Unusual rapid hydroboration of alkenes using diborane in chlorohydrocarbon solvents

Article abstract of DOI:10.1016/S0040-4039(00)01320-4

Diborane dissolves in chlorohydrocarbon solvents, such as dichloromethane, 1,2-dichloroethane and 1,1,2,2-tetrachloroethane to form ~0.5 M solutions of B₂H₆. In these solutions, diborane is in equilibrium with solvent-BH₃ adducts (8-15%) and these solutions hydroborate representative olefins almost instantaneously, even at -16°C. The alcohol products can be obtained by standard oxidation of these organoboranes. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01320-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 9361–9364
Unusual rapid hydroboration of alkenes using diborane in
†
chlorohydrocarbon solvents
Josyula V. B. Kanth and Herbert C. Brown*
H.C. Brown Center for Borane Research, Purdue University, West Lafayette, IN 47907-1393, USA
Received 19 July 2000; revised 1 August 2000; accepted 3 August 2000
Abstract
Diborane dissolves in chlorohydrocarbon solvents, such as dichloromethane, 1,2-dichloroethane and
1
,1,2,2-tetrachloroethane to form ꢀ0.5 M solutions of B H . In these solutions, diborane is in equilibrium
2 6
with solvent–BH adducts (8–15%) and these solutions hydroborate representative olefins almost instantan-
3
eously, even at −16°C. The alcohol products can be obtained by standard oxidation of these organo-
boranes. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: diborane; dichloromethane; 1,2-dichloroethane; 1,1,2,2-tetrachloroethane; olefins; hydroboration.
The hydroboration of olefins using diborane in the absence of solvent is sluggish requiring
1
elevated temperatures and long reaction periods. However, in coordinating solvents such as
2
diglyme, ether and THF, the reaction is rapid and quantitative. Whereas in non-coordinating
solvents such as hydrocarbons, hydroborations do not occur or they occur very slowly.
Recently, during our hydroboration studies of representative olefins using some new, highly
reactive amine–borane adducts in various solvents, unusual rate retardation was observed in
3
dichloromethane solvent with some amine–borane adducts. It is quite intriguing as with other
borane–Lewis base adducts such as Me S:BH , the rates of hydroborations in dichloromethane
2
3
4
are comparable to those in n-pentane and diethyl ether. In order to understand this unusual
slow hydroboration reaction in dichloromethane with the amine–borane adducts, we carried out
detailed studies and explained the rate retardation considering dipolar interactions between
dichloromethane and amine–borane, as described in Eq. (1).
(1)
*
Corresponding author. Fax: (765) 494-0239; e-mail: hcbrown@purdue.edu
†
Communication no. 4 from the H.C. Brown Center for Borane Research, dedicated to Professor Harry Wasserman
on his 80th birthday.
0
040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01320-4

