Org. Synth. 2002, 79, 103
DOI: 10.15227/orgsyn.079.0103
DICYCLOHEXYLBORON TRIFLUOROMETHANESULFONATE
[
Methanesulfonic acid, trifluoro-, anhydride with dicyclohexylborinic
acid
]
Submitted by Atsushi Abiko
1
Checked by Eric M. Flamme and William R. Roush.
1. Procedure
Caution! Borane-dimethyl sulfide complex is foul smelling.
All operations using dimethyl sulfide must be carried out in
a well-ventilated hood.
Dicyclohexylboron trifluoromethanesulfonate.
2 An
oven-dried, 250-mL, round-bottomed
flask containing a
stir bar and capped with a
rubber
septum is charged with
cyclohexene
(33.4 mL, 0.33 mol)
(Note 1)
and
dry diethyl ether (100 mL)
(Note 2), and kept at 0°C under
nitrogen.
Borane-dimethyl sulfide complex
(16.6
mL, 0.16 mol)
(Note 1) is
added dropwise during 30 min with stirring. The reaction mixture is stirred for 3
hr at 0°C, then the solid is allowed to settle without stirring. The supernatant organic
solution is removed as much as possible by syringe (Notes
3,
4), and the residual solid
is dried under reduced pressure
(Note 5) to give
dicyclohexylborane
(
26.3-28.3 g,
92-99% yield)
(Note 6), which is used
without purification for the preparation of the triflate.
The solid is suspended in
100 mL of dry
hexane
(Note 2) and
trifluoromethanesulfonic acid (24.0 g,
0.16 mol)
(Note 1) is added dropwise
via a glass syringe (Notes 7,
8) over 30 min with constant stirring, during
which time vigorous gas evolution occurs; the solid gradually dissolves, and the solution
develops a yellow-orange color (Note 9). Stirring is continued
at room temperature for 1 hr, then the reaction mixture is left for 1-2 hr without
stirring. A semi-solid phase separates and the top layer is transferred via cannula
into a dry, 250 mL round-bottomed flask
(Note 10).
The flask is placed in a −20°C freezer for 36 hr. Large crystals form and the
mother liquor is transferred via cannula to another dry, 100-mL, round-bottomed
flask. The crystals are dried under reduced pressure at 0°C for 5 hr giving
40.8 g (78%) of
dicyclohexylboron
trifluoromethanesulfonate
. The mother liquors are concentrated
to dryness under nitrogen (N2) using a rotary
evaporator
(Note 11). The residue is redissolved
in
20 mL of dry hexane
and crystallized in a −20°C freezer as described above to give an additional
7 g of product. The total yield
of
dicyclohexylboron trifluoromethanesulfonate
is 47.8 g (92%) (Notes 12,
13). The isolated crystals are dissolved
in
hexane
in a graduated
cylinder equipped with a ground glass joint and a stopcock-equipped syringe inlet
to make a 1 M stock solution (Notes 14, 15).
2. Notes
1.
Cyclohexene
was purchased from Wako
Pure Chemical Ltd. Japan, or Aldrich Chemical Company, Inc.
,
and used after distillation from
lithium aluminum
hydride
.
Borane-dimethyl
sulfide complex was obtained from Aldrich Chemical Company, Inc.
,
and was used as received.
3
Trifluoromethanesulfonic acid was purchased from
Wako Pure Chemical Ltd. Japan or Aldrich Chemical Company,
Inc.
, and used without purification. The checkers used a freshly
opened ampule of
trifluoromethanesulfonic acid
for each run.
2.
Diethyl ether
was freshly distilled from
benzophenone ketyl
.
Hexane
was distilled from
lithium aluminum hydride
.
3.
The supernatant organic solution might contain active
borane
. The solution that was removed
was treated with
methanol
to destroy any active
borane
.
The waste containing
dimethyl sulfide
must be treated appropriately before being discarded.
4.
Care must be taken to avoid loss of product that is easily taken
up in the syringe. The checkers found it more convenient to perform the reaction in
a
tared, round-bottomed Schlenk flask. Solvent was then removed
from the solid
dicyclohexylborane
by filtration using a positive
nitrogen flow.
5.
Dicyclohexylborane
bumps during the drying step, and the checkers found that it is advantageous to use
a
vacuum adapter containing a
glass frit
and an
auxiliary cold trap to prevent contamination of the
vacuum line with the product.
6.
The submitter obtained
27.2-30.0
g (
96-105% yield)
of
dicyclohexylborane
.
7.
Trifluoromethanesulfonic acid
is highly corrosive, and a
glass syringe must be used for this
operation.
Plastic syringes are rapidly destroyed by
trifluoromethanesulfonic acid
, leading
to safety hazards if used for this procedure.
8.
