Diels-Alder dimer

In the absence of air or peroxides, only cycHc dimers are formed in the thermal dimerization of isoprene (33). Six cycHc dimers are formed in good yields four substituted cyclohexenes (3—6) and two dimethylcyclooctadienes (7—8). The latter two are, of course, not Diels-Alder dimers. There is some evidence that the isoprene dimerization mechanism differs from the usual Diels-Alder route. 

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

Tile behavior of /3-moiiooxo derivatives of 4-chlomaiioiies (27) toward morpholine was rather complex (98JOC9840). Tlius, the proposed thio-ketoiie 5-sulhde intermediates 28 would dimerize into either 1,2,4,5-tetrathianes 29 in a two-step manner or to 1,3,4,5,6-oxatetrathiocins 30 by a [5 + 3] cycloaddition. Meanwhile, the formation of oxadithiins 31 and 1,2,4-trithiolanes 32 is suggestive of the disproportionation of 28 into the thioke-tones 33 and the thioketone 5 -disulhdes 34. Tlie oxadithiins 31 correspond to a Diels-Alder dimer of 33, and the 1,2,4-trithiolanes 32 correspond to cycloadducts of 33 and 34. 

The use of cyclopentadiene as a diene provides a route to the [2.2.1]bicycloheptane skeleton, which is of considerable theoretical interest. Cyclopentadiene, however, exists as its Diels-Alder dimer at room temperature and must be cracked thermally to... 

Despite the body of evidence in favor of the Mayo mechanism, the formation of diphenylcyclobutanes (90, 91) must still be accounted for. It is possible that they arise via the 1,4-diradical 94 and it is also conceivable that this diradical is an intermediate in the formation of the Diels-Alder adduct 95 (Scheme 3.64) and could provide a second (minor) source of initiation. Direct initiation by diradicals is suggested in the thermal polymerization of 2,3,4,5,6-pentafluorostyrene where transfer of a fluorine atom from Diels-Alder dimer to monomer seems highly unlikely (high C-F bond strength) and for derivatives which cannot form a Diels-Alder adduct. 

It has been proposed that transfer to monomer may not involve the monomer directly but rather the intermediate (110) formed by Diels-Alder dimerization (Scheme 6.28). 70 Since 110 is formed during the course of polymerization, its involvement could be confirmed by analysis of the polymerization kinetics. 

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... 

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. 

A third approach to the borole system bearing alkyl substituents (99) has recently been developed93 but because these substituents are smaller, the Diels-Alder dimer (100) is the actual product (Eq. 32). 

Diels-Alder condensation, in VDC polymer degradation, 25 712 Diels-Alder dimer, 23 381-382, 383 Diels-Alder maleic addition product,... 

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. 

The self-condensation of 3,4-dichlorothiophene 1,1-dioxide (89) in refiuxing xylene yields trace amounts of 2,3,7,8-tetrachlorodibenzo-thiophene 5,5-dioxide (90). The major product (50%) is the benzothio-phene (91) formed by loss of SO2 and HCl from the Diels-Alder dimer of 89. Further treatment of 91 with 89 in refiuxing trichlorobenzene again yields 90 (23%). Compound 90 is reported to possess insecticidal properties. ... 

The product is reported to melt at 102°. This material has n.m.r. peaks (CDCI3 solution) at 2.14 and 6.19p.p.m. with relative intensities of 3 1. The infrared spectrum (CHCI3 solution) shows the strongest absorption at 1670 cm accompanied, among others, by four more bands at 1390, 1280, 1005, and 835 cm. The product has ultraviolet maxima (CHCI3 solution) at 260 m (c 2600), 400 m/i (e 1120), and 572 rn/i (e 288). It is reported that the material undergoes slow Diels-Alder dimerization. ... 

The initiation mechanism for styrene has been established [Graham et al., 1979 Kaufman, 1979 Kothe and Fischer, 2001 Mayo, 1968 Olaj et al., 1976, 1977a,b]. It involves the formation of a Diels-Alder dimer (XII) of styrene followed by transfer of a hydrogen atom from the dimer to a styrene molecule (Eq. 3-63). Whether formation of the... 

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. 

Hexachlorocyclopentadiene is fluorinated at the saturated carbon atom upon heating to 280 =C with mercury(II) fluoride. The products are 10% l,2,3,4,5-pentachloro-5-fluorocyclopenta-l,3-dicne (3) and its Diels-Alder dimer 4 in 20 % yield. The dimer 4 is monomerized upon sublimation in vacuo through a 350 CC heated Pyrex tube. The overall yield of 1,2,3,4.5-pentachloro-5-fluorocyclopenta-l,3-diene (3) so obtained is close to 30%.57... 

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-... 

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 ... 

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). 

The Diels-Alder dimerization of 2,6,6-trimethyl-2,4-cyclo-hexadienone to l,4,6,6,9,9-hexamethyl-A3U-tricyclo-[6.2.2.0 2 7]-dodecane-5,10-dione2 facilitates its separation from the major alkylation product, 2,6-dimethylanisole. 

Kinetic stability toward Diels-Alder dimerization. 1996... 

The synthesis of 3-methylmetalloles is more difficult because they are unstable as monomers. Thermolysis of the tertiary carbamate corresponding to the alcohol 48 (under the experimental conditions of equations 22 or 23) gives the Diels-Alder dimer of... 

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. 

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