Electrochemical Approach for Direct C-H Phosphonylation of Unprotected Secondary Amine

Article abstract of DOI:10.1021/acs.orglett.9b02707

Direct α-phosphonylation of an unprotected secondary amine in a single step is of practical importance to amino phophophates. However, this protocol is limited due to the high redox barrier of unprotected amine. In this paper, we report C-H phosphonylation of an unprotected secondary amine via an electrochemical approach in the presence of catalytic carboxylate salt. This metal-free and exogenous oxidant-free method furnishes diverse target molecules with satisfactory yield under mild reaction conditions. Successful application of the protocol in a gram-scale experiment demonstrates the potential utility for further functionalization.

Full text of DOI:10.1021/acs.orglett.9b02707

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Electrochemical Approach for Direct C−H Phosphonylation of
Unprotected Secondary Amine
Min Huang,† Jie Dai,‡,§ Xu Cheng,*,‡,§ and Mengning Ding*,†
†Key Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, ‡Jiangsu Key Laboratory of Advanced
§
Organic Materials, School of Chemistry and Chemical Engineering, and National Demonstration Centre for Experimental
Chemistry Education, Nanjing University, Nanjing 210023, China
S
* Supporting Information
ABSTRACT: Direct α-phosphonylation of an unprotected
secondary amine in a single step is of practical importance to
amino phophophates. However, this protocol is limited due to
the high redox barrier of unprotected amine. In this paper, we
report C−H phosphonylation of an unprotected secondary
amine via an electrochemical approach in the presence of
catalytic carboxylate salt. This metal-free and exogenous
oxidant-free method furnishes diverse target molecules with
satisfactory yield under mild reaction conditions. Successful application of the protocol in a gram-scale experiment demonstrates
the potential utility for further functionalization.
irect α-phosphonylation of amine could provide amino
derivation, a direct phosphonylation of unprotected secondary
amine would be desired.
D
phosphonates, mimics of amino acids with broad
pharmaceutical applications.1 It was, however, unexpected
that most of the direct phosphonylation protocols require
tertiary amine or amide as starting materials, in the presence of
various catalysts and stoichiometric oxidant2 (Scheme 1a).
Electrochemical organic synthesis has recently experienced
tremendous growth, leading to a variety of novel and
unexpected transformations3 with transitional-metal catalyst,4
organocatalyst,5 and even catalyst-free6 conditions. Compared
to conventional synthetic methods, the electrochemical
strategy could also be conveniently employed to construct
C−C,7 C−O,8 C−N,9 and C−S10 bonds under much more
mild conditions. Despite the progress on the electrochemical
C−P bond formation from trisubstituted amine and imine
made in recent years through electrochemical strategies,11 the
C−H phosphonylation from unprotected secondary amine has
been left unexplored. Lei12 and co-workers demonstrated
electrochemical dehydrogenative aromatization of a series of
N-heterocycles. In this chemistry, a mediated dehydrogenation
of unprotected cyclic amine was proposed as the operative
mechanism, whereas the dehydrogenation took place at the Pt
anode to produce aromatized compounds as the major
product. Inspired by such correlation between anodic materials
and the electron transfer pathway, we achieved a readily anodic
oxidation of secondary free amine, affording the C−H
phosphonylation by the combination of a suitable anode and
Brønsted base catalyst. Mechanistic investigations further show
that the electro-organic reaction follows a Brφnsted base
catalysis pathway (Scheme 1b).
Scheme 1. Directed C−H Phosphonylation of Amine
Two possible reasons could contribute to this substrate
preference: (i) the trisubstituted amine was more electron-
donating and could more readily undergo single electron
transfer with oxidative reagent or catalyst, and (ii) the iminium
ion intermediate of amide or trisubstituted amine was highly
reactive toward nucleophiles, facilitating the intermolecular
bond formation. In comparison, the similar conversion
involving unprotected amine remains unexplored. As the
unprotected amine could provide broad diversity during
We started the study using 1,2,3,4-tetrahydroisoquinoline
(1a) as the starting material and diethyl phosphate (2a) as a
model nucleophile to trap the dehydrogenative intermediate.
