A new and convenient approach for the synthesis of P-stereogenic intermediates bearing a tert-butyl(methyl)phosphino group

Article abstract of DOI:10.1007/s11164-017-2923-6

Chiral organophosphorus compounds possessing a P-stereogenic center have been widely used as agricultural chemicals, pharmaceuticals, organocatalysts, and ligands for transition-metal catalysis. P-Stereogenic intermediates bearing a tert-butyl(methyl)phosphino group are important for the preparation of several commonly used diphosphine ligands. However, previously reported synthetic methods used hazardous starting materials and are difficult to operate. In order to overcome these limitations, a new and convenient synthesis for a number of P-stereogenic intermediates possessing a tert-butyl(methyl)phosphino group has been developed. The reported route relies on an air- and moisture-stable secondary phosphine oxide prepared from a readily available starting material, trichlorophosphane.

Full text of DOI:10.1007/s11164-017-2923-6

Res Chem Intermed
DOI 10.1007/s11164-017-2923-6
A new and convenient approach for the synthesis of
P-stereogenic intermediates bearing a tert-
butyl(methyl)phosphino group
1
2
1
•
Yang Liu
Zhenfeng Zhang
•
Boqiang Ding
1
•
Delong Liu
1
1,2
•
Yangang Liu
•
Wanbin Zhang
Received: 8 January 2017 / Accepted: 24 February 2017
Ó Springer Science+Business Media Dordrecht 2017
Abstract Chiral organophosphorus compounds possessing a P-stereogenic center
have been widely used as agricultural chemicals, pharmaceuticals, organocatalysts,
and ligands for transition-metal catalysis. P-Stereogenic intermediates bearing a
tert-butyl(methyl)phosphino group are important for the preparation of several
commonly used diphosphine ligands. However, previously reported synthetic
methods used hazardous starting materials and are difficult to operate. In order to
overcome these limitations, a new and convenient synthesis for a number of
P-stereogenic intermediates possessing a tert-butyl(methyl)phosphino group has
been developed. The reported route relies on an air- and moisture-stable secondary
phosphine oxide prepared from
trichlorophosphane.
a
readily available starting material,
Keywords P-stereogenic Á Organophosphorus Á Ligand Á Phosphine borane Á
Secondary phosphine oxide
Introduction
Chiral organophosphorus compounds possessing a P-stereogenic center have
attracted considerable attention due to their increasingly common use as agricultural
chemicals, pharmaceuticals, organocatalysts, and especially for their application as
ligands for transition-metal catalysis [1–5]. Since the pioneering work of Knowles
&
&
Zhenfeng Zhang
zhenfeng@sjtu.edu.cn
Wanbin Zhang
wanbin@sjtu.edu.cn
1
2
School of Pharmacy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, China
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800
Dongchuan Road, Shanghai, China
123

Y. Liu et al.
and his discovery of diphosphine DiPAMP [6], various P-stereogenic ligands have
been developed with different P-containing groups and backbones [7]. However, it
was not until 30 years had passed that the next type of promising P-stereogenic
ligands possessing a unique (tert-butyl)methylphosphino group were developed.
The large discrimination between the tert-butyl and methyl groups provided these
ligands with powerful stereocontrol, while the electron-rich properties of the dialkyl
substituents greatly improved catalytic activity. Using BisP* as a foundation,
Imamoto et al. have developed a series of ligands bearing different backbones such
as MiniPhos, TrichickenfootPhos (TCFP), MaxPhos, QuinoxP*, BenzP*, 3H-
QuinoxP*, and so on (Fig. 1). These ligands are able to coordinate with a variety of
different transition-metals and have been successfully applied in many asymmetric
catalyses such as hydrogenations, hydrosilylations, allylic substitutions, hydroacy-
lations, borylations, conjugated additions, cross-couplings, etc. [8–17].
In this respect, significant efforts have been made for their enantioselective
preparation (Fig. 1). Initially, the ligands BisP*, MiniPhos and TCFP were
synthesized using an asymmetric desymmetrization of an intermediate dimethyl-
substituted phosphine borane [8–10, 13]. A s-BuLi-(-)-sparteine complex was used
stoichiometrically and is only available as one enantiomer. The MaxPhos ligand
bearing an N-atom connection between the two phosphino groups was synthesized
via a key aminophosphine borane intermediate [14]. Obviously, this method cannot
be extended to the preparation of other ligands with different backbones. In order to
solve this problem, a more general intermediate, (tert-butyl)methylphosphine
borane, was synthesized and applied in the preparation of QuinoxP*, BenzP*, and
Fig. 1 Synthesis of P-stereogenic diphosphine ligands
1
23

