Organic Syntheses, Vol. 75, 129
Checked by Sherry R. Chemler and William R. Roush.
1. Procedure
A. (2-Bromoallyl)diisopropoxyborane. A flame-dried,
200-mL, three-necked, round-bottomed flask is equipped with a
magnetic stirring bar,
rubber septum,
125-mL dropping funnel, and an
Allihn-type condenser. To the top of the condenser, a
3-L bag (Note
1),
water aspirator through a
calcium chloride tube, and
allene gas cylinder (Note
2) are connected with
two three-way stopcocks as shown in
f.htmigure 1. The flask and the bag are evacuated twice using a water aspirator, and filled with
allene gas each time (Note
3). The stopcock between the bag and the reaction flask is closed off (Note
4), and the flask is cooled to −20°C in a cooling bath made of dry ice and a 9 : 1 mixture of
ethylene glycol and acetone.
Boron tribromide, BBr3 (25 g, 9.4 mL, 100 mmol) is introduced through the rubber septum via cannula (Note
5). The stopcock separating the bag from the reaction vessel is then opened. Gas absorption starts immediately and is complete within 10 min to give a dark red solution. When the
allene is completely consumed, the gas cylinder is replaced by a
nitrogen inlet tube and
nitrogen is introduced into the flask. The mixture is stirred at −20°C for 30 min and then
30 mL of dry dichloromethane (Note
6) is added. Next,
33 mL (0.23 mol) of diisopropyl ether (Note
7) in
50 mL of dry dichloromethane is introduced from the dropping funnel over 1 hr (
exothermic!). The mixture is stirred at −20°C for 30 min, then at room temperature for 2 hr, and finally under reflux for 1 hr. The mixture is cooled to room temperature and all volatile components are removed under reduced pressure using a water aspirator (Note
8). The vessel is filled with
nitrogen, and the residue is transferred to a distillation flask via a cannula using
nitrogen. Distillation of the residue under reduced pressure gives
16.4-17.1 g (
66-69%) of
(2-bromoallyl)diisopropoxyborane as a clear liquid, bp
39-43°C (0.4 mm) (Note
9),(Note
10),(Note
11),(Note
12).
Figure 1
2. Notes
1. A
Tedlar bag, made of polyvinyl fluoride, was used. These bags were purchased from SKC, Inc. (US).
2. Cylinders of
allene were purchased from PCR (Japan) or Matheson Gas Products (US).
3. This step is very important. If all the air is not replaced by
allene, the reaction with
BBr3 sometimes does not go to completion.
4.
Allene must not be allowed to condense in the reaction vessel prior to the introduction of the
BBr3.
Boron tribromide reacts rapidly and exothermically with liquid
allene to give a black mixture that yields very little of the desired product. In one experiment in which the connection between the Tedlar bag and the reaction vessel was not closed while the vessel was cooled to −20°C, the yield of
(2-bromoallyl)-diisopropoxyborane was
9%.
5.
BBr3 (≥99.99% purity) was purchased in glass ampoules from Wako Pure Chemical Industries, Ltd., or Aldrich Chemical Company, Inc. The submitters transferred the reagent into a storage vessel equipped with
septum inlets before use, as follows. The ampoule stem was scored with a file and broken off in a nitrogen-filled glove bag. A septum was then placed over the opening. While BBr
3 can be transferred by syringe, it is advisable to use a cannula to avoid problems with the plunger freezing. The beveled point of a
2-mm Teflon tube obtained from Nippon Rikagaku Kikai Co., Ltd. was inserted through the pre-punctured septum on the ampoule leaving the tip above the liquid level. The other end of the tube was inserted through a septum on the storage vessel that was vented through a
bubbler. The tube was placed below the liquid level in the ampoule, and the
BBr3 was transferred into the storage vessel by applying a positive
nitrogen pressure through a hypodermic needle. When all the liquid had been transferred, the Teflon tube and the bubbler vent were withdrawn from the ampoule and inserted into the measuring vessel.
Graduated cylinders or centrifuge tubes with standard taper joints are excellent measuring vessels when fitted with a
two-inlet adapter obtained from Aldrich Chemical Company, Inc. In the same way, the required amount of
BBr3 was transferred from the storage vessel to the measuring vessel, and then from the measuring vessel to the reaction flask. The submitters recommend that if the
BBr3 is not from a freshly opened ampoule, it should be distilled under
nitrogen before use.
3 The checkers purchased
25-g lots of BBr3 and used the entire ampoule in each run, thereby avoiding the need to store and redistill any excess, unused reagent. The checkers transferred the BBr
3 in a dry box under an atmosphere of dry
nitrogen into a glass vessel, and then into the reaction vessel via cannula as specified by the submitters.
