Simon
Doherty
*a,
Julian G.
Knight
a,
Jack R.
Ellison
a,
Peter
Goodrich
b,
Leanne
Hall
b,
Christopher
Hardacre
*b,
Mark J.
Muldoon
b,
Soomin
Park
b,
Ana
Ribeiro
c,
Carlos Alberto Nieto
de Castro
c,
Maria José
Lourenço
c and
Paul
Davey
d
aNUCAT, School of Chemistry, Bedson Building, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, UK. E-mail: simon.doherty@ncl.ac.uk; Fax: +44 (0)191 222 6929; Tel: +44 (0)191 222 6537
bThe QUILL Research Centre, School of Chemistry and Chemical Engineering, Queen's University Belfast, Belfast BT9 5AG, UK. E-mail: c.hardacre@qub.ac.uk; Fax: +44 (0)28 9097 4687; Tel: +44 (0)28 9097 4592
cDepartmento de Química e Bioquímica e Centro de Ciências Moleculares e Materiais Faculdade de Ciências, Universidade de Lisboa, 1749-016, Lisboa, Portugal
dGivaudan, Schweiz AG, Überlandstrasse 138, CH-8600 Dübendorf, Switzerland
First published on 9th December 2013
The asymmetric Diels–Alder reaction between N-acryloyloxazolidinone and cyclopentadiene and the Mukaiyama-aldol reaction between methylpyruvate and 1-phenyl-1-trimethylsilyloxyethene have been catalysed by heterogeneous copper(II)-bis(oxazoline)-based polymer immobilised ionic liquid phase (PIILP) systems generated from a range of linear and cross linked ionic polymers. In both reactions selectivity and ee were strongly influenced by the choice of polymer. A comparison of the performance of a range of Cu(II)-bis(oxazoline)-PIILP catalyst systems against analogous supported ionic liquid phase (SILP) heterogeneous catalysts as well as their homogeneous counterparts has been undertaken and their relative merits evaluated.
Scheme 1 Asymmetric Diels–Alder reaction between N-acryloyloxazolidinone and cyclopentadiene catalysed by 10 mol% Cu(II)-bis(oxazoline) complexes. |
Polymers with chirality incorporated into the backbone have been widely studied as catalyst supports for a range of asymmetric C–C bond forming reactions including Diels–Alder cycloadditions, oxidations and hydrogenations.4–8 In contrast, the concept of immobilising an ionic liquid in the form of a cation-decorated polymer, e.g. in the form of a polyelectrolyte, to combine the favourable properties of ionic liquids with the advantages of heterogenisation as well as overcome leaching and improve long term stability in asymmetric catalysis has been far less studied.9,10 Selected examples include the use of polymer-supported imidazolium based ILs in organocatalysed nitroaldol reactions11 and enzyme-catalysed transesterifications.12 A Ru-BINAP complex immobilised on a polymer-supported pyrrolidinium tetrafluoroborate has been applied to the asymmetric hydrogenation of methylacetoacetate.13 Therein, high ee's were obtained but the activity was lower than in methanol under homogeneous conditions. Rhodium and ruthenium complexes of phosphorylated BINAP have been immobilised in polymer electrolytes and used to catalyse the asymmetric hydroformylation of vinyl acetate and styrene14 and the asymmetric hydrogenation of dimethyl itaconate,14 respectively; the latter gave activities and selectivities that matched those obtained under homogeneous conditions.
Having recently developed an efficient and recyclable peroxometalate-based polymer immobilised ionic liquid phase oxidation catalyst using a cation-decorated ring opening metathesis-derived polymer as support,15 we now report that copper(II)-bis(oxazoline)-derived PIILP catalysts A and B either rival or outperform their homogeneous or SILP counterparts for the Diels–Alder (Scheme 1) and the Mukaiyama-aldol reaction (Scheme 2). The ionic polymer structures tested (IP-1–6) as well as polystyrene (P-7) used to support catalysts A and B are shown in Fig. 1.