A new and convenient approach for the synthesis of…
3
H-QuinoxP* [11, 12, 15–17]. However, a very flammable starting material, PH3,
was used, and so the reactions are hazardous to conduct. Therefore, a more
convenient operation for the large-scale synthesis of (tert-butyl)methylphosphine
borane, as well as other P-stereogenic intermediates possessing a tert-
butyl(methyl)phosphino group, is still greatly desired.
Recently, we have been interested in the application of P-stereogenic ligands
bearing a unique (tert-butyl)methylphosphino group [18–28]. The demand to
develop new ligands prompted us to search for a more efficient method for the
synthesis of (tert-butyl)methylphosphine borane and related intermediates. Inspired
by the stability of secondary phosphine oxides to air and moisture [29–32], we have
recently developed a new and convenient route for the synthesis of (tert-
butyl)methylphosphine boranes using such an intermediate from the commonly
used starting material trichlorophosphane. In addition, some other P-stereogenic
intermediates such as (tert-butyl)methylphosphine oxide and tert-
butyl(methyl)phosphinous acid borane can be easily prepared.
Experiment
1
Commercially available reagents were used without further purification. H NMR
1
3
400 MHz), C NMR (101 MHz) and P NMR (162 MHz) spectra were recorded
31
(
on a Varian MERCURY plus-400 spectrometer.
Tert-butyldichlorophosphane (2)
To a dried flask containing trichlorophosphane (40 mL, 0.46 mol, 2.5 eq) was
added dropwise tert-butylmagnesium chloride in THF (1.0 M, 180 mL, 0.18 mol).
The reaction mixture was maintained at 20–30 °C during addition and stirred at
room temperature for a further 24 h. After filtration under an atmosphere of
nitrogen, the volatile solvent and unreacted trichlorophosphane were evaporated.
The desired product 2 was obtained by distillation under nitrogen atmosphere as a
colorless solid (14.6 g, 51% yield).
1
H NMR (400 MHz, CDCl ): d (ppm) = 1.23 (d, J = 14.8 Hz, 9H, C(CH3)3);
3
PH
13
3
1
P NMR (162 MHz, CDCl ): d (ppm) = 200.0 (s); C NMR (100 MHz, CDCl3):
3
d (ppm) = 24.4 (CH ), 23.8 (d, J = 20.9 Hz, C(CH3)3).
3
PC
Ethyl tert-butylphosphinate (3)
To a solution of tert-butyldichlorophosphane (10.8 g, 67.9 mmol) in THF (50 mL)
was added anhydrous ethanol (75 mL) at room temperature. After completion of the
addition, the reaction mixture was stirred at room temperature for 12 h. The volatile
solvent was removed by a rotary evaporator and the residue was distilled under
reduced pressure to obtain the desired product 3 as a colorless oil (8.6 g, 84% yield).
1
H NMR (400 MHz, CDCl ): d (ppm) = 6.71 (d, J = 514.4 Hz, 1H, PH),
3
PH
4
.24–4.02 (m, 2H, OCH ), 1.34 (t, J = 7.2 Hz, 3H, CH CH ), 1.11 (t,
2 HH 2 3
3
1
JPH = 17.6 Hz, 9H, C(CH3)3); P NMR (162 MHz, CDCl ): d (ppm) = 50.5 (s).
3
123

