Synthesis of tertiary phosphine oxides by alkaline hydrolysis of quaternary phosphonium zwitterions using excess t-BuOK and stoichiometric water

Article abstract of DOI:10.1080/00397911.2020.1868006

Hydrolysis of quaternary arylphosphonium zwitterions bearing COO^? and those in situ generated from the corresponding salts bearing Ac or OH at the aryl ring by using excess t-BuOK and stoichiometric water affords tertiary arylphosphine oxides in moderate to excellent yield, in contrast to hydrolysis of these zwittertion or salts in aqueous NaOH that mainly provides phosphine oxides with the loss of the aryl group. Under the t-BuOK/water conditions, hydrolysis of carbonyl stabilized ylides Ph₃P = CHCOR (R = Ph, Me, and OEt), which partially exist as phosphonium enolates, prefers to produce Ph₂P(O)CH₂COR. Further reduction of Ph₂P(O)CH₂COMe by PhSiH₃ allows the preparation of Ph₂PCH₂COMe in 43% yield.

Full text of DOI:10.1080/00397911.2020.1868006

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Synthesis of tertiary phosphine oxides by alkaline
hydrolysis of quaternary phosphonium zwitterions
using excess t-BuOK and stoichiometric water
Chun-Hong Zhong & Wenhua Huang
To cite this article: Chun-Hong Zhong & Wenhua Huang (2021) Synthesis of tertiary
phosphine oxides by alkaline hydrolysis of quaternary phosphonium zwitterions using excess
t-BuOK and stoichiometric water, Synthetic Communications, 51:10, 1537-1546, DOI:
10.1080/00397911.2020.1868006
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SYNTHETIC COMMUNICATIONSV
2021, VOL. 51, NO. 10, 1537–1546
https://doi.org/10.1080/00397911.2020.1868006
Synthesis of tertiary phosphine oxides by alkaline
hydrolysis of quaternary phosphonium zwitterions using
excess t-BuOK and stoichiometric water
Chun-Hong Zhong and Wenhua Huang
Department of Chemistry, Tianjin University, Tianjin, P. R. China
ABSTRACT
ARTICLE HISTORY
Hydrolysis of quaternary arylphosphonium zwitterions bearing COOꢀ
and those in situ generated from the corresponding salts bearing Ac
or OH at the aryl ring by using excess t-BuOK and stoichiometric
water affords tertiary arylphosphine oxides in moderate to excellent
yield, in contrast to hydrolysis of these zwittertion or salts in aque-
ous NaOH that mainly provides phosphine oxides with the loss of
the aryl group. Under the t-BuOK/water conditions, hydrolysis of car-
bonyl stabilized ylides Ph3P ¼ CHCOR (R ¼ Ph, Me, and OEt), which
partially exist as phosphonium enolates, prefers to produce
Ph2P(O)CH2COR. Further reduction of Ph2P(O)CH2COMe by PhSiH3
allows the preparation of Ph2PCH2COMe in 43% yield.
Received 1 June 2020
KEYWORDS
Alkaline hydrolysis;
phosphorus ylide;
quaternary phosphonium
salt; quaternary
phosphonium zwitterion;
tertiary phosphine oxide
GRAPHICAL ABSTRACT
Ph
O
Ph P R¢
Ph P R¢
t
(1) -BuOK (excess), THF, rt
(2) H2O (1.0 equiv)
R
R
−
−
p
m
p
o
p
R = -Ac, -COO , -COO , -OH, -OH; R' = Me, Ph.
t
O
(1) -BuOK (excess), THF, rt
O
X
O
Ph P
2
Ph P
3
(2) H2O (1.0 equiv)
X = Me, Ph, OEt.
X
Introduction
Tertiary phosphine oxides have been widely used as carbanion sources in the Horner-
Wittig reaction,[1] as precursors to synthesize tertiary phosphines[2–6] including chiral
tertiary phosphines,[7] as organocatalysts in asymmetric synthesis,[8] and as ligands to
form complexes with transition metals.[9–12] They have also been applied in the synthe-
sis of photoelectronic materials.[13] Considering the phosphorus starting materials, ter-
tiary phosphine oxides can be synthesized from (1) secondary phosphine oxides by
CONTACT Wenhua Huang
huangwh@tju.edu.cn Department of Chemistry, Tianjin University, 135 Yaguan Road,
Tianjin 300354, P. R. China.
Supplemental data for this article can be accessed on the publisher’s website
ß 2020 Taylor & Francis Group, LLC