Scheme 2 Asymmetric Mukaiyama-aldol reaction between methylpyruvate and 1-phenyl-1-trimethylsilyloxyethene catalysed by 10 mol% Cu(II)-bis(oxazoline) complex A. |
Copper(II)-bis(oxazoline) complexes were selected for immobilisation on the basis that they catalyse a host of asymmetric C–C bond forming reactions and because they have been studied on a variety of support materials which will enable the relative merits of PIILP catalysis to be assessed. The benchmark Diels–Alder reaction between N-acryloyloxazolidinone and cyclopentadiene catalysed by 10 mol% copper(II)-bis(oxazoline) complexes was initially conducted in CH2Cl2, Et2O and 1-ethyl-3-methylimidazolium bis{trifluoromethyl)sulfonyl}imide ([C2mim][NTf2]) under homogeneous, biphasic and SILP conditions, full details of which are summarised in Table 1. With the exception of Et2O, regardless of the choice of solvent employed, catalyst A produced significantly higher ee's and endo selectivities than catalyst B. For both catalysts, under homogeneous conditions, the presence of an IL either as an additive (entries 4 and 5) or as a bulk solvent (entry 3) resulted in higher conversions and ee's compared with similar reactions conducted in molecular solvents (entries 1 and 2).
Entry | Solvent/support | Time (min) | Catalyst A | Catalyst B | ||||
---|---|---|---|---|---|---|---|---|
Conv.a,b (%) | endo eeb (%) | % endob | Conv.a,b (%) | endo eeb (%) | % endob | |||
a Conversion at 20 °C. b Determined by HPLC. | ||||||||
1 | CH2Cl2 | 15 | 78 | 70(S) | 88 | 44 | 16(R) | 82 |
2 | Et2O | 15 | 32 | 14(S) | 87 | 38 | 15(R) | 80 |
3 | IL | 1 | 100 | 90(S) | 89 | 100 | 19(R) | 88 |
4 | IL/Et2O | 5 | 100 | 91(S) | 89 | 100 | 18(R) | 78 |
5 | CH2Cl2/IL | 5 | 64 | 82(S) | 96 | 100 | 16(R) | 70 |
6 | SiO2 | 5 | 32 | 16(S) | 88 | 76 | 4(R) | 84 |
7 | SiO2/IL | 5 | 100 | 87(S) | 85 | 100 | 8(R) | 84 |
8 | CNT | 5 | 64 | 66(S) | 87 | 44 | 20(R) | 83 |
9 | CNT/IL | 5 | 100 | 92(S) | 87 | 100 | 3(R) | 80 |
With the aim of comparing the efficiency of polymer immobilised ionic liquid supports against conventional SILP-systems, catalysts A and B were also supported on SiO2 or multi-walled carbon nanotubes (CNT) by wet impregnation from dichloromethane using [C2mim][NTf2] as the ionic liquid. Reactions were conducted in diethyl ether as the solvent due to the low rates of reaction for the homogeneous catalysts (entry 2) and the low solubility of [C2mim][NTf2] in diethyl ether compared with dichloromethane, which should reduce ionic liquid/catalyst leaching. A significant increase in both the ee and conversion was observed for reactions conducted using the SILP catalysts (entries 7 and 9) in comparison to the analogous heterogeneous reactions conducted in the absence of a [C2mim][NTf2] film (entries 6 and 8). In the case of the SiO2 support (entry 6) the very low conversion and ee could be due to the role of the surface silanols which can promote cyclopentadiene dimerisation and act as a non-chiral active catalyst.16 Moreover, both SiO2 and CNT supported systems based on catalyst A gave better conversions and markedly higher ee's (entries 7 and 9) than that obtained for homogeneous reactions using molecular solvents (entries 1 and 2). In all cases, the configuration of the Diels–Alder adduct was identical to those obtained under homogenous conditions in both ionic liquid and molecular solvents. These results indicate that heterogenisation of a chiral catalyst in an ionic environment can lead to a marked improvement in performance compared with the corresponding homogeneous and ‘non-ionic’ heterogeneous systems; however, this enhancement appears to be catalyst specific. Although support effects can influence the reaction the enhancement observed in the bulk IL and SILP systems could be, in part, due to the large excess of the less coordinating [NTf2]− anion compared with the [OTf]− anion in the original catalyst.17 Further studies are required to fully understand the role of the anion and nature of the support.
