Benzocyclobutene Diels-Alder reaction with

The cobalt(I)-mediated [2 + 2 + 2] cycloaddition of 1,5-diynes with mono-alkynes provides access to benzocyclobutene derivatives (Scheme 24). Thermal rearrangement of benzocyclobutenes into o-quinodimethane and subsequent Diels-Alder reaction with an alkene moiety allow the formation of a tricyclic compound. 

Several stereochemical facets show up on Diels-Alder reactions of 7,8-disubstituted o-quinodimeth-anes. A seminal study of addition reactions of 7,S-diphenylbenzocyclobutenes provided strong evidence for a conrotatory opening of the four-membered ring, followed by a supra-supra-facial endo-selective Diels-Alder reaction with TV-phenylmaleimide, as exemplified by the transformation (607) - (609) (Scheme 137). Based on this result, it is now generally agreed that the trans relation of substituents at C-7 and C-8 in benzocyclobutenes translates, via the transient ( , )-o-quinodimethanes, e.g. (608), into the cis disposition in the corresponding cycloadducts. 

It is well known that benzocyclobutenes undergo thermal ring-opening to o-quinodimethane systems which readily undergo the Diels-Alder reaction with dienophilcs to yield tctrahydronaphthalene derivatives, and it has again... 

The Diels-Alder reaction of 3,6-dimethoxybenzocyclobutene 30 with some dienophiles was studied (Scheme 9). A mixture of benzocyclobutene 30, A -phenyl-maleimide 54, and benzene was heated at 210 in a sealed tube. The adduct 55 was obtained in 53% yield. The reaction of 30 and cyclohexenone 56 similarly gave the adduct 57 in 36% yield. In the case of dienone 58, however, the desired product could not be obtained. On the other hand, the Diels-Alder reaction with tranj-enone 59 afforded adduct 60 in a good yield (76% yield). Therefore, we felt confident that the reaction of benzocyclobutene 30 with enone (+)-31 would give the desired adduct with a tetracyclic skeleton. 

Cocyclotrimerization, if applied to 1,5-hexadiynes 7, leads to benzocyclobutenes 8. After smooth thermal ring opening, these give highly reactive ort/io-quinodimethanes 9, which can be trapped by Diels-Alder reaction with an alkene or hetero-carbon multiple bond. If these dienophiles are in a side chain of the primary diync, the conversion leads in a one-pot version to carbo- and heterocyclic polycyclic systems with defined trans stereochemistry at the newly... 

Introdncing the sulphonic acid functionality can be performed via different pathways. Pioneering work has been done by Nakajima and Kondo . Via a Diels-Alder reaction with benzocyclobutene, they introduced a benzene moiety and afterwards they performed a snlphonation. They achieved an acidity of 1.44 mmol HVg. Other reports " describe the preparation of a solid catalyst by the co-condensation or grafting with 3-mercaptorpropyltriethoxysilane which is subsequently oxidized by H2O2, H2SO4 or HNO3. 

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 Diels-Alder reaction of o-quinodimethanes (from benzocyclobutenes) with nitrogen-ubstituted enes has also been applied to alkaloids synthesis (see p. 280f. T. Kametani, 1972, 1973, 1974 W. Oppolzer, 1978 A). 

For the synthesis of estradiol methyl ether 4-319, the cydobutene derivative 4-317 was heated to give the orthoquinonedimethane 4-318 which cydized in an intramolecular Diels-Alder reaction [109]. The thermally permitted, conrotatory elec-trocyclic ring-opening of benzocyclobutenes [110] with subsequent intramolecular cycloaddition also allowed the formation of numerous complex frameworks (Scheme 4.70). 

The cure mechanism of the BCB/BMI blend is believed to differ from that of the BCB oli omer. Following the thermally induced ring opening of the strained four member ring of the benzocyclobutene it can then react with the dienophile introduced to the system by the BMI resin via a Diels-Alder reaction An illustration of that reaction is shown below. 

A study of benzocyclobutene polymerization kinetics and thermodynamics by differential scanning calorimetry (DSC) methods has also been reported in the literature [1]. This study examined a series of benzocyclobutene monomers containing one or two benzocyclobutene groups per molecule, both with and without reactive unsaturation. The study provided a measurement of the thermodynamics of the reaction between two benzocyclobutene groups and compared it with the thermodynamics of the reaction of a benzocyclobutene with a reactive double bond (Diels-Alder reaction). Differential scanning calorimetry was chosen for this work since it allowed for the study of the reaction mixture throughout its entire polymerization and not just prior to or after its gel point. The monomers used in this study are shown in Table 3. The polymerization exotherms were analyzed by the method of Borchardt and Daniels to obtain the reaction order n, the Arrhenius activation energy Ea and the pre-exponential factor log Z. Tables 4 and 5 show the results of these measurements and related calculations. 

From this work, the authors concluded that the reaction of two benzocyclobutene groups with each other is exothermic to the extent of 221.7 12.5 kJ. By comparison, the reaction of one mole of benzocyclobutene moieties with one mole of double bonds in a Diels-Alder fashion is exothermic by 184.1 12.5 kJ. Thus, the thermal dimerization of benzocyclobutenes is thermodynamically favored over the cycloaddition reaction of a benzocyclobutene with a double bond by about 37-38 kJ. By contrast, the authors found that for monomers that could react either by a benzocyclobutene dimerization or by a benzocyclobutene - double bond cycioaddition (e.g. monomer 13 in Table 3) the reaction products appeared to be dominated by the cycioaddition pathway. From this data, it was concluded that the Diels-Alder reaction was kinetically... 

The reaction of the bisbenzocyclobutene 41 with the bismaleimide 44 is similar to the reaction of 41 with the activated diacetylene 43. From the DSC thermogram in Fig. 24 it is apparent that there is one major exotherm peak at 258.8 °C associated with this reaction. This could be interpreted as either being the polymerization exotherm peak for a benzocyclobutene-maleimide Diels-Alder reaction or else it is the coincidental overlap of the exotherm peaks associated with each pure component. This latter case is a distinct possibility since both the pure bisbenzocyclobutene 41 and the pure bismaleimide 44 have... 

Replacing the hydrogen in 68 with a phenyl group leads to the secondary acetylenic monomer 70. It was believed that this disubstituted acetylene would suppress the reaction of acetylene with itself and insure that there was an acetylene functionality available for reaction with the o-quinodimethane at 200 °G The DSC of 68 showed the presence of a single exothermic peak at 263 °C which the authors felt was adequate evidence for the occurrence of a Diels-Alder reaction between the acetylene and benzocyclobutene. Unfortunately they did not report on any control experiments such as that between diphenylacetylene and simple benzocyclobutene hydrocarbon or a monofunctional benzocyclobutene in order to isolate the low molecular weight cycloaddition product for subsequent characterization. The resulting homopolymer of 68 had a Tg of 274 °C and also had the best thermooxidative stability of all of the acetylenic benzocyclobutenes studied (84% weight retention after 200 h at 343 °C in air). 

The maleimide group can undergo a variety of chemical reactions. The reactivity of the double bond is a consequence of the electron withdrawing nature of the two adjacent carbonyl groups which create a very electron-deficient double bond, and therefore is susceptible to homo- and copolymerizations. Such polymerizations may be induced by free radicals or anions. Nucleophiles such as primary and secondary amines, phenates, thiophenates, carboxylates, etc. may react via the classical Michael addition mechanism. The maleimide group furthermore is a very reactive dienophile and can therefore be employed in a variety of Diels Alder reactions. Bisdienes such as divinylbenzene, bis(vinylbenzyl) compounds, bis(propenylphenoxy) compounds and bis(benzocyclobutenes) are very attractive Diels Alder comonomers and therefore some are used as constituents for BMI resin formulations. An important chemical reaction of the maleimide group is the ENE reaction with allylphenyl compounds. The most attractive comonomer of this family is DABA particularly when tough bismaleimide resins are desired. 

Figure 3.19 Schematic illustration of the synthetic pathway to benzene sulfonic acid PMOs the first step involves a Diels-Alder reaction of the ethene bridges with benzocyclobutene in the second step the benzene moieties were sulfonated.

The next three reactions (Scheme 3.38), cotrimerization of enediyne 146 with bis(trimethylsilyl)acetylene 150, thermolysis of the benzocyclobutene derivative 151, and the intramolecular Diels-Alder reaction of the intermediate 152, were... 

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