Direct conversion of sec-phosphine oxides to sec-phosphine-boranes using BH3

Article abstract of DOI:10.1016/j.tetlet.2021.152837

An effective and mild synthetic method for secondary phosphine-boranes from secondary phosphine oxides was developed. The slow addition of borane-tetrahydrofuran to a solution of secondary phosphine oxides at ambient temperature gave secondary phosphine-boranes with high yield. The use of air-sensitive secondary phosphines, which are common substrates for secondary phosphine-borane syntheses, is avoided in this mild process. Moreover, generation of by-products, such as secondary hydroxy-phosphine-boranes, is also minimized.

Full text of DOI:10.1016/j.tetlet.2021.152837

Tetrahedron Letters 67 (2021) 152837
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Direct conversion of sec-phosphine oxides to sec-phosphine-boranes
using BH3
⇑
Masatoshi Yamada , Mitsutaka Goto, Mitsuhisa Yamano
Process Chemistry, Pharmaceutical Sciences, Takeda Pharmaceutical Company Limited, 17-85, Jusohonmachi 2-chome, Yodogawa-ku, Osaka 532-8686, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
An effective and mild synthetic method for secondary phosphine-boranes from secondary phosphine oxi-
des was developed. The slow addition of borane-tetrahydrofuran to a solution of secondary phosphine
oxides at ambient temperature gave secondary phosphine-boranes with high yield. The use of air-sensi-
tive secondary phosphines, which are common substrates for secondary phosphine-borane syntheses, is
avoided in this mild process. Moreover, generation of by-products, such as secondary hydroxy-phos-
phine-boranes, is also minimized.
Received 5 December 2020
Revised 29 December 2020
Accepted 8 January 2021
Available online 1 February 2021
Keywords:
Ó 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND
license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Secondary phosphine-borane
Secondary phosphine oxide
Borane complex
Introduction
phosphine oxides must have strong reducing activity, such as
DIBAL-H [9], TMDS/Ti(iPrO)4 systems [10], and LiAlH4 [11], due to
Most secondary phosphine-boranes possess good air-stability,
high crystallinity, and availability as an equivalent of secondary
phosphines, which are difficult to handle due to air-sensitivity,
besides many of them being amorphous or liquids.[1] Therefore,
secondary phosphine-boranes are remarkably useful precursors
to synthesize various tertiary phosphine derivatives that are one
of the most important classes of ligands for catalysts in transi-
tion-metal-catalyzed reactions.[2] In fact, a rapidly increasing
number of new phosphine ligands have been prepared via sec-
ondary phosphine-boranes, such as chiral phosphine-phosphite
ligands [3], triphenyl phosphine analogues [4], P-OP ligands [5],
and BINAP analogues [6]. In ligand syntheses, tertiary phosphine-
boranes are often obtained from the secondary phosphine-boranes,
as the corresponding tertiary phosphine ligands are obtained by
the removal of BH3 with an easy operation [7].
Thus, secondary phosphine-boranes have been the target com-
pounds to synthesize in a number of phosphine ligand syntheses. A
great number of synthetic methods for secondary phosphine-bor-
anes are known, however the classic method is the procedure that
uses secondary phosphines or secondary phosphine chlorides, gen-
erally air sensitive or toxic compounds, as starting substrates
(Scheme 1) [8]. Conversely, secondary phosphine oxides are a good
substrate because of their good crystallinity and stability in air.
However, the reducing reagents for reduction of secondary
the high strength of the phosphorus-oxygen double bond.
Imamoto et al. firstly developed a direct conversion from phos-
phine oxides to phosphine-boranes with LiAlH4 in the presence of
CeCl3, and NaBH4 [12]. They reported that phosphine-boranes had
a great utility for the synthesis of chiral phosphine ligands [13]. It
was described that trivalent cerium played dual roles in the reac-
tion. One is for activating the phosphine oxides through coordina-
tion so that deoxygenation can occur with LiAlH4 and another is for
activating NaBH4 to enhance reaction with the intermediate phos-
phines to form phosphine-boranes. Despite these respectable
approaches, a simpler direct conversion method from phosphine
oxides into phosphine-boranes is desirable from an industrial
viewpoint. The direct conversion of five-membered ring phosphine
oxides into the phosphine-boranes was firstly reported with harsh
conditions, such as the use of excess BH3 reagent at high tempera-
ture [14]. Stankevic et al. reported that secondary hydroxy-phos-
phine-boranes, a by-product of reduction, were simultaneously
generated when the weak reducing agent BH3 was used in the
direct conversion of biaryl phosphine oxides [15]. For a solution
to the problem, they found that the selectivity of the conversion
towards the formation of secondary phosphine-boranes could be
improved by the addition of a small amount of water to the reac-
tion mixture [15]. However, such a reaction system is expected
to be fraught with drawbacks, such as a low reproducibility,
because of decomposition of BH3 by water in the reaction mixture
for large scale synthesis. Therefore, we undertook an investigation
aimed at developing an effective synthetic method to produce sec-
ondary phosphine-boranes using a BH3 reagent in mild conditions.
