Chiral Br?nsted Acid Catalyzed Enantioselective Phosphonylation of Allylamine via Oxidative Dehydrogenation Coupling

Article abstract of DOI:10.1021/acs.orglett.6b01514

A new strategy for the synthesis of chiral α-amino phosphonates by enantioselective C-H phosphonylation of allylamine with phosphite in the presence of a chiral Br?nsted acid catalyst has been developed. This protocol successfully integrates direct C-H oxidation with asymmetric phosphonylation and exhibits high enantioselectivity.

Full text of DOI:10.1021/acs.orglett.6b01514

Letter
pubs.acs.org/OrgLett
Chiral Brønsted Acid Catalyzed Enantioselective Phosphonylation of
Allylamine via Oxidative Dehydrogenation Coupling
Ming-Xing Cheng,† Ran-Song Ma,† Qiang Yang,† and Shang-Dong Yang*,†,‡
†State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. China
‡State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, Lanzhou, 730000, P. R. China
S
* Supporting Information
ABSTRACT: A new strategy for the synthesis of chiral α-amino phosphonates by enantioselective C−H phosphonylation of
allylamine with phosphite in the presence of a chiral Brønsted acid catalyst has been developed. This protocol successfully
integrates direct C−H oxidation with asymmetric phosphonylation and exhibits high enantioselectivity.
hiral α-amino phosphonates have attracted considerable
imines (aza-Pudovik reaction).5−7 Another widely used
Cattention in modern pharmaceutical chemistry due to approach is the asymmetric Kabachnik−Fields reaction.8 For
example, the List group developed a direct catalytic asymmetric
Kabachnik−Fields reaction to furnish β-branched α-amino
phosphonates using chiral phosphoric acid as catalyst.8b More
recently, the Feng group also reported a selective approach for
the preparation of α-amino phosphonates using a chiral
scandium-N,N′-dioxide complex.8c To date, the development
of an alternative protocol, which enables efficient synthesis of
chiral α-amino phosphonates, remains requisite. Over the past
few decades, Li’s cross-dehydrogenative coupling (CDC)
reactions provided a highly efficient and versatile protocol for
the construction of C−C and C−heteroatom bonds via reactive
imine intermediates generated from C−H oxidations,9 which
has also been proven to be an economical alternative route for
the synthesis of α-amino phosphonates.10 In the past several
years, inspired by the first asymmetric CDC reaction of Cu(I)-
catalyzed alkynylation of N-phenyltetrahydroisoquinoline,11
asymmetric CDC reactions involving in different chiral control
strategies have been disclosed and made significant progress.12
These processes fulfill the ideals of efficiency as well as green
chemistry, and they represent cutting-edge synthetic technol-
ogy in modern organic chemistry. Herein, we report a general
chiral phosphoric acid catalyzed enantioselective synthesis of
unsaturated α-amino phosphonates via aerobic sp3 C−H
oxidations. This protocol starts from readily availble allylamine,
uses a simple catalytic system, and provides access to a series of
their intriguing biological activities such as anti-HIV (human
immunodeficiency virus)1 and antibacterial activities2 as well as
protease3 and phosphatase inhibitory properties4 (Scheme 1A).
Because biological activity depends on their absolute
configuration at the α-carbon atom, the development of an
efficient way for the preparation of optically pure α-amino
phosphonates is highly desired. The most straightforward
strategy for the enantioselective synthesis of chiral α-amino
phosphonates is the asymmetric addition of phosphites to
Scheme 1. New Strategy for the Synthesis of Chiral α-Amino
Phosphonate
Received: May 26, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01514
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
chiral allylic α-amino phosphonates which are difficult to obtain
from known process (Scheme 1B).
such as 3,5-CF3C6H3, 3,5-MeC6H3, and 3,4-MeC6H3 (1i−1k)
were also examined, and the best result was obtained in 49%
yield with 62% ee. Further screening revealed that the
pentafluorobenzene substituted chiral phosphoric acid (1l)
improved the ee value to 82%, although the yield was
unsatisfying. Whereas, using a larger substituent such as
triphenylsilyl- (1m) and 2,4,6-triisopropylbenzene (1n), we
respectively obtained a racemic product and lower enantiose-
lectivity. Other complicated chiral phosphoric acids screening
has still not afforded good results (1o−1s). Based on foregoing
results, it was proven that electron-withdrawing substituents
were superior to electron-donating substituents and small steric
hindrance groups were superior to large steric hindrance groups
to obtain higher enantioselectivity. Finally, we selected 1l as the
catalyst for further screening, including various solvents,
concentrations, and reaction times. To our delight, by using
1l (10 mmol %) as catalyst and Ag2CO3 (1.1 equiv) as oxidant
in 1.5 mL of m-xylene for 0.1 mmol of 2a with 0.1 mmol of
diisopropyl phosphite at 50 °C under an argon atmosphere for
168 h (see Supporting Information for more details), the
desired product of 4a was obtained in 88% yield with 84% ee.
With the optimized conditions in hand, we explored the
scope of the reaction. We first screened the influence of the N-
substituent on the allylamine (Table 1). Results showed that
Initially, we studied the oxidative phosphonylation reaction
of diisopropyl phosphite (2.0 equiv) with 4-bromo-N-
cinnamylaniline (2a) in the presense of Ag2CO3 (1.1 equiv)
