Visible-Light-Mediated Z -Stereoselective Monoalkylation of β,β-Dichlorostyrenes by Photoredox/Nickel Dual Catalysis

Article abstract of DOI:10.1055/a-1374-9384

Metal-catalyzed alkylation of 1,1-dihalovinyl moiety commonly suffers from both a lack of stereoselectivity and the overreaction leading to the dialkylation product. The methodology described herein features a new pathway to alkylate stereoselectively β,β-dichlorostyryl substrates to provide the Z -trisubstituted olefin only with fair to good yields. This cross-coupling reaction bears on the smooth and photoinduced formation of a C-centered radical that engages in a nickel-catalyzed organometallic cycle to form the key C _sp2-C _sp3bond.

Full text of DOI:10.1055/a-1374-9384

SYNLETT
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart
2021, 32, A–F
Cluster
A
Modern Nickel-Catalyzed Reactions
M. Abdellaoui et al.
Cluster
Synlett
Visible-Light-Mediated Z-Stereoselective Monoalkylation of
,-Dichlorostyrenes by Photoredox/Nickel Dual Catalysis
Mehdi Abdellaoui
Alexandre Millanvois
Z
R1
Ni
PC
Cl
O
O
Etienne Levernier
Y : K[18C6],
Si
R1
Y
Cyril Ollivier*
0
0
0
0
-
0
0
0
3
-
2
4
1
6-4
4
6
7
Cl
Cl
Na
blue LIGHT
O
O
R2
R2
PC: photocatalyst
Louis Fensterbank*
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0
0
0
-
0
0
0
3
-
0
0
0
1
-
7
1
2
0
silicates
mono-alkylated products
1,1-dichlorostyrenes
Institut Parisien de Chimie Moléculaire, Sorbonne
Université, UMR CNRS 8232, 4 Place Jussieu,
Paris Cedex 05, France
cyril.ollivier@sorbonne-universite.fr
louis.fensterbank@sorbonne-universite.fr
Published as part of the Cluster
Modern Nickel-Catalyzed Reactions
Corresponding Authors
Received: 18.12.2020
Accepted after revision: 26.01.2021
Published online: 27.01.2021
served traces of the E-stereoisomer and/or of the dialkylat-
ed side product that proved difficult to separate (Scheme 1,
eq. d).
DOI: 10.1055/a-1374-9384; Art ID: St-2020-k0632-c
Abstract Metal-catalyzed alkylation of 1,1-dihalovinyl moiety com-
monly suffers from both a lack of stereoselectivity and the overreaction
leading to the dialkylation product. The methodology described herein
features a new pathway to alkylate stereoselectively ,-dichlorostyryl
substrates to provide the Z-trisubstituted olefin only with fair to good
yields. This cross-coupling reaction bears on the smooth and photoin-
duced formation of a C-centered radical that engages in a nickel-cata-
lyzed organometallic cycle to form the key Csp2–Csp3 bond.
