Electrocyclic reactions benzocyclobutene

A significant acceleration of the electrocyclic ring opening of benzocyclobutene derivatives has been disclosed under the influence of a -silicon atom.55 This effect has been associated with the adjacent anion-driven electrocyclic reactions such as oxy-Cope rearrangement. 

This approach to cyclohexadienes can be considered an extension of that described for the precursor to type I cyclohexadiene in which the vinyl group is part of a cyclic stmcture. An example is tte low-temperature photochemical generation of the bis-exomethylene compound (196) from the benzocyclobutene (195), which was followed by an electrocyclic reaction to give compound (197). Thermolysis of (198) also leads to an assumed bis-exo-methylene intermediate (199), wldch electrocyclizes with involvement of die pyridinium ring to afford die alkaloid ellipticene (200) after dehydrogenation. ... 

An important electrocyclic reaction is the ring-opening of benzocyclobutenes to give o-quinodimethanes. The resulting diene is an excellent substrate for reaction with a dienophile in a Diels-Alder reaction (see Section 3.1.2). For example, in a synthesis of the steroid estrone, the benzocyclobutane 337, prepared by a cobalt-mediated cyclotrimerizafion, was converted on heating to the o-quinodimethane... 

Only a few facts about the chemical reactivity of the parent [6]radialene (5) are known - certainly because it is not easyto handle - but they reveal its character as a triple 1,3-diene system (catalytic hydrogenation, triple 1,4-addition of Br2, [4+2] cycloaddition reactions [5, 6]). Much more is known about the alkyl-substituted radialenes 113 and 72, in particular due to the detailed investigations of Hopf and coworkers [88]. Because of the presence of three hexa-2,4-diene subunits in the latter radialenes, it is not surprising that isomerization pathways via sigmat-ropic and electrocyclic reactions exist. Thus, in a gas-phase thermolysis of 113, products 125-129 were formed in relative yields that depended on the reaction temperature (e.g., 127 was the major product at 260 C and 129 at 360 °C). The mechanistic scenario includes the isomerization 113 125 by three consecutive 1,5-H shifts, and the sequence 127 128 129 [88]. The permethylated [6]radi-alene (72) is thermally much more stable than 113 the product mixture obtained from its pyrolysis at 350 "C was dominated by benzocyclobutene 130 (an analog of 127), which, however, could be isolated in only 17% yield [88] (Scheme 4.27). 

Malacria and coworkers [274] used an intermolecular trimerization of alkynes to gain efficient access to the skeleton of the phyllocladane family. Thus, the Co-cata-lyzed reaction of the polyunsaturated precursor 6/4-4 gave 6/4-5 in 42% yield. Here, six new carbon-carbon bonds and four stereogenic centers are formed. The first step is formation of the cyclopentane derivative 6/4-6 by a Co-catalyzed Conia-ene-type reaction [275] which, on addition o f his( Iri me ill y I si ly 1) e thy ne (btmse), led to the benzocyclobutenes 6/4-7 (Scheme 6/4.2). The reaction is terminated by the addition of dppe and heating to reflux in decane to give the desired products 6/4-5 by an electrocyclic ring opening, followed by [4+2] cycloaddition. 

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

Benzocyclobutene (BCB) pciymerizBs at much lower temperatures of about 250 °C than the closely related biphenylene. The reaction undergoes electrocyclic ring ojKiung to form o-xylylene which polymerizes to produce cyclic dimer andpoly-o-xylylene [92]. 

While the electrocyclic ring opening to o-quinodimethanes is the major reaction pathway in the irradiation of substituted benzocyclobutenes (cf. Example 6.14), the irradiation of unsubstituted benzocyclobutene yields 1,2-dihydropentalene (119) and 1,5-dihydropentalene (120) as major products. The mechanism shown with prebenzvalene (118) as primary photochemical intermediate has been proposed to explain the formation of the isomeric dihydropentalenes (Turro et al.. 1988). Supporting calculations that yield the same mechanism for the benzene-to-fulvene transformation have been published (Dreyerand Klessinger, 1995). 

In the presence of very reactive dienes, such as o-quinodimethanes, even electron-rich dienophiles, such as oxime ethers, can be reacted, as exemplified in Ae intramolecular cycloaddition of (143 Scheme 65), which is created in situ by an initial (2 + 2 -t- 2] cycloaddition reaction of (140) with (141) and subsequent electrocyclic ring opening of the resulting benzocyclobutene (142). This strategy has been applied recently to a novel isoquinoline synthesis. 

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. 

A mechanophore (blue in Fig. 2a) is a strategically designed chemical entity which responds to mechanical force in a predictable and useful manner (Fig. 2d-f). The polymer strand here acts as an actuator to transmit macroscopic force to the target. For a fully extended polymer chain, the maximum tension force is at the middle point of the chain contour. So the mechanophore should be incorporated into the middle of the chain with its active bond along the chain contotu (Fig. 2a) [15, 29, 32]. Examples of mechanochemical reactions include homolytic scission of weak bonds (diazo [33]), electrocyclic ring-opening (benzocyclobutenes [29], spiropyrans [32, 34 5], gem-dichlorocyclopropanes [46-49], ge/n-difluorocyclo-propanes [30, 50], and epoxide [51]), cycloreversion reactions (cyclobutane derivatives [52-56], Diels-Alder adducts [57, 58], 1,3-dipolar adducts [59, 60], and 1,2-dioxetanes [61]), dative bond scission [62-64], and flex-activated reactions [34, 65, 66], as recently reviewed by Bielawski [67]. 

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. 

There is a large literature that describes the electrocyclic ring opening of cyclobutenes and benzocyclobutenes as one step in a cascade of reactions that lead to fused aromatic rings. An example of this class of reactions is Suzuki and... 

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