Demonstration of fourfold rotational barriers in push-pull ethylenes

Sandström, Jan; Sjöstrand, Ulf; Wennerbeck, Ingegerd

doi:10.1021/ja00455a064

Cited by 20 publications

(12 citation statements)

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“…There is a long history of reducing the strength of the π-component of double bonds by a substitution pattern such as DDC=CAA, where D is a π-donor (NH 2 , OH, halogen), and A a π-acceptor (CN, COH, BH 2 , CF 3 ). The donor and acceptor substituents stabilize respective partial positive and negative ion character on the carbons they bond to, and thereby entail a zwitterionic contribution at the expense of the covalent π-bond (34)(35)(36)(37). The argument given is basically a valence bond theory one; a molecular orbital explanation of the effect of these "push-pull" systems may also be provided.…”

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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“…Specifically, with turning the acceptor part of the molecule out of the π-symmetry plane, the donor part of the molecule becomes increasingly nonplanar as well. In fact, each heteroatom, having approximately the planar sp 2 configuration in the S 0 minimum, becomes more pyramidal, i.e., closer to sp 3 , in the CdC twisted configurations; see Figures 1,2,4,and 5, and the Supporting Information. The mentioned large-amplitude motions are significantly more pronounced for molecules without internal constraints, such as 4-6.…”

Section: Resultsmentioning

confidence: 99%

“…2 The D + A -polarization can bring about quite a significant extension of the originally short CdC bond, which could consequently lead to a remarkablesmore than 20 kcal‚mol -1 slowering of the usually high barriers to rotation about CdC bonds, 3 so that the corresponding dynamic processes would be observable by NMR. 4 However, registered experimental barriers depend significantly on the choice of model compounds. NMR dynamic phenomena in molecules with relatively rigid donor fragments, e.g.…”

Section: Introductionmentioning

confidence: 99%

Correlated MO Study of the Low-Barrier Intramolecular Motions in Donor−Acceptor Ethenes

et al. 2005

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Correlated MP2 and MCSCF MO calculations of several model push-pull ethenes in most cases indicate no great participation of excited singlet and triplet electronic configurations in either the minima or the transition structures for the suggested facilitated intramolecular rotation about the polarized C=C bond. This situation changes significantly only in molecules with sulfur atoms in the molecule as either donors or acceptors. The outstanding contribution of sulfur atoms as either donors or acceptors is a significant increase of push-pull ethene molecular polarizabilities. Thus, within the studied small series of mostly planar push-pull ethenes, polarizability appears a better indicator of rapid intramolecular motions about the C=C bond than straight polarity. Substituents with larger steric demands around the C=C bond are shown to likely preclude its complete turnaround, thus prompting a ramification of the interpretations of dynamic NMR phenomena in sterically constrained push-pull ethenes as large-amplitude librations resulting from strong rovibrational and relatively weak electronic coupling. These librations, as shown by complete vibrational mode analysis of corresponding rotational transition structures, cover in fact certain sectors of the intuitively suggested full rotations similar to those about C-C single bonds.

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“…Sandström and co-worker have found that barriers to rotation around the double bonds in such systems are not very high. 45 Modeling of the photoreaction path…”

Section: Conversion Back Tomentioning

confidence: 99%

Arresting consecutive steps of a photochromic reaction: studies of β-thioxoketones combining laser photolysis with NMR detection

Pietrzak

Dobkowski

Gorski

et al. 2014

Phys. Chem. Chem. Phys.

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Photochromism of monothiodibenzoylmethane has been studied in a number of environments at different temperatures. Direct laser irradiation of a sample located in the NMR magnet allowed in situ monitoring of the phototransformation products, determining their structure, and measuring the kinetics of the back reaction. These observations, along with the data obtained using electronic and vibrational spectroscopies for rare gas matrix-isolated samples, glasses, polymers, and solutions, as well as the results of quantum-chemical calculations, provide insight into the stepwise mechanism of the photochromism in β-thioxoketones. At low temperature in rigid matrices the electronic excitation leads to the formation of the -SH exorotamer of the (Z)-enethiol tautomer. In solutions, further steps are possible, producing a mixture of two other non-chelated enethiol forms. Photoconversion efficiency strongly depends on the excitation wavelength. Analysis of the mechanisms of the photochromic processes indicates a state-specific precursor: chelated thione-enol form in the excited S2(ππ*) electronic state. The results show the potential of using laser photolysis coupled with NMR detection for the identification of phototransformation products.

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Demonstration of fourfold rotational barriers in push-pull ethylenes

Cited by 20 publications

References 1 publication

Stabilizing a different cyclooctatetraene stereoisomer

Stabilizing a different cyclooctatetraene stereoisomer

Correlated MO Study of the Low-Barrier Intramolecular Motions in Donor−Acceptor Ethenes

Arresting consecutive steps of a photochromic reaction: studies of β-thioxoketones combining laser photolysis with NMR detection

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