Photoredox-Catalyzed C-H Arylation of Internal Alkenes to Tetrasubstituted Alkenes: Synthesis of Tamoxifen

Article abstract of DOI:10.1021/acs.orglett.7b03223

Visible-light-induced direct C-H arylation of S,S-functionalized internal alkenes, that is, α-oxo ketene dithioacetals and analogues, has been efficiently realized with aryldiazonium salts (ArN₂BF₄) as coupling partners and Ru(bpy)

Full text of DOI:10.1021/acs.orglett.7b03223

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX-XXX
Photoredox-Catalyzed C−H Arylation of Internal Alkenes to
Tetrasubstituted Alkenes: Synthesis of Tamoxifen
Quannan Wang,†,‡ Xiaoge Yang,† Ping Wu,†,‡ and Zhengkun Yu*,†,§
†Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, P. R. China
‡University of Chinese Academy of Sciences, Beijing 100049, P. R. China
§State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
354 Fenglin Road, Shanghai 200032, P. R. China
S
* Supporting Information
ABSTRACT: Visible-light-induced direct C−H arylation of
S,S-functionalized internal alkenes, that is, α-oxo ketene dithio-
acetals and analogues, has been efficiently realized with aryl-
diazonium salts (ArN2BF4) as coupling partners and Ru(bpy)3Cl2·
6H2O as photosensitizer at ambient temperature. The strategy to
activate the internal olefinic C−H bond by both the alkylthio
and electron-withdrawing functional groups was investigated. The synthetic protocol was successfully applied to the synthesis of
all-carbon tetrasubstituted alkenes including tamoxifen.
etrasubstituted alkenes are important structural motifs
multisubstituted ketene unit and the low reactivity of the internal
Tin many natural products, pharmaceuticals, and func- olefinic C−H bonds.
tional organic materials1 and can also be used as versatile
organic intermediates.2 However, their regio- and stereoselective
synthesis has remained a great challenge.3 Although hetero-
atom-group-substituted tetrasubstituted alkenes have been
documented for diverse synthetic applications,4 all-carbon tetra-
substituted alkenes have demonstrated prominent importance
as anticancer agents. For example, tamoxifen5a has been
the most widely used anticancer drug for clinical treatment
of breast cancer. Intrigued by the superior bioactivity of
Tamoxifen and its analogs,5b construction of all-carbon tetra-
substituted alkenes has recently been paid much attention.6
Transition-metal catalyzed multicomponent reactions of
alkynes7 and intramolecular carboarylation or tandem cycliza-
tion of aryl-tethered alkynes8 have usually been employed for
this purpose. Allenes,9 diazo reagents,10a,b and N-tosylhydra-
zones10c could act as the effective building blocks of all-carbon
substituted alkenes. In principle, cross-coupling of an activated
ethylene or ketene unit can be utilized to synthesize tetrasub-
stituted alkenes. In this regard, halo-,11a oxygen-,11b boron-,11c and
silicon-functionalized11d tetrasubstituted alkenes have been
used as the substrates. Recently, direct C−H functionalization
has attracted much attention due to high atom-economy of
the synthetic methods and ready availability of the starting
materials.12 Vinyl(2-pyridyl)silanes have been successfully used
to synthesize tetrasubstituted alkenes by multistep Pd-catalyzed
Heck and Hiyama couplings.6g,13 Tamoxifen was prepared
by copper−palladium-catalyzed decarboxylative cross-coupling
of 3,3-diarylacrylic acids with aryl halides via triarylethylene-
type intermediates.14 Although C−H functionalization of tri-
substituted alkenes seems to be the straightforward route
to tetrasubstituted alkenes, only a few examples have been
documented6d,15,16 due to the steric hindrance around the
Photoredox catalysis is emerging as a promising research area
in organic synthesis17 and has recently been paid much atten-
tion to render C−H functionalization under mild, “green”
conditions.18 It has been known that 1,1-diborylalkenes,19a
1,1-dihaloalkenes,6g,19b and gem-silylborylalkenes19c can undergo
transition-metal-catalyzed cross-coupling with electrophiles
to form all-carbon tetrasubstituted alkenes. Liebeskind−Srogl
cross-coupling employing the reactions of thioesters with
boronic acids has been well explored for C−S cleavage.20,21a
Featuring the typical structure of gem-di(heteroatom)-trisub-
stituted alkenes, 1,1-di(alkylthio)alkenes, that is, α-oxo ketene
dithioacetals,21b can be considered as a class of building blocks
for all-carbon, tetrasubstituted alkenes.22
During the ongoing investigation of C−H activation of
internal alkenes, we have developed a strategy to activate an
internal olefinic C−H bond by introducing two alkylthio groups
and one electron-withdrawing group (EWG) to the two ends of
an olefinic CC bond,21a which has been successfully applied in
the C−H alkylation15b−f and alkenylation16 of trisubstituted
α-oxo (cyano) ketene dithioacetals. Unfortunately, C−H arylation
of these internal alkenes has not yet been achieved under
traditional transition-metal catalysis to date. In our previous
work, C−H alkylation and trifluoromethylation of α-oxo ketene
dithioacetals could be realized through an iron-15c or copper/
silver15f-catalyzed radical pathway. Thus, we reasonably envi-
sioned that direct C−H arylation of α-oxo ketene dithioacetals
might occur throuh a radical pathway under photocatalytic
conditions. As radical precursors, aryldiazonium salts have been
applied as the coupling partners in photoredox catalysis.23
Received: October 16, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03223
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Organic Letters
Letter
Herein, we report visible light-induced direct C−H arylation of
α-oxo ketene dithioacetals as well as construction of all-carbon
tetrasubstituted alkenes including tamoxifen.
