Cubane

Penta cyclo [ 4.2.0.02,5.03,8.04,7 ] octane

Shimmering, colorless diamonds

Fixed

1.29 g · cm -3

130-131 ° C

200 ° C ( decomposition)

146.7 Pa ( 25 ° C)

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Cubane is a synthetic hydrocarbon, the molecule consists of eight are arranged in the form of a cube carbon atoms to a corresponding hydrogen atom. The carbon skeleton enclosing a cavity, so Cuban belongs to the class of cage compounds, more specifically to the Platonic hydrocarbons. It was up to its first synthesis in 1964, initially as in the theory exist and because of the unusually sharp 90 -degree bond angles of the carbon atoms as unstable. Through this 90 -degree angle Cuban saves a lot of energy in these bonds, so it can serve as a base of high-energy fuels and explosives (see also tetranitro Cuban, octanitrocubane ).

History

The representation of Cuban in a thirteen- step synthesis by Philip Eaton was first published in 1964. The original synthesis starts from 2-cyclopenten- 1-one ( 1), which is initially in a monobrominated Wohl-Ziegler reaction with N -bromosuccinimide to give (2 ). The Tribromverbindung (3 ) is obtained by Weiterbromierung with elemental bromine, from which the actual output connection of the Cuban - synthetic 2- Bromcyclopentadienon (4 ) is obtained by dehydrohalogenation:

This starting compound (2- Bromcyclopentadienon ) is dimerized in a first step on a Diels -Alder reaction (2). The two carbonyl groups in (2) are protected with ethylene glycol (2a). Subsequently, the endo -position acetal group is selectively hydrolyzed to give the compound (3). This is followed by an intramolecular photochemical [2 2 ] cycloaddition of bromo ketone (4 ) to:

The bromo ketone thus obtained is rearranged via a Favorskii rearrangement by ring contraction to the carboxylic acid ( 5). The free carboxylic acid (5) is transferred to a peroxycarboxylic ester (6) and then decarboxylated thermally (7):

Finally, the remaining acetal group is also hydrolyzed to migrate the bromoketone (8 ) in a second Favorskii reaction and converting the resulting carboxylic acid ( 9) in the corresponding peroxycarboxylic (10). The target molecule cubane ( 11 ) is obtained after a thermal decarboxylation of (10 ):

Synthesis

The structure of the cubane structure can be done in a simpler five-level synthesis for first Cuban -1 ,4 -dicarboxylic acid. The starting material for the synthesis here cyclopentanone, which is converted in the first step by means of ethylene glycol to the cyclic ketal. By bromination produces a triple brominated Cyclopentanonketal, which is converted by dehydrohalogenation, and Diels -Alder reaction in a polycyclic Diels-Alder intermediate. An intramolecular photochemical [2 2 ] cycloaddition reaction leading to a cubane partial structure which is completed by a reaction under reflux in the presence of sodium hydroxide solution. The total yield is over all of the reaction steps such as 25%. The synthesis has been carried out up to the kilogram scale.

Cuban -1 ,4 -dicarboxylic acid is a base compound for the synthesis of other substituted cubane compounds .. The decarboxylation to Cuban takes place in two steps via the tert- butyl peresters. Nearly quantitative synthesis is achieved by the photo-degradation of Thiohydroxamsäureesters.

Properties

Physical Properties

Cuban is a solid, crystalline substance that occurs in two polymorphic crystal forms at room temperature. At room temperature, the crystal form II is available which converts at 121.9 ° C in a first-order phase transition in the crystal form I. This crystal form is in the plastically crystalline form. This means that the connection is located between the two phase transitions in a mesomorphic state. At 131.8 ° C, the liquid phase is achieved. The vapor pressure function is obtained after August according log10 (P) = A / T B (P in Torr T in K) where A = 2200 and B = 8, the link is with a standard enthalpy of ΔfHsolid = 542 kJ · mol 1 or ΔfHgas = 622 kJ · mol -1 strongly endothermic. The standard enthalpy ΔcHsolid is -4833.27 kJ · mol -1. At room temperature Cuban crystallizes in a trigonal crystal structure with space group R3 with one molecule per unit cell. The specific means of electron diffraction bond lengths are for the C -C bond 157.27 ± 0.19 pm, for the C -H bond of 111.8 ± 0.8 pm and differ only slightly from those in cyclobutane with 155.1 pm for the C -C bond and 109 pm for the C -H bond.

Chemical Properties

Despite the highly strained bonds, the connection is stable. A measurable decomposition is observed at temperatures above 200 ° C. The activation energy for the thermolysis is relatively high with a value of 180.5 kJ · mol -1. Compared to light, air and water Cuban is also relatively stable. Cuban can undergo intramolecular, metal- bond rearrangements. The reaction in the presence of silver or palladium catalysts cuneane.

With rhodium catalysts syn tricyclooctadiene is first formed, which can be converted to cyclooctatetraene at 50-60 ° C thermal.

Use

The Italian oil company Agip took Cuban to obtain a higher calorific value of gasoline for motor sports. However, since this special mixture is expensive and would thus be only large racing stables available, the regulations were in most leagues adapted and prescribed only normalized fuel without high-energy additives.