Benzocyclobutenes, pyrolysis

It was also found that the ring expansion could be accomplished photo-chemically, from either phenyldiazomethane or triplet phenylcarbene.7 Both the thermal and photochemical ring expansions were found to be reversible,5c, thus providing rare examples of carbene-to-carbene interconversions. One remarkable example of this reversibility is the interconversion of the isomeric tolylcarbenes upon pyrolysis — the ultimate products of which include styrene and benzocyclobutene (Scheme 3).6,8,9... 

Dienes can also be generated under the conditions of Diels-Alder reaction from precursors. An example of exceeding reactive diene is quinodimethane which has been synthesized from the pyrolysis of benzocyclobutene. 

An investigation established a relationship between the intensity of the [M - C02] ion in the spectra of substituted isochroman-3-ones (29) and the yield of the corresponding benzocyclobutene (29a), obtained from flash vacuum pyrolysis of (29 equation 5) (77JOC2989). 

In some instances the relatively easy homolytic cleavage of the C-SOz bond could be used for an efficient synthesis of polycyclic hydrocarbons, particularly cyclophanes and benzocyclobutene via pyrolysis of sulfones at temperatures of 300-900°C (see [203] for references on these reactions). An example [399] is ... 

In relation to the six-membered transition state proposed for the pyrolysis of a-chloro-o-xylene which gives benzocyclobutene and HC1, the gas-phase elimination of a-phenylethyl chloride was carried out in the presence of in order to see whether... 

Pyrolysis of isochromanones gives benzocyclobutene in good yield The latter have been converted to various heterocyclic compounds and steroids... 

Hansen, H.-J. and Schmid, H., Aromatic sigmatropic rearrangements, Chem. Brit., 5, 111, 1969. Trahanovsky, W.S. and Mullen, P.W., Formation of 2-indanone and benzocyclobutene from the pyrolysis of phenyl propargyl ether, J. Am. Chem. Soc., 94, 5911, 1972. 

Benzocyclobutene. The reaction of a,a -dibromo-o-xylene (1) in dry DMF (20°, 16 hr.) with Na2Te gives l,3-dihydrobenzo[c]teUurophene (2) in moderate yield. Pyrolysis of (2) at 500° under reduced pressure in a stream of He over quartz wool in a silica tube of a flow system gives benzocyclobutene (3) in 74% yield by loss of tellurium. ... 

Finally, the high-temperature pyrolysis of benzocyclobutene gives styrene. While this could be the result of cleavage of the stronger cyclobutene bond followed by a hydrogen shift. Chapman and Trahanovsky provided evidence based on carbon and deuterium labeling that reversion to a tolyl carbene and its subsequent rearrangement could be responsible for a substantial amount of product. ... 

The pyrolysis of dispiro[2.0.2.4]deca-1,5-diene gives tetralin, benzocyclobutene, and styrene in a 2 2 1 ratio, respectively, along with traces of 4-ethynylspiro[2.5]-hept-l-ene (Scheme 11.66). ... 

Pyrolysis of isochromanones in the gas phase at 500 °C, with resultant loss of carbon dioxide, has afforded a new synthesis of 4,5-dimethoxy-, 4,5-methylenedioxy-, and 4-methoxy-benzocyclobutene. Photosolvolysis of 2-(3,5-dimethoxyphenyl)ethyl methanesulphonate in 50% aqueous methanol gave inter alia small amounts of 3,5-dimethoxybenzocyclobutene. 

Other examples of the use of intramolecular Diels-Alder reactions in preparative chemistry have appeared the o-quinodimethane (186), produced as a transient intermediate in the pyrolysis of the analogous benzocyclobutene, cyclizes to give (187 65%), the desulphurization of which (Raney nickel) affords a potential intermediate for the preparation of tetracyclic diterpenes. Elevated temperatures result in the isomerization of (188) into the internal [4 -I- 2] adduct (189) in which it is clear that a cyano-group has acted as a dienophile (189) is the result of a subsequent double-bond shift, and the structure was validated by crystallographic structure analysis. Bridged y-lactams have been successfully prepared by the intramolecular Diels-Alder reactions of NN-diallyl- and iV-(l,l-dimethylprop-2-yn-l-yl)amides of various penta-2,4-diene carboxylic acids. Thus, pyrolysis of (190) gave an 80%... 

Cava has noted that pyrolysis of the sulphones (187) gives the benzocyclobutenes (188). The reaction presumably proceeds by extrusion of sulphur dioxide and recyclization of the intermediate o-quinonemethides. Extrusion of tellurium from the readily available heterocycles (189) and (190) at 500 C gives the benzo- and naphtho-cyclobutenes (191) and (192). The monoazabiphenylenes (193) and (194) have been prepared by pyrolysis of the triazaphenanthrenes (195) and (196). ... 

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). 

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