a) Minato & Tamao, 1987: arylation of dichlorostyrenes
Cl
PdCl2(dppb)
Cl
MeO
+
Cl
MeO
97%
MgBr
b) Negishi, 2004: alkynylation of dichlorostyrenes
Key words cross-coupling, stereoselective alkylation, radical, dichlo-
rostyrenes, bis(catecholato)silicates, dual catalysis, counterion, nickel
TMS
TMS
Cl
Ph
PdCl2(dppf)
Ph
Ph
+
Cl
The substitution of the carbon–chlorine bond of ,-di-
chlorostyryl derivatives1,2 was firstly approached by Minato
and Tamao3,4 who developed a palladium-catalyzed aryla-
tion that advantageously provided the Z-monoarylated
product (Scheme 1, eq. a). Later, Negishi and co-workers de-
veloped a palladium-catalyzed alkynylation5,6 employing
zinc acetylides as nucleophilic partners to afford the Z-mo-
nosubstituted products with high yields and stereoselec-
tively (Scheme 1, eq. b). However, the formation of the side
dialkynylated product remains the main weakness of this
methodology. Concerning the alkylation attempts of ,-di-
chlorostyryl substrates, they also suffered from overreac-
tion and the disubstituted products were often observed.7,8
An alternative was considered by Figadère and Alami and
consisted in the iron-catalyzed version of the Kumada
cross-coupling but the formation of the dialkylated com-
pound remained competitive (Scheme 1, eq. c).9 It was not
until the work of Roulland and co-workers10 that a promis-
ing answer to the problem of a stereoselective monoalkyla-
tion was provided. Indeed, Roulland designed a Suzuki-type
reaction with 9-BBN alkyl boranes that relies on the use of a
bidentate phosphine ligand for palladium with a large bite
angle to achieve the monoalkylation with high stereoselec-
tivity and in excellent yields. However, once again, they ob-
+
Cl
10%
TMS
ClZn
87%
TMS
c) Figadère & Alami, 2004; Fe-catalyzed alkylation of dichlorostyrenes
n-Bu
Cl
Fe(acac)3
n-Bu
51%
MeO
MeO
+
Cl
MeO
H
n-BuMgBr
+
20%
n-Bu
d) Roulland, 2007: alkylation of dichlorostyrenes with alkyl boranes
Cl
Pd2(dba)3
Cl
Xantphos ligand
MeO
+
Cl
88% Z/E = 93:7
base
MeO
9-BBN
e) This work:
O
R
4CzIPN
R
Cl
NiCl2·dme, dtbbpy
O
Si
Ar
Ar
+
O
blue LED
O
Y
1
Cl
Cl
2
3
Scheme 1 Literature on metal-catalyzed arylation, alkynylation, and
alkylation of ,-dichlorostyrenes
© 2021. Thieme. All rights reserved. Synlett 2021, 32, A–F
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
M. Abdellaoui et al.
Cluster
Synlett
In contrast, to the best of our knowledge, nickel-cata-
lyzed monoalkylation of such electrophiles has not yet been
reported in the literature. Indeed, the closest example was
the enantioselective alkylation of (Z)--bromo-styryl deriv-
atives developed by Reisman11 to afford the Z-trisubstituted
alkenes with retention of the stereochemistry of the double
bond. However, this methodology did not involve the sub-
stitution of dihalogenated styryl derivatives.
Considering those elements and based on our continu-
ing interest in photoredox catalysis12–15 and in photore-
dox/Ni dual catalysis,16–20 we considered involving bis(cate-
cholato)silicates in a photoredox/Ni-catalyzed vinylation
reaction.21 Indeed, thanks to their low oxidation potential
(between 0.3 and 0.9 V vs SCE) bis(catecholato)silicates are
known as efficient radical precursors.22 Their uses in dual
catalysis were investigated by our group22,23 and the group
of Molander for the alkylation of aryl and alkenyl ha-
lides.24,25 To illustrate the versatility of this methodology,
our group also investigated the Csp3–Csp3 bond formation,26
the formation of ketones through cross-coupling with acyl
chlorides,27 and the alkylation of alkenyl halides including
one preliminary example of alkylation of p-methoxy ,-di-
chlorostyrene.28 We wished to further investigate this valu-
able transformation and examined the reactivity of various
dichlorostyryl substrates 1 towards silicates 2 to furnish
cross-coupling (Z)-styrylchloride products 3 (Scheme 1, eq.
e).29
and 2c-Na (55%) were obtained. Similarly, 15-C-5-chelated
sodium silicates 2a′ were isolated in 45% yield. All these so-
dium silicates can be kept several months without degrada-
tion if stored under air- and moisture-free conditions.
