Nickel-Catalyzed Electrosynthesis of Aryl and Vinyl Phosphinates

Article abstract of DOI:10.1002/ejoc.202000422

A mild and useful nickel-catalyzed electrochemical phosphonylation of aryl and vinyl bromides is described. We show that alkyl H-phenylphosphinates can be coupled electrochemically with functionalized aryl and vinyl bromides using very simple conditions (Fe/Ni anode, bench-stable nickel pre-catalyst, undivided cell, galvanostatic electrolysis) to furnish the corresponding aryl and vinyl phosphinates in satisfactory to good yields. Couplings can also be applied to heteroaromatic bromides with some limitations like increased propensity to hydro-dehalogenation.

Full text of DOI:10.1002/ejoc.202000422

Communication
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EurJOC
European Journal of Organic Chemistry
Electrochemical Phosphonylation
Nickel-Catalyzed Electrosynthesis of Aryl and Vinyl
Phosphinates
Farah Daili,[a] Abdelhakim Ouarti,[a] Marine Pinaud,[a] Ibtihal Kribii,[a] Stéphane Sengmany*[a]
Erwan Le Gall,[a] and Eric Léonel*[a]
Abstract: A mild and useful nickel-catalyzed electrochemical catalyst, undivided cell, galvanostatic electrolysis) to furnish the
phosphonylation of aryl and vinyl bromides is described. We corresponding aryl and vinyl phosphinates in satisfactory to
show that alkyl H-phenylphosphinates can be coupled electro- good yields. Couplings can also be applied to heteroaromatic
chemically with functionalized aryl and vinyl bromides using bromides with some limitations like increased propensity to
very simple conditions (Fe/Ni anode, bench-stable nickel pre- hydro-dehalogenation.
our knowledge, no example of vinyl phosphinates preparations
have been described through electrochemical routes. Since
phosphinates derivatives are known as interesting phosphates
bioisosteres,[7] the development of new synthetic approaches is
desirable. Therefore, as a part of our activity devoted to electro-
chemical nickel-catalyzed C(sp2)–P couplings, we report herein
as an extension of our previous work,[6a] the synthesis of aryl
and vinyl phosphinates using a Fe/Ni sacrificial anode and a
nickel catalyst in an undivided cell under constant current elec-
trolysis.
The development of more and more sustainable chemical
processes has gained much attention in the past years due to
the ever-increasing awareness of environmental issues. Under
the combined impetus of historic electroorganic and main-
stream chemists, organic electrosynthesis, a non-conventional
tool for alternative synthetic transformations, has been recently
subjected to a tremendous development.[1] In such an electro-
chemical approach, electrons are employed as a green and non-
toxic reductant or oxidant surrogates. In addition, in a strong
willingness to put experimental simplicity at a center stage, in-
tensive efforts have been devoted to the development of useful
electrosynthetic procedures. In this context, electrochemical
methods employing undivided cells, galvanostatic mode, and
avoiding an external source of heating can be considered as
particularly relevant. This is the case of electrooxidative or tran-
sition metal-catalyzed electroreductive cross-couplings, ranging
from the well-documented C(sp2)–C(sp2) bond formation,[2] to
C(sp2)–heteroatom such as nitrogen,[3] oxygen,[4] sulfur[5] or
phosphorus.[6] In 2018, our group reported the reductive
C(sp2)–P cross-coupling of aryl bromides and dialkyl phosphites
for the preparation of aryl and heteroaryl phosphonates.[6a] The
cheap and affordable bench-stable nickel bromide bipyridine
was used as pre-catalyst and electrolyses were performed un-
der galvanostatic mode in acetonitrile as less toxic solvent. Very
recently, Xiang and co-workers[6b] described the synthesis of
aryl and heteroaryl phosphonates, and some phosphinates and
phosphine oxides (Scheme 1). However, it can be noted that
only a few examples of aryl phosphinates and, to the best of
Scheme 1. Nickel-catalyzed C–P couplings.
