Generation of bis(pentafluorophenyl)borane-dimethyl sulfide complex as a solution of hexane and its application to hydroboration of alk-1-yne with pinacolborane

Article abstract of DOI:10.1016/j.tetlet.2007.09.176

A solution of bis(pentafluorophenyl)borane-dimethyl sulfide complex in hexane was generated by redistribution between tris(pentafluorophenyl)borane and borane-dimethyl sulfide complex. In the resulting solution a stoichiometric hydroboration of alk-1-yne

Full text of DOI:10.1016/j.tetlet.2007.09.176

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Tetrahedron Letters 48 (2007) 8475–8478
Generation of bis(pentafluorophenyl)borane–dimethyl sulfide
complex as a solution of hexane and its application to
hydroboration of alk-1-yne with pinacolborane
Masayuki Hoshi,* Kazuya Shirakawa and Mitsuhiro Okimoto
Department of Applied and Environmental Chemistry, Kitami Institute of Technology, 165 Koen-cho, Kitami,
Hokkaido 090-8507, Japan
Received 9 August 2007; revised 20 September 2007; accepted 27 September 2007
Available online 3 October 2007
Abstract—A solution of bis(pentafluorophenyl)borane–dimethyl sulfide complex in hexane was generated by redistribution between
tris(pentafluorophenyl)borane and borane–dimethyl sulfide complex. In the resulting solution a stoichiometric hydroboration of
alk-1-yne with pinacolborane proceeded well at room temperature to afford (E)-alk-1-enylboronic acid pinacol ester in high yield.
Bis(pentafluorophenyl)borane–dimethyl sulfide complex served as a mediator for the hydroboration.
Ó 2007 Elsevier Ltd. All rights reserved.
Tris(pentafluorophenyl)borane [B(C6F5)3] is a unique
Lewis acid, that is, an air-stable, water-tolerant, ther-
mally robust compound. Due to its distinctive features,
it has extensively been used as a Lewis acid catalyst in
organic synthesis.1 In connection with pentafluorophen-
yl substituted-boron compounds, Piers and co-workers
reported that bis(pentafluorophenyl)borane [HB(C6F5)2]
is an extremely active hydroborating agent towards
alkenes and alkynes.2 Hydroboration with HB(C6F5)2
in benzene proceeded at a much faster rate than that
with other hydroborating agents. Regarding the prepa-
ration of HB(C6F5)2, two routes were recommended.
One route was a three-step procedure: (1) synthesis of
Me2Sn(C6F5)2 by reaction of Me2SnCl2 with C6F5Li,
(2) synthesis of ClB(C6F5)2 by transfer of pentafluoro-
phenyl group from Me2Sn(C6F5)2 to BCl3, (3) synthesis
of HB(C6F5)2 by treatment of ClB(C6F5)2 with Me2-
SiCl(H). The other route was that the reaction of
B(C6F5)3 with Et3SiH was carried out in benzene at
60 °C for 3 days. The former requires a great deal of
time and skill, while the latter is simple but takes much
time. Thus, a simple and easy preparation of HB(C6F5)2
is desirable for enhancing its utility. Herein we describe
a straightforward generation of bis(pentafluorophenyl)-
borane–dimethyl sulfide complex [HB(C6F5)2ÆSMe2] as a
solution in hexane and an application of it as a mediator
for hydroboration of alk-1-yne with 4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (pinacolborane).
Exchange reactions have been reported between
B(C6F5)3 and other organometallics (main-group metal
compounds).2b,3 We explored the possibility of a prepa-
ration of HB(C6F5)2 employing redistribution between
B(C6F5)3 and borane–dimethyl sulfide complex
(BH3ÆSMe2), both of which are commercially available.
The redistribution reaction was examined in some
solvents, such as hexane, benzene, 1,2-dichloroethane,
CDCl3 and DME, using 11B NMR spectroscopy. When
using hexane as the solvent, the predominant generation
of HB(C6F5)2 was observed. Thus, a mixture of
B(C6F5)3 (0.05 mmol) and BH3ÆSMe2 (0.05 mmol) in
hexane (1 mL) was stirred for 1 h at room temperature
to give a clear liquid and a white solid sticking to the
glass surface of the reaction flask. The 11B NMR spec-
troscopy of the clear liquid revealed only a doublet at
d À10.9 (J = 107 Hz), indicating that HB(C6F5)2 had
been formed in hexane (Fig. 1). The signal of HB(C6F5)2
was found further upfield than that reported in the liter-
ature,2 probably due to the complexation between
HB(C6F5)2 and Me2S. The white solid was dissolved
with CDCl3, and then the solution was analyzed by
11B NMR spectroscopy. The spectrum exhibited three
Keywords: Bis(pentafluorophenyl)borane–dimethyl sulfide complex;
Tris(pentafluorophenyl)borane; Borane–dimethyl sulfide complex;
Hydroboration; Pinacolborane.
*
Corresponding author. Tel./fax: +81 157 26 9403; e-mail: hoshi-m@
chem.kitami-it.ac.jp
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.09.176

