Hexaethinylbenzol

Peralkynylated Tetraazaacene Derivatives

The synthesis of a decaethynylated tetraazapentacene is described. It was obtained by a combination of condensation reactions giving the two pyrazine rings and subsequent consecutive Stille-type couplings. This is the first example of any higher (hetero)acene that is peralkynylated. The presence of the four nitrogen atoms removes the peri interaction of the substituted alkyne groups, giving this rock-stable and highly twisted heteroacene.

How do redox groups behave around a rigid molecular platform? Hexa(ferrocenylethynyl)benzenes and their "electrostatic" redox chemistry

A new family of hexakis(ferrocenylethynyl)benzenes was synthesized by Negishi coupling from ethynylferrocenes and C(6)Br(6) and can be reversibly oxidized to stable hexaferrocenium salts (see picture, Ar(F)=[3,5-C(6)H(3)(CF(3))(2)]). Their cyclic voltammograms show a single six-electron wave, three distinct two-electron waves, or a cascade of six single-electron waves, depending on the electrolyte counterion and number of methyl substituents on the ferrocenyl groups.

Two-dimensional polymers: just a dream of synthetic chemists?

In light of the considerable impact synthetic 2D polymers are expected to have on many fundamental and applied aspects of the natural and engineering sciences, it is surprising that little research has been carried out on these intriguing macromolecules. Although numerous approaches have been reported over the last several decades, the synthesis of a one monomer unit thick, covalently bonded molecular sheet with a long-range ordered (periodic) internal structure has yet to be achieved. This Review provides an overview of these approaches and an analysis of how to synthesize 2D polymers. This analysis compares polymerizations in (initially) a homogeneous phase with those at interfaces and considers structural aspects of monomers as well as possibly preferred connection modes. It also addresses issues such as shrinkage as well as domain and crack formation, and briefly touches upon how the chances for a successful structural analysis of the final product can possibly be increased.

Oligofunctional amphiphiles featuring geometric core group preorganization: synthesis and study of Langmuir and Langmuir–Blodgett films

Based on the principle of supramolecular preorganization, a new type of oligofunctional amphiphile, of which compounds 1-4 are representative structures, has been designed and synthesized. The typical feature of their structure is a highly rigid and geometrically well-defined central unit composed of ethynylene substituted aromatic spacers with different numbers of amphiphilic segment groups (also of a rigid geometric design) attached to it. The molecules form well-defined Langmuir films when spread from a solution at the air/water interface or when a 10(-4) M aqueous CaCl2 solution was used as the subphase. By analysis of the surface pressure-surface area (pi-A) isotherms, information on the packing behavior and orientation of the amphiphilic molecules depending on the molecular structure could be obtained. Morphological characterization of the dynamic process of monolayer compression at the air/water interface was carried out by Brewster angle microscopy, illustrating several phase states visualized as snap shots. Thin monolayer films produced on a 10(-4) M aqueous CaCl2 subphase can be transferred to a mica solid support by the Langmuir-Blodgett technique. Tapping mode atomic force microscopy reveals a surface topography of the monofilms composed of 1 and 3 that differ in roughness and also in the properties of elasticity, hardness and adhesive strength. X-Ray crystal structure analysis of three relevant intermediate compounds of the synthesis were successfully determined giving an indication of the potential structural features inherent in the new amphiphiles.

Six- and Eightfold Palladium-Catalyzed Cross-Coupling Reactions of Hexa- and Octabromoarenes

Palladium-catalyzed sixfold coupling of hexabromobenzene (20) with a variety of alkenylboronates and alkenylstannanes provided hexaalkenylbenzenes 1 in up to 73 % and 16 to 41 % yields, respectively. In some cases pentaalkenylbenzenes 21 were isolated as the main products (up to 75 %). Some functionally substituted hexaalkenylbenzene derivatives containing oxygen or sulfur atoms in each of their six arms have also been prepared (16 to 24 % yield). The sixfold coupling of the less sterically encumbered 2,3,6,7,10,11-hexabromotriphenylene (24) gave the desired hexakis(3,3-dimethyl-1-butenyl)triphenylene (25) in 93 % yield. The first successful cross-coupling reaction of octabromonaphthalene (26) gave octakis-(3,3-dimethyl-1-butenyl)naphthalene (27) in 21 % yield. Crystal structure analyses disclose that, depending on the nature of the substituents, the six arms are positioned either all on the same side of the central benzene ring as in 1 a and 1 i, making them nicely cup-shaped molecules, or alternatingly above and below the central plane as in 1 h and 23. In 27, the four arms at C-1,4,6,7 are down, while the others are up, or vice versa. Upon catalytic hydrogenation, 1 a yielded 89 % of hexakis(tert-butylethyl)benzene (23). Some efficient accesses to alkynes with sterically demanding substituents are also described. Elimination of phosphoric acid from the enol phosphate derived from the corresponding methyl ketones gave 1-ethynyladamantane (3 b, 62 % yield), 1-ethynyl-1-methylcyclohexane (3 c, 85 %) and 3,3-dimethylpentyne (3 e, 65 %). 1-(Trimethylsilyl)ethynylcyclopropane (7) was used to prepare 1-ethynyl-1-methylcyclopropane (3 d) (two steps, 64 % overall yield). The functionally substituted alkynes 3 f-h were synthesized in multistep sequences starting from the propargyl chloride 11, which was prepared in high yields from the dimethylpropargyl alcohol 10 (94 %). The alkenylstannanes 19 were prepared by hydrostannation of the corresponding alkynes in moderate to high yields (42-97 %), and the alkenylboronates 2 and 4 by hydroboration with catecholborane (27-96 % yield) or pinacolborane (26-69 % yield).

[FeCp]+-induced hexafunctionalization of hexamethylbenzene with dendrons for the direct synthesis of redox-active iron-centered metallodendrimers

Phenol tri- and nonaallyl dendrons (3 and 7, respectively) were functionalized at the focal position to give the new triallyl dendrons 4 and 6 and the nonaallyl dendrons 11 and 13 that contain a iodoalkyl or a bromobenzyl termini. All these dendrons were used for the [FeCp]+-induced hexafunctionalization of hexamethylbenzene in [FeCp(eta6-C6Me6)][PF6] (1) under mild conditions in the presence of KOH. These reactions directly yielded the 18-allyl and 54-allyl dendrimers 9, 10, and 14 with a [FeCp(eta6-arene)]+ unit located at the dendrimer core. Cyclic voltammetry studies were recorded in THF and DMF with these metallodendrimers and compared with those of analogous dendrimers or complexes of smaller size that contain a [FeCp(eta6-arene)]+ unit at the core. The decreased rate of heterogeneous electron transfer when the dendritic size increases first disclosed by Diederich and Gross is confirmed. The variation of the redox potential of the Fe(II/I) redox system with increasing dendritic size is negligible even in a solvent of high dielectric constant such as DMF. This trend is attributed to fact that the involved "redox" orbital is buried on the metal center, well protected by the shell of alkyl chains (electron-reservoir nature), unlike in ferrocene. The chemical irreversibility increases in THF as the dendrimer size increases, due to more facile ligand substitution with THF at the 19-electron level when the chain bulk increases.