Selective P-C bond cleavage of tertiary phosphine boranes by sodium

Article abstract of DOI:10.1080/10426507.2021.1948851

Herein reported is the facial modification of tertiary phosphine boranes R₃PBH₃ by selective cleaving the P-Ph bond by sodium in which the phosphide borohydride R₂PNa(BH₃) is quantitatively generated and could be easily quenched by electrophiles to furnish a series of new phosphine boranes in high yields.

Full text of DOI:10.1080/10426507.2021.1948851

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gpss20
Selective P-C bond cleavage of tertiary phosphine
boranes by sodium
Jian-Qiu Zhang, Jing-Jing Ye & Li-Biao Han
To cite this article: Jian-Qiu Zhang, Jing-Jing Ye & Li-Biao Han (2021): Selective P-C bond
cleavage of tertiary phosphine boranes by sodium, Phosphorus, Sulfur, and Silicon and the Related
Elements, DOI: 10.1080/10426507.2021.1948851
To link to this article: https://doi.org/10.1080/10426507.2021.1948851
View supplementary material
Published online: 08 Jul 2021.
Submit your article to this journal
Article views: 27
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
https://www.tandfonline.com/action/journalInformation?journalCode=gpss20

PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
https://doi.org/10.1080/10426507.2021.1948851
SHORT COMMUNICATION
Selective P-C bond cleavage of tertiary phosphine boranes by sodium
Jian-Qiu Zhanga,b , Jing-Jing Yea,b, and Li-Biao Hana,b
b
aNational Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan; Division of Chemistry, Faculty of Pure
and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki, Japan
ABSTRACT
ARTICLE HISTORY
Received 8 March 2021
Accepted 23 June 2021
Herein reported is the facial modification of tertiary phosphine boranes R3PBH3 by selective cleav-
ing the P-Ph bond by sodium in which the phosphide borohydride R2PNa(BH3) is quantitatively
generated and could be easily quenched by electrophiles to furnish a series of new phosphine
boranes in high yields.
KEYWORDS
Phosphine borane; C-P
bond cleavage; sodium
GRAPHICAL ABSTRACT
Introduction
and directly obtained from Ph3PꢀBH3. Although the metallation
of phenylphosphine boranes R2PhP(BH3) with lithium and
potassium generating phosphide boranes R2P-(BH3)Mþ (M ¼
Li, K) has been reported,[13–15] employing the much cheaper
and readily available sodium is a better choice. Thus, in this
protocol, we report a direct modification of R2PhP(BH3) with
sodium, followed by the addition of alkyl halides, enabling
access to a series of new phosphine boranes in good to excel-
lent yields.
Tertiary phosphines are extensively applied as ligands in transi-
tion metal-catalyzed reactions,[1] as organocatalysts in synthetic
chemistry[2] and as reducing reagents in Staudinger,[3]
Mitsunobu,[4] Wittig reaction,[5] etc. However, phosphines R3P
inherently bearing a lone pair of electrons at the phosphorus
atom, in particular alkyl functionalized phosphines, are air-sen-
sitive and easily oxidized to the corresponding phosphine
oxides, leading to great difficulties in handling and purification
of these compounds.[6] Conversion of the phosphines to the
corresponding air-stable phosphine boranes (R3PꢀBH3) upon
treatment with a borane reagent (BH3ꢀTHF or BH3ꢀSMe2) is a
general and efficient strategy to slow down their oxidation,
since the borane group is also easily removed by a base.[7–9]
Based on our previously established works on the P-Ph bond
cleavage of triphenylphosphine oxide Ph3P(O) and triphenyl-
phosphine Ph3P by sodium dispersion (sodium finely dispersed
in paraffin oil with lm-scale sizes, hereafter abbreviated as
SD), the corresponding reactive intermediate Ph2P(ONa) and
Ph2PNa was generated, respectively. And finally a range of
new phosphine oxides Ph2P(O)R and phosphines Ph2PR were
easily obtained after quenching by electrophiles RX (Scheme
1A).[10–12] Thus, we wondered if this strategy would be also
applied to triphenylphosphine borane Ph3PꢀBH3 to react with
sodium to generate the corresponding intermediate sodium
phosphide boranes Ph2PNa(BH3) via selective P-Ph bond
cleavage (Scheme 1B). Since Ph3PꢀBH3 is a common and inex-
pensive phosphine borane, if the above reaction proceeds read-
ily, the other functionalized phosphine boranes that are
Results and Discussion
The study began by treating 0.2 mmol of triphenylphosphine
borane (Ph3PꢀBH3) 1a with 0.42 mmol of SD at room tem-
perature, the solution immediately changed to brown. After
stirring the mixture for 2 h, as monitored by 31P NMR spec-
tra (Scheme 2), the original signal at d 21.6 (JB-P ¼ 53.5 Hz)
of Ph3PꢀBH3 totally disappeared and a new signal at d ꢁ 31.1
(JB-P ¼ 35.6 Hz) emerged, indicating that Ph2PNa(BH3) 2a
was generated almost quantitatively. Furthermore, the final
product diphenyl(n-octyl)phosphine borane 3a (Ph2Pn-
Oct(BH3)) with a signal at d 16.5 (JB-P ¼ 50.2 Hz) was
obtained in quantitative yield by simple quenching with one
equivalent of n-octyl bromide (n-OctBr) at room tempera-
ture. It was interesting that no byproduct via P-B bond
cleavage was detected. And 1H NMR spectra verified the
formation of 3a (Scheme 2B).
This interesting result led us to explore the reactivity of
triphenylphosphine sulfide (Ph3P(S)) upon treatment with
difficult to prepared by the traditional methods[13] will be easily SD. Thus, similarly, to a solution of 0.2 mmol of Ph3P(S) in
CONTACT Li-Biao Han
libiao-han@aist.go.jp
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan
Division of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki, Japan.
Supplemental data for this article can be accessed online at https://doi.org/10.1080/10426507.2021.1948851.
ß 2021 Taylor & Francis Group, LLC