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M. Hoshi et al. / Tetrahedron Letters 48 (2007) 8475–8478
À18.8 (J = 107 Hz) as well as a doublet at d À10.9
(J = 107 Hz). The signal of the triplet might be
H2B(C6F5)ÆSMe2 and the signal of the quartet was
assigned to BH3ÆSMe2. Consequently, using an excess
amount of BH3ÆSMe2 was found to be unfavourable
for generating a solution of HB(C6F5)2ÆSMe2 in hexane.
With a straightforward generation of HB(C6F5)2ÆSMe2
in hand, we then investigated its utilization. Srebnik
et al. described that a stoichiometric hydroboration of
alkyne with pinacolborane proceeded very slowly
without using catalyst.5 We have previously reported
that dicyclohexylborane-mediated hydroboration of
alk-1-ynes (1) with pinacolborane proceeds under neat
conditions at room temperature, affording (E)-alk-1-
enylboronic acid pinacol esters (2) in good to excellent
yields.6 Since HB(C6F5)2 hydroborates 1 immediately to
produce the corresponding (E)-alk-1-enylbis(pentafluoro-
phenyl)borane,2 HB(C6F5)2 may well function as a
mediator similar to dicyclohexylborane. We envisioned
a mechanism for HB(C6F5)2ÆSMe2-mediated hydrobor-
ation of 1 with pinacolborane. Thus, hydroboration of
1 with HB(C6F5)2ÆSMe2, generated by the redistribution
between B(C6F5)3 and BH3ÆSMe2, would give (E)-alk-1-
Figure 1. 11B NMR spectrum of the clear liquid after the reaction of
B(C6F5)3 with BH3ÆSMe2 in hexane (160 MHz; BF3ÆOEt2).
signals: a triplet at d À8.0 (J = 127 Hz), a singlet at d
À2.3 and a doublet at d 26.2 (J = 180 Hz) in a ratio of
1:3:1.5 (Fig. 2). The signal of the triplet was assigned
to H2B(C6F5) and the signal of the singlet was in agree-
ment with that of B(C6F5)3. It is recognized that dimeric
dialkylborane displays a signal in the d 25–30 region.4
The signal of the doublet was thus assigned to
HB(C6F5)2, which was dimeric species in solution. It
should be noted that HB(C6F5)2ÆSMe2 can be obtained
conveniently as a solution in hexane, despite the coexis-
tence of other boranes. We also examined the reaction
of B(C6F5)3 (0.05 mmol) with an excess amount of
BH3ÆSMe2 (more than 0.05 mmol) in order to improve
the formation of HB(C6F5)2ÆSMe2. The 11B NMR spec-
troscopy of the reaction mixture (liquid phase) exhibited
a triplet at d À16.2 (J = 114 Hz) and a quartet at d
enylbis(pentafluorophenyl)borane–dimethyl
sulfide
complex, whose alk-1-enyl group would be replaced
with the hydride of pinacolborane to yield product 2
together with regeneration of HB(C6F5)2ÆSMe2, which
would complete the reaction cycle (Scheme 1). This led
us to examine hydroboration of hex-1-yne (1a) with pina-
colborane in a solution of HB(C6F5)2ÆSMe2 in hexane.
We were pleased to find that the hydroboration pro-
ceeded at room temperature to give (E)-hex-1-enylbo-
ronic acid pinacol ester (2a)7 in constant yield after
stirring for 6 h. Accordingly, we probed the optimum
conditions for the reaction of B(C6F5)3 with BH3ÆSMe2
in order to obtain product 2a in high yield. Table 1
shows the results in which the reaction was conducted
varying the amounts and rate of B(C6F5)3 and
BH3ÆSMe2 in hexane at room temperature. As can be
Figure 2. 11B NMR spectrum of the residual white solid after the reaction of B(C6F5)3 with BH3ÆSMe2 in hexane (160 MHz; BF3ÆOEt2).