⇑
Corresponding author at: Chemical R&D Division, SPERA PHARMA, Inc., 17-85,
Jusohonmachi 2-chome, Yodogawa-ku, Osaka 532-0024, Japan.
E-mail address: masatoshi.yamada@spera-pharma.co.jp (M. Yamada).
https://doi.org/10.1016/j.tetlet.2021.152837
0040-4039/Ó 2021 The Author(s). Published by Elsevier Ltd.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

M. Yamada, M. Goto and M. Yamano
Tetrahedron Letters 67 (2021) 152837
Scheme 1. Features of secondary phosphine-boranes.
Herein we report our recent progress, which includes the first
selective synthesis protocol for secondary phosphine-boranes by
stoichiometric reduction with BH3 under a controlled slow addi-
tion rate of the BH3 reagent. We believe that our efficient approach
is suitable for the construction of various tertiary phosphine
ligands via the useful preparation of secondary phosphine-boranes.
Fig. 1. Structure of the complex consisting of one molecule of bis(3,5-dimethyl-
phenyl)phosphine oxide (1a) and two molecules of bis(3,5-dimethylphenyl)hy-
droxy-phosphine-borane (3a).
Results and discussion
We chose bis(3,5-dimethylphenyl)phosphine oxide (1a) as the
substrate for an exploratory study, to evaluate the selectivity to
produce bis(3,5-dimethylphenyl)phosphine-borane (2a). It has
been known that the reaction of a secondary phosphine oxide with
BH3 gives two products, the secondary phosphine-borane and the
secondary hydroxy-phosphine-borane [15]. The borane reduction
of 1a was performed at room temperature in toluene with borane
tetrahydrofuran complex (BH3-THF). The HPLC area% ratio of 2a
and 3a in the resulting mixture was 11% and 79%, respectively
(Scheme 2).
The liquid chromatography mass spectrometry (LC/MS) spec-
trum of 3a is shown as Fig. S1 in the Supporting Information. More-
over, we prepared a single crystal from the reaction mixture [16].
The single crystal was identified as a complex crystal, composed
of two molecules of 3a and one molecule of 1a, by single-crystal
X-ray diffraction analysis (Fig. 1).
Through an examination to improve the selectivity for 2a on the
borane reduction of 1a with BH3-THF, we found that the ratio of 2a
to 3a was greatly variable depending on the addition rate of the
BH3-THF solution to the reaction mixture. Therefore, we investi-
gated the influence of the addition rate of BH3-THF solution in
toluene d8 by 31P NMR study and HPLC analysis (Table 1).
The selectivity for 2a was further evident from the 31P NMR
spectra obtained upon mixing 1a with BH3-THF at various addition
rates (Table 1, spectra a-g). When slow addition of BH3-THF solu-
tion (1.00 mmol/min, 2 mL) was carried out, the HPLC yield of 2a
was maintained at least at 94% (Table 1, entries 1–3) and no spec-
trum of 3a was observed by 31P NMR (spectra a-c). With an
increase in addition rate of BH3-THF, a decrease in the selectivity
of 2a became apparent, simultaneous with an increase in the trans-
formation to 3a (Table 1, entries 4–6, spectra d-f). Moreover, the
HPLC yield of 3a was about 50% when the addition rate of BH3-
THF was between 4.00 mmol/min and 12.00 mmol/min. The spec-
trum of 3a was also observed at around d 90 ppm by 31P NMR
(spectra e-g). [17] From these results, we postulate that when con-
ducting fast addition of BH3, the excess BH3 accumulates in the
reaction mixture and the BH3 coordinates with bis(3,5-dimethyl-
phenyl)hydroxy-phosphine, minor tautomer of 1a, leading to the
byproduct 3a being formed preferentially in the kinetically con-
trolled reaction conditions (Scheme 3). Therefore, the optimum
addition rate of the BH3-THF to obtain 2a selectively was deter-
mined to be 0.10–0.12 mmol/min for a 1 mmol scale of 1a (slow
addition method).
