Diels-alder dimerization reaction

The thermally induced Diels-Alder dimerization reaction producing vinylcyclohexene is very difficult to prevent except by lowering the storage... 

The amine-catalyzed Diels-Alder dimerization reaction of a, 3-unsaturated ketones in water was developed by Barbas et al. to form cyclohexanone derivatives (Eq. 12.12). They believe that the reaction proceeds via the in situ formation of 2-amino-1,3-butadiene and iminium-activated enone, as the diene and dienophile, respectively. 

The thermally induced Diels-Alder dimerization reaction producing vinylcyclohexene is very difficult to prevent except by lowering the storage temperature (246). Since the reaction rate increases about ninefold for every 20°C increase in temperature (Table 6), care must be taken to keep butadiene at a low temperature. Table 6. Effects of Temperature on Dimerization Rate of Butadiene ... 

In 2005, Nikaido et al. [9] reported the first example of cobalt-phosphine-catalyzed intermolecular Diels-Alder dimerization reaction (Scheme 16.7). The dimerization of piperine derivative (28) in the presence of cobalt-phosphine complexes appear to be more efficient than that of using only cobalt, which, combined with the result under purely thermal conditions, indicates that the addition of phosphine ligand changes the inhibition of cobalt to the reaction into promotion. Triphenylphosphine (PPhs) is the most favorable phosphine ligand of the reaction. 

Yorimitsu and Oshima reported that allenyl alcohol 19 can act as a 1-isopropylidene-2-propenyl metal equivalent via a retro-allylation process (Scheme 5.28) [22]. Similarly, the reactions with alcohols 20 furnish the corresponding arylated 1,3-dienes 21 (Scheme 5.29). However, the products undergo Diels—Alder dimerization reactions, which can be suppressed by addition of... 

In contrast to those unreactive dienes that can t achieve an s-cis conformation, other dienes are fixed only in the correct s-cis geometry and are therefore highly reactive in the Diels-Alder cycloaddition reaction. 1,3-Cyclopentadiene, for example, is so reactive that it reacts with itself. At room temperature, 1,3-cycIopentadiene dimerizes. One molecule acts as diene and a second molecule acts as dienophile in a self Diels-Alder reaction. 

The ortho-quinone methides are difficult to isolate due to their high reactivity, which leads to rapid Diels-Alder dimerization or trimerization (Fig. 7.26). At 150°C, a partial retro-Diels-Alder reaction of the trimer can occur to form ortho-quinone methide and bis(2-hydroxy-3,5-dimethylphenyl) ethane (dimer).51... 

This preparation is based on a procedure published by the submitters. 9-Phenylphenanthrene has been prepared previously by the reaction of phenyllithium with 9-chlorophelianthrene, by the high-temperature dehydrogenation with palladimn on charcoal of the Diels-Alder dimer of 1-phenyl-1,3-butadiene, and by the acid-catalyzed cyclization of the alcohol formed from the reaction of 2-biphenylylmagnesium iodide and 2-phenoxy-acetophenone. ... 

A review of Diels-Alder reactions of fullerenes with acyclic and cyclic dienes has been presented. The addition of substituted pyrimidine o-quinodimethanes (75) to [60]fullerenes yields novel organofullerenes (76) bearing a pyrimidine nucleus covalently attached to the Ceo cage (Scheme 26). The Diels-Alder dimerization of cyclopenta[/]phenanthrene (77) with isobenzindene (78) yields the dimer (79) in 85% yield (Scheme 27). Further evidence has been supplied to support an early reorganization of the r-network in the dimerization of 2-methoxycarbonylbuta-1,3-diene. The Lewis acid-catalysed Diels-Alder reactions of acrylate derivatives of new carbohydrate-based chiral auxiliaries with cyclohexadiene show excellent endo. exo... 

This simple view is clearly true for some reactions, e.g., the Diels-Alder dimerization of cyclopentadiene, where the rate constant in ethanol is the same as in hexane, and only a factor of three larger than in the gas phase. In contrast, for the example mentioned above of the 8 2 reaction (1), the reaction proceeds fifteen orders of magnitude faster in the gas phase than in methanol. For the Sfjl reaction of tert-butyl iodide, however, the gas phase rate constant can be estimated to be about 86 orders of magnitude slower than the solution phase rate constant. It is thus for ionic reactions that the tremendous changes in the rate constant upon solvation are seen. We are therefore specifically interested in those gas phase ion-molecule reactions that are the counterparts to the well-known solution phase reactions. 

Diels-Alder dimer or its reaction with styrene is the rate-determining step in initiation is not completely established. The dependence of Rp on [M] is closer to third-order than second-order, indicating that Eq. 3-63b is the slow step. The Diels-Alder dimer has not heen isolated, but ultraviolet spectroscopy of the reaction system is entirely compatible with its presence. There are indications that the photopolymerization of neat styrene proceeds by a similar mechanism. 

