Organocatalytic Phosphonylation of in Situ Formed o-Quinone Methides

Article abstract of DOI:10.1021/acs.orglett.7b03019

A new class of Br?nsted acid catalysts based on N-heterocyclic phosphorodiamidic acids (NHPAs) has been developed. The NHPA catalyst promotes phospha-Michael addition reaction of trialkylphosphites to in situ generated ortho-quinone methides (o-QMs) for t

Full text of DOI:10.1021/acs.orglett.7b03019

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX-XXX
Organocatalytic Phosphonylation of in Situ Formed o‑Quinone
Methides
Hai Huang and Jun Yong Kang*
Department of Chemistry and Biochemistry, University of Nevada Las Vegas, 4505 South Maryland Parkway, Las Vegas, Nevada
89154-4003, United States
S
* Supporting Information
ABSTRACT: A new class of Brønsted acid catalysts based on N-
heterocyclic phosphorodiamidic acids (NHPAs) has been
developed. The NHPA catalyst promotes phospha-Michael
addition reaction of trialkylphosphites to in situ generated ortho-
quinone methides (o-QMs) for the construction of diaryl
phosphonates in moderate to excellent yields with 1.5 mol %
catalyst. Diastereoselective synthesis of P-chiral phosphinate esters
is achieved with the use of dialkyl phenylphosphonites.
rganophosphonate compounds and their derivatives have
shown a wide range of applications in medicinal
co-workers disclosed a Pd-catalyzed deprotonative cross-
coupling reaction of benzyl phosphonates with aryl halides.13
This protocol, however, is restricted to only benzyl diisopropyl
phosphonate substrate due to the use of a nucleophilic base.
Recently, the Anand group14 and our laboratory15 revealed 1,6-
hydrophosphonylation of para-quinone methides (p-QMs) for
the construction of diaryl phosphonates under metal-free
conditions. Nonetheless, for the facile synthesis and inherent
instability of p-QMs,16 a di-tert-butyl group on p-QM derivatives
is necessary, which may decrease the synthetic value of the
product with extra steps for removal of the protecting group.
Considering the prevalent applications in many different fields,
the development of an efficient, mild synthetic protocol for diaryl
phosphonates under metal-free conditions is highly desirable.
While the p-QMs have been extensively used in the phospha-
Michael reaction,14,15 ortho-quinone methides (o-QMs), iso-
meric p-QM counterparts, have remained underexplored
Michael acceptors. Since the pioneering work by Fries and
Kann in 1907,17 o-QMs have been regarded as highly versatile
synthetic intermediates in organic synthesis.18 They have been
utilized in various synthetic transformations such as the [4 + n]
cycloaddition reaction,19 Michael addition reaction,19c 6π-
electrocyclization,20 and others.18b,d Among the synthetic
transformations, Michael addition reaction of o-QMs or aza-o-
QMs with different nucleophiles, including carbon, nitrogen,21
sulfur, and oxygen nucleophiles,22 has become an efficient
synthetic strategy for the direct synthesis of diverse ortho-
hydroxybenzyl or 2-aminobenzyl compounds. Despite the
successful application of carbon and various heteroatom
nucleophiles,19h,i,23 phospha-Michael reaction of trialkylphos-
phites with o-QMs has remained dormant since its discovery, due
to the challenge of in situ transformation of P(III) to P(V).24 This
O
chemistry,1 agrochemistry,2 and organic synthesis.3 In particular,
diaryl phosphonates exhibit a broad spectrum of significant
biological activities such as human prostatic acid phosphatase
inhibition A,4 leukocyte elastase inhibition B,5 and calcium
antagonistic activity C6 (Figure 1). They are also used for the
Figure 1. Representative examples of functional diaryl phosphonates
and their derivatives.
