Chemoselective Synthesis of α-Amino-α-cyanophosphonates by Reductive Gem-Cyanation-Phosphonylation of Secondary Amides

Article abstract of DOI:10.1021/acs.orglett.9b01257

A novel approach to α-amino-α-cyanophosphonates has been developed. The method features a Tf₂O-mediated reductive geminal cyanation/phosphonylation of secondary amides. Mild reaction conditions, high bond-forming efficiency, inexpensive readily available starting materials, and good to excellent yields with wide functional group compatibility constitute the main advantages of this method. The protocol can be run on a gram scale.

Full text of DOI:10.1021/acs.orglett.9b01257

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Chemoselective Synthesis of α‑Amino-α-cyanophosphonates by
Reductive Gem-Cyanation−Phosphonylation of Secondary Amides
Ting-Ting Chen, Ai-E Wang,*,†,‡ and Pei-Qiang Huang*,†,‡
†Department of Chemistry and Fujian Provincial Key Laboratory of Chemical Biology, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen 361005, Fujian, China
‡State Key Laboratory of Applied Organic Chemistry Lanzhou University, Lanzhou 730000, China
S
* Supporting Information
ABSTRACT: A novel approach to α-amino-α-cyanophosphonates has been developed. The method features a Tf2O-mediated
reductive geminal cyanation/phosphonylation of secondary amides. Mild reaction conditions, high bond-forming efficiency,
inexpensive readily available starting materials, and good to excellent yields with wide functional group compatibility constitute
the main advantages of this method. The protocol can be run on a gram scale.
However, compared to the tremendous number of reports on
the preparation of α-aminophosphonic acid derivatives6 and α-
aminonitriles,5a,c very few examples are known for the synthesis
of α-amino-α-cyanophosphonic acid derivatives (Scheme 1).7
Moreover, all of those methods are based on specific
multifunctional starting materials, and it takes three to four
steps to prepare them. Consequently, the development of a
general and efficient method for the synthesis of α-amino-α-
cyanophosphonic acids from simple and readily accessible
starting materials is highly desirable.
Amides are a class of readily available and stable compounds
which are widespread in nature.8 Due to their low electro-
philicity, breakthroughs for the chemoselective and direct
conversion of amides into functional groups at lower oxidation
states have not been achieved until recent years.9−17 In this
respect, several groups have reported the reductive cyanation14
and alkylation/cyanation15 of amides for the synthesis of α-
amino nitriles (Scheme 2). Using Schwartz’s reagent, Zhao has
developed a reductive phosphonylation of amides for the direct
transformation of amides into α-aminophosphonates.16 In
addition, our group has reported the reductive bisphosphony-
lation of amides for the synthesis of α-amino bisphosphonates.17
In this context, we envisioned that the reductive geminal
cyanation/phosphonylation of amides, with a C−C bond and a
C−P bond formed at the carbonyl carbon, would provide an
attractive method for the efficient synthesis of α-amino-α-
cyanophosphonic acid derivatives. As part of our ongoing
interest in developing new synthetic methods based on
activation of amides with trifluoromethanesulfonic anhydride
(Tf2O),12a,b,d−f,h,14e,g,17 we report herein the Tf2O-mediated
α-Aminophosphonic acids are isosteres of α-amino acids in
which the planar carboxyl group is replaced by a tetrahedral
phosphonic acid moiety. α-Aminophosphonic acids and their
phosphonate esters as well as phosphono peptides exhibit
diverse biological activities and have found wide applications in
the areas of biological and medicinal chemistry as well as
agriculture.1 They have also been used as important synthetic
intermediates2 and organocatalysts3 in organic synthesis.
On the other hand, α-aminonitriles are common structural
motifs found in many bioactive natural products and
pharmaceutical agents.4,5c They are also versatile building
blocks that can be readily converted to α-amino acids, α-
amino carbonyl compounds, vicinal diamines, β-amino alcohols,
as well as a wide range of nitrogen heterocycles.5
Owing to the wide range of activity and importance of α-
aminophosphonic acids and α-aminonitriles, the preparation of
trifunctional α-amino-α-cyanophosphonic acid derivatives 1
(Scheme 1), which combine both of the above characteristic
structures, is therefore very attractive.
