Diels-Alder , adducts

The most important classes of functionalized [60]fullerene derivatives, e.g. methanofullerenes [341, pyrrolidinofullerenes [35], Diels-Alder adducts [34i] and aziridinofullerene [36], all give rise to a cancellation of the fivefold degeneration of their HOMO and tlireefold degeneration of their LUMO levels (figure Cl.2.5). This stems in a first order approximation from a perturbation of the fullerene s 7i-electron system in combination with a partial loss of the delocalization. 

The effect of ligands on the endo-exo selectivity of Lewis-acid catalysed Diels-Alder reactions has received little attention. Interestingly, Yamamoto et al." reported an aluminium catalyst that produces mainly exo Diels-Alder adduct. The endo-approach of the diene, which is normally preferred, is blocked by a bulky group in the ligand. 

Only the results of the major (>90%) endo isomer of the Diels-Alder adduct are shown. 

Due to the prolonged reaction times in organic solvents, cKmerisation of the diene occurs during the reaction, resulting in contaminated product mixtures after work-up. In contrast the reactions in water yield quantitatively the H-NMR-pure Diels-Alder adducts. 

Unfortunately, 4.44 did not react with cyclopentadiene in the way that was desired. Instead, another reaction occurred, ultimately leadir to an unexpected Diels-Alder adduct 4.47 that could be isolated... 

Fortunately, in the presence of excess copper(II)nitrate, the elimination reaction is an order of magnitude slower than the desired Diels-Alder reaction with cyclopentadiene, so that upon addition of an excess of cyclopentadiene and copper(II)nitrate, 4.51 is converted smoothly into copper complex 4.53. Removal of the copper ions by treatment with an aqueous EDTA solution afforded in 71% yield crude Diels-Alder adduct 4.54. Catalysis of the Diels-Alder reaction by nickel(II)nitrate is also... 

Endo-exo ratios of the micelle-catalysed reactions have been determined by adding 0.25 mmol of 5.1c and 0.5 mmol of 5.2 to a solution of 5 mmol of surfactant and 0.005 mmol of EDTA in 50 ml of water in carefully sealed 50 ml flasks. The solutions were stirred for 7 days at 26 C and subsequently freeze-dried. The SDS and CTAB containing reaction mixtures were stirred with 100 ml of ether. Filtration and evaporation of the ether afforded the crude product mixtures. Extraction of the Diels-Alder adducts from the freeze-dried reaction mixture containing C12E7 was performed by stirring with 50 ml of pentane. Cooling the solution to -18 C resulted in precipitation of the surfactant. Filtration and evaporation of the solvent afforded the adduct mixture. Endo-exo ratios... 

We have demonstrated that due to inhomogeneous distribution of both reaction partners in the micelles, the pseudophase model leads to erroneous estimates of the second-order rate Constantin the micellar pseudophase, so that conclusions regarding the medium of the reaction cannot be derived through this model. However, analysis of substituent effects and endo-exo ratios of the Diels-Alder adducts indicate that the reaction experiences a water-like medium. 

Analysis The nitro compound looks like a Diels-Alder adduct, so we know where to put the double bond ... 

The simplest of all Diels-Alder reactions cycloaddition of ethylene to 1 3 butadi ene does not proceed readily It has a high activation energy and a low reaction rate Substituents such as C=0 or C=N however when directly attached to the double bond of the dienophile increase its reactivity and compounds of this type give high yields of Diels-Alder adducts at modest temperatures... 

To deduce the identity of the diene and dienophile that lead to a particular Diels-Alder adduct we use curved arrows in the reverse fashion to undo the cyclohexene derivative Start with the tt component of the double bond in the SIX membered ring and move electrons in pairs... 

Cyclic dienes yield bridged bicyclic Diels-Alder adducts... 

Give the structure of the Diels-Alder adduct of 1 3 cyclohexadiene and dimethyl... 

Two constitutional isomers of molecular formula CgHi20 are formed in the following reac tion Ignoring stereochemistry suggest reasonable structures for these Diels-Alder adducts... 