Initially, the reaction of α-benzoyl ketene di(methylthio)acetal
(1a) with 4-bromophenyldiazonium tetrafluoroborate (2a) was
conducted to screen the reaction conditions (eq 1). With 3 mol %
ketene di(methylthio)acetals reacted with 2a to yield 3n−q in
decent yields (71−82%). The di(ethylthio)acetal analogues of 1a
and 1n, that is, 1r and 1s, were reacted with 2a to give 3r (71%)
and 3s (73%), respectively, demonstrating a steric effect of the
ethylthio group. The cyclic α-benzoyl ketene dithioacetal sub-
strate also reacted well with 2a to yield 3t (71%). tert-Butyl((1-
methoxyvinyl)oxy)dimethylsilane was also tested as the sub-
strate, but it did not react with 4-Br-phenyldiazonium salt (2a) to
form the target product. In the reaction mixture, only the homo-
coupling product of 2a was detected.
Then the protocol generality was explored by extending the
scope of aryldiazonium salts (2) (Scheme 2). Because α-acetyl
Ru(bpy)3Cl2·6H2O as the photosensitizer, the reaction of 1a
with 2a in a 1:2 molar ratio afforded the target C−H arylation
product 3a in 60% yield in DMSO at ambient temperature under
visible-light irridiation of a 15 W white LED bulb. Both the
photosensitizer loading and substrate concentrations affected the
reaction efficiency. Adding a base such as Na2CO3, K2CO3, or
NaHCO3 did not promote the reaction. Shorter reaction times
such as 5 and 10 h were applied to the reaction, affording 3a in
62% and 67% isolated yields, respectively. The highest isolated
yield (78%) was obtained at a 1:2.2 molar ratio of 1a/2a in the
presence of 3.5 mol % of Ru(bpy)3Cl2·6H2O. A 4 mmol scale
reaction also efficiently afforded 3a (66%). Notably, the reaction
could not occur without either the photosensitizer or visible light
irradiation (see the Supporting Information).
a
Scheme 2. Scope of Aryldiazonium Salts 2
Next, the scope of α-oxo ketene dithioacetals (1) was investigated
under the optimized conditions (Scheme 1). The halo-substituted
a
Scheme 1. Scope of α-Oxo Ketene Dithioacetals
a
Conditions: 1 (0.3 mmol), 2 (0.66 mmol), Ru(bpy)3Cl2·6H2O
(0.0105 mmol), DMSO (3 mL), 0.1 MPa N2, 15 W white LED,
25 °C, 15 h. 1 (0.45 mmol), 2 (0.3 mmol), Ru(bpy)3Cl2·6H2O
(0.003 mmol). 1 (0.45 mmol), 2 (0.3 mmol), Ru(bpy)3Cl2·6H2O
(0.015 mmol). Yields refer to the isolated products.
b
c
ketene di(methylthio)acetal (1n) could efficiently react with 2a
to form 3n (82%) which might be easily transformed to a
tamoxifen-type alkene, α-acetyl ketene dithioacetals were used as
the trisubstituted alkene substrates to react with various
aryldiazonium salts. Reacting 1n with phenyldiazonium tetra-
fluoroborate (2b) led to 4a in 61% yield, and the yield was
improved to 71% by altering the molar ratio of 1:2 from 1:2.2 to
1.5:1. Both 4-F- and 3-F-phenyldiazonium salts reacted well to
give 4b (73%) and 4c (68%), respectively. However, the reaction
of 2-F-phenyldiazonium salt (2e) formed unsymmetrical alkenyl
azobenzene 4d′ (44%), exhibiting steric and electronic effects.