2
J.-Q. ZHANG ET AL.
Scheme 1. Selective P-C bond cleavage by SD.
Scheme 3. Reaction of Ph3P(S) with SD.
Table 1. P-C bond cleavage of phosphine boranesa.
Entry
R1
R2
R3X
Yield/3
31P NMR, d/ppm
1
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Et
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
n-Bu
Et
n-BuBr
n-BuCl
n-BuI
n-OctBr
PhCH2Br
Br(CH2)4Br
i-PrCl
i-PrI
t-BuBr
n-BuBr
n-BuBr
3b, 92%
3b, 86%
3b, 86%
3a, 92%
3c, 94%
3d, 95%
3e, n.r.
3e, 87%
3f, n.d.
3g, 69%
3h, 72%
17.23/16.95
17.22/16.92
17.27/16.93
17.16/17.01
19.16/18.79
16.5
2b
3
4
5
6c
7
–
8
25.86/25.49
–
16.67/16.30
20.43/20.07
9
10d
11d
aReaction conditions: Phosphine borane
1 (0.2 mmol), THF (1.0 mL), SD
(0.42 mmol), rt, 2 h. Then R3X (0.2 mmol), rt, 0.5 h, the yield was estimated
by 31P NMR based on 1 used.
b12 h at the 1st step.
cR3X (0.1 mmol).
d30 h at the 1st step.
and Ph3P, because no signal of Ph2PSNa and
Ph2PNa appeared.
Scheme 2. Reaction of Ph3P·BH3 1a with SD monitored by 31P NMR and the 1H
NMR spectra of 3a.
Because SD failed to cleave P-Ph bond of Ph3P(S), further
emphasis was only placed on investigating the reactions of
phosphine boranes with SD. As depicted in Table 1, the
intermediate sodium phosphide borohydride Ph2PNa(BH3)
in situ generated from the reaction of Ph3P(BH3) with SD
was capable of reacting with a series of aliphatic halides to
deliver new phosphine boranes with good efficiency. For
example, all kinds of primary alkyl halides (chloride, brom-
ide, iodide) could readily trap Ph2PNa(BH3) to give the new
phosphine boranes 3 in high yields (Table 1). Especially the
less reactive n-butyl chloride, which did not react with
Ph2PONa at room temperature,[11] now could react with
Ph2PNa(BH3) efficiently, demonstrating that the reactivity of
1.0 mL of THF was added dropwise SD at room tempera-
ture, and the reaction was monitored by 31P NMR spectros-
copy. As we can see from Scheme 3, after 1 hour of reaction
time, the P-Ph bond of Ph3P(S) (d ¼ 42.8 ppm) did not react
with sodium at all and the P ¼ S double bond was broken
by SD, giving rise to the desulfurization product Ph3P (d ¼
ꢁ4.9 ppm) in 4% 31P NMR yield. The yield of Ph3P
increased up to 31% after 16 hours (Scheme 3). Worth notic-
ing is that almost no product stemming from P-Ph bond
cleavage could be detected during the whole process. In con-
trast to the reactions of phosphine borane, these results dis-
closed an interesting fact that the P ¼ S bond is more easily Ph2PNa(BH3) is higher than that of Ph2PONa. The quench-
to be broken by SD than the P-Ph bond in both Ph3P(S) ing reaction proceeds smoothly with benzyl bromide and

PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
3
1,4-dibromobutane, affording the corresponding phosphine a syringe filter the reaction mixture was quenched with n-
borane 3c and 3d in 94% and 95% yield, respectively OctBr (0.2 mmol, 35 mL) and stirred at room temperature
(entries 5 and 6). However, except for isopropyl iodide for 0.5 h. The final mixture was added to 2.0 mL EtOAc and
(entry 8), the reactions with isopropyl chloride (entry 7) and washed with H2O (3 ꢃ 1 mL). Then the organic layer was
dried over anhydrous MgSO4 and the solvent was removed
under reduced pressure. The crude product was purified by
GPC to afford 3a (56.2 mg, colorless oil, 0.18 mmol, 90%
tert-butyl bromide (entry 9) did not take place, perhaps due
to inert C-Cl bond of i-PrCl and large steric hindrance of t-
BuBr. In addition, by prolonging the reaction time, treat-
ment of diarylphosphine borane Ph2Pn-Bu(BH3) and dialky-
l(aryl)phosphine borane PhPEt2(BH3) with SD could also
lead to the corresponding R2PNa(BH3) intermediate. Finally,
the new generated phosphine boranes were produced in
moderated yields (Table 1, entries 10 and 11).
Unfortunately, the coupling reactions of Ph2PNa(BH3) with
aromatic chloride such as p-chlorotoluene gave complicated
results and no expected product could be detected even in
the presence of a palladium catalyst.[11]
1
yield); H NMR (400 MHz, CDCl3): d ¼ 7.70–7.65 (m, 4H),
7.47–7.40 (m, 6H), 2.23–2.16 (m, 2H), 1.57–1.47 (m, 2H),
1.41–1.33 (m, 2H), 1.29–1.23 (m, 8H), 0.86 (t, J ¼ 6.8 Hz,
3H); 13C NMR (100 MHz, CDCl3): d ¼ 132.2 (d, JPC
¼
9.1 Hz), 131.2 (d, JPC ¼ 1.8 Hz), 129.8 (d, JPC ¼ 54.4 Hz),
128.9 (d, JPC ¼ 9.6 Hz), 31.9, 31.2 (d, JPC ¼ 13.7 Hz), 29.1
(d, JPC ¼ 7.6 Hz), 25.9, 25.6, 23.1, 22.7, 14.2; 31P NMR
(162 MHz, CDCl3): d ¼ 16.5 (JBP ¼ 57.2 Hz).
General experimental procedures for the reaction of phos-
phine boranes with sodium
Conclusions
A Schlenk tube was charged with phosphine borane
R3PꢀBH3 1 (0.2 mmol) and 1.0 mL of THF; then under stir-
ring SD (0.42 mmol, 42 mL) was added to the solution and
stirring of the mixture was continued at room temperature
for the indicated time (2 h, 12 h, 16 h, 30 h). Then the excess
of sodium was removed by a syringe filter followed by
quenching of the reaction mixture with RX (0.2 mmol) and
stirring at room temperature for 0.5 h. The crude mixture
was analyzed by 31P NMR and the estimated yield of 3 was
calculated by integration based on the phosphine borane
1 used.
In conclusion, we have developed an efficient transformation
of arylphosphine boranes via the selective P-Ph bond cleav-
age by using SD, which not only completes our previously
established systematical studies on the P-C bond cleavage by
sodium, but also provides a new approach to alkylated phos-
phine boranes.
Experimental
Materials
The solvent THF and reagents alkyl halides were purchased
and used without further purification. Triphenylphosphine bor-
ane 1a and SD (ꢂ10 mol/L) was provided by Tokyo Chemical
Industry Co., Ltd. Ph2n-BuP(BH3) and Et2PhP(BH3) was pre-
pared from Ph2n-BuP and Et2PhP by treating with BH3·THF
solution, respectively. The Supplemental Materials contains
Acknowledgements
This work was primarily supported by joint research between AIST
and Maruzen Petrochemical Co., Ltd.
1
sample H, 13C and 31P NMR spectra for the product, 3 (mul-
ORCID
tiple runs, Figures S1–S18).
Jian-Qiu Zhang
Li-Biao Han
http://orcid.org/0000-0002-5220-3200
http://orcid.org/0000-0001-5566-9017
Instrumentation
1H, 31P and 13C NMR spectra were recorded with a JEOL
JNM-ECS400 instrument operating at 400 MHz (1H),
References
[1] Adrio, L. A.; Hii, K. K. Application of Phosphine Ligands in
Organic Synthesis. Organomet. Chem. 2009, 35, 62–92. DOI: 10.
1039/b801380m.
[2] Guo, H.; Fan, Y. C.; Sun, Z.; Wu, Y.; Kwon, O. Phosphine
Organocatalysis. Chem. Rev. 2018, 118, 10049–10293. DOI: 10.
1021/acs.chemrev.8b00081.
1
100 MHz (13C), and 162 MHz (31P). Chemical shifts for H
NMR are referred to internal Me4Si (0 ppm) and reported as
follows: chemical shift (d ppm), multiplicity, coupling con-
stant (Hz) and integration. Chemical shifts for 31P NMR are
given relative to H3PO4 (85% solution in D2O, 0 ppm).
€
[3] Bednarek, C.; Wehl, I.; Jung, N.; Schepers, U.; Brase, S. The
Staudinger Ligation. Chem. Rev. 2020, 120, 4301–4354. DOI:
10.1021/acs.chemrev.9b00665.
Synthesis
[4] Fletcher, S. The Mitsunobu reaction in the 21st century. Org.
Chem. Front. 2015, 2, 739–752. DOI: 10.1039/C5QO00016E.
[5] Byrne, P. A.; Gilheany, D. G. The Modern Interpretation of the
Wittig Reaction Mechanism. Chem. Soc. Rev. 2013, 42,
6670–ꢁ6696. DOI: 10.1039/C3CS60105F.
Experimental procedure for the preparation of
Ph2Pn-Oct(BH3) 3a
A Schlenk tube was charged with triphenylphosphine borane
Ph3PꢀBH3 (0.2 mmol, 55.2 mg) and 1.0 mL of THF; then
under stirring, SD (0.42 mmol, 42 mL) was added to the solu-
tion, and stirring of the mixture was continued at room
temperature for 2 h. After removing the excess of sodium by
[6] Stewart, B.; Harriman, A.; Higham, L. J. Predicting the Air
Stability of Phosphines. Organometallics 2011, 30, 5338–5343.
DOI: 10.1021/om200070a.
[7] Imamoto, T.; Oshiki, T.; Onozawa, T.; Kusumoto, T.; Sato, K.
Synthesis and Reactions of Phosphine-Boranes. Synthesis of