A very large number of Diels-Alder reactions are recorded in the chemical literature many of which involve relatively complicated dienes dienophiles or both On the basis of your knowl edge of Diels-Alder reactions predict the constitution of the Diels-Alder adduct that you would expect to be formed from the following combinations of dienes and dienophiles... 

A method for making ben2onitri1e by dehydrogenation of the Diels-Alder adduct of butadiene and acrylonitrile also has been described (79). Ben2onitri1e also can be made on a small scale by the dehydration of ben2amide ia an iaert solvent with phosphoms oxychloride or ben2enesulfonyl chloride and an organic amine (80,81). 

Bis(bexacbIorocycIopentadieno)cycIooctane. The di-Diels-Alder adduct of hexachlorocyclopentadiene [77 7 ] and cyclooctadiene (44) is a flame retardant having unusually good thermal stabiUty for a chlotinated aUphatic. In fact, this compound is comparable ia thermal stabiUty to brominated aromatics ia some appHcations. Bis(hexachlorocyclopentadieno)cyclooctane is usedia several polymers, especially polyamides (45) and polyolefins (46) for wire and cable appHcations. Its principal drawback is the relatively high use levels required compared to some brominated flame retardants. 

Maleic acid and fumaric acid can also be, and are often, incorporated in alkyd resins in the form of the Diels-Alder adduct of rosin. The adducts are tribasic acids which provide pendent carboxyl groups in the resin molecules, which can be saponified to give ionic, and, in turn, water-soluble characteristics to the resin. However, the resultant alkyds often have poorer color retention, toughness, gloss retention, and exterior durabiUty. 

Chlorendic anhydride is the common name of the Diels-Alder adduct of maleic anhydride and hexachlorocyclopentadiene, 3,4,5,6,7,7-hexachloroendomethylene-l,2,3,6-tetrahydrophthahc anhydride (HET). The resultant resins from HET contribute to the flame retardancy of the alkyd coatings. HET gives a greater reaction rate than phthaUc anhydride, to the extent that at 204—210°C the reaction rate approximates that of phthaUc anhydride at a temperature of 238°C (8). However, the resins tend to develop darker color, particularly at high processing temperature. Tetrachlorophthahc anhydride [117-08-8] made by conventional chlorination of phthaUc anhydride, would also impart flame retardancy to its alkyds. However, it is appreciably less soluble in the usual processing solvents than is phthaUc anhydride, and is reported to be of appreciably lower chemical reactivity (8). 

Vitamin D2 reacted with maleic anhydride to give a mono Diels-Alder adduct, which hydrolyzed to yield a dicarboxyhc acid. Acetylation of the alcohols, esterification of carboxyHc acids, and hydrogenation gave a compound that, when ozonized, gave a saturated ketone, This molecule... 

In a novel approach to vitamin K, Hoffmann-La Roche has exploited the potential acidity at C-3 as a means to attach the side chain of vitamin (36). Menadione was reacted with cyclopentadiene to yield the Diels-Alder adduct. The adduct is treated with base and alkylated at C-3 with phytyl chloride. A retro Diels-Alder reaction yields vitamin K. Process improvements in this basic methodology have been claimed by Japanese workers (37). 

A process has been disclosed in which the mixture of naphthoquinones is reacted with a diene such as butadiene. Owing to the fact that the undesked product is an unsubstituted naphthoquinone, this dieneophile readily reacts to form a Diels-Alder adduct. By appropriate control of reaction parameters, Htde reaction is observed with the substituted naphthoquinone. Differential solubiUty of the adduct and vitamin allows for a facile separation (57,58). 

Table 3. Diels-Alder Adducts from Cyclopentadiene...

The reactions of pyrroles with dimethyl acetylenedicarboxylate (DMAD) have been extensively investigated. In the presence of a proton donor the Michael adducts (125) and (126) are formed. However, under aprotic conditions the reversible formation of the 1 1 Diels-Alder adduct (127) is an important reaction. In the case of the adduct from 1-methylpyrrole, reaction with a further molecule of DMAD can occur to give a dihydroindole (Scheme 48) (82H(19)1915). 

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). 

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