A new method for the synthesis of diamantane by hydroisomerization of binor-S on treatment with sulfuric acid

A new method was developed for the direct synthesis of the second representative of the homologous series of diamond-like hydrocarbons, diamantane, in 65% yield by hydroisomerization of the norbornadiene dimer, endo-endo-heptacyclo[8.4.0.02,12.03,8.04,6.05,9.011,13]tetradecane (binor-S) on treatment with concentrated sulfuric acid (98%). In the presence of H2SO4 of lower concentration (75–80%), the reaction stops after the hydrogenation step giving endo-endo-pentacyclo[7.3.1.12,5.18,10.03,7]tetradecane in 68% yield with excellent selectivity (100%).


Introduction
Among the highly diverse polycyclic and cage compounds, an important place is occupied by diamond-like compounds called diamondoids, whose lower representatives belong to the homologous series C 4n+6 H 4n+12 . Owing to the rigid structure, diamondoids typically have high thermal stability and high reactivity compared with aliphatic and alicyclic saturated hydrocarbons and show peculiar chemical behavior.
Crude oil is known to be the main natural source of diamondoids. In the oil and gas field exploration, the presence of diamondoids is used to evaluate the field maturity. Whereas the synthesis and chemical reactivity of adamantane, the first member of the diamondoid homologous series, which is pro-duced on an industrial scale (prepared by AlBr 3 or AlCl 3 -induced skeletal isomerization of a petrochemical monomer, hydrogenated dicyclopentadiene) [1], have been studied rather extensively, the chemical behavior of diamantane, the second member of the diamandoid homologous series, has been poorly studied. The main cause of this situation is the lack of facile methods for its synthesis.
As can be seen from Scheme 1, the synthesis of diamantane (1) from binor-S (2) is a two-step process, in which the hydrogenation performed in the first step is most complex and has always been an obstacle to the generation of large amounts of diamantane. In view of the foregoing, we set ourselves the task to develop a one-pot method for the synthesis of diamantane (1) from binor-S (2).

Results and Discussion
In this study, we developed a new method for the synthesis of pentacyclo  8,10 ]tetradecane (tetrahydrobinor-S, 3c) and diamantane (1) ( Table 1). An increase in the sulfuric acid ratio to binor-S (2) ( [2]/[H 2 SO 4 ] = 1:20-50) and rising the temperature to 40 °С lead to decreased product yield due to resinification. When the H 2 SO 4 ratio to binor-S (2) is 1:5, the conversion of compound 2 decreases to 10%. On the other hand, when the reactions are carried out in CS 2 or without any solvent, the selectivity to diamantane (1) increases to 100%, with the maximum yield being 65% (Table 1, entry 12). A portion of binor-S (2) is converted to resinous products. When the reaction was ultrasonically assisted, the reaction time decreased to 2 h with the yield of diamantane (1) being retained (62%).
In order to answer the question of what is the hydrogen source in the hydroisomerization of binor-S (С 14 H 16 , 2) containing 4 hydrogen atoms less than diamantane (С 14 H 20 , 1), we carried out a series of control experiments using deuterated sulfuric acid (98%) in cyclohexane (С 6 H 12 , experiment A), in deuterated cyclohexane (C 6 D 12 , experiment B), or in carbon disulfide (CS 2 , experiment C).
In experiment А, the major isomer 1-D 2 , which is formed upon hydroisomerization of binor-S (2), contains two deuterium atoms. Two more hydrogen atoms are probably provided by cyclohexane. Unexpectedly, the reaction also gave undeuterated diamantane (1), which may be due to deuterium exchange with hydrogen of cyclohexane under the action of D 2 SO 4 . The major product 1-D 3 , which is formed in experiment B with D 2 SO 4 in C 6 D 12 contains three deuterium atoms. The expected isomer with four deuterium atoms is formed in a minor amount. Evidently, binor-S (2) acts as the hydrogen source for the isomer C 14 H 17 D 3 , 1-D 3 . Our attempt to carry out the deuteration of diamantane (1) with D 2 SO 4 in carbon disulfide for 7 h at 20 °C was unsuccessful. Evidently, the deuterium exchange, resulting in the formation of diamantanes 1-D 7 and 1-D 8 containing 7 and 8 deuterium atoms, occurs at the hydroisomerization step (experiment C).  [13]. Since 75-80% H 2 SO 4 contains 20-25% water, the participation of water as a hydrogen source in the reaction cannot be ruled out either.
Attempts to perform hydroisomerization of binor-S (2) to diamantane (1) on treatment with nitric or orthophosphoric acid were unsuccessful, with the starting binor-S (2) being recovered unchanged. The reaction of hydrocarbon 2 with hydrochloric acid proceeds with the addition of HCl to the cyclopropane ring and results in the formation of a mixture of monoand dichloro derivatives, the synthesis of which has been reported [13,14]. When sulfuric acid is replaced by an ionic liquid prepared from triethylamine and sulfuric acid [15], the reaction follows a different route: Starting binor-S (2) is converted to two isomeric hexacyclic hydrocarbons, hexacyclo

Conclusion
Thus, we developed a new one-pot method for the synthesis of diamantane (1) by hydroisomerization of binor-S (2) on treatment with concentrated sulfuric acid (98%) in carbon disulfide ]tetradecane (2, 0.368 g, 2 mmol) and the solvent were charged into a glass reactor (V = 100 mL). Then, concentrated (98%) sulfuric acid (1.96 g, 20 mmol) was added in portions with vigorous stirring. When the whole amount of H 2 SO 4 has been added, the reaction mixture was stirred at 20 °С for 15 h. After completion of the reaction, 10% NaOH was added to the reaction mixture, the organic phase was separated, and filtered through a silica gel layer (with petroleum ether as the eluent). The solvent was distilled off and the residue was recrystallized from a 1:1 ethyl acetate/cyclohexane mixture. The characteristic data and graphical spectra of diamantane are almost identical with the literature data [25].  11,13 ]tetradecane (2, 0.368 g, 2 mmol) was charged into a glass reactor (V = 100 mL) and dissolved in cyclohexane (10 mL). Then, 75-80% sulfuric acid (1.96 g, 20 mmol) was added in portions with vigorous stirring. When the whole amount of H 2 SO 4 has been added, the reaction mixture was stirred at 20 °С for 7 h. After completion of the reaction, 10% NaOH was added to the reaction mixture, the organic part was separated, and filtered through a silica gel layer (with petroleum ether as the eluent). The solvent was distilled off and the residue was recrystallized from a 1:1 ethyl acetate/cyclohexane mixture. Colorless crystals; 68% yield; mp 104-106 °C; 1

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

Funding
The results were obtained with the financial support of the Russian Ministry of Education and Science (project no. 2019-05-595-000-058) on unique equipment at the 'Agidel' Collective Usage Center (Ufa Federal Research Center, Russian Academy of Sciences), by the Scholarship of the President of the Russian Federation to young scientists and postgraduates (SP-1601.2018.1) and carried out within the RF state assignment, reg. no. АААА-А19-119022290009-3.