G. Clar 6n Rule versus Hückel 4n + 2 Rule 3464 H. Hydrocarbons versus Heteroatomic Systems 3465 IV. Hidden Treasures of Kekulé Valence Structures 3466 A. Conjugated Circuits 3467 B. Innate Degree of Freedom 3470 C. Clar Structures 3472 V. Graph Theoretical Approach to Chemical Structure 3473 A. Metric 3473 B. Chemical Graphs 3473 C. Isospectral Graphs 3473 D. Embedded Graphs 3475 E. Partial Ordering 3476 VI. On Enumeration of Benzenoid Hydrocarbons 3477 VII. Kekulé Valence Structures Count 3479 A. Non-branched Cata-condensed Benzenoids 3481 B. Branched Cata-condensed Benzenoids 3482 C. Benzenoid Lattices 3482 D. Peri-condensed Benzenoids 3483 E. Miscellaneous Benzenoids 3484 F. The Approach of Platt 3485 G. Computer Programs for Calculating K 3485 H. Transfer-Matrix Method 3486 I. Use of Recursion Relations 3486 J. Use of Signed Matrices 3487 VIII. Enumeration of Conjugated Circuits 3488 IX. Approximate Approaches versus Ambitious Computations 3490

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Aromaticity of polycyclic conjugated hydrocarbons.

A different story of pi-delocalization--the distortivity of pi-electrons and its chemical manifestations.

A σ-bond between sp2- and sp-carbon centers can be formed either by vinylation of a 1-alkyne or an organometallic alkynyl, or by alkynylation of a vinyl organometallic reagent.1–3 Several copper(I) alkynides undergo successful coupling reactions with halogen compounds in which the halogen is linked to an sp2-carbon center (vinylic, allenic, aromatic and heteroaromatic carbon). In all cases, however, a stoichiometric amount of a copper(I) salt appears to be necessary, which makes the synthesis less attractive for performance on a large scale.4–6

2.4 – Coupling Reactions Between sp2 and sp Carbon Centers

Through space NMR shieldings of aromatic (benzene, mono-substituted and annelated benzenes, ferrocene, [14]- and [18]-annulenes, phenylenes and tetra- to heptahelicene) and anti-aromatic molecules (cyclobutadiene and pentalene) were assessed by ab initio molecular-orbital calculations. Employing the nucleus-independent chemical shifts (NICS) concept, these through space NMR shieldings were visualized as iso-chemical-shielding surfaces (ICSSs) and can be applied quantitatively to determine the stereochemistry of proximal nuclei. In addition, the distances in A at ICSS values of ±0.1 ppm in-plane and perpendicular-to-center of the aromatic ring system were employed as a simple means to compare and estimate qualitatively the aromaticity of the systems at hand.

Visualization of through space NMR shieldings of aromatic and anti-aromatic molecules and a simple means to compare and estimate aromaticity

Oxidative coupling of bis(2-ethynylphenyl)ethyne gives the title annulene (I, R = H), the corresponding trimer, hexabenzononadehydro[30]annulene, and a pentamer, decabenzopentadehydro[50]annulene X-ray and NMR anal of I (R = H) shows it to be a nonplanar, unstrained, and chiral, but rapidly enantiomerizing mol The synthesis of I (R = Me2CH) allowed the detn of the barrier to enantiomerization through a planar structure: 93 kcal/mol-1 All of these compds explode violently on heating A detailed examn of this decompn focused on I (R = H): 245 Deg dec; DH = -95 kcal/mol-1 (DSC); 10% mass loss (TGA); evolved gases: CH4 and H2 (Raman spectroscopy, gas phase NMR); LD-FTMS indicates oligomerization with loss of H2 and CH4 TEM anal of the essentially pure carbon residue revealed extensive formation of ordered carbon layers of tube- and onion-type, in addn to graphite and amorphous carbon This observation suggests exploration of this novel process in the designed construction of carbon allotropes

Synthesis, Crystal Structure, and Explosive Decomposition of 1,2:5,6:11,12:15,16- Tetrabenzo-3,7,9,13,17,19-hexadehydro[20]annulene: Formation of Onion- and Tube-like Closed-Shell Carbon Particles

Novel Synthesis of the Angular [3]Phenylene (Terphenylene) by Cobalt-Catalyzed Cyclization of Bis(2-ethynylphenyl)ethyne: a Molecule with an Internal Cyclohexatriene Ring

Cobalt-katalysierte Cyclisierung von Bis(2-ethinylphenyl)ethin-neue Synthese von gewinkeltem [3]Phenylen (Terphenylen), einem Molekül mit einer Cyclohexatrien-Einheit†

Rainer Diercks

Papers

Novel Synthesis of the Angular [3]Phenylene (Terphenylene) by Cobalt-Catalyzed Cyclization of Bis(2-ethynylphenyl)ethyne: a Molecule with an Internal Cyclohexatriene Ring

Cobalt-katalysierte Cyclisierung von Bis(2-ethinylphenyl)ethin-neue Synthese von gewinkeltem [3]Phenylen (Terphenylen), einem Molekül mit einer Cyclohexatrien-Einheit†