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C.-H. ZHONG, AND W. HUANG
Scheme 1. Mechanism of alkaline hydrolysis of quaternary phosphonium salts.
arylation[14–18] with aryl halides or pseudohalides, and phosphinylation of alkynes[19,20]
or arynes,[21] (2) phosphinic chloride by nucleophilic substitution with Grignard
reagent,[22,23], (3) tertiary phosphine by oxidation,[24–26] and (4) quaternary phosphon-
ium salts by alkaline hydrolysis in aqueous NaOH.[27–30] The last route is based on a
–
long-established mechanism[31]: nucleophilic attack by OH at phosphorus to form a
–
hydroxyphosphorane followed by deprotonation by OH to form an oxyanionic phos-
phorane which undergoes fragmentation to produce a phosphine oxide and concur-
rently expel a carbanion which after protonation gives an alkane or arene (Scheme 1).
The easiness of R4 group to leave depends on the stability of carbanion ꢀR4, and is gen-
erally in the order: benzyl > phenyl > methyl.[32] Therefore, if R4 is the easiest group to
leave, the synthesis of phosphine oxides bearing R4 by this route will be challenging.
Recently we have developed a metal-free method for the synthesis of aryltriphenyl-
phosphonium bromides from Ph3P and aryl bromides bearing a wide range of
functional groups.[33] Phosphonium salt 1a (Scheme 2a) can also be prepared from
p-BrC6H4Ac and Ph3P by this method. After hydrolysis of 1a in aqueous NaOH, it was
hoped that a two-step, transition metal-free route would be found to synthesize phos-
phine oxide 2a. Unfortunately, attempts at hydrolysis of 1a in 3 M NaOH (aq) at room
temperature afforded no 2a but four degradation products: 3 (47%), PhCOMe (20%),
Ph3PO (22%), and Ph2HPO (21%) (Scheme 2a). According to the mechanism shown in
Scheme 1, PhCOMe and Ph3PO form from intermediates I and II followed by leaving a
4-acetylphenyl anion, which is the more stable than a phenyl anion (path a, Sch. 2b).
Product 3 is probably from ligand coupling[34] of intermediate II involving a phenyl
migration and the leave of Ph2POꢀ which upon protonation affords Ph2HPO (path b,
Scheme 2b). In paths both a and b, the electron-withdrawing acetyl group plays a key
role, leading to the failure to produce 2a.
We reasoned that if 1a is at first deprotonated by a strong, excess base such as
t-BuOꢀ to ensure the formation of phosphonium enolate zwitterion IV and then a stoi-
chiometric water is added to in situ generate ꢀOH, the following hydrolysis of IV
would form intermediate V and after deprotonation by t-BuOꢀ intermediate VI
(Scheme 3). There would be two decomposition pathways for VI: (1) the loss of a phe-
nyl anion to form enolate VII, which upon protonation produces 2a (path a), and (2)
the loss of dianion VIII to form Ph3PO (path b). However, dianion VIII is expected to
be highly unstable, so path b would be inhibited, leading to the formation of 2a as the
major product. Herein we report the utilization of this strategy to synthesize tertiary
phosphine oxides from either stable quaternary phosphonium zwitterions or those zwit-
terions generated in situ from the corresponding phosphonium salts.

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(a)
(b)
Scheme 2. (a) Alkaline hydrolysis of (4-acetylphenyl)triphenylphosphonium bromide 1a. (b)
Mechanism for alkaline hydrolysis of 1a.
Scheme 3. Hydrolysis of (4-acetylphenyl)triphenylphosphonium bromide 1a in the presence of
t-BuOK
Results and discussion
When phosphonium salt 1a was treated with 3.2 equiv of t-BuOK in THF and then
water (1.0 equiv) in THF, phosphine oxide 2a was isolated in 53% yield (Table 1),

