Dimerization cyclobutadiene

Cyclobutadiene owes its observed instability much to the kinetic property. Cyclobutadiene dimerizes in the argon matrix above 35 K [69] and only exists for 2-10 ms under low pressure [70, 71], However, cyclobutadiene is stabilized by bulky substituents (Scheme 30). The ferf-butyl derivative was quantitatively prepared even at a high temperature (130 °C) [72]. Yellow crystals of the cyclobutadiene fused by two seven-membered rings did not decompose below 240 °C [73]. 

The adduct generated from the benzvalene and butadiene was brominated and dehydrobrominated to a cyclohexadiene which seems to be a suitable precursor of tetrahedrane. However, its photolysis yields (2+2)intramolecular cycloadducts and its thermolysis gives only a cyclobutadiene dimer (99)99). 

Matrix-isolated diene (96) undergoes photochemical transformation into the isomeric compounds (97) and (98) as the primary photoproducts. Maier and his coworkers have observed the photoisomerization of the cyclobutadiene dimer (99) into the pentacyclic compound (100). The u.v. irradiation of the Dewar benzene (101) a.R ords the paracyclophane (102, 13%). The prismane (103) can be produced by the irradiation of the Dewar benzene (104). The intermediacy of the benzene (105) in the transformation is likely. ... 

In a study of valence isomerizations of the diazabicyclooctadiene-diazacyclooctatriene system, it was found that the bicyclic compound 88 at 180° generated 4,5-dimethyl-3,6-diphenylpyridazine. Similarly, irradiation of the tricyclic structure afforded quantitatively 3,4,5,6-tetramethylpyridazine and the cyclobutadiene dimer. The bicyclic... 

Cyclooctatetraene (COT), a An non-aromatic hydrocarbon, is only one of the many (CH)s compounds, but it is a central character. The other isomers, many of which interconvert with COT by thermal and photochemical pathways, include bicyclo[4.2.0]octa-2,3,7-triene (BOT), semibullvalene (SB), barrelene (B), tricyclo[3.3.0.0 ]octa-3,7-diene (TOD), the cyclobutadiene dimers (CBD), tetracyclo[4.2.0.0. " 0 ]oct-7-ene (TOE), cubane (C), tetracyclo[4.2.0.0. 0 ] octene - the intramolecular Diels-Alder isomer of BOT (IDA), tetracy-... 

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. ... 

At lower pressures, benzene, acetylene and the dihydropentalenes (see above) are formed probably as a result of highly vibrationally excited COT. COT was envisioned to occur by a retro 2 + 2 cleavage to the 5 yn-cyclobutadiene dimer followed by ring opening to BOT and then on to COT. 

Cyclobutadiene dimerizes at temperatures as low as - 200°C to give the two products shown. Explain mechanistically. 

Cyclobutadiene itself is not stable at room temperature. Several derivatives with stabilizing groups have been prepared by the acid-catalyzed dimerization of alkjmes (R. Gompper, 1975). Less substituted cyclobutadienes could be obtained by photolytic reactions in solid matrix at low temperatures (G. Maier, 1973, 1974). 

Shielding and Stabilization. Inclusion compounds may be used as sources and reservoirs of unstable species. The inner phases of inclusion compounds uniquely constrain guest movements, provide a medium for reactions, and shelter molecules that self-destmct in the bulk phase or transform and react under atmospheric conditions. Clathrate hosts have been shown to stabiLhe molecules in unusual conformations that can only be obtained in the host lattice (138) and to stabiLhe free radicals (139) and other reactive species (1) similar to the use of matrix isolation techniques. Inclusion compounds do, however, have the great advantage that they can be used over a relatively wide temperature range. Cyclobutadiene, pursued for over a century has been generated photochemicaHy inside a carcerand container (see (17) Fig. 5) where it is protected from dimerization and from reactants by its surrounding shell (140). 

