a Department of Chemistry ,PO Box 912,University of W ales ,Cardi†,UK CF13T B
b L everhulme Centre for Innovative Catalysis ,Department of Chemistry ,University of L iverpool ,L iverpool ,UK L 693BX
c Department of Chemistry ,L oughborough University ,L oughborough ,L eicestershire ,UK L E113T U
d ICI Synetix ,R&T Division ,PO Box 1,Billingham ,T eesid
e ,UK T S231L B
Manganese-exchanged MCM-41modiÐed with a chiral salen is an e†ective enantioselective heterogeneous epoxidation catalyst for cis -stilbene.
Interest in asymmetric synthesis continues to increase and this has highlighted the need for the design of highly selective asymmetric catalysts.Most of this research activity concerns homogeneous catalysts,but there is now increased interest in the identiÐcation of suitable heterogeneous asymmetric cata-lysts,since such catalysts readily overcome the problems typi-cally encountered with homogeneous systems,namely product recovery and catalyst separation.To date,three approaches have been taken in the design of heterogeneous enantio-selective catalysts:(i )the use of a chiral support for an achiral metal catalyst,1(ii )modiÐcation of an achiral heterogeneous catalyst using a chiral cofactor,2h 4and (iii )the immobilisation of a homogeneous catalyst.5Most attention and success has been observed with enantioselective hydrogenation cata-lysts.2h 6
One of the biggest challenges remaining for catalytic chem-ists is the design of heterogeneous enantioselective oxidation catalysts;this is a topic that retains intense research interest,but,to date,is si
gniÐcantly less advanced than the research into enantioselective hydrogenation catalysts.Interest in het-erogeneous oxidation catalysts was stimulated by the dis-covery that the microporous titanium silicalite TS-1is an e†ective catalyst for epoxidation reactions at 25È60¡C using hydrogen peroxide as oxidant,and this has led to the obser-vation that a broad range of microporous and mesoporous materials are active oxidation catalysts.7However,to date these titanium-containing systems have not been successfully modiÐed to act as heterogeneous enantioselective catalysts.In contrast,there has been signiÐcant research interest in homo-geneous enantioselective epoxidation catalysts and in this area there has been signiÐcant success.8h 11In particular,chiral manganese salen complexes are very e†ective as asymmetric homogeneous epoxidation catalysts for cis -substituted aryl alkenes.11h 13The identiÐcation of heterogeneous counterparts has been somewhat slower and for manganese complexes three approaches have been adopted:(i )supporting Mn Èsalen alkene epoxidation catalysts on polymers,14h 16(ii )ion exchange of manganese complexes into the intra-crystalline space of zeolites,e .g .zeolite Y 17or mesoporous materials,18and (iii )encapsulation of manganese complexes within zeolites by synthesis using “ship-in-a-bottleÏmethodology.19Consider-able progress has been made using polymer-supported systems,14but the use of inorganic supports has received far less attention.Preformed manganese complexes have been
ion-exchanged in zeolites but only using manganese complex-es with achiral nitrogen ligands,e .g .bipyridyl.20Ogunwumi and Bein 19have demonstrated that the “ship-in-a-bottleÏapproach can be successfully used to synthesize the asym-metric manganese salen complex inside the cages of the zeolite EMT.The molecular cross section of the manganese salen complex is too large for the ion-exchange method to be employed successfully.Although this approach does produce an asymmetric epoxidation catalyst,the pore size of the zeolite can limit the e†ective range of substrates that can be utilized.Here we demonstrate the synthesis of a heterogeneous enantioselective epoxidation catalyst for the larger substrate cis -stilbene by combining manganese(III )salen with the meso-porous Al-MCM-41(Scheme 1).This work extends the meth-odology previously identiÐed in our studies concerning the enantioselective aziridination of alkenes using Cu 2`-exchanged zeolite Y modiÐed with bis(oxazolines).21
Al-MCM-41was synthesized according to literature methods 22and ion exchanged with aqueous (0.2
Mn(OAc)2
M,25¡C,24h),Ðltered,washed with water and vacuum dried.This procedure was repeated twice and the resulting material was calcined (550¡C,24h)(Mn content 2.0%by weight).The calcined Mn-exchanged A
l-MCM-41was heated under reÑux with the chiral salen ligand,(R ,R )-([)-N ,N @-bis(3,5-di-tert -butylsalicylidene)-1,2-cyclohexanediamine (Aldrich),in (24h,Mn :salen \1:1),cooled to 0¡C and washed
CH 2Cl 2
with This procedure resulted in incorporation of
CH 2Cl 2
.10%of the chiral salen ligand (determined by TGA and solu-tion analysis).The Mn-exchanged Al-MCM-41:salen catalyst (2.0Mn%by weight,Mn :salen \1:0.1mol ratio)was inves-tigated for the epoxidation of cis -stilbene using iodosylben-zene as oxidant;the results,together with those of control
Scheme 1
New J .Chem .,1998,Pages 1167È11691167
Table 1Epoxidation of stilbene at 25¡C using Mn-exchanged MCM-41
Selectivity c (%)
Epoxide    ans d Entry Catalyst Time a /h Conversion b ,c yield c (%)cis trans (%)1None
25000002Mn(OAc)2
e ,freactive materials studies
24100  1.5010003Mn Èsalen 110086297178complex e 4Al-MCM-41e 2400ÈÈÈ5Mn-MCM-41e 2453010006Mn-MCM-41210069584270]salen e 7Mn-MCM-412610035010025]salen e ,g 8Solution e 200ÈÈÈ9Mn-MCN-4123718613930reused e 10
Mn-MCM-412
100
52
63
37
54
recalcined ]salen e
a Reaction time.