preparation of chemiluminescence materials D7 and flame
retardants E8 (Figure 1). In addition, they serve as versatile
building blocks for the synthesis of vinyl-based functional
compounds such as fluorescent materials9 and OLED emitters.10
Although the Michaelis−Arbuzov reaction represents a classic
method for diaryl phosphonate synthesis, this procedure is
limited to alkyl halide substrates and elevated reaction temper-
ature.11 To overcome these limitations, much effort has been
devoted to develop new synthetic methods for constructing
diaryl phosphonates. For example, the Chakravarty group
reported FeCl3-mediated Friedel−Crafts-type arylation of α-
hydroxy phosphonates with various arenes, which requires a
stoichiometric amount of FeCl3.12 On the other hand, Walsh and
Received: September 26, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03019
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
oxidation process is of utmost importance to establish a catalytic
cycle and typically requires extra nucleophilic additives,24 which
can make the reaction more complex. In this regard, in situ
generation of the advantageous nucleophiles for the trans-
formation of P(III) to P(V) would be an ideal strategy.
Scheme 2. Scope of Phospha-Michael Reaction
Our group has a long-standing interest in the development of
N-heterocyclic phosphines (NHPs) and their derivatives for the
discovery of new synthetic transformations.15,25,26 With the
continued efforts to develop NHP-derived catalysts, we recently
synthesized N-heterocyclic phosphorodiamidic acids (NHPAs)
and discovered their exceptional catalytic activity in the phospha-
Michael reaction. Herein, we report a novel NHPA-catalyzed
phospha-Michael addition reaction of o-QMs with trialkylphos-
phites and dialkyl phenylphosphonites for the synthesis of diaryl
phosphonates and phosphinates, respectively. To the best of our
knowledge, this transformation demonstrates the first diaster-
eoselective phospha-Michael addition reaction of dialkyl phenyl-
phosphites to o-QMs.
a
Reaction conditions: 1 (0.1 mmol), 2 (0.1 mmol), and NHPA1 (1.5
b
c
mol %) in DCM (0.5 mL) at rt for 18 h. Isolated yield. Reaction run
for 48 h. 1.0 mL of DCM was used.
d
With the NHPAs (NPHA1−4) in hand (see Supporting
Information (SI) for the NHPA synthesis), we explored the
potential of NHPAs as organocatalysts in the phospha-Michael
reaction of 2-(hydroxy(phenyl)methyl)phenol 1a with triethyl-
phosphite 2a (Scheme 1). To our delight, the desired diaryl
example, 1q and 1r with aliphatic substituents were also suitable
substrates for this reaction to provide the alkyl-substituted benzyl
phosphonates 3q, 3r in 86% and 78% yields, respectively.
Saligenol 1s was smoothly converted to benzyl phosphonate 3s
with 91% product yield. In an effort to challenge the synthesis of
tetrasubstituted diaryl phosphonates, no target product was
observed when diaryl methyl tertiary alcohol 1t was treated with
2a, probably due to the steric hindrance (Scheme 2, 3t). Finally,
different alkylphosphites such as P(OMe)3 2b and P(OiPr)3 2c
were evaluated, and they also afforded the corresponding diaryl
phosphonates 3u, 3v in 99% and 94% yields, respectively.
Having early success in the phosphonylation of o-QMs with
trialkyl phosphites, we explored the reactivity of dialkyl
phenylphosphonites under the same reaction conditions
(Table 1). This reaction allows a regio- and diastereoselective
transformation, providing only 1,4-addition products with good
diastereoselectivity (4:1 dr). First, we investigated the scope of
a
Scheme 1. Optimization of the Reaction Conditions
a
Reaction conditions: 1a (0.1 mmol), 2a (0.1 mmol), and acids (1.5
b
mol %) in DCM (0.5 mL) at rt for 18 h. Isolated yield.
