Benzo cyclobutadiene

It is of interest to consider at this point some of the specific molecules in Scheme 9.2 and compare their chemical properties with the calculated stabilization energies. Benzo-cyclobutadiene has been generated in a number of ways, including dehalogenation of... 

Push-pull stabilized benzocyclobutadienes are not known, but several benzo-cyclobutadienes are isolable as stable transition metal complexes9, notably 29, synthesized by Pettit and his co-workers41. The diene character of the uncomplexed... 

Fig. 3a. ORTEP diagram of the crystal structure of 181-1 b Projection of 181-1 normal to the benzo-cyclobutadiene-diide-rings c Projection of 181-11 normal to the benzocyclobutadiene-diide-rings...

Naphthobenzocyclobutadienes are already known, but the first stable benzo-cyclobutadiene (817) has only now been prepared by thermal rearrangement of the bis-alkyne (816), and the anthra[6] cyclobutadiene (818) has been synthesized to two routes, one of which involves cyclization of a bis-alkyne. 

Liberation of cyclobutadiene from its tetrahapto-iron tricarbonyl complex is believed to proceed via a dihapto iron complex, and the first such complex to be prepared by an independent method is [824 Fp — T -CpFe(CO)2]. On oxidation with iodide ion in the presence of diphenylisobenzofuran the adduct (825) of benzo-cyclobutadiene is obtained. A dinuclear complex with cyclobutadiene acting as a bridging ligand between two iron atoms has also been prepared. ... 

The main disadvantage of the Pauling-Wheland valence bond approach is the fast growth of the number of canonical structures to be considered as the sizes of molecules increase. For example, benzo-cyclobutadiene, which has just two more jr-electrons than benzene, has 14 canonical structures, compared to only 5 canonical structures for benzene, while... 

Intramolecular cycloadditions between cyclobutadiene and an oxygen-tethered unactivated alkene (alkyne) offers an attractive route to benzo[c]furans (Scheme 26, <96JA9196>). 

The simplest 4 system is cydobutadiene (in which n = 1), which is an extremely unstable compound. It can be stabilized by metal complexation. One method of stabilizing cydobutadiene is by means of weU-chosen substituents, for example, diethyl 2,4-bis (diethylamino)cyclobutadiene-l,3-dicarboxylate (20) is reasonably stable, melting at 52 °C, and the ring is square planar. Benzo-annelation has also been used as a method of stabihzation. Biphenylene (21) has long been known, and l,2-diphenylnaphtho[f)]cydobutadiene (22) is now known (Figure 7.10). 

Electronic spectra of samples of tetramethylcyclobutadiene prepared from different precursors differ among themselves, again indicating intermolecular interactions within the matrix [34]. Thus if pyridazine or phthalic derivatives are also present the products are coloured whereas when o-di(hydroxymethyl)benzene or 3,4-benzo-dihydrofuran are the coproducts the mixtures are colourless [34]. Cyclobutadiene and its alkyl derivatives either absorb at about 300-305 nm or have no bands above 300 nm, consistent with rectangular singlet structures [34-37]. 

Of some interest in this connection is the fact that luminescence was observed in the conversion of the benzo derivative of the cyclobutadiene dimers to benzo-COT when 9,10-dibromoanthracene was present. Finally, interesting polycyclic materials with the cyclobutadiene dimer skeleton have been examined. ... 

Toda and co-workers have published a full paper on the preparation of the benzo-l,2 4,5-dicyclobutadiene (141), and have described some more of its reactions. Addition of diphenylcyclopropenone in refluxing xylene gave the novel quadricyclane derivative (142), which rearranged on further heating. Reaction of (141) with tetra-cyanoethylene resulted in an olefin metathesis reaction with one of the cyclobutadiene double bonds to give (143), but with dimethyl maleate, ° addition took place... 

CBD = cyclobutadiene COT = cyclooctatetraene DMCHD = 2,3-dimethylenecyclohexane-l,4-diyl DMCPD = 4,5-dimethylenecyclopentane-1,3-diyl MBQDM = /n-benzo-quinodimethane TM = trimethylene TMB = tetramethylene-benzene TME = tetramethyleneethane TMM = trimethylene-methane TST = transition state theory. 

An X-ray structure determination for the stable benzocyclobutadiene (49) indicates, as expected, that the canonical form shown best describes the structure. Sophisticated calculations have been carried out for a wide range of benzo-fused cyclobutadiene derivatives. The agreement between calculated and experimental transition energies and moments for the few known compounds is impressive enough to suggest that predictions for the (as yet) unknown compounds are reliable. Because of the discrepancy between the calculated spectra and the data reported for the tentatively claimed 1,2-diphenylphenan-thro[l]cyclobutene (50), the alternative structure (51) has been proposed. ... 

Some n.m.r. spectra for benzo- and dibenzo-cyclobutadiene dications indicate that these species are fully delocalised six- and ten-electron aromatic systems respectively. Flash-pyrolytic extrusion of nitrogen from benzocinnoline analogues has been employed in the generation of the fused cyclobutadiene derivatives (52), (53), and (54). 1,2-Dimethylbenzocyclobutadiene gives the... 

The reactions may proceed via a cationic intermediate (87). The reactions provide another source of cyclobutadienes. Heterogeneous Group VIII transition metals deformylate benzilic acid and its derivatives to the corresponding ketones. Solutions of Na2PtCl4 in acetic acid have similarly been shown to deformylate benzilic acid, possibly via a r-complex (88), to benzo-phenone. Benzhydrol does not deformylate under these conditions, but does undergo hydrogen-deuterium exchange. ... 

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