Immobilized cyclopentadiene in the Diels Alder / retro-Diels Alder concept

Immobilized cyclopentadiene on polystyrene has been synthesized and studied in [4π + 2π]cycloadditions with a variety of dienophiles. Reasonable to excellent conversions were obtained. The cyclopentadiene Diels-Alder adducts turned out to be generally too stable to allow effective cycloreversion both under dynamic (under vacuum) and static (in a solvent) thermal conditions. A detailed study of the immobilized cyclopentadiene benzoquinone adduct showed a remarkable inreactivity of the enone double bond showing the limitations of the solid phase approach in the Diels-Alder/Retro-Diels-Alder synthetic strategy.


Introduction
The Diels Alder/retro-Diels Alder concept is a powerful synthetic strategy in stereo-and enantioselective routes to functionalized cycloalkenes. 1,2We have extensively studied this concept for the endo-tricyclo[5.2.1.0 2,6]decadienone system 1 2d,3 (Scheme 1).This tricyclic enone is in essence the Diels Alder adduct of cyclopentadiene and the elusive cyclopentadienone 2. The annulated cyclopentenone moiety in 1 can be chemically transformed, e.g. by conjugated additions to afford functionalized tricyclic decadienones 3. The reversibility of the Diels Alder reaction now allows regeneration of an enone moiety affording functionalized cyclopentenones 4.This cycloreversion can be accomplished thermally, either in solution (static thermolysis) or in the gas phase (dynamic thermolysis).For gas phase reactions the Flash Vacuum Thermolysis (FVT) technique has been particularly successful for those substrates that have a sufficiently high volatility. 4,5Since the formation of cyclopentenones 4 from cyclopentadienone 2 by direct conjugate additions is not possible due to the instability of cyclopentadienone 2 above -100°C,

Scheme 1
In principle, the Diels Alder/retro-Diels Alder concept can be translated into a process that is carried out on solid support.Such a solid-phase process could possibly lead to a combinatorial synthesis of cycloalkenes.This approach would feature the immobilized component 5 as the protecting cyclic diene in the initial Diels Alder reaction affording the resin-bound adduct 7 when reacted with dienophile 6 (Scheme 2).Chemical modification of the immobilized tricyclic system 7 to 8 is followed by thermal cycloreversion cleaving the modified dienophile 9 from the immobilized cyclic diene 5 that in principle could be reused.In this paper, the synthesis and reactivity of immobilized cyclopentadiene 5a and some selected adducts, is reported.Only a few papers on immobilized cyclopentadiene have been reported hitherto.A detailed study of the general application of immobilized cyclopentadiene in the Diels Alder/retro-Diels Alder synthetic strategy has not yet been undertaken.In 1985, Mirua et al. 6 disclosed a first example of polymer-supported cyclopentadiene 5a.Rotello et al. 7 reported the Diels Alder reaction between buckminsterfullerene C 60 and cyclopentadiene linked to the Merrifield resin.Recently, Wanner and Koomen 8 showed the ability of immobilized cyclopentadiene 5a to react with the vulnerable nitroso functionality of nitrosoadenosine triacetate.Rotello et al. 9 also investigated cyclopentadiene immobilized on silica gel as an extension of their studies on the purification of buckminsterfullerenes.

