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Supported chiral catalysts: the role of the polymeric networka a a a , a*´B. Altava , M.I. Burguete , E. Garcıa-Verdugo , S.V. Luis , M.J. Vicent ,

bJ.A. Mayorala ´Department of Inorganic and Organic Chemistry, ESTCE, University Jaume I, E-12080 Castellon, Spain

b ´Facultad de Ciencias, Instituto de Ciencia de los Materiales de Aragon (C.S.I.C.), Universidad de Zaragoza, E-50009 Zaragoza,Spain

Received 15 June 2000; received in revised form 28 November 2000; accepted 10 December 2000

Abstract

In the preparation of chiral-supported catalysts, the immobilization process can produce changes in the behavior of theresulting resin-supported species. The polymeric matrix plays important roles that affect the activity, selectivity and stabilityof the final catalysts. Pseudodilution effects very often favor the activity of the supported species, and the presence of thehydrophobic matrix increases the stability of the water-sensitive active sites. The steric interaction between the polymericbackbone and the groups in the chiral auxiliary can modify, even dramatically, the stereochemical outcome of the reaction. 2001 Elsevier Science B.V. All rights reserved.

Keywords: Asymmetric catalysts; Heterogeneous catalysts; Aminoalcohols; Bisoxazoline; TADDOLs

1. Introduction will center on them for our discussion [1–3].The most general strategy for the preparation of

The preparation of polymer-supported enan- chiral-supported catalysts starts from the analy-tioselective catalysts represents one of the most sis of a well-known and efficient enantioselec-interesting applications of polymers in organic tive homogeneous process [4]. Structural modi-chemistry. Accordingly, much effort has been fications of one or several of the components ofmade to prepare immobilized chiral auxiliaries the catalyst are then examined in order to allowable to form the expected chiral catalysts. its incorporation into a polymeric matrix and toDifferent inorganic and organic supports have develop a heterogeneous counterpart of thatbeen assayed for the immobilization process, catalyst [5–7]. When using this approach, thebut organic polymers and, in particular, poly- role of the polymeric backbone is very oftenstyrene–divinylbenzene copolymers and related neglected, being considered as a mere inertmaterials, continue to mainly be used and we matrix to which the ‘important’, reactive frag-

ment is attached. This is not true, however, andvery significant changes in activity, stability and

*Corresponding author. Tel.: 1 34-964-728-239; fax: 1 34-selectivity can be observed as a consequence of964-728-214.

E-mail address: [email protected] (S.V. Luis). the immobilization process.

1381-5148/01/$ – see front matter 2001 Elsevier Science B.V. All rights reserved.PI I : S1381-5148( 01 )00036-0

26 B. Altava et al. / Reactive & Functional Polymers 48 (2001) 25 –35

Enantiopure products used for the preparation resin-bound amino acids in a parallel synthesisof chiral auxiliaries can be very expensive, as approach (see Scheme 1) [4,7–12].can many of the products used as supports. A Heterogeneous chiral aluminium Lewis acidsreasonable economic balance for the use of can easily be prepared from these supportedthese chiral-supported materials requires the aminoalcohols by reaction with EtAlCl [13].2

development of catalytic processes for which The resulting aluminum species can be used asonly minor amounts of costly materials are catalysts for the Diels–Alder reaction betweenused. Recovery and recycling of the supported methacrolein and cyclopentadiene (Scheme 2).species is a second requirement and this is The corresponding N-benzylated aminoal-considered to be one of the main advantages for cohols were prepared as their homogeneousthe use of heterogeneous systems. A third factor analogues in order to compare the activities ofis the use of readily available starting materials both soluble and supported aluminium speciesfrom the chiral pool. In this context, we have (Fig. 1).carried out much of our work with the deriva- A lower activity, when compared with thetives of two simple families of chiral com- analogous homogeneous counterparts, has verypounds: amino acids (aminoalcohols, bisox- often been reported for polymer-bound species.azolines or peptidomimetic structures) and tar- This has generally been ascribed to diffusionaltaric acid (in particular TADDOLs). problems and attempts to minimize this draw-

back include the use of macroporous networksand the anchoring of the reactive sites throughlong spacers [6c,6d]. Nevertheless, an increase2. Supported aminoalcoholsin activity can also be observed as a direct

Supported aminoalcohols represent a very consequence of the presence of the polymericinteresting class of heterogeneous chiral aux- network. Some results are shown in Table 1 foriliaries that have been widely used for a variety aminoalcohols 11–17. Those results show that,of catalytic and non-catalytic reactions. Poly- for supported pyrrolidinol and ephedrine, quan-mer-bound aminoalcohols derived from simple titative conversions for the Diels–Alder reactionamino acids can be prepared through different are obtained at much shorter reaction times thatapproaches, including direct grafting onto a for the homogeneous analogues. The mostMerrifield resin, polymerization of the corre- remarkable data were obtained for the prolinolsponding vinylic monomers and modification of derivatives: only the resin-bound species (14

Scheme 1.