4
J.-Q. ZHANG ET AL.
New Bidentate Ligands with Homochiral Phosphine Centers via
Optically Pure Phosphine-Boranes. J. Am. Chem. Soc. 1990,
112, 5244–5252. DOI: 10.1021/ja00169a036.
Organophosphine Oxides by Sodium. J. Org. Chem. 2020, 85,
14166–14173. DOI: 10.1021/acs.joc.0c01642.
[12] Ye, J.; Zhang, J.-Q.; Saga, Y.; Onozawa, S. Y.; Kobayashi, S.;
Sato, K.; Fukaya, N.; Han, L.-B. Ready Approach to
Organophosphines from ArCl via Selective Cleavage of C–P
Bonds by Sodium. Organometallics 2020, 39, 2682–2694. DOI:
10.1021/acs.organomet.0c00295.
[13] Pellon, P. Phosphine-Boranes in Synthesis. Borane as an
Efficient Protecting Group in the Preparation of Functionalized
Phosphines. Tetrahedron Lett. 1992, 33, 4451–4452. DOI: 10.
1016/S0040-4039(00)60107-7.
€
[8] Ohff, M.; Holz, J.; Quirmbach, M.; Borner, A. Borane
Complexes of Trivalent Organophosphorus Compounds.
Versatile Precursors for the Synthesis of Chiral Phosphine
Ligands for Asymmetric Catalysis. Synthesis 1998, 1998,
1391–1415. DOI: 10.1055/s-1998-2166.
[9] Demchuk, O. M.; Jasinski, R.; Strzelecka, D.; Dziuba, K.; Kula,
K.; Chrzanowski, J.; Krasowska, D. A Clean and Simple Method
for Deprotection of Phosphines from Borane Complexes. Pure
Appl. Chem. 2018, 90, 49–62. DOI: 10.1515/pac-2017-0313.
[10] Zhang, J.-Q.; Ye, J.; Huang, T.; Shinohara, H.; Fujino, H.; Han,
L.-B. Conversion of Triphenylphosphine Oxide to
[14] Gourdel, Y.; Ghanimi, A.; Pellon, P.; Le Corre, M. Activation of
Phosphines with Borane. Tetrahedron Lett. 1993, 34,
1011–1012. DOI: 10.1016/S0040-4039(00)77478-8.
Organophosphorus via Selective Cleavage of C-P, O-P, and C-
H Bonds with Sodium. Commun. Chem. 2020, 3, 1–9. DOI: 10.
1038/s42004-019-0249-6.
ꢀ
ꢁ
ꢁ
[15] Stankevic, M.; Wojcik, K.; Jaklinska, M.; Pietrusiewicz, K. M.
Eur. J. Org. Chem. In Situ Dearomatisation/Alkylation of
Arylphosphane Derivatives. 2012, 2012, 2521–2534. DOI: 10.
1002/ejoc.201200096.
[11] Zhang, J.-Q.; Ikawa, E.; Fujino, H.; Naganawa, Y.; Nakajima, Y.;
Han,
L.-B.
Selective
C-P(O)
Bond
Cleavage
of