8. A −78°C trap to collect all volatile materials should be placed between the aspirator and the reaction vessel during this evaporative distillation.
9. The submitters obtained
18.4 g of product (
70%) starting from
26.5 g (10 mL, 106 mmol) of BBr3.
10. As
(2-bromoallyl)diisopropoxyborane is thermally labile, the distillation should be carried out below 100°C. It is not very sensitive to air but decomposes slowly. It is recommended that it be handled under an inert atmosphere and stored in a
refrigerator. The spectral properties are as follows:
1H NMR (CDCl
3, 400 MHz) δ: 1.16 (d, 12 H, J = 6.0), 2.25 (br s, 2 H), 4.42 (septet, 2 H, J = 6.0), 5.31 (d, 1 H, J = 0.6), 5.48 (d, 1 H, J = 1.3);
13C NMR (CDCl
3, 100 MHz) δ: 24.4, 28-32 (br), 65.7, 116.3, 131.3;
11B NMR (CDCl
3, 128 MHz) δ: 28.04; HRMS for C
9H
19BBrO
2 (M
+ + 1) calcd 249.0661, found 249.0667.
11. The checkers determined the purity of the reagent to be ca. 94% by manual integration (cut and weigh) of the three
11B resonances observed in the sample: δ 17.42 (4.3 %), δ 28.04 (94.3 %) and δ 47.49 (1.4 %) respectively
12. The checkers found that the
1H NMR spectrum of
(2-bromoallyl)-diisopropoxyborane is concentration dependent. When the NMR spectrum was measured at a concentration of 20 μL of product in ca. 0.5 mL of CDCl
3,
(2-bromo-allyl)diisopropoxyborane was observed along with a substantial amount of a second material that had
1H resonances for the vinylic and allylic protons that were very similar in chemical shift to the vinylic and allylic resonances of the desired product. However, when the
1H NMR spectrum of a much more concentrated solution (ca. 250 μL in 0.250 mL of CDCl
3) was measured, the resonances of the contaminant were barely apparent. The origin of this phenomenon has not been conclusively determined.
15. The product contained traces of
ethyl levulinate and was redistilled to give an analytical sample:
1H NMR (CDCl
3, 400 MHz) δ: 1.47 (s, 3 H), 2.02-2.07 (m, 1 H), 2.09-2.48 (m, 1 H), 2.61 (d, 1 H, J = 8.5), 2.63 (dd, 1H, J = 7.6, J = 1.6), 2.85 (AB q, 2 H, J
AB = 15.0, δν = 16.0), 5.65 (d, 1 H, J = 1.9), 5.74 (t, 1 H, J = 0.9);
13C NMR (CDCl
3, 100 MHz) δ: 26.3, 29.1, 32.4, 51.1, 85.1, 122.7, 125.9, 176.1; IR (neat) cm
−1: 1780 (C=O), 1635 (C=C); HRMS for C
8H
15O
2NBr (M + NH
4+) calcd 236.0286, found 236.0287. Anal. Calcd. for C
8H
11O
2Br: C, 43.86; H, 5.06. Found: C, 43.57; H, 4.97.
3. Discussion
The haloboration reaction of 1-alkynes proceeds stereo- and regioselectively to give 2-halo-1-alkenylboranes that can be used for the stereoselective synthesis of haloalkenes and di- or trisubstituted alkenes.
4 Although isolated double bonds do not undergo the haloboration reaction with haloboranes,
5 allene5 and conjugated dienes
6 give the 1 : 1 adducts. The bromoboration reaction of
allene with
tribromoborane proceeds very rapidly, but the resulting
(2-bromoallyl)dibromoborane is difficult to isolate, because it readily polymerizes during distillation. On the other hand, this intermediate can be converted into dialkoxyborane derivatives by the reaction with ethers such as
diisopropyl ether or
anisole; the resulting (2-bromoallyl)dialkoxyboranes are stable and readily isolated by distillation. Alcohols have also been used for the conversion of bromoboranes to the corresponding alkoxyboranes.
7 However, the reaction with ethers is preferred because evolution of
hydrogen bromide is avoided. This method is especially effective when the product is sensitive to acids, as in the case reported here. Diisopropoxyborane derivatives have the same reactivity as that of the diphenoxyborane derivatives described in a previous paper;
8 however, diisopropoxyborane derivatives are easier to isolate because of their lower boiling points. As described here,
(2-bromoallyl)diisopropoxyborane can be prepared by the bromoboration reaction of
allene with
tribromoborane, followed by addition of
isopropyl ether.
Isopropyl bromide thus generated does not cause any problems and can be removed readily.
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