samecurve tendencies to different additives. The yields with additives were much higher than those without additive. At260 °C, the yield was only 39.56% in the absence of additive, while it was 72.15%, 84.20%, 95.90% assisted withNaHSO4, ZnCz, FeCl3, respectively. This is due to the catalyst H+ in the reaction. The dissociation of NaHSO4generated H*. The hydrolysis of ZnCl2 and FeCl3 also gave H*. Additionally, the self- dissociation of NCW would beintensified by the introduction of additives NaHSO4, ZnCl2 and FeCl3. As a result, the acetophenone yield increasedgreatly in the presence of additives. On the other hand, ZnCl2 and FeCl3 played the role of Lewis acids to promote thehydration. And FeCl3 is stronger than ZnCl2 in terms of Lewis acidity. As a consequence, the catalytic ability isFeCl3 > ZnCl2 > NaHSO4 on the same condition in NCW system.In the lower temperature ranges (220 -260。C), the yield increased gradually with increasing temperature. But itdropped considerably after reaching a maximum around 260 °C. The shitof yield over the temperature range was aresultof two effects: firstly, with temperature increasing, the ionization constant of water increased and NCW offered more H*,0]070-so010一30-20-中国煤化工THCNMHGTanpenauerecFig. 1. Yield dependence of acetophenone on temperaure at 90 min with various additives.s. Li et al./Chinese Chemical Ltters 22 (2011) 393- -396395,p-cp"PhPhPtScheme 1. Formation of phenylnaphthalenes from phenylacetylene.h Pb、H-一=-PhPh、sScheme 2. Formation of triphenylbenuzene isomers from pbenylacetylene.which accelerated the hydration. Around 260 °C, the ionization constant of water approached to the maximum [18,19].That meant water can contribute most H+ at this temperature, resulting the highest yield at 260。C. Secondly, the increasein temperature (>260 °C) speeded up side reactions and decreased the selectivity towards acetophenone. Theexperiments Katritzky et al. [14] completed may explain it. Based on their results, hydration of phenylacetylene wasdominant, while higher temperature was favorable to the cycloaddition of phenylacetylene, giving various by-products,such as x-naphthalene, B- naphthalene, 1,3,5-triphenylbenzene and 1,2,4-triphenylbenzene (see Schemes 1 and 2).Fig. 2 ilustrates the influence of reaction time on the yield of acetophenone. The curves indicated that 120 min wasthe appropriate reaction time and on the same condition, FeCl3 presented the best catalytic power. High yield of96.68% was obtained when the reaction time was 120 min with the additive FeCl3. The yield dropped to some degreewhen the reaction endured longer than 120 min. This was due to the conversion of acetophenone [14] (see Scheme 3),which underwent acid-catalyzed ionic reactions with longer time in NCW. For example, the condensation ofacetophenone gave triphenylbenzene, 1,3-diphenylbut-2-en-1-one.Generally, the hydration of alkynes in organic solvent may follow two pathways [12]: one is proposed on the basisof mercuric salts as catalysts [6]. Another mechanism is presented by using acid-Bronsted catalysts [6]. Ferric ionexhibits strong Lewis acidity due to its small ionic radius and high coordination number, and is able to accept electroneasily. And the main hydration product is acetophenone. Based on the two facts, a possible mechanism similar to theacid-Bronsted catalyzed mechanism was speculated for the hydration of phenylacetylene in NCW (see Scheme 4).Firstly, ferric ion combines with the triple bond, resulting in the activation of the triple bond and giving a short-lived πcomplex. Then H* in the solution combines with π electron in the complex and a carbocation intermediate was formed20中国煤化工TYHCNMHGTmclninFig. 2. Yield dependence of acetopbenonc on time at 260 °C with various additives.396s. Li et al./Chinese Chemical Ltters 22 (2011) 393 -396PhCOCH2CH2COPhays >rH-Ph，hPhScbeme 3. Pathways for the conversion of acetopbenone.H~CH3Scheme 4. Mechanism of acid-Brpnsted mediated bydration of aklynes.by the elimination of ferric ion. Next, the carbocation intermediate is attacked by water and yields an enol, which is notstable in the acid environment and transforms into a ketone by tautomerization. In this pathway, the hydration productacetophenone is obtained.3. ConclusionA green procedure for the manufacture of acetophenone was employed for the hydration of phenylacetyleneassisted with additive in NCW. High yield of 96.68% was obtained at 260。C, 120 min in the presence of FeClz. Apossible reaction mechanism was proposed based on the product detected.AcknowledgmentThis research is supported by tbe National Natural Science Foundation of China (Nos. 21073064, 21003049).References[1] N. Akiya, PE. Savage, Ind. Eng. Chem. Res. 40 (200) 1822.[2] P.E. Savage, Chem. Rev. 99 (1999) 603.[3] A.R. Katritzky, S.M. Allin, Acc. Chem. Res. 29 (1996) 399.[4] M. Siskin, A.R. Katrizky, Chem. Rev. 101 (200) 825.[5] A.R. Katrizky, D.A. Nichols, M. Siskin, et al. Chem. Rev. 101 (2001) 837.[6] L. Hintermann, A. Labonne, Synthesis 8 (2007) 1121.[7] Y. Fukuda, K. Uchimoto, J. Org, Chem.56 (1991) 3729.[8] Y. Fukuda, H. Shiragami, K. Uchimoto, et al. J. Org. Chem.56 (1991) 5816.[91 J. Blum, H. Huminer, H. Alper, J. Mol. Catal. 75 (1992) 153.[10] PW. Jennings, J.W. 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