Results and Discussion
Rotello et al. 7 immobilized cyclopentadiene by stirring a solution of sodium cyclopentadienylide in THF with the Merrifield resin 10 at -20°C.A loading of 52% was thus accomplished, i.e. 52% of available chloride of Merrifield resin 10 had been substituted by cyclopentadiene.A significant improvement of Rotello's procedure was achieved by simply increasing the reaction temperature from -20°C to 45°C and using THF as the solvent.A loading of 70% was now achieved (Table 1, entry 1).The higher temperatures did not adversely affect the stability of the sodium cyclopentadienylide solution, as long as the reaction was performed under a strict inert atmosphere. 10It should be noted that the Merrifield resin used here is different from the one applied by Rotello, regarding the degree of cross-linking and the number of available sites (see experimental part).The use of lithium as the cation did not lead to better results (Table 1, entries 2,4).This observation is in agreement with the loading of 43% reported by Wanner and Koomen, 8 when they used lithium cyclopentadienide in DMF.The immobilization reaction was monitored with infrared spectroscopy.The Merrifield resin gave a sharp absorption at 1265 cm -1 due to C-Cl-stretching.This absorption decreased in intensity when the reaction proceeded.The loading was determined by two methods.The first method constitutes the quantitative determination of the chloride that is released in the substitution of the Merrifield resin with the cyclopentadienyl anion.After the immobilization reaction, the resin was washed with water.The combined aqueous washings contained the chloride anions substituted by cyclopentadiene.The quantity of these chloride anions was determined by titration in triplo using the Vollhard titration. 11Assuming that this quantity of chloride corresponds to an equimolar amount of reacted cyclopentadiene, the loading can now be easily calculated.The second method is based upon a report by Bing Yan and co-workers 12 who showed that elemental analysis of resins provides reliable information on the presence of certain elements, e.g.nitrogen.For this purpose, the immobilized cyclopentadiene 5a was reacted with tetracyanoethylene (Scheme 4).Elemental analysis of the resulting immobilized adduct 11b provided the nitrogen content of the resin, thus giving the amount of tetracyanoethylene that had reacted.As it is assumed that tetracyanoethylene reacts quantitatively with cyclopentadiene, the cyclopentadiene loading can now easily be calculated.

Scheme 4
In a typical immobilization reaction, the titration method showed that 1.19 mmole of available chloride per gram of Merrifield resin was substituted by cyclopentadiene, resulting in a loading of 1.15 mmole cyclopentadiene per gram of resin 5a.Elemental analysis of the tetracyanoethylene adduct showed that 1.25 mmole cyclopentadiene per gram of 5a had reacted.These results show that the immobilized cyclopentadiene had indeed reacted quantitatively with tetracyanoethylene.In conclusion, elemental analysis of the tetracyanoethylene adduct 11b gives a reliable estimation of the loading of cyclopentadiene on a polystyrene resin and is a good alternative for the chloride titration method.
Two examples of a Diels Alder reaction applying immobilized cyclopentadiene on a polystyrene-based resin were known at the time this research started.Miura and co-workers 6 described the cycloaddition of immobilized cyclopentadiene with maleic anhydride in nitrobenzene at 180°C.Rotello et al. 7,9 disclosed the cycloaddition with a mixture of C 60 and C 70 buckminsterfullerenes.
Besides the successful cycloaddition of immobilized cyclopentadiene 5a with tetracyanoethylene as described above, the Diels-Alder reactivity of 5a was studied for a variety of dienophiles (Scheme 5).All dienophiles used were commercially available except for Nphenethylmaleimide 13 and diethyl maleate.
The Diels Alder reactions were generally carried out overnight in DMF at room temperature.Only diethyl maleate required an elevated temperature to achieve acceptable conversions.This in accordance with the relatively low reactivity of this dienophile towards cyclopentadiene in solution.The formation of the Diels Alder adducts was confirmed by IR analysis.The benzoquinone adduct 11c, for example, showed a C=O absorption at 1670 cm -1 .The maleic anhydride adduct 11a showed typical anhydride C=O absorptions at 1860 and 1773 cm -1 and C-O stretches at 1221 cm -1 , whereas the diethyl maleate adduct 11d had a characteristic ester absorption at 1728 cm -1 .Weak CN-absorptions were observed at 2200 cm -1 in the IR-spectra of the tetracyanoethylene adduct 11b.The yields were determined by measuring the weight increase of the resin or more accurately by elemental analysis when nitrogen was present in the dienophiles and hence in the polymer-supported adducts.The results listed in Table 2 show that all cycloaddition reactions can be performed with high efficiency.In solution, monosubstituted cyclopentadienes rapidly isomerize at room temperature through a 1,5 hydride shift (Scheme 6).This isomerisation is catalyzed by alkali cyclopentadienylide. 14,15 As a result, the reaction of an alkyl halide with sodium cyclopentadienylide always leads to a mixture of three possible alkylcyclopentadiene isomers 12a-c. 15,16,17In Diels Alder cyclizations, alkylcyclopentadienes generally afford mixtures of the corresponding cycloadducts 13a-c, in which the alkylgroup may be attached at a maximum of three different positions (Scheme 6).