Y. Liu et al.
1
3
C NMR (100 MHz, CDCl ): d (ppm) = 62.4 (d, J = 7.7 Hz, CH CH ), 30.8 (d,
3
PC
2
3
JPC = 96.0 Hz, C(CH ) ), 22.5 (d, JPC = 1.4 Hz, C(CH ) ), 16.1 (d, J = 5.6 Hz,
CH CH ).
3 3
3 3
PC
2
3
Tert-butyl(methyl)phosphine oxide (4)
To a solution of ethyl tert-butylphosphinate (6.0 g, 40 mmol) in Et O (60 mL) was
2
added methylmagnesium iodide in Et O (1 M, 40 mL, 1.0 eq) at 10 °C. After
2
completion of the addition, the reaction mixture was gradually raised to 20 °C and
stirred for a further 4 h. After cooling to 0–5 °C, the reaction mixture was treated
with an aqueous solution (10 mL) of K CO (11 g, 80 mmol, 2.0 eq). After
2
3
filtration, the filtrate was evaporated to obtain the desired product 4 as a colorless
solid (2.9 g, 60% yield).
1
H NMR (400 MHz, CDCl ): d (ppm) = 6.75 (dq, J = 224 Hz, JHH = 3.6
3
PH
Hz, 1H, PH), 1.58 (dd, JPH = 13.2 Hz, J = 3.6 Hz, 3H, PCH ), 1.17 (d,
3
HH
3
1
J
= 16.8 Hz, 9H, C(CH3)3); P NMR (162 MHz, CDCl ): d (ppm) = 48.6 (s);
PH
3
1
3
C NMR (100 MHz, CDCl ): d (ppm) = 30.6 (d, J = 71.4 Hz, C(CH ) ), 23.7
3
PC
3 3
(
d, JPC = 3.0 Hz, C(CH ) ), 9.7 (d, JPC = 62.2 Hz, PCH3).
3 3
Tert-butyl(methyl)phosphine borane (5)
To a solution of tert-butyl(methyl)phosphine oxide (3.0 g, 25 mmol) was added
dropwise a solution of BH ÁTHF (1 M, 75 mL, 3.0 eq) at 0 °C. The reaction
3
mixture was raised to reflux temperature and stirred for 12 h. After careful removal
of the volatile solvent, the residue was purified by flash chromatography on silica
gel using hexane/dichloromethane (5:1) as the eluent to give tert-
butyl(methyl)phosphinous acid borane 5 as a colorless solid (2.8 g, 83% yield)
and tert-butylmethylphosphine borane 6 as a colorless solid (0.3 g, 10% yield).
1
H NMR (400 MHz, CDCl ): d (ppm) = 1.43 (d, J = 8.8 Hz, 3H, PCH ), 1.15
3
PH
3
3
1
(
(
d, JPH = 14.2 Hz, 9H, PC(CH ) ); P NMR (162 MHz, CDCl ): d (ppm) = 120.7
q, JPB = 68 Hz); C NMR (100 MHz, CDCl ): d (ppm) = 31.5 (d, J = 40.4
3 3
3
13
3
PC
Hz, C(CH ) ), 24.8 (d, J = 4.0 Hz, C(CH ) ), 12.1 (d, J = 36.8 Hz, PCH3).
3
3
PC
3 3
PC
(
R)-tert-butyl(methyl)phosphine borane ((R)-6)
To a solution of tert-butyl(methyl)phosphine borane (1.2 g, 10 mmol) in THF
5 mL) was added dropwise a solution of n-BuLi in hexane (1.6 M, 6.3 mL,
0 mmol, 1.0 eq) at 0 °C. After stirring for 0.5 h, (R)-a-methylbenzylisocyanate
1.5 g, 10 mmol, 1.0 eq) was slowly added to the above solution at 0 °C. The
(
1
(
reaction mixture was warmed to room temperature and stirred for 12 h. After
quenching with water, the solution was extracted with hexane (30 mL 9 3). The
organic phases were combined, washed with brine, dried over Na SO , and
2
4
evaporated. The residue was precipitated to give a white solid by the addition of
hexane to its solution of ethyl acetate. The solid (0.93 g, 3.4 mmol) was dissolved in
DMF (5 mL) and treated with a solution of KOH (38 mg, 0.7 mmol, 0.2 eq) in
methanol (4 mL). The reaction mixture was stirred at room temperature for 12 h.
1
23