Suitable crystals of the sodium bis(catecholato)benzyl
silicate 2a-Na were obtained by vapor diffusion using an ac-
etone/diethyl ether mixture (Figure 1). [C22H21NaO5Si] crys-
tallized in a tetragonal crystal system with a = 19.0046(7)
Å, b = 19.0046(7) Å, c = 11.5788(5) Å,  =  =  = 90° (Z = 8).32
Sodium ions are coordinated to one oxygen atom of four
catechol ligands and one acetone molecule completes the
coordination sphere. The O1–5–Na1 bond lengths are span-
ning from 2.288(3) Å to 2.355(2) Å. Finally, an intramolecu-
lar -cation interaction between the centroid of the benzyl
and Na1 can be observed with a distance of 3.2712(9) Å.
[18-C-6]-Potassium silicates 2a (R = Bn), 2b (R = –(CH2)2OAc),
2c (R = cyclohexyl), and 2d (R = –CH2NHPh) were prepared
as in previously described procedures.22,30 At the occasion
of this study, we also introduced the previously unknown
sodium silicates 2-Na since they could result in altered re-
activity. Sodium salts as additives have indeed resulted in
optimized reactivity in some nickel-catalyzed cross-cou-
pling reactions.11,31
Figure 1 X-ray crystal structure of 2a-Na, thermal ellipsoids are drawn
at 50% probability level. Hydrogen atoms are omitted for clarity.
Dashed lines denote bonds between neighboring silicates.
Sodium silicates 2-Na were prepared as previously for
potassium silicates (Scheme 2).22 The desired alkoxysilane
is reacted with catechol (2 equiv.) and with 1 equiv. of sodi-
um methoxide in methanol. 15-Crown-5 (1 equiv.) can also
be added in the reaction mixture to provide sodium sili-
cates 2′. Gram quantities of 2a-Na (55% yield), 2b-Na (64%),
The Si coordination of 2a-Na can be described as a
slightly distorted square pyramid (SP) structure with the
benzyl group occupying the apical site (Si–C13 bond length
= 1.879(2) Å). The four basal positions are occupied by the
two oxygen atoms of two catechol ligands. We can notice
that the Si atom is located just a little above the plane de-
fined by these fourth atoms (distance Si–plane centroid =
0.4517(6) Å). For 2a-Na, the dihedral angle method permits
to calculate a displacement of 89.3% (TBP → SP, TBP: trigo-
nal bipyramidal).33 This is also underlined by the close val-
ues of all the 4 basal bond Si–O distances from 1.735(2) to
1.759(2) Å. Compared to an idealized SP with four angles of
86°, a slight distortion is observed with angle deviations
from –2.5° to 2.1°. Interestingly, 2a-Na forms a polymeric
assembly along the c-axis that originates from multiple in-
teractions between Na and oxygen atoms of the catechols of
two silicates (see Supporting Information). Sodium cyclo-
hexyl silicate 2c-Na also crystallized, and an XRD structure
R
O
O
Si
MeONa (1 equiv.)
O
Na
O
R-Si(OR')3
2a-Na, 55% R = Bn
2b-Na, 64% R = (CH2)3O(CO)Me
2c-Na, 55%, R = c-Hex
MeOH
rt, 3 h
+
OH
R
O
O
OH
Si
15-C-5 (1 equiv.)
MeONa (1 equiv.)
O
O
O
O
R' = Et, Me
O
Na
O
O
2a′, 45%, R = Bn
Scheme 2 Synthesis of sodium silicate 2-Na and 2′
© 2021. Thieme. All rights reserved. Synlett 2021, 32, A–F

C
M. Abdellaoui et al.
Cluster
Synlett
analysis was obtained34 and proved to be very similar to the
corresponding potassium one.22
Finally, oxidation potentials of sodium silicates were de-
termined by cyclic voltammetry and proved consistent
with the low values we already observed for the potassium
ones22 i.e., Eox = +0.77 V vs SCE for 2a-Na, +0.88 V for 2b-Na,
and +0.81 V for 2c-Na.
with a fluoride. In contrast, the dibrominated reagent 1a-Br
led in our conditions to a complex mixture in which only a
marginal amount of monoalkylated was detected. Interest-
ingly, only the presumably less reactive dichlorinated sub-
strate 1a positively reacted in this dual catalysis reaction
(44% of adduct 3aa) and was then chosen as model sub-
strate for the optimization of the reaction conditions. To
our delight, the alkylation occurred selectively without any
formation of side products such as the dialkylated product
or the E-stereoisomer. The Z-stereochemistry of 3aa was as-
signed by analogy with subsequent findings (vide infra).
