Cyclobutadiene

Cyclobutadiene is an organic compound with the formula Template:Chem2. It is very reactive owing to its tendency to dimerize. Although the parent compound has not been isolated, some substituted derivatives are robust and a single molecule of cyclobutadiene is quite stable. Since the compound degrades by a bimolecular process, the species can be observed by matrix isolation techniques at temperatures below 35 K. It is thought to adopt a rectangular structure.<ref name="Kollmer"/><ref name="Cram2003"/>

Structure and reactivity

The compound is the prototypical antiaromatic hydrocarbon with 4 pi electrons (or π electrons). It is the smallest [n]-annulene ([4]-annulene). Its rectangular structure is the result of a pseudo<ref>Albright, Burdett and Whangbo, Orbital Interactions in Chemistry 2nd ed. pp 282ff. </ref>- (or second order) Jahn–Teller effect, which distorts the molecule and lowers its symmetry, converting the triplet to a singlet ground state.<ref name="Senn"/> The electronic states of cyclobutadiene have been explored with a variety of computational methods.<ref>Balkova, A.; Bartlett, R. J. J. Chem. Phys. 1994, 101, 8972–8987.</ref> The rectangular structure is consistent with the existence of two different 1,2-dideutero-1,3-cyclobutadiene valence isomers. This distortion indicates that the pi electrons are localized, in agreement with Hückel's rule which predicts that a π-system of 4 electrons is not aromatic.

In principle, another situation is possible. Namely, cyclobutadiene could assume an undistorted square geometry, if it adopts a triplet spin state. In this case the molecule will be aromatic in agreement with Baird's rule. While a theoretical possibility, the triplet form of the parent cyclobutadiene and its substituted derivatives remained elusive for decades. However, in 2017, the square triplet excited state of 1,2,3,4-tetrakis(trimethylsilyl)-1,3-cyclobutadiene was observed spectroscopically, and a singlet-triplet gap of E_ST = 13.9 kcal/mol (or 0.6 eV per molecule) was measured for this compound.<ref>Template:Cite journal</ref>

Synthesis

Several cyclobutadiene derivatives have been isolated with steric bulky substituents. Orange tetrakis (tert-butyl)cyclobutadiene arises by thermolysis of its isomer tetra-tert-butyltetrahedrane. Although the cyclobutadiene derivative is stable (with respect to dimerization), it decomposes upon contact with Template:Chem2.<ref>Template:Cite journal</ref><ref name="Irngartinger"/>

Trapping

Samples of cyclobutadiene are unstable since the compound dimerizes at temperatures above 35 K by a Diels-Alder reaction.<ref>Template:Cite book</ref> By suppressing bimolecular decomposition pathways, cyclobutadiene is well-behaved. Thus it has been generated in a hemicarceplex.<ref name="Cram2003"/> The inclusion compound is generated by photodecarboxylation of bicyclopyran-2-one.<ref>Template:Cite journal</ref> When released from the host–guest complex, cyclobutadiene dimerizes and then converts to cyclooctatetraene.

After numerous attempts, cyclobutadiene was first generated by oxidative degradation of cyclobutadieneiron tricarbonyl with ammonium cerium(IV) nitrate.<ref name="emerson"/><ref name="henery"/> When liberated from the iron complex, cyclobutadiene reacts with electron-deficient alkynes to form a Dewar benzene:<ref name="pettit"/>

Cyclobutadiene to Dewarbenzene conversion

The Dewar benzene converts to dimethyl phthalate on heating at 90 °C.

One cyclobutadiene derivative is also accessible through a [2+2]cycloaddition of a di-alkyne. In this particular reaction, the trapping reagent is 2,3,4,5-tetraphenylcyclopenta-2,4-dienone and one of the final products (after expulsion of carbon monoxide) is a cyclooctatetraene:<ref name="lee"/>

Acetylene-Acetylene [2 + 2] Cycloadditions Chung-Chieh Lee 2006

See also

• Butadiene
• Cyclobutene

Further reading

• Template:Cite journal