Zinc-Catalyzed Phosphonylation of Alcohols with Alkyl Phosphites

Article abstract of DOI:10.1021/acs.orglett.0c00932

In the presence of a catalytic amount of either Zn(acac)₂ or bis(2,2,6,6-tetramethyl-3,5-heptanedionato)zinc(II) (Zn(TMHD)₂), primary, secondary, and tertiary alcohol substituents on a wide range of substrates, including acyclic and cyclic structures, carbohydrates, steroids, and amino acids, reacted with dimethyl phosphite to afford the corresponding H-phosphonate diesters in high to excellent yields.

Full text of DOI:10.1021/acs.orglett.0c00932

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Letter
Zinc-Catalyzed Phosphonylation of Alcohols with Alkyl Phosphites
Yuki Saito, Soo Min Cho, Luca Alessandro Danieli, and Shu Kobayashi*
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ABSTRACT: In the presence of a catalytic amount of either Zn(acac)2 or bis(2,2,6,6-tetramethyl-3,5-heptanedionato)zinc(II)
(Zn(TMHD)2), primary, secondary, and tertiary alcohol substituents on a wide range of substrates, including acyclic and cyclic
structures, carbohydrates, steroids, and amino acids, reacted with dimethyl phosphite to afford the corresponding H-phosphonate
diesters in high to excellent yields.
hosphorylation of alcohols is a fundamental organic
above 150 °C were needed for the reaction, and substrates
Preaction in both life science and synthetic organic were limited to alcohols with simple alkyl chains.34,35 Catalytic
phosphorylation/phosphitylation/phosphonylation reactions
with readily available and nonactivated reagents under mild
reaction conditions to provide sustainable synthetic access to
various kinds of phosphates/phosphites/phosphonates with
minimal waste are clearly desired.36
Herein, we report highly active Zn complex-catalyzed
phosphonylation reactions of a wide range of alcohols using
alkyl phosphites as phosphonylating reagents to provide H-
phosphonate diesters under mild reaction conditions.
We used dimethyl phosphite 2 as a phosphonylation reagent
not only because it is an inexpensive and readily available
reagent but also because the corresponding H-phosphonates
would be easily converted into phosphates and thiophosphates
under suitable oxidation conditions. Various Lewis acids were
examined using cyclohexanol (1a) as a model alcohol in the
presence of MS 4A (Table 1).
chemistry. In biological systems, it is known that phosphor-
ylation of serine hydroxyl groups in proteins drastically changes
the properties of the protein and triggers various kinds of
biological events in the cell.1−3 In the field of organic
chemistry, phosphorylation of alcohols is the most straightfor-
ward way to access organophosphate molecules. Such
phosphates are found in natural products and pharmaceutical
agents as well as in functional materials.4−9 For example, the
glucocorticoid class of steroids is phosphorylated when used
for medical treatment to improve water solubility.10 One of the
most important applications is the synthesis of oligonucleo-
tides, which are oligomers of nucleotides linked by phosphates;
these systems have gained much attention in recent years
because they offer a new generation of therapeutic modal-
ity.11−15
Phosphorylation of alcohols can be achieved by substitution
on the phosphorus atom, and synthetic chemists have devoted
much effort to designing efficient phosphorylating reagents and
activating reagents such as phosphoryl chlorides and
pyrophosphates to effect such transformations.16−25 On the
other hand, P(III) is sometimes more reactive than P(V), and
phosphorus compounds are used as starting materials to react
with alcohols providing phosphites/phosphonates, which are
readily oxidized to phosphates efficiently. For example, the use
of phosphoramidites and H-phosphonates26−28 has enabled
highly reliable methods to be developed that are currently used
for the manufacture of oligonucleotides, mainly in solid-phase
synthesis.29−33 In spite of intense efforts, however, a drawback
to the use of such compounds is that they require an excess of
activating reagent and produce large amounts of waste.
