Diels-Alder reactions reactivity

The FMO model is also valuable in rationalizing several observations about relative reactivities in the Diels-Alder reaction. Reactivity is enhanced with electron-withdrawing groups on the olefin, as shown by the data in Table 11.5 for reaction of cyclopentadiene with cyano-substituted alkenes. Reactivity is also enhanced by any electron-donating groups on the diene, as shown by the data in Table 11.6 for the reaction of some dienes with tetr acy anoethy lene. 

The Intramolecular Diels-Alder Reaction Reactivity and Stereocontrol... 

A few years after the first articles of Breslow had appeared, Grieco elegantly demonstrated that the astonishing rate and selectivity enhancements of Diels-Alder reactions in water can be exploited sirccessfully in organic synthesis. He extensively studied the reactivity of dienes containing... 

Fortunately, azachalcone derivatives (2.4a-g, Scheme 2.4) turned out to be extremely suitable dienophiles for Lewis-add catalysed Diels-Alder reactions with cyclopentadiene (2.5). This reaction is outlined in Scheme 2.4 and a large part of this thesis will be devoted to the mechanistic details of this process. The presence of a chromophore in 2.4 allows kinetic studies as well as complexation studies by means of UV-vis spectroscopy. Furthermore, the reactivity of 2.4 is such that also the... 

Interestingly, the rate constants for Diels-Alder reaction of the ternary complexes with 3.9 are remarkably similar. Only with 2,2 -bipyridine and 1,10-phenanthroline as ligands, a significant change in reactivity is observed. It might well be that the inability of these complexes to adopt a planar geometry hampers the interaction between the copper ion and the dienophile, resulting in a decrease of the rate of the catalysed Diels-Alder reaction. 

In summary, we have demonstrated that it is possible to extend the scope of Lewis-acid catalysis of Diels-Alder reactions in water, by employing a chelating auxiliary. We envisage that analogues of 4.39 capable of undergoing a Mamrich reaction with 4.50 can be treated with reactive dienes in the presence of a Lewis-acid catalyst in water. 

Simple cyclobutanes do not readily undergo such reactions, but cyclobutenes do. Ben-zocyclobutene derivatives tend to open to give extremely reactive dienes, namely ortho-c]uin(xlimethanes (examples of syntheses see on p. 280, 281, and 297). Benzocyclobutenes and related compounds are obtained by high-temperature elimination reactions of bicyclic benzene derivatives such as 3-isochromanone (C.W. Spangler, 1973, 1976, 1977), or more conveniently in the laboratory, by Diels-Alder reactions (R.P. Thummel, 1974) or by cycliza-tions of silylated acetylenes with 1,5-hexadiynes in the presence of (cyclopentadienyl)dicarbo-nylcobalt (W.G, Aalbersberg, 1975 R.P. Thummel, 1980). 

The allenyl moiety (2,3-aikadienyl system) in the carbonylation products is a reactive system and further reactions such as intramolecular Diels-Alder and ene reactions are possible by introducing another double bond at suitable positions of the starting 2-alkynyl carbonates. For example, the propargylic carbonate 33 which has l,8(or 1.9)-diene-3-yne system undergoes tandem carbonylation and intramolecular Diels-Alder reaction to afford the polycyclic compound 34 under mild conditions (60 C, 1 atm). The use of dppp as ligand is important. One of the double bonds of the allenyl ester behaves as part of the dieneflSj. 

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... 

The balance between aromatic and aUphatic reactivity is affected by the type of substituents on the ring. Furan functions as a diene in the Diels-Alder reaction. With maleic anhydride, furan readily forms 7-oxabicyclo [2.2.1]hept-5-ene-2,3-dicarboxyhc anhydride in excellent yield [5426-09-5] (4). 

Vinylboranes are interesting dienophiles in the Diels-Alder reaction. Alkenylboronic esters show moderate reactivity and give mixtures of exo and endo adducts with cyclopentadiene and 1,3-cyclohexadiene (441). Dichloroalkenylboranes are more reactive and dialkylalkenylboranes react even at room temperature (442—444). Dialkylalkenylboranes are omniphilic dienophiles insensitive to diene substitution (444). In situ formation of vinyl-boranes by transmetaHation of bromodialkylboranes with vinyl tri alkyl tin compounds makes possible a one-pot reaction, avoiding isolation of the intermediate vinylboranes (443). Other cycloadditions of alkenyl- and alkynylboranes are known (445). 

Isoprene is highly reactive both as a diene and through its allyhc hydrogens, and its reactions are similar to those of butadiene (qv) (8). Apart from polymerisation, the most widely investigated isoprene reactions are the formation of six-membered rings by the Diels-Alder reaction ... 

Maleic anhydride has been used in many Diels-Alder reactions (29), and the kinetics of its reaction with isoprene have been taken as proof of the essentially transoid stmcture of isoprene monomer (30). The Diels-Alder reaction of isoprene with chloromaleic anhydride has been analy2ed using gas chromatography (31). Reactions with other reactive hydrocarbons have been studied, eg, the reaction with cyclopentadiene yields 2-isopropenylbicyclo[2.2.1]hept-5-ene (32). Isoprene may function both as diene and dienophile in Diels-Alder reactions to form dimers. 

The second mechanism, proposed by Mayo (116), involves the Diels-Alder reaction of two styrene molecules to form a reactive dimer (DH) followed by a molecular assisted homolysis between DH and another styrene molecule. 

Flame Retardants. Although the use of chlorinated derivatives of DCPD has been restricted in the pesticide area, some are widely used in flame and fire retardant chemicals (see Flame retardants). The starting material is the fliUy chlorinated DCPD cracked to monomeric hexachlorocyclopentadiene, which is then converted via a Diels-Alder reaction with maleic anhydride to a reactive bicycHc anhydride (9), known as chlorendic anhydride [115-27-5]. 

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). 

Diels-Alder reactions in the presence of Lewis acids represent a case in which the Lewis acid is often used in catalytic quantities. The complexed ester (ethyl acrylate in the example given below) is substantially more reactive than the uncomplexed molecule, and the reaction proceeds through the complex. The reactive complex is regenerated by exchange of the Lewis acid from the adduct. 

Cycloaddition involves the combination of two molecules in such a way that a new ring is formed. The principles of conservation of orbital symmetry also apply to concerted cycloaddition reactions and to the reverse, concerted fragmentation of one molecule into two or more smaller components (cycloreversion). The most important cycloaddition reaction from the point of view of synthesis is the Diels-Alder reaction. This reaction has been the object of extensive theoretical and mechanistic study, as well as synthetic application. The Diels-Alder reaction is the addition of an alkene to a diene to form a cyclohexene. It is called a [47t + 27c]-cycloaddition reaction because four tc electrons from the diene and the two n electrons from the alkene (which is called the dienophile) are directly involved in the bonding change. For most systems, the reactivity pattern, regioselectivity, and stereoselectivity are consistent with describing the reaction as a concerted process. In particular, the reaction is a stereospecific syn (suprafacial) addition with respect to both the alkene and the diene. This stereospecificity has been demonstrated with many substituted dienes and alkenes and also holds for the simplest possible example of the reaction, that of ethylene with butadiene ... 

It has long been known that the Diels-Alder reaction is particularly efficient and rapid when the dienophile contains one or more electron-attracting groups. These substituent effects are illustrated by the data in Table 11.3. In the case of the diene, reactivity is increased by electron-releasing substituents. Some illustrative data are given in Table 11.4. 

Table 11.3 Relative Reactivity toward Cyclopentadiene in the Diels-Alder Reaction...

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