Diels-Alder mechanism

Structure and Mechanism of Formation. Thermal dimerization of unsaturated fatty acids has been explaiaed both by a Diels-Alder mechanism and by a free-radical route involving hydrogen transfer. The Diels-Alder reaction appears to apply to starting materials high ia linoleic acid content satisfactorily, but oleic acid oligomerization seems better rationalized by a free-radical reaction (8—10). 

B. l,3>2>Dioxaphospholens.—The kinetics of the addition of trialkyl phosphites to benzil have been investigated spectrophotometrically. The second-order reaction of trimethyl phosphite in dioxan has activation parameters of A// = 8.4 kcal mol and AS = — 47.5 e.u. In benzene the rate constant increases linearly with low concentrations of added organic acid and decreases linearly with low concentrations of added base. The Diels-Alder mechanism is considered unlikely on the basis of these data, and the slow step is considered to be nucleophilic addition of the phosphite to the carbon of the carbonyl group (see Scheme). 

The stability of C60 and C70 solutions in vegetable oils has been studied in air and under inert atmosphere, after thermal processing and under the action of UV radiation. In all cases it has been found that C60 and C70 are prone to form adducts with the fatty acid chains of the vegetable oils. The adducts are formed both by radical and Diels-Alder mechanisms. The pharmaceutical valency and potential of such adducts has also been discussed. 

Thus, the addition of fullerenes to the fatty chains cannot be exclusively through a Diels-Alder mechanism, but involves also a radical mechanism. 

Burger et al. first introduced 2//-l,3-thiazete as a new heterocyclic system. By utilizing the stabilizing effect of trifluormethyl groups on strained-ring systems, they succeeded in reducing the size of a substituted oxathiazine ring to the thiazete system. The reaction proceeds via a thermally induced retro Diels-Alder mechanism with subsequent electrocyclization (Eq. 39). 

Diels-Alder mechanism. It was found that those polymers which had the least weight loss in the isothermal aging study were derived from those monomers which by DSC polymerized to the greatest extent by a Diels-Alder mechanism. It was inferred therefore that if similar considerations apply to the copolymers of bismaleimides and bisbenzocyclobutenes then these materials too might be the result of predominantly Diels-Alder type polymerizations. To the extent to which these conclusions are correct, the enhanced thermal stability of the bisbenzocyclobutene/bismaleimide copolymers is likely to be due to their being Diels-Alder polymers of some sort. 

Sie67 allowed methyl linoleate to react with gaseous styrene at 28o°C and isolated an addition compound by molecular fractionation of the reaction products in the cascade fractionating still (Fig. 78). From the analytical data of the addition compound he concluded that a Diels-Alder mechanism was involved in the copolymerization reaction of styrene with the primarily conjugated linoleic ester (c/. Table XVI) ... 

A similar approach to the synthesis of tetracyclic indole alkaloid derivatives has been described [182], and the use of reactive chiral iminium ions allows the realisation of stereoselective aza Diels-Alder reactions even in aqueous solution [183,184]. Nevertheless it should be noted that reactions of electron-rich dienes with imines e. g. derived from amino acids do not necessarily proceed via a Diels-Alder mechanism. They may as well undergo a domino-Mannich-Michael sequence which also efficiently leads to useful nitrogen heterocycles [185-188]. 

Dichloro-l-phenyl-2 (I 7/)-pyrazinonc underwent 3,6-Mdging alkylation (by a Diels-Alder mechanism) to give 4,6-dichloro-2-phenyl-2,5-diazabicyclo [2.2.2] oct-5-en-3-one (52) H2 C=CH2 (25 atm), PhMe, 110°C, sealed, 16 h >86% [as somewhat unstable crude material, characterized by mass spectrometry (MS) and NMR] 374 also many analogues and derived products.374,375... 

The discussion up to now has dealt primarily with the interactions between the orbitals of the isolated molecules in ground-state geometries. In the case of the Diels-Alder mechanism, the influence of geometrical distortions was discussed briefly, and here this subject will be taken up in more detail. 

Now we have a rather unusual Vollhardt co-trimerization. We can start by making the cobalt heterocycle but we cannot then do the Diels-Alder mechanism. Instead the cobalt must combine the reagents to form a seven-membered ring from w hich it withdraws as it couples its two ligands. The hydrogens in the product are cis because they were cis in the enamine but there is no stereoselectivity in which side of the new ring is occupied by the cobalt. Fortunately, this doesn t matter as both diastereoisomers give the same product after the cobalt is removed. 

Similar to vinylcyclopropane, the parent system, bi(cyclopropyl), does not even react with the strongest dienophiles according to a bishomo-Diels-Alder mechanism. Even the perfect preorientation of the three-membered rings in the synperiplanar conformation is insufficient to permit the reaction. 

Heating 1,3,5-triazine (1) with an enamine hydrochloride bearing clcctron-acccpting substituents on Cl, i.e. 14, gives 4,5-disubstituted pyrimidines 15.28 This class of reactions is proposed to proceed by a Diels-Alder mechanism and is discussed further in Section 2.3.2.3. 

Reactions of oxazoles indicate that they are not fully aromatic substances and that the azadiene system present readily reacts with dienophiles by the Diels-Alder mechanism. The bicyclic adducts formed in these condensations undergo facile aromatization, particularly in acid media, by elimination of a molecule of water, an alcohol, hydrocyanic acid, a nitrile, or hydrogen, forming substituted pyridine bases. The course of the reaction is highly dependent on the substituents on the oxazole ring, the nature of the dienophile, and the reaction conditions. 