A new and convenient approach for the synthesis of…
Water (10 mL) and an aqueous solution of HCl (0.5 M, 1 mL) were added and the
reaction mixture was extracted with hexane (20 mL 9 5). The combined organic
phase was washed with brine, dried over Na SO , and evaporated. After filtration
2
4
and evaporation, the residue was purified by flash chromatography on silica gel
using hexane/dichloromethane (5:1) as the eluent to give (R)-tert-butylmethylphos-
phine borane (R)-6 as a colorless solid (0.25 g, 21% yield for two steps).
1
H NMR (400 MHz, CDCl ): d (ppm) = 4.42 (dm, J = 355 Hz, 1H, PH), 1.31
3
PH
(
dd, JPH = 10.8 Hz, J = 6.4 Hz, 3H, PCH ), 1.22 (dd, J = 14.4 Hz, 4.8 Hz,
HH 3 PH
3
1
13
9
H, C(CH ) ); P NMR (162 MHz, CDCl ): d (ppm) = 12.7 (bs); C NMR
3
3
3
(
JPC = 34.4 Hz, C(CH ) ), 2.1 (d, JPC = 35.2 Hz, PCH3).
100 MHz, CDCl ): d (ppm) = 26.1 (d, JPC = 2.4 Hz, C(CH ) ), 25.8 (d,
3
3 3
3 3
(
S)-tert-butyl(methyl)phosphine borane ((S)-6) can also be obtained by using (S)-
a-methylbenzylisocyanate, which is also commercially available. The ee of tert-
butyl(methyl)phosphine borane was determined by HPLC using a chiral column
after converting to a benzyl derivative.
(S)-2-(tert-butyl(methyl)phosphanyl)-3-(di-tert-butylphosphanyl)quinoxaline (7)
To a solution of (R)-tert-butylmethylphosphine borane ((R)-6) (0.36 g, 3.0 mmol,
.7 eq) in THF (5 mL) was added n-BuLi in hexane (1.6 M, 2.0 mL, 3.2 mmol,
.8 eq) at 0 °C. After stirring for 0.5 h, the solution was slowly added to a
2
2
suspension of 2-di-tert-butylphosphino)-3-chloroquinoxaline (0.35 g, 1.1 mmol) in
DMF (5 mL) at 0 °C, and stirred for a further 4 h. After addition of water (10 mL),
the mixture was extracted with ethyl acetate (15 mL 9 2). The combined organic
phases were washed with brine (20 mL) and evaporated to give a yellow pasty oil,
which was treated with TMEDA (3 mL) in ethyl acetate (5 mL) for 1 h at room
temperature. After evaporation, the residue was purified by flash chromatography on
silica gel using hexane/ethyl acetate (30:1) as the eluent to afford (S)-2-(tert-
butylmethylphosphino)-3-(di-tert-butylphosphino)quinoxaline (0.39 g, 91% yield)
as a yellow solid. The pure compound was obtained by recrystallization from ethyl
acetate/methanol (1/5).
1
H NMR (400 MHz, CDCl ): d (ppm) = 8.10–8.00 (m, 2H, ArH), 7.75–7.63 (m,
3
2
C(CH ) ), 1.20 (d, J = 11.6 Hz, 9H, C(CH ) ), 1.16 (d, J = 12.0 Hz, 9H,
H, ArH), 1.42 (d, JPC = 5.2 Hz, 3H, PCH ), 1.35 (d, J = 11.8 Hz, 9H,
3 PC
3
3
PC
3 3
PC
3
1
C(CH ) ); P NMR (162 MHz, CDCl ): d (ppm) = 22.2 (d, J = 106 Hz,
3
3
3
PP
P(C(CH ) ) ), -14.0 (d, J = 106 Hz, P(C(CH ) )CH ).
3
3 2
PP
3 3
3
Result and discussion
Starting material availability is very important for a large-scale procedure. A more
common and stable compound, trichlorophosphane (1), was chosen instead of the
hazardous phosphane which has been used in the previously reported methods [33].
However, it also creates a problem concerning the selectivity of the three P–Cl
bonds. Therefore, the bulky tert-butyl group was substituted prior to the methyl
group by using the commercially available Grignard reagent tert-butylmagnesium
123

