Benzocyclobutadiene dimerization

Comparable behaviour has been observed with 343, the unsubstituted analogue of 339. When the bis(trimethylsilyl) derivative 342 is hydrolysed with dilute base, the product obtained is the benzocyclobutadiene dimer 344 presumably formed by the pathway indicated . 

Chapman and co-workers have reported the direct spectroscopic observation of benzocyclobutadiene itself by reaction of cis-1,2-di-iodobenzocyclobutene with zinc powder at 230 °C. Initial cooling of the resultant vapour deposited a mixture of dimeric material and traws-l,2-di iodobenzocyclobutene. The benzocyclobutadiene was trapped in an argon matrix at 20 K on either a caesium iodide or a sapphire plate for spectroscopic examination. The infrared spectrum showed a band at 700 cm which is probably due to one of the C—H bond deformations of the four-membered ring. The ultraviolet spectrum showed at 243, 246.5,256, 270, 281.5, and 289 nm. On warming above 75 K, the known benzocyclobutadiene dimer (150) was formed and identified by comparison with authentic material. 

Like benzocyclobutadiene, benzazetes are reactive dienophiles, but fail to react as dienes except in the special case of dimerization. Thus 2-phenylbenzazete is inert to cyclopentene, but-2-yne, iV-phenylmaleimide and iV,iV-dimethylaminopropyne. 

The high yield reduction of 1,2-dihalides to produce olefins has been employed to advantage to prepare reactive olefins. Electron transfer in electrochemistry is proportional to the diffusion coefficient, which is related in a much less sensitive way to temperature changes than is chemical reactivity. Thus it may become possible to synthesize and study electro-chemically species whose chemical reactivity is high by working at low temperatures. Electroreduction of 1,2-dibromobenzocyclobutene (144) in acetonitrile or butyroni-trile/TEAP or chemical reduction using the biphenyl radical anion resulted in the formation of benzocyclobutadiene (145)128. Efforts to observe the electrochemically generated anion radical or dianion of benzocyclobutadiene indicated that dimerization to 146 was faster than further reduction (equation 84). 

The 1,2-cycloaddition reaction can take place in an intramolecular manner (equation 63), although in this example the initial excitation involves the aromatic group . A reaction of a different type is thought to be involved in the first stage of the formation of azulene or naphthalene photodimers from diphenylacetylene (equation 64), though here it is claimed that an intermediate benzocyclobutadiene species has been detected . The intermediate isomer of diphenylacetylene is formed via the triplet state and is relatively long-lived at — 10 °C. The major dimers formed are 1,2,3-triphenylazulene and 1,2,3-triphenylnaphthalene hexaphenylbenzene and octaphenylcubane are also produced . 

It is of interest to consider at this point some of the specific molecules in Scheme 8.2 and compare their chemical properties with the calculated stabilization energies. Benzocyclobutadiene has been generated in a number of ways, including dehalogenation of dibromobenzocyclobutene. " Chemically, benzocyclobutadiene reacts as a polyene having a quinodimethane stmcture and is a reactive diene in Diels-Alder cycloadditions, dimerizing or polymerizing readily. ... 

A reinvestigation of the photochemistry of diphenylacetylene by Ota has provided evidence for the intermediate formation of the isomeric 2-phenyl-benzocyclobutadiene as an unstable green crystalline compound which is a precursor of the dimers 1,2,3-triphenylazulene and 1,2,3-triphenylnaphthalene. 

The benzene-fused analogue (189) has also been prepared in 14% yield by a Wittig reaction <72JA7087>. This could be oxidized to the sulfoxide and sulfone by H2O2 in acetic acid. Neither the sulfoxide nor the sulfone showed any tendency to dimerize by a Diels-Alder mode. This is in contrast to simple thiophene 5-oxides, and is probably because any such dimerization would result in the formation of a benzocyclobutadiene structure. Photolysis of the sulfone leads to a head-to-head dimer. The thiophene ring in (189) is very reactive. Bromination with pyridinium bromide... 

Dehalogenation (1, 1278-1279 3, 335 5, 755). Benzocyclobutadiene (2) has been observed directly by IR spectroscopy by dehalogenation of cis-1,2-diiodobenzocyclobutene (2) with zinc under argon in a special apparatus. It is formed as a matrix of the dimer. ... 

As a reactive dienophile ortbo-benzyne also participates in the ene reaction. Thus, alkenes with allylic hydrogen can undergo concerted reaction to give substituted benzenes. However, the yields are rarely good. Cycloaddition of ortho-benzyne to alkynes should in principle give benzocyclobutadienes. Such intermediates are highly unstable and not surprisingly are not isolated. Instead, the products, formed in low yield, derive from further reaction with another molecule of ortho-benzyne or by dimerization (Scheme 7.32). 

Reaction is thought to proceed via initial ring-opening of the cyclobutene ring to give a 3,5-diene-l, 7-diyne. In accord with this, octa-3,5-dienc-l,7-diyne is unstable and is rapidly converted, in high yield, into a dimer of benzocyclobutadiene [103]. 

Oxidation of benzocyclobutadiene complexes also appears to provide benzocyclobutadiene since its dimer, or a diene adduct, have been isolated [9, 109],... 

Of interest in this connection is the fact that cw,cw-3,5-octadiene-l,7-diyne itself has been observed spectroscopically in the hydrolysis of its 1,8-bistrimethylsilyl precursor. However, after only a few minutes at room temperature, a tetracyclic, rearranged, dimer of benzocyclobutadiene was isolated (Scheme 9.3). 

The desilylation of the disilyl derivative of diendiyne with tetrabutylammo-nium fluoride (TBAF) results in the formation of diendiyne. The diendiyne then undergoes electrocyclization reactions to benzocyclobutadiene, followed by its dimerization and yielding of all the four possible angular dimers, with nearly equal proportions in 66% isolated yield. However, in the presence of excess of cyclopentadiene, the Diels-Alder reaction occurs to give the endo isomer (Scheme 24.27) [1]. 

A number of cases are known where ethylene derivatives undergo n cycloaddition to form cyclobutanes. Tetracycanoethylene [TCNE (NC)2C=C(CN)2] and fluorinated ethylenes are particularly prone to behave in this way. These reactions were discovered at the laboratories of duPont de Nemours Co. and have been studied in great detail by Bartlett and his collaborators. As we have already seen, concerted cis dimerizations of this kind should be less favorable than an path via an intermediate 1,4-butadiyl biradical all these reactions in fact take place in this way. An interesting example is the reaction of TCNE with bismethylenecyclobutene (230). The normal Diels-Alder reaction to form (231) is inhibited because this would be an antiaromatic cyclobutadiene derivative and because the transition state leading to it would be isoconjugate with benzocyclobutadiene. The product is therefore the spiran (232), formed as indicated by an process. 

On exposure to tetrabutylammoniun fluoride (TBAF) for desilylation, the dienediynes 38 underwent interesting cascade cyclizations to form unusual structures. For example, an initial electrocyclic reaction of the desilylated dienediyne 39 generated the enediallene 40, which in turn underwent a second electrocyclic reaction to afford the benzocyclobutadiene 41 (Scheme 10). Dimerization of 41 in a Diels-Alder manner furnished 42 and subsequently the angular dimer 43. 

Interestingly, the presence of a phenyl substituent on the four-membered ring of the benzocyclobutadienes 50 also changed the course of dimerization. The linear dimers 52 were produced presumably via an initial formation of the syn dimers 51. 

---