In order to further explore the effect of an ionic environment, catalysts A and B were supported on IPs 1–6 and non-ionic polymer P-7 using wet impregnation from dichloromethane. The efficacy of the resulting PIILP catalysts for the asymmetric Diels–Alder reaction conducted between N-acryloyloxazolidinone and cyclopentadiene in diethyl ether was investigated and compared with the corresponding systems modified with a thin film of IL; the results of which are summarised in Table 2 and will be compared with the systems reported in Table 1. In agreement with previous studies performed using these catalysts in ionic liquids3 the active PIILP catalyst was formed after only 5 min by stirring a dichloromethane solution of Cu(OTf)2 and bis(oxazoline) in the presence of ionic polymer; for comparison much longer aging times (>3 h) are generally required to achieve efficient and reproducible catalysis in molecular solvents.1
Entry | Polymer | Solvent | Catalyst A | Catalyst B | ||||
---|---|---|---|---|---|---|---|---|
Conv.a,b (%) | endo eeb (%) | % endob | Conv.a,b (%) | endo eeb (%) | % endob | |||
a Conversion at 20 °C after 5 minutes. b Determined by HPLC. | ||||||||
1 | IP-1 | Et2O | 86 | 90(S) | 88 | 100 | 35(R) | 78 |
2 | IP-1 | IL/Et2O | 100 | 99(S) | 95 | 100 | 27(R) | 76 |
3 | IP-2 | Et2O | 80 | 48(S) | 92 | 100 | 32(R) | 79 |
4 | IP-2 | IL/Et2O | 65 | 31(S) | 85 | 100 | 26(R) | 74 |
5 | IP-3 | Et2O | 69 | 28(S) | 86 | 100 | 38(R) | 73 |
6 | IP-3 | IL/Et2O | 77 | 69(S) | 89 | 100 | 15(R) | 76 |
7 | IP-4 | Et2O | 100 | 99(S) | 93 | 100 | 41(R) | 76 |
8 | IP-4 | IL/Et2O | 60 | 80(S) | 91 | 100 | 31(R) | 73 |
9 | IP-5 | Et2O | 27 | 7(S) | 88 | 100 | 16(R) | 92 |
10 | IP-5 | IL/Et2O | 98 | 9(S) | 88 | 100 | 18(R) | 79 |
11 | IP-6 | Et2O | 47 | 44(S) | 84 | 100 | 38(R) | 80 |
12 | IP-6 | IL/Et2O | 91 | 84(S) | 91 | 99 | 18(R) | 91 |
13 | P-7 | Et2O | 0 | 0 | 0 | 0 | 0 | 0 |
14 | P-7 | IL/Et2O | 74 | 84(S) | 88 | 100 | 30(R) | 73 |
Reference systems generated by adsorbing catalysts A and B onto the non-ionic polymer (P-7) were completely inactive when reactions were conducted in diethyl ether in the absence of additional ionic liquid (entry 13), even after the catalyst was aged for 3 h. In contrast, significant conversions were obtained for reactions performed with PIILP systems based on IP-1–6. In general, higher ee's were obtained with PIILP-systems generated by immobilisation of catalysts A and B onto the in-house synthesised polymers IP-1–4 (entries 1–8) compared with those based on commercially available IP-5–6 (entries 9–12). Moreover, for both catalysts the most efficient PIILP systems were those based on IP-4 with catalyst A giving cycloadduct endo-(2S) in 99% ee and catalyst B giving cycloadduct endo-(2R) in 41% ee at 100% conversion (entry 7). The former is the highest ee reported, to date, for a copper(II)-catalysed reaction of this substrate combination at room temperature under heterogeneous or homogeneous conditions. In addition, the ee of 90% (S) obtained with PIILP catalyst based on IP-1 and catalyst A (entry 1) also matched that obtained in neat ionic liquid and ionic liquid-diethyl ether (Table 1, entries 3–4). The significant difference between the reactions using the ionic and non-ionic polymers is thought to be due to the reduced binding of the complex on the latter support. In this regard, leaching of up to 50% of the copper into the diethyl ether phase for polymer P-7 together with a strong interaction of the cyclopentadiene with the polystyrene support were thought to be responsible for catalyst inactivity.
