Triprismane (CH)6

Ein DaMocles Projekt von

• Heiko Hofmann • Nina Hundertmark • Fatema Rad • Regina Sander Contents

Introduction…………………………………………...page 1 Chemical and physical informations………………...page 2 Stability………………………………………………..page 3 Preparation……………………………………………page 4 - 6 Research and Development…………………………..page 7 - 8 References……………………………………………..page 9 Introduction

Triprismane is the smallest and probably most famous member of a class of extraordinary and interesting polyhederanes, the [n]-prismanes. The smallest member of this hydrocarbon family was proposed by Ladenburg in 1869 as structure for benzene, and more than a century ago, the existence of triprimane became part of a discussion about cage compounds. Formally these cage compounds consist of an even number of methine units positioned at the corners of a regular prism. It's very high symmetry and their complex structure makes them not only difficult to preparate, but also highly strained. The conservation of orbital symmetry, publicised by Woodward and Hoffmann, is the most important facet about the durableness of triprismane and its thermodynamic stability. In 1965 Woodward won the nobel prize of chemistry for these rules.

Seite 1 von 9 1.) Chemical and physical informations

[3]-prismane [4]-prismane [5]-prismane [6]-prismane (ΔHf = 1338,9 kJ/mol) (ΔHf = 719,7 kJ/mol) (ΔHf = 602,5 kJ/mol) (ΔHf = 686,2 kJ/mol)

ΔHf(Benzol) = 961,9 kJ/mol)

These interpretations have been predicted by MM2 calculations as they are also used by programmes like ChemDraw.

Empirical formula C6H6 CAS RN (Registry 650-42-0 N°) Name: IUPAC: teracylo[2.2.0.02,6.03,5]-hexane prismane, [3]-prismane, triprismane, ladenburg-Benzene. Prisman (germ.)

[3]-prismane in words and figures:

Boiling Point 79,8+-7°C (1bar) Density 1,577+-0,06 g/cm3

ΔHvap 30,73+-0,8kJ/mol Flashing Point -31,6+-11,7°C Molar Mass 78,11 g/mol Vapour Pressure 97,0 Torr (25°C)

These dates have all been calculated. A lot of the physical dates couldn't get ascertained because of the lack of research resources so that calculated appreciations of values are easier to obtain.

Seite 2 von 9 Stability:

Triprismane is an explosive liquid substance that is stable at room temperature, because the needed thermal [π2s+π2s]-bond cracking is prohibited by symmetrical reasons. On a temperature of 90°C triprismane breaks down with a half-life period of 11h into 61% benzene and 39% not definable constituents.

As mentioned before, Hoffmann and Woodward ascertained the orbital symmetry to be conservated when a bond is cracked thermical, but this set of rules also allows predictions about the stereochemistry of pericyclic reactions based on orbital symmetry, such as electro cyclic reactions, cycloadditions or sigma tropic reactions:

• In an open chain system that contains 4n-electrons, the orbital symmetry of the HOMO defuses that a bonding interaction between the termini must involve an overlap between the orbital envelopes on opposite faces of the system. This needs a contrarotatory process.

• In an open system with 4n+2-electrons, a terminal bonding interaction within ground state molecules needs an overlap of the orbital envelopes on the same side of the system. This is only achievable by disrotatory displacements.

• In a photochemical reaction an electron in the HOMO of the reacting molecule is promoted to an excited state, which leads to a reversal of terminal symmetry relationships and reversal of stereospecifity.

If a reaction fulfils these rules, it is symmetry allowed, otherwise it is symmetry forbidden and will unlikely happen due to the fact that it requires a lot of energy.

Seite 3 von 9 2.) Preparation

Retro synthesis of triprismane:

The way of retro synthesis over the horizontal cleavage does not work. 5 can not be cyclized to [3]-prismane. However the vertical cleavage leads to the aimed product.

Another possibility to get [3]-prismane is to open one of the cylobutane rings like shown above. Unfortunately the step from 8 back to 7 is very unlikely because 8 has other options to stabilize itself.

9 and 10 can be realized in synthesis. Indeed 9 was the starting material for the first synthesis of [3]-prismane as we can see below.

Seite 4 von 9 Synthesises of triprismane:

First synthesis of [3]-prismane derivates

Experiments to get [3]-prismane by irritating benzene at 254 nm failed. In the resulting mixture 9 and 10 was found but no [3]-prismane.

