Hydrolysis of Phosphonothioates with a Binaphthyl Group: P-Stereogenic O-Binaphthyl Phosphonothioic Acids and Their Use as Optically Active Ligands and Chiral Discriminating Agents

Article abstract of DOI:10.1021/acs.orglett.8b00147

The hydrolysis of phosphonothioates with a binaphthyl group afforded the first example of O-(2′-hydroxy)binaphthyl phosphonothioic acids in good to high yields and >95:5 diastereoselectivity. The reaction proceeds via an axis-to-center chirality-transfer reaction. The ability of these acids to act as chiral molecular auxiliaries was demonstrated by using them as optically active ligands for the asymmetric ethylation of benzaldehyde and as a chiral discriminating agent for chiral aliphatic amines.

Full text of DOI:10.1021/acs.orglett.8b00147

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Hydrolysis of Phosphonothioates with a Binaphthyl Group:
P‑Stereogenic O‑Binaphthyl Phosphonothioic Acids and Their Use as
Optically Active Ligands and Chiral Discriminating Agents
Kazuma Kuwabara,† Yuuki Maekawa,† Mao Minoura,‡ and Toshiaki Murai*,†
†Department of Chemistry and Biomolecular Science, Faculty of Engineering, Gifu University, Yanagido, Gifu 501-1193, Japan
‡Department of Chemistry, Graduate School of Science, Rikkyo University, Nishi-ikebukuro, Toshima-ku, Tokyo 171-8501, Japan
S
* Supporting Information
ABSTRACT: The hydrolysis of phosphonothioates with a
binaphthyl group afforded the first example of O-(2′-
hydroxy)binaphthyl phosphonothioic acids in good to high
yields and >95:5 diastereoselectivity. The reaction proceeds via
an axis-to-center chirality-transfer reaction. The ability of these
acids to act as chiral molecular auxiliaries was demonstrated by
using them as optically active ligands for the asymmetric ethylation of benzaldehyde and as a chiral discriminating agent for chiral
aliphatic amines.
P-Stereogenic organophosphorus compounds1 have attracted
Scheme 1. Axis-to-Center Chirality Transfer from
Phosphorothioates and Phosphonothioates
increasing attention due to their diverse applications in a variety
of research areas related to chirality. One characteristic feature
of phosphorus is that it can adopt three- and four-coordinate
geometries, and a range of optically active compounds that
contain tri- and pentavalent phosphorus atoms have been
designed, synthesized, and applied. For example, trivalent P-
stereogenic organophosphorus compounds play important
roles as optically active ligands for transition metals.2 Moreover,
pentavalent organophosphorus compounds with heteroatom-
containing functional groups such as OH, SH, or SeH may
serve as P-stereogenic chiral acids,3 and their importance has
been noted in research areas related to biology.4 More
interestingly, if such compounds possess chirality not only on
the phosphorus atom but also on the carbon chains attached to
the phosphorus atom, they may provide well-organized chiral
environments. In fact, such types of derivatives, which exhibit
carbon chains with central chirality, have been developed,5 but
their applications as chiral molecular tools remain somewhat
underdeveloped. Therefore, we have focused our attention on
the binaphthyl group, which is able to distinguish asymmetric
environments.6 In particular, we have recently developed an
axis-to-center chirality-transfer strategy7 for the synthesis of P-
stereogenic compounds from organophosphorus substrates
bearing a binaphthyl group. For example, the reaction of
phosphorothioates I with a THF solution of [Bu4N]F
containing 10% of H2O furnished acid salts III with high
efficiency and high enantiomeric excess (Scheme 1a).7a In this
reaction, fluoride II may be initially formed, but the high
electrophilicity of the phosphorus atom in II may cause further
hydrolysis.
center, and acidic protons. Herein, we report the diaster-
eoselective synthesis of O-binaphthyl phosphonothioic acids via
axis-to-center chirality transfer, and their applications as
optically active ligands in the asymmetric ethylation of
benzaldehyde and as chiral discriminating agents for chiral
aliphatic amines.
Initially, we reacted phosphonothioate 1c with metal
hydroxides (Table 1). The hydrolysis of 1c occurred at room
temperature to give the desired product 2c in 88% yield with a
diastereoselectivity of >95:5, albeit prolonged reaction times
(>140 h) were required when LiOH was used (entry 1). In this
reaction, the axial chirality of the binaphthyl group was clearly
transferred to the central chirality at the phosphorus atom, and
products derived from a further hydrolysis of 2c did not form.