As supporting catalysts A and B in a thin IL film coated onto CNT and SiO2 supports was shown to have a beneficial effect on reaction activity/selectivity (Table 1), the influence of combining [C2mim][NTf2] with polymers 1–7 was also studied. Encouragingly, in some cases, the introduction of ionic liquid led to a marked increase in catalyst performance. The most significant increase in ee and conversion was obtained for catalyst A immobilised on IP-6 which gave cycloadduct endo-(2S) in 84% ee and 91% conversion in the presence of ionic liquid compared with 44% ee and 47% conversions in the absence of [C2mim][NTf2] (entries 11–12). An enhancement in ee was also achieved when a thin film of [C2mim][NTf2] was added to PIILP systems based on catalyst A immobilised on IP-1 and IP-3 (entries 1–2 and 5–6) with IP-3 giving the most marked enhancement in ee from 28% to 69% (Δee = 41%); however, there was little change in endo/exo-selectivity and conversion. While the addition of ionic liquid to the catalyst immobilised on P-7 also resulted in a marked improvement in conversion (entry 14), it was not possible to quantify the enhancement in ee as both catalysts are completely inactive in the absence of ionic liquid (entry 13). In contrast, the increase in ee from 90% to 99% for the IP-1/catalyst A/[C2mim][NTf2] combination was accompanied by an increase in endo/exo selectivity from 88% to 95%; the performance of this system matched that of IP-4 in diethyl ether. Although IP-5 also showed a marked increase in conversion in the presence of [C2mim][NTf2], there was no significant change in ee which remained very poor, (entry 10). Conversely, for IPs 2 and 4 the presence of a thin layer of ionic liquid led to a significant reduction in ee and endo selectivity (entries 4 and 8). PIILP systems based on catalyst B and IP's 1–6 showed much smaller changes in ee but larger variations in endo/exo selectivity in the presence of an IL film in comparison to the changes observed with catalyst A under comparable conditions. Moreover, in contrast to catalyst A, with the exception of IP-5, all of the IPs showed a decrease in ee upon the addition of an IL thin film. Despite this decrease in ee, all of the PIILP catalyst-IL combinations gave higher or equivalent ee's compared with the same reactions conducted under homogeneous conditions in dichloromethane (Table 1).
While exceptionally high ee's and endo/exo ratios have been achieved using PIILP and PIILP/IL catalysts, the disparate and unpredictable variations in enantioselectivity and endo/exo selectivity as a function of the catalyst and ionic support or support-ionic liquid combination highlights the importance of developing a rational understanding of how catalyst–support interactions influence catalyst efficacy.
Reasoning that a polymer immobilised ionic liquid phase support should effectively retain the catalyst, reusability experiments were conducted on PIILP catalysts derived from A and IP-1 and IP-6 as well as the corresponding catalysts immobilised onto SiO2, with and without a thin film of IL (Table 3). Both PIILP catalysts recycled poorly with a significant decrease in ee and conversion upon successive recycles (entries 1–3 and 6–7).
Entry | Support | Run | Conv.a,b (%) | endo eeb (%) | % endob |
---|---|---|---|---|---|
a Conversion at 20 °C after 5 minutes. b Determined by HPLC. | |||||
1 | 1 | 86 | 90(S) | 88 | |
2 | IP-1 | 2 | 68 | 73(S) | 86 |
3 | 3 | 30 | 7(S) | 85 | |
4 | IP-1/IL | 1 | 100 | 99(S) | 95 |
5 | 2 | 98 | 74(S) | 77 | |
6 | IP-6 | 1 | 47 | 44(S) | 86 |
7 | 2 | No reaction | |||
8 | IP-6/IL | 1 | 91 | 84(S) | 91 |
9 | 2 | 41 | 88(S) | 92 | |
10 | SiO2 | 1 | 90 | 54(S) | 85 |
11 | 2 | 83 | 43(S) | 84 | |
12 | SiO2/IL | 1 | 100 | 87(S) | 86 |
13 | 2 | 100 | 84(S) | 85 | |
14 | 3 | 93 | 81(S) | 86 |
ICP analysis of the diethyl ether used to extract the product between recycles revealed that this procedure removed 4.2% and 8.0% of the copper from IP-1 and IP-6 respectively. Ligand leaching of 10.2% from IP-1 and 14.3% from IP-6 was also determined by HPLC. The addition of an IL film to both PIILP catalysts resulted in an increase in conversion and ee (entries 4–5 and 8–9) and a significant reduction in copper leaching to 0.2% for systems based on both IP-1 and IP-6, respectively. Moreover, for the PIILP-IL systems the level of ligand leaching was below the limit of quantification by HPLC analysis. However, these levels of copper/ligand leaching observed with PIILP catalysts in the presence or absence of an IL film do not explain the significant drop in conversion and/or ee observed upon recycle. In addition, no significant change in the polymer structure as determined by infra-red spectroscopy was observed on recycle (see ESI† for data on IP-1 which showed the largest decrease in activity).