B. Altava et al. / Reactive & Functional Polymers 48 (2001) 25 –35 27

Scheme 2.

neous systems, the enantioselectivities observedwere very low except for the polymeric catalyst14 for which a 14% ee was observed. Accordingto this, the next step in our study was to analyzethe effect of the nature of the polymeric back-bone on the stereoselectivity of the abovereaction. For this purpose, we prepared a seriesof prolinol-containing polymers (18–20) ob-tained through grafting onto a Merrifield resinwith a low degree of crosslinking (18G) or bypolymerization of the appropriate vinylic mono-Fig. 1.

mers [14]. Suspension (18S) and bulk (18M)polymerizations were carried out using DVB as

and 15) were revealed to be active catalysts for the crosslinking agent. More flexible crosslink-this reaction. The former results can be ex- ing agents containing oxygen atoms were alsoplained by considering the formation in solution assayed in the polymerization process (19, 20).of aggregates of low activity. The presence of The corresponding aluminium catalysts werethe polymeric backbone to which the active prepared from the polymers and used for thesites are bound makes the formation of such above Diels–Alder reaction (Fig. 2). Someaggregates more difficult, making the supported results are summarized in Table 2.aluminium species more active. This effect is As can be seen, the polymer obtained byrelated to the so-called pseudodilution effect suspension polymerization showed a lower ac-[13]. tivity, which is most likely due to diffusion

In all cases, for homogeneous and heteroge- problems within the polymeric matrix. Alter-

Table 1Results obtained for the Diels–Alder reaction between methacrolein and cyclopentadiene using aluminium catalysts 10–17

Homogeneous t Conversion Heterogeneous t Conversioncatalysts (min) (%) catalysts (min) (%)

10 240 1 14 15 9811 240 – 15 105 7012 300 97 16 60 9513 1140 96 17 30 97


presence of oxygen donor atoms in the back-bone seems to disfavor the selectivity as a resultof the coordination of a large non-chiral frag-ment to the catalytic site [17]. The effect islower for polymer 20 where simultaneouscoordination of the oxygen atoms of the chain ishindered.

Fig. 2.

3. Supported bisoxazolines

A similar effect, i.e. an increase in theTable 2Results obtained for the Diels–Alder reaction between methac- enantioselectivity observed when the supportedrolein and cyclopentadiene in the presence of Al catalysts derived chiral auxiliary is prepared by polymerizationfrom resins 18–20 [14]

instead of grafting, was also obtained for bisox-a bPolymer D.C. Al loading Yield Exo/endo % azolines, a second family of amino acid deriva-(%) (mmol /g) (%) ratio ee

tives [18]. As can be seen in Scheme 3,18G 2 0.74 98 92.8 14

supported bisoxazolines can be prepared by18S 20 1.14 76 90:10 10grafting through functionalization of the central18M1 20 0.90 95 88:12 12

18M2 90 0.56 97 90:10 25 methylene bridge or by polymerization of the19 90 0.16 93 85:15 2 appropriate vinylic derivatives. The presence of20 90 0.35 98 85:15 11

two polymerizable units in the monomerica Degree of crosslinking (molar percentage).b bisoxazolines allows polymerization both in theAfter 3 h.

presence or absence of additional crosslinkingagents (DVB or related derivatives) and/orcomonomers (styrene) [19]. In this case the

natively, a clustering of chiral groups during benchmark reaction considered was the Cu-cata-polymerization, which would produce a situa- lyzed cyclopropanation of styrene (Scheme 4).tion more similar to that found in solution, The results shown in Table 3 again demon-could also be considered. Such problems can be strate that a decrease in the activity and selec-avoided by the use of monoliths [15] prepared tivity, relative to the homogeneous system, ofby bulk polymerization. In this case, the activity the resulting catalyst is obtained when theand selectivity of the monolithic polymers hav- polymer obtained by grafting (22) is used. Theing lower degrees of crosslinking were similar use of polymers prepared using additional cross-to those observed for the system prepared via linking agents (25a,b) follows similar trends.grafting. However, when very high degrees of Again, the use of crosslinking agents containingcrosslinking were used (90% of crosslinking oxygen atoms, which seems to compete foragent) a significant increase in enantioselectivity coordination to the metal centre, is detrimentalwas observed. This indicates that polymeriza- for the catalytic properties of the resulting Cu-tion in the presence of a chiral monomer can loaded resin (25b).affect the polymeric backbone so as to favor the On the contrary, when monolithic polymerscreation of more appropriately ordered chiral are prepared in the absence of other crosslinkingcavities [16]. This effect can only be observed agents (25c), the results obtained nicely re-when a very rigid matrix is created. Such produce those obtained for the homogeneousrigidity is lacking in other resins with lower analogue (23). As a matter of fact, when thedegrees of crosslinking. On the other hand, the vinylic bisoxazoline derivative 24 is the only


