Diels Alder reaction polymerization

Concerning the Diels-Alder reaction, polymeric supports were mostly involved in solid phase synthesis where the diene and the dienophile are grafted onto the polymer. Compared to these procedures, only few examples involving supported catalysts were reported in the literature. 

Discussion of ladder polymers also enables us to introduce a step-growth polymerization that deviates from the simple condensation reactions which we have described almost exclusively in this chapter. The Diels-Alder reaction is widely used in the synthesis of both ladder and semiladder polymers. In general, the Diels-Alder reaction occurs between a diene [XVI] and a dienophile [XVll] and yields an adduct with a ring structure [XVlll] ... 

Aqueous ring-opening metathesis polymerization (ROMP) was first described in 1989 (90) and it has been appHed to maleic anhydride (91). Furan [110-00-9] reacts in a Diels-Alder reaction with maleic anhydride to give exo-7-oxabicyclo[2.2.1]hept-5-ene-2,3—dicarboxylate anhydride [6118-51 -0] (24). The condensed product is treated with a soluble mthenium(Ill) [7440-18-8] catalyst in water to give upon acidification the polymer (25). Several apphcations for this new copolymer have been suggested (91). 

Sorbic acid is oxidized rapidly in the presence of molecular oxygen or peroxide compounds. The decomposition products indicate that the double bond farthest from the carboxyl group is oxidized (11). More complete oxidation leads to acetaldehyde, acetic acid, fumaraldehyde, fumaric acid, and polymeric products. Sorbic acid undergoes Diels-Alder reactions with many dienophiles and undergoes self-dimerization, which leads to eight possible isomeric Diels-Alder stmctures (12). 

Polymerization. CPD dimerizes spontaneously and exothermically at ambient temperature to DCPD. At temperatures above 100°C, CPD can be made to polymerize noncatalytically via a series of consecutive Diels-Alder reactions to trimer, tetramers, and higher oligomers. Eor example, the trimers, 3a,4,4a,5,8,8a,9,9a-octahydro-4,9 5,8-dimethanobenz-lJT-[ iQdene, [7158-25-0] (3) and l,4,4a,4b,5,8,8a,9a-octahydro-l,4 5,8-dimethano-lJT-fluorene [35184-08-8] (4), are formed ia the ratio 87 13 by the monomer adding to the dimer (19). 

Elastomers. Ethylene—propylene terpolymer (diene monomer) elastomers (EPDM) use a variety of third monomers during polymerization (see Elastomers, ethyiene-propylene-diene rubber). Ethyhdenenorbomene (ENB) is the most important of these monomers and requires dicyclopentadiene as a precursor. ENB is synthesized in a two step preparation, ie, a Diels-Alder reaction of CPD (via cracking of DCPD) with butadiene to yield 5-vinylbicyclo[2.2.1]-hept-2-ene [3048-64-4] (7) where the external double bond is then isomerized catalyticaHy toward the ring yielding 5-ethyhdenebicyclo[2.2.1]-hept-2-ene [16219-75-3] (ENB) (8) (60). 

Benzo[Z)]furans and indoles do not take part in Diels-Alder reactions but 2-vinyl-benzo[Z)]furan and 2- and 3-vinylindoles give adducts involving the exocyclic double bond. In contrast, the benzo[c]-fused heterocycles function as highly reactive dienes in [4 + 2] cycloaddition reactions. Thus benzo[c]furan, isoindole (benzo[c]pyrrole) and benzo[c]thiophene all yield Diels-Alder adducts (137) with maleic anhydride. Adducts of this type are used to characterize these unstable molecules and in a similar way benzo[c]selenophene, which polymerizes on attempted isolation, was characterized by formation of an adduct with tetracyanoethylene (76JA867). 