The drop in catalyst activity upon recycle for PIILP systems based on catalyst A is thought to be mainly attributed to the build-up of cyclopentadiene dimer18 on the support surface, as has been previously observed in other SILP catalysed Diels–Alder reactions.3 This proposal was supported by a reduction in conversion from 86% to 32% and a drop in ee from 90% to 19% upon addition of cyclopentadiene dimer (10 molar equivalents with respect to substrate) to the ethereal layer of a freshly prepared PIILP system derived from IP-1 and catalyst A. While the PIILP and PIILP-IL catalysts performed poorly upon recycle the SiO2-based SILP system maintained high conversions and good ee's upon recycle.3 This was thought to be associated with more efficient removal of the cyclopentadiene dimer from the hydrophilic silica than from the surface of the lipophilic IP. In this regard, studies are currently underway to explore the effect on recycle efficiency of increasing the hydrophilicity of the polymer by introducing PEG-derived co-monomers or PEG-functionalised pyrrolidinium monomers.
Encouraged by the enhancement in performance obtained with selected PIILP catalysts, testing was extended to include a comparison of the Mukaiyama-aldol reaction (Scheme 2) catalysed by 10 mol% copper(II)-bis(oxazoline) catalyst A under homogeneous conditions in various solvents and under heterogeneous conditions with SILP and PIILP-based catalysts, full details of which are summarised in Table 4. Complete conversion was achieved within 1 min at room temperature with catalyst A in the ionic liquid whereas only moderate conversions were obtained after 15 min in dichloromethane and diethyl ether (entries 1–3). A markedly higher ee in the ionic liquid compared with dichloromethane and diethyl ether was also observed; however, this was offset by the formation of a minor amount (∼10%) of by-product, identified by 1H NMR spectroscopy as 3-hydroxy-1,3-diphenyl-butan-1-one. Interestingly, this by-product was only generated during reactions conducted in ionic liquid and there was no evidence for its formation during reactions catalysed by the PIILP, PIILP/IL or SILP systems. This by-product results from a Mukaiyama-aldol reaction between 1-phenyl-1-trimethylsiloxyethene and acetophenone, the latter of which is generated via hydrolysis of the 1-phenyl-1-trimethylsiloxyethene substrate.2 In addition, both SILP catalysts gave complete conversion in short reactions times with only a slight reduction in ee compared with the homogenous reaction conducted in ionic liquid (entries 5–6). In contrast to the SILP-based catalysts, in general, the ee's and conversions obtained with the PIILP and PIILP/IL system were lower than found under homogeneous conditions. For the PIILP and PIILP/IL catalysts derived from IP-2, ee's of 86% and 90% were obtained, respectively, which are comparable with those obtained under homogeneous IL or SILP conditions, albeit it with slightly lower conversions (entries 9 and 10). For comparison, the reference system generated by supporting catalyst A on the non-ionic polymer P-7 were completely inactive even in the presence of an IL film (entries 15–16). A high ee has previously been achieved for the Mukaiyama-aldol reaction between ketene thioacetal and methyl pyruvate under heterogeneous conditions using an insoluble polymer-bound copper(II)-bis(oxazoline) catalyst; however, while the ee of 91% was comparable to that of the 93% obtained under homogeneous conditions the heterogeneous system was less active and required markedly longer reaction times to reach similar conversions.19
Entry | System | Time (min) | Conv.a (%) | eeb (%) config |
---|---|---|---|---|
a Conversion at 20 °C determined by IH-NMR. b Determined by HPLC. | ||||
1 | CH2Cl2 | 15 | 43 | 82(S) |
2 | Et2O | 15 | 32 | 80(S) |
3 | IL | 1 | 100 | 89(S) |
4 | IL/Et2O | 5 | 100 | 84(S) |
5 | SiO2/IL | 5 | 100 | 86(S) |
6 | CNT/IL | 5 | 100 | 85(S) |
7 | IP-1 | 5 | 49 | 24(S) |
8 | IP-1/IL | 5 | 47 | 11(S) |
9 | IP-2 | 5 | 63 | 86(S) |
10 | IP-2/IL | 5 | 56 | 90(S) |
11 | IP-4 | 5 | 56 | 13(S) |
12 | IP-4/IL | 5 | 69 | 48(S) |
13 | IP-6 | 5 | 36 | 40(S) |
14 | IP-6/IL | 5 | 55 | 63(S) |
15 | P-7 | 5 | 0 | — |
16 | P-7/IL | 5 | 0 | — |
