Planar tetrakoordinierter Kohlenstoff in einem neutralen gesättigten Kohlenwasserstoff: theoretischer Entwurf und Charakterisierung

Abstract

Absolut planar ist, wie Molekülorbitalrechnungen ergaben, das zentrale Kohlenstoffatom im gespannten polycyclischen Käfigkohlenwasserstoff Dimethanospiro[2.2]octaplan umgeben (siehe Struktur). Die entsprechenden C‐C‐Bindungen sind aber nicht ungewöhnlich lang, was auf eine hohe inhärente Stabilität dieser allerdings bislang noch nicht synthetisierten Verbindung schließen läßt.

Carbon Flatland: Planar Tetracoordinate Carbon and Fenestranes

“…More recently Radom computationally explored the alkaplanes, a hypothetical family of strained hydrocarbon cage compounds, and found that the octaplane 4 with S 4 symmetry should contain a central carbon atom with opposite bond angles close to 180°. − Furthermore, it was shown that structures 5 ( D 2 h ) as well as 6 ( D 2 h ) contain a planar tetracoordinate carbon . This concept, to place a carbon atom in the center of a saturated or unsaturated hydrocarbon skeleton, is based on the robust resistance of the ligand system to larger distortions from tetrahedral or planar arrangements, which reinforces planarizing distortions for the central carbon atom.…”

Three-Dimensional Hydrocarbon Cores Based on Multiply Fused Cyclopentane and Indane Units: Centropolyindanes

“…Development of propellane chemistry since Ginsburg's pioneering work comprised various studies on [3.3.3]propellanes . A particular impact has been the first synthesis of triquinacene reported by Woodward et al, which triggered several longstanding efforts to provide improved syntheses of this prototypical triquinane tris(homotriene), − understand its electronic nature, − and perform its dimerization to pentagon−dodecahedrane, the latter goal having never been achieved. ,,,,− Other outstanding landmarks in this field of polyquinanes and polyquinenes were the successful syntheses of the parent all- cis -[5.5.5.5]fenestrane (“staurane”) − and the related all- cis -[5.5.5.5]fenestra-2,5,8,11-tetraene and related staurane derivatives. − The most prominent motif for research into fenestranes has been the challenge to flatten (“planarize”) the geometry of the central, tetracoordinate carbon atom. − Important variants of this theme comprised small-ring congeners, which have become experimentally accessible down to [4.4.4.5]fenestranes in the all- cis series − but also include the highly strained cis , cis , cis , trans stereoisomers of different ring sizes. − …”

“…11°. These distortions and the flattening effect, in particular, are remarkable in view of the discussion on “planar” tetracoordinated carbon. ,− ,,− In the same vain, conversion of suitably bridgehead-functionalized fenestrindanes to more highly unsaturated “fenestrindenes” appears to be a challenge still. Thus, the hypothetical, fully unsaturated analogue 195 and the likewise hypothetical aromatic fenestrindene dication 196 and dianion 197 promise to give an experimental answer to early calculations on the degree of flattening in the related, nonbenzoannelated [5.5.5.5]fenestrenes (Scheme 36). − In contrast to the polyolefinic [5.5.5.5]fenestrenes, conversion of which into the corresponding dication should be extremely difficult, the benzoannelated analogues offer at least a good chance to generate the electron-poor dication 196 , which would provide an aromatic 10π-electron and electron-withdrawing periphery around the central (flattened or even planarized) carbon atom.…”

Two-Dimensional Be2C with Octacoordinate Carbons and Negative Poisson’s Ratio

“…in 1970. , Later, Schleyer and co-workers theoretically designed the first ptC molecule (1, 1-dilithiocyclopropane) in 1976 . Since then, ptC chemistry has attracted extensive studies. − A large number of ptC species have been proposed theoretically − and some global minimum structures, such as CAl 4 – , CAl 4 2– , and CAl 3 Si – , have been observed experimentally. More interestingly, in addition to ptC, many molecules containing planar pentacoordinate carbon (ppC) , and hexacoordinate carbon (phC) − have been predicted theoretically.…”