Cyclobutadiene is not square

Masamune, Satoru; Souto-Bachiller, Fernando A.; Machiguchi, Takahisa; Bertie, John E.

doi:10.1021/ja00483a043

Cited by 76 publications

(34 citation statements)

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“…[24][25][26] }, which can be connected in a reasonable manner, we may argue that the crystal structure is not really associated with a local disorder ( Figure 3). We observe two significant tendencies when Me 2 2 transforms into Me 2 3&Me 2 CBD S /CO 2 toward Me 2 CBD R : 1) the C2ÀC3 and C6ÀO1 bonds expand and 2) the C3-C4-C5-C6 ring with a trapezoidal form in Me 2 2 tends toward a square geometry in Me 2 CBD S (see the Supporting Information and Figure S1 for details).…”

Section: Resultsmentioning

confidence: 90%

“…[32][33][34] These considerations inspired us to design unprecedented experiments in which the G 4 C system was used as a host protective matrix for the synthesis of Me 2 CBD in aqueous solution. Previous or very recent results showed that cyclobutadiene, [25] cyclobutadieneiron-tricarbonyl, [35] or tetrasilacyclobutadiene [36] can be stabilized within a highly protective hydrophobic environment of bulky hydrocarboneous substituents, allowing crystallographic characterization. Herein, of very special interest is the stability of the parent Me 2 CBD and its unperturbed precursors Me 2 2 and Me 2 3 under protection of the G 4 C matrix.…”

Section: Resultsmentioning

confidence: 99%

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Unprecedented Synthesis of 1,3‐Dimethylcyclobutadiene in the Solid State and Aqueous Solution

Legrand

Gilles

Petit

et al. 2011

Chemistry A European J

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Cyclobutadiene (CBD), the smallest cyclic hydrocarbon bearing conjugated double bonds, has long intrigued chemists because of its chemical characteristics. The question of whether the molecule could be prepared at all has been answered, but the parent compound and its unperturbed derivatives have eluded crystallographic characterization or synthesis "in water". Different approaches have been used to generate and to trap cyclobutadiene in a variety of confined environments: a) an Ar matrix at cryogenic temperatures, b) a hemicarcerand cage enabling the characterization by NMR spectroscopy in solution, and c) a crystalline guanidinium-sulfonate-calixarene G(4)C matrix that is stable enough to allow photoreactions in the solid state. In the latter case, the 4,6-dimethyl-α-pyrone precursor, Me(2)1, has been immobilized in a guanidinium-sulfonate-calixarene G(4)C crystalline network through a combination of non-covalent interactions. UV irradiation of the crystals transforms the entrapped Me(2)1 into a 4,6-dimethyl-Dewar-β-lactone intermediate, Me(2)2, and rectangular-bent 1,3-dimethylcyclobutadiene, Me(2)CBD(R), which are sufficiently stable under the confined conditions at 175 K to allow a conventional structure determination by X-ray diffraction. Further irradiation drives the reaction towards Me(2)3&Me(2)CBD(S)/CO(2) (63.7 %) and Me(2)CBD(R) (37.3 %) superposed crystalline architectures and the amplification of Me(2)CBD(R). The crystallographic models are supported by additional FTIR and Raman experiments in the solid state and by (1)H NMR spectroscopy and ESI mass spectrometry experiments in aqueous solution. Amazingly, the 4,6-dimethyl-Dewar-β-lactone, Me(2)2, the cyclobutadiene-carboxyl zwitterion, Me(2)3, and 1,3-dimethylcyclobutadiene, Me(2)CBD, were obtained by ultraviolet irradiation of an aqueous solution of G(4)C{Me(2)1}. 1,3-Dimethylcyclobutadiene is stable in water at room temperature for several weeks and even up to 50 °C as demonstrated by (1)H NMR spectroscopy.

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Section: Resultsmentioning

confidence: 90%

Section: Resultsmentioning

confidence: 99%

Unprecedented Synthesis of 1,3‐Dimethylcyclobutadiene in the Solid State and Aqueous Solution

Legrand

Gilles

Petit

et al. 2011

Chemistry A European J

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“…For a long time, many theoretical and experimental chemists have scrutinized CBD in comparison with benzene to reveal the nature of aromaticity/ antiaromaticity and localization/delocalization [2,3]. Although the structure of CBD has not been established experimentally, many theoretical and experimental results suggest that it has the rectangular D 2h structure rather than the D 4h square structure in the singlet ground state (S 0 ) [4][5][6][7][8][9][10][11]. The distortion from D 4h to D 2h is due to the second-order Jahn-Teller effect [12].…”

Section: Introductionmentioning

confidence: 99%

Tetra-radical and ionic S1/S0 conical intersections of cyclobutadiene

Sumita

Saito

2010

Chemical Physics

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“…An interesting example of the latter is the highly unstable cyclobutadiene molecule (I), which has four n electrons. By analogy to benzene, one might anticipate a square, resonance-stabilized structure, 2; however, it is well established both experimentally (6)(7)(8) and computationally (9-1 1) that cyclobutadiene is a rectangular diene, 1. As March has pointed out, ".…”

Section: Introductionmentioning

confidence: 99%

Anomalous stabilizing and destabilizing effects in some cyclic π-electron systems

Politzer¹,

Grice²,

Murray³

et al. 1993

Can. J. Chem.

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Ab initio computational studies have been carried out for three molecules that are commonly classed as antiaromatic: cyclobutadiene (1), 1,3-diazacyclobutadiene (7), and 1,4-dihydropyrazine (6). Their dinitro and diamino derivatives were also investigated. Stabilizing or destabilizing energetic effects were quantified by means of the isodesmic reaction procedure at the MP2/6-31G*//HF/3-21G level, and calculated molecular electrostatic potentials (HF/STO-5G//HF/3-21G) were used as a probe of electron delocalization. Our results do not show extensive delocalization in the π systems of any one of the three parent molecules. The destabilization found for 1 and 7 is attributed primarily to strain and to repulsion between the localized π electrons in the C=C and C=N bonds, respectively. However, 6 is significantly stabilized, presumably due to limited delocalization of the nitrogen lone pairs. NH2 groups are highly stabilizing, apparently because of lone pair delocalization. NO2 is neither uniformly stabilizing nor destabilizing.

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Cyclobutadiene is not square

Cited by 76 publications

References 0 publications

Unprecedented Synthesis of 1,3‐Dimethylcyclobutadiene in the Solid State and Aqueous Solution

Unprecedented Synthesis of 1,3‐Dimethylcyclobutadiene in the Solid State and Aqueous Solution

Tetra-radical and ionic S1/S0 conical intersections of cyclobutadiene

Anomalous stabilizing and destabilizing effects in some cyclic π-electron systems

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