b As determined by decomposition of iodosylbenzene to iodobenzene,using HPLC.
c Conversions,yields an
d selectivity deter-mined by HPLC,using APEX ODS reverse-phas
e column.d Enantiomeric excess determined by chiral HPLC using (R ,R )Whelk-O 1column,92%hexanen ÈPr i OH.e Reactions were conducted in unless otherwise noted with molar ratio o
f cis -stilbene :
CH 2Cl 2catalyst :iodosylbenzene \7:1:0.13.f Reaction conducted in g trans -Stilbene used as substrate.CH 3
Mn(OAc)2
,of MCM-41or salen ligand,is not a particularly active cata-lyst;only 1.5%yield of the epoxide is formed after reaction for 24h at 25¡C and only the trans -epoxide is formed (entry 2).ModiÐcation of manganese in solution by the chiral salen ligand as expected leads to a signiÐcant rate enhancement,but the cis -epoxide is also formed (entry 3)and the trans -epoxide is formed with Interestingly,immobilization of the manganese by Al-MCM-41(entry 5)leads to an increase in reactivity when compared with the non-manganese-exchanged parent material (Al-MCM-41,entry 4),whereas non-immobilised Mn 3`in the absence of the salen ligand is inac-tive (entry 2).This e†ect suggests that the Al-MCM-41is occupying part of the manganese coordination sphere and that this restricts the cis to trans transformation.ModiÐcation of the Mn-exchanged Al-MCM-41with the salen ligand leads to a further enhancement in reactivity and the cis :trans ratio of the epoxide.The trans epoxide is formed with f 70%(entry 6),which is very similar to that observed for the equivalent homogeneous reactions (entry 3).The turnover rates,based on the amount of Mn Èsalen,for the heter-ogeneous catalyst were found to be higher than those for the homogeneously catalysed reaction,this is in contrast to pre-vious studies when comparable turnover rates have been observed.In the present study the turnover rates [mol epoxide (mol Mn Èsalen)~1/h ~1]are found to be 26for the heter-ogeneou
s reaction and 7for the ans -Stilbene (entry 7)was found to be a signiÐcantly less reactive substrate and f the resulting trans -epoxide was signiÐ-cantly decreased.The use of Mn-exchanged Al-MCM-41:salen catalyst for this epoxidation does not result in the formation of signiÐcant levels of by-products as has been observed when manganese bypyridyls have been used as cata-lysts,19and typically only deoxybenzoin is observed at low levels (ca .5È10%,based on iodosylbenzene)although some decomposition of iodosylbenzene is observed (ca .30%).
A further set of experiments was carried out to examine the reusability of the Mn-exchanged Al-MCM-41:salen catalyst.Following the reaction,the Mn-exchanged Al-MCM-41:salen catalyst was recovered by Ðltration and the solid reused in a new catalytic reaction (entry 9);although the reac-tivity and enantioselectivity declined,epoxide was still formed,and the cis :trans ratio was practically unchanged.Recalcina-tion of the recovered material and addition of new salen ligand essentially restored the catalytic activity and the enan-tionselection (entry 10).Use of the solution following Ðltration did not give any reaction (entry 8),and furthermore this solu-tion contained no Mn 3`although some salen leaching was observed.These experiments demonstrate that the reaction occurring with Mn-exchanged Al-MCM-41:salen is wholly catalysed heterogeneously.
This study demonstrates that Mn-exchanged Al-MCM-41when modiÐed by a chiral salen ligand can be
an e†ective enantioselective heterogeneous epoxidation catalyst,and these results extend the generality of the methodology described in our earlier studies of asymmetric enantioselective aziridination using Cu 2`-exchanged zeolites.21
Acknowledgements
thank ICI Katalco,Robinson Bros.,the EPSRC and the We DTI/LINK program on asymmetric synthesis for funding.
References
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solution (61.3g)was stirred with water (160g)and then
sodium aluminate (2.77g)was added slowly.Silica (16.7g,fumed),
tetramethylammonium silicate solution (46.7g,25wt.%)and tetramethylammonium hydroxide solution (8.8g,25wt.%)were added with stirring.The mixture was reacted in an autoclave (100¡C,24h)after which time the solid material was collected,washed with water and calcined (550¡C;4h;air,12h).BET:N 2,adsorption Èdesorption measurements at [178¡C conÐrmed N 2that
this material was mesoporous.Average pore diameter:38Ó.The powder X-ray di†raction pattern of the solid sample gave results in agreement with spectra published in the literature.
Received 29th June 1998;
L etter 8/04970J
New J .Chem .,1998,Pages 1167È11691169

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