Pentylpyridinium Tribromide: A Vapor Pressure Free Room Temperature Ionic Liquid Analogue of Bromine

Abstract

The synthesis and characterization of the room temperature ionic liquid pentylpyridinium tribromide (2) is described. Tribromide 2 was used as a vapor pressure free bromine analogue for the bromination of ketones, aromatics, alkenes, and alkynes. The brominations were carried out in the absence of organic solvents and in most cases the only extraction solvent needed was water. Selectivities and reactivities were shown to be superior to current protocols. The spent reagent pentylpyridinium bromide (1) was easily recycled.

Key words

pentylpyridinium tribromide - room temperature ionic liquid - solvent-free brominations

References

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• 8b

  Bromide 1 is a white hygroscopic solid that melts in humid air affording an ‘RTIL’ although, strictly speaking, it is not.

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• 10b

  For the sake of comparison, the viscosities of bicycle oil and olive oil at r.t. are approximately 10^-2 Pa s and 10^-1 Pa s, respectively.

• Similar examples that demonstrate the complete elimination of residual vapor pressure of strong acids in functional RTIL are:
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• Representative Procedures
• 12a

  p -Bromophenol: To crystalline phenol (1.03 g, 10.9 mmol) was added dropwise 2 (4.04 g, 10.3 mmol) over 20 min. The first few drops of 2 caused the phenol crystals to melt and to form a limpid reaction solution. The resulting orange syrup was left stirring for another 20 min during which time the color changed to yellow. Extraction with Et₂O (4 × 20 mL) yielded a colorless oil (1.66 g, 93%) and spectroscopic data corresponded to pure p-bromophenol. The yellowish ionic liquid phase was dried under high vacuum, identified as pure 1 (1 is liquid when moist) [8] by ¹H NMR (1.99 g, 8.6 mmol), and reacted with 1 equiv of Br₂ to regenerate 2. Note: We do not recommend the use of recycled 2 from monobrominations that could contain traces of HBr in acid-sensitive reactions.

• 12b

  1,2-Dibromocyclohexane: Tribromide 2 (5.00 g, 12.8 mmol) was added dropwise to neat cyclohexene (1.05 g, 12.8 mmol) over 25 min yielding a viscous orange mixture which was left stirring for 2 h. To the resulting yellow biphasic system (heavy phase: 1,2-dibromocyclohexane) was added water (10 mL) for easier phase separation. The organic product was dried with a small amount of Na₂SO₄, filtered and left for 5 min under high vacuum to remove traces of bromine leaving a colorless liquid (2.60 g, 84%). Spectroscopic data corresponded to a pure sample of 1,2-dibromocyclohexane.

• For examples, see:
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• 13b Bora U. Bose G. Chaudhuri MK. Dhar SS. Gopinath R. Khan AT. Patel BK. Org. Lett.  2000,  2:  247 

9

Pentylpyridinium Tribromide (2). Bromine (64.48 g, 404.7 mmol) was added over 30 min to solid crushed pentylpyridinium bromide (1, 93.07 g, 404.4 mmol) under mechanical stirring and cooling in a water bath affording a deep red liquid. After stirring for 2 h the liquid was left in vacuo overnight. Yield: 156 g (99%); mp 0 °C; κ = 8.09 mS cm^-1; d = 1.79 g cm^-3; η_35-500 0.038 Pa s (all at 25° C). ¹H NMR (CD₂Cl₂): δ = 0.80-1.00 (m, 3 H), 1.30-1.55 (m, 4 H), 2.00-2.15 (m, 2 H), 4.60-4.75 (m, 2 H), 8.10-8.20 (m, 2 H), 8.50-8.65 (m, 1 H), 8.80-8.95 (m, 2 H). ¹³C NMR (400.14 MHz, CD₂Cl₂): δ = 13.62 (s), 22.12 (s), 28.20 (s), 31.33 (s), 63.14 (s), 129.08 (s), 144.42 (s), 145.86 (s). Anal. Calcd for C₁₀H₁₆NBr₃: C, 30.80; H, 4.14; N, 3.59. Found: C, 30.95; H, 4.11; N, 3.71.