Diazadioxa[8]circulenes: Planar Antiaromatic Cyclooctatetraenes

Hensel, Thomas; Trpcevski, Denis; Lind, Christopher Trier; Grosjean, Rémi; Hammershøj, Peter; Nielsen, Christian; Brock‐Nannestad, Theis; Nielsen, Bjarne E.; Schau‐Magnussen, Magnus; Minaev, B. F.; Baryshnikov, Gleb V.; Pittelkow, Michael

doi:10.1002/chem.201303194

Cited by 88 publications

(73 citation statements)

References 31 publications

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“…Important in this story is a theoretical design paper by Baldridge and Siegel (24), the independent synthesis of some planar COTs (25,26), shown in Fig. 4, some others in which the aromaticity of fused rings is balanced with steric and electronic constraints (27)…”

Section: Can One Make the (Zzze) Isomer The Stable Form Of A Substmentioning

confidence: 99%

Stabilizing a different cyclooctatetraene stereoisomer

Lei

Xie

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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An unconventional cis-cis-cis-trans or (Z,Z,Z,E) structure B of cyclooctatetraene (COT) is calculated to lie only 23 kcal/mol above the well-known tub-shaped (Z,Z,Z,Z) isomer A; one example of this type of structure is known. The barrier for B returning to A is small, 3 kcal/mol. However, by suitable choice of substituents, the (Z,Z,Z,E) isomer can be made to lie in energy below the tub-shaped structure. Steric, clamping, and electronic strategies are proposed for achieving this. In the steric strategy, the C 8 H 4 (CH 3 ) 2 (C( t Bu) 3 ) 2 structure B is predicted to lie 21 kcal/mol below structure A, which is separated from form B only by a small barrier. A simple clamping strategy, effective for COT planarization, does not influence the A/B isomerization much. But, if the clamping group is aromatic (a fused benzene, pyrrole, thiophene, furan), the subtle interplay of potential aromaticity with clamping can be used to confer persistence if not stability on the (Z,Z,Z,E) isomer. An electronic strategy of a different kind, push-pull substitution on the COT ring, was not very effective in stabilizing the B form. However, it led us to vicinal amine-borane-substituted normal COTs that proved to be quite good at activating H 2 in a frustrated Lewis pair scenario. The molecule is most certainly not aromatic, in accord with Hückel's rule (2). It features four cis-or Z double bonds in a characteristic nonplanar D 2d tub shape (3). From a combined femtosecond rotational coherence spectroscopy and theoretical study we have the molecule's precise gas-phase structure, with bond lengths r e (C-C) = 1.470, r e (C=C) = 1.337, and r e (C-H) = 1.079 Å (4). Potential ring inversion and bond-bond switching in COT have fascinated the organic community for decades (5-7). The planar D 4h structure of COT is a transition state (TS) for the ring inversion (calculated barrier height of 10-14 kcal/mol) (8-12). The D 8h bondswitching TS lies a few kilocalories per mole higher. The accepted automerization surface is summarized schematically in Fig. 1.Motivated by the recent determination of the structures of some asymmetrically substituted COTs (13), we began a systematic study of COTs. Some peculiar geometries, far away from the tub shape, emerged; in time we identified them as cis-cis-cistrans (Z,Z,Z,E) isomers of COT. Stabilizing these structures is the aim of this article.(Z,Z,Z,E)-COT structures already appear in the theoretical and experimental literature. Conesa and Rzepa in 1998 predicted this isomer to lie 21-29 kcal/mol above the conventional all-Z structure, with a small barrier to return to all-Z COT (14). In their studies of Möbius aromaticity, Havenith et al. (15) optimized a C 8 H 8 (Z,Z,Z,E) structure, finding it again 21-29 kcal/mol above (Z,Z,Z,Z). And, in a comprehensive paper on the photochemistry and thermal rearrangements of COT, Garavelli et al. (16) found the (Z,Z,Z,E) isomer as a reaction intermediate, 24 to 25 kcal/mol above normal COT. The primary interest in their wideranging paper was in the photochemistry o...

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Section: Can One Make the (Zzze) Isomer The Stable Form Of A Substmentioning

confidence: 99%

Stabilizing a different cyclooctatetraene stereoisomer

Lei

Xie

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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show abstract

“…In our previous studies [28][29][30][31][32][33][34][35][36][37][38][39][40][41] the structure and electronic absorption spectra of the heteroannelated octatetraenes were described in detail but the vibronic effects have been discussed only qualitatively. As a good example of vibronic effects manifestation the electronic absorption spectrum of the tetraoxa [8]circulene 1 (Fig.…”

Section: Introductionmentioning

confidence: 99%

“…Therefore their structural features, functionalization possibilities and spectral properties are studied extensively in the numerous recent researchers [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41].…”

Section: Introductionmentioning

confidence: 99%

DFT simulation of the heteroannelated octatetraenes vibronic spectra with the Franck–Condon and Herzberg–Teller approaches including Duschinsky effect

et al. 2015

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“…Optically active tetraphenylene derivatives (68) bearing t-butyls were obtained in 75-86% yields and in moderate enantioselectivities (54-64%). Nevertheless, a highly enantiomerically pure 68a (≥95% ee) was obtained after recrystallization.…”

Section: Oxidative Coupling Reaction Of Dilithiobiphenyl Derivativesmentioning

confidence: 99%

“…These [8] circulenes have a significant antiaromatic character, as can be predicted using computational methods [68].…”

Section: Tetrameric Condensationsmentioning

confidence: 99%