Ionic and Radical Diels-Alder Reactions

Conjugated cations, anions and radicals can give the Diels-Alder reaction. In such a case, the two cr bonds are formed in two separate steps (stepwise 

Extensive studies by Gorman and Gassman have shown that an allyl cation can be a 27r-electron component in a normal electron-demand cationic Diels-Alder reaction and, since a carbocation is a very strong electron-withdrawing group, the allyl cation is a highly reactive dienophile [19a, 21]. 

Anionic Diels-Alder reactions have been studied less extensively with the interest having been focused mainly on the cycloaddition of enolates of a,/l-unsaturated ketones with electron-poor olefins [24] (Equations 1.8 and 1.9). These reactions are fast and stereoselective and can be regarded as a sequential double Michael condensation, but a mechanism involving a Diels-Alder cycloaddition seems to be preferred [24b,f, 25]. 

The first evidence of an anionic Diels Alder reaction was given by Rickborn [25a]. The reaction of anthrone with N-methylmaleimide in CHCI3 or THE occurs with low yield [26] (Equation 1.10), while in DMF or in the presence of catalytic amounts of amine (EtsN, Py) the reaction is completed in a few minutes [25]. 

Radical Diels-Alder reactions have been used mainly to synthesize polycyclic molecules. These reactions, like those that involve cations and anions as components, proceed quickly but generally do not give high yields. Thus, the tricyclic enone 14 is the result of an intramolecular Diels-Alder reaction of quenched vinyl radical intermediate 13 obtained by treating the iododienynone 12 with n-tributyltin hydride/2,2 -azobisisobutyronitrile (AIBN) [28] (Equation 1.11). 

The cycloaddition is ascribable to the oxyanion of hydrogen-bonded enolate (ArO —HNEt3 ) rather than to the hydrogen-bonded enol (ArOH—NEtj). An enantioselective version of the reaction was achieved by using a homochiral amine [27]. Similarly the reactions with less reactive dienophiles such as dimethyl fumarate, fumaronitrile, maleonitrile and methyl acrylate give the Diels-Alder adducts quantitatively when the cycloadditions are carried out in THF or CHCI3 in the presence of EtjN, while in MeOH Michael adducts were isolated. Experimental evidence supports the hypothesis that the base-catalyzed cycloadditions of anthrone with dienophiles are concerted Diels-Alder processes [25b]. 

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Any molecule that can at least formally be synthesized by a Diels-Alder reaction is a potential candidate for the mass spectral RDA reaction. In addition, the RDA reaction is not restricted to positive radical ions, but it may also occur from even-electron ions as well as from negative radical ions, NT. These findings remind us of the fact that ionic reactions are determined by the intrinsic properties and the internal energy of the ions, and thus only indirectly by the ionization technique used for their creation. 

By the end of the 1950s the main features of ionic and radical reactions were reasonably well understood, but pericyclic reactions were not even recognized as a separate class. Diels-Alder reactions, and a good many others, were known individually. Curly arrows were used to show where the bonds went to in these reactions, but the absence of a sense of direction to the arrows was unsettling. Doering provocatively called them no-mechanism reactions in the early 1960s. 

The reaction of 1,2-dihydrophosphinine oxides 150A and 150B with tetra-cyanoethylene (TCNE) does not follow the Diels-Alder reaction, but rather follows the reaction of two moles of dihydrophosphinine (both of the two double bond isomers) with one mole TCNE to give a 2 1 cyclic adduct, via the 1 1 adduct, due to the ionic or radical nature of TCNE. The... 

In general, die influence of diene conformation on the rate of silico Diels-Alder reaction speaks for a concerted mechanism of the [4+2] cycloaddition. This mechanism is confirmed by the small influence of solvent polarity and radical trt s on the reaction rate, which speaks against a pronotmced ionic or radical multiple step mechanism. On the other hand, trans-piperylene reacts over 1000 times faster with the standard silaethene than does tra s,tra s-hexadiene, wh eas the opposite is true for ethenes [27]. This fact contradicts dramatically the carbon-analogy of silaethenes, fiom which either trans-piperylene reacts too fast, or trans.trans-hexadiene too slow with the silaethene. 