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5
During these studies, a closer look was taken at the reactivity of diborane in
dichloromethane, as no useful data has been reported on the hydroboration of olefins using
diborane in dichloromethane. To our delight, it was noted that diborane dissolves in
11
dichloromethane to form solutions ꢀ0.5 M in B H . More interestingly, the B NMR
2
6
examination showed the presence of ꢀ8–10% of the CH Cl :BH adduct (+2.4ppm, q). Also,
2
2
3
this solution of diborane in dichloromethane hydroborated 1-octene instantaneously, even at
−
16°C (ice–salt mixture), to the trialkylborane stage (Eq. (2)).
(2)
The hydroboration of other representative olefins, such as 1-decene, 2-methyl-1-butene,
b-pinene, cyclohexene, a-pinene and 2,3-dimethyl-2-butene were also carried out under similar
conditions. The less hindered 2-methyl-1-butene, b-pinene and cyclohexene were instantaneously
hydroborated to the trialkylborane stage, while hindered a-pinene and 2,3-dimethyl-2-butene
were hydroborated to the monoalkyl stage instantaneously at −16°C, with further hydroboration
6
being slow. The results are presented in Table 1.
Table 1
a
Hydroboration of representative olefins using diborane in dichloromethane
b
c
d
Olefin
Reaction time (min)
Product(s)
Yield (%)
e
1
1
2
-Octene
5
5
5
5
5
1-Octanol/2-octanol (94/6)
1-Decanol/2-decanol (94/6)
2-Methyl-1-butanol/2-methyl-2-butanol (99/1)
Myrtanol
Cyclohexanol
Isopinocampheol
89
91
86
90
92
88
83
e
-Decene
-Methyl-1-butene
e
e
b-Pinene
f
Cyclohexene
g
a-Pinene
2
10
g
,3-Dimethyl-2-butene 10
2,3-Dimethyl-2-butanol
a
To a 0.5 M solution of diborane in dichloromethane at −16°C, an olefin was added and the contents were stirred
at the same temperature.
b
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Completion of reaction was checked by B NMR.
c
Product alcohols were obtained after standard oxidation using NaOH/H O . The ratio of the alcohols was
2
2
established by GC analysis.
d
Isolated yield.
e
To a 0.5 M (5.0 mL, 2.5 mmol) solution of diborane in dichloromethane 15 mmol of olefin were added.
To a 0.5 M (5 mL, 2.5 mmol) solution of diborane in dichloromethane 10 mmol of olefin were added.
To a 0.5 M (5.0 mL, 2.5 mmol) solution of diborane in dichloromethane 5 mmol of olefin were added.
f
g
In order to understand this unusual reactivity more thoroughly, similar experiments were also
conducted in a few other chlorohydrocarbons such as chloroform, 1,2-dichloroethane, and
1
,1,2,2-tetrachloroethane. When diborane gas was passed into chloroform, in addition to
dissolved diborane (+17.6, 62%) and CHCl :BH (+2.2, 8%), considerable amounts of an
3
3
1
1
7
impurity (+27.2, d, 30%) was noted by B NMR examination, due to a reaction between
chloroform and diborane (with hydrogen evolution). However, such impurities are absent when

9363
diborane is passed into 1,2-dichloroethane and 1,1,2,2-tetrachloroethane. The results were
summarized in Table 2. These solutions also hydroborated simple, unhindered olefins instantan-
eously at −16°C. These hydroborations also proceed at lower temperatures, as low as −78°C.
However, the reaction time is longer at such temperatures.
Table 2
11
a
Various signals observed in B NMR examination of diborane in chlorohydrocarbons
b
b
Chlorohydrocarbon
Diborane (% , l ppm) Chlorocarbon:BH (% , l ppm) Other signals (%, l ppm)
3
Dichloromethane
Chloroform
92 (+17.4, m)
62 (+17.6, m)
85 (+17.2, m)
8 (+2.4, q)
8 (+2.2, q)
15 (+2.4, q)
9 (+2.2, q)
–
c
30 (+27.2, d)
–
–
1,2-Dichloroethane
1,1,2,2-Tetrachloroethane 91 (+17.4, m)
a
Into chlorohydrocarbon (5.0 mL), diborane (20 mmol, excess) was passed slowly at −16°C.
Yield (%) was established by B NMR.
Amount increased with time when kept at 0°C.
b
c
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Large-scale hydroboration reactions can be conveniently carried out by taking the olefin to be
hydroborated in a chlorohydrocarbon solvent, such as dichloromethane, 1,2-dichloroethane or
,1,2,2-tetrachloroethane, and passing in diborane gas at an ice–salt mixture temperature,
1
providing instantaneous hydroboration. The regioselectivities of these hydroborations are
similar to those observed for THF:BH and Me S:BH (Table 1). The organoboranes obtained
3
2
3
after hydroboration can be converted into the corresponding alcohols by standard oxidation
procedures (NaOH/H O ), with the addition of THF, methanol or ethanol as co-solvents, as
2
2
these chlorohydrocarbons are completely immiscible with the aqueous phase.
In conclusion, for the first time we have observed that instantaneous hydroborations can be
carried out in chlorohydrocarbons with diborane. More interestingly, these hydroborations
readily take place at lower temperatures (instantaneous at −16°C, with slower reactions at
−
78°C). This for the first time provides a temperature handle to hydroboration in such solvents
and we are currently looking at applications for selective hydroborations.
Acknowledgements
Financial support from the Herbert C. Brown Center for Borane Research, Purdue University
is gratefully acknowledged.
References
1
. (a) Hurd, D. T. J. Am. Chem. Soc. 1948, 70, 2053. (b) Whatley, A. T.; Pease, R. N. J. Am. Chem. Soc. 1954, 76,
35.
8
2
. (a) Brown, H. C.; Subba Rao, B. C. J. Am. Chem. Soc. 1956, 78, 2582. (b) Brown, H. C.; Subba Rao, B. C. J.
Org. Chem. 1957, 22, 1135. (c) Brown, H. C.; Subba Rao, B. C. J. Am. Chem. Soc. 1959, 81, 6423. (d) Brown, H.
C.; Murray, K. J.; Murray, L. J.; Snover, J. A.; Zwiefel, G. J. Am. Chem. Soc. 1960, 82, 4233. (e) Brown, H. C.
Hydroboration; W.A. Benjamin: New York, 1962. (f) VanNieuwenhze, M. S. In Encyclopedia of Reagents for
Organic Synthesis; Paquette, L. A., Ed.; Wiley: New York, 1995; Vol. 2, p. 1017. (g) Brown, H. C. Organic
Synthesis via Boranes; Aldrich Chemical Co., Inc.; Milwaukee, WI, 1997; Vol. I.