The situation changed when using benzene derivates carrying bulky substitutes. The first success to synthesize a [3]-prismane derivate was done by Viehe and co- workers. They used the photoisomization of 1,2,3-triflouro-4,5,6-tris-tert-butyl- benzene 11.

Later this rearrangement process was not only observed for a hydrocarbon 15, which isomerised to 16-18, but also for numerous other highly substituted benzene derivatives. The steric hindrance between the substitutes of the substrates is a perquisite for the process, because it enforces non-planarity of the aromatic precursor.

Seite 5 von 9 First synthesis of [3]-prismane

The first synthesis of [3]-prismane on its own was discovered by Katz and co- Workers in 1973. Starting by treating lithium pentadienide 19 with dichloromethane and methyl lithium they got benzvalene 9. When it was reacted with N-phenyl- triazolindione 20 the 1:1 adduct 23 was formed. It was shown that this step begins with the formation of the zwitterions 21 leading to the delocalized cation 22, which can reclose to the observed adduct 23. Hydrolysis followed by oxidation of 23 provided the azo compound 24 which on ultraviolet irradiation extruded nitrogen leading to a yield of 4-6%. Total yield of isolated product referred to the benzvalene 9 was 1,8 %.

In other experiments by irradiating 24 at 366 nm in deuterated cyclohexane the process results in formation of benzene, Dewar benzene, benzvalene, prismane and 1,2-diazacyclooctatetrene. The product ratio was found to be markedly dependent upon the oxygen content of the sample.

Seite 6 von 9 3.) Research and Development:

The today's research is to be examined nowadays thereby the most important sources of information to check chemical structure and reactive theories. Chemists endeavour to produce solid molecules with high energy and tension in order to convert them into specific different substances whilst at the same time tension energy is set free. But it is very difficult to predict the reaction of such tension bond. Because of its low stability this prismane has not been exactly analysed yet. But chemists from Heidelberg were able to synthesise a stable prismane derivate. The tiny crystals were searched at – 178° C and it was observed that all three and four carbon rings of these bonds had the form of a “banana”. These crystals always appear with smaller carbon bond angles than tetraeder angles (109°).

Seite 7 von 9 Nature uses such molecular tension to show the reaction of different active substances as well, such as the carbon three rings in pyrethrin an insecticide out of kinds of pyrethrum or in penicillin an antibiotic, a molecule of four rings of three carbon atoms and one nitrogen atom. Experts assume they are the effective motive power for the chemical reaction.

There is a long list of possible uses for tension systems. As mentioned before all these molecules are highly energetic and therefore some of these cage molecules can be used for fuel admixtures for racing cars or rockets, where fuel with high density of energy is needed. Cage molecules are also suitable for tests of reaction ways. As we know chemical reactions exist of various single steps. To develop new chemical synthesis many

single steps must be researched and thus cage molecules are predestined. Scientists were successful in creating the first double-over-bridged prismane derivate. To start with they chose 1 and 8 for this product. They added AlCl3, CH2Cl2 and the product cyclobutadiene-AlCl3-sigma complexes 6 or 10 developed and when they put DMAD and DMSO into the basis substance, the double-over-bridged dewar-benzene was produced and if the product 8 is heated for 72h and irradiated at 280 nm prismane 3 is achieved with 15% yield. To explain the exact mechanism there has to be made kinetic studies.

Seite 8 von 9 References:

1) Henning Hopf; Classics in Hydrocarbon Chemistry Synthesis, Concepts, Perspectives; Wiley VCH, 41-47 2) J.Am.C.Soc.; 95; 8; 1973; 2738-2739 3) P. S. Engel and C. Steel, Acc. Chem. Res., 6, 1973, 275, 4) Advanced Chemistry Development (ACD/Labs) Software V8.14 for Solairs (1994-2005 ACD/Labs) 5) Angewandte Chemie 1992, 104, Nr.7; 1990, 102, Nr.12 6) Die Erhaltung der Orbitalsymmetrie,R. B. Woodward u. R. Hoffmann, 2. Nachdr. der 1. Aufl. Erschienen: Weinheim/Bergstr. : Verl. Chemie, 1972 7) Orbital Symmetrie and reaction mechanism, E. Amitai Halevi Berlin [u.a.] Springer, 1992

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