AgSbF6-controlled diastereodivergence in alkyne hydroarylation: Facile access to Z- and E-alkenyl arenes

Article abstract of DOI:10.1039/c4cc03602f

AgSbF₆-controlled diastereodivergent hydroarylation reactions were developed. Unprecedented and remarkable switching of the E/Z-stereoselectivity could be obtained by adjusting the AgSbF₆ loading. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc03602f

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AgSbF6-controlled diastereodivergence in alkyne
hydroarylation: facile access to Z- and E-alkenyl
arenes†
Cite this: Chem. Commun., 2014,
50, 8028
Received 13th May 2014,
Accepted 3rd June 2014
Minsik Min, Donghee Kim and Sungwoo Hong*
DOI: 10.1039/c4cc03602f
www.rsc.org/chemcomm
AgSbF6-controlled diastereodivergent hydroarylation reactions were
developed. Unprecedented and remarkable switching of the E/Z-
stereoselectivity could be obtained by adjusting the AgSbF6 loading.
The transition metal-catalyzed C–H bond functionalization is a
rapidly evolving research field as atom and step-economical
tools that are useful in organic synthesis and total synthesis.
Among these processes, alkyne hydroarylation has been shown
to be a highly efficient route for the synthesis of alkenyl arenes
because it allows for the direct synthesis of functionalized
alkenes directly from simple arenes and alkynes.1,2
Scheme 1 Overall reaction scheme for the alkyne hydroarylation.
Directing groups, such as amides, esters, ketones, carbamates,
phosphine oxides, or sulfoxide-substituted aromatics, were shown to
undergo hydroarylation with alkynes in the presence of ruthenium(II)3 for AgSbF6-controlled stereodivergence in a class of hydroarylation
or rhodium(III)4 complexes as catalysts, yielding trisubstituted alkenes. reactions between arenes and alkynes.
Recently, significant progress was made toward E-stereoselective (syn
We began our investigation of the alkyne hydroarylation
addition) coupling in hydroarylation reactions via a chelation-assisted using chromones 1, which are prevalent in a plethora of natural
concerted metalation–deprotonation pathway.3,4 Subsequent coordi- and bioactive compounds.5 After surveying some potential
native insertion of the alkyne into the metal–carbon bond provides a catalytic systems,6 we found that a catalytic system consisting
metallacycle intermediate, which was then protonated by an organic of [Ru( p-cymene)Cl2]2 (5 mol%), and AgSbF6 (16 mol%) in
acid to give the corresponding E-alkene derivative in a stereoselective combination with Cu(OAc)2 (10 mol%) and AcOH (2.0 equiv.)
manner (Scheme 1a).
in 1,2-dichloroethane (DCE) at 100 1C provided 2a in a 94%
Despite the successes reported thus far, this type of directing combined yield (Table 1, entry 1). The newly generated alkene was
group-assisted hydroarylation approach has been limited to yielding E configured, thus suggesting that the addition step proceeded in a
the E-alkene products, and universal access to the Z-alkenyl arenes syn manner. The catalyst system was found to be applicable to the
remains a distinct challenge. In this regard, new catalytic systems to reactions of chromones bearing useful substrate functional groups,
override the directing group-controlled E-stereoselective coupling and Table 1 outlines the scope of the hydroarylation reaction under
would be highly valuable for the efficient synthesis of Z-alkene the optimized reaction conditions. The use of chromones bearing
products. During studies of transition metal-catalyzed hydro- substituents at the 6-position as substrates yielded a mixture of the
arylations of alkynes, we observed unprecedented switching in E and Z configurations probably due to steric effects (entries 11,
the E/Z-stereoselectivity through the action of AgSbF6 under the 12, 13). To our surprise, if the same reaction was carried out in the
reaction conditions (Scheme 1b). Herein, we describe a method presence of additional 20 mol% of AgSbF6, a completely different
stereoisomeric pattern was observed, and the Z-selective products
Center for Catalytic Hydrocarbon Functionalizations, Institute of Basic Science