Cobalt-Catalyzed Migrational Isomerization of Styrenes

Article abstract of DOI:10.1021/acs.orglett.9b04305

An efficient cobalt-catalyzed migrational isomerization of styrenes was developed using the thiazoline iminopyridine (TIP) ligand. This reaction is operationally simple and atom-economical using readily available starting materials to access trisubstituted alkenes. Even when using a 0.1 mol % catalyst loading, the reaction could be conducted in neat and completed in 1 h with excellent conversion and high E stereoselectivity.

Full text of DOI:10.1021/acs.orglett.9b04305

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Letter
Cobalt-Catalyzed Migrational Isomerization of Styrenes
Jiajin Zhao,† Biao Cheng,† Chenhui Chen, and Zhan Lu*
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ABSTRACT: An efficient cobalt-catalyzed migrational isomer-
ization of styrenes was developed using the thiazoline iminopyr-
idine (TIP) ligand. This reaction is operationally simple and atom-
economical using readily available starting materials to access
trisubstituted alkenes. Even when using a 0.1 mol % catalyst
loading, the reaction could be conducted in neat and completed in
1 h with excellent conversion and high E stereoselectivity.
Scheme 1. Isomerization of Simple α-Alkyl Styrene
risubstituted olefins are ubiquitous structural motifs
Tfound in many bioactive natural products, agrochemicals,
and pharmaceuticals.1 Notably, several classical approaches for
the synthesis of trisubstituted alkenes have been well-defined,
such as Wittig reaction, Horner−Wadsworth−Emmons
reaction, Julia olefination, Peterson olefination, etc.2 Further-
more, metal-catalyzed coupling reactions and olefin metathesis
are also well-established methods for their synthesis.3 Never-
theless, these approaches provided stoichiometric chemical
waste and suffered from poor reactivity and E/Z selectivity.2−4
Due to its great atom economy, migrational isomerization of
easily accessible terminal olefins is an ideal method for the
synthesis of trisubstituted olefin. A main feature of this method
is migration of the original carbon−carbon double bond, which
is driven by a thermodynamically favored termination step.5
Over the past decades, tremendous efforts have been
dedicated to the development of efficient catalytic strategies
based on noble metal complexes6 including ruthenium,7
rhodium,7j,8 palladium,9 iridium,10 and others.11 Recently, the
earth-abundant metal catalysts also play an important role in
this research area.12 Although a wide range of efficient metal-
catalyzed olefin migrational isomerization reactions have been
developed, we find that the highly efficient and selective
migrational isomerization of simple α-alkyl styrenes to
trisubstituted styrenes is still a challenge and has not been
well explored (Scheme 1). In 2009, the RajanBabu group
described a palladium catalyst exhibits a high preference for the
formation of the trisubstituted alkenes from a simple α-alkyl
styrene (Scheme 1a).9a However, the E/Z selectivity (2/1−
19/1 E/Z) and the scope of substrate (only 2 examples) need
to be improved. In 2012, the Grotjahn group reported that
using a ruthenium complex as a catalyst could catalyze the
migrational isomerization of simple α-alkyl styrenes (Scheme
1b).7c In 2015, in the process of studying for the selective
oxidation of aromatic olefins, the Xiao group found the
isomerization of a simple α-alkyl styrene to trisubstituted
alkenes with Z-selectivity (Scheme 1c).13 Unfortunately, the
yield of olefin isomerization could not exceed 80% and the
scope of substrates was no more than two in both cases. In
Received: November 30, 2019
https://dx.doi.org/10.1021/acs.orglett.9b04305
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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Letter
a
2016, Norton and co-workers described an elegant radical
isomerization reaction using Co(dmgBF2)2(THF)2 under 6
atm of H2 pressure in which seven examples on α-alkyl styrenes
with functional groups were reported with 52−>95% yields.
However, simple α-alkyl styrenes have not been mentioned.12j
In 2018, the Zhao group presented an unprecedented
enantioselective isomerization of homoallylic and bishomoal-
lylic secondary alcohols (Scheme 1d).8b This catalytic system
was also applied to the isomerization of simple α-alkyl styrenes
with an excellent yield (80−90%) and E-selectivity (>10/1 E/
Z). However, the recovery of starting materials was not
mentioned and the scope of substrate has not been well
explored.