and the chiral phosphoric acid 1a (10 mmol %) in toluene at 50
°C under an Ar atmosphere. Fortunately, target product 4a was
obtained in 27% yield with 42% ee after 72 h (Scheme 2).
Scheme 2. Screening of Catalysts for Enantioselective
a
Phosphonylation of Allylamine
a
Table 1. Effect of N-Substituent
entry
R
product reaction time (h) yield (%) ee (%)
1
2
3
4
5
4-BrC6H4
4-OMeC6H4
C6H5
4a
4b
4c
4d
4e
168
92
88
84
64
71
−
75
120
120
120
85
Ts
N.R.
N.R.
CBZ
−
a
All reactions were carried out in the presence of 0.1 mmol of
allylamine and 0.1 mmol of diisopropyl phosphite in 1.5 mL of solvent
under an Ar atmosphere unless otherwise specified. Yield of isolated
product. The ee was determined by HPLC analysis using chiral
columns. N.R. = No Reaction.
a
All reactions were carried out in the presence of 0.1 mmol of 2a and
0.2 mmol of 3 in 2.0 mL of toluene under an Ar atmosphere for 72 h
the presence of p-methoxy decreased the enantioselectivity
(Table 1, entry 2). Moreover, when using Ts (p-Toluenesul-
fonate) and CBZ (Carbobenzoxy) as the N-protecting group,
the corresponding product was not obtained (Table 1, entries
4−5). These results demonstrated that the electrical of N-
substituent on the allylamine was vital for reaction activity and
high enantioselectivity.
A broad range of aromatic as well as heteroaromatic
allylamines are suitable substrates for the asymmetric reaction.
High enantioselectivities and yields were obtained (Scheme 3).
We first explored the scope in terms of aromaticity. Electron-
withdrawing or -donating groups as well as halogen substituents
were well tolerated (4f−4p), especially when allylamine with a
2-trifluoromethylbenezne substituent was used, we obtained the
desired product in 92% yield with 90% ee (4i). The reaction
with a multisubstituted aromatic also afforded higher
enantioselectivities (4q−4r). When using a naphthalene
substituent instead of a benzene substituent, the products
could be isolated in moderate yields and high enantioselectiv-
ities (4s−4t). However, when the long chain substituent was
tested, the desired product was obtained in a lower yield and
unless otherwise specified. Yield of isolated product. The ee was
b
determined by HPLC analysis using chiral columns. Reaction was
carried out in the presence of 0.1 mmol of 2a and 0.1 mmol of 3 in 1.5
mL of m-xylene under an Ar atmosphere for 168 h.
Encouraged by this result, a series of substituted chiral
phosphoric acids (1b−1s) were synthesized to catalyze the
reaction. When using 1b instead of 1a, the reaction proceeded
smoothly and achieved better enantioselectivity (63% ee). It
was demonstrated that the substituents on the skeleton were
vital to improve enantioselectivity. Next, we examined various
aryl substituents at the 3,3′-positions of the chiral phosphoric
acids (1c−1e). When the substituents 4-MeC6H4, 4-CF3C6H4,
and 4-ClC6H4 were at the 3,3′-positions of the chiral
phosphoric acid, ee values were only slightly increased and
electron-withdrawing 4-CF3C6H4 and 4-ClC6H4 were superior
to electron-donating 4-MeC6H4 to obtain higher yields and
enantioselectivities. Subsequently when the substituents were
changed to 2-PhC6H4, 2-FC6H4, and 2-acetyl-C6H4 (1f−1h),
the results remained unsatisfying. Next, multisubstituted aryls
B
DOI: 10.1021/acs.orglett.6b01514
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Organic Letters
Letter
a
Scheme 3. Scope of Allylamine
Scheme 5. Plausible Reaction Mechanism
enantioselectivity by proximity effect. Finally, protonation gives
rise to the desired product and the catalytic cycle completes.
In summary, we have reported the first general chiral
Brønsted acid catalyzed oxidative phosphonylation of an sp3
C−H bond. This protocol benefits from easily accessible
starting material and a simple catalytic system and offers easy
and high efficient access to synthesize bioactive chiral α-amino
phosphonates with high enantioselectivities. Further studies
related to C−H bond oxidative functionalization are underway
in our laboratory.
a
All reactions were carried out in the presence of 0.1 mmol of 2f−2x
and 0.1 mmol of 3 in 1.5 mL of solvent under an Ar atmosphere. Yield
of isolated product. The ee was determined by HPLC analysis using
chiral columns.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b01514.
enantioselectivity (4u). To our delight, heteroaromatic
substituents were suitable substrates for our enantioselective
phosphonylation reaction. The corresponding heteroaromatic
phosphonates 4v and 4w could be isolated in good yields and
high enantioselectivities. The allylamine with a ferrocene
substituent could be used as a substrate in the reaction to
obtain 4x in 92% ee although the yield was unsatisfying.
The optical phosphonylation data and HPLC analysis data of
phosphonate 4b were found to be in good agreement with
those reported in the literature; thus, the absolute configuration
could be determined by comparison.7b The product 4b also can
be converted to the corresponding α-amino phosphonate
without loss of enantioselectivity (Scheme 4).
■
S
Experimental details and characterization data for all new
compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: yangshd@lzu.edu.cn.
Notes
The authors declare no competing financial interest.
Scheme 4. Deprotection of N-substituent
ACKNOWLEDGMENTS
■
We are grateful for the NSFC (Nos. 21272100, 21472076, and
21532001) and Program for Changjiang Scholars and
Innovative Research Team in University (IRT_15R28)
financial support.
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