SOME STABLE CYCLOBUTADIENES

There are some simple cyclobutadienes that are stable at room temperature for varying periods of time. These either have bulky substituents or carry certain other stabilizing substituents such as seen in tri-tert-butylcyelobutadiene (57). Sueh... 

There are some simple cyclobutadienes that are stable at room temperature for varying periods of time. These either have bulky substituents or carry certain other stabilizing substituents such as seen in tri-tert-butylcyclobutadiene (83). ° Such compounds are relatively stable because dimerization is sterically hindered. Examination of the NMR spectrum of 83 showed that the ring proton (8 = 5.38) was shifted upfield, compared with the position expected for a nonaromatic proton, for example, cyclopentadiene. As we will see (pp. 89-90), this indicates that the compound is antiaromatic. 

Examination of the cyclobutadiene system indicates that it possesses four 7c-electrons and is thus an unstable 4 system. Cyclobutadiene itself only exists at very low temperatures, though some of its derivatives are stable to some extent at room temperature. Cyclobutadiene is a rectangular diene. Loss of two electrons through the departure of two chloride ions from the 3,4-dichlorocyclobutene derivative creates a 2jc-electron aromatic system, the square, stable cyclobutenyl dication (Scheme 1.2). 

Stability of methylenecyclopropene (stable below — 75°C) stands in contrast to cyclobutadiene, an isomer, which appears to dimerize in a diffusion controlled process The triapentafulvenes or calicenes " constitute another type of methylenecyclopropene which gives some indication of dipolar character and thus a degree of aromaticity. These compounds link the positively charged cyclopropenyl cation to a cyclopentadienyl... 

Structural studies of cyclobutadiene and some of its derivatives reveal a pattern of alternating single and double bonds and a rectangular, rather than a square, shape. Bond distances in a stable, highly substituted derivative of cyclobutadiene illustrate this pattern of alternating short and long ring bonds. 

Division of the Hess-Schaad resonance energy by the number of tt electrons gives the resonance energy per tt electron (REPE) of the compound. A compound with a substantially positive REPE value (greater than, say, 0.01 jS ) is predicted to be aromatic. A compound with a near-zero REPE is nonaromatic. A compound with a substantially negative REPE is predicted to be antiaromatic, being less stable than if its double bonds were isolated from one another. Some antiaromatic hydrocarbons are cyclobutadiene and fulvalene. 

The thermal reaction of cobalt polymers 4.28 with isocyanates at 120 °C leads to 2-pyridone-containing polymers such as 4.30 [70]. Well-characterized, yellow polyesters 4.31 containing skeletal (cyclobutadiene)cobalt moieties in the main chain have been prepared by interfacial polycondensation approaches [73]. The use of solubilizing alkoxy substituents R afforded materials with Mn = 5,400-16,300 (PDI = 1.3-1.8). Analogous materials to 4.31 with a 1,3-disposition of the main-chain substituents on the cyclobutadiene ligands have also been studied [73, 74]. Thermotropic liquid crystallinity was detected by polarizing microscopy, with, in some cases, mesophases stable over the temperature range from about 110 to >250°C. 

The first attempt at the synthesis of cyclobutadiene (I), also by Willstatter 94) in 1905 was not successful. Since then numerous workers have attempted to prepare cyclobutadiene and its derivatives. Although no stable simple derivatives have been obtained, evidence has accumulated which appears to point to the intermediacy of cyclobutadienes in some reactions (7,8, 92a). 

It has already been pointed out that the bond-bisection requirement is imprecise.Moreover, even if this restriction is accepted provisionally, the problem reappears as soon as we get to the higher homologs of bicyclobutane benzvalene, in which the bicyclobutane moiety is fused onto ethylene, and naphth-valene, in which it is fused to a benzene ring. Both of these molecules are much less stable thermodynamically than their respective aromatic valence isomers, benzene and naphthalene, but - like cyclobutadiene - are remarkably resistant to thermal isomerization. Naphthvalene will be discussed at some length in a subsequent chapter, in connection with its unusual photochemical properties. We will here restrict our attention to the reluctance of benzvalene to undergo thermal isomerization to benzene. 

In some case, the photolysis products of dienes and trienes depends on the excited state in which photochemical cyclisation occurs (singlet or triplet). To populate triplet state, suitable triplet sensitizer are used if required. The. 3-Methylene-l, 5-hexadiene gives a cyclobutadiene derivative through singlet excited state and tricyclic ring compound from triplet excited state. This difference of photochemical process is because of life-time of the excited states. Higher the life-time of reacting intermediate more will be possibility to form more stable radicals. 

Bimolecular (15-17) and diradical (9-10) mechanisms have been invoked to explain results of some acetylene thermal polymerization. Though a thermal [2+2] reaction (15,16) is not favoured accortog to Woodward-Hoffmann rules, a stepwise pathway involving diradical species can be considered (29), Such a reaction for triple bonds would give cyclobutadiene derivatives, which cannot be isolated, except where these rearrange or react to give stable compounds (30), Thereby, we propose a bimolecular mechanism in which cyclobutadiene derivatives should be considered as transient species and would allow us to explain some rearrangements (scheme 3). 

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