[a] F. Daili, A. Ouarti, M. Pinaud, I. Kribii, S. Sengmany, E. Le Gall, E. Léonel
Electrosynthèse, Catalyse et Chimie Organique, Université Paris-Est Créteil,
CNRS, ICMPE, UMR7182,
We first examined the electrochemical coupling of 4-bromo-
benzotrifuoride and methyl H-phenylphosphinate using tri-
ethylamine as a base, Fe/Ni (64/36) rod as the anode, Ni foam
as the cathode, NBu4Br as the supporting electrolyte, 10 %
NiBr2bpy as the pre-catalyst, at 200 mA constant current in
acetonitrile at room temperature (Table 1). It was anticipated
that a large excess of the phosphorus compound would be
2 rue Henri Dunant, 94320 Thiais, France
E-mail: leonel@icmpe.cnrs.fr
sengmany@icmpe.cnrs.fr
http://www.icmpe.cnrs.fr/spip.php?rubrique68
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.202000422.
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mandatory, as previously reported[6a] (entry 1). However, phosphinic acid failed, with no coupling product being de-
whereas the expected coupling product could be successfully tected after 1.5 h electrolysis.
With the most efficient C(sp2)–P reaction conditions in hands
(entry 12), the coupling of various aryl bromides with methyl
H-phenylphosphinate was then evaluated (Table 2). It was
found that the electrochemical process tolerates a large range
of functional groups borne by the aromatic ring, including elec-
tron-withdrawing (trifluoromethyl 2a–a′, cyano 2b–b′, oxo 2c–
c′, ester 2d) and electron-donating groups [1,2-(methylenedi-
oxy) 2g, methoxy 2h–h′′, dimethylamino 2i, alkyl 2j–j′ and 2k].
Reactions proceed over a period of 1 to 1.5 h in the majority
of cases (see the supporting information for details). The pres-
ence of a second halogen atom is also compatible with the
process (chlorine 2e–e′, fluorine 2f–f′′). As already noticed pre-
viously,[6a] it is important to point out that with the chlorinated
substituents, neither the double coupling product, nor the
hydro-dechlorination product are observed, showing a particu-
larly selective reaction at the C-Br bond. This interesting result
allows a potential second coupling using a conventional chemi-
cal method (e.g. Suzuki-Miyaura). In the case of some sterically-
hindered ortho-substituted phenyl bromides (2a′′ and 2c′′),
coupling products are formed in reduced amounts at the ex-
pense of hydro-dehalogenation side-products. The coupling
was then extended successfully to fused polyaromatic bromides
obtained after short time electrolysis, a flash chromatography
on silica gel did not allow to separate the desired product from
the remaining methyl H-phenylphosphinate. This issue was ad-
dressed by reducing the amount of phosphinate to 1.2 equiva-
lents (entry 2), thus leading to the desired product in 45 %
yield. When iron/nickel alloy anode was replaced either by pure
iron (entry 3) or pure nickel (entry 4), the cross-coupling was
much less efficient. In these cases, it was observed the signifi-
cant formation of the hydro-dehalogenation product of bromo-
benzotrifluoride (with nickel anode) or low conversion despite
3 h electrolysis (with an iron anode). However, the addition of
DMF (20 %) to acetonitrile improved both the solubility of the
nickel pre-catalyst and allowed to reduce the reaction time
from 2 to 1.5 h without impact on the yield (entry 5). As antici-
pated, no reaction occurred in the absence of electric current
(entry 6). By examining other supporting electrolytes (entries
7–10), it was found that the yield can be slightly improved us-
ing lithium chloride or a 2:1 mixture of tetrabutylammonium
bromide and -iodide as supporting electrolyte. However, the
mixture of tetrabutylammonium halides was maintained for the
rest of the study, not only because it is less hygroscopic than
lithium chloride, but also because it promotes less reductive
homocoupling of bromobenzotrifluoride. In the last series of
experiments, the proportion of DMF was reduced to 10 % (entry
11) and the amount of phosphorus reagent was increased from
1.2 to 1.5 equivalent to furnish 57 % of the expected coupling
product (entry 12). Additionally, it can be mentioned the yield
decreases slightly when imposing the electric current at 100
(entry 13) or at 300 mA (entry 14), 54, and 48 % respectively. It
is noteworthy that phosphorus reagent needs to be employed
in its ester form. Indeed, a control electrolysis involving the re-
action of 4-bromobenzotrifluoride with the commercial phenyl-
Table 2. Scope of aryl bromides.[a,b]
Table 1. Optimization of the experimental conditions.
Entry Anode
Solvent
Electrolyte
I [mA] Yield [%][a]
[b]
1
2
3
4
5
6
7
8
Fe/Ni
Fe/Ni
Fe
MeCN
MeCN
MeCN
MeCN
NBu4Br
NBu4Br
NBu4Br
NBu4Br
200
200
200
200
200
0
200
200
200
200
200
–
45
< 10
< 10
45
n.r.