Aqueous hydrofluoric acid dissolved in acetonitrile is a good catalyst for intramolecular Diels-Alder reactions [9] This reagent promotes highly stereoselective cyclizations of different triene esters (equation 8) The use of other acids, such as hydrochloric, acetic, and trifluoroacetic acid, results in complete polymerization of the starting trienes [9] (equation 8)... 

Collins and coworkers applied the bis(tetrahydroindenyl)zirconium triflate 32, which is used as a polymerization catalyst, to the asymmetric Diels-Alder reaction [50] (Scheme 1.61). A remarkable solvent effect was observed - although only a low optical yield was obtained in CH2CI2, high optical purity (91% ee) was realized in 2-nitropropane by use of only 1 mol% of the catalyst. The catalyst is also effective for crotonoyloxazolidinone, giving the cycloadduct in 90% ee. 

The use of catalysts for a Diels-Alder reaction is often not necessary, since in many cases the product is obtained in high yield in a reasonable reaction time. In order to increase the regioselectivity and stereoselectivity (e.g. to obtain a particular endo- or exo-product), Lewis acids as catalysts (e.g. TiCU, AICI3, BF3-etherate) have been successfully employed." The usefulness of strong Lewis acids as catalysts may however be limited, because they may also catalyze polymerization reactions of the reactants. Chiral Lewis acid catalysts are used for catalytic enantioselective Diels-Alder reactions. ... 

The Diels-Alder reaction was thought for many years to be only slightly influenced by catalysts. However, in 1960, Yates and Eaton (6) clearly demonstrated that with certain dienophiles, the presence of a molar equivalent of aluminum chloride can cause a remarkable acceleration of the reaction. Providing the diene is not polymerized (7) or otherwise destroyed by the catalyst, the modification can be fruitfully employed to carry out the reaction at lower temperature and for shorter times. 

When the cnolate of an enone is brought into reaction with an enone, usually a carbocyclic system is prepared by two consecutive Michael additions (M1MIRC reactions). Due to the lower temperatures employed and the absence of diene polymerization these reactions are useful alternatives for Diels-Alder reactions and proceed in general with high diastereoselectivities. When neither enolate nor enone is cyclic a monocyclic system is formed 338 which can be converted into a bicyclic system when the Michael addition is followed by an aldol reaction339. When, however, the enolate is cyclic a bicyclic or a tricyclic system is formed340 341. 

The use of zeolites is particularly advantageous for self-Diels-Alder reactions of gaseous dienes because it reduces the polymerization of the reactant. An example is the cyclodimerization of 1,3-butadiene to 4-vinylcyclohexene [20a] carried out at 250 °C with satisfactory conversion when non-acidic zeolites, such as large-pore zeolites Na-ZSM-20, Na- S and Na-Y, are used. 

Diels-Alder reactions of vinylpyrazoles 45 and 46 only occur with highly reactive dienophiles under severe conditions (8-10 atm, 120-140 °C, several days). MW irradiation in solvent-free conditions also has a beneficial effect [40b] on the reaction time (Scheme 4.11). The indazole 48, present in large amounts in the cycloaddition of 45 with dimethylacetylenedicarboxylate, is the result of an ene reaction of primary Diels-Alder adduct with a second molecule of dienophile followed by two [l,3]-sigmatropic hydrogen shifts [42]. The MW-assisted cycloaddition of 46 with the poorly reactive dienophile ethylphenyl-propiolate (Scheme 4.11) is significant under the classical thermal reaction conditions (140 °C, 6d) only polymerization or decomposition products were detected. 

The study was extended to the inverse electron-demand Diels-Alder reaction between the (E)-l-carboalkoxybutadienes 21 with ethylvinylether 22 (Figure 5.1). No reaction was observed in any case either the starting materials were recovered or polymeric material was produced. 

Starting from 27, cyclo-Cig was prepared in the gas phase by laser flash heating and the neutral product, formed by stepwise elimination of three anthracene molecules in retro-Diels-Alder reactions, was detected by resonant two-photon-ionization time-of-flight mass spectrometry [23]. However, all attempts to prepare macroscopic quantities of the cyclocarbon by flash vacuum pyrolysis using solvent-assisted sublimation [50] only afforded anthracene and polymeric material. 