Scheme 3.

Scheme 4.

Table 3Results obtained in the cyclopropanation reaction of styrene using Cu-loaded polymers 22 and 25

Polymer Cu loading Run Yield trans /cis % ee % ee(mmol /g) (%) ratio (trans) (cis)

a23 – 1 32 2.3 50 4022 0.44 1 18 1.9 26 2125a 0.19 1 11 2.5 18 1825b 0.18 1 32 2.0 8 825c 0.39 1 28 1.5 46 42

2 24 1.5 46 4125d 0.01 1 28 1.9 61 55

2 27 1.7 55 52a Non-polymeric analogue.


monomer used in the polymerization process compounds to give resins such as 31 or 33 has(25d), the results obtained are even better than been carried out through grafting proceduresfor the soluble counterpart. It is interesting to (Fig. 3). These kinds of supported chiral aux-note the high activity of the polymeric catalyst iliaries containing four nitrogen atoms seemderived from 25d, even if only a minor part of well suited to form chiral complexes withthe bisoxazoline moieties are available for cop- copper. Accordingly, their copper complexesper complexation. All the catalysts prepared were assayed as catalysts for the above-men-from monolithic resins showed good stability. tioned cyclopropanation of styrene (Scheme 4).

Some interesting results are shown in Table 4[22].

In general, when homogeneous copper4. Supported chiral polyamides andcatalysts are used, an induction period for thepolyaminesreaction is observed and the desired reaction

Simple C2 symmetric peptidomimetics [20] starts to proceed only after a period of 1–3 h. Asuch as 30 and 32 can be prepared easily and greater activity is shown, however, by theefficiently from natural amino acids and some supported catalysts, for which an inductionof them have been used as chiral auxiliaries period is not observed. The induction period[21]. Reduction of the amide functionalities observed in solution seems to be related to thewith B H can also be carried out efficiently on formation of aggregates that are disrupted with2 6

these compounds to give the corresponding the formation of polar compounds as a result ofchiral polyamines. Immobilization of the chiral the non-catalyzed decomposition of ethyl

diazoacetate (27), one of the components of thereaction. Again, the pseudodilution effect of thepolymeric matrix precludes aggregate formationand produces more active catalysts. The enan-tioselectivities found for this process, however,were lower than those obtained with the morerigid bisoxazoline ligands.

5. Supported tartaric acid esters

Esterification of tartaric acid represents themost simple approach to the preparation ofsupported tartaric acid derivatives, and several

Fig. 3. authors have used this route for the preparation

Table 4Results obtained in the cyclopranation of styrene catalyzed by copper complexes of ligands 30–33

Homogeneous systems Heterogeneous systemsa aLigand Time (h) Yield (%) ee (%) Ligand Time (h) Yield (%) ee (%)

30 3.0 – – 31 1.5 12 –4.0 16 0 24.0 28 0

32 1.0 – – 33 1.0 17 –1.5 19 12 23.0 31 4

a Calculated for the cis-cyclopropanes.


and non-catalytic enantioselective processes[7,25,26]. Enantioselectivities observed in re-actions carried out in the presence of TADDOLligands are greatly affected by the exact natureof the substituents at the a positions of thealcohol moieties and at the C-2 position of thedioxolane ring [27]. Accordingly, every effortwas made to prepare homogeneous andheterogeneous TADDOLs having the samegroups at those positions and differing only inthe presence or absence of the polymeric back-

Scheme 5. bone. The general approach is outlined inScheme 6.