45
Ni
Fe/Ni
Fe/Ni
Fe/Ni
Fe/Ni
Fe/Ni
Fe/Ni
Fe/Ni
Fe/Ni
Fe/Ni
Fe/Ni
MeCN/DMF (8:2) NBu4Br
MeCN/DMF (8:2) NBu4Br
MeCN/DMF (8:2) NEt4BF4
MeCN/DMF (8:2) NaI
MeCN/DMF (8:2) LiCl
MeCN/DMF (8:2) NBu4Br/NBu4I
MeCN/DMF (9:1) NBu4Br/NBu4I
MeCN/DMF (9:1) NBu4Br/NBu4I 200
MeCN/DMF (9:1) NBu4Br/NBu4I
MeCN/DMF (9:1) NBu4Br/NBu4I
44
48
48
50
9
10
11
12
13
14
57[c]
54[c]
48[c]
100
300
[a] Typical procedure: undivided cell, iron/nickel (64/36) anode, nickel cath-
ode, tetrabutylammonium bromide/tetrabutylammonium iodide (0.15 mmol/
0.068 mmol), acetonitrile/DMF (18/2 mL, v/v), NiBr2bpy (10 mol-%), (het-
ero)aromatic bromide (2 mmol), methyl H-phenylphosphinate (3 mmol), tri-
ethylamine (4 mmol), constant current: 200 mA, room temperature. [b] Iso-
lated yield. [c] Hydro-dehalogenation product as main side-product.
[a] Isolated yield. [b] 2 equiv. of methyl H-phenylphosphinate were used. The
coupling product could not be separated of the excess of methyl
H-phenylphosphinate. [c] 1.5 equiv. of methyl H-phosphinate instead of
1.2 equiv.
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such as bromonaphthalene derivatives (2m–o) or 9-bromo-
phenanthrene (2p). However, the reaction of 9-bromoanthra-
cene failed, probably due to a steric effect. Some heteroaro-
matic bromides were next assessed. Only electron-rich hetero-
aromatic bromides such as 3-bromothiophene and 3-bromo-
furan were able to couple successfully with methyl H-phenyl-
phosphinate in 50 and 33 % yields respectively (products 2q
and 2r) together with hydro-dehalogenation products. On the
contrary, electron-poor heteroaromatic bromides like 2-, 3- and
4-bromopyridine, or 5-bromopyrimidine resulted in traces of
coupling products, hydro-dehalogenation products being
mainly observed.
Table 4. Scope of alkyl H-phenylphosphinates.[a,b]
The optimized conditions were then extended to commer-
cially available vinyl bromides (Table 3). Thus, ꢀ- and α-bromo-
styrenes, 1- and 2-bromoprop-1-ene and 2-bromobut-2-ene
were successfully coupled with methyl H-phenylphosphinate in
moderate to good yields (products 3a, 3b, 3c, 3d and 3e). In
the case of 1-bromoprop-1-ene (product 3c), the yield is partic-
ularly limited due to the formation of a larger amount of hydro-
dehalogenation product as well as to the volatility of the cou-
pled product. Moreover, it can be mentioned that the coupling
of ꢀ-bromostyrene, 1-bromoprop-1-ene, or 2-bromobut-2-ene
with methyl H-phenylphosphinate was achieved without no-
ticeable isomerization of the C-C double bond (products 3a, 3c,
and 3e).
[a] Typical procedure: undivided cell, iron/nickel (64/36) anode, nickel cath-
ode, tetrabutylammonium bromide/tetrabutylammonium iodide (0.15 mmol/
0.068 mmol), acetonitrile/DMF (18/2 mL, v/v), NiBr2bpy (10 mol-%), 4-bromo-
benzotrifuoride (2 mmol), alkyl H-phenylphosphinate (3 mmol), triethylamine
(4 mmol), constant current: 200 mA, room temperature. [b] Isolated yield. [c]
Diastereomeric ratio (dr) was determined by 1H NMR.
In summary, a mild and useful nickel-catalyzed electrochemi-
cal phosphonylation of aryl and vinyl bromides has been devel-
oped. Alkyl H-phenylphosphinates were coupled electrochemi-
cally with functionalized aryl and vinyl bromides using very sim-
ple conditions (Fe/Ni anode, bench-stable nickel pre-catalyst,
undivided cell, simple power supply) to furnish the correspond-
ing aryl and vinyl phosphinates in satisfactory to good yields.