Y. Liu et al.
chloride. Besides the THF solvent present in the Grignard reagent, no other
additional solvent was added. The Grignard reagent can also be obtained by treating
tert-butyl chloride with magnesium, which significantly reduces the cost. Moreover,
the tert-butyldichlorophosphane (2) can be purified by distillation. Since the
product’s melting point is only slightly lower than room temperature and it boils
readily, an air condenser was utilized and heating of the distillate from outside the
condenser was needed. The product is highly sensitive to moisture because of the
two reactive P–Cl bonds, therefore this step must be carried out under a nitrogen
atmosphere. Subsequently, an additional step to synthesize ethyl tert-butylphosphi-
nate (3) was carried out to avoid the over-substitution of the methyl group. We made
use of this simple but efficient method to obtain the phosphinate in high yield. It is
worth mentioning that, even when excess ethanol was used, only the desired product
was obtained, accompanied by the release of hydrogen chloride. Although
methylation with methylmagnesium bromide gave poor results, the methylation
with methylmagnesium iodide gave an excellent yield of the desired product. After
workup and filtration, the desired tert-butyl(methyl)phosphine oxide (4) was
obtained in good yield, and no further optimization was carried out. According to
reported work, converting the protecting group from oxygen to borane may be
possible using LiAlH –NaBH –CeCl and (COCl) –NaBH systems [34, 35].
4
4
3
2
4
Indeed, reduction together with the boronation were completed in one step using a
single reagent, a BH ÁTHF complex [36]. The desired tert-butyl(methyl)phosphine
3
Scheme 1 Preparation and application of the P-stereogenic intermediates
1
23

A new and convenient approach for the synthesis of…
borane (6) was obtained in racemic form and in poor yield, while another important
intermediate, tert-butyl(methyl)phosphinous acid borane (5), was obtained in high
yield (Scheme 1). Differing from the phosphine oxide (4) and phosphine borane (6)
which act as P-nucleophiles, the phosphinous acid borane (5) can be easily activated
by sulfonylation to act as a P-electrophile. Compound 5 was further reduced to 6 in
good yield according to a reported method [31, 37].
In order to obtain the optically pure tert-butyl(methyl)phosphine borane, a
chemical resolution was conducted according to a literature procedure [33]. After
treatment with (S)-(1-isocyanatoethyl)benzene, the racemate was converted to two
diastereoisomers which can be separated by choromatography. The (R)-tert-
butyl(methyl)phosphine borane was finally prepared by removing the acyl group
under hydrolysis conditions. The enantioisomer with (S)-configuration can also be
obtained by using (R)-(1-isocyanatoethyl)benzene which is commercially available.
Substitution of (R)-6 with 2-chloro-3-(di-tert-butylphosphanyl)quinoxaline afforded
the useful ligand, (S)-3H-QuinoxP* (7), in good yield [17] (Scheme 1).
Conclusion
Using an easy-to-operate procedure, some important and versatile P-stereogenic
intermediates bearing a tert-butyl(methyl)phosphino group, either in racemic or
optically pure form, were synthesized from simple and accessible starting materials
and reaction reagents.
Acknowledgements This work was partially supported by the National Natural Science Foundation of
China (No. 21572131). We gratefully acknowledge Prof. Tsuneo Imamoto and Dr. Masashi Sugiya for
helpful discussion. We also thank the Instrumental Analysis Center of SJTU for characterization.
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