Molecular Tweezers for Hydrogen: Synthesis, Characterization, and Reactivity

Sumerin, Victor; Schulz, Felix; Atsumi, Michiko; Wang, Cong; Nieger, Martin; Leskelä, Markku; Repo, Timo; Pyykkö, Pekka; Rieger, Bernhard

doi:10.1021/ja806627s

Cited by 359 publications

(291 citation statements)

References 31 publications

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“…The most interesting thing about these molecules is that they are reactive, activating H 2 and other bonds heterolytically. Linked amine-borane units have so far been relatively rare in FLP chemistry, but they are known (39)(40)(41)(42). We were thus encouraged to study the interaction of our substituted COTs with H 2 .…”

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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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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“…[39] Repo et al haben kürzlich über ein ähnliches intramolekular verknüpftes Amin-Boran-System (68/69) berichtet, das von Tetramethylpiperidin abgeleitet ist (Schema 24). [40] Dieses System ist in der Lage, H 2 reversibel zu aktivieren. Details werden in Abschnitt 6.3 diskutiert.…”

Section: Carbene Für Die Flp-aktivierung Von Hunclassified

Frustrierte Lewis‐Paare: metallfreie Wasserstoffaktivierung und mehr

2009

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Die Kombination sterisch gehinderter Lewis‐Säuren und ‐Basen führt nicht zur üblichen Neutralisationsreaktion unter Bildung der “klassischen” Lewis‐Säure/Base‐Addukte. Stattdessen stehen die Lewis‐Acidität und ‐Basizität solcher “frustrierten Lewis‐Paare” (FLPs) gemeinsam für die Durchführung ungewöhnlicher Reaktionen zur Verfügung. Typische Beispiele für FLPs bestehen aus inter‐ und intramolekularen Kombinationen sperriger Phosphine und Amine mit stark elektrophilen RB(C6F5)2‐Komponenten. Viele frustrierte Lewis‐Paare sind in der Lage, Wasserstoff heterolytisch zu spalten. Die resultierenden H+/H−‐Paare (z. B. stabilisiert in Form der entsprechenden Phosphonium‐Kation/Hydridoborat‐Anion‐Salze) fungieren als metallfreie Katalysatoren für die Hydrierung sperriger Imine, Enamine, Enolether usw. FLPs reagieren auch mit Alkenen, Carbonylverbindungen und einer Vielzahl anderer kleiner Moleküle, darunter auch Kohlendioxid, in vermutlich kooperativen Dreikomponentenreaktionen. Auf dieser Beobachtung lassen sich neue Synthesestrategien aufbauen.

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“…9 Since then, many different FLP systems have been developed 10 and reactivity with a variety of substrates has been observed, including B-H bonds, 11 CO2, 12,13 unsaturated bonds 10 and even catalytic, metal-free hydrogenation. [14][15][16][17][18] The silylene LSi (L = CH{(C=CH2)(CMe)(2,6-i Pr2C6H3N)2}) will oxidatively add ammonia to give four-coordinate LSi(H)NH2 19 and is the only reported main group molecule that has been successfully used in the activation of PH3, yielding LSi(H)PH2. 20 Phosphenium cations [R2P] + are a class of phosphorus-containing molecules that have a rich history of reactivity, including reactions with Lewis bases, 21-23 inorganic and organic unsaturated bonds, 24 and as ligands on transition metals.…”

Section: Introductionmentioning

confidence: 99%

Synthesis, Reactivity, and Computational Analysis of Halophosphines Supported by Dianionic Guanidinate Ligands

et al. 2012

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The reported chemistry and reactivity of guanidinate supported group 15 elements in the +3 oxidation state, particularly phosphorus, is limited when compared to their ubiquity in supporting metallic elements across the periodic table. We have synthesized a series of chlorophosphines utilizing homo-and heteroleptic (dianionic)guanidinates and have completed a comprehensive study of their reactivity. Most notable is the reluctancy of these four-membered rings to form the corresponding Nheterocyclic phosphenium cations, the tendency to chemically and thermally eliminate carbodiimide and the scarcely observed ring expansion by insertion of a chloro(imino)phosphine into a P-N bond of the P-N-C-N framework.Computational analysis has provided corroborating evidence for the unwillingness of the halide abstraction reaction by demonstrating the exceptional electron acceptor properties of the target phosphenium cations and the underscoring strength of the P-X bond.

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Molecular Tweezers for Hydrogen: Synthesis, Characterization, and Reactivity

Cited by 359 publications

References 31 publications

Stabilizing a different cyclooctatetraene stereoisomer

Stabilizing a different cyclooctatetraene stereoisomer

Frustrierte Lewis‐Paare: metallfreie Wasserstoffaktivierung und mehr

Synthesis, Reactivity, and Computational Analysis of Halophosphines Supported by Dianionic Guanidinate Ligands

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