Polymer-Supported Oxidation Reactions Two Contrasting Experiments for the Undergraduate Laboratory Alan J. Buglass and John S. Waterhouse Cambridge College of Arts and Technology, East Road, Cambridge, CBI IPT. United Kingdom
Polymer-supported (PS) reagents are finding an important place in synthetic organic chemistry ( 1 - 4 ) . This paper describes the use of PS oxidizing agents in some experiments which are now part of various advanced practical courses a t CCAT. The experiments described illustrate the common advantages of 1's reagents such as ease of operation and selectivity. They alsocontrast the behariour of macroreticular resins and gel resins. Macroreticular resins have stable pore structures throughout the polymer network, and reactant molecules can readily penetrate the structure. Gel resins, on the other hand, require a solvent to swell them before reactant molecules can oenetrate. Suitable solvents for swelling gel resins described here are dipolar aprotic solvents such as tetrahvdrofuran (THF). We have adapted commercially a\,ailahlk resins as polymer supports since it is often not possible for undergraduates to synthesize specialized resins during the course of a normal practical session. The reactions discussed are ( 1 ) the epoxidation of cyclohexene and (,2 .1 the oxidation of alcohols. The former uses a swellable gel resin, whereas the latter uses a macroreticular resin. Standard reagents bound to polymers are used in each case.
Epoxidation of Cyclohexene Using Polymergound Peroxyacid The experiment has been developed to illustrate the preparation of peroxy-acid resins from readily available weak cation exchange resins and their use for the epoxidation of cyclohexene. The cation exchange resins are gel resins, and their structural details are given in Table 1.
@ = polymer backbone No special equipment is required for the experiment described here. which uses Duolite C433 as the ion exchanee
resin and THF as the solvent for the epoxidation reaction. Dried heads of ion exchanee resin are oxidized hv the method of Tagaki (6) using p-&enesulfonic acid and hydrogen neroxide. The oeroxv-acid content of the oxidized resin can be measured h i iodibmetry (6) (Table 1).The course of the eooxidation reaction is followed hv GLC. and the product, epoxycyclohexane, can be isolated by filtering off the spent reagent and removing the solvent. Tshle 2, third entry, shnwsrhe r~sultsobtainedfrom the hasicexperiment, which ran be readily extended to investi~atethe effects of the following: (1) the dt.gree of cross-linking of the resin (Table
Table 1. Results ot Peroxldatlon of Carboxyllc A d d Ion Exchange Rerlns Peroxy-acid Content/mmol g-'
Resin Duolite C433b Duollte C43Sr Amberlite IRC 50d
5.0 2.5-3.0 0.5
' B B M on dry resin. aE%~ntiallya homopolymer of acrylic aoid moss-linked with 3% dlvinylbsnlans (DVB). CEssentiallya homopolymerof methacrylic acid msslinked wm -3% DVB. dE~mntlallya homopolymerof memaorylic acid cmss-linkedwlm -7% DVB.
Table 2. Results of Epoxldatlon ot Cyclohexene with PeroxyAcid Resbsd
Resin
Solvent
Temp P C )
Time (min)
Dwiite C433 Duolite C433 Duolite C433 Duolite C433 Duolite C433 Duolite C435 Duoiite C436
dioxan dioxan THF THF benzene dioxan THF
25 30 25 30 25 25 25
90 60 120 75 90 120 120
COn"m8lon Of Cyclohexene
(%p
98 95 98 (807 95
5 94 92
1); (2) the presence of methyl groups in the polymer backbone (Tahle 1); (3) the effects of different solvents on the epoxidation reaction (Table 2). The low peroxy-acid content of oxidized Amberlite IRCSO (Tahle 1) results from the comparatively high degree of cross-linking of the resin. This greatly reduces the ability of the resin to swell and hence imoedes oenetration of reagent int(8 the polymer matrix. Much higher peroxy-acid contents are ohtained from the less hiehlv crnss-linked resins 1)uolite C433 and C436. As these r&s have the same degree of cross-linking, the lower peroxy-acid content obtained from Duolite C436 must be due to the presence of methyl groups in the polymer backbone. The peroxy-acid content for Duolite C433 represents about 50%conversion of the carboxylic acid groups. Data for epoxidation reactions are given in Table 2. They clearly show the necessity for a good swelling solvent. The polar solvents dioxan and THF, both of which swell the resin beads, are suitable for the reaction, hut benzene, which does not swell the resin, gives very Door yields. Duolite C433 and C436appear to give.comparibie results for epoxidation, hut, owine tu the lower peroxs-acid content of the latter, more of it has to he used so i t iC not as convenient as Duolite 433, which is the resin of choice for the experiment described below. Volume 64 Number 4
April 1987
371
Oxidation of Alcohols Using Polymer-Bound Chromium(VI) Oxide The experiment has been developed to illustrate the general advantages of using the important oxidizing agent CrOs (7) supported on the commercial macroreticular resin Amherlyst A-26. This resin is a strongly basic anion exchanger and in acidic solution the CrOs is probably bound as HCrOdto the cationic sites of the resin (-NR3+). The standard experiment, based upon a previously reported method (8)shows quite clearly the smooth oxidation of secondary alcohols to ketones using the polymer-bound Cr03. Thus 9-hydroxyfluorene, diphenylmethanol, and 1phenylethanol can be oxidized to 9-fluorenone, henzophenone, and acetophenone, respectively.