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C.-H. ZHONG, AND W. HUANG
Table 1. Synthesis of tertiary phosphine oxides by alkaline hydrolysis.a
aReaction conditions: phosphonium salt 1 (0.5 mmol), THF (3.4 mL), t-BuOK (1.6 mmol), rt, 10 min, then water (0.5 mmol)
in THF (3 mL), rt, 3 h. For comparison, the yields obtained by hydrolysis using 3 M NaOH are listed in the parentheses.
c
nd ¼ not detected. bPh3PO and PhCOOH were obtained in 83% and 89% yields, respectively. Ph3P ¼ CHCOPh was iso-
e
lated in 73% yield. dPh3PO was isolated in 58% yield. Ph3PO was isolated in 98% yield.
indicating the success of our strategy. This strategy is also applicable to aryltriphenyl-
phosphonium salts bearing a p- or m-COOꢀ group at the aryl ring as phosphine oxides
2b and 2c were in 65% and 61% yields, respectively. The hydrolysis of 1b and 1c in
3 M NaOH gave 2b and 2c in much lower yields probably due to the protonation of
COOꢀ to COOH, leading to the loss of the aryl bearing COOH during the hydrolysis.
This suggests that it is necessary to use a strong base like t-BuOꢀ to prevent COOꢀ
from the formation of COOH. For phosphonium salt 1d, however, phosphine oxide 2d
was isolated in only 7% yield; its hydrolysis in 3 M NaOH did not produce 2d but
Ph3PO (83%) and PhCOOH (89%). At this stage, we do not know the exact reason why
both reaction conditions did not work for this specific substrate. Fortunately, phosphon-
ium salt 1e bearing an o-OH provided phosphine oxide 2e in 76% yield, whereas its
hydrolysis in 3 M NaOH gave 2e in only 6% yield. Although the hydrolysis of phos-
phonium salt 1f in 3 M NaOH gave 2f in 75% yield, using t-BuOK/H2O improved the
yield of 2f to 95%. Phosphonium salt 1g bearing a methyl substituent at phosphorus
could also afford the corresponding phosphine oxide 2g in a reasonable yield, still
higher than that obtained by 3 M NaOH. These results indicate that although the phen-
olic OH itself is an electron-donating in the context of conjugation, its deprotonation
by a strong base like t-BuOꢀ to ensure the formation of stronger electron-donating Oꢀ
could further improve the yield of phosphine oxide bearing this phenolic OH. When
phosphonium salt 1h was hydrolyzed in 3 M NaOH, phosphine oxide 2h was not
detected and instead the stabilized ylide, i.e., Ph3P ¼ CHCOPh, was isolated in 73%

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Scheme 4. Alkaline hydrolysis of carbonyl-stabilized phosphonium ylides.
yield, indicating that this ylide is relatively stable to 3 M NaOH. It has been reported[35]
that Ph3P ¼ CHCOPh can be prepared from 1h by treating with 2 M NaOH (aq). By
using t-BuOK/H2O, however, 2h could be obtained in 85% yield probably via the for-
mation of Ph3P ¼ CHCOPh which was further hydrolyzed to give 2h under stronger
basic condition. Alkaline hydrolysis of some acidic phosphonium salts has been sug-
gested[36] to proceed through the corresponding phosphonium ylides, especially those
stabilized ylides. For the hydrolysis of benzylphosphonium salt 1i, neither using 3 M
NaOH nor using t-BuOK/H2O led to the formation of phosphine oxide 2i; instead,
Ph3PO was isolated in 98% and 58% yield, respectively, possibly due to the instability of
the formed semi-stabilized ylide.[37]
Phosphonium salts 1aꢀg can be readily prepared by our recently developed
method[33] in the absence or presence of a catalytic amount of NiBr2. This method
coupled with their hydrolysis by using t-BuOK/water provides a two-step sequence to
tertiary arylphosphine oxides bearing acidic groups such as OH, COOH, and Ac at the
aryl ring. This two-step sequence compares quite favorably to the known synthetic
methods requiring the use of either stoichiometric ZnCl2 and a ligand for 2a,[38] or a
Pd catalyst for 2b and 2c,[39] or a three-step sequence for 2eꢀg.[40,41] For the synthesis
of 2h, our hydrolysis method also compares very well with the reported method by the
Arbuzov reaction at 160 ꢁC starting from Ph2POMe and BrCH2COPh.[42]
With the success in the hydrolysis of phosphonium salt 1 h through the ylide
Ph3P ¼ CHCOPh, we next investigated the direct hydrolysis of two stabilized phos-
phorus ylides 4 and 5. After treated with t-BuOK/H2O, phosphorus ylide 4 provided
phosphine oxide 2j in 65% yield (Scheme 4A). It is noteworthy that COOEt could sur-
vive the reaction condition and the transesterification product was not observed.
Phosphorus ylide 5 also underwent smoothly hydrolysis to give phosphine oxide 2k in
80% yield on a 1 mmol scale; the preparation on a larger scale (10 mmol) gave 2k in a
higher yield (95%), indicating that our hydrolysis strategy is practical (Scheme 4B).
Attempts to reflux the ylide 5 in 3 M NaOH led to the formation of phosphine oxide
2k in only 8% yield, and Ph3PO in 57% yield (see supporting information). This is

