Synthesis of polycyclic hydrocarbons C14H20 by hydrogenation of exo- exo-, exo-endo-, endo-exo-, and endo-endo- hexacyclo[9.2.1.0.0.0.0]tetradec-12-enes with H2SO4 and isomerization of the products to diamantane induced by ionic liquids

A new method was developed for hydrogenation of unsaturated hexacyclic norbornadiene dimers, exo-exo-, exo-endo-, endo-exo-, and endo-endohexacyclo[9.2.1.0.0.0.0]tetradec-12-enes, using sulfuric acid (98%), giving pentacyclo[8.2.1.1.0.0]tetradecanes, which were subjected to skeletal rearrangement under the action of ionic liquids to form diamantane in up to 84% yield. Introduction Diamantane (pentacyclo[7.3.1.1.0.0]tetradecane, C14H20 1) is the second representative of the homologous series of diamondoids. It is promising for the preparation of medicinal agents, polymer materials, and solvent-resistant rubbers and can serve as the raw material for the synthesis of thermally stable synthetic lubricating oils and transmission fluids [1-3]. The known methods for diamantane synthesis (1) are based on the skeletal isomerization of strained, thermodynamically less stable C14H20 polycyclic hydrocarbons [4-8].

The most promising field of application of ILs is homogeneous and heterogeneous catalysis, as noted in a number of recent reviews and monographs [20][21][22][23]. Inorganic ionic liquids may have Brønsted or Lewis acidity or behave as superacids. In particular, superacid properties are inherent in AlCl3-containing melts, which makes them attractive for the use in catalysis [24][25][26]. According to published data [27,28], the ionic liquid In a previous study [29], we performed the first synthesis of diamantane by hydroisomerization of hydrogenated norbornadiene [4π+2π]-dimers (NBDs), that is, , exo-endo- (4), and endo-exohexacyclo[9.2.1.0 2,10 .0 3,8 .0 4,6 .0 5,9 ]tetradecanes (5) (with two carbon atoms less in the molecules than in the diamantane molecule) induced by ionic liquids. Therefore, the goal of the present study was to develop a method for hydrogenation of both the double bond and the three-carbon ring of hydrocarbons 6-9 to obtain hydrocarbons (C14H20), iso-compositional with diamantane, and to perform 3 subsequently a skeletal rearrangement of these products to diamantane (1) under the action of ionic liquids.
It is noteworthy that complete conversion of the starting hydrocarbons 6-9 depends on the duration of the reaction. The required reaction time is 7 h in the case of exo-exo- (6) and endo-exo-(7) hexacyclo[9.2.1.0 2,10 .0 3,8 .0 4,6 .0 5,9 ]tetradec-12-enes, 10 h for the exoendo isomer (8), and 15 h for the endo-endo-isomer (9). 4 In order to confirm the involvement of H2SO4 into hydrogenation of dimers 6-9, we carried out a control experiment on hydrogenation of NBD 6 with deuterated sulfuric acid (96-98%; 99% atomic fraction of D) in cyclohexane. According to gas chromatography/mass spectrometry analysis data, the molecular weight (m/z) of the obtained product (10а) was 190 Da, which corresponds to the molecular formula C14H18D2. According to 13  When compound (6) was hydrogenated with H2SO4 in carbon disulfide, the hydrocarbon conversion decreased to 38%, which means that not only cyclohexane, but also the proper hydrocarbon (6) participates in hydrogenation.
As can be seen from the structure of products (10) and (11), the reactions of hydrocarbons (6) and (8) with H2SO4 were not accompanied by skeletal rearrangements and gave products of the expected structure. As regards hydrocarbons (7) and (9), which contain the most shielded three-carbon ring, they reacted with sulfuric acids to give products structurally identical to adducts (10) and (11), obtained from hydrocarbons (6) and (8), respectively. Evidently, hydrogenation of compounds (7) and (9) proceeds 5 by a more complex pathway. The first step is protonation of the double bond, which is followed by the hydride ion transfer from cyclohexane (or from hydrocarbon 7), thus completing hydrogenation. Then the carbocation skeletal rearrangement takes place, which starts with the protonation of compound (3) at the cyclopropane ring to give the carbocation (K + ).  (10) from hydrocarbon (7).
Compound (11) is formed from hydrocarbon (9) by a similar pathway.
Attempts to perform hydrogenation of hydrocarbons 6-9 using hydrochloric, nitric, or orthophosphoric acid were unsuccessful: after the reaction, the starting NBDs 6-9 were recovered unchanged.
In a previous study [31], we accomplished direct synthesis of diamantane (1) using H2SO4 (98%) from the heptacyclic norbornadiene dimer, binor-S, which has four H atoms less in the molecule than diamantane. This fact indicates that H2SO4 promotes hydrogenation and isomerization of binor-S. In the case of reaction of hexacyclic dimers 6-9 with sulfuric acid, the reaction stops after hydrogenolysis of the three-carbon ring to give pentacyclotetradecanes 10 and 11.
In the next stage of investigation, we carried out skeletal isomerization of iso-   Table 1. The highest yield of diamantane 1 was obtained when the ionic liquid prepared from On the other hand, according to published data [32], the increase in the diamantane (1) yield was placed into a glass reactor (V=100 mL) and dissolved in cyclohexane (10 mL).
After the whole amount of H2SO4 was added, the reaction mixture was stirred at 0-20 о С for 7-10 h. After completion of the reaction, a 10% solution of NaOH was added to the reaction mixture, and the organic phase was separated and filtered through a silica gel layer (elution with petroleum ether). The solvent was distilled off.
Preparation of diamantane: Hydrocarbon (10) or (11) (1 mmol) and pre-synthesized ionic liquid (3 mmol) were charged into a glass reactor (V=50 mL) under argon. The reaction was conducted with continuous stirring at 50 о С for 6 h. Then the reactor was cooled down to room temperature, and the reaction mixture was extracted with petroleum ether and filtered through a silica gel layer (elution with petroleum ether).
The characteristic data and graphical spectra of diamantane are almost identical with the literature data [29].

Supporting Information
Supporting Information File 1 Experimental procedures, NMR, and mass spectral data. Funding