For catalyst B smaller changes in the Diels–Alder cycloadduct ee were observed over the range of polymers studied. However, for all PIILP and PIILP-IL systems studied significantly higher conversions were obtained than the corresponding homogeneous reactions (Table 1) which highlights a positive/synergistic influence of the catalyst–polymer–IL interaction compared with the IL–catalyst interaction. It has been well-documented that for catalyst B the metal–ligand complex exists as an equilibrium of several different geometries, all of which are active catalysts,26 and thus the presence of such multiple active geometries could be responsible for the higher conversions observed in comparison to catalyst A. Moreover, these active catalysts can give cycloadducts with opposite configurations which would result in a reduction in the product ee. Such a number of active catalyst geometries could also be the origin of the large differences in endo selectivity which has not been previously observed for similar reactions conducted using SILP based silica and carbon supports.3
As previously established, an ionic environment in SILP conditions is required for higher catalyst performance, therefore, it is also possible that the number and distribution of ionic sites in the polymer can influence the selectivity of the reaction.27 In this study, the concentration of ions at the surface of the polymers is significantly higher than the loading of the Cu. For example, catalyst A/B loadings of 0.1 mmol g−1 on the support are significantly lower than surface concentration of [NTf2]− present. For example, IP-5 has an anion concentration of 0.99 mmol g−1. It may be expected that if the anion were the major determining factor with respect to enantioselectivity, increasing anion concentration would lead to high ee's; however, the converse is observed with higher ee's found for IP-1 and IP-4 which have [NTf2]− concentrations of 0.68 mmol g−1 and 0.76 mmol g−1, respectively, compared with IP-5. This provides further evidence that it is the structure of the support and the support–IL–catalyst interactions that determine the catalyst activity/selectivity rather than simply the concentration of ions present.
Although these PIILP systems showed excellent initial activity/selectivity, deleterious results were observed upon recycle which appeared to be due to the polymer interactions with the highly reactive cyclopentadiene and its dimer. In this regard, it should be possible to develop a highly active and recyclable catalytic system by tailoring the ionic polymer to minimise these negative interactions.
SI1320 silica support was obtained from GRACE Davison and calcined at 400 °C prior to use. Carbon nanotubes were obtained from Bayer Material Science. Commercial polymers IRA-400 and IRA-900 were purchased from Aldrich as Cl− salts and exchanged into the [NTf2]− form using LiNTf2 forming the corresponding IPs 5 and 6. Polystyrene P-7 was used as received.
All polymers were ground using a mortar and pestle and particles of <250 μm were used for catalyst immobilisation. For the Diels–Alder reaction the endo selectivity and conversions were determined by HPLC and the ee's based on the endo isomer were calculated from the HPLC profile using a Chiralcel OD-H column (hexane:propan-2-ol 90:10 flow rate 1 cm3 min−1 at 210 nm. The retention times of the endo enantiomers were major (2S)-enantiomer tR ∼ 18 min and minor (2R)-enantiomer tR ∼ 20 min.
For the Mukaiyama-aldol reaction, conversions and selectivity were determined by 1H-NMR. Enantioselectivity was calculated from the HPLC profile using a Chiralcel OD-H column (hexane:propan-2-ol 96:4 flow rate 1 cm3 min−1 at 254 nm. The retention times of the enantiomers were major (2S)-enantiomer tR ∼ 23 min and minor (2R)-enantiomer tR ∼ 19 min.
Following concentration of the Diels–Alder reaction mixture, the crude adduct was dissolved in 5 cm3 of diethyl ether and filtered through a small plug of silica gel to afford unpurified product and analysed directly.
Following concentration of the Mukaiyama-aldol reaction mixture, the residual oil was redissolved in THF (5 cm3) and the corresponding silyl-adduct was hydrolysed by stirring for 1 h using 2 M HCl (10 cm3). The THF was removed under vacuum and the keto-alcohol was extracted from the aqueous by washing with Et2O (3 × 5 cm3). The crude product was analysed directly.
For reactions conducted in diethyl ether, the catalyst was activated in dichloromethane which was then removed under vacuum followed by addition of diethyl ether (5 cm3) and stirred for 30 min after which the substrates were added. The reaction was worked up as previously described to afford the unpurified products which were analysed directly.
Footnote |
† Electronic supplementary information (ESI) available: 1H NMR, 13C NMR and 19F NMR spectra for the polymer precursors synthesised. TGA, SEM and FTIR traces for the polymers used. HPLC traces of exo and endo-cycloadducts and aldol adducts formed. See DOI: 10.1039/c3gc41378k |
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