Xylylenes Diels-Alder reactions

Although 1, -elimination of o xylene polyhalides with sodium iodide, zinc, copper, or iron metal is a fundamental method for the formation of o-xylylene intermediates, it is difficult to carry out the reaction under mild conditions such as at room temperature. o Trimethylsilylmethyl)-benzyltrimethylammonium halides were devised for this purpose and were shown to generate the o-xylylene at room temperature. However, we have successfully generated o-xylylene at room temperature by the reaction of a, 0,-dibromo o-xylene with metallic nickel(51). The Diels-Alder reaction of... 

The long-lived isomeric xylylene cation radical then undergoes either coupling to the adducts in equation (72) or back electron transfer followed by Diels-Alder reaction of the resulting neutral xylylenes and chloranil. 

Since the [6]radialenes are triple-diene systems, it comes as no surprise that they have been used in multiple Diels-Alder reactions. In fact, after a first 1 1 addition with 150, leading to 161, has taken place, the reaction could proceed in two fashions—a linear course of addition leading to a para-xylylene 162, and an angular route which produces an ortho-xylylene intermediate 163 (equation 19>I<12 103. 

The two new bonds can be obtained by a Diels-Alder reaction. First, deprotonation gives an enolate that has an ortho-xylylene resonance structure. Diels-Alder reaction followed by retro-Diels-Alder reaction gives the product. 

Since the [6]radialenes are triple-diene systems, it comes as no surprise that they have been used in multiple Diels-Alder reactions. In fact, after a first 1 1 addition with 150, leading to 161, has taken place, the reaction could proceed in two fashions—a linear course of addition leading to a ara-xylylene 162, and an angular route which produces an crf/ic-xylylene intermediate 163 (equation 19)102-103 Whereas for the hexamethyl compound 150 only products formed by the linear route have been detected with a sizeable number of dienophiles (X=X inter alia TCNE, maleic anhydride, benzoquinone, 1,4-naphthoquinone, acrolein, methyl acrylate ), the parent system 4 undergoes threefold Diels-Alder addition in a star-shaped manner leading to 164 with dimethyl acetylenedicarboxylate and to 165 with fumaroyl chloride followed by methanolysis (equation 20). ... 

The equilibrium between the cyclobutene and the 1,3-butadiene is shifted in favor of the cyclobutene in benzocyclobutenes. The electrocyclic ring opening of a benzocyclobutene, an aromatic compound, leads to an o-xylylene, a nonaromatic and reactive compound. o-Xylylenes are useful intermediates in Diels-Alder reactions. 

Make C1-C5, C4-C6. Break C1-C4. The two new cr bonds have a 1,3-re-lationship in a new six-membered ring, suggesting that a Diels-Alder reaction has taken place. Disconnect the C1-C5 and C4-C6 bonds of the product to see the immediate precursor to the product, an o-xylylene. The double-bodied arrow (=>), a retro synthetic arrow, indicates that you are working backward from the product. 

The barrier to converting the s-trans conformation to the s-cis conformation contributes to the overall activation barrier for Diels-Alder reactions. Structural factors that increase the proportion of diene in its s-cis conformation increase the rate of the Diels-Alder reaction, and factors that increase the proportion of diene in its s-trans conformation decrease the rate of the reaction. Cyclopentadiene is one of the best dienes for the Diels-Alder reaction partly because it cannot rotate out of its s-cis conformation. In fact, cyclopentadiene undergoes [4 + 2] cycloaddition to itself so readily that it lasts only a few hours at 0 °C. o-Xylylenes are especially good dienes both because of their enforced s-cis geometry and because a nonaromatic starting material is transformed into an aromatic product. By contrast, dienes in which one of the double bonds is cis are poor substrates for Diels-Alder reactions because steric interactions between the in substituents in the s-cis conformation are particularly severe, and dienes whose s-trans conformation is enforced do not ever undergo the Diels-Alder reaction. 

A six-membered ring with two new cr bonds in a 1,3-relationship may indicate a [4 + 2] cycloaddition (Diels-Alder or hetero-Diels-Alder reaction). When the new ring is fused to an aromatic one, benzyne or an o-xylylene may have been a reactive intermediate. 

An interesting variation of the rDA reaction is the retro-dimetalla-Diels-Alder reaction that generates two reactive intermediates, o-xylylene (257) and the metal-metal double bonded dimer (256), via a dinu-clear elimination. Performing s equilibrium reaction in the presence of reagents that react with (256), such as trialkylphosphines, yields the dinuclear monophosphine adducts. Consumption of (256) allows the o-xylylene to dimerize as shown in equation (113). ... 

