Cobalt-Catalyzed Z to e Isomerization of Alkenes: An Approach to (E)-β-Substituted Styrenes

Article abstract of DOI:10.1021/acs.orglett.0c00072

An efficient cobalt-catalyzed Z to E isomerization of β-substituted styrenes using the amido-diphosphine ligand was developed, delivering the (E)-isomers with good functional tolerance and high stereoselectivity. The reaction could be scaled up to gram-scale with a catalyst loading of 0.1 mol %, using a mixture of (Z)- and (E)-alkene as the starting material. Preliminary mechanistic studies indicated that cobalt(I)-hydride and a benzylic-cobalt species were probably involved in the reaction, as supported by experiments and DFT calculations.

Full text of DOI:10.1021/acs.orglett.0c00072

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Letter
Cobalt-Catalyzed Z to E Isomerization of Alkenes: An Approach to
(E)‑β-Substituted Styrenes
Hongmei Liu,† Man Xu,† Cheng Cai, Jianhui Chen,* Yugui Gu, and Yuanzhi Xia*
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ABSTRACT: An efficient cobalt-catalyzed Z to E isomerization of β-
substituted styrenes using the amido-diphosphine ligand was developed,
delivering the (E)-isomers with good functional tolerance and high
stereoselectivity. The reaction could be scaled up to gram-scale with a
catalyst loading of 0.1 mol %, using a mixture of (Z)- and (E)-alkene as
the starting material. Preliminary mechanistic studies indicated that
cobalt(I)-hydride and a benzylic-cobalt species were probably involved in
the reaction, as supported by experiments and DFT calculations.
he 1-propenylbenzenes are important building blocks in
synthetic organic chemistry,1 as such structural motifs are
Scheme 1. Methods for Z to E Isomerization of Alkenes
T
generally contained in many naturally occurring compounds,2
having wide applications in the pharmaceutical chemistry,3
pesticide,4 flavor and fragrance industry5 (Figure 1). A number
Figure 1. Selected industrially relevant targets containing 1-
propenylbenzenes scaffold.
of elegant methods have been established for the preparation
of alkenes, including Wittig reaction,6 Julia olefination,7 olefin
metathesis,8 cross-coupling reaction9 and alkyne semihydroge-
nation.10 However, these protocols are always suffered from
poor E/Z selectivity. The two isomers are difficult to separate
from mixtures, and may lead to different reactivity,11 regio-12
or diastereoselectivity13 in the functionalization reactions.
Hence, the development of efficient and convenient method
for geometrical isomerization between E/Z mixtures of alkenes
is highly valuable.14
Although contra-thermodynamic in nature, the E to Z
isomerization of alkenes has been established via photo-
chemical procedures.15 Meanwhile, isomerizing of cis-alkene to
trans-alkene could be achieved in the presence of strong acids,
halogens, or elemental selenium, but these methods were not
applicable to simple alkenes (Scheme 1a).14b,16 In 2002, the
Spencer group developed a systermatic Z to E isomerization of
E/Z-mixtures of β-substituted styrenes with the involvement of
a π-allylic palladium catalyst; however, the catalytic system was
not suitable for electron-deficient substrates and required a
relatively high catalyst loading (10 mol %) (Scheme 1b).17 In
the presence of tributyltin hydride (2.2 equiv), Jung and co-
workers described a Z to E isomerization of (Z)-alkenes
employing a palladium-hydride catalysis under reflux con-
ditions.18
Owing to its high natural abundance, biocompatibility, and
unique catalysis, earth-abundant transition metal catalysts such
as iron, cobalt, nickel, and copper have attracted much
attention in the past two decades.1b−e,19 The earth-abundant
transition metals20 have been considered as an alternative to
Received: January 7, 2020
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© XXXX American Chemical Society
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noble metals21 for alkene isomerization. In combination with
appropriate ligands, cobalt complexes have been demonstrated