Further probing of the electron-withdrawing substituent effect
revealed that a 4-NO2 substituent led to exclusive formation of
alkenyl azobenzene 4e′ (82%) (eq 2), depicting a direct route
to unsymmetrical azobenzenes through C−H diazo reaction.24
Aryldiazonium salts bearing an electron-withdrawing substituent
such as 4-Cl, 4-MeCO, or 4-CO2Me reacted with 1n to afford
a
Conditions: 1 (0.3 mmol), 2a (0.66 mmol), Ru(bpy)3Cl2·6H2O
(0.0105 mmol), DMSO (3 mL), 0.1 MPa N2, 15 W white LED, 25 °C,
15 h. Yields refer to the isolated products.
α-benzoyl ketene di(methylthio)acetals efficiently underwent
the reactions with 2a, affording the target products 3b−f in
67−76% yields, while 4-CO2Me as the substituent lessened the
yield of 3g to 66%. Electron-donating methyl and methoxy
substituents exhibited different impacts on the formation of
3h−k (63−72%). 2-Naphthoyl had a negative steric impact on the
reaction. Unexpectedly, α-(2-thienoyl)ketene di(methylthio)-
acetal reacted efficiently to give 3m (77%). Moreover, aliphatic
α-acetyl-, cyclopropylcarbonyl-, ester-, and cyano-functionalized
B
DOI: 10.1021/acs.orglett.7b03223
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Organic Letters
Letter
was performed in the presence of 4.0 equiv of 2,2,6,6-tetramethyl-
1-piperidinyloxy (TEMPO) or 2,6-di-tert-butyl-4-methylphenyl
(BHT) under the standard conditions. These radical-trapping
reagents completely inhibited or deteriorated the reaction,
forming no 3a in the case of using TEMPO or affording 3a in
24% yield in the presence of BHT, implicating a radical pathway
involved in the catalytic cycle. It is noteworthy that the adducts
between TEMPO or BHT and the aryl radicals were not detected
by HRMS analysis. The kinetic isotope effect (KIE) experi-
ments were carried out by means of the reactions of 1a and
its deuterated form 1a[D] with 2a. A secondary isotope effect25
was observed with kH/kD = 1.1, indicating that cleavage of the
internal olefinic C−H bond in 1a was not involved in the rate-
determining step in the overall catalytic cycle. Light on/off experi-
ments revealed that the reaction only proceeded with irradiation of
the 15 W LED, ruling out the radical-chain propagation mechanism.
Eventually, internal alkenes 1 were applied as the building
blocks to prepare tamoxifen-type all-carbon tetrasubstituted
alkenes. First, α-acetyl ketene di(methylthio)acetal (1n) was trans-
formed to 1,1-S,S-functionalized tetrasubstituted alkene 4a
(Scheme 2), which was then successively arylated by PhB(OH)2
and 4-(2-dimethylaminoethoxy)phenylboronic acid through
Liebeskind−Srogl cross-coupling,22a affording 6b and 11 in
87% and 63% yields, respectively (Scheme 3). Luche reduction26
4f−h (70−78%), while the electron-donating groups OCF3,
OMe, t-Bu, and Me on the aryl functional group of 2
deteriorated the reaction efficiency, leading to 4i−m in
58−69% yields, and 2-methyl exhibited a negative steric effect
on the yield of 4n (47%). In a similar manner, excess α-acetyl
ketene di(ethylthio)acetal reacted with various aryldiazonium
salts to yield the target products 4o−s in decent yields, whereas
the same reactions only afforded the products in 30−60%
yields under the standard conditions. Phenyldiazonium salt
(2b) reacted less efficiently with α-benzoyl ketene dithio-
acetals to form 4t (58%) and 4u (60%) (Scheme 2), while
the reactions of the same internal alkenes with 2a gave 3a
(78%) and 3t (71%) (Scheme 1), respectively. In the cases of
α-(4-CF3-benzoyl), acetyl, and ester ketene dithioacetals, 2b
only showed a moderate reactivity to form 4v−y (54−62%),
but alteration of the reaction conditions by using excessive
amounts of the internal alkene substrates led to remarkable
yield enhancement for 4v (54% to 70%) and 4y (58% to 82%).