In order to shorten the reaction time, more basic metal
hydroxides such as NaOH, KOH, and CsOH were examined,
but the reaction did not reach completion, and the starting
material 1c was partially recovered (entries 3−5). In contrast, a
reaction at higher temperature (70 °C) accelerated the reaction
We envisaged that acids possessing skeletons analogous to II,
provided that they are sufficiently stable to be isolated, could
serve as highly versatile chiral molecular tools given the
presence of an axially chiral binaphthyl group, a P-stereogenic
Received: January 14, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00147
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Organic Letters
Letter
a
Table 1. Hydrolysis of Cyclohexylphoshonothioate 1c
Table 2. Hydrolysis of a Range of Phoshonothioic Acid
a
Esters 1
time
(h)
yield
b
recovery of 1c
b
c
entry
base
temp
rt
(%)
(%)
dr
1
2
3
4
5
LiOH
LiOH
NaOH
KOH
144
18
88
78
67
47
48
0
0
>95:5
>95:5
>95:5
>95:5
>95:5
70 °C
rt
rt
rt
144
144
144
33
53
52
CsOH
a
The reaction was carried out with 1 (0.5 mmol) and LiOH (5.0
equiv) in THF/H2O (1 mL/1 mL) at room temperature unless
b
c
otherwise noted. Isolated yield. Determined by 31P NMR analysis of
the crude reaction mixtures.
(18 h) under retention of the high diastereoselectivity (entry
2).
We then subjected a range of phosphonothioates 1 to these
hydrolysis conditions (Table 2). The hydrolysis of methyl-
phosphonothioate 1a reached completion within 30 min to
quantitatively afford the corresponding product 2a with slightly
reduced diastereoselectivity (entry 1). Longer reaction times
were required for the reaction of 1b, albeit 2b was obtained
almost exclusively as a single diastereoisomer (entry 2). The
reaction of 1d, which carries a tert-butyl group, did not proceed
at room temperature, and the starting material 1d was
completely recovered, while the reaction at 100 °C proceeded
smoothly to give the corresponding product 2d in 94% yield
with high diastereoselectivity (entry 3). Arylphosphonothioates
1e−1j were also susceptible to these hydrolysis conditions
(entries 4−9). We also discovered that the substituents at the
ortho-position of the aromatic groups affected the diaster-
eoselectivity. Esters with no ortho-substituents (entries 4 and 8)
afforded 2e and 2i with a diastereoselectivity of ∼73:27,
whereas substrates 1f, 1g, 1h, and 1j exhibited higher
diastereoselectivity (entries 5, 6, 7, and 9). Among these, esters
1h and 1j, which carry 1-napththyl and mesityl groups,
respectively, furnished 2h and 2j with a diastereoselectivity of
>95:5 (entries 7 and 9).
The hydrolysis of phosphonate 3b and phosphonoselenoate
4b also proceeded smoothly to quantitatively furnish the
corresponding acids 5b and 6b (Scheme 2). Due to the fast
tautomerization, the phosphorus atom in 5b is achiral, while the
product 6b exhibited high levels of diastereoselectivity.
However, at this point, the reaction pathway of the present
reaction still remains unclear. Such substitution reactions8
generally proceed via SN2-type reactions and/or SN1-type
reactions that may involve pentacoordinate intermediates, but
the environment around the phosphorus atom might affect the
reaction pathway. Indeed, the high influence of the substituents
at the phosphorus atom with respect to reaction times and
diastereoselectivity of the reactions imply that the reaction may
proceed in an SN2-type fashion.
a
The reaction was carried out with 1 (0.5 mmol) and LiOH (5.0
equiv) in THF/H2O (1.0 mL/1.0 mL) at room temperature unless
b
c
otherwise noted. Isolated yield. Determined by 31P NMR analysis of
d
the crude reaction mixtures. The reaction was carried out in 1,4-
dioxane/H2O (0.6 mL/0.6 mL) at 100 °C.
Scheme 2. Hydrolysis of Phosphonate 3b and
Phosphonoselenoate 4b
The absolute configuration of the major isomer of 2i was
unequivocally determined by a single-crystal X-ray diffraction
analysis (Figure 1). The tetrahedral phosphorus atom in 2i
exhibited an R-configuration. The bond length of the PS
bond (1.9355(5) Å), is closer to typical PS double bonds
which suggests the presence of a PS double bond while the
proton should reside on the oxygen atom.
Subsequently, we tested the resulting products 2, 5b, and 6b
with respect to their potential to serve as optically active
ligands. For that, the ethylation of benzaldehyde (7) with Et2Zn
in the presence of Ti(OiPr)4 was chosen as a model reaction.11
(∼1.9 Å)9 than to typical P−S single bonds (∼2.1 Å),3 10
p,
B
DOI: 10.1021/acs.orglett.8b00147
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Organic Letters
Letter
Table 4. Asymmetric Ethylation of 8 in the Presence of a
a
Range of Phosphonothioic Acids 2
Figure 1. Molecular structure of 2i with thermal ellipsoids set to 50%
probability.