M. Hoshi et al. / Tetrahedron Letters 48 (2007) 8475–8478
SMe2
8477
B(C6F5)3
+ BH3
O
B
H
R
O
HB(C6F5)2
SMe2
C
C
H
2
HC CR
1
O
O
SMe2
BH
(C6F5)2B
H
H
C
C
R
Scheme 1. Reaction mechanism for HB(C6F5)2ÆSMe2-mediated hydroboration of alk-1-yne with pinacolborane.
Table 1. Optimization of hydroboration of 1a with pinacolborane using HB(C6F5)2ÆSMe2 generated in situ by redistribution between B(C6F5)3 and
a
BH3ÆSMe2
O
BH
O
O
B
H
O
CC4H9-n (1a)
n
-C6H14
HC
C
C
B(C6F5)3
+ BH3
SMe2
C4H9-n
H
r.t., 6 h
r.t.
2a
Entry
B(C6F5)3 (mol %)
BH3ÆSMe2 (mol %)
React. timeb (h)
Yield of 2ac (%)
1
2
3
4
5
6
7
8
9
0
1
2
2
2
2
3
3
3
3
0
1
1
2
3
4
2
3
3
4
0
1
1
1
1
1
1
1
Trace
47
78
83
84
87
87
92
86
0.5
1
10
90
a Reaction conditions: pinacolborane (1 mmol), 1a (1 mmol) and n-C6H14 (1 mL).
b A period of stirring a mixture of B(C6F5)3 and BH3ÆSMe2.
c Conversion yield according to GC analysis.
Table 2. Hydroboration of 1 with pinacolborane in the presence of a catalytic amount of HB(C6F5)2ÆSMe2 generated in situ by redistribution
a
between B(C6F5)3 and BH3ÆSMe2
O
BH
O
O
B
H
R
O
n
-C6H14
CR (1)
r.t., 6 h
HC
C
C
B(C6F5)3
+ BH3 SMe2
H
r.t. 1 h
2
Entry
R
Yield of 2b (%)
1
2
3
4
5
n-Bu
t-Bu
Cl(CH2)3
Cyclohexenyl
Ph
87
95
80
91
83
a The reaction was carried out in n-C6H14 (4 mL) using B(C6F5)3 (0.12 mmol), BH3ÆSMe2 (0.12 mmol), pinacolborane (4 mmol) and an alk-1-yne
(4 mmol).
b Isolated yield.

