A simple and convenient method for the preparation of diborane from tetrabutylammonium borohydride and benzyl chloride for application in organic synthesis

Article abstract of DOI:10.1016/j.tetlet.2007.07.173

Diborane is readily generated in situ at 25 °C in toluene using the Bu₄NBH₄/PhCH₂Cl and Bu₄NBH₄/I₂ reagent systems. The reagent prepared in this way is used for the reduction of carbonyl compounds and hydroboration-oxidation of olefins to obtain the corresponding alcohols in good yields.

Full text of DOI:10.1016/j.tetlet.2007.07.173

Tetrahedron Letters 48 (2007) 6966–6969
A simple and convenient method for the preparation
of diborane from tetrabutylammonium borohydride
and benzyl chloride for application in organic synthesis
Mariappan Periasamy,* G. P. Muthukumaragopal and Nalluri Sanjeevakumar
School of Chemistry, University of Hyderabad, Central University P.O., Hyderabad 500 046, India
Received 20 May 2007; revised 13 July 2007; accepted 25 July 2007
Available online 31 July 2007
Abstract—Diborane is readily generated in situ at 25 ꢀC in toluene using the Bu4NBH4/PhCH2Cl and Bu4NBH4/I2 reagent systems.
The reagent prepared in this way is used for the reduction of carbonyl compounds and hydroboration–oxidation of olefins to obtain
the corresponding alcohols in good yields.
ꢁ 2007 Elsevier Ltd. All rights reserved.
The reactive forms of reducing agents such as LiBH4 and
diborane require solvents like THF, dioxane and di-
glyme.1–4 However, these solvents have a high tendency
to form peroxides. Also, it is difficult to recover these
solvents after aqueous work-up. Aqueous or alcoholic
solvents can be used in the case of NaBH4 but there
are some limitations. Raber and Guida5 reported the
reduction of carbonyl compounds by tetrabutylammo-
nium borohydride (Bu4NBH4) in dichloromethane.
However, an excess of reagent and a long reaction time
was required for the completion of the reduction.
Recently, it was reported from this laboratory that the
CBS (Corey, Bakshi, Shibata) oxazaborolidine catalyst
could be easily prepared from borane generated using
Bu4NBH4/CH3I and a,a-diphenyl-2-pyrrolidine metha-
nol (DPPM) in THF.6 We have examined the use of
other alkyl halides for the generation of diborane from
Bu4NBH4, since methyl iodide is toxic. Accordingly, we
carried out the reaction of Bu4NBH4 with benzyl chlo-
ride or iodine in toluene and diborane was readily gener-
ated in >90% yield in this way. We report here the
reduction of aldehydes, ketones, carboxylic acids, acid
chlorides and esters to the corresponding alcohols in
good yields using these reagent systems. Also, the
Bu4NBH4/PhCH2Cl reagent system is useful for the
hydroboration–oxidation of olefins to give the corre-
sponding alcohols in good yields.
The tetraalkylammonium borohydrides are readily solu-
ble in organic solvents and they have low reactivity as
reducing agents. It is considerably easier to handle these
reagents compared to other metal borohydrides. For
instance, they can be recrystallized from ethyl acetate.7
We have observed that the addition of benzyl chloride
to tetrabutylammonium borohydride in THF at 25 ꢀC
for 30 min followed by addition of Ph3P gives Ph3P:BH3
(11B NMR: d ꢀ37.6 ppm) in 90% yield. This indicates
the formation of borane in the reaction of Bu4NBH4
with PhCH2Cl (Scheme 1).
The Bu4NBH4/PhCH2Cl system also provides a conve-
nient source of diborane gas. This method can be used
for preparing various Lewis acid–BH3 complexes. For
example, Ph3P:BH3 can be prepared in 90% yield by
passing B2H6, generated by Bu4NBH4/PhCH2Cl in
toluene, through a solution of Ph3P in THF. We have
also observed that the diborane generated in this way
PhCH2Cl
H3B:THF + Bu4NCl + PhCH3
Ph3P
Bu4NBH4
THF
RT, 15-30 min
Keywords: Tetrabutylammonium borohydride; Reduction; Hydro-
boration; Oxidation.
Ph3P:BH3
90% yield
*
Corresponding author. Tel.: +91 40 23134814; fax: +91 40
23012460; e-mail: mpsc@uohyd.ernet.in
Scheme 1.
0040-4039/$ - see front matter ꢁ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.07.173