Scheme 1. Synthetic Routes to α-Amino-α-cyanophosphonic
Acid Derivatives
Received: April 10, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01257
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
As shown in Table 2, under the optimized conditions, a variety
of secondary amides underwent reductive geminal cyanation/
phosphonylation with TMSCN and diethyl phosphite to give
the corresponding α-amino-α-cyanophosphonates in good
isolated yields. The electronic properties of aroyl amides do
not have an obvious influence on the reaction. High yields were
obtained for benzamides bearing electron-donating (1b, 1d, 1e)
or electron-withdrawing (1j−o) groups at the para position of
the phenyl ring (84−98% yields). The steric effects of the
substituents seem to have some influence. A slight decrease in
yields was observed for benzamides 2c and 2h, which have
substituents at the ortho position of the ring (71% and 82%
yields, respectively). The reaction is widely functional group
tolerant and highly chemoselective. Consistent with the
observations by Charette,11b−d not only halogen (1f−i), nitro
(1k), cyano (1l), and azo (1m) groups but also ester (1o) and
ketone (1n) groups are largely compatible with this Tf2O-
mediated process to furnish the desired α-amino-α-cyano-
phosphonates in high yields. 2-Naphthamide 2p was also a
suitable substrate for the reaction.
The N-substituents on the secondary aroyl amides could vary
from isopropyl to other sec-alkyl (1q−s) and n-alkyl (1t−y)
substituents and benzyl (1z) or allyl (1aa) groups, and some
functional groups such as ether (1s and 1u), ester (1v), and
halogen (1w and 1x) groups are tolerated. Interestingly, for N-
(4-bromobutyl)benzamide 2y, the tandem cyanation/phospho-
nylation and cyclization occurred to afford the α-amino-α-
cyanophosphonate 1y. The introduction of an N-tert-butyl and
N-phenyl group to the secondary amide, however, completely
abolished product formation (1ab and 1ac). Furthermore, the
branched nonenolizable pivalamide 2ad is also a suitable
substrate for the reaction. No desired product was obtained
for enolizable alkanamides. Instead of imidoyl cyanide, enamine
derivative was formed after reaction with TMSCN.
Scheme 2. Reported Methods for the Direct Conversion of
Amides to α-Aminonitriles and α-Aminophosphonates and
Our Proposal for the Synthesis of α-Amino-α-
cyanophosphonates from Amides
reductive geminal cyanation/phosphonylation of secondary
amides for the preparation of α-amino-α-cyanophosphonates.
The initial investigation began with the reaction of Tf2O-
activated N-isopropylbenzamide 2a with TMSCN and diethyl
phosphite (Table 1). When a solution of amide 2a (1.0 equiv)
Table 1. Reaction Optimization on Reductive Geminal
a
Cyanation/Phosphonylation of Secondary Amides
b
entry phosphite (equiv)
base
temp (°C) yield (%)
c
1
2
3
4
5
6
7
8
2.0
2.0
2.0
2.0
2.0
2.0
1.5
1.3
1.3
1.1
1.3
1.3
None
None
None
0 to rt
rt
45
rt
rt
rt
rt
rt
rt
rt
81 (73 )
81
55
90
DBU (2.0 equiv)
c
K2CO3 (2.0 equiv)
Na2CO3 (2.0 equiv)
K2CO3 (2.0 equiv)
K2CO3 (2.0 equiv)
Na2CO3 (2.0 equiv)
K2CO3 (2.0 equiv)
K2CO3 (1.8 equiv)
K2CO3 (1.5 equiv)
96 (90 )
Besides diethyl phosphite, reductive geminal cyanation/
phosphonylation of amides with different phosphites was also
studied. The reaction with dimethyl phosphite and diisopropyl
phosphite gave the expected products 1ae and 1af in 92% and
85% yield, respectively, while no product was obtained for di-
tert-butyl and diphenyl phosphite (1ag and 1ah). Diphenyl-
phosphine oxide is inert for the reaction (1ai).
c
96 (89 )
c
96 (92 )
c
95 (92 )
c
9
93 (85 )
c
10
11
12
94 (88 )
c
rt
rt
95 (90 )
c
93 (88 )
To explore the practical utility of this transformation in
organic synthesis, a gram-scale preparation of α-amino-α-
cyanophosphonate 1z from amides 2z (1.06 g, 5.00 mmol)
was performed, which furnished the desired product 2z in 80%
yield.