Scheme 6
Similarly, immobilization of cyclopentadiene on a resin most likely also gives rise to three isomerizing structures.As a consequence, the produced immobilized cycloadducts 12 may be linked at three different positions to the polystyrene-based resin.It is likely that the mode of linkage has a direct impact on both the subsequent chemical transformations carried out on these polycyclic structures and the eventual cycloreversion process.Therefore, it is desirable to know whether there is any regioselectivity in the [4π + 2π]-cycloadditions of immobilized cyclopentadiene.Hence, attempts were made to analyze the composition of a possible isomeric mixture 11c (Figure 1), obtained from the reaction of immobilized cyclopentadiene 5a and benzoquinone.A realistic structure determination is only possible after cleavage of the complete cycloadducts from the resin.In order to cleave the entire tricyclic structures from the resin, a cleavable linker must be introduced between the polymeric backbone and cyclopentadiene.The Wang resin was chosen, because it has an additional cleavable methoxyphenyl linker, which is lacking in the Merrifield resin.Cleavage from the Wang resin under acidic conditions normally occurs by breaking an ester bond.However, if the benzoquinone adduct of cyclopentadiene is immobilized on the Wang resin as in structure 17, only an ether bond is present for cleavage.This bond can readily be broken by using trifluoroacetic acid or boron tribromide. 18The resulting mixture of isomers 18 can then be analyzed, e.g. by GC/MS.
In order to immobilize cyclopentadiene on the Wang resin 14, the hydroxyl groups of this resin first need to be substituted by chloride.This was accomplished by treatment of 14 with dichlorotriphenylphosphorane in dichloromethane in the presence of imidazole, affording resin 15. 19 Cyclopentadiene was now easily immobilized on the Wang resin using the same conditions as those that are optimal for linking to the Merrifield resin (Scheme 7).Subsequent cycloaddition of benzoquinone and the obtained resin 16 gave the Wang-immobilized adduct 17 with a loading of 0.75 mmole per gram of resin.