1542
C.-H. ZHONG, AND W. HUANG
possibly because the ylide 5 might be in an equilibrium with phosphonium hydroxide
IX (Scheme 4C) in aqueous NaOH. The decomposition of IX according to the general
mechanism (Scheme 1) is favorable for the loss of the carbonylmethyl group instead of
a phenyl group and the formation of Ph3PO. Under t-BuOK/H2O conditions, the ylide
5 mainly exists as phosphonium enolate zwitterion X, which prefers to lose a phenyl
anion rather than enolate dianion during hydrolysis, leading to the formation of phos-
phine oxide 2k. As the ylides 4 and 5 are commercially available, their hydrolysis under
t-BuOK/H2O provides a facile access to phosphine oxides 2j and 2k, which are usually
prepared by the Arbuzov reaction at 160 ꢁC[42] and lithiation of Ph2P(O)Me using BuLi
followed by treating with EtOCOMe[43], respectively. Moreover, 2k could be reduced to
phosphine 6 in 43% yield by using Beller’s method[6] (Scheme 4B). The synthesis of 6
from acetone and Ph2PCl has been reported[44] but suffers from the use of LDA and
low temperature (ꢀ78 ꢁC).
Conclusions
In summary, we have developed a method for hydrolysis of quaternary phosphonium
zwitterions including carbonyl-stabilized phosphorus ylides by using excess t-BuOK and
stoichiometric water to generate or keep a negatively charged substituent on phos-
phorus, making it difficult to leave during the hydrolysis. This method should extend
the utility of alkaline hydrolysis of quaternary phosphonium salts by providing ready
access to phosphine oxides bearing functional groups such as OH, COOH, COOEt,
COPh and Ac.
Experimental
1H and 13C NMR spectra were recorded on a Bruker Avance 400 NMR spectrometer.
HRMS spectra were recorded on a Varian 7.0 T FTMS. The 1 M solution of t-BuOK in
THF and THF (SuperDry) were purchased from a local company. The preparation of
phosphonium salts 1b, 1d, and 1f has been reported in our previous paper.[33] The
same procedure was used for the preparation of phosphonium salt 1a. Phosphonium
salts 1c, 1e, and 1g were prepared by the same procedure except that a catalytic amount
of NiBr2 was added. Phosphonium salts 1h and 1i as well as ylides 4 and 5 were pur-
chased from a local company.
Typical procedure for the synthesis of 1-(4-(diphenylphosphoryl)phenyl)ethanone
2a using t-BuOK/water
To a round-bottom flask (25 mL) containing phosphonium salt 1a (231 mg, 0.5 mmol)
and THF (3.4 mL), a solution of t-BuOK in THF (1 M, 1.6 mL) was added. The resulting
mixture was stirred at rt for 10 min, then a solution of water (9 mg, 0.5 mmol) in THF
(3 mL) was added. After stirred further for 3 h, the reaction mixture was treated with
1 M HCl (15 mL), and then extracted with CH2Cl2 (10 mL ꢂ 3). The combined extract
was evaporated under reduced pressure, and the residue was isolated by preparative
TLC (CH2Cl2/iPrOH, 30/1, v/v) to give phosphine oxide 2a as a yellow solid in 53%

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1
(84 mg) yield. H-NMR (400 MHz, DMSO-d6) d 2.62 (s, 3 H), 7.54ꢀ7.70 (m, 10 H),
7.75 ꢀ 7.82 (m, 2 H), 8.09 (d, J ¼ 7.4 Hz, 2 H); 13C-NMR (100 MHz, DMSO-d6) d 27.4,
128.7 (d, JC-P ¼ 11.8 Hz), 129.4 (d, JC-P ¼ 11.9 Hz), 132.0 (d, JC-P ¼ 10.0 Hz), 132.4 (d,
JC-P ¼ 10.0 Hz), 132.6 (d, JC-P ¼ 103.3 Hz), 132.8 (d, JC-P ¼ 2.7 Hz), 138.1 (d, JC-P
¼
99.5 Hz), 139.7 (d, JC-P ¼ 2.8 Hz), 198.2; 31P-NMR (162 MHz, DMSO-d6) d 25.09. These
NMR data are consistent with those reported.[45 In addition, the hydrolysis of phos-
phonium salt 1a in 3 M NaOH provided no phosphine oxide 2a but p-AcC6H4Ph 3
(46 mg, 47%), PhCOMe (60 mg, 20%), Ph3PO (30 mg, 22%), and Ph2HPO (21 mg, 21%),
1
which are identified by comparing with authentic samples. NMR data for 3: H-NMR
(400 MHz, DMSO-d6) d 2.62 (s, 3 H), 7.41 ꢀ 7.47 (m, 1 H), 7.49 ꢀ 7.54 (m, 2 H), 7.75 (d,
J ¼ 7.2 Hz, 2 H), 7.83 (d, J ¼ 8.5 Hz, 2 H), 8.05 (d, J ¼ 8.5 Hz, 2 H); 13C-NMR (100 MHz,
DMSO-d6) d 27.2, 127.3, 127.5, 128.8, 129.4, 129.6, 136.1, 139.4, 145.0, 197.9. These
NMR data are also consistent with those reported.[46]
Full experimental detail, and NMR spectra (1H and 13C). This material can be found
via the “Supplementary Content” section of this article’s webpage.
Funding
We thank the National Natural Science Foundation of China for their financial support of our
program [Grant No. 21272170].
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