Regioselective Synthesis of Methyl-Substituted Adamantanes for Promoting Oxidation Stability of High-Density Fuels

Abstract

Alkyl-adamantane fuels have drawn tremendous attention for aerospace vehicles due to their high density and high oxidation stability. Here, we reported a regioselective route to synthesize methyl-substituted adamantanes including 1,3,5-trimethyladamantane and 1,3,5,7-tetramethyl-adamantane with single configuration via alkylation of 1,3-dimethyl-adamantane, with yields of ∼60 and ∼80%, respectively. The reaction kinetics of alkylation has been investigated, and the reaction rate constants and apparent activation energies were calculated based on the experimental kinetic data. The oxidation stability of methyl-substituted adamantane family were first evaluated via pressure differential scanning calorimetry and rapid small-scale oxidation test, and the results indicate that the oxidation stability is correlated to the carbon type available on the molecule in the order of 1,3,5,7-tetramethyl-adamantane > 1,3,5-trimethyl-adamantane > 1,3-dimethyl-adamantane > 1-methyl-adamantane. Notably, 1,3,5,7-tetramethyl-adamantane with four quaternary carbons and no tertiary carbon presents supreme oxidation stability than JP-10 and decalin. Finally, we used 1,3,5,7tetramethyl-adamantane as an additive and achieved in improving the oxidation stability of JP-10. This work presents methylsubstituted adamantanes as good high-density and high-oxidation-stability fuel components and suggests that methyl groups substituted on tertiary carbon of adamantane has a beneficial effect on improving oxidation stability.

Exhaustive One‐Step Bridgehead Methylation of Adamantane Derivatives with Tetramethylsilane

“…The introduction of the last methyl group needed to convert AdMe 3 to AdMe 4 is comparably slower and the reaction needs 24 or 48 extra hours until completion. This observation is backed up by recently‐reported kinetic measurements which showed an increase of activation energy of 7.2 kJ.mol −1 from the methylation of AdMe 2 to AdMe 3 and AdMe 3 to AdMe 4 [21] . This effect does not originate from steric effects due to the introduction of methyl groups since they are located too far from the available tertiary C−H bonds and geometrically positioned in a way that should limit steric strain.…”

“…There is strong evidence in the literature that the existence of non‐stabilized siliconium ions such as Me 3 Si + is highly unlikely in these conditions [33,37] . The recently published results on the TMS methylation of AdMe 2 are suggesting such a siliconium ion pathway, but we express our skepticism concerning this mechanism [21] . We therefore do not expect the reaction mechanism to proceed by a simple pathway of methyl transfer from TMS, but rather by an intricate interaction between AlCl 3 and TMS.…”

“…Therefore, this method has not been proven to be convenient for the isolation of pure polymethyladamantanes and remained of limited synthetic interest. Simultaneously to our studies, the TMS methylation protocol was described on AdMe 2 with kinetic measurements [21] …”

Synthesis and Performance of Strained Multicyclic Hydrocarbons as Highly Potential High-Energy-Density Fuels

“…27,28,40,41 It should be noted that the high reactivity of strained fuels with oxygen may not favor long-term stability, so the incorporation of anti-oxidants is needed for practical application. 37,38 4. CONCLUSIONS In this work, highly strained cyclopropane-containing fuels have been synthesized by Simmons−Smith cyclopropanation of polycyclic alkenes.…”

“…The C–C bonds of the three-membered cyclic structure are highly distorted, so the strained fuels may be highly active and prone to react with oxygen to possess excellent ignition performance. For example, the highly strained quadricyclane and 1,4-dicyclopropylbuta-1,3-diyne are very flammable and hypergolic with oxidants such as HNO 3 and N 2 O 4 . ,, We adopted the PDSC method with the non-isothermal mode to the reaction with oxygen. − Figure shows intensive exothermic signals in air atmosphere, illustrating the reaction of fuel with oxygen. The onset temperature is in the order of fuel 1 < fuel 2 < fuel 3 < fuel 4 < JP-10, suggesting that the strained fuels are easier to react with oxygen for ignition in order of fuel 1 > fuel 2 > fuel 3 > fuel 4 > JP-10.…”