Heteroaryldiazonium salts such as quinolone-3-diazonium
and methyl thiophene-2-carboxylate-3-diazonium salts were
also examined, but their reactions with 1a only afforded the
corresponding target products in 21% and 15% yields, respec-
tively. The molecular structures of compounds 4e′ and 4u were
further confirmed by the X-ray single crystal crystallographic
determinations (see the SI for details).
Scheme 3. Synthesis of Tamoxifen
To verify the strategy for activating the internal olefinic C−H
bond, α-acetyl ketene monothioacetal (5)22a was applied in
the photocatalytic reactions with aryldiazonium salts 2 (eq 3).
of 11 gave tamoxifen analogue 12 in 58% yield ((Z)-12,
29%; (E)-12, 29%), and subsequent dehydration/hydrogena-
tion6d yielded tamoxifen (13) in 75% yield (Z/E = 1.4/1).
An alternative route was also developed for the transformation of
alkenes 1 to all-carbon tetrasubstituted alkenes. By means of two
successive Liebeskind−Srogl cross-couplings with arylboronic
acids, alkenes 1 were first transformed to 1,1-diaryl-trisubstituted
alkenes 9, which were then subject to photocatalytic C−H
arylation with 2 to afford the target products 10a−g in 51−62%
yields (eqs 4 and 5).
As compared with the transformations of the corresponding
α-acetyl ketene dithioacetals to form 3a (78%), 4a (61%), and 4l
(60%), the reaction efficiency to form tetrasubstituted alkenes 6a
(66%), 6b (50%), and 6c (50%) was obviously decreased. These
results have suggested that the di(alkylthio) functional groups
are necessary for internal alkenes 1 to undergo the photocatalytic
C−H arylation efficiently with 2. Under the standard conditions,
all-carbon trisubstituted 1,1-diphenyl-1-butene (7) reacted with
2a to give all-carbon tetrasubstituted alkene 8 in 37% yield, while
its α-acetyl analog 9a reacted to form 10a in a much higher yield
(62%), revealing the crucial role of the EWG, that is, α-oxo, in the
activation of the internal olefinic C−H bond (eq 4).
Control experiments were conducted to probe the reaction
mechanism (see the SI for details). The reaction of 1a with 2a
C
DOI: 10.1021/acs.orglett.7b03223
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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In summary, efficient photoredox-catalyzed direct C−H
arylation of internal alkene α-oxo ketene dithioacetals with
aryldiazonium salts has been realized to form 1,1-S,S-function-
alized tetrasubstituted alkenes. Combined with Liebeskind−
Srogl cross-coupling, tamoxifen-type alkenes were prepared.
The present protocol provides a new route to all-carbon tetra-
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ASSOCIATED CONTENT
* Supporting Information
Experimental materials and procedures, NMR of compounds,
and X-ray crystallographic analysis for compounds 4e′ and 4u.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b03223.
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X-ray crystallographic data for compounds 4e′ and 4u
(CIF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zkyu@dicp.ac.cn.
ORCID
(16) (a) Yang, X. G.; Liu, Z. Q.; Sun, C. L.; Chen, J. P.; Yu, Z. K. Chem. -
Eur. J. 2015, 21, 14085. (b) Yu, H. F.; Jin, W. W.; Sun, C. L.; Chen, J. P.;
Du, W. M.; He, S. B.; Yu, Z. K. Angew. Chem., Int. Ed. 2010, 49, 5792.
(17) (a) Xuan, J.; Zhang, Z.-G.; Xiao, W. J. Angew. Chem., Int. Ed. 2015,
54, 15632. (b) Bergonzini, G.; Schindler, C. S.; Wallentin, C.-J.;
Jacobsen, E. N.; Stephenson, C. R. J. Chem. Sci. 2014, 5, 112.
(18) (a) Yi, H.; Niu, L. B.; Song, C. L.; Li, Y. Y.; Dou, B. W.; Singh, A.
K.; Lei, A. W. Angew. Chem., Int. Ed. 2017, 56, 1120. (b) Metternich, J.
B.; Gilmour, R. J. Am. Chem. Soc. 2016, 138, 1040. (c) Yang, F. Z.;
Koeller, J.; Ackermann, L. Angew. Chem., Int. Ed. 2016, 55, 4759.
(d) Fabry, D. C.; Ronge, M. A.; Zoller, J.; Rueping, M. Angew. Chem., Int.
Quannan Wang: 0000-0002-4136-0016
Xiaoge Yang: 0000-0002-3832-5316
Ping Wu: 0000-0003-2650-9043
Zhengkun Yu: 0000-0002-9908-0017
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Basic Research Program of China
(2015CB856600) and the National Natural Science Foundation
of China (21472185) for support of this research.
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