as efficient catalysts for position isomerization of alkenes.22−30
In the 1970s, Orchin and co-workers found cobalt hydride
complex [CoH(Co)4] could catalyze isomerization of ally
benzene and proposed a Co−H insertion/β-H elimination
pathway based on deuterium-labeling experiments.22 Sub-
sequently, Satyanarayana and Periasamy disclosed migration
isomerization of olefins over one position employing a CoCl2/
PPh3/NaBH4 system.23 In 2009, Oshima and Yorimitsu
developed a cobalt-catalyzed isomerization of 1-alkenes to
(E)-2-alkenes with a NHC ligand in the presence of a Grignard
reagent.24 In 2014, Holland25 and Hilt26 independently
reported cobalt-catalyzed (Z)-selective isomerization of
terminal alkenes with bulky β-diketiminate and Ph2PH as the
ligand, respectively. Employing a radical based hydrogen atom
transfer (HAT) mechanism, Shenvi and co-workers described
a chemoselective isomerization of terminal alkenes with
catalytic amounts of the Co-salen complex and organosilane.27
Later, the Norton group found that Co(dmgBF2)2(THF)2
could generate a low concentration of H· donor under H2
pressure, which can promote radical isomerization of terminal
alkenes.28 In 2018, Liu and Jiao developed a regioselective
olefin isomerization under kinetic control with designed
pyridine-amine-phosphine cobalt complexes using ammonia
borane as the reductant, delivering less hindered olefin
products with high selectivity.29 In combination of visible
a
Table 1. Optimization of Conditions
b
entry
solvent
time
ligand
product (E/Z)
1
2
3
4
5
6
7
8
9
dioxane
dioxane
dioxane
dioxane
dioxane
THF
1 h
1 h
1 h
1 h
1 h
1 h
1 h
1 h
12 h
1 h
−
16/84
31/69
27/73
97/3
25/75
50/50
97/3
Xantphos
DPEphos
PNP
PN
PNP
PNP
PNP
PNP
PNP
DCM
toluene
toluene
toluene
98/2
98/2
98/2(99%)
c
d
10
a
Unless otherwise noted, the reaction was conducted with 1a (0.5
mmol, E/Z 15/85), CoCl2 (0.0125 mmol), ligand (0.015 mmol), and
NaHBEt3 (0.038 mmol) in solvent (1 mL) at room temperature for 1
h. The E/Z ratio of product was determined by H NMR analysis.
The reaction was conducted with 1a (1 mmol, E/Z 15/85), CoCl2
(0.025 mmol), PNP (0.03 mmol), and NaHBEt3 (0.075 mmol) in
toluene (1 mL) at room temperature for 1 h. Isolated yield.
b
1
c
d
̈
light with cobalt catalysis, Konig and co-workers reported a
thermodynamic and kinetic isomerization of alkenes, in which
the regioselectivity could be controlled by switching the
ligand.30 Very recently, Schoenebeck and co-workers devel-
oped an additive-free double bond migration of terminal
alkenes via a nickel-catalyzed intramolecular 1,3-hydrogen
atom relocation strategy.31 However, geometrical isomer-
ization of alkenes with earth abundant transition metal catalyst
is still rarely reported.32 In this current report, we present an
efficient cobalt-catalyzed Z to E isomerization of E/Z-mixtures
of alkenes, producing (E)-isomers with high stereoselectivity
(Scheme 1c).
To initiate this study, the E/Z-mixture of anethole (1a) (E/
Z 15/85) was subjected to the catalytic system of CoCl2 and
NaBHEt3 in dioxane (0.5 M) (Table 1, entry 1). After stirring
at room temperature for 1 h, the E/Z ratio of the alkene was
almost unchanged (E/Z 16/84). A slight increase in E/Z
selectivity was observed when Xantphos or DPEphos was used
as the ligand in the reaction (entries 2−3). Interestingly, the
use of the amido-diphosphine ligand (PNP) led to a significant
raise in E/Z selectivity (entry 4, 97/3 E/Z), while bidentate
amido-monophosphine ligand (PN) showed poor reactivity
(entry 5). Screening of solvents (entries 6−8) showed that the
E/Z selectivity could be slightly improved to 98/2 in toluene.
Extending the reaction time to 12 h did not lead to further
improvement in the stereoselectivity (entry 9). When
increasing the concentration of 1a, the reactivity was not
effected and gave the desired (E)-antethole in 99% isolated
yield with 98/2 stereoselectivity (entry 10).
with E/Z selectivity equal to or greater than 98/2 (1a−1p).
The methoxy substituent at the meta- or ortho-position of the
phenyl ring have no influence on the stereoselectivity (1b−1c).