We presume that direct transformation of 1a to 2a or 3a pro-
ceeds as shown in Scheme 3. Keglevich et al. have already reported
a mechanistic consideration of borane reduction via phosphine for
cyclic phosphine oxides.[14] Additionally, Stankevic et al. have
reported that at least three equivalents of borane are necessary
for the complete conversion.[15] Therefore, we tried to compare
results with the equivalents of BH3-THF by a 31P NMR study
(Fig. 2).
The 31P NMR study for the reaction was conducted using a reac-
tion mixture of 1a in toluene d8 at room temperature for 0.5 h with
a slow addition method. In the case of using one equivalent BH3,
1a, as the major component, and bis(3,5-dimethylphenyl)phos-
phine (4a), as an intermediate, were observed (Fig. 2 spectrum
c). However, it was confirmed that by applying a slow addition
method, the direct transformation from 1a to 2a could be com-
pleted with 2 equivalents of BH3 without observing the spectra
of 1a and 4a by 31P NMR (Fig. 2, spectrum b). Moreover, when
using three equivalents of BH3, the reaction proceeded without
any problems (Fig. 2, spectrum c). As a result, we demonstrated
that the reaction could be completed using the theoretical equiva-
lents (2 equiv) of BH3, in agreement with Keglevich’s mechanistic
consideration [14].
To investigate the effect of the solvent system, we examined the
borane reduction of 1a under a 4.00 mmol/min addition rate of
BH3, which confirmed the formation of 2a and 3a (Table 2). Nonpo-
lar solvents, toluene and dichloromethane (DCM), showed low
selectivity for 2a and some 1a remained (Table 2, entries 1 and
2). Otherwise, in the polar solvents, tetrahydrofuran (THF) and ace-
tonitrile (MeCN), a moderate selectivity for 2a was observed
(Table 2 entries 3 and 4). Among the polar solvents, MeCN showed
high selectivity (90.5%) for 2a. Therefore, we selected MeCN as the
reaction solvent for the synthesis of 2a.
Scheme 2. Borane reduction of 1a with BH3-THF. aReaction conditions: To
solution of bis(3,5-dimethylphenyl)phosphine oxide (1a, 4 mmol) in toluene was
added 5.0 equiv of 1.0 M borane tetrahydrofuran complex tetrahydrofuran solution
and the mixture stirred at room temperature for 2.5 h.
a
Next, we tried to use other borane reagents, 1.0 M borane
dimethylsulfide complex (BH3-DMS) tetrahydrofuran solution
and 1.0 M borane pyridine complex (BH3-Py) tetrahydrofuran solu-
2

M. Yamada, M. Goto and M. Yamano
Tetrahedron Letters 67 (2021) 152837
Table 1
Results of the reduction of 1a for different BH3-THF addition rates.a
Entry
Addition rate (mmol/min)b
HPLC (area%)c
31P-NMRc
1a
2a
3a
1.4
1.0
3.4
7.3
17.9
50.8
50.7
1
2
3
4
5
6
7
0.10
0.12
1.00
2.00
3.00
4.00
12.00
0.5
0.6
0.4
0.5
0.5
3.3
4.2
95.6
95.5
94.0
89.5
78.9
43.8
42.9
a)
b)
c)
d)
e)
f)
g)
a
Reaction conditions: bis(3,5-dimethylphenyl)phosphine oxide (1a, 1 mmol) was treated with 2.0 equiv of 1.0 M borane tetrahydrofuran complex tetrahydrofuran solution
in toluene d8 (1 mL) at room temperature.
b
Addition rate of BH3-THF by auto syringe pump.
All reaction times were 0.5 h after complete addition of BH3.
c
Scheme 3. Plausible reaction pathway for borane reduction of 1a with BH3-THF.
tion, for the synthesis of 2a by the slow addition method. BH3-DMS
and BH3-Py were prepared from 2.0 M BH3-DMS tetrahydrofuran
solution and 8.0 M BH3 pyridine solutions, respectively. However,
the reaction did not proceed when using BH3-Py (Table 3, entry
4). In the case of using BH3-DMS, the reaction proceeded smoothly
but the required reaction time for completion was longer than for
BH3-THF (Table 3, entries 1–3). Therefore, it was confirmed that
BH3-THF was the optimum reagent for synthesis of 2a.