M. Periasamy et al. / Tetrahedron Letters 48 (2007) 6966–6969
6967
effectively reduces various functional groups such as
aldehydes, ketones, carboxylic acids and acid chlorides
readily at 25 ꢀC (Scheme 2, Tables 1 and 2). However,
esters required 12 h for completion of the reaction.
The process is simple and amenable to scale-up.
commercially. However, each has limitations and all
are air sensitive.11 Previously, hydroboration of olefins
has been reported using tetrabutylammonium boro-
hydride in chloroform under refluxing conditions.12
The R4NþBH4ꢀ=Me3SiCl system has been used for the
conversion of olefins to alcohols without any oxidizing
agent.13 We have observed that the Bu4NBH4/PhCH2Cl
reagent system can be used for hydroborating olefins
under ambient conditions in toluene/THF mixture and
the corresponding alcohols were obtained in good yields
after H2O2/OHꢀ oxidation (Scheme 3, Table 3).
We also observed that addition of a small amount of
THF (2 mL) was advantageous. It is also very important
to note that the tetraalkylammonium halide by-product
can be readily removed and recovered from the reaction
products. The recovered tetraalkylammonium halide
can be used directly for the preparation of tetraalkyl-
ammonium borohydride, which makes this method
economical.
In conclusion, the Bu4NBH4/PhCH2Cl and Bu4NBH4/
I2 reagent systems were used for reduction of various
carbonyl compounds to the corresponding alcohols
under mild conditions and in good yields (82–94%).
These reagents are also useful for the hydroboration of
Diverse hydroborating agents such as BH3:THF,
BH3:S(CH3)2, 9-BBN and thexylborane are available
OH
O
OH
Bu4NBH4/ I2
Bu4NBH4
Ar
R'
Ar
R
Ar
R'
PhCH2Cl
Toluene,
RT
Toluene,
RT
Ar = C6H5, 4-ClC6H4,
4-MeOC6H4,
Ar = C6H5, 4-ClC6H4,
4-NO2C6H4
Ar = C6H5, 4-ClC6H4,
4-MeOC6H4
4-NO2C6H4
R' = H, CH3
R' = H, CH3
R = H, CH3, OH, OMe
Scheme 2. Reduction of representative carbonyl compounds with Bu4NBH4/PhCH2Cl and Bu4NBH4/I2.
Table 1. Reduction of representative carbonyl compounds with Bu4NBH4/PhCH2Cl8
Entry
1
Substratea
CHO
Time (min)
15
Productb
CH2OH
Yieldc (%)
86
1a
1b
6a
6b
CHO
CH2OH
2
3
15
30
92
91
Cl
Cl
CHO
CH2OH
1c
2a
2b
6c
7a
7b
MeO
MeO
OH
CH3
OH
CH3
O
4
5
15
15
91
94
CH3
O
CH3
Cl
Cl
6
7
8
9
2c
3
90
120
15
7c
6a
8
82
89
90
82
O
OH
CH2OH
COOH
CH2COCl
CH2CH2OH
CH2CH2OH
4
CH2CO2Me
5
720
8
a All the reactions were carried out on 5 mmol scale with 7 mmol of the Bu4NBH4/PhCH2Cl reagent.
b Products were characterized by IR, 1H NMR, 13C NMR and by comparison with reported data.9
c Yields of isolated products.

6968
M. Periasamy et al. / Tetrahedron Letters 48 (2007) 6966–6969
10
Table 2. Reduction of representative carbonyl compounds with Bu4NBH4/I2
Entry
Substratea
Time (min)
Productb
CH2OH
Yieldc (%)
90
CHO
1
1a
1b
15
6a
6b
CHO
CH2OH
CH2OH
2
3
15
15
94
89
Cl
Cl
CHO
1d
2a
6d
7a
O2N
O2N
O
OH
CH3
4
15
91
CH3
COOH
CH2OH
5
6
3
5
30
6a
8
89
86
CH2CH2OH
CH2CO2Me
720
a All the reactions were carried out on 5 mmol scale with 6 mmol of the Bu4NBH4/I2 reagent.
b Products were characterized by IR, 1H NMR, 13C NMR and by comparison with reported data.9
c Yields of isolated products.
olefins to afford the corresponding alcohols in good
yields (80–86%) after oxidation. Hence, this reagent sys-
Acknowledgements
tem has considerable potential for further synthetic
exploitation.
We are thankful to the DST and CSIR for financial sup-
port and research fellowships. The DST support under
the FIST program for the 400 MHz NMR is gratefully
acknowledged. We are also thankful to the UGC for
support under the University of Potential for Excellence
Programme.
OH
Bu4NBH4
OH
PhCH2Cl
Toluene 20 mL
THF 5 mL
RT, 4 h
92%
8%
References and notes
H2O2/NaOH
1. House, H. O. Modern Synthetic Reactions, 2nd ed.;
Scheme 3.
Benjamin Inc.: Philippines, 1972, pp 1–256.
Table 3. Hydroboration of alkenes using Bu4NBH4/PhCH2Cl14
Entry
Substratea
Time (h)
Productb
Yieldc (%)
86
OH
1
9
3
8
OH
2
3
10
4
4
7c
82
80
OH
11
12
a All the reactions were carried out on 7.5 mmol scale with 9 mmol of the Bu4NBH4/PhCH2Cl reagent.
b Products were characterized by IR, 1H NMR, 13C NMR and by comparison with reported data.9
c Yields of isolated products.