The reaction of
dicyclohexylborane
and
trifluoromethanesulfonic acid
is highly exothermic. On one occasion, the checkers cooled
the reaction in an
ice bath during the addition period, with
no effect on product yield. The submitter reports that he once experienced a sudden
vigorous reaction under cooling conditions, probably due to accumulation of unreacted
triflic acid. It thus appears safer to add the acid at room temperature, slowly, so
that it reacts immediately.
9.
The submitter obtained a colorless solution of
boron
triflate, but the checkers observed development of a yellow-orange color
upon addition of
triflic acid. Regardless of
the color, the triflate solution could be used without a decrease in yields.
10.
The small semi-solid yellow layer (about 2 mL) was left behind.
11.
The
rotary evaporator was back-flushed
with dry N2, and it is recommended that a drying column be inserted between the evaporator
and the
water aspirator.
12.
Dicyclohexylboron trifluoromethanesulfonate
exhibited the following spectroscopic properties:
1H NMR (400 MHz, CDCl
3) δ: 67 (m,
10 H), 1.19 (m, 12 H)
;
13C NMR (100 MHz, CDCl
3) δ:
118.3 (q, J = 316), 30.5, 26.9, 26.6,
26.5
;
11B
NMR (115 MHz, hexane/C
6D
6: δ relative to BF
3:Et
2O)
δ 59.2 (br s)
; lit.
2b
11B NMR δ 59.6 (br
s)
.
13.
The submitter obtained the product in
89% yield (one crop) by using this crystallization procedure.
14.
An alternative isolation procedure follows. When the reaction
between
dicyclohexylborane
and
trifluoromethanesulfonic acid
was complete, the liquid layer was transferred via cannula to a
tared,
graduated cylinder, the top of which had been modified with a
stopcock-equipped
syringe inlet and a ground glass joint. This flask was directly attached
to a
rotary evaporator
(Note 10) for removal
of solvent. The resulting orange-yellow oil was dried under high vacuum with an
auxiliary
cold trap in order to obtain an accurate weight of the product, which
solidified into a solid mass (
23.2 g,
81% yield). This material was
dissolved in
hexane
to give
a 1 M solution that was stored in a refrigerator. Upon cold storage,
dicyclohexylboron
trifluoromethanesulfonate
crystallizes from solution, but the
crystals easily redissolve when the solution is stirrred at room temperature before
use. Reagent solutions prepared in this way gave excellent results in the aldol reaction,
p. 116.
15.
A stock solution of
dicyclohexylboron
trifluoromethanesulfonate
in
hexane
is reasonably stable, if stored at ≈4°C under
nitrogen.
It may be purified by storing the
hexane
solution at −20°C, collecting the crystals that form, and redissolving
this material in a calculated amount of
dry hexane
to give a 1 M solution.
Handling and Disposal of Hazardous Chemicals
The procedures in this article are intended for use only by persons with prior training in experimental organic chemistry. All hazardous materials should be handled using the standard procedures for work with chemicals described in references such as "Prudent Practices in the Laboratory" (The National Academies Press, Washington, D.C., 2011 www.nap.edu). All chemical waste should be disposed of in accordance with local regulations. For general guidelines for the management of chemical waste, see Chapter 8 of Prudent Practices.
These procedures must be conducted at one's own risk. Organic Syntheses, Inc., its Editors, and its Board of Directors do not warrant or guarantee the safety of individuals using these procedures and hereby disclaim any liability for any injuries or damages claimed to have resulted from or related in any way to the procedures herein.
3. Discussion
Dialkylboron trifluoromethanesulfonates (triflates) are particularly useful reagents
for the preparation of boron enolates from carbonyl compounds, including ketones,
thioesters and acyloxazolidinones.
4 Recently, the combination of
dicylohexylboron trifluoromethanesulfonate
and
triethylamine
was found
to effect the enolization of carboxylic esters.
5
The boron-mediated asymmetric aldol reaction of carboxylic esters is particularly
useful for the construction of anti β-hydroxy-α-methyl carbonyl units.
6 The present procedure
is a slight modification of that reported by Brown, et al.
2
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
Dicyclohexylboron trifluoromethanesulfonate:
Methanesulfonic
acid, trifluoro-, anhydride with
dicyclohexylborinic acid
(13); (145412-54-0)
Borane- dimethyl sulfide complex:
Methyl
sulfide, compd. with
borane (1:1);
Borane,
compd. with
thiobis[methane] (1:1); (13292-87-0)
Cyclohexene (8,9); (110-83-8)
Dicyclohexylborane:
Borane, dicyclohexyl-
(8,9); (1568-65-6)
Trifluoromethanesulfonic acid: HIGHLY CORROSIVE:
Methanesulfonic
acid, trifluoro- (8,9); (1493-13-6)
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