9
3
364
. (a) Brown, H. C.; Kanth, J. V. B.; Zaidlewicz, M. Organometallics 1999, 18, 1311. (b) Brown, H. C.; Kanth, J. V.
B.; Dalvi, P. V.; Zaidlewicz, M. J. Org. Chem. 1999, 64, 6263. (c) Brown, H. C.; Kanth, J. V. B.; Zaidlewicz, M.
J. Org. Chem. 1998, 63, 5154. (d) Brown, H. C.; Kanth, J. V. B.; Dalvi, P. V.; Zaidlewicz, M. J. Org. Chem. 2000,
65, 4655.
4
5
6
. Lane, C. F. J. Org. Chem. 1974, 39, 1437.
. Ref. 2g, page 18. (b) Kanth, J. V. B.; Brown, H. C. Inorg. Chem. 2000, 39, 1795.
. Typical procedure for the preparation of diborane solutions in chlorohydrocarbon solvents with hydroboration of
representative olefins. The procedures followed for 1,2-dichloroethane and 1,1,2,2-tetrachoroethane are identical to
that used for dichloromethane described below. 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
5b
was connected to a diborane generation set-up, as described elsewhere. The flask was kept at −16°C (ice–salt
temperature) and dichloromethane (5.0 mL) was introduced into the flask. Diborane gas (20 mmol, excess),
5b
generated as described elsewhere, was slowly passed into the reaction mixture. After the complete generation of
11
diborane gas, the bubbler was disconnected. The B NMR examination of the reaction mixture showed the signals
corresponding to dissolved diborane (92%, l +18.1) and the borane–dichloromethane complex (8%, +1.1(q)). The
concentration of active hydride was established to be 0.5 M in B H , using hydride analysis. Another oven-dried
2
6
RB flask, provided with a septum inlet and stirring bar, was cooled to −16°C under dry nitrogen. The flask was
charged with 2.5 mL of a 0.5 M solution of diborane in dichloromethane at −16°C (ice–salt temperature). a-Pinene
11
(
5 mmol) was added slowly at that temperature and the contents were stirred for 10 min. The B NMR of the
methanolized reaction mixture showed signals due to IpcB(OMe) (65%, major) and Ipc BOMe (35%, minor). The
2
2
active hydride in the reaction mixture was destroyed using careful addition of methanol (3.0 mL). Once the hydride
evolution subsided, the hydrogen was flushed out and aqueous NaOH (3.0 M, 2.0 mL) was added, followed by
H O (6 mmol). The organic layer was separated and the aqueous layer was extracted with ether and the combined
2
2
organic extract was washed with brine and dried over anhydrous MgSO . Evaporation of solvent gave essentially
4
pure isopinocampheol (0.68 g, 88% yield), the analytical data of which matched with the authentic sample.
11
7
. The B NMR examination of diborane in chloroform showed signals corresponding to dissolved diborane (+17.6,
2%) and CHCl :BH , in addition to a signal at +27.2 (d, 30%). The amount of this product increased with time.
6
3
3
However, when this mixture reacted with 1-octene, in addition to trialkylborane, the signal at +27.2 was also
11
observed in B NMR examination, indicating its non-reactivity towards 1-octene under the present conditions.
We tentatively assign this signal due to the formation of (Cl C) BH. Further studies on the structure and reactivity
3
2
of this species are in progress.
.