Acids 5b, 2b, and 6b (20 mol %) were able to promote the
ethylation to give 8 in good yield (Table 3). The enantiomeric
a
Table 3. Asymmetric Ethylation of 7
a
The reaction was carried out with 7 (0.5 mmol), Ti(OiPr)4 (1.2
equiv), 2 (20 mol %), and Et2Zn (3.0 equiv) in CH2Cl2 (1.0 mL) at 0
b
c
°C for 1 h. Ratio of the diastereomers of ligands: >95:5. The
enantiomeric excess was determined by HPLC on a chiral stationary
i
phase (CHIRALPAK OD-H, PrOH/hexane = 10/90, flow rate 1.0
d
mL/min). The reaction was carried out at −20 °C.
a
The reaction was carried out with 7 (0.5 mmol), Ti(OiPr)4 (1.2
equiv), ligand (20 mol %), and Et2Zn (3.0 equiv) in toluene (1.5 mL)
b
at 0 °C for 1 h. Ratio of the diastereomers of ligands 2b and 6b:
c
>95:5. The enantiomeric excess was determined by HPLC on a chiral
i
stationary phase (CHIRALPAK OD-H, PrOH/hexane = 10/90, flow
rate 1.0 mL/min).
excess of 8 derived from 2b and 6b was higher than that from
5b, which reflects the importance of the central chirality at the
phosphorus atom for the asymmetric reactions. We then used a
range of phosphonothioic acids 2 as optically active ligands
(Table 4). Among these, the acids that carry aromatic rings
with ortho-substituents enhanced the enantioselectivity. In
particular, acid 2j, which carries a mesityl group, furnished 8
in up to 94% ee. Many types of optically active ligands with a
binaphthyl group catalyze the asymmetric alkylation of aromatic
aldehydes with zinc reagents. To achieve high enantioselectiv-
ity, multiple biphanthyl groups were incorporated in the
ligands,12 or substituents were introduced at the 3,3′-positions
of the binaphthyl group.13 Unlike these precedents, the present
optically active ligands possess axial chirality and central
chirality at the phosphorus atom, and the enantioselectivity
was controlled by the achiral substituents on the chiral
phosphorus atom.14
Figure 2. 1HNMR spectra of 1:1 mixtures of (A) 9a + 2j (0.02 mmol
each), (B) 9a + 5j, (C) 9b + 2j, and (D) 9c + 2j in CDCl3 (0.5 mL).
Finally, the efficiency of phosphonothioic acids 2 to
discriminate the chirality of chiral aliphatic amines15 was
tested, as this is an important research field to develop readily
available chiral agents that can quickly discriminate enantiomers
of racemic mixtures.16 The 1H NMR spectra of a 1:1 mixture of
2j and racemic 1-phenethylamine (9a) clearly showed two
doublet signals of the diastereomeric salts at 1.05 and 1.13 ppm
(Δδ = 0.08) derived from 2j and 9a (Figure 2A). To
demonstrate the importance of the chirality on the phosphorus
atom for the chiral discrimination, phosphonic acid 5j was
mixed with 9b, and the resulting salts also showed two similar
doublets at 1.12 and 1.18 ppm (Δδ = 0.06) (Figure 2B), but
the signals are broader than in the case of 2j. The critical
importance of the P-stereogenic centers was highlighted by the
discrimination of 2-aminobutane (9b) and 3-methylpyperidine
(9c). The 1H NMR spectra of the 1:1 mixtures of 5j and 9b or
9c were broad, and methyl signals corresponding to each
diastereomer were not observed. In contrast, a 1:1 mixture of 2j
and 9b exhibited two triplets and two doublets at 0.64 and 0.82
ppm (Δδ = 0.18) and at 0.93 and 1.00 ppm (Δδ = 0.07),
1
respectively (Figure 2C). Likewise, the H NMR spectra of a
mixture of 2j and 9c showed two doublets at 0.59 and 0.65 ppm
C
DOI: 10.1021/acs.orglett.8b00147
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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chromatographic detection of simple alkyl amines such as 9b
and 9c is nontrivial due to the lack of UV-active functional
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In summary, we have demonstrated that the hydrolysis of
phosphonothioates 1 that carry a binaphthyl group proceeds via
axis-to-center chirality transfer with high efficiency and
diastereoselectivity. In these reactions, only one P−O bond in
the starting material 1 is cleaved selectively. The resulting acids
2 can be used as optically active ligands for the Ti-mediated
asymmetric ethylation of benzaldehyde with Et2Zn. The
enantiomeric excess of the product was optimized by changing
the substituents on the phosphorus atom in 2 without the need
to modify the substituents on the binaphthyl group. The utility
of acids 2 was further demonstrated by using them as chiral
discriminating agents for racemic aliphatic amines. Further
studies on the axis-to-center chirality-transfer reactions and the
applications of the resulting products as chiral molecular tools
are currently in progress in our laboratories.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00147.
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Experimental details and data (PDF)
Accession Codes
CCDC 1816837 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*mtoshi@gifu-u.ac.jp
ORCID
Toshiaki Murai: 0000-0003-4945-0996
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by JSPS KAKENHI Grants
JP16H01139 (Middle Molecular Strategy) and JP16H04146.
■
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