We then examined the effect of the cation in this reac-
tion. No improvement of yield was observed from free po-
tassium silicate 2a-K (39% of 3a, entry 4), sodium silicates
with 15-C-5 (2a′, 39%, entry 5) or without 15-C-5 (2a-Na,
42%, entry 6). Nevertheless, the fact that the plain sodium
silicate 2a-Na could be engaged successfully opened inter-
esting perspectives. Focusing on silicate 2a, we then investi-
gated other possibilities to increase the yield of 3aa (Table
1). While doubling the loading in silicate (entry 7) or its
concentration (entry 8) did not have a significant impact on
the yield, a longer reaction time (72 h) with double amount
of 2a provided the best yield of 3a (70%, entry 10). We also
checked that nickel catalysis was necessary for this trans-
formation: no radical addition – -elimination of a chlorine
radical takes place22 to deliver 3aa (entry 11)
All ,-dichlorostyrenes 1 were synthesized by a com-
mon Corey–Fuchs–Ramirez olefination from the corre-
sponding aldehydes35 and were obtained with good yields.
For our catalytic system, we considered the same as in our
previous studies,28 namely nickel(II)·dme in the presence of
2,6-di-tert-butyl bipyridine as catalytic system and 1,2,3,5-
tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN)36 as
photocatalyst. 4CzIPN is endowed with a very high reduc-
tion potential after photoexcitation (+1.59 V vs SCE)37 and a
long-lived excited state (5.1 s) which enables the efficient
oxidation of the silicate (Eox < 0.9 V vs SCE) to afford the cor-
responding C-centered radical.
We first concentrated on the cross-coupling of 1-vinyl-
naphthalene derivatives 1a as electrophiles with benzyl sil-
icates 2a (see Table 1). We began our investigation by delin-
eating the range of dihalogenostyrenes that could handle
this approach (Table 1, entries 1–3). The difluorinated re-
agent 1a-F did not react despite the presence of silicon on
the radical precursor likely to establish a strong interaction
Table 1 Optimization of the Cross-Coupling Reaction
4CzIPN (1 mol%)
NiCl2·dme (2 mol%)
O
X
O
Si
Y
O
O
X
dtbbpy (2 mol%)
DMF, blue LEDs
rt
X
1a-X
2a
3aa
Entry
Reaction time (h)
X
[Si] counterion Y
Silicate (equiv.)
Yield of 3aa (%)a
1
2
48
48
48
48
48
48
48
48
72
72
48
F, 1a-F
K[18-C-6], 2a
K[18-C-6], 2a
K[18-C-6], 2a
K, 2a-K
1.5
1.5
1.5
1.5
1.5
1.5
3
NRb
mixture
44
Br, 1a-Br
Cl, 1a
Cl, 1a
Cl, 1a
Cl, 1a
Cl, 1a
Cl, 1a
Cl, 1a
Cl, 1a
Cl, 1a
3
4
39
5
Na[15-C-5], 2a′
Na, 2a-Na
39
6
42
7
K[18-C-6], 2a
K[18-C-6], 2a
K[18-C-6], 2a
K[18-C-6], 2a
K[18-C-6], 2a
47
8c
9
3
51
1.5
3
50
10
11d
70
1.5
0
a NMR yield was determined using 1,3,5-trimethoxybenzene as internal standard.
b NR: no reaction detected.
c Variation of the concentration (0.1–0.2 M)
d Reaction performed without nickel catalyst.