Although Ishihara and co-workers reported catalytic phosphor-
ylation of alcohols with phosphoric acid, reaction temperatures
We started by examining the use of early transition-metal
catalysts that are known to be hard Lewis acids, such as
Sc(OTf)3, La(OTf)3, and Hf(OTf)4 (entries 1−3). However,
the best yield for the first screening was only 19% with
La(OTf)3. Accordingly, we turned our attention to late-
transition-metal Lewis acids (entries 4 and 5). Interestingly,
the use of Co(acac)2 led to a moderate yield of the desired
product 3a. Further extensive evaluation of various Lewis acids
finally revealed that Zn(acac)2 had good catalyst activity,
Received: March 13, 2020
https://dx.doi.org/10.1021/acs.orglett.0c00932
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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Table 1. Optimization of Reaction Conditions
Scheme 1. Substrate Scope of the Reaction with Simple
Alcohols*
entry
catalyst
x (mol %)
solvent
T (°C) yield (%)
1
2
3
4
5
6
7
8
Sc(OTf)3
10
10
10
10
10
10
10
10
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
THF
60
60
60
60
60
60
60
60
60
60
40
40
40
40
40
40
40
40
rt
<5
19
<5
58
<5
77
56
46
64
77
87
65
41
40
18
68
88
<5
94
La(OTf)3
Hf(OTf)4
Co(acac)2
Cu(TMHD)2
Zn(acac)2
Zn(OTf)2
ZnEt2
Zn(Salicylate)2
Zn4(TFA)6O
Zn(acac)2
Zn4(TFA)6O
Zn(acac)2
Zn(acac)2
Zn(acac)2
Zn(acac)2
Zn(acac)2
Zn(acac)2
Zn(acac)2
9
10
2.5
10
10
11
12
13
14
15
16
17
18
19
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
MeCN
BTF
BTF
BTF
BTF
a
b
ac
,
a
b
MS 5A (200 mg) was added instead of MS 4A (100 mg). In the
c
absence of MS 4A. Compound 2 (2 equiv) was used.
affording the target compound in 77% yield (entry 6). We
tested other Zn catalysts, and it was revealed that square-planar
Zn complexes such as Zn(acac)2 and Zn(salicylate)2 gave
higher yields than other Zn salts, although most of the Zn salts
showed higher catalytic activity than other metal salts (entries
7−9). Interestingly, the Zn oxo-cluster Zn4(TFA)6O, which is
known to be a good catalyst for transesterification,37,38 showed
catalytic activity that was comparable to that of Zn(acac)2
(entry 10). Further comparison of these active Zn salts was
performed with decreased reaction temperature. It was found
that the reaction proceeded smoothly even at 40 °C, and Zn
(acac)2 showed higher catalytic activity than the Zn cluster
catalyst (entries 11 and 12). With a decreased amount of
catalyst, the effect of solvent was examined (entries 13−16).
While the use of acetonitrile as solvent resulted in a significant
decrease in yield, probably due to undesired coordination of
the solvent to the catalyst, the use of THF gave almost the
same result as with toluene. The use of benzotrifluoride (BTF)
gave the best yield of 3a, and further improvement of the yield
could be achieved by the addition of 200 mg of MS 5A instead
of 100 mg of MS 4A (entries 16 and 17). The yield was low in
the absence of molecular sieves (entry 18). Finally, decreasing
the reaction temperature to room temperature and reducing
the amount of dimethyl phosphite to 2 equiv suppressed the
overreaction of 3a and improved the yield to 94% (entry 18).
With optimized conditions in hand, we next examined the
scope of the reaction with structurally simple alcohols (Scheme
1). We initially focused on cyclic alcohols with different ring
sizes, and found that cyclopentanol and cycloheptanol afforded
the desired H-phosphonate diesters 3b and 3c, respectively, in
high yields. Substituents on the cyclohexyl ring did not affect
the reactivity, and phosphonylated menthol 3d was obtained in
excellent yield. Bicyclic structures were also applicable in this
*
The reactions were performed on 0.3 mmol scale unless otherwise
a
b
noted. Reaction was performed in 1.2 mmol scale. Reaction was
performed at 0 °C.
reaction, and H-phosphonate diesters 3e and 3f were obtained
in high yields. Furthermore, structurally complex steroid 1g
reacted smoothly under the standard reaction conditions to
afford 3g in excellent yield. Linear secondary alcohols also
showed similar reactivity to cyclic alcohols. In addition to
simple alkyl-substituted alcohol 1h, phenyl- and vinyl-
substituted alcohols 1i and 1j also afforded the corresponding
H-phosphonate diesters in excellent yields.
For primary and secondary alcohols, the former showed
higher reactivity than the latter, and overreaction of the
products resulted in decreased final yields. This problem could
be solved by simply decreasing the reaction temperature to 0
°C. Under these reaction conditions, simple alkyl alcohols 3-
B
https://dx.doi.org/10.1021/acs.orglett.0c00932
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phenylpropanol (1k) and n-hexanol (1l) gave the desired
products in excellent yields. However, benzyl alcohol (1m)
suffered from the required compromise between reactivity and
selectivity, and the desired product 3m was obtained in 75%
yield. In contrast, alcohol 1n, bearing a primary alkyl chloride,
gave the corresponding product 3n in good yield while
retaining the potentially electrophilic chloride group. Polyether
1o and alkene 1p were also tolerated under the reaction
conditions, and 3o and 3p, respectively, were obtained in high
yields.