Table 1. Substrate Scope of Diastereoselective Phospha-
Michael Reaction
phosphonate 3a was obtained in 99% yield by NMR and 91%
isolated yield with only 1.5 mol % NHPA1. Further modification
of the NHPAs with electron-donating groups on the nitrogen
atom (NHPA2, NHPA4) decreased the catalytic activity
whereas that of electron-deficient groups maintained the
excellent catalytic reactivity (NHPA3). These outcomes strongly
support that the pKa of NHPAs can be systemically modified. We
also tested the known Brønsted acids NHPA527 and BPA, but
they were inferior to our NHPA1 (see SI for more conditions).
Having established the optimized reaction conditions, we
investigated the scope of the reaction described in Scheme 2. The
electronic effects of the substrates on this transformation are
negligible whereas the steric effects significantly influence the
product yields in this phospha-Michael reaction. For example,
the reaction tolerates both electron-donating and -deficient
groups, providing the corresponding products in high to
excellent yields (Scheme 2, 3a−e, 3g−m). In contrast, ortho-
substituted substrates 1f (4,6-ditert-Bu) and 1n (2-MeO)
provided the target products in 25% and 64% yields, respectively
(Scheme 2, 3f, 3n). A polycyclic aromatic compound 1o also
proved to be a suitable substrate, producing 2-naphthyl
phosphonate 3o in 75% yield. A heteroaromatic substrate 1p
also succeeded in providing the desired adduct 3p in 78% yield.
We also explored the scope of alkyl, aryl-mixed substrates. For
a
b
c
entry
R
R1
R2
product
yield (dr)
1
Me
Et
H
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
4a
4b
4c
4d
4e
4f
85% (4:1)
84% (3:1)
81% (4:1)
80% (4:1)
76% (4:1)
74% (4:1)
84% (4:1)
82% (4:1)
81% (4:1)
69% (4:1)
2
H
3
iPr
H
4
Me
Me
Me
Me
Me
Me
Me
Me
4-Me
4-Cl
4-Br
H
5
6
7
4-MeC6H4
4-MeSC6H4
4-FC6H4
4g
4h
4i
8
H
9
H
10
11
H
4-PhenylC6H4
2-Naph
4j
d
H
4k
58% (99:1)
a
Reaction condition: 1 (0.1 mmol), 2 (0.1 mmol) and NHPA1 (1.5
b
c
mol %) in DCM (0.5 mL) at rt for 18 h. Isolated yield. dr value
d
1
determined by H NMR on the crude reaction mixture. dr value of
the isolated product by flash column chromatography on a silica
column.
B
DOI: 10.1021/acs.orglett.7b03019
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Organic Letters
Letter
phosphonite nucleophiles. When dimethyl phenylphosphonite
2d was employed, the target phosphinate product 4a was isolated
in 85% yield with 4:1 dr (Table 1, entry 1). Other nucleophiles
such as ethyl and isopropyl phosphonites 2e, 2f also provided the
desired phosphinate products 4b, 4c in 84% and 81% yields with
3:1 and 4:1 dr values, respectively (Table 1, entries 2−3). Next,
we examined the scope of Michael acceptors with various
substituents and they all yielded the corresponding products in
good yields (Table 1, entries 4−10). Finally, a solubility-based
purification of diastereomers was successfully demonstrated.
Purification via flash column chromatography of a crude mixture
of 2-naphthyl phosphinate (4k) with 4:1 dr allowed the isolation
of the major diastereomer in 58% yield with 99:1 dr (Table 1,
entry 11). With the demonstration of an efficient column
purification of the diastereomers, we further examined different
phosphinate products 4a−e by performing second column
chromatography. In general, they were all succeeded in the
isolation of the major diastereomers. This flash column
chromatography was particularly useful for the purification of
diastereomers 4a, 4e (dr >99:1) (see SI for proposed model for
diastereoselectivity).