The reaction of a mixture of (E)- and (Z)-β-methylstyrene
delivered the (E)-isomer with 98/2 selectivity (1d). This
catalytic system could tolerate alkyl, phenyl, fluoro, chloro,
bromo, protected phenol, aniline, and thiophenol groups at the
para-position of the phenyl ring, yielding the desired (E)-
alkenes with 98/2 or even 99/1 selectivity (1e−1o). The
introduction of a strongly electron-deficient substituent such as
the trifluoromethyl group in the phenyl ring also led to
excellent E/Z selectivity (1p, E/Z 99/1). The geometrical
isomerization of β-alkyl styrenes proceeded smoothly to give
the (E)-isomers with up to 99/1 selectivity (1q−1s). Pure (Z)-
stilbene converted to (E)-stilbene under the standard
conditions with 99/1 selectivity (1t). Other aryl alkenes,
such as 2-naphthalene, thiophene, furan, and pyridine, were
also tolerated to afford the corresponding (E)-isomers (1u−
1x), while the reactions of five-membered aryl alkenes afforded
the products with a slightly lower stereoselectivity. Further-
more, industrially relevant substrates, such as nothosmyrnol
(1y), methyl eugenol (1z), asarone (1aa), and isoelemicin
(1ab), were successfully converted to the corresponding trans-
products in quantitative yields with high stereoselectivity (E/Z
≥ 98/2), respectively.
A gram-scale reaction was carried out with a catalyst loading
of 0.1 mol % at room temperature for 1 h, producing the
desired (E)-product in quantitative yields without compromis-
ing the stereoselectivity (Scheme 3a). This is much more
efficient compared to other catalytic systems with noble metals
for geometrical isomerization.17,18 To further showcase the
synthetic utility, contrast experiments of alkene epoxidation
were performed (Scheme 3b). trans-Anethole [(E)-1a, 98/2 E/
Z] could be converted to trans-β-methylstyrene oxide (trans-2,
With the optimal set of conditions in hand, the substrate
scope of Z to E isomerization was shown in Scheme 2. Except
for (Z)-stilbene (1t), all alkene substrates were conveniently
prepared by Wittig olefination, and the E/Z ratios of the
starting materials were shown in the parentheses. A variety of
β-methylstyrenes containing electron-rich or -deficient groups
were suitable for this catalytic system, giving the (E)-products
B
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Scheme 2. Substrate Scope for Co-Catalyzed Z to E
Isomerization of Styrenes
Scheme 4. Preliminary Mechanistic Experiments
a
isomerization product [D-(E)-1ab], while no deuterium was
found at the C1 position (benzylic position), ruling out the
formation of π-allyl metal intermediate. Interestingly, both C1
and C2 were deuterated in the recovered 4-methoxy styrene
(Scheme 4a). When the reactions of 1a were further performed
in the presence of different radical initiators, 1,1-diphenyl
ethene, di-tert-butylhydroxytoluene, and 9,10-dihydro-
anthracene, respectively (Scheme 4b), no influence on the
results was found, indicating that the radical based HAT
pathway is unlikely. Furthermore, the reaction of pure (E)-
anethole yielded the product with 98/2 E/Z selectivity (the
same as the reaction of E/Z-mixture of 1a), which
demonstrated that the Z to E isomerization process was
reversible and controlled by thermodynamic properties
(Scheme 4c).
a
The reaction was conducted with E/Z-mixtures of alkene (1 mmol,
According to the above results, the plausible mechanism was
proposed in Scheme 5. First, a ligated cobalt hydride species
E/Z ratios were shown in the parentheses), CoCl2 (0.025 mmol),
PNP (0.030 mmol), and NaHBEt3 (0.075 mmol) in toluene (1 M) at
room temperature for 1 h. Isolated yields. The E/Z ratios were
1
determined by H NMR analysis.
Scheme 5. Proposed Mechanism
Scheme 3. Gram-Scale Reaction and Epoxidation of E/Z-
Mixtures and (E)-Alkene
(A) is generated by treating CoCl2 and PNP ligand with
NaBHEt3.33 Then (Z)-alkene coordinates with species A to
form complex B prior to its selective insertion into the Co−H
bond to form a benzylic cobalt species C. Finally, the
formation of an (E)-alkene and regeneration of cobalt hydride
species A occur by β-H elimination.