© 2021. Thieme. All rights reserved. Synlett 2021, 32, A–F

D
M. Abdellaoui et al.
Cluster
Synlett
Based on all the results from Table 1, the standard con-
ditions (48 h and 1.5 equiv. of silicate) appear to be a good
compromise in terms of reaction time and cost to reach a
decent yield in this cross-coupling reaction. Subsequently,
we engaged under those conditions various [18-C-6]-potas-
sium and sodium silicates 2 and 2-Na with ,-dichlorosty-
renes 1 under the Ni/4CzIPN dual catalysis conditions to ex-
plore the scope of this reactivity (Scheme 2).
The results of Scheme 3 illustrate a certain versatility of
this cross-coupling reaction between the gem-dichlorosty-
ryl precursors 1 and the silicates 2. Indeed, the reaction oc-
curs with an alkyl silicate (primary and secondary), a ben-
zylic one, and even the -methyl-anilino-silicate with vari-
ous dichlorostyrenes 1, and provides previously unknown
coupling products with fair to good yields. In some cases,
the use of the sodium silicates 2-Na compared to the potas-
sium ones 2 proved more rewarding in terms of yields for
coupling products 3 (products 3ab and 3bc). The coupling
does not depend on the electron-withdrawing or electron-
donating character of the substituent of the dichlorosty-
rene. Indeed, substrates that own a methoxy group or a tri-
fluoromethyl group in para position react similarly with the
primary alkyl acetoxypropyl radical (2b) or the more nucle-
ophilic -methyl-aniline one (2d) (Scheme 3).
Of note, the more sensitive precursors, piperonal and
nitro substrates failed to give any adducts (3fb and 3gb)
and it is also important to specify that only the Z-isomer, as
determined by NOESY experiments, of each adduct 3 was
isolated and that no dialkylation product was observed. In
the reactions of amino silicate 2d, some (Z)-chlorostyrene
was formed as side product in 15–20% yield that was highly
suggestive of a protodenickelation based on the proposed
mechanism of Scheme 4.
[Si]
R
R
2
Cl
1
N
N
0
Ni
Cl
*
PC
PC
Ar
hν
PC
N
N
N
NiI
R
Cl NiII
Cl
N
N
Cl
NiI Cl
I
Ar
Cl
N
R
Ar
R
Cl
N
N
R
NiIII
Cl
Cl
General reaction
4CzIPN
3
II
Ar
O
Si
O
R
O
Ar
NiCl2·dme
dtbbpy
Cl
O
R
Scheme 4 Plausible mechanism for the cross-coupling reaction
Ar
Ar
Y
DMF, blue LED, rt
Cl
Cl
1
2
3
Y =
K
[18-C-6] Na
Because we know that nickel is necessary in these reac-
tions to obtain products 3 and based on the literature
mechanism proposals of related Ni/photoredox dual cataly-
sis reactions,18–20,38 it appears reasonable to invoke the trap-
ping by nickel of a C-centered radical originating from the
photoinduced oxidation of the silicates 2. The nickel inter-
mediate that would be involved is the nickel(II) complex I
that originates from the oxidative addition into the C–Cl
bond of the substrates 1 (Scheme 4). The observed Z-selec-
tivity might be ascribed to the presence of the bidentate li-
gand on nickel that would generate a significant steric hin-
drance around the metallic center and that possibly orients
the oxidative addition on the more accessible C–Cl bond,
with no aryl group in syn. Even though the effect of the
bulkiness around the metallic center seems clear, the insen-
sitivity of the coupling to the substituent on the aryl moiety
might appear strange but is reminiscent of previous find-
ings in arylative couplings.23,24 This would be consistent
with a radical addition on nickel(II) that is rather insensi-
tive to electronic effects.39–42 The resulting nickel(III) com-
plex II then undergoes reductive elimination to provide
products 3 and a nickel(I) salt that would be reduced by the
photoredox cycle to Ni(0).