Table 1. Optimization for Isomerization of Alkenes
While studying cobalt-catalyzed alkene hydrofunctionaliza-
tion,14 we found that alkene isomerization occurred which is
quite similar to results observed by other research groups.12
Herein, we developed migrational isomerization of 1,1-
disubstituted aromatic alkenes using a thiazoline iminopyridine
(TIP) cobalt dichloride complex with high efficiency and
stereoselectivity.
Results and Discussion. At the beginning of screening, 1-
(but-1-en-2-yl)-4-methoxybenzene (1a) was chosen as a model
substrate. The NaBHEt3 was used as the activator. Since
bis(imino)pyridine ligands and bis(oxazoline)pyridine ligands
played an important role in cobalt-catalyzed alkene trans-
formation,14a we first screened these two ligands. Only 66%
and 24% yields of isomerization product were observed,
respectively (entries 1 and 2, Table 1). The reaction of 1a
using an oxazoline iminopyridine (OIP) cobalt complex (Lc·
CoCl2, 5 mol %) as a catalyst in neat at room temperature for 1
h could be smoothly performed to afford the isomerized
product E-2a in 88% yield and hydrogenation product 3a in
6% yield, as well as with 6% recovery (entry 3). Then, the
substitution on the OIP ligand was screened to enhance the
reactivity and selectivity of isomerization. The use of the Ld
ligand with the bulkier substitution 2,6-diisopropyl group on
imine did slow down the isomerization reaction (entry 4). The
use of Le with only one smaller methyl group on imine
dramatically inhibited the hydrogenation product, however,
slightly decreased the reactivity (entry 5). The use of Lf with a
2-tert-butyl group on imine did increase the reactivity (entry
6). Using the corresponding more electron-rich thiazoline
iminopyridine ligand Lg15 could dramatically improve the
isomerization reaction which afforded 2a in 96% yield with 2%
yield of 3a and 2% recovery (entry 7). The reaction using 1
mol % of catalyst could afford 2a in 96% yield without loss of
reactivity (entry 8). Due to the lack of the amount of
reductant, the hydrogenation reaction was also inhibited. The
reaction with a 0.1 mol % catalyst loading in 4 mmol scale
could afford the desired product in 96% yield, and the turnover
number was up to 960 (entry 9). So, the suitable stereo-
chemical outcome on the ligand did affect the reactivity, and
the side hydrogenation reaction could be inhibited by
decreasing the amount of catalyst and reductant. The
stereoscopic configuration was confirmed by NOE analysis.
For operational simplicity, the standard conditions were
chosen to be alkene (0.5 mmol), Lg·CoCl2 (1 mol %), and
NaBHEt3 (3 mol %) in neat at room temperature for 1 h.
Under the optimized conditions, the substrate scope was
explored in Scheme 2. Various functional groups were well
tolerated, such as ether, halide, protected alcohol, ester, and
amine. Electron-donating substituents on the phenyl ring, such
as methyl, methoxyl, and dimethylamino, could be well
a
The reaction was conducted using alkene (0.5 mmol), cobalt
complex (x mmol %), and NaHBEt3 (3x mol %) in neat at room
temperature for 1 h in glovebox. Determined by H NMR using
TMSPh or CH2Br2 as an internal standard. Determined by GC and a
calibration curve to achieve accurate integration/conversion. 4 mmol
b
1
c
d
scale.
tolerated, and the conversions were all >94% (2b−f, 2n−p).
Electron-withdrawing substituents on the phenyl ring did not
influence the conversion, but decreased the ratio of E/Z (2g−
m). The ortho-, meta-, and para-substituents on benzene rings
(2c−m) resulted in similarly remarkable conversion and an
excellent E/Z ratio. The conversion and stereoselectivity were
not obviously influenced by the length of the alkyl chain (2q).
Intracyclic olefins (2r) could also be obtained with 98%
conversion. Sulfur-containing heterocycles thiophene (2s) and
ferrocene (2t) could be well tolerated to give the
corresponding products in 97% and 96% conversion,
respectively. Other than 1,1-disubstituted alkenes, monosub-
stituted alkenes could also be converted to internal alkenes
(2v−x) with excellent benzylic selectivity. 1,2-Disubstituted
alkenes could also be converted to a benzylic selective product
(2y) with E-stereoselectivity.