Scheme 7
Immobilized adduct 17 was then treated with trifluoroacetic acid in dichloromethane, followed by thorough washing of the resin.The combined washings were concentrated, affording a complex mixture.The mixture was analysed with GC-MS in order to establish whether isomers of tricyclic structure 18 were present.The gas chromatogram showed three peaks in a ratio of 2:1:1.The mass spectra associated with these peaks contained parent ions of 280 m/z and similar fragmentation patterns, confirming the cleavage of three isomers of tricyclic structure 18.The abundance of one isomer over the other two isomers indicates that attachment of cyclopentadiene to the resin at one position is preferred.This is in accordance with the observation that the 5-substituted alkylcyclopentadiene 12a is the kinetically preferred product in solution. 14,15The resulting immobilized Diels Alder adduct is most likely the bridgehead substituted tricyclic structure III (Figure 1).However, this cannot be confirmed unambiguously by mass spectroscopy alone.In conclusion, some selectivity in the formation of the immobilized cyclopentadiene adducts is observed.Apparently, there is no effect of the polystyrene backbone on the isomeric ratio.
In order to study the efficiency of the cycloreversion of the immobilized cycloadducts 11a,c,d,g and h, two methods were applied, viz.thermolysis of the resin beads under vacuo and trapping of the volatile products at -78°C (dynamic procedure) and heating the resin beads in a high boiling solvent (static procedure).In the first approach the immobilized cycloadducts were heated for three hours at a pressure of 0.05 mbar and applying temperatures varying between 150 to 250ºC.Higher temperatures were not applied as considerable decomposition (blackening) of the resin took place.At 150°C, cycloreversion was only observed for adduct 11c, affording benzoquinone in small amounts.The other adducts did not undergo cycloreversion until a temperature of 200ºC was attained.The yield of cleavage products at this temperature was generally rather low as shown in Table 3, except for benzoquinone.Maleic anhydride could also be obtained in a good yield, albeit only at 250°C.This dynamic thermolysis approach may have the disadvantage that less volatile dienophiles produced may not be completely released from the resin and therefore lower the observed yield.In order to quantify this problem, the residual resin was swollen in dichloromethane, stirred and filtered.Careful analysis of this filtrate showed the absence of any cycloreversion product indicating that the rational given above does not explain the observed low yields.Finally, the same heating protocol was repeated, but this did not result in an additional amount of dienophile.
The immobilized benzoquinone adduct 11c was also subjected to the static thermolysis protocol.Thus, adduct 11c was heated at reflux in a high-boiling solvent, such as dimethylformamide or o-dichlorobenzene.After filtration of the resin, the filtrate was analyzed with GC and TLC to establish whether any cycloreversion product had been cleaved off from the resin.The results listed in Table 4 reveal that no cycloreversion could be achieved at all, using this approach.Apparently, the temperature needed for cycloreversion of solid phase cyclopentadiene adducts 11c under static thermal conditions is considerably higher than required under dynamic thermal conditions.Because of these negative results with 11c, the other cycloadducts 11 were not studied under these conditions.Recapitulating, the temperature required for cycloreversion of cycloadducts 11 is generally too high and highly dependant on the nature of the dienophile to be released.Besides, the only successful cycloreversion reactions were performed under dynamic thermolysis conditions.However, with respect to a combinatorial extension of the Diels-Alder/retro-Diels-Alder concept, cycloreversion under static thermolysis conditions, i.e. using a solvent, would be preferable.Cycloreversion of immobilized cyclopentadiene adducts by thermolysis is possible, but the results are not satisfactory for a combinatorial approach.
The benzoquinone adduct 11c was now selected for a study on conceivable chemical transformations.Such a modified adduct would produce a new cyclohexene compound when subjected to thermolysis.If successful this would substantiate the viability of the Diels Alder/retro-Diels Alder concept on solid phase as depicted in Scheme 2.
The immobilized benzoquinone adduct 11c was subjected to the alkaline epoxidation reaction (Scheme 8).Although this reaction proceeds readily in solution, 20 no epoxidation took place at all as was shown by infrared spectroscopy.In addition, thermolysis of the resulting resin afforded benzoquinone as the only cycloreversion product.A possible explanation might be the poor swelling capacity of the solvent mixture used, viz.acetone and water.The active sites on the resin may not be accessible for the basic hydroperoxide, blocking the epoxidation.However, other epoxidation methods using more suitable solvents (Table 5, entries 2,3), including one method especially designed for solid-phase synthesis (Table 5, entry 4), 21 did not alter the result.The desired epoxide 19 was not formed.Hence, it can be concluded that other factors than a poor swelling capacity prevent the epoxidation of immobilized adduct 11c.Other reactions that work well for non-immobilized tricyclic cyclopentadiene benzoquinone adducts, viz.aromatisation of the quinone system or a [4+2]-Diels Alder reaction with dimethylbutadiene were also tried with immobilized adduct 11c.However, only starting material was recovered.These negative results reveal that the enone moiety in immobilized adduct 11c is unexpectedly unreactive.
Theoretically, it is possible that the double bond has undergone a second Diels Alder reaction with another immobilized cyclopentadiene moiety, to give the cross-linked polystyrene resin 20, as shown in Figure 2. The high degree of cross-linking in resin 20 should be obvious from deteriorated swelling properties.However, when immobilized adduct 11c was suspended in dichloromethane, no reduced swelling compared to the Merrifield resin was observed.Additional experiments had to be performed to obtain more information on the possible existence of cross-linked structure 20.Both the bis-cyclopentadiene benzoquinone adduct 22 and its immobilized counterpart 23 were synthesized for comparison.The infrared spectral data of these adducts, listed in Table 6, provide the information required to unambiguously exclude the formation of cross-linked resin 20.The mono-adduct 21 of cyclopentadiene and benzoquinone showed a carbonyl absorption at 1662 cm -1 (entry 1), whereas absorption at 1689 cm -1 is characteristic for the di-adduct 22 (entry 2).Thus, an apparent distinction can be made between the presence of a mono-or a di-adduct, based on infrared spectral data.The immobilized cyclopentadiene adduct 11c only showed a strong absorption at 1663 cm -1 (entry 3), which strongly suggests that this adduct is present only as the mono-adduct.Finally, conclusive information was obtained from the reaction of immobilized cyclopentadiene 5a and the monoadduct 21 of cyclopentadiene and benzoquinone (Scheme 9).The resulting immobilized structure showed a strong carbonyl absorption band at 1688 cm -1 , caused by the formation of the di-adduct 23.This proves that the immobilized di-adduct also shows a carbonyl absorption at 1688 cm -1 in the IR-spectrum.The complete absence of this absorbance in immobilized adduct 11c thus excludes the formation of the benzoquinone di-adduct 20.In conclusion, cyclopentadiene was successfully immobilized on the Merrifield resin, affording a monomeric, thermally stable polymeric diene.A loading of up to 1.2 mmole of cyclopentadiene per gram of resin was achieved.The Diels Alder reactivity of the immobilized cyclopentadiene turned out to be reasonably good and comparable with that in solution.The retro-Diels Alder reaction, however, is not generally feasible; it is highly dependent on the nature of the cycloadduct.Chemical modification of the immobilized cycloadducts using methods that generally succeed in solution, were not always successful.Interestingly, the immobilized benzoquinone adduct 11c, for example, appeared totally inert to reactions that proceed in high yields for the non-immobilized benzoquinone adduct 21.The reason for this remarkable inreactivity of the enone moiety in 11c is unknown As a final conclusion, the Diels Alder/retro-Diels Alder concept is not generally feasible for immobilized cyclopentadiene on polystyrene.The temperatures required to accomplish the cycloreversion of the immobilized cyclopentadiene adducts 11 is generally too high, both under dynamic and static conditions, to be of practical use.Thus, immobilized cyclopentadiene 5a and its adducts cannot be applied in a combinatorial approach towards pharmaceutically interesting intermediates.