a
Reaction conditions: amide 2a (0.50 mmol), Tf2O (0.55 mmol), 2-
F-Pyr (0.60 mmol), CH2Cl2 (0.2 M), 0 °C, 10 min; TMSCN (0.60
b
mmol), 0 °C to rt, 3 h; HPO(OEt)2, 10 h. Determined by 1H NMR
c
with 1,3,5-trimethoxybenzene as internal standard. Isolated yield.
It is worth mentioning that most of the products obtained are
stable compounds, which can be purified by flash chromatog-
raphy and kept stable even after storage at room temperature for
one year, except for ortho-substituted 1c and 1h, which are labile.
Interestingly, 1w and 1x, which are prone to form aziridiniums,19
are also stable.
In conclusion, we have developed a novel and general method
for the preparation of α-amino-α-cyanophosphonates. The
method features a Tf2O-mediated reductive geminal cyanation/
phosphonylation of secondary amides. The reaction is mild,
efficient, and tolerant of a wide range of functional groups,
providing the corresponding α-amino-α-cyanophosphonates in
good to excellent yields. Further applications of this method are
currently underway in our laboratory.
and 2-F-Pyr. (1.2 equiv) in CH2Cl2 was treated successively with
Tf2O (1.1 equiv, 0 °C, 10 min), TMSCN (1.2 equiv, 0 °C to rt, 3
h), and HPO(OEt)2 (2.0 equiv, 0 °C to rt, 10 h), the expected α-
amino-α-cyanophosphonate product 1a was isolated in a yield of
73% (entry 1). Addition of HPO(OEt)2 in the presence of
organic/inorganic bases (2.0 equiv) improved the yield in most
cases (entries 4−6, see the SI for details), probably due to the
influence of the base on the tautomerism, favoring the phosphite
form.18 The highest yields were obtained with K2CO3 and
Na2CO3, providing product 1a in 90% and 89% isolated yield,
respectively (entries 5 and 6). Further optimization showed that
K2CO3 was the optimal base, in the presence of which the
amount of diethyl phosphite can be decreased to 1.3 equiv
without much influence on the yield (entry 8).
B
DOI: 10.1021/acs.orglett.9b01257
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Organic Letters
Letter
a
Table 2. Substrate Scopes for Reductive Geminal Cyanation/Phosphonylation of Secondary Amides
a
Reaction conditions: amide (0.50 mmol), Tf2O (0.55 mmol), 2-F-Pyr (0.60 mmol), CH2Cl2 (0.2 M), 0 °C, 10 min; TMSCN (0.60 mmol), 0 °C
b
to rt, 3 h; HPO(OR)2 (0.65 mmol), K2CO3 (1.0 mmol), 10 h. Molecular ion peaks were not observed in their HRMS spectra, while [M − HCN]+
peaks appeared as the most intensive ones. (See the SI for details). Run on 5.0 mmol scale.
c
C
DOI: 10.1021/acs.orglett.9b01257
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(8) The Amide Linkage: Structural Significance in Chemistry,
Biochemistry and Materials Science; Greenberg, A., Breneman, C. M.,
Liebman, J. F., Eds.; Wiley: New York, 2003.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.9b01257.
■
S
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and 1H/13C/31P NMR spectra of new compounds (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: aiewang@xmu.edu.cn.
*E-mail: pqhuang@xmu.edu.cn.
ORCID
Ai-E Wang: 0000-0002-6891-4840
Pei-Qiang Huang: 0000-0003-3230-0457
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support of this work from the National Key R&D
Program of China (Grant No. 2017YFA0207302), the National
Natural Science Foundation of China (Nos. 21672176 and
21332007), the PCSIRT of Ministry of Education, and the
Natural Science Foundation of Fujian Province of China (No.
2014J01062) is gratefully acknowledged.
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DEDICATION
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This work is dedicated in honor of the 90th birthday of Professor
Qing-Yun Chen.
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Sprachmann, J.; Gonzalez, L.; Maulide, N. Org. Lett. 2017, 19, 2662.
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