Organic Syntheses, CV 9, 95
9-BORABICYCLO[3.3.1]NONANE DIMER
[9-Borabicyclo[3.3.1]nonane, dimer]
Submitted by John A. Soderquist
1 and Alvin Negron.
Checked by Daniel M. Berger and Larry E. Overman.
1. Procedure
CAUTION! The manipulation and handling of air-sensitive compounds requires the use of special techniques. While no difficulties have been encountered with the present procedures, the preparer should consult References
3 and
2 prior to carrying out these syntheses.
A
2-L, three-necked, round-bottomed flask containing a
magnetic stirring bar is fitted with a
250-mL addition funnel and a distillation assembly set for downward distillation to a
500-mL receiver flask.
Rubber septa are used to isolate the system from atmospheric contact. Under a nitrogen purge, vented to an exhaust
hood through a
mercury bubbler, the entire system is thoroughly flame-dried (Note
1). After the 2-L flask is cooled to room temperature, it is charged with
500 mL of pure, dry 1,2-dimethoxyethane (Note
2) and
153 mL (1.53 mol) of borane-methyl sulfide complex (Note
3) employing a double-ended needle to effect the transfer. With a similar technique,
164 g (1.52 mol) of 1,5-cyclooctadiene (Note
4) is transferred to the addition funnel. To the stirred
borane solution,
1,5-cyclooctadiene is added dropwise over ca. 1 hr to maintain a reaction temperature of 50–60°C during which time a small amount of
dimethyl sulfide (bp
38°C) distills slowly from the reaction mixture. After the addition is completed, the addition funnel is replaced with a
glass stopper and approximately 300 mL of the solution is distilled to reach a final distillation temperature of 85°C, indicating the complete removal of
dimethyl sulfide from the reaction mixture (Note
5). If the distillate temperature does not reach 85°C,
150 mL of additional 1,2-dimethoxyethane is added and the distillation is continued until the distillate temperature reaches 85°C. The distillation assembly is replaced with a
rubber septum and
1,2-dimethoxyethane is added to the reaction flask to bring the total liquid volume to 1 L. The mixture is warmed to effect the dissolution of the solid and allowed to cool very slowly to 0°C, which results in the formation of crystalline
9-borabicyclo[3.3.1]nonane (9-BBN) dimer. The supernatant liquid is decanted from the product using a double-ended needle and the
9-BBN dimer is dissolved in
1 L of fresh 1,2-dimethoxyethane. After the flask is cooled to 0°C, the supernatant liquid is removed as above and the large needles are dried under reduced pressure for 12 hr at 0.1 mm to give
158–165 g (
85–89%) of product (mp
152–154°C, sealed capillary) (Note
6),(Note
7),(Note
8).
2. Notes
1. Alternatively, the apparatus can be dried for 4 hr at 150°C, assembled hot and purged with dry
nitrogen.
2.
1,2-Dimethoxyethane, available from the Aldrich Chemical Company, Inc., was predried over
calcium hydride and distilled from
sodium/benzophenone prior to use. The solvent was used directly after purification or stored in an ampule bottle, available from the Aldrich Chemical Company, Inc., under a
nitrogen atmosphere.
3.
Borane-methyl sulfide complex, obtained from the Aldrich Chemical Company, Inc., was used directly without additional purification. However, titration of the reagent was carried out with
glycerol as described
3 to determine its actual molarity. Older samples of this reagent can be distilled under aspirator vacuum to obtain pure reagent.
5. Failure to remove the
dimethyl sulfide from the reaction mixture increases the solubility of the
9-BBN dimer and lowers the overall yield to ca.
65%.
6. The spectra of the product are as follows:
1H NMR (300 MHz, C
6D
6) δ: 1.44–1.57 (m, 4 H), 1.58–1.74 (m, 12 H), 1.83–2.07 (m, 12 H). A standard HETCOR experiment revealed that protons on each of the methylene carbons were superimposed upon one another to give rise to these downfield multiplets;
13C NMR (75 MHz, C
6D
6) δ: 20.2 (br, C-1,5), 24.3 (C-3,7), 33.6 (C-2,4,6,8);
11B NMR (96 MHz, C
6D
6) δ: 28.
7. The
9-BBN dimer so prepared is reasonably air-stable so that exposure to the atmosphere for 1 month lowered the mp to ca.
146–151°C.
8. Purification of commercial
9-BBN and other samples can be effected by recrystallization from
1,2-dimethoxyethane. Insoluble impurities can be removed from hot
1,2-dimethoxyethane solutions of
9-BBN by decantation of the solution to a second dry flask. To prevent clogging of the double-ended needle during the transfer process it is important to keep the ends of needle below the liquid surfaces. We have found that the receiver vessel should be charged with a small quantity of fresh, hot
1,2-dimethoxyethane prior to decantation and that a portion of this material should be transferred under a positive pressure of
nitrogen to the
9-BBN solution to warm initially the transfer needle. Subsequently, the hot
9-BBN solution can be transferred without difficulty.
Waste Disposal Information
All toxic materials were disposed of in accordance with "Prudent Practices in the Laboratory"; National Academy Press; Washington, DC, 1995.
3. Discussion
As a dialkylborane,
9-borabicyclo[3.3.1]nonane (9-BBN) is unrivaled in both stability and selectivity.
8 It has been distilled (bp
195°C, 12 mm) and exhibits a strong characteristic IR absorption band at 1560 cm
−1 (B-H-B) for the bridged dimeric structure.
5 The crystal structure of
9-BBN dimer has been determined
9 and the drawing above approximates the conformational features of this compound. The
13C NMR properties of 9-BBN adducts have been studied extensively.
10
Since the 9-methoxy derivative of
9-BBN is a common by-product of several reactions of
9-BBN,
11 its efficient conversion back to
9-BBN has been described.
12 Such a process enables one to recycle
9-BBN in reactions which require its high regioselectivity in hydroboration reactions and the related organoborane conversions.
The selective transformations of
9-BBN are numerous and varied, with derivatives being readily prepared through both hydroboration and organometallic methodology.
8 It has been used for the preparation of isomerically-pure boracycles,
11,13 the highly enantioselective reduction of aldehydes and ketones,
14 the preparation of new selective borohydride reducing agents,
15 C-C bond-forming transformations,
16 and radiopharmaceutical labeling.
17 Its reactivity has made it the reagent of choice for many organoborane conversions.
18 The stability and distinctive spectral properties of
9-BBN have provided the initial key information to unravel the details of hydroboration reactions.
8,19
This preparation is referenced from:
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