Frustrated Lewis Pairs: Metal‐free Hydrogen Activation and More

Stephan, Douglas W.; Erker, Gerhard

doi:10.1002/anie.200903708

Cited by 1,832 publications

(576 citation statements)

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“…There are examples in the literature showing that discrete molecules comprising Lewis acid-base pairs separated at a distance that they do not undergo neutralization ('frustrated Lewis acid-base pairs') can activate hydrogen and act as hydrogenation catalysts 21,[45][46][47][48][49][50] . On the basis of this known activity of frustrated Lewis acid-base pairs in organic molecules acting as metal-free hydrogenation catalysts, a reasonable proposal to rationalize the catalytic activity of Gr materials as hydrogenation catalysts would be the existence on the Gr layer of similar type of frustrated Lewis acid-base pairs consisting of the case of Gr by independent acid and basic pairs located on the Gr sheet at an adequate distance to promote the splitting of H 2 molecules.…”

Section: Resultsmentioning

confidence: 99%

Graphenes in the absence of metals as carbocatalysts for selective acetylene hydrogenation and alkene hydrogenation

et al. 2014

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Catalysis makes possible a chemical reaction by increasing the transformation rate. Hydrogenation of carbon-carbon multiple bonds is one of the most important examples of catalytic reactions. Currently, this type of reaction is carried out in petrochemistry at very large scale, using noble metals such as platinum and palladium or first row transition metals such as nickel. Catalysis is dominated by metals and in many cases by precious ones. Here we report that graphene (a single layer of one-atom-thick carbon atoms) can replace metals for hydrogenation of carbon-carbon multiple bonds. Besides alkene hydrogenation, we have shown that graphenes also exhibit high selectivity for the hydrogenation of acetylene in the presence of a large excess of ethylene.

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Section: Resultsmentioning

confidence: 99%

Graphenes in the absence of metals as carbocatalysts for selective acetylene hydrogenation and alkene hydrogenation

et al. 2014

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“…The full B-N bond formation reminded us of the remarkable story of frustrated Lewis pairs (FLPs), pioneered by Stephan (38). The most interesting thing about these molecules is that they are reactive, activating H 2 and other bonds heterolytically.…”

Section: Sidewaysmentioning

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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“…This type of bond cleavage has come to be termed "frustrated Lewis pair" (FLP) bond activation. 20,21 In the hydrogenation chemistry, the role of an H2•B(C6F5)3 adduct (I, E = H) is still in question, 22 and proposals that involve the formation of a weak "encounter complex" II, stabilized by secondary C-H•••F interactions, prior to FLP activation of H2 via III have strong computational support. 23,24 Whether the path to III involves an EH•B(C6F5)3 adduct I or an encounter complex II, the bond activation transition state leads to an ion pair IV which proceeds to product upon transfer of hydride from the [HB(C6F5)3] -to the substrate carbon, regenerating the B(C6F5)3 catalyst.…”

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confidence: 99%