To explore various substrates for the reaction, we applied it
using various phosphine oxides. The results of the direct transfor-
mation to phosphine-boranes by the BH3-THF slow addition
method are summarized in Table 4.
Fig. 2. 202 MHz 31P NMR spectra of 1a in the presence of 1.0–3.0 equiv. of BH3.
selectivity (Table 4, entries 1–5). On the other hand, when a
tertiary phosphine oxide (triphenyl phosphine oxide) was used
as a substrate in this system, the reaction did not proceed at all.
Conclusion
The various diaryl secondary phosphine oxides (1) transformed
to the corresponding secondary phosphine-boranes (2) with high
In conclusion, a straightforward synthesis of diaryl secondary
phosphine-boranes, starting from secondary phosphine oxides
3

M. Yamada, M. Goto and M. Yamano
Tetrahedron Letters 67 (2021) 152837
Table 2
Results of the reduction of the 1a by BH3-THF in various solvents.a
Entry
Solvent
HPLC (area%)b
1a
2a
3a
1
2
3
4
toluene
DCM
THF
3.3
4.0
0.5
0.8
43.8
45.3
77.6
90.5
50.8
47.5
19.2
3.0
MeCN
a
Reaction conditions: bis(3,5-dimethylphenyl)phosphine oxide (1a, 1 mmol) was treated with 2.0 equiv of 1.0 M borane tetrahydrofuran complex tetrahydrofuran solution
at room temperature. The 1.0 M borane tetrahydrofuran complex tetrahydrofuran solution was added to the 1a solution at a 4.00 mmol/min addition rate using an auto
syringe pump.
b
All reaction times were 0.5 h after complete addition of BH3.
Table 3
Results of the reduction of 1a by various borane reagents.a
Entry
BH3 reagent
Time (min)
HPLC (area%)
1a
2a
3a
1
2
3
4
BH3-THF
BH3-DMS
BH3-DMS
BH3-Py
30
30
100
30
n.d.b
11.7
n.d.b
98.6
96.3
83.6
94.2
n.d.b
0.4
1.4
1.5
n.d.b
a
Reaction conditions: bis(3,5-dimethylphenyl)phosphine oxide (1a, 1 mmol) was treated with 2.0 equiv. of the indicated 1.0 M borane reagent at room temperature. The
indicated 1.0 M borane reagent was added to the 1a solution at an addition rate of 0.10 mmol/min using an auto syringe pump.
b
No detection.
Table 4
Result of the reduction of the various phosphine oxides by BH3-THF.a
Entry
R
HPLC (area%)b
Yield (%)c
1
2
3
1
2
3
4
5
3,5-dimethylphenyl
phenyl
4-methylphenyl
4-methoxyphenyl
1-naphthyl
n.d.d
n.d.d
n.d.d
n.d.d
n.d.d
94.6
95.2
94.9
97.3
92.6
0.6
95.3 (2a)
85.8 (2b)
85.7 (2c)
85.0 (2d)
87.1 (2e)
1.4
0.4
n.d.d
1.5
a
Reaction conditions: The desired phosphine oxides (1, 1 mmol) were treated with 2.0 equiv. of 1.0 M borane tetrahydrofuran complex tetrahydrofuran solution at room
temperature. The 1.0 M borane tetrahydrofuran complex tetrahydrofuran solution was added to the solution of 1 at a 0.10 mmol/min addition rate using an auto syringe
pump.
b
All reaction times were 0.5 h after complete addition of BH3.
Yield is isolated yield after column chromatography.
No detection.
c
d
(1), with high selectivity has been found and a practical synthetic
method has been developed. It has provided secondary
phosphine-boranes (2) with excellent yield, suppressing the gener-
ation of secondary hydroxy-phosphine-boranes (3) by using a slow
addition of BH3-THF. In further work, we intend to use this
methodology to construct various chiral/achiral phosphine ligands
from secondary phosphine oxides (1).