M. Periasamy et al. / Tetrahedron Letters 48 (2007) 6966–6969
6969
2. Burkhardt, E. R.; Matos, K. Chem. Rev. 2006, 106, 2617–
2650, and references cited therein.
3. Brown, H. C.; Krishnamurthy, S. Tetrahedron 1979, 35,
567–607.
4. (a) Kanth, J. V. B.; Periasamy, M. J. Org. Chem. 1991, 56,
5964–5965; (b) Prasad, A. S. B.; Kanth, J. V. B.;
Periasamy, M. Tetrahedron 1992, 48, 4623–4628.
5. Raber, D. I.; Guida, W. C. J. Org. Chem. 1976, 41, 690–
696.
6. Anwar, S.; Periasamy, M. Tetrahedron: Asymmetry 2007,
17, 3244–3247.
7. Brandstorm, A.; Junggren, U.; Lamm, B. Tetrahedron
Lett. 1972, 31, 3173–3176.
8. General procedure for the reduction of carbonyl compounds
utilizing the Bu4NBH4/PhCH2Cl reagent system (Table 1):
Bu4NBH4 (1.79 g, 7 mmol) in toluene (25 mL) at 25 ꢀC,
was treated with benzyl chloride (0.84 mL, 7 mmol)
followed by slow addition of the substrate (5 mmol) to
be reduced and the reaction mixture was stirred until the
starting material had disappeared (TLC analysis). The
mixture was carefully quenched with 2 N HCl (5 mL) and
extracted with ether (3 · 25 mL). The combined organic
extracts was washed with brine (25 mL) and dried over
anhydrous Na2SO4. The solvent was evaporated under
reduced pressure and the residue purified on a silica gel
column to obtain the pure product using hexane/ethyl
acetate (97:3) as eluent.
9. (a) De Vries, J. G.; Van Bergen, T. J.; Kellogg, R. M.
Synthesis 1977, 246–247; (b) Santaniello, E.; Farachi, C.;
Manozcchi, A. Synthesis 1987, 912–914; (c) Lane, C. F.;
Daniels, J. J. Org. Synth. 1972, 52, 59; (d) Basavaiah, D.;
Reddy, J. G.; Rao, K. V. Tetrahedron: Asymmetry 2004,
15, 1881–1888; (e) CRC Handbook of Chemistry and
Physics, 85th ed., The Chemical Rubber Co.: Boca Raton,
Florida, 2004.
toluene (15 mL) and THF (2 mL) in a two-neck RB flask
was stirred at 25 ꢀC for 5 min under an N2 atmosphere and
then cooled to 0 ꢀC. I2 (0.76 g, 3 mmol) dissolved in
toluene (10 mL) was added slowly over 10 min. The
substrate to be reduced (5 mmol) was added and the
reaction mixture was warmed to 25 ꢀC and stirred until the
starting material had disappeared (TLC analysis). The
mixture was carefully quenched with 2 N HCl (5 mL) and
extracted with ether (3 · 25 mL). The combined organic
extracts was washed with brine (25 mL) and dried over
anhydrous Na2SO4. The solvent was evaporated under
reduced pressure and the residue was purified by silica gel
column chromatography to afford the corresponding
alcohol using hexane/ethyl acetate (97:3) as eluent.
11. Brown, H. C.; Subba Rao, B. C. J. Am. Chem. Soc. 1959,
81, 6423–6428.
12. Narasimhan, S.; Swarnalakshmi, S.; Balakumar, R. Indian
J. Chem. 1998, 37B, 1189–1190.
13. Baskaran, S.; Gupta, V.; Chidambaram, N.; Chandrasek-
aran, S. J. Chem. Soc., Chem. Commun. 1989, 903–904.
14. General procedure for the hydroboration of olefins with the
Bu4NBH4/PhCH2Cl reagent system (Table 3): A mixture
of Bu4NBH4 (2.31 g, 9 mmol) in toluene (20 mL) and THF
(5 mL) was treated with benzyl chloride (1.1 mL, 9 mmol)
and the mixture stirred for 5 min at 25 ꢀC. Styrene
(0.9 mL, 7.5 mmol) was added and the reaction mixture
was further stirred for 4 h. Excess hydride was destroyed
by the dropwise addition of 2 mL of water, followed by
the addition of aqueous NaOH (5 mL). The reaction flask
was cooled to 5 ꢀC and the borinic acid was oxidized at
25 ꢀC by slow addition of H2O2 (5 mL). The mixture was
extracted with ether (3 · 25 mL) and the combined organic
extracts washed with brine (25 mL) and dried over
anhydrous Na2SO4. The solvent was evaporated under
reduced pressure and the residue was purified by silica gel
column chromatography to afford the corresponding
alcohol using hexane/ethyl acetate (97:3) as eluent.
10. General procedure utilizing the Bu4NBH4/I2 reagent system
(Table 2): A mixture of Bu4NBH4 (1.54 g, 6 mmol) in