Stabilizing a different cyclooctatetraene stereoisomer

Abstract

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...

Rational Design of a Carbon‐Boron Frustrated Lewis Pair for Metal‐free Dinitrogen Activation

“…Throughout this paper, we used the ωB97X‐D/cc‐pVTZ (density functional theory method using the ωB97X‐D functional with correlation‐consistent polarized valence triple‐zeta basis set) method to carry out calculations unless otherwise noted (details are provided in Supplementary Information). Such a method has been recently used to investigate the possibility of amine‐borane‐substituted cyclooctatetraenes for H 2 activation in a frustrated Lewis pair scenario by Hoffmann, Schaefer, Lei and their co‐workers . As aromaticity has been one of the most important factors for stabilization in chemistry, our ongoing interest in aromaticity as well as FLP chemistry stimulated us to design a serious [4+2] cycloadditions by using various unsaturated species (Figure b).…”

“…Such a method has been recently used to investigate the possibility of amine-borane-substituted cyclooctatetraenes for H 2 activation in af rustrated Lewis pair scenario by Hoffmann, Schaefer,L ei and their co-workers. [46] As aromaticity has been one of the most important factors for stabilization in chemistry,o ur ongoing interest in aromaticity [47][48][49][50][51][52][53][54] as well as FLP chemistry [55][56][57] stimulated us to design as erious [4+ +2] cycloadditionsb y using various unsaturated species ( Figure 1b). The computed Gibbs free energy for Equation (1) is + 11.7 kcal mol À1 ,s uggesting an endothermic reaction.…”

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“…Houk and coworkers claimed that hyperconjugative antiaromaticity could increase the reactivity on Diels–Alder reactions . Recently, Lei, Schaefer, Hoffman, and coworkers theoretically reported stabilization of cyclooctatetraene (COT) stereoisomer. The vicinal amino-/borane-substituted COTs were proposed to activate H 2 in an FLP scenario.…”

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