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M. Hoshi et al. / Tetrahedron Letters 48 (2007) 8475–8478
Foster, K. L.; Beck, V. H.; Piers, W. E. J. Org. Chem.
1999, 64, 4887–4892; (d) Gevorgyan, V.; Liu, J.-X.; Rubin,
M.; Benson, S.; Yamamoto, Y. Tetrahedron Lett. 1999, 40,
8919–8922; (e) Parks, D. J.; Blackwell, J. M.; Piers, W. E.
J. Org. Chem. 2000, 65, 3090–3098; (f) Gevorgyan, V.;
Rubin, M.; Benson, S.; Liu, J.-X.; Yamamoto, Y. J. Org.
Chem. 2000, 65, 6179–6186; (g) Gevorgyan, V.; Rubin, M.;
Liu, J.-X.; Yamamoto, Y. J. Org. Chem. 2001, 66, 1672–
1675; (h) Rubin, M.; Schwier, T.; Gevorgyan, V. J. Org.
Chem. 2002, 67, 1936–1940.
seen from Table 1, using each 3 mol % of B(C6F5)3 and
BH3ÆSMe2 and running the redistribution for 1 h gave
the best result (92% yield) (entry 8). To support our pro-
posed mechanism, we conducted the hydroboration of
1a with pinacolborane in the presence of a catalytic
amount
of
(E)-hex-1-enylbis(pentafluorophenyl)-
borane–dimethyl sulfide complex in place of
HB(C6F5)2ÆSMe2.8 The hydroboration proceeded in the
same way as a result, giving product 2a in a similar yield
to that when HB(C6F5)2ÆSMe2 was employed. This fact
indicates that HB(C6F5)2ÆSMe2-mediated hydroboration
involves the transfer of alk-1-enyl group from boron to
boron and the concomitant transfer of hydride.
2. (a) Parks, D. J.; Spence, R. E. vonH.; Piers, W. E. Angew.
Chem., Int. Ed. Engl. 1995, 34, 809–811; (b) Parks, D. J.;
Piers, W. E.; Yap, G. P. A. Organometallics 1998, 17,
5492–5503.
3. For examples, see: (a) Lee, C. H.; Lee, S. J.; Park, J. W.;
Kim, K. H.; Lee, B. Y.; Oh, J. S. J. Mol. Catal. A: Chem.
1998, 132, 231–239; (b) Walker, D. A.; Woodman, T. J.;
Hughes, D. L.; Bochmann, M. Organometallics 2001, 20,
3772–3776.
4. Soderquist, J. A.; Brown, H. C. J. Org. Chem. 1980, 45,
3571–3578.
5. Pereira, S.; Srebnik, M. Organometallics 1995, 14, 3127–
3128.
Hydroboration of several types of 1, which bear struc-
turally and electronically diverse substituents, with pina-
colborane was carried out under the above optimized
conditions, and the results are summarized in Table 2.
The reactions proceeded smoothly under the optimum
conditions for 1a to provide the corresponding products
29 in high yields.
6. Shirakawa, K.; Arase, A.; Hoshi, M. Synthesis 2004,
1814–1820.
In conclusion, we have found that a solution of
HB(C6F5)2ÆSMe2 in hexane is generated by the redistri-
bution reaction between B(C6F5)3 and BH3ÆSMe2 at
room temperature for 1 h. Moreover, it has been
demonstrated that a stoichiometric hydroboration of 1
with pinacolborane can be promoted by using a catalytic
amount of HB(C6F5)2ÆSMe2 generated in situ to provide
the corresponding products 2 in high yields. It is note-
worthy that not only dicyclohexylborane6,10 but also
HB(C6F5)2ÆSMe2 is capable of transferring an alk-1-enyl
group from boron to boron.
7. Analytical data for 2a agreed very closely with those in the
literature.5
8. After the redistribution reaction between B(C6F5)3
(0.04 mmol) and BH3ÆSMe2 (0.04 mmol) in hexane
(1 mL) at room temperature for 1 h, a solution of
HB(C6F5)2ÆSMe2 in hexane, thus obtained, was trans-
ferred to another reaction flask by syringe under argon. To
the stirred solution was added hex-1-yne (0.08 mmol) at
0 °C, and the reaction mixture was stirred for 1 h at room
temperature to form a solution of (E)-hex-1-enylbis-
(pentafluorophenyl)borane–dimethyl sulfide complex (11B
NMR: d 41.1) in hexane. Hex-1-yne (0.92 mmol) and
pinacolborane (1.00 mol) were added to the solution at
0 °C, and the mixture was stirred for 6 h at room
temperature.
References and notes
9. Analytical data for 2 were closely in agreement with those
in the literature.5
10. (a) Arase, A.; Hoshi, M.; Mijin, A.; Nishi, K. Synth.
Commun. 1995, 25, 1957–1962; (b) Hoshi, M.; Arase, A.
Synth. Commun. 1997, 27, 567–572.
1. For examples, see: (a) Piers, W. E.; Chivers, T. Chem. Soc.
Rev. 1997, 26, 345–354, and references cited therein; (b)
Ishihara, K.; Yamamoto, H. Eur. J. Org. Chem. 1999,
527–538, and references cited therein; (c) Blackwell, J. M.;