Note
The research described was totally carried out at the former
Takeda Pharmaceutical Company Limited (TAKEDA), Department
of Process Chemistry, 17-85, Jusohomachi 2-chome, Yodogawa-
ku, Osaka 532-8686, Japan and funded by TAKEDA. Currently,
Masatoshi Yamada and Mitsuhisa Yamano belong to the Chemical
4

M. Yamada, M. Goto and M. Yamano
Tetrahedron Letters 67 (2021) 152837
phosphine oxide 1a (1.03g, 4.00 mmol) was. The reaction mixture was stirred
at room temperature for 2.5 h and purification by column chromatography on
silica gel (toluene, then EtOAc) was performed to give a white powder. The
powder was dissolved in tetrahydrofuran. The solvent was evaporated under
atmospheric pressure to give colorless crystals. Crystal structure: see
Supporting Information (CCDC963954).
R&D Division, SPERA PHARMA, Inc., 17-85, Jusohonmachi 2-chome,
Yodogawa-ku, Osaka 532-0024, Japan. Mitsutaka Goto belong to
Oncology and External Supply Small Molecule Quality, API, Global
Quality, TAKEDA, 17-85, Jusohonmachi 2-chome, Yodogawa-ku,
Osaka 532-8686, Japan.
[17] M. Stankevic, M.K. Pietrusiewicz, Synthesis 8 (2005) 1279–1290.
[19] M. Yamada, M. Goto, M. Yamano, JP Patent JP 2003140671, 2003.
The authors declare no competing financial interest.
Declaration of Competing Interest
Further reading
[18] General Information and Methods: All solvents for general isolated products
and chromatography were reagent grade. 1.0 M borane tetrahydrofuran
complex tetrahydrofuran solution, 2.0 M borane dimethylsulfide complex
tetrahydrofuran solution, and 8M borane pyridine complex were purchased
from Sigma-Aldrich Corporation. Diphenyl phosphine oxide (1b) was
purchased from Tokyo Chemical Industry Co., Ltd. Triphenyl phosphine
oxide was purchased from Wako Pure Chemical Industries, Ltd. Bis(3,5-
dimethylphenyl)phosphine oxide (1a), bis(4-methylphenyl)phosphine oxide
(1c), bis(4-methoxyphenyl)phosphine oxide (1d), and dinaphthylphosphine
oxide (1e) were synthesized based on literature procedures. [19] Chromatog-
raphy was performed using Silicagel 60 (20-230 mesh) with toluene. All
reactions were monitored by HPLC on an ODS (Octadecylsilyl) column:
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgments
We thank to the following TAKEDA colleagues, Mr. Kokichi
Yoshida, Mr. Hideo Hashimoto, and Dr. David Cork for helpful dis-
cussions, Ms. Keiko Higashikawa and Mr. Mitsuhiro Nishitani for
help with the X-ray single-crystal structure analysis, Mr. Kenichi
Oka and Ms. Yoko Fujioka for help with LC/MS analyses.
Phenomenex KinetexÒ
5lm C18 100A (conditions: ultraviolet (UV) detector
278 nm, mobile phase 1 vol.% aq. acetic acid/acetonitrile (HPLC grade)=25
vol/75 vol, flow 1.0 mL/min). NMR spectra were recorded at 500 MHz(1H),
126 MHz (13C) and 202 MHz (31P) on a Bruker DPX-500 spectrometer.
Chemical shifts are given in ppm (d) and coupling constants (J) are recorded
in hertz (Hz). LC/MS analysis for hydroxy-phosphine-boranes (3) was
performed on an electrospray ionization (ESI) mass spectrometer with UV
detection. Representative Procedure for the Preparation of secondary phos-
phine-boranes (2a-2e): Bis(3,5-dimethylphenyl)phosphine oxide 1a (259 mg,
1.00 mmol) was charged to a three necked round bottom flask, which was
then evacuated and filled back with nitrogen three times. Under the nitrogen
atmosphere, acetonitrile (1 mL) was added to the flask, and then 1.0 M borane
tetrahydrofuran complex tetrahydrofuran solution (2 mL, 2.00 mmol) was