out under suction, hut suction should he applied lone. enough between wa-hmgr to remove rurplur solvent. The resin is then added toTHF ( 6 ml., i n a small, stoppered flask set in a thermostat bath at 52. Y'. Cvrlohexene (0.5ml.. 5.0 mmol) is ~~, then added, and the mixture is either stirred or shaken gently. The reaction is followed by GLC (PEG 20M 100 'C) and is allowed to continue until the eyclohexene peak is 1% or less of its original height. The epoaycyelohexane can be isolated in the following manner. The rection mixture is diluted with ether (20 mL) and filtered. The resin beads are washed with several portions of ether, and the solvent is then removed from the combined filtrate and washineson a rotary evaporator. The yield of eponycyelahexane is typieall~0.40 g (80%),bp 129-130 OC. ~
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Chromium( VI) Oxide Oxidations Preparation of Polymer-Bound Chromium(VI) Oxide Amherlyst A-26 resin (10 g) is stirred with saturated aqueous ehromium(Vl)oxide (20mL) for 30 min at room temperature,using a magnetic stirrer and follower. The resulting activated polymer is collected by suction filtration and washed with 100 mL of water, followed by 20 mL of acetone. It is then dried by leaving it in the Buchner funnelunder suction for 30 minor preferably by allowing it to air-dry overnight. It is much better to prepare a sufficientquantity of polymer-bound reagent in advance of oxidation experiments.
Using thin-layer chromatography (TLC) and infrared (IR) s ~ e c t r o s c o ~ivt ,can be seen that significant oxidation has o&urred aft& 20 min and oxidation is normally complete after 1h. A further experiment illustrates the specific nature of these PS oxidations with regard to alcohols. Thus a typical primary alcohol, such as benzyl alcohol, is oxidized to henzaldehyde only, as can be shown by TLC and IR spectroscopy. No benzoic acid. from the oxidation of benzaldehvde. " . can be detected in the reaction mixture. Another further e x ~ e r i m e nshows t how the PS reaction is sensitive toward sol;ent polarity. Thus benzyl alcohol is oxidized much more readily in a relatively nonpolar solvent such as petroleum spirit (bp 60-80 "C) than in the polar a ~ r o t i solvent c THF. TLC and IR s~ectroscoov show incomplete reaction after a n hour u s i n i t h e latter-solvent. This result is in sharp contrast to reactions involving eel-type resins descrihedabove, where relatively polar soTventsare required to swell the resin in order to facilitate reaction.
Epoxidation of Cyclohexene Caution: It has been reported (5,fi)that aliphatic peroxyacid resins are liable to detonate when dry; however, our repeated attempts to detonate the dried resins failed. I n fact, the method described here keeps the peroxy-acid resin wet and swollen a t all times, and under these conditions no problems have been reported. A solution of 60%hydrogen peroxide (20mL,0.35 mol) is cooled to -5 "C (ice-salt bath) and p-toluenesulfonic acid (25 g, 0.15 mol) is
added slowly with stirring. The resulting solution is added to dried Duolite C433 (3.0 g), and the mixture is allowed to stand overnight at 40 OC. (Alternatively,the mixture can stand at room temperature for one week, if more convenient.) The resin heads are filtered under suction and washed several times with water until the washines are free from acid and hvdroeen peroxide (test with acidic potassium iodide solution). The deadsire then immediately washed several times with THF to remove the water. It is important that the beads should not be allowed to dry ~~~~~~
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The Basic Experiment: Oxidation of Secondary Alcohols The alcohol and the oxidizing polymer are mixed with toluene (35 mL) at the rate of 5.5 mmol of alcohol to every 5 g of polymer. The mixture is heated under reflux, with stirring,for one hour. A quiekfit Ehrlenmeyer flask and stirrer hotplate with magnetic follower can be used for this operation. The progress of the reaction is followed using TLC on fluorescent silica gel (Polygram SIL G 40 X 80 mm sheets, eluted with 1:3 acetnne:hexane) and IR spectroscopy by evaporating several drops of the reaction mixture on a sodium chloride disk. When the reaction has reached completion, as judged by the TLC or IR monitor, the polymer is removed by filtration and the solvent is removed hv use of a rotarv. evaoorator. The oroduet can then he . purified hy the UWRI methods. Swtnhle alcohols for oxidation are 9-hydroxyfluorene.diphenylmerhnnol, and I-phenylathanol.Yieldsarceeding RWoarerommon. Further Experiments Oxidation of Primary Alcohols A orimalv alcohol such as b e n d alcohol can he oxidized usine the pame method as descrihed in the ham eaperrment. The major product is checked wing the TLC and IR spectroscopy methods dewribed previuusly, and authentic sampler uf benzaldehyde and benzoic acid are used in the identification. Dependence of Oxidation on Soluent The oxidation described above can be carried out using other solvents, such as the polar aprotic solvent THF and the nonpolar petroleum spirit (bp 60-80 'C). In each case the reaction mixture is examined for extent of reaction using the TLC and IR spectroscopy techniques described previously. 1. Hdgo,P. Chem. Britain 1978.14.237: 2. Frkhet, J.M. T e t r o h d m n 1981.37.663. 8.
Akdah.A.:Shenington,D. C . Chom. Re". 1981,81,N7.
4. Hdge, P.; Sherrington. D. C., Ed. Polymer-Supported Renetions in Organic Chon& try: wi1ey-1nterscienco:New Y0.k. 1980. 6. Harrim. C. R.;H d a . P. J. Chem. Soe. Parkin 11976.695. 6. Tsksgi. T. J. Applied Polymer Sei. 1915,19,1M8. 7. McCme, W. Errsic 01gonir Rsoctions:Heydon: London,1973; p 133. 8. Wade, L. G.: Still. L. M. J. Chem Edur. 1980.67.237.
