Cyclobutadiene is an organic compound with the formula . 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.
Structure and reactivity
The compound is the prototypical
antiaromaticity with 4
pi electron. It is the smallest
n-
annulene (4-annulene). Its rectangular structure is the result of a pseudo
[Albright, Burdett and Whangbo, Orbital Interactions in Chemistry 2nd ed. pp 282ff. ]- (or second order) Jahn–Teller effect, which distorts the molecule and lowers its symmetry, converting the
triplet state to a
singlet state ground state.
The electronic states of cyclobutadiene have been explored with a variety of computational methods.
[Balkova, A.; Bartlett, R. J. J. Chem. Phys. 1994, 101, 8972–8987.] The rectangular structure is consistent with the existence of two different 1,2-dideutero-1,3-cyclobutadiene
. 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
aromaticity.
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 aromaticity 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 EST = 13.9 kcal/mol (or 0.6 eV per molecule) was measured for this compound.
Synthesis
Several cyclobutadiene derivatives have been isolated with
steric effect. Orange tetrakis (
Butyl group)cyclobutadiene arises by thermolysis of its isomer tetra-
tert-butyl
tetrahedrane. Although the cyclobutadiene derivative is stable (with respect to dimerization), it decomposes upon contact with .
Trapping
Samples of cyclobutadiene are unstable since the compound dimerizes at temperatures above 35 K by a Diels-Alder reaction.
By suppressing bimolecular decomposition pathways, cyclobutadiene is well-behaved. Thus it has been generated in a
carceplex.
The inclusion compound is generated by photodecarboxylation of bicyclopyran-2-one.
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.
The Dewar benzene converts to dimethyl phthalate on heating at 90 °C.
One cyclobutadiene derivative is also accessible through a 2+2cycloaddition 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:
- [File:CyclobutadienSynthDessyWhite.png|400px|Acetylene-Acetylene Cycloadditions Chung-Chieh Lee 2006]]
See also
Further reading
