[4]Annulene system. Direct proof for its rectangular geometry

Delbaere, L. T. J.; James, Michael N.G.; Nakamura, Nobuo; Masamune, Satoru

doi:10.1021/ja00840a072

Cited by 69 publications

(19 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: 91%

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: 91%

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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“…Ever since KekulØ first attempted the preparation of CBD in 1872, [1] this illusive molecule posed ac hallenge to experimental and theoretical chemists. [3] Thef undamental questions that surrounded CBD dealing with its aromaticity, enthalpy of formation, reactivity in cycloaddition reactions, isomerization and ground state multiplicity led to intensive research efforts, [2,3] but some of the basic theoretically predicted properties of CBD remain without experimental support even today.O ne basic property which remains to be explored, is the spectroscopic observation of CBDslow-lying triplet diradical excited state. [3] Thef undamental questions that surrounded CBD dealing with its aromaticity, enthalpy of formation, reactivity in cycloaddition reactions, isomerization and ground state multiplicity led to intensive research efforts, [2,3] but some of the basic theoretically predicted properties of CBD remain without experimental support even today.O ne basic property which remains to be explored, is the spectroscopic observation of CBDslow-lying triplet diradical excited state.…”

mentioning

confidence: 99%

“…[2] Owing to the high reactivity of CBD,i tw as not until nearly ahundred years later that CBD derivatives could be synthesized and characterized. [3] Thef undamental questions that surrounded CBD dealing with its aromaticity, enthalpy of formation, reactivity in cycloaddition reactions, isomerization and ground state multiplicity led to intensive research efforts, [2,3] but some of the basic theoretically predicted properties of CBD remain without experimental support even today.O ne basic property which remains to be explored, is the spectroscopic observation of CBDslow-lying triplet diradical excited state.…”

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confidence: 99%

Spectroscopic Observation of the Triplet Diradical State of a Cyclobutadiene

et al. 2017

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Tetrakis(trimethylsilyl)cyclobuta-1,3-diene (1)w as subjected to at emperature-dependent EPR study to allowt he first spectroscopic observation of at riplet diradical state of ac yclobutadiene (2). From the temperature dependent EPR absorption area we derive asinglet!triplet (1!2)energy gap, E ST ,o f1 3.9 kcal mol À1 ,i na greement with calculated values. The zero-field splitting parameters D = 0.171 cm À1 ,E= 0cm À1 are accurately reproduced by DFT calculations.T he triplet diradical 2 is thermally accessible at moderate temperatures.It is not an intermediate in the thermal cycloreversion of cyclobutadiene to two acetylene molecules.

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“…Cyclobutadiene (CBD) [1][2][3][4][5][6][7][8] is an important compound for representing ac yclic 4p system with Hückel antiaromaticity and Baird aromaticity. [9][10][11][12] In the ground state (i.e., S 0 state), this cyclic 4p ring system undergoes second-order Jahn-Teller distortion to circumvent antiaromaticity, [13] resulting in arectangular shape with significant bond alternation (Figure 1a).…”

Section: Introductionmentioning

confidence: 99%

An Element‐Substituted Cyclobutadiene Exhibiting High‐Energy Blue Phosphorescence

et al. 2021

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1,3,2,4‐Diazadiboretidine, an isoelectronic heteroanalogue of cyclobutadiene, is an interesting chemical species in terms of comparison with the carbon system, whereas its properties have never been investigated experimentally. According to Baird's rule, Hückel antiaromatic cyclobutadiene acquires aromaticity in the lowest triplet state. Here we report experimental and theoretical studies on the ground‐ and excited‐state antiaromaticity/aromaticity as well as the photophysical properties of an isolable 1,3,2,4‐diazadiboretidine derivative. The crystal structure of the diazadiboretidine derivative revealed that the B2N2 ring adopts a planar rhombic geometry in the ground state. Yet, theoretical calculations showed that the B2N2 ring turns to a square geometry with a nonaromatic character in the lowest triplet state. Notably, the diazadiboretidine derivative has the lowest singlet and triplet states lying at close energy levels and displays blue phosphorescence.

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[4]Annulene system. Direct proof for its rectangular geometry

Cited by 69 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

Spectroscopic Observation of the Triplet Diradical State of a Cyclobutadiene

An Element‐Substituted Cyclobutadiene Exhibiting High‐Energy Blue Phosphorescence

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