added to the stirred solution of 1a at a constant rate (0.10 mmol/min) at room
temperature. The reaction mixture was stirred at room temperature for 0.5 h
and was purified by column chromatography on silica gel (toluene) to afford
244 mg (95% yield, white solid) of bis(3,5-dimethylphenyl)phosphine-borane
2a. 1H NMR (500 MHz, toluene-d8) d 7.29-7.31 (m, 4H), 6.72 (s, 2H), 5.96 (dq,
JPH = 375 Hz, 1H), 1.99 (s, 12H), 1.40-2.00 (brm, 3H). 13C NMR (126 MHz,
toluene-d8) d 138.43, 138.35, 132.92, 130.59, 126.97, 20.56. 31P NMR (202
MHz, toluene-d8) d 2.40 (m). Diphenylphosphine-borane (2b): Yield, 86%
(176 mg, colorless oil). 1H NMR (500 MHz, toluene-d8) d 7.43-7.45 (m, 5H),
6.95-6.99 (m, 5H), 5.80 (dq, JPH = 376 Hz, 1H), 1.48-2.01 (brm, 3H). 13C NMR
(126 MHz, toluene-d8) d 132.88, 130.92, 128.56, 126.87. 31P NMR (202 MHz,
toluene-d8) d 1.54 (m). Bis(4-methylphenyl)phosphine-borane (2c): Yield,
86% (196 mg, white solid). 1H NMR (500 MHz, toluene-d8) d 7.41-7.45 (m,
4H), 6.84-6.85 (m, 4H), 5.90 (dq, JPH = 375 Hz, 1H), 1.54-2.05 (brm, 3H). 13C
NMR (126 MHz, toluene-d8) d 141.29, 141.27, 132.89, 132.81, 129.49, 129.41,
124.19, 123.73, 20.81. 31P NMR (202 MHz, toluene-d8) d-0.16 (m). Bis(4-
methoxyphenyl)phosphine-borane (2d): Yield, 85% (222 mg, white solid). 1H
NMR (500 MHz, toluene-d8) d 7.43-7.48 (m, 4H), 6.61-6.62 (m, 4H), 5.94 (dq,
JPH = 374 Hz, 1H), 1.55-2.11 (brm, 3H). 13C NMR (126 MHz, toluene-d8) d
162.14, 162.11, 134.46, 118.34, 117.85, 114.46, 114.36, 54.34. 31P NMR (202
MHz, toluene-d8) d -2.57 (m). Di-1-naphthylphosphine-borane (2e): Yield,
87% (304 mg, white solid). 1H NMR (500 MHz, toluene-d8) d 8.20-8.23 (m,
2H), 7.42-7.51 (m, 8H), 7.17-7.24 (m, 4H), 6.18 (dq, JPH = 376 Hz, 1H), 1.82-
2.15 (brm, 3H). 13C NMR (126 MHz, toluene-d8) d 134.61, 133.03, 132.94,
128.62, 128.45, 127.72, 127.67, 126.67, 124.44, 123.99. 31P NMR (202 MHz,
toluene-d8) d 2.20 (m). Representative Procedure for the Preparation of
secondary hydroxy-phosphine-boranes (3a-3e): Bis(3,5-dimethylphenyl)
phosphine oxide 1a (254 mg, 1.00 mmol) was charged to a three necked
round bottom flask, which was then evacuated and filled back with nitrogen
three times. Under the nitrogen atmosphere, acetonitrile (1 mL) was added to
the flask, and then 1.0 M borane tetrahydrofuran complex tetrahydrofuran
solution (2 mL, 2.00 mmol) was added to the stirred solution of 1a at a
constant rate (4.00 mmol/min) at room temperature. The reaction mixture
was then stirred at room temperature for 0.5 h and analyzed by LC/MS to
confirm bis(3,5-dimethylphenyl)hydroxy-phosphine-borane 3a. LC/MS (ESI):
m/z [M-H]- calcd for C16H22BOP 271.1429, found 271.1428. Diphenylhy-
droxy-phosphine-borane (3b): LC/MS (ESI): m/z [M-H]- calcd for C12H14BOP
215.0803, found 215.0798. Bis(4-methylphenyl)hydroxy-phosphine-borane
(3c): LC/MS (ESI): m/z [M-H]- calcd for C14H18BOP 243.1116, found
243.1109. Bis(4-methoxyphenyl)hydroxy-phosphine-borane (3d): LC/MS
(ESI): m/z [M-H]- calcd for C14H18BO3P 275.1014, found 275.1008. Di-1-
naphthylhydroxy-phosphine-borane (3d): LC/MS (ESI): m/z [M-H]- calcd for
C20H18BOP 315.1116, found 315.1106.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2021.152837.
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was evacuated and filled back with nitrogen three times. Under the nitrogen
atmosphere, 1.0 M borane tetrahydrofuran complex tetrahydrofuran solution
(20 mL, 20.0 mmol) was added to the flask, and then bis(3,5-dimethylphenyl)
5