The gram-scale reaction could be smoothly carried out to
afford 2a and 2c in 90% and 86% yield, respectively (eq 1,
Scheme 3). The isomerization product could be further
derivatized (eqs 2−5, Scheme 3). The trisubstituted alkenes E-
2c could undergo epoxidation to afford epoxides 4c in 69%
yield with >99/1 dr, which is quite challenging to obtain. An
analogous reaction using 2c with a 4/1 E/Z ratio furnished 4c
B
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a
Scheme 2. Scope of Isomerized Alkenes
Scheme 3. Gram-Scale Reaction and Further Derivatizations
Scheme 4. Mechanistic Studies
a
The reaction was conducted using alkenes (0.5 mmol), Le·CoCl2
(0.005 mmol) and NaHBEt3 (0.015 mmol) in neat at room
temperature for 1 h in glovebox. The conversions were determined
1
by H NMR using TMSPh or CH2Br2 as an internal standard or by
Scheme 5. Proposed Mechanism
GC analysis. Stereoscopic configuration was confirmed by NOE
b
c
analysis. Without note, E/Z > 20/1. E/Z = 18/1. Using 5 mol %
catalyst loading. 3 h. 5 h. 12 h.
d
e
f
in 72% yield with 4/1 dr. The cyclopropane 5c with high dr
(99/1) could be obtained in 72% yield via cyclopropanation of
trisubstituted alkenes E-2c. However, when the same reaction
partners were studied using 2c with an E/Z ratio of 4/1, 5c was
afforded in 73% yield with 4/1 dr.
To probe the possible mechanism of the migrational
isomerization of simple α-alkyl styrene, the deuterium reaction
of D-1b was performed under standard conditions to afford
98% conversion of D-2b with 49% D on the methyl group and
53% D on the vinyl group (eq 6, Scheme 4). The partial
absence of deuterium at the vinyl group illustrated that alkene
insertion and β-hydride elimination were reversible. The
reaction of 1,1-diphenylethylene as a radical trap could be
carried out smoothly which might exclude a radical pathway
with hydrogen atom transfer (eq 7, Scheme 4).
underwent β-hydride elimination to regenerate cobalt hydride
species and afford trisubstituted alkenes.
In summary, we have developed a cobalt-catalyzed isomer-
ization of alkenes, which could convert 1,1-disubstituted
aromatic alkenes to trisubstituted alkenes with high stereo-
selectivity. The thiazoline iminopyridine ligand was found as a
suitable ligand for this efficient transformation. The mono-
substituted and the 1,2-disubstituted alkenes could also be
suitable under the same conditions to deliver benzylic selective
products with excellent conversion and high stereoselectivity.
Based on previous reports16 and our experimental outcomes,
an alkene insertion/β-hydride elimination pathway via a cobalt
hydride intermediate for this protocol is shown in Scheme 5.
The reduction of TIP·CoCl2 by NaBHEt3 could afford cobalt
hydride species A, which underwent alkene insertion to deliver
cobalt alkyl species C. This cobalt alkyl species C then
C
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b04305.
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Experimental procedures, characterization data for all
compounds (PDF)
́
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AUTHOR INFORMATION
Corresponding Author
■
Zhan Lu − Zhejiang University, Hangzhou, China;
orcid.org/0000-0002-3069-079X; Email: luzhan@
zju.edu.cn
Other Authors
Jiajin Zhao − Zhejiang University, Hangzhou, China
Biao Cheng − Zhejiang University, Hangzhou, China
Chenhui Chen − Zhejiang University, Hangzhou, China
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.9b04305
Author Contributions
†J.Z. and B.C. contributed equally.
Notes
́
(i) Krompiec, S.; Pigulla, M.; Kuznik, N.; Krompiec, M.; Marciniec,
B.; Chadyniak, D.; Kasperczyk, J. J. Mol. Catal. A: Chem. 2005, 225,
91. (j) Krompiec, S.; Kuznik, N.; Penczek, R.; Rzepa, J.; Mrowiec-
The authors declare no competing financial interest.
́
́
Białon, J. J. Mol. Catal. A: Chem. 2004, 219, 29.
ACKNOWLEDGMENTS
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2009, 74, 4565. (b) Gauthier, D.; Lindhardt, A. T.; Olsen, E. P. K.;
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Financial support was provided by NSFC (21922107 and
21772171), Zhejiang Provincial Natural Science Foundation of
China (LR19B020001), Center of Chemistry for Frontier
Technologies, and the Fundamental Research Funds for the
Central Universities.
(c) Mamone, P.; Grunberg, M. F.; Fromm, A.; Khan, B. A.; Gooßen,
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L. J. Org. Lett. 2012, 14, 3716.
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