Couplings were successfully applied to heteroaryl bromides.
However, the limitation was observed with electron-poor
heteroaryl bromides, which lead majorly to the corresponding
hydro-dehalogenation products.
Table 3. Scope of vinyl bromides.[a,b]
Experimental Section
To a 25 mL undivided electrochemical cell, fitted by an iron/nickel
(64/36) anode surrounded by a nickel foam as the cathode (surface:
15 cm2, Nitech, porosity 500 μm) were added acetonitrile (18 mL),
N,N′-dimethylformamide (2 mL), tetrabutylammonium bromide/
tetrabutylammonium iodide (0.15 mmol/0.068 mmol) and 1,2-di-
bromoethane (100 μL, 1.16 mmol). The mixture is electrolyzed at a
constant current intensity of 200 mA for 15 minutes at room tem-
perature. Then, the current is stopped and were sequentially added:
NiBr2bpy (75 mg, 0.2 mmol, 10 mol-%), (hetero)aryl or vinyl bromide
(2 mmol, 1.0 equiv.), phosphorus reagent (3 mmol, 1.5 equiv.) and
triethylamine (560 μL, 4 mmol, 2.0 equiv.). The solution is electro-
lyzed at 200 mA for 1–2 h (GC monitoring). A saturated EDTA aque-
ous solution (30 mL) was added, and the resulting solution is ex-
tracted with AcOEt (3 × 30 mL). The combined organic layers were
washed with H2O (2 × 100 mL) then with a saturated NaCl aqueous
solution (100 mL), dried with Na2SO4, filtered and the solvents evap-
orated under vacuum. The crude product was purified by flash chro-
matography on silica (granulometry 70–200 μm), eluted with a gra-
dient mixture of solvents (CH2Cl2/AcOEt or CH2Cl2/acetone) to
afford the pure product.
[a] Typical procedure: undivided cell, iron/nickel (64/36) anode, nickel cath-
ode, tetrabutylammonium bromide/tetrabutylammonium iodide (0.15 mmol/
0.068 mmol), acetonitrile/DMF (18/2 mL, v/v), NiBr2bpy (10 mol-%), vinyl
bromide (2 mmol), methyl H-phenylphosphinate (3 mmol), triethylamine
(4 mmol), constant current: 200 mA, room temperature. [b] Isolated yield.
[c] E/Z ratio was determined by 31P NMR.
The successful coupling between aryl/vinyl bromides and
methyl H-phenylphosphinate prompted us to examine the reac-
tivity of other alkyl H-phenylphosphinates in the reaction.
Therefore, starting phosphorus compounds were efficiently
prepared by esterification of phenylphosphinic acid with alkyl
chloroformates according to a standard procedure.[8] Methyl,
ethyl, propyl, n-butyl, isobutyl, and (1R,2S,5R)-menthyl H-phos-
phinates thus obtained were subsequently used in C-P cross-
couplings (Table 4).
All alkyl H-phenylphosphinates led to the corresponding
coupling products in moderate to good yields (2a and 4a–d).
Acknowledgments
It is of note that (1R,2S,5R)-menthyl H-phenylphosphinate was The financial support of this work by the CNRS, the University
also successfully coupled with 4-bromobenzotrifluoride to fur- Paris-Est Créteil and the University Paris-Est (PhD grant to F. D.)
nish 4e as diastereoisomers mixture in 62:38 ratio.
is gratefully acknowledged.
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Keywords: Electrosynthesis · Cross-coupling · Nickel ·
Phosphinates · Synthetic methods
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Received: March 30, 2020
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Electrochemical Phosphonylation
F. Daili, A. Ouarti, M. Pinaud,
I. Kribii, S. Sengmany,* E. Le Gall,
E. Léonel* .............................................. 1–5
Nickel-Catalyzed Electrosynthesis of
Aryl and Vinyl Phosphinates
A mild nickel-catalyzed electrochemi- plings from alkyl H-phenylphosphin-
cal approach for phosphonylation of ates and aryl or vinyl bromides. This
aryl and vinyl bromides is reported. electrochemical method enabled to
The used of iron/nickel alloy sacrificial prepare more than 30 new (hetero)aryl
anode process in undivided cell and and vinyl phenylphosphinates with
under constant current electrolysis al- good yields.
lowed to achieve C(sp2)–P cross-cou-
doi.org/10.1002/ejoc.202000422
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