Scheme 4. Proposed Mechanism
Scheme 5. Synthetic Utility of Diaryl Phosphonate 3a
Nucleophilic additives are usually required in the phospha-
Michael reaction with trialkylphosphites for the transformation
of P(III) to P(V).24 In contrast, our NHPA-catalyzed phospha-
Michael reaction of o-QM with P(OEt)3 generates the target
Michael adducts without the nucleophile additives. We reasoned
that the in situ generated H2O molecule by dehydration of the o-
hydroxybenzyl alcohols can act as an internal nucleophile to
transform P(III) to P(V). To test our hypothesis, control
experiments with drying agents such as molecular sieves and
MgSO4 were performed, and reduced product yields (66−78%)
of 3a by NMR were observed (Scheme 3, eq 1). These outcomes
Scheme 3. Control Experiments
the synthetic transformation of 3a. The phenol group on 1a was
readily oxidized to γ-ketophosphonate 5a with PhI(OAc)2. The
conversion of the phenolic hydroxyl group 3a to the
corresponding aryl triflate 5b in the presence of Tf2O and
Et3N proceeded smoothly, and the sequential Suzuki cross-
coupling reaction of 5b with PhB(OH)2 delivered the target
product 5c in 85% yield. McKenna reaction conditions28 for
dealkylation of the ethylphosphonate 3a provided only cyclic
phosphonate 5d as a potential halogen-free flame retardant.8 The
treatment of 3a with allyl bromide under basic conditions
afforded allyloxy substituted diaryl phosphonate 5e in moderate
yield. Finally, considering the significant application of the cyclic
strongly suggest that the H2O molecule plays an important role
in the reaction process. Additionally, an in situ NMR study of this
phospha-Michael reaction of 1a with P(OiPr)3 2c was conducted
in CD2Cl2 solvent which also supports that the H2O molecule
serves as an internal nucleophile to generate the target product
3v and iPrOH (Scheme 3, eq 2).
To gain insights into the catalytic cycle of this transformation,
a plausible mechanism is proposed on the basis of the results
from the control experiments (Scheme 4). The NHPA-catalyzed
dehydration of o-hydroxybenzyl alcohol 1a results in the
generation of o-QM intermediate A, which is activated by a
hydrogen bond with the NHPA1. The following phospha-
Michael addition reaction with P(OEt)3 2a generates a
phosphonium intermediate B, which then is attacked by a H2O
nucleophile to produce the diarylphosphonate product 3a and
EtOH.
phosphonates as precursors of stabilized C-centered radicals,29
a
one-pot cyclization of 3a in the presence of SOCl2 and K2CO3
was conducted and the desired cyclic phosphonate product 5f
was obtained in 93% yield.
In summary, we have developed a novel N-heterocyclic
phosphorodiamidic acid (NHPA) organocatalyst for the
phospha-Michael reaction of o-QMs with trialkylphosphites.
This NHPA catalyst has demonstrated its high catalytic efficiency
(1.5 mol % catalyst) in phosphonylation of o-QMs to synthesize
versatile diaryl phosphonates under mild reaction conditions. In
addition, this organocatalytic system enables diastereoselective
synthesis of diaryl phosphinates employing dialkyl phenyl-
phosphonites. A series of control experiments and an in situ
NMR study suggest that a H2O molecule generated by
dehydration of o-hydroxybenzyl alcohol serves as an internal
nucleophile for the transformation of P(III) to P(V). On the
To demonstrate the synthetic utility of the versatile diaryl
phosphonate adducts, we first tested a large-scale experiment
with 1a (1.0 g, 5.0 mmol), and it afforded the target Michael
adduct 3a (1.44 g) in 90% yield (Scheme 5). Next, we explored
C
DOI: 10.1021/acs.orglett.7b03019
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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.orglett.7b03019.
■
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Experimental details (PDF)
Spectral data (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: junyong.kang@unlv.edu.
ORCID
Jun Yong Kang: 0000-0002-7178-2981
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by University of Nevada Las Vegas.
Maciej Kukula at SCAAC is acknowledged for mass spectra data.
Dr. Nagaraju Molleti (Stockholm University) is acknowledged
for his early, related work. The authors thank Prof. Rich G. Cater
(OSU) and Dr. Mike Standen (Valliscor) for helpful discussions.
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