an initial investigation with a tertiary alcohol also suffered from
low reactivity, which led us to explore the use of more active
Zn catalysts for more demanding substrates. After extensive
studies, we finally identified sterically hindered bis(2,2,6,6-
tetramethyl-3,5-heptanedionato)zinc(II) (Zn(TMHD)2) as
the most active catalyst. With the new active catalyst in
hand, we investigated the scope of the reaction with
challenging alcohols (Scheme 3). To our delight, a sterically
Scheme 3. Substrate Scope of the Reaction with Demanding
Alcohols
We then explored the potential of our methodology by using
carbohydrates as substrates (Scheme 2). First, 1-O-methyl-2,3-
Scheme 2. Substrate Scope of the Reaction with
Carbohydrate Derivatives
hindered tertiary alcohol gave 3x in quantitative yield. Acyclic
tertiary alcohol 1y also gave the desired product 3y in good
yield. Unexpectedly, this catalyst possessed extraordinary
functional group tolerance. Using steroid 1z, which features a
free phenol group, the secondary alcohol reacted selectively to
smoothly afford 3z in excellent yield. Substrates with
carbamate and ester groups were also tolerated, with CbZ-
protected serine ester 1aa giving the desired H-phosphonate
diester 3aa in high yield. Notably, in this case, the carbamate
moiety was tolerated in this reaction system.
As described previously, the product 3g could be easily
oxidized to biologically important trialkyl phosphate 4g in high
yield by a slightly modified, reported procedure (Scheme 4).38
This result suggests that combining the currently described
method with the oxidation protocol would allow access to a
wide variety of trialkyl phosphates with high efficiency.39
a
b
Reaction was performed at 0 °C. Dimethyl phosphite (3 equiv) was
used.
acetonide-protected ribose was employed under the standard
reaction conditions. To our delight, the desired product 3q was
obtained in high yield. The protecting group at the 2,3-
positions could be changed to Bn groups without significant
loss of reactivity, and 3r was obtained in good yield.
Encouraged by the successful results of the ribose substrates,
we next investigated 2-deoxyribose substrates. 5-DMTr-
protected 2-deoxyriboses 1s and 1t worked well, and the
target H-phosphonate diesters 3s and 3t, respectively, were
obtained in good yields. The stereocenter at the 1-position (α-
and β-) did not affect the reactivity or selectivity. Similarly, 3-
BnO-protected 2-deoxyriboses 1u and 1v afforded 5-
phosphonylated compounds 3u and 3v in good yields. In
addition to pentose derivatives, protected glucose 1w also gave
the corresponding H-phosphonate diester 3w in high yield.
Apart from primary and secondary alcohols, to our
knowledge, there has been no successful example of the use
of tertiary alcohols in reported catalytic systems. In our study,
Scheme 4. Oxidation of H-Phosphonate
In conclusion, we have developed a highly active and
selective Zn-catalyzed phosphonylation of alcohols with
dimethyl phosphite. The reaction proceeded smoothly even
at 0 °C to give the target H-phosphonate diesters in high to
excellent yields. Various alcohols including primary, secondary,
and even tertiary alcohols of acyclic and cyclic structures,
carbohydrates, steroids, and amino acids reacted smoothly with
excellent functional group tolerance, and interestingly, a
sterically hindered Zn complex demonstrated the best activity
C
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00932.
Synthesis and characterization of products; general
procedure for the phosphonylation reaction; copies of
NMR charts (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Shu Kobayashi − Department of Chemistry, Graduate School of
̅
Science, The University of Tokyo, Tokyo 113-0033, Japan;
orcid.org/0000-0002-8235-4368; Email: shu_kobayashi@
chem.s.u-tokyo.ac.jp
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Bird, L.; Lowes, L. P.; Alfano, L.; Gomez, A. M.; Lewis, S.; Kota, J.;
Malik, V.; Shontz, K.; Walker, C. M.; Flanigan, K. M.; Corridore, M.;
Kean, J. R.; Allen, H. D.; Shilling, C.; Melia, K. R.; Sazani, P.; Saoud, J.
B.; Kaye, E. M.; Eteplirsen Study, G. Eteplirsen for the Treatment of
Duchenne Muscular Dystrophy. Ann. Neurol. 2013, 74, 637−647.
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Authors
Yuki Saito − Department of Chemistry, Graduate School of
Science, The University of Tokyo, Tokyo 113-0033, Japan
Soo Min Cho − Department of Chemistry, Graduate School of
Science, The University of Tokyo, Tokyo 113-0033, Japan
Luca Alessandro Danieli − Department of Chemistry,
Graduate School of Science, The University of Tokyo, Tokyo
113-0033, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00932
(16) Deutsch, A.; Ferno, O. Use of dibenzylphosphoryl chloride for
phosphorylations. Nature 1945, 156, 604−604.
Notes
The authors declare no competing financial interest.
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phosphoryl transfer using TiCl4 catalysis. Org. Lett. 2002, 4, 3671−
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ACKNOWLEDGMENTS
■
This work was partially supported by a Grant-in-Aid for
Science Research from the Japan Society for the Promotion of
Science (JSPS), The University of Tokyo, and MEXT, Japan.
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