Extrapolation from the known reactivity of cyclobutadiene would suggest that azetes should be highly reactive towards dimerization and as dienes and dienophiles in cycloaddition reactions and the presence of a polar C=N should impart additional reactivity towards attack by nucleophiles. Isolation of formal dimers of azetes has been claimed as evidence for the intermediacy of such species, but no clear reports of their interception in inter-molecular cycloaddition reactions or by nucleophiles have yet appeared. 

This dimerization is an extremely fast reaction and limits the lifetime of cyclobutadiene, except at very low temperatures. 

When uradiated, fluonnated isomers of Dewar benzene yield pnsmane derivatives that rearrange thermally to benzene Photolysis of hexakis(mfluororaethyl)benzvalene ozonide gives tetrakis(tnfluoromethyl)cyclobutadiene and its dimer [J47]... 

Cyclo butadiene is highly reactive and shows none of the properties associated with aromaticity. In fact, it was not even prepared until 1965, when Rowland Pettit of the University of Texas was able to make it at low temperature. Even at —78 °C, however, cyclobutadiene is so reactive that it dimerizes by a Diels-Alder reaction. One molecule behaves as a diene and the other as a dienophile. 

The analogous dimerization of alkynes over Fe(C0)5 is not applicable, so clearly a different route towards alkynylated derivatives of 25 was needed. Comparison of 25 to cymantrene suggests that metallation of the hydrocarbon ligand should be the route of choice for the synthesis of novel substituted cyclobutadienes. In the literature, addition of organolithium bases (MeLi, BuLi) to the CO ligands with concomitant rearrangement had been observed [25]. But the utilization of LiTMP (lithium tetramethylpiperidide, Hafner [26]) or sec-BuLi as effectively non-nucleophilic bases led to clean deprotonation of the cyclobuta-... 

Monoiodination of a zirconacyclopentadiene with one equivalent of iodine followed by the addition of one equivalent of CuCl gives the dimer of the cyclobutadiene and the Diels—Alder product in the presence of methyl maleate. This indicates the formation of a l-iodo-l,3-dienyl copper compound and the subsequent elimination of Cul to give a cyclobutadiene equivalent. Direct reductive elimination of zirconacydopentadienes affording cyclobutadienes has not yet been observed. 

Let us return to the thermal decomposition of Fe(CO)(l,3-C4H6)2. Once the calibration constant is known, the enthalpy of the net process 9.10 can be calculated as the product of s and the area (A + B). The next step is to correct this value to 298.15 K by using heat capacity data. This exercise is, however, complicated by the cyclobutadiene polymerization. Brown et al. analyzed the reaction products by mass spectrometry and found several oligomers, in particular the dimer (C4H6)2 and the trimer (C4H6)3 [163]. With such a mixture, it is difficult to ascribe the observed enthalpy change to a well-defined chemical reaction. This is discussed in the paper by Brown and colleagues, who were nevertheless able to recommend a value for the standard enthalpy of formation of the iron-olefin... 

The (tetraphenylcyclobutadiene)PdX2 dimer reacts with a variety of metal complexes via transfer of the cyclobutadiene ligand to another metal. These reactions and other ligand transfer reactions have been reviewed by Efraty15. 

Reduction of (312) has been found to afford the dimer (313) which upon heating rearranged to yield the unprecedented di(benzopentalene) complex (314). The regio- and stereo-specificity of the conversion (313) into (314) implies a metal-mediated pathway for the process (see Scheme 100). The first observable cis-bis(alkyne)cyclobutadiene rearrangement [see (315) to (316)] has been reported. 

It turns out that cyclobutadiene is not a perfect square (two bonds are longer than the others), but it is essentially planar. Not surprisingly, it is very unstable and dimerizes extremely readily. It only exists at very low temperatures either in a matrix with an inert solvent (where the molecules are kept apart), or at room temperature as an inclusion compound in a suitable host molecule. Azacyclobutadiene (azete) is also extremely unstable, for similar reasons. 

---