Cyclobutadiene has long fascinated organic chemists. It is the 4e analogue of the 6e benzene molecule, yet it could hardly be more different. Despite nearly a century of effort, cyclobutadiene analogues were only first prepared in the 1970s, reflecting its strong antiaromatic character.

Per-trimethylsilylcyclobutadiene 1 offers opportunities to probe the properties of the cyclobutadiene ring as the bulky substituents diminish dimerization and polymerization of the reactive π-bonds. Kostenko and coworkers have now reported on the triplet state of 1.¹ They observe three EPR signals of 1 at temperatures above 350 K, and these signals increase in area with increasing temperature. This is strong evidence for the existence of triplet 1 in equilibrium with the lower energy singlet. Using the variable temperature EPR spectra, the singlet triplet gap is 13.9 ± 0.8 kcal mol^-1.

The structures of singlet and triplet 1 were optimized at B3LYP-D3/6-311+G(d,p) and shown in Figure 1. The singlet is the expected rectangle, with distinctly different C-C distance around the ring. The triplet is a square, with equivalent C-C distances. Since both the singlet and triplet states are likely to have multireference character, the energies of both states were obtained at RI-MRDDCI2-CASSCF(4,4)/def2-SVP//B3LYPD3/6-311+G(d,p) and give a singlet-triplet gap of 11.8 kcal mol^-1, in quite reasonable agreement with experiment.

singlet

triplet

Figure 1. Optimized geometries of singlet and triplet 1.

References

1. Kostenko, A.; Tumanskii, B.; Kobayashi, Y.; Nakamoto, M.; Sekiguchi, A.; Apeloig, Y., "Spectroscopic Observation of the Triplet Diradical State of a Cyclobutadiene." Angew. Chem. Int. Ed. 2017, 56, 10183-10187, DOI: 10.1002/anie.201705228.

InChIs

1: InChI=1S/C16H36Si4/c1-17(2,3)13-14(18(4,5)6)16(20(10,11)12)15(13)19(7,8)9/h1-12H3
InChIkey=AYOHYRSQVCLGKR-UHFFFAOYSA-N

Cyclobutadiene is the prototypical antiaromatic compound. McMahon has examined the
effect of ethynyl substitution on this ring, with a long term eye towards the possibility of these types of species being involved in the synthesis of fullerenes.¹

All of the possible ethynyl-substituted cyclobutadiene species (1-7) were optimized at B3LYP/6-31G(d) and CCSD/cc-pVDZ in their singlet and triplet states.

The structures of singlet and triplet 7 are shown in Figure 1. The geometries provided by the two different methods are quite similar. They show a rectangular form for the singlets and a delocalized, nearly square ring for the triplets.

7singlet

7triplet

Figure 1. CCSD/cc-pVDZ optimized structures of singlet and triplet 7.

The computed singlet-triplet gap decreases with each ethynyl substituent. B3LYP, which overestimates the stability of triplets, predicts that 6 and 7 will be ground state triplets, while CCSD predicts a singlet ground state for all 7 species, with the gap decreasing steadily from 11.5 to 8.2 kcal mol^-1, a value that is also probably underestimated.

This change in the singlet-triplet gap reflects a stronger stabilizing effect of each ethynyl group to the cycnobutadiene ring for the triplet than for the singlet state. This is seen in the homodesmotic stabilization energies.

Lastly, NICS(1)_zz values are positive for all of the singlets and negative for the triplets. The positive values for the singlets reflect their antiaromatic character, also seen in the alternant bond distances around the ring. The NICS values of the singlets decrease with increasing substitution. The negative NICS values of the triplets reflects aromatic character, as seen in the non-alternant distances around the ring. Interestingly, the triplet NICS values decrease with increasing ethynyl substitution, suggesting decreased aromaticity, even though the homodesmotic reactions suggest increasing stabilization with substitution.