of functionalized polymers that have been used The phenolic TADDOL derivatives 37 are thefor different stereoselective reactions [9,23]. In key compounds for this strategy [26]. Selectiveour case, we observed that esterification of benzylation at the phenol group gives way totartaric acid with a Merrifield resin gave poly- the formation of soluble 38, while the use of amers with the general structure 34 in which Merrifield resin as the alkylating agent affordsdifunctionalization of the diacid had taken place the polymer-bound derivatives 40G through the[24]. From 34, aluminum and titanium catalysts usual grafting procedure. The use of 4-chloro-35 and 36 were prepared (Scheme 5) and methyl styrene allows us to obtain vinylicassayed for the Diels–Alder reaction between monomers 39 which could be polymerizedmethacroleine and cyclopentadiene (Scheme 2). under a variety of conditions to give resins 40SThe enantioselectivities observed were low in and 40M. Reaction of these TADDOLs with

iboth cases, but the most remarkable observation TiCl (OPr ) afforded the corresponding Ti2 2

was the very large increase in stability, in catalysts which were assayed for the Diels–particular for the titanium species (36). Catalyst Alder reaction between cyclopentadiene and 3-36 could be used up to six times, maintaining a crotonoyl-1,3-oxazolidin-2-one (Scheme 7).reasonable activity at longer reaction times. The The best results were obtained, in terms offinal reused catalyst can be washed, dried and enantioselectivity, with TADDOLs havingreactivated again by reaction with additional bulky aryl groups at the a positions, such asTiX species. This regenerated catalyst showed 2-naphthyl or 3,5-dimethylphenyl. Some illus-4

a good activity for the first runs, but displayed a trative results are shown in Table 5. Thus, forfaster deactivation curve. the compounds having 2-naphthyl groups, the

results follow the expected trends: the resinobtained via grafting (40G-a) shows a slightdecrease in activity and enantioselectivity rela-6. Supported TADDOLstive to that observed for the homogeneous

The low enantioselectivities observed in the system (38a). An improvement in activity isformer experiments suggested that more rigid obtained with the related monolithic polymerand sterically crowded ligands derived from (40M-a), for which diffusion problems aretartaric acid would be necessary for the prepara- reduced, but the enantioselectivity was compar-tion of more efficient enantioselective catalysts. able. Similar trends are found for the homoge-TADDOL derivatives seem to meet all these neous and grafted derivatives containing 3,5-requirements and, accordingly, have been much dimethylphenyl groups (38b and 40G-b). How-used as chiral auxiliaries for different catalytic ever, a very remarkable result was obtained with


Scheme 6.

Scheme 7.

Table 5Results obtained for the Diels–Alder reaction between cyclopentadiene and 41 using Ti-TADDOLates derived from compounds 38 and 40

Catalyst Ar Yield (%) endo/exo ratio ee (%)

38a 2-Naphthyl 96 74:26 61 (2S,3R)40G-a 2-Naphthyl 50 83:17 40 (2S,3R)40M-a 2-Naphthyl 96 79:21 40 (2S,3R)38b 3,5-(CH ) C H 100 71:29 38 (2R,3S)3 2 6 3

40G-b 3,5-(CH ) C H 98 71:29 17 (2R,3S)3 2 6 3

40M-b 3,5-(CH ) C H 100 80:20 18 (2S,3R)3 2 6 3

the monolithic derivative 40M-b. In this case, ture leading to a different kind of asymmetricthe major isomer obtained was the 2S,3R instead induction [28].of the 2R,3S isomer obtained with the other two Additionally, the polymeric monoliths con-derivatives. Thus, the morphology of the poly- taining Ti-TADDOLates also showed very out-meric backbone can even determine the topicity standing stability. Thus, for instance, the poly-of the resulting products! Polymerization to meric catalyst derived from 40M-b maintainedform part of a very rigid, cross-linked matrix the same activity and selectivity 3 months aftercan modify the steric interactions between the being prepared (see Fig. 4). Even when thestructural components of the TADDOL struc- catalyst (containing a Ti-alkoxylate) was ex-


problems are avoided. Much additional work isrequired in order to better understand the role ofthe polymeric matrix in these systems, and to beable to design novel and more efficient sup-ported enantioselective catalysts for practicalapplication. The development of monolithicpolymers represents a very useful and promisingcontribution to this field.

Acknowledgements

Fig. 4. Time-dependent activity for the Ti-loaded monolithic Financial support for this work was providedcolumn 40M.

by the Spanish CICYT (projects MAT96-1053and MAT99-1176), Generalitat Valenciana (pro-

posed to air for an additional month, the re- ´ ´ject GV-99-64) and Fundacio Caixa Castello-sulting species maintained its activity, but lost Bancaixa (project P1B97-10). We would like tosome selectivity. warmly thank Prof. Mayoral and his group at

the University of Zaragoza for their contributionto many parts of this work.

7. Conclusion

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