Amine-Boranes: Novel Syntheses and Application as Green Hypergolic Propellants

Reported herein is the identification of amine-boranes as high-performance hypergolic propellant fuels utilized to power chemical rockets engines in spacecrafts, satellites, and missiles. Amine-boranes, while matching the performance standards of state-of-the-art carcinogenic hydrazine-based fuels, are non-toxic and non-volatile. To access these hypergolic propellants, a trans-amination-based route has been developed converting ammonia borane to amine-boranes in excellent yields in refluxing tetrahydrofuran. To aid the large-scale synthesis of amine-boranes via trans-amination, a novel ammonia-mediated synthesis of \u3e98% pure ammonia borane in 92% yield has been achieved. Mechanistic studies point towards ammonia acting as a reagent rather than a catalyst or co-solvent alone. Building on this study, a water-promoted environmentally benign synthesis of \u3e99% ammonia borane has also been developed with 2-methyltetrahydrofuran as the solvent. Further described herein is a direct synthesis of amine boranes in near quantitative yields from sodium borohydride, ammonium sulfate, and the corresponding amines in refluxing tetrahydrofuran. A third-generation amine-borane synthesis utilizing readily available reagents such as baking soda, water, sodium borohydride, and amine in tetrahydrofuran at room temperature has also been developed. This mild protocol has allowed access to amine-boranes bearing functional groups such as hydroxyl, thiol, alkene, amide, ester, nitro, etc. which were inaccessible using earlier routes. Moreover, in addition to broadening the scope of amine protection in organic synthesis, these functionalized amine-boranes have been shown to effectively self-assemble on silica and gold surfaces, expanding the utility of amine-boranes as organic ligands for surface functionalization