References

(1) Esselman, B. J.; McMahon, R. J., "Effects of Ethynyl Substitution on Cyclobutadiene," J. Phys. Chem. A 2012, 116, 483-490, DOI: 10.1021/jp206478q

InChIs

1: InChI=1/C4H4/c1-2-4-3-1/h1-4H
InChIKey=HWEQKSVYKBUIIK-UHFFFAOYAI

2: InChI=1/C6H4/c1-2-6-4-3-5-6/h1,3-5H
InChIKey=XFHXCHFBSJDBGT-UHFFFAOYAG

3: InChI=1/C8H4/c1-3-7-5-6-8(7)4-2/h1-2,5-6H
InChIKey=YSSBCMLQKUIAEP-UHFFFAOYAI

4: InChI=1/C8H4/c1-3-7-5-8(4-2)6-7/h1-2,5-6H
InChIKey=IRAQOGPKMAXTQY-UHFFFAOYAN

5: InChI=1/C8H4/c1-3-7-5-6-8(7)4-2/h1-2,5-6H
InChIKey=YSSBCMLQKUIAEP-UHFFFAOYAI

6: InChI=1/C10H4/c1-4-8-7-9(5-2)10(8)6-3/h1-3,7H
InChIKey=BVCIPPXDFXWTJR-UHFFFAOYAQ

7: InChI=1/C12H4/c1-5-9-10(6-2)12(8-4)11(9)7-3/h1-4H
InChIKey=HEUYILYXFVQGOW-UHFFFAOYAO

Henry Rzepa’s response¹ to the reported detection and x-ray structure of 1,3-dimethylcyclobutadiene² has now been published. He takes a different tack than those take by Alabugin³ and Scheschkewitz⁴ in refuting the analysis of this work (see this earlier post). Rzepa discuses computations to evaluate the possible lifetime of 1,3-dimethylcyclobutadiene in the vicinity of CO₂. In particular, he examines the barrier for the allowed [4+2] cycloaddition to give back the lactone 1 (Reaction 1), which was photolyzed in the experiment to produce the cyclobutadiene and CO₂ species in the first place.

The gas phase free energy barrier at 175 K (the experimental condition) computed at ωB97XD/6-311G(d,p) is 16.8 kcal mol^-1, which is sufficiently high to limit this back reaction. Embedding this into a water continuum lowers the barrier to 12.9 kcal mol^-1.

But the experiment has these species embedded inside a calixarene host along with guanidinum
cations. The cation could associate with the CO₂ (indicated in Reaction 1 as X), and inclusion of a guanidinium in the gas phase, reduces the barrier to 3.3 kcal mol^-1. Rerunning this computation now with a water continuum produce an intermediate zwitterion formed by making the C-C bond, and the second step makes the C-O bond.

Finally, modeling the reaction with guanidium inside a calixarene host leads to a barrier of 8 kcal
mol^-1, 10.5 kcal mol^-1 with water continuum. Rzepa concludes that recombination of 1,3-dimethylcyclobutadiene and CO₂ to give 1 should be too fast on the timescale of the experiment for observation of the cyclobutadiene. This argument, along with the two previous papers, strongly casts doubt on the original claim.

I should point out that Henry has deposited all the structures in a nice enhanced table. You may need a subscription to get to this – I have not checked the access conditions.

References

(1) Rzepa, H. S., "Can 1,3-dimethylcyclobutadiene and carbon dioxide co-exist inside
a supramolecular cavity?," Chem. Commun. 2011, ASAP, DOI: 10.1039/C0CC04023A

(2) Legrand, Y.-M.; van der Lee, A.; Barboiu, M., "Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix," Science 2010, 329, 299-302, DOI: 10.1126/science.1188002.

(3) Alabugin, I. V.; Gold, B.; Shatruk, M.; Kovnir, K., "Comment on "Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix"," Science, 2010, 330, 1047, DOI: 10.1126/science.1196188.