Ar
Ph
MeO
Cl
F3C
3da
3aa 40%, 42%a
3ca
70%
54%
3ba 48%
33%a
O
MeO
Ar
O
Cl
3ab 45%, 51%a
3cb
35%
64%
3bb
O
MeO
3eb 54%b
F3C
O
O2N
3gb
NRc
NRc
3db 51%
3fb
MeO
Ar
Cl
3cc 60%
3bc 30%
60%a
3ac
70%
MeO
Ph
Ar
N
F3C
H
Cl
30%
3dd
3ad 43%
3bd 57%
Scheme 3 Reaction of gem-dichlorostyrenes 1 with various silicates 2
(isolated yieds). Reagents and conditions: 1 (1 equiv), 2 (1.5 equiv),
4CzIPN (1 mol%), NiCl2·dme/dtbbpy (2 mol%), rt for 48 h. a NMR yield
was determined using 1,3,5-trimethoxybenzene as internal standard. b
See also ref. 28. c NR: no reaction detected, starting material was recov-
ered.
Based on the clean but inefficient conversion of some of
these reactions, we also wished to explore other conditions
to run them. Notably we used an in-house flow setting43,44
(see the Supporting Information for details) in order to re-
© 2021. Thieme. All rights reserved. Synlett 2021, 32, A–F

E
M. Abdellaoui et al.
Cluster
Synlett
duce the reaction times. We tested silicate 2c and naphthyl
electrophile 1a as a model reaction based on its efficiency
in batch after 48 h of irradiation (70% of 3ac, Scheme 3). In-
terestingly, within a residence time of 50 min, we reached
32% yield in 3ac and 50% within 90 min (Scheme 5). There-
fore, the flow conditions open promising perspectives for
this methodology.
Funding Information
The authors thank the Agence Nationale de la Recherche (Grant No.
ANR-17-CE07-0018, HyperSilight (PhD grant to EL)), the Centre Na-
tional de la Recherche Scientifique (CNRS), and Sorbonne Université
for financial support.
A
g
enceNationale
d
elaRecherche(A
N
R-17-C
E
0
7-0
0
1
8)Ce
n
treNatio
n
aldel
a
R
echerch
e
S
cie
n
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e()Sorb
o
n
ne
U
n
iversiét
(
)
Acknowledgment
We are grateful to J. Forté, G. Gontard and Lise-Marie Chamoreau for
X-ray diffraction analyses and helpful discussions.
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/a-1374-9384. Su
p
p
orti
n
gInformatio
n
Su
p
p
ortingInformatio
n
References and Notes
Scheme 5 Continuous-flow conditions
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7810.
(3) Minato, A.; Suzuki, K.; Tamao, K. A. J. Am. Chem. Soc. 1987, 109,
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2016, 138, 475.
Finally, we investigated the possibility of the postfunc-
tionalization of the alkylated adducts 3 by using our meth-
odology. We engaged different monoalkylated products 3
with silicates 2 (3bb with benzyl silicate and 3bd with ace-
toxypropyl silicate) and we only recovered the starting ma-
terial after 2 days of irradiation. All in all, this absence of re-
activity confirms that our methodology is inoperative to
achieve the substitution of the two C–Cl bonds. Neverthe-
less, Roulland’s and Tamao’s works showed that the post-
functionalization of the monosubstituted product was ef-
fective by engaging them in their palladium-catalyzed al-
kylation or arylation sequences.2,3
To conclude, we have developed an innovative way to al-
kylate gem-dichlorostyrenes with fair to good yields. Con-
trary to most of the previous methodologies employing pal-
ladium catalysts, this cross-coupling reaction advanta-
geously combines the earth-abundant nickel metal with a
cheap organic dye to achieve smoothly the substitution at
room temperature (where all the previous methods men-
tioned are performed under reflux). Furthermore, the main
advantages of this reaction are its total Z-stereoselectivity
and the absence of overreaction to widen the scope of this
methodology. Otherwise, one of the aims of the study was
the approach of the silicate’s counterion and its effect on
the photoredox catalysis. The second interest of this study
is the introduction of sodium silicates 2-Na that improves
the atom economy compared to the [18-C-6]-chelated po-
tassium silicate analogues 2. If this work provided interest-
ing and promising insights, the influence of the counterion
remains nonexhaustive and will be pursued by our group.
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