Experimental Section
General Procedures.Melting points were determined using a Reichert thermopan microscope (uncorrected), or using a Büchi Melting Point B-545 (corrected).100 MHz 1 H-NMR spectra were recorded on a Bruker AC-100 spectrometer, 200 MHz 1 H-NMR spectra were recorded on a Bruker AC-200 spectrometer and 300 MHz 1 H-NMR and all 13 C-NMR spectra were recorded on a Bruker AC-300 spectrometer using tetramethylsilane (TMS) as an internal standard.FTIR spectra were recorded on an ATI Mattson -Genesis Series FTIR spectrophotometer.DRIFT spectra were recorded using a BioRad FTS-25 spectrometer equipped with an in situ Diffuse Reflectance Infrared Fourier Transform (DRIFT) accessory.For high-resolution mass spectra a double focussing VG7070E mass spectrometer was used.GC/MS was performed on a Varian Saturn GC/MS (Ion trap) equipped with a DB-5 column (30m x 0,25mm).Elemental analyses were performed using a Carlo Erba Instruments CHNS-O EA 1108 element analyzer.Gas chromatographic (GC) analyses were performed on a Hewlett-Packard HP5890II gas chromatograph (flame ionisation detector, FID) equipped with an HP-3396II integrator, using a capillary column (HP-1, 25 m x 0.31 mm x 0.17 µm) and nitrogen at 2 ml/min (0.5 atm) as the carrier gas.Diethyl ether was distilled from sodium hydride.Tetrahydrofuran was distilled from sodium and benzophenone.Heptane and dichloromethane were distilled from calcium hydride.Ethyl acetate was distilled from potassium carbonate.Other solvents were distilled if necessary.The Merrifield resin used was obtained from Fluka and cross-linked with 1% divinylbenzene and contained 1.7 mmole Cl/g resin.

the tricyclodecadienone system 1
in fact constitutes a synthetic equivalent of cyclopentadienone 2.

Scheme 3 Table 1 .
Linking of Cyclopentadiene to the Merrifield resin a Cyclopentadiene (in mmole/g functionalised resin) that reacted with tetracyanoethylene.b Percentage of chloride in Merrifield resin that is substituted by cyclopentadiene.

Table 2 .
Diels Alder reactions with cyclopentadiene immobilized on Merrifield resin Calculated by weight increase, except for entries 2, 5, 8 (calculated by elemental analysis).
b c Loading of Diels Alder adduct, expressed as mmole/g modified resin.dReaction performed at 80°C.

Table 3 .
Results of the cycloreversion reactions under dynamic conditions a not determined.

Table 4 .
Attempted cycloreversion of 11c in a solvent under static thermal conditions a microwave, 500W.

Table 5 .
Methods tried for the epoxidation of immobilized benzoquinone adduct 11c

Table 6 .
Infrared spectral data of benzoquinone adducts
© ARKAT USA, Inc Chlorinated Wang-resin(15).Cleavage from Wang resin(17).Resin 17 (117 mg) was stirred in a 1:1 mixture of dichloromethane and trifluoroacetic acid/water (95:5) for 2.5 h.The resin was filtered and washed with methanol and dichloromethane.The filtrate was concentrated in vacuo, affording a mixture containing three products in a ratio 2:1:1, which were identified as three isomers of 18.