Pericyclic reactions are a class of reactions that include some of the most powerful synthetically useful reactions such as the Diels-Alder reaction. Pericyclic reactions often proceed with simultaneous reorganization of bonding electron pairs and involve a cyclic delocalized transition state. They differ from ionic or free radical reactions as there are no ionic or free radical intermediates formed during the course of the reaction. They proceed by one-step concerted mechanisms and have certain characteristic properties (although there are some exceptions to all these rules). 

In most Diels-Alder reactions, no catalyst is needed, but Lewis acids are effective catalysts in many cases, particularly those in which Z in the dienophile is a C=0 or C=N group. A Lewis acid catalyst usually increases both the regioselec-tivity of the reaction (in the sense given above) and the extent of endo addition, °° and, in the case of enantioselective reactions, the extent of enantioselectivity. It has been shown that InCla is an effective catalyst for aqueous Diels-Alder reactions,which is suitable for ionic Diels-Alder reactions, and there are other Lewis acid catalysts that are effective in water. °° Brpnsted acids have also been used to accelerate the rate of the Diels-Alder reaction.Lanthanum triflate [La(OTf)3] has been reported as a reusable catalyst ° ° and Me3SiNTf2 has been used as a green Lewis acid catalyst. ° Cationic Diels-Alder catalysts have been developed, particularly oxazaborolidine catalysts. ° Some Diels-Alder reactions can also be catalyzed by the addition of a stable cation radical, for... 

Pericyclic reactions656,657 are the second distinct class of the three, more or less exclusive categories of organic reactions—ionic (Chapters 4 and 5), pericyclic (this Chapter) and radical (Chapter 7). Their distinctive features are that they have cyclic transition structures with all the bond-making and bond-breaking taking place in concert, without the formation of an intermediate. The Diels-Alder reaction and the Alder ene reaction are venerable examples. 

Methods for the synthesis of central hydroisoquinolines or pyrrolo-hydroisoquinoHnes of manzamine A can be classified into 11 types of reactions (1) intermolecular Diels-Alder reaction, (2) intramolecular Diels-Alder reaction, (3) photochemical reaction, (4) radical reaction, (5) ionic cyclization, (6) intramolecular Michael reaction, (7) intramolecular Maimich reaction, (8) intramolecular [3 + 2] cycloaddition, (9) intermolecular [3 + 2] cycloaddition, (10) Pauson-Khand reaction, and (11) eneyne metathesis. 

Method of synthesis maleic anhydride and diamines are reacted in the presence of catalyst such as triethylamine, these are further cured to form crosslinked polymers. Thermal curing is promoted by the presence of radical or ionic initiators. BMI can also be synthesized by Diels-Alder reaction (see ref.) Jiang, B Hao, J Wang, W Jiang, L Cai, X, Eur. Polym. J., 37, 463-70,2001. 

By the mid-1960s, ionic and radical reactions were well studied and their mechanisms largely understood. There were a number of reactions that did not fit into either of these categories. One example is the Diels-Alder reaction, identified in the 1930s. This reaction is notable for its stereocontrol, implying a highly ordered transition state and a concerted mechanism. Another example, electrocyclic ring closure, was identified by R. B. Woodward in his work on the synthesis of vitamin Bj. These reactions and others like them were called no-mechanism reactions. 

The chemistry of vulcanisation and the technological properties of the vulcanisates obtained by the different cure systems available are discussed. Sulphur vulcanisation, crosslinking reactions involving free radicals, electron deficient species, ionic addition, ene-addition and diels-alder addition and vulcanisation by dehalogenation and dehydrohalogenation are described. 429 refs. 

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