Graphite felt was employed as both the anode and the cathode
Received: August 1, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02707
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
material with controlled cell potential. At first, LiClO4 was
applied as an electrolyte, and desired product 3a was detected,
giving 46% yield. Due to the property of LiClO4 as a Lewis
acid, we evaluated Cu(OTf)2 as an additional Lewis acid
additive. Unexpectedly, with 10 mol % of Cu(OTf)2 loading,
the desired 3a was not detected at all. So, we subjected a
catalytic amount of base into the reaction, and NaOAc was
proven as the optimum choice, giving 3a in 76% isolated yield.
Several tetraalkylammonium salts were also employed as
alternative electrolytes instead of LiClO4, and Bu4NClO4
could give an acceptable yield of 64%. Subsequently, other
solvents (including DMF, DCM, and MeOH) and electrode
materials were tested, as summarized in Table 1.
Scheme 2. Substrates of Unprotected Cyclic Secondary
Amine
a
Table 1. Optimization of Reaction Conditions
additive
(20 mol %)
b
entry electrolyte
solvent
electrode yield (%)
1
2
3
4
5
6
7
8
LiClO4
LiClO4
LiClO4
LiClO4
Bu4NClO4
Bu4NBr
Bu4NF
LiClO4
LiClO4
LiClO4
LiClO4
LiClO4
LiClO4
LiClO4
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DMF
CH2Cl2
MeOH
MeCN
MeCN
MeCN
C+/C−
C+/C−
C+/C−
C+/C−
C+/C−
C+/C−
C+/C−
C+/C−
C+/C−
C+/C−
C+/C−
C+/Pt−
Pt+/C−
Pt+/Pt−
46
ND
88 (76 )
76
64
32
35
48
15
ND
22
30
27
20
Cu(OTf)2
NaOAc
K3PO4
c
NaOAc
NaOAc
NaOAc
lutidine
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
9
10
11
12
13
14
a
Conditions: 1a (0.5 mmol), 2a (1 mmol), supporting electrolyte
(0.1 mmol), solvent (20 mL), graphite felt (C) anode and cathode, rt,
b
6.6 V, 6 h. Yields were determined by 31P NMR using trimethyl
c
phosphate as internal standard. Isolated yield.
With the optimized conditions (Table 1, entry 3), we
explored the substrate scope for this dehydrophosphonlyation
reaction with a series of unprotected secondary amines
(Scheme 2). Initially, the substrates with both electron-
donating and -withdrawing groups at position 6 were proved
feasibly, affording the desired products (3b−3h) in moderate
to excellent yield. We found that N-heterocycles with an
electron-donating group can achieve a relatively higher yield
(3b−3f). In addition, N-heterocycles substituted with 3,5-
dimethylisoxazole and o-methyl(phenyl)sulfane at position 6
also lead to favorable yields (3g, 3h). These results indicate
that there is no inhibition by the heterocyclic substituent in
this electro-organic system. Different groups were introduced
into position 7 of N-heterocycles. The result shows that
electron-donating groups favor a higher yield, which agrees
with that of position 6. In addition, the halogen atoms Br and F
are well-tolerated, and target molecules (3j, 3k) were easily
achieved with a moderate yield. For the substitution of methyl,
methyl formate, and cyanide, the desired molecules can also be
achieved with moderate yield (3i, 3l, 3m). At a specific length,
a trifluoromethyl at position 8 was able to furnish the
a
Reaction conditions: 1 (0.5 mmol), 2 (1.0 mmol), NaOAc (0.1
mmol), LiClO4 (0.1 mmol), CH3CN (20 mL), 6.6 V, rt, graphite felt.
Isolated yields.
corresponding product (3o), and two methoxyls at positions 6
and 7 (3n) even gave an excellent yield up to 80%.
Simultaneously, some substrates with substitution at position
2 were investigated. In these cases (3p, 3q), the yield was 73
and 67%, respectively. A substrate changing its aromatic ring
from benzene into pyridine (3r) was amenable to this
transformation, as well. In this case, even with a bulkier steric
hindrance at the α-position, the target C−P was constructed
successfully with an acceptable yield (3s). Finally, the small
steric hindrance of dimethyl phosphonate (3t) suited the
system well and afforded the field at a close level with that of
the standard reaction. Because of the bulky steric hindrance of
dibenzyl phosphite, 3u afforded a relatively low yield.