To better understand the experimental observations, DFT
calculations were carried out to shed light on the details of the
transformation (Scheme 6a).34 It was found that when the
cobalt(I)-hydride (IN1) is involved,29 it complexes with (Z)-
1a forming IN2 favorably. The migratory insertion occurs very
facilely via TS1, in which no N−Co coordination bond but
interestingly a N−H → Co agostic interaction (Co−H = 2.28
Å; see SI for details) was found, affording intermediate IN3
dr 97/3) smoothly, whereas the reaction of E/Z mixtures of 1a
gave a mixture of cis- and trans-epoxidation products.
According to literature reports,21d the isomerization reaction
could occur via a metal-hydride catalysis, HAT mechanism, or
π-allyl metal intermediate. A series of preliminary experiments
were conducted to gain insight into the catalytic cycle (Scheme
4). When 1 equiv of deuterated 4-methoxy styrene (D-3) was
added to the reaction of 1ab, a 41% proportion of the
deuterium was incorporated at the C2 position of the
C
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the catalytic cycle, which was supported by the experiments
and DFT calculations. Further investigation of novel catalytic
transformations mediated via β-elimination with an earth
abundant transition metal catalyst are undergway in our
laboratory.
Scheme 6. DFT Results (Relative Free Energies Are in kcal/
mol)
ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00072.
Experimental and computational details, data, and
spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Jianhui Chen − Wenzhou University, Wenzhou, China;
orcid.org/0000-0002-5080-1880; Email: cjh@
wzu.edu.cn
Yuanzhi Xia − Wenzhou University, Wenzhou, China;
orcid.org/0000-0003-2459-3296; Email: xyz@
wzu.edu.cn
Other Authors
Hongmei Liu − Wenzhou University, Wenzhou, China
Man Xu − Wenzhou University, Wenzhou, China
Cheng Cai − Wenzhou University, Wenzhou, China
Yugui Gu − Wenzhou University, Wenzhou, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00072
Author Contributions
containing a delocalized benzylic anion moiety. From IN3, the
barrier for β-H elimination via TS2 is 12.2 kcal/mol and the
product complex IN4 is formed slightly endergonically. The
dissociation step is endergonic by 12.3 kcal/mol, from which
the (E)-1a is formed and IN1 is regenerated. The calculated
barriers in Scheme 6a are consistent with the mild conditions
and fast transformation. The E isomer is thermodynamically
more stable than the Z isomer by 2.4 kcal/mol, explaining well
the distribution of the stereoisomers in reactions from the Z/E
mixture and absolute E configuration. To understand the
different deuterium incorporation in 1a and 3 in Scheme 4a,
the energies for alkene insertion into Co−H bond were
compared. Calculations revealed that the insertion of substrate
1a occurs more selectively via TS1 than via TS1′ by 2.6 kcal/
mol (R = Me, Scheme 6b), as a more stable delocalized
benzylic anion is formed via the former TS (see SI for details).
However, when 4-methoxy-styrene is used (R = H), the energy
of TS3 is only 0.8 kcal/mol lower than that of TS3′. These
energies are in good agreement with the deuterium labeling
experiments.
In conclusion, we have developed an efficient and mild
method for Z to E isomerization of a variety of β-substituted
styrenes prepared from Wittig olefination. In a combination of
CoCl2, PNP ligand, and NaBHEt3, the isomerization
proceeded smoothly to give the desired (E)-isomers with
high stereoselectivity at room temperature. This catalytic
reaction could be scaled up easily with a low catalyst loading of
0.1 mol %, implying the potential practical application of the
method. Preliminary mechanistic studies suggested that a
cobalt(I)-hydride and a benzylic-cobalt species are involved in
†H.-M.L. and M.X. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support was provided by the NSFC (No. 21801191,
21572163, and 21873074), Wenzhou Science & Technology
Bureau (G20180016), College Students Innovation and
Entrepreneurship Training Program (No. JWSC2018079),
and Laboratory open project (No. JW19SK65) of Wenzhou
University.
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E
https://dx.doi.org/10.1021/acs.orglett.0c00072
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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2016, 138, 8588−8594.
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(34) All calculations were done at the (SMD)M06/6-311+G(d, p)/
SDD//BP86/6-31G(d)/LANL2DZ level of theory. Computational
details are given in the Supporting Information.
F
https://dx.doi.org/10.1021/acs.orglett.0c00072
Org. Lett. XXXX, XXX, XXX−XXX