In addition an enzymatic prenylation of the sesquiterpene hydroquinone 29 to the corresponding sesterpene 28 might be a possible alternative to the Diels-Alder mechanism. 

In the first step, cyclooctatetraene undergoes slow valence tautomerism to form a bicyclooctatriene. Due to similar thermodynamic stability, these two isomers will exist in equilibrium. In step 2, in the presence of Cz (CN) t, the bicyclooctatriene undergoes addition to yield a three-ringed product. This type of addition, involving the 1,4 addition of an alkene to a conjugated diene, is the Diels-Alder reaction. A simple scheme for the Diels-Alder mechanism is given below ... 

The precise mechanism of this transformation was not deduced. An electron-transfer mechanism may represent the predominant pathway or it may operate in conjunction with a thermal Diels-Alder mechanism. It is important to note that cyclization of 203 to 205, when carried out on large scale, is facilitated by benzoquinone and other electron acceptors. 

In 1988, Vedejs and Fields prepared 3-thiazolines from 5-methoxy-2-methyl-oxazole 249 and a series of thioaldehydes, generated in situ from their cyclopenta-diene adducts. Thus heating 249 and thioaldehyde 250 in a sealed tube at 140°C for 48 h afforded a 95% yield of 251 as a 1 1 mixture of diastereomers (Fig. 3.74). The unactivated oxazole, 5-methyl-2-phenyloxazole was unreactive under the same conditions. A Diels-Alder mechanism was proposed for this reaction but no intermediate Diels-Alder adduct was observed. A nitrile ylide pathway was discounted, since these reactions proceed at room temperature if the thioaldehyde is generated photolytically and oxazoles do not form nitrile ylides under those conditions. Thioformaldehyde, thioacetaldehyde, thiobenzaldehyde, and thioace-tone all successfully underwent similar reactions to provide thiazohnes. 

This mechanism is distinct from a Diels-Alder mechanism, which may also occur though, if so, in low yield. A chromanone mechanism has also been proposed and leaves no unsaturation upon crosslink formation. Though this has now become the most likely mechanism, the six-membered ene reaction mechanism may still play a role in crosslink formation but, again, at a low yield. 

Dimer acids, or dibasic fatty acids, are formed either by thermal polymerization or carbon-to-carbon linking of fatty acid chains. Thermal polymerization of monomeric fatty acids like dehydrated castor oil or linoleic acids carried out at 205°C-300°C in an inert atmosphere yields dimer acid. Whereas the latter is established through a Diels-Alder mechanism, involving addition of one molecule of non-conjugated linoleic acid and one molecule of thermally conjugated linoleic acid or dehydrated castor oil [10]. A review of dimerization provides a comprehensive idea about the manufacture of dimer acids [11]. 

Figure 7 - Concerted and non-concerted Diels-Alder mechanisms...

In the ENB production, DCPD is thermally cracked to CPD, which subsequently reacts, via the Diels-Alder mechanism, with butadiene to form 5-vinylbicyclo[2.2. l]-hept-2-ene [3048-64-4] (VNB) (7) (58,59). The vinyl double bond is then isomerized in the presence of a selective super-base catalyst such as NaK amide to yield 5-ethylidenebicyclo[2.2.1]-hept-2-ene [16219-75-3] (ENB) (8) (60-62). 

Thermal polymerization kinetics that obey the Diels-Alder mechanism were studied. Expressions for rate of propagation and molecular weight were derived. The rate of propagation varies as the cube of monomer concentration. Subcritical damped oscillations can be expected under certain conditions. Reaction in a circle representation of free radical reactions was used to analyze the stability of reactions. Oscillations in concentration were discussed for systems of three and four reactions in a circle and the results generalized to n reactions in a circle. 

When energy is supplied to a mixture of a conjugated diene and an alkene, a ring-forming reaction takes place to produce a cyclohexene. The alkene in this reaction is called a dienophile, because it has demonstrated an affinity for the diene. The product of the reaction is often called the adduct, which is another word for product in a reaction between two molecules. Our discussion of the Diels-Alder mechanism begins with the arrow formalism, which points out which bonds are broken and shows where the new bonds are made in the forward reaction (Rg. 12.51). 

Figure 3.16 Diels-Alder mechanism of polymerisation of polyenoic fatty acids (R , = residues of hydrocarbon chains,...

There are two possibilities at this point. The most conservative is that MCH is a poor model for AH this allows the postulated MAH reaction of AH to be retained as the initiation process in styrene. The least conservative is that the entire Diels-Alder mechanism for styrene s initiation is wrong. ... 

There are other problems with the Diels-Alder mechanism. For example, it appears from UV evidence that AH builds up to its steady-state concentration so slowly that an induction period should be observed in the rate of polymerization. However, none has been reported, although an induction period in chain transfer by AH is easily observed (6a). 

Candidate for Initiation of Polymerization by 1,4-Diradicals As was described above, my research group has been involved for some time with efforts to establish the Diels-Alder mechanism for initiation in the thermal polymerization of styrene suggested by Mayo. However, we also have argued that some of the initiation in styrene might be due to 1,4-diradicals. (See Figure IB.)... 

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