Several different companies have greened various steps of the process. In VNB production by-products come from competing Diels-Alder reactions and polymerization, largely of cyclopentadiene. The reaction is usually carried out in a continuous tube reactor, but this results in fouling, due to polymerization, at the front end, where the dicyclopentadiene is cracked to cyclopentadiene at temperatures over 175 °C. Whilst fouling does not have a very significant effect on yield, over time it builds up. 

On the other hand, numerous examples are already known in which monomeric metaphosphoric esters are generated by thermolysis reactions. Most worthy of mention in this context is the gas phase pyrolysis of the cyclic phosphonate 150 which leads via a retro-Diels-Alder reaction to butadiene and monomeric methyl metaphosphate (151) 108,109, no). While most of the phosphorus appears as pyrophosphate and trimeric and polymeric metaphosphate, a low percentage (<5%) of products 152 and 153 is also found on condensation of the pyrolyzate in a cold trap containing diethylaniline or N,N,N, N,-tetraethyl-m-phenylene-diamine. The... 

Dehydration of P-nitro alcohols provides an important method for the preparation of nitroalkenes. Because lower nitroalkenes such as nitroethylene, 1-nitro-1-propene, and 2-nitro-l-propene tend to polymerize, they must be prepared carefully and used immediately after preparation. Dehydration with phthalic anhydride is the most reliable method for such lower nitroalkenes.42,43 Such lower nitroalkenes have been used as important reagents for Michael acceptors or dienophiles in the Diels-Alder reaction, which will be... 

A similar conclusion has been drawn during an examination of the Diels-Alder reaction of 6-demethoxy-/J-dihydrothebaine with methylvinylketone using microwave irradiation [110]. When performed under conventional heating conditions, extensive polymerization of the dienophile was observed whereas reaction is much more cleaner under microwave activation (Eq. 61). 

Linders et al. [73] studied the Diels-Alder reaction of 6-demethoxy-/J-dihydrothe-baine with an excess of methyl vinyl ketone (used both as reactant and solvent), which gives a mixture of two isomeric adducts. When the reaction was performed using classical heating extensive polymerization occurred, whereas much less poly-... 

Diels-Alder reactions [31] and 1,3-dipolar cycloadditions [32, 33] have been performed by use of this methodology. For example, Diaz-Ortiz described the hetero-Diels-Alder and 1,3-dipolar cycloaddition reactions of ketene acetals. The reactions were improved and products were isolated directly from the crude reaction mixture without polymerization of the ketene acetals [34],... 

In an attempt to prepare new diprenorphine analogs, Linders, in one of the first examples of microwave-induced organic reactions, reported the reaction between methyl vinyl ketone and 6-demethoxy-/J-dihydrothebaine (48) [53], The Diels-Alder reaction, when performed under classical conditions, led to extensive polymerization of the dienophile. A dramatic improvement was achieved when the cycloaddition was conducted in a modified microwave oven at the reflux temperature of methyl vinyl ketone. By use of these conditions adducts 49 and 50 were obtained in a 3 2 ratio, according to HPLC (Scheme 9.12). 

As mentioned above, the electrochemical oxidation of a diene yields 1,2- and 1,4-addition products when the reaction is carried out in the presence of a nucleophile such as methanol or acetic acid. When the oxidation is carried out in the absence of the nucleophile it usually yields a polymeric compound as the major product. The formation of a small amount of the Diels-Alder adduct is, however, observed when the reaction is carried out in CH2CI2 with graphite anode. One of the proposed reaction pathways is shown in equation 68, though it is not clear whether the cyclohexadienyl radical serves as a diene (as shown in equation 6) or a dienophile in the Diels-Alder reaction. 

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