Not only the case of vinyl chloride but also styrene shows that the observed chain transfer to monomer is not the simple reaction described by Eq. 3-112. Considerable evidence [Olaj et al., 1977a,b] indicates that the experimentally observed Cm may be due in large part to the Diels-Alder dimer XII transferring a hydrogen (probably the same hydrogen transferred in the thermal initiation process) to monomer. 

CpC2 gives the monomer CpC. It was suggested that one of these dimers might be the result of a Diels-Alder type reaction. 

The analogy of these dimerization processes to thermal Diels-Alder type reactions which sometimes also yield cyclobutane structures is worth noting and may be taken as one of the arguments for a diradical structure of the transition in the latter process. Also, it may be pointed out that the photoexcited state involved is presumably the same one involved in the well-known photochemical trans-cis interconversion of such olefins. 

Diets-Alder catalysis.2 This cation radical enhances the reactivity of a neutral or electron-rich eis-1,3-diene in Diels-Alder reactions. Thus 1,3-cyclohexadiene undergoes Diels-Alder dimerization only at temperatures around 200°. The presence of 5-10 mole % of this salt effects dimerization even at —78°, with the usual endo/ exo selectivity (5 1). It also permits facile condensation of 1,3-cyclohexadiene with a hindered dienophile such as 2,5-dimethyl-2,4-hexadiene (equation 1) the dimer of the former diene is a minor product (20% yield). 

The most important chain-transfer reaction is transfer to the Diels-Alder dimer. Added chemical transfer agents, mainly mercaptans, allow the regulation of molecular weight and molecular weight distribution of commercial polymers. The com-... 

Arsenin, antimonin and bismin react readily with dienophiles in the Diels-Alder reaction, and the reaction with hexafluoro-2-butyne and other acetylenic dienophiles has been used to trap these heterocycles as barrelenes (115), in particular the unstable bismin (equation 21). The reactivity increases with increasing size of the heteroatom and this may be related to decreased aromatic character of the heterobenzenes with increasing size of the heteroatom. In fact, bismin and antimonin are so reactive that at low temperatures (< -10 °C and < - 50 °C respectively) both exist as Diels-Alder dimers (116 equation 20). 

However, it has been recently suggested that oxirane intermediates also play a part in this reaction, producing some of the minor products (Scheme 59)152. Dienes do not appear to be good substrates for this reaction, at least not with triplet oxygen, as the cation-radical Diels-Alder dimerization is much faster unless the alkene is sterically hindered153. 

The diterpenoid 1 from the heartwood of Callitris macleayana is the Diels-Alder dimer of the dienone 2a. The acetate 2b of the alcohol 2a is readily available by oxidation of 5-isopropyl-2-methylphenol with lead tetraacetate, but all attempts to hydrolyse 2b to 2a failed. "Dimeric indans" were obtained under acidic conditions, while use of potassium hydroxide in methanol at room temperature for 15 minutes followed by acidification with 1M hydrochloric acid, extraction and repeated chromatography over silica gave the three products 3, 4 and 5 in the relative distribution 11.7, 63.7 and 24.7%. The same products were formed in similar proportions when the reaction temperature was varied from 0°C to 64°C treatment of any of the products 3, 4 or 5 with potassium hydroxide in methanol also gave a mixture of 3,4 and 5. 

In addition to the cyclodienes discussed above, the carboxidation of 1,3-cyclohex-adiene having conjugated double bonds was also tested. In this case, the reaction is strongly complicated by the Diels-Alder side reaction. The main part of the diene is consumed by the dimerization process, and only 25-30% is involved in the oxidation, yielding cyclic ketones. 

Polystyrene. The polymerization of styrene is most commonly done under free radical conditions. Peroxides are used to initiate the reaction at low temperatures. At 100°C styrene acts as its own initiator. Below 80°C the termination mechanism primarily involves combination of radicals. Above 80°C both disproportionation and chain transfer with the Diels-Alder dimer are important. 

The metathesis polymerisation of dicyclopentadiene, an inexpensive monomer (commercially available cyclopentadiene dimer produced by a Diels-Alder addition reaction containing ca 95 % endo and ca 5 % exo form), leads to a polymer that may be transformed into a technically useful elastomer [144-146, 179] and thermosetting resin [180,181]. The polymerisation has characteristics that make it readily adaptable to the reaction injection moulding ( rim ) process [182], The main feature of this process comes from the fact that the polymerisation is carried out directly in the mould of the desired final product. The active metathesis catalyst is formed when two separate reactants, a precatalyst (tungsten-based) component and an activator (aluminium-based) component, are combined. Monomer streams containing one respective component are mixed directly just before entering the mould, and the polymerisation into a partly crosslinked material takes place directly in this mould (Figure 6.5) [147,168,183-186],... 

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