Flash vacuum pyrolysis of a p-chlorobenzoate ester (35) has also led to useful chemistry <90TL1487>. The reaction produced oxazole-4,5-xylylene (36), which could be trapped with thio-phenol or SO2 to give the corresponding adducts in 39% and 25% yield, respectively (Scheme 5). The unstable intermediate also gave a Diels-Alder adduct with methyl acrylate. The o-xylylene oxazole derivative and the furan analogue were unique among those of several heterocycles studied in that they underwent Diels-Alder reactions instead of polymerization. 

Ujaque, G. Lee. P.S. Houk, K.N. Henteniann. M.F. Danishefsky. S.J. The origin of endo stereoselectivity in the hetero-Diels-Alder reactions of aldehydes with ortho-xylylenes CH-tc, tc-tc, and steric effects on stereoselectivity. Chern. Eur, J. 2002. 8. 3423-3430. 

The prominence of the Diels-Alder reaction of quino-2,3-dimethanes (also known as ortfto-quinodimelhanes and < rt/j( -xylylenes) has been established for nearly six decades since the seminal work by Cava [1,2], Several excellent reviews are available [3-7]. The logical extension of this chemistry to the synthesis and Diels-Alder reactions of indole-2,3-quinodimethanes (2,3-dimethylene-2,3-dihy-dro-l/7-indoles) has been recognized as an efficient route to indoles, carbazoles, and related fused indoles [8],... 

The Diels-Alder reaction of various dienophiles with the highly reactive diene o-xylylene has been utilized extensively in the construction of various polycyclic ring systems. Although calculations show that the o-xylylene species may actually be best represented as a biradicaloid 1 [97], its reactivity parallels that of a highly reactive diene 2 (see Figure 7.1). The high reactivity presumably results from attaiiunent of aromaticity in the Diels-Alder cycloadduct. 

The final product 11.34 also contains two silyl groups. These are also very useful as they undergo facile electrophilic /p o-substitution. As two of the alkynes are part of a diyne, the reaction produces a bicyclic compound. Remarkably, even benzocyclobutenes 11J5 can be formed in good yield (Scheme 11.15). These are especially useful as thermolysis results in an electrocyclic ring opening to an o-xylylene, which can be trapped in situ in a (classical) Diels-Alder reaction. 

The past year has seen the mushrooming of the use of intramolecular Diels-Alder reactions in total synthesis. Especially prominent are routes proceeding via u-xylylene intermediates. The thermolysis of benzocyclobutanes is one method for generating such intermediates and Kametani and his co-workers have exploited this approach in a number of new routes to natural products (Scheme... 

Nicolaou and Oppolzer and their respective co-workers have generated o-xylylenes by thermal extrusion of sulphur dioxide from benzosulpholanes and trapped them by intramolecular Diels-Alder reactions [e.g. (124) - (125) and (126)-> (127)]. Vollhardt and his co-workers had earlier adopted a strikingly... 

Ultrasound has been exploited in the zinc mediated generation of a number of reactive intermediates. Most of these are only normally attainable in very low yield as a result of the forcing conditions necessary for their generation. On the other hand Han and Boudjouk showed that ortho-xylylene (35) was readily available from o-dibromoxylene. Trapping the intermediate tetraene with methyl vinyl ketone gave an 87 % yield of the product (36) arising from Diels-Alder reaction with the exocyclic diene [154] (Scheme 72). 

Diels-Alder reactions, 133, 135 epoxidation, 69-72, 516 grafting on polyethylene, 462 hydroformylation, 44 hydrogenation, 41, 42 isomerization catalysts, 133, 484 isomerization during polymerizations, 484 isomerization kinetics, 484 isopropyl alcohol radical reaction, 207 MA copolymerization, 532, 534, 541 Michael reactions, 63-66 nitrone adducts, 224, 225 olefin copolymerization, 288 olefin ene reactions, 162 phenanthrene adducts, 181 plasticizers use, 14 production—synthesis, 14, 78-81 radical copolymerization, 270, 275-277, 307, 315, 317, 333, 345, 365, 379 radical polymerization, 239, 264, 287 reaction with allyl alcohol, 46 reaction with sodium bisulfite, 53 styrene copolymerization, 365, 483 tetraalkyl methylenediphosphonate adduct, 66 transesterification, 46 /7-xylylene copolymerization, 359 dialkyl stannyl, PVC stabilizer, 275 diaryl, synthesis from MA, 80 pyridinium, betaine intermediate, 216... 

Intramolecular Diels-Alder Routes. In recent years the intramolecular variant of the Diels-Alder reaction has proved an extremely powerful tool for ring construction. There is usually a considerable rate acceleration over the inter-molecular variant and in addition two or more rings are generated in a single step. In the past few years this strategy has proved particularly useful in the synthesis of steroids through intramolecular Diels-Alder reaction of o-xylylene intermediates [e.g. (23) - (24)] and this work has now been reviewed by one of the pioneers of... 

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