(4) Scheschkewitz, D., "Comment on "Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix"," Science, 2010, 330, 1047, DOI: 10.1126/science.1195752.

InChIs

1: InChI=1/C7H8O2/c1-4-3-7(2)5(4)6(8)9-7/h3,5H,1-2H3
InChIKey=GLYAMHMFKKLRAL-UHFFFAOYAT

1,3-dimethylcyclobutadiene: InChI=1/C6H8/c1-5-3-6(2)4-5/h3-4H,1-2H3
InChIKey=ADQGKIKNUMJFSL-UHFFFAOYAU

Earlier this year, Barboiu made the astonishing claim of the x-ray characterization of 1,3-dimethylcyclobutadiene, brought about by the photolysis of 4,6-dimethyl-α-pyrone encapsulated in a guanidinium-sulfonate-calixarene crystal (Reaction 1).¹ I had not blogged on this paper because Henry Rzepa did a quite thorough analysis of it in this blog post. Now, a couple of rebuttals have appeared and it is time to examine this study.

Alabugin calls in question whether the reaction has in fact proceeded beyond 2.² They note that in the x-ray crystal structure, the distance between a carbon of the purported cyclobutadiene ring and the carbon of CO₂ is only 1.50 and 1.61 Å. Barboiu called this a “strong van der Waals contact”, but this is a distance much more attributable to a covalent bond. In fact, the shorter distance is in fact shorter than some of the other C-C distances in the structure that Barboiu calls covalent! Perhaps more bizarre is that the putative CO₂ fragment is highly bent: 119.9&;deg;, a value inconsistent with CO₂ but perfectly ordinary for an sp² carbon.  In fact, B3LYP/6-31G** computations suggest that bending CO₂ this much costs about 75 kcal mol^-1 – and tack on another 7 kcal mol^-1 to make the two C-O distances unequal (as found in the x-ray structure!). Thus, Alabugin suggests that only 2 has been formed, and notes that the cleavage to 3 would likely require light of much higher energy that that used in the Barboiu experiment.

Scheschkewitz argues that the x-ray data can be better interpreted as suggesting only the Dewar β-lactone 2 is present, though in its two enantiomeric forms.³ There is no evidence, he suggests of any cyclobutadiene component at all.

It should be noted that Barboiu stands⁴ by his original work and original assignment, claiming that these types of x-ray experiments are quite difficult and large error bars in atom positions are inherent to the study.

Henry Rzepa has blogged again on this controversy and has a paper coming out on this soon. I shall update when it appears. Henry notes in one of the comments to his blog that a TD-DFT computations does show that the Dewar β-lactone 2 is transparent from 320-500nm.

References

(1) Legrand, Y.-M.; van der Lee, A.; Barboiu, M., "Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix," Science 2010, 329, 299-302, DOI: 10.1126/science.1188002.

(2) Alabugin, I. V.; Gold, B.; Shatruk, M.; Kovnir, K., "Comment on "Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix"," Science, 330, 1047, DOI: 10.1126/science.1196188.

(3) Scheschkewitz, D., "Comment on "Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix"," Science 2010, 330, 1047, DOI: 10.1126/science.1195752.

(4) Legrand, Y.-M.; van der Lee, A.; Barboiu, M., "Response to Comments on "Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix"," Science, 330, 1047, DOI: 10.1126/science.1195846.

InChIs

1: InChI=1/C7H8O2/c1-5-3-6(2)9-7(8)4-5/h3-4H,1-2H3
InChIKey=IXYLIUKQQQXXON-UHFFFAOYA

2: InChI=1/C7H8O2/c1-4-3-7(2)5(4)6(8)9-7/h3,5H,1-2H3
InChIKey=GLYAMHMFKKLRAL-UHFFFAOYAT

3: InChI=1/C6H8/c1-5-3-6(2)4-5/h3-4H,1-2H3
InChIKey=ADQGKIKNUMJFSL-UHFFFAOYAU