The catecholborane was also prepared using the borate 2
solid (mp: 99-102 °C, lit.8b 97-103 °C) in quantitative
yields.
with other reactive borane-Lewis base complexes, such as
i
THF:BH
Bu N:BH
3
, i-Am
Et N:BH
similar to that used for borate 2 and Me
2
S:BH
3
, PhEtPr N:BH
3
, PhEt
following procedures
S:BH . Borane-
2
N:BH
3
, i-Pr
2
-
Preparation of Catecholborane from the Borate 2 and
Diborane Gas. An oven-dried RB flask provided with a
septum inlet, stirring bar, and gas bubbler was assembled
hot and cooled to room temperature under a stream of
nitrogen. The gas bubbler was connected to a diborane
i
3
3
3
and i-Pr
2 3
EtN:BH
2
3
tetrahydrofuran readily reacts with borate 2, even at room
temperature, and the catecholborane was obtained in 80%
yield after the removal of volatile tetrahydrofuran. With the
amine-borane complexes, the initial equilibration with
borate 2 readily occurs at room temperature. However, the
liberated amine from the amine-borane after this initial
reaction complexes with the borate 2, as observed by the
9
generation set-up as described elsewhere. The flask was
charged with the borate 2 (17.31 g, 50 mmol) in a
temperature bath that was kept at 100 °C. Diborane gas (40
mmol, excess), generated as described elsewhere, was passed
into the reaction mixture slowly. The diborane gas was
1
1
11
B NMR examination, which showed peaks corresponding
readily reacted with the borate 2. The B NMR examination
to the formation of amine complexes of borate 2, and this
requires additional heating to achieve further reaction.
The catecholborane obtained after distillation using above-
mentioned procedures has been stored at 0 °C, and no
appreciable disproportionation was noted. However, when
catecholborane was stored at room temperature, the amount
of borate seemed to increase, similar to the observations
made earlier.2
showed clean formation of catecholborane in addition to a
minor amount of unreacted borate 2 (10%). After the
complete generation and absorption of diborane gas, the gas
bubbler was removed, and the reaction flask was fitted with
a distillation set-up. Distillation under reduced pressure (49
°C/50 mmHg, lit. 50 °C/50 mmHg) provided catecholborane
in 80% yield (12.83 g) and in 98% chemical purity. The
residue in the distillation flask was identified as the borate
2f
c
In conclusion, we have developed new, economical,
convenient, and environmentally benign procedures for the
preparation of the valuable organoborane reagent, cat-
echolborane, which is finding increasing application in
organic synthesis. These procedures, which are free of
problems, such as vigorous reaction and evolution of excess
hydrogen, associated with the currently used procedure
should encourage more practicing organic and organometallic
chemists to use this versatile reagent.
2
, which can be mixed with further amounts of borate 2 and
used again for the generation of catecholborane.
Preparation of Catecholborane from the Borate 2 and
Diborane Gas in Toluene. The procedures followed for all
solvents, such as toluene, n-heptane, tetralin, diphenyl ether,
etc., were essentially the same, and the procedure followed
using toluene as solvent is representative.
An oven-dried RB flask provided with a septum inlet,
stirring bar, and gas bubbler was assembled hot and cooled
to room temperature under a stream of nitrogen. The gas
bubbler was connected to a diborane generation set-up as
Experimental Section
9
described elsewhere. The flask was charged with the borate
2
General. All manipulations and reactions with air-
sensitive compounds were carried out in an inert atmosphere
(17.31 g, 50 mmol) in dry toluene (30 mL), and the reaction
mixture was kept at 90 °C (bath temperature). Diborane gas
(40 mmol, excess), generated as described elsewhere, was
passed into the reaction mixture slowly, during 4 h. The
diborane gas was slowly absorbed into the borate-toluene
mixture. The B NMR examination showed clean formation
of the catecholborane in 86% yield in addition to a minor
amount of unreacted borate 2 (14%).
Preparation of Catecholborane from the Borate 2 and
Diborane Gas in a Glyme. The procedures followed in the
glymes, such as di-, tri-, and tetraglyme, were essentially
the same, and the procedure followed using tetraglyme as
solvent is representative.
(
dry nitrogen, argon, etc). The special techniques employed
2g
in handling air-sensitive materials are described elsewhere.
The glassware was oven-dried for several hours, assembled
1
1
while hot, and cooled in a stream of dry nitrogen gas.
NMR spectra were recorded on a 300 MHz multinuclear
instrument. The chemical shifts δ are in ppm relative to BF
OEt . Hydride analysis studies were carried out using the
gasimeter. Triglyme and tetraglyme (anhydrous), catechol,
Me S:BH were procured from Aldrich; boric acid was
obtained from (Mallinckrodt); all were used as obtained. All
of the solvents, toluene, n-heptane, THF, and dichlo-
romethane, were distilled prior to use.
B
11
3
:
2
2
3
Preparation of Tri-O-phenylene bis-borate (2). This
An oven-dried RB flask provided with a septum inlet,
stirring bar, and gas bubbler was assembled hot and cooled
to room temperature under a stream of nitrogen. The gas
bubbler was connected to a diborane generation set-up as
8b
was prepared closely following the literature procedure. An
oven-dried RB flask provided with a septum inlet and a
stirring bar fitted with Dean-Stark apparatus was charged
with catechol (16.52 g, 150 mmol) and boric acid (6.18 g,
9
described elsewhere. The flask was charged with the borate
5
0 mmol) in toluene (50 mL). The contents were gently
refluxed until all of the H O (5.4 g, 300 mmol) formed during
the reaction had been collected in the Dean-Stark apparatus
∼3-4 h). The completion of reaction was also confirmed
by B NMR, which showed clean formation of borate 2
+22.3 ppm, s), with no other boron species present. The
solvent toluene was distilled-out to obtain borate 2 as a white
2 (17.31 g, 50 mmol) in dry tetraglyme (30 mL), and the
reaction mixture was kept at 70 °C. Diborane gas (30 mmol,
small excess), generated as described elsewhere, was passed
into the reaction mixture slowly. The diborane gas was
readily absorbed into the borate-tetraglyme mixture. The
2
(
11
1
1
(
B NMR examination showed clean formation of cat-
echolborane in addition to a minor amount of unreacted
552
•
Vol. 4, No. 6, 2000 / Organic Process Research & Development