The dehydrophosphonlyation approach was then applied to
acyclic molecules (Scheme 3). Target products were
B
DOI: 10.1021/acs.orglett.9b02707
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3. Substrates of Unprotected Acyclic Secondary
a
Amine
Figure 1. Cyclic voltammogram (CV) investigations of direct
electrochemical C−H phosphonylation. CV characteristics of the
reaction background (solvent MeCN and electrolyte LiClO4),
phosphate 2a, 2a in NaOAc, amine 1a, 1a in NaOAc, and a mixture
of 1a and 2a in NaOAc (best mimicking the reaction conditions).
Scheme 4. Plausible Reaction Pathway
a
Reaction conditions: 4 (0.5 mmol), 2 (1.0 mmol), NaOAc (0.1
mmol), LiClO4 (0.1 mmol), CH3CN (20 mL), 6.6 V, rt, graphite felt.
Isolated yields.
successfully achieved (5a−5e). First, N-benzylcyclohexan-
amine (4a) was subjected to the reaction and afforded 5a in
35% yield. Then the methoxy was introduced to the aryl para-
position. For the substrate N-(4-methoxybenzyl)-
cyclohexanamine, the reaction occurred and provided an
acceptable yield of 53% (5b). Cyclohexyl connected to N was
next replaced by tertiary butyl (5c) and cyclopentyl (5d),
which also led to a desired product at the same level as 5a.
Specifically, the primary amine (4-methoxyphenyl)-
methanamine was employed to the system, and the target
secondary amine product was achieved (5e).
To demonstrate the synthetic application of this electro-
chemical protocol, a scale-up reaction was carried out. The
reaction was designed using 5 mmol 1a and 10 mmol 2a. All of
the additives were scaled up corresponding to the amount we
optimized. The reaction time was prolonged to 28 h, and the
reaction proceeded smoothly. Finally, the target product 3a
was gained with a 67% isolated yield.
To gain further mechanistic insights about this electro-
organic system, we studied the voltammogram properties of 1a,
2a, NaOAc, and the reaction mixture to provide key
electrochemical parameters and information regarding the
reaction pathway. As shown in Figure 1, diethyl phosphonate
2a was electrochemically inert whether alone (blue curve) or
with 20 mol % of NaOAc (green curve). However, substrate 1a
underwent an anodic oxidation with an obvious anodic current
after the onset of ∼0.75 V vs Ag/AgCl, and a remarkably
increasing Faradaic current was observed with an increasing
oxidative potential. Compared with 1a (red curve), the anodic
current of the mixture of 1a and NaOAc (magenta curve) was
generally enhanced with similar onset potentials, indicating the
benefit of NaOAc addition to slightly promote the electron
transfer.
rise to a radical cation intermediate A. After consequent
deprotonation, carbon radical B is formed and further
undergoes a second electron transfer to give intermediate C,
which is attacked by phosphite 2a catalyzed by Bronsted base
sodium acetate and furnished final product 3a. On the cathode,
hydrogen evolution occurs and an acetate anion is regenerated,
producing the high value hydrogen gas which is detected by a
gas detector (see Supplementary Figure S3) and giving the
atomic economy to the whole electro-organic reaction.
In summary, a green and direct electrochemical C−H
phosphonylation of unprotected secondary amine was
developed under mild react conditions without transition
metal, catalyst, and exogenous oxidant. For the substrate scope,
a series of cyclic and chain secondary amines were well
compatible. The scale-up experiment also showed the
promising use of this protocol in mass production. An acetate
anion was important to facilitate the electron transfer and
nucleophilic addition of phosphite.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02707.
Synthesis of starting compounds, detail process of
electrochemistry reaction, characterization data (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Based on our synthetic and mechanistic investigations, a
plausible reaction pathway is summarized and outlined in
Scheme 4. Initially, 1a is oxidized at the anode electrode, giving
*E-mail: chengxu@nju.edu.cn.
C
DOI: 10.1021/acs.orglett.9b02707
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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*E-mail: mding@nju.edu.cn
ORCID
Xu Cheng: 0000-0001-6218-611X
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the support by the Fundamental Research
Funds for the Central Universities in China (020514380136
and 020514380195) and Natural Science Foundation of
Jiangsu Province (BK20180321).
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