Benzocyclobutene alkene

Table 18. Properties of AB-benzocyclobutene/alkene monomers and homopolymers...

In a recent re-examination of the thermolysis of benzocyclobutenes for the in situ generation of o-quinodimethanes, the resultant IMDA diastereoselectivity was highly dependent on the nature of the hydroxyl protective group.90 The intramolecular 4 + 2-cycloaddition of o-quinodimethanes (83), derived from ene-bis(sulfinylallenes) (82), with electron-deficient and electron-rich alkenes produced the corresponding polycyclic aromatic compounds (84) (Scheme 22).91 The enantioselective Diels-Alder... 

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. 

Oppolzer, in a series of key ptqiers in 1974, showed that the amide substituents in the benzocyclo-butenes (39) can rotate outward to give the dienamides (40) as intermediates. Intramolecular trying by the remote alkene groups afforded the tricyclic derivatives (41) in very good to excellent yields. This woik clearly established the value of benzocyclobutenes in the preparation of complex polycyclic target... 

A major initial limitation of the benzocyclobutene approach to o-quinodimethanes was the lack of efficient, large-scale syntheses for many specifically substituted derivatives. Fortunately, recent developments have lemov much of this impediment. Q>nceptually, the synthesis of benzocyclobutenes from aromatic precursors can be envisaged in only a limited number of ways. These include [2 -i- 2] cycloadditions involving benzynes and alkenes, intramolecular cyclization on to a benzyne, cyclizations involving arene anions, and electrocyclic closure of o-quinodimethanes. Benzocyclobutene derivatives can also be prepared by aromatization of bicyclo[4.2.0]octanes. Detailed discussion of variations to these approaches can be found in the cited reviews. The cobalt catalyzed co-oligomerization of 1,5-hexadiynes with al-kynes, especially bis(trimethylsilyl)acetylene, has also been employed for the preparation of specifically substituted benzocyclobutenes. In the latter case the cyclobutenes are often not isolated but converted directly to o-quinodimethanes and subsequent products. ... 

The diazotization of anthranilic acid, a classic route to benzyne, when carried out in the presence of vinyl acetate, vinyl ethers or 1,1-dichloroethylene gives the expected benzocyclobutenes in about 40% yield. Despite the rather moderate yields this method represents a convenient route to multigram quantities of the parent compounds, benzocyclobutenol and benzocyclobutenone. The latter is easily converted to benzocyclobutene-1,2-dione. The diazotization sequence applied to 2-amino-3,6-dimeth-oxybenzoic acid and 1,1-dichloroethylene results in a 80% yield of 3, imethoxycyclobuten-l-one. Trapping of benzynes with other ethylene derivatives, and especially more substituted alkenes, has given generally poorer, variable results. ... 

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... 

In general, [2 + 2] cycloadditions to simple alkenes (to form benzocyclobutenes) proceed in only moderate to low yields, and by a stepwise rather than a concerted mechanism As expected, because of the electrophilic nature of benzyne, the reactions proceed best with alkenes bearing electron-donating substituents. Despite these limitations, the reaction sometimes offers a simple, direct, one-step route to useful synthetic intermediates. 

Arynes react readily with simple alkenes to give either benzocyclobutenes or substituted benzenes (Scheme 7.31). The formation of benzocyclobutenes by [2+2] cycloaddition reaction of the aryne to the alkene proceeds best for strained and electron-rich carbon-carbon (C=C) double bonds. For example, dicyclopentadiene reacts to give the ex o-isomer of the corresponding four-membered ring in good yield. The addition to cyanoethene (acrylonitrile) and the reaction with the electron-rich ethoxyethene (ethyl vinyl ether) gives the cyano- and ethoxy-benzocyclobutenes in 20% and 40% yields, respectively. The latter reaction almost certainly involves nucleophilic addition of the enol ether to the electrophilic aryne followed by coUapse... 

Alkenes also could be intermolecular reaction partners of the diynes 29, and thus benzocyclobutenes were formed [46]. Two important observations were made in this context ... 

Scheme 4.18 With alkenes as intermolecular partners benzocyclobutene substructures are formed.

Ramberg-Backlund reactions, discussed in more detail in Chapters 1 and 2, continue to be of great significance in the synthesis of strained alkenes. Bicyclo-[2,2,l]heptenes, [4,2,2]propellanes, 1,4-tetramethylene- and 1,4-penta-methylene-Dewar-benzene, and the ethylene ketal of hydrindane-3,6-dien-l-one have been prepared, utilizing this rearrangement as the final step. 2,5-Dialkyl-sulpholans have been converted, in synthetically useful yields (20— 67%), into 1,2-dialkyl-cyclobutenes upon reaction with Bu Li in THF followed by reduction with LiAlH4. This most interesting reaction has been applied to the preparation of a wide variety of fused benzocyclobutenes. ... 

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