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into it. The flask was fitted with a cold water condenser and placed
in an oil bath at 1258C for 18–24 h. After which time, the reaction
solvent was cautiously decanted from the insoluble mixture, and
the remaining residue was extracted with hot toluene. The solvent
was then filtered through a pad of silica gel. After evaporation of
the solvent, crude compounds were purified by silica gel column
chromatography or recrystallization.
Synthesis of compound 1
As a general procedure, 22.4 g (100 mmol) 1,3-dimethylimidazoli-
um iodide and 4.68 g (120 mmol) sodium borohydride was stirred
for 18 h at 1258C to afford crude product. The crude product was
recrystallized from water to give the pure product 1 as a white
1
solid. Yield: 70%; m.p. 1448C; H NMR (400 MHz, CDCl3): d=0.69–
1.33 (m, 3H, BH3), 3.74 (s, 6H, CH3), 6.81 ppm (s, 2H, CH); 13C NMR
(101 MHz, CDCl3): d=119.95, 35.96 ppm; IR (KBr): n˜ =3133, 3167,
3051, 2985, 2953, 2275, 1574, 1479, 1445, 1266, 1232, 1187, 1122,
876, 736, 702, 655 cmꢀ1; HRMS (ESI): m/z: calcd for C5H10BN2:
109.0937 [M]+; found: 109.0935; elemental analysis calcd (%) for
C5H11BN2: C 54.61, H 10.08, N 25.47; found: C 54.56, H 10.25,
N 25.58.
Figure 6. High-speed camera photos that show a spatially resolved ignition
event for a droplet of 5 falling into 100% HNO3.
may be attributed to the fuel-rich allyl substituent in the struc-
ture. For the liquid compound 5, its low Tg (ꢀ708C), high ther-
mal stability (Td =1608C), short ID time (15 ms), high specific
impulse (303.5 s), and low volatility makes it a potentially safer
fuel to replace hydrazine derivatives in liquid bipropellants.
Synthesis of compound 2
As a general procedure, 23.8 g (100 mmol) 1-methy-3-ethyl imida-
zolium iodide and 4.68 g (120 mmol) sodium borohydride was
stirred for 18 h at 1258C to afford the crude product, which was
purified by column chromatography (20% EtOAc/hexane) to afford
1
2 as a white solid. Yield: 68%; H NMR (400 MHz, CDCl3): d=0.70–
Conclusions
1.40 (m, 6H, CH3, BH3), 3.73 (s, 3H, CH3), 4.16 (q, 2H, CH2),
6.85 ppm (d, 2H, CH); 13C NMR (101 MHz, CDCl3): d=170.33–
172.02,120.18, 118.17, 43.72, 35.79, 15.46 ppm; IR (KBr): n˜ =3161,
3123, 2973, 2933, 2873, 2275, 1574, 1479, 1441, 1273, 1229, 1131,
753 cmꢀ1; HRMS (ESI): m/z: calcd for C6H12BN2:123.1094 [M]+;
found: 123.1089; elemental analysis calcd (%) for C6H13BN2:
C 58.12, H 10.57, N 22.59; found: C 57.98, H 10.68, N 23.12.
In summary, six imidazolylidene-borane compounds were syn-
thesized and characterized. These compounds exhibited good
water-stability and hypergolic reactivity with WFNA as the oxi-
dizer. As a potential hypergolic fuel, compound 5 showed
excellent properties including a low melting point, high ther-
mal stability, low viscosity, and a very short ID time, and there-
fore has potential as a hypergolic fuel in bipropellant formula-
tions.
Synthesis of compound 3
As a general procedure, 25.2 g (100 mmol) 1-methy-3-propyl imi-
dazolium iodide and 4.68 g (120 mmol) sodium borohydride was
stirred for 18 h at 125 8C to afford the crude product, which was
purified by column chromatography (20% EtOAc/hexane) to afford
3 as a colorless liquid. Yield: 64%; 1H NMR (400 MHz, CDCl3): d=
0.75–1.35 (m, 6H, CH3, BH3), 1.80 (m, 2H, CH2), 3.73 (s, 3H, CH3),
4.06 (t, 2H, CH2), 6.86 ppm (s, 2H, CH), 13C NMR (101 MHz, CDCl3):
d=170.45–171.97, 120.04, 118.92 50.26, 35.74, 23.40, 10.93 ppm; IR
(KBr): n˜ =3125, 2965, 2877, 2295, 1572, 1474, 1412, 1384, 1251,
1221, 1127, 864, 728, 636, 577 cmꢀ1; HRMS (ESI): m/z: calcd for
C7H14BN2: 137.1250 [M]+; found: 137.1246; elemental analysis calcd
for C7H15BN2: C 60.92, H 10.95, N 20.30; found: C 60.75, H 11.03,
N, 20.19.
Experimental Section
Chemicals
The organic solvents were of analytical grade. 1-Methylimidazole
(99%), 1-ethylimidazole (99%), 1-propylimidazol, 1-butylimidazole
(99%), and 1-allylimidazole (99%) were purchased form Sigma–
Aldrich. Iodomethane (99%), iodoethane (99%), and sodium boro-
hydride (97%) were purchased from J&K Scientific. All of the chem-
icals were used without further purification.
Synthesis of imidazolylidene-borane compounds
Synthesis of compound 4
Iodomethane or iodoethane (1.2 equiv) was added slowly in por-
tions to a dichloromethane solution of the appropriate imidazole
(5m) over the course of 15–30 min. The reaction mixture was
allowed to stir for 1 h, after which time the mixture was concen-
trated by rotary evaporation. Then, the remaining residue was
washed by ethyl acetate three times and dried under vacuum to
give the salt sample. The sample was used directly. Then a 250 mL
flask was charged with 100 mmol of imidazolium salt and 100 mL
of toluene, and then 120 mmol of sodium borohydride was added
As a general procedure, 26.6 g (100 mmol) 1-methy-3-butyl imida-
zolium iodide and 4.68 g (120 mmol) sodium borohydride was
stirred for 18 h at 125 8C to afford the crude product, which was
purified by column chromatography (20% EtOAc/hexane) to afford
4 as a colorless liquid. Yield: 65%; 1H NMR (400 MHz, CDCl3): d=
0.70–1.31 (m, 6H, CH3, BH3), 1.37 (m, 2H, CH2), 1.76 (m, 2H, CH2),
3.73 (s, 2H, CH3), 4.10 (d, 2H, CH2), 6.81 ppm (d, 2H, CH); 13C NMR
(101 MHz, CDCl3): d=120.01, 118.81, 48.55, 35.81, 32.19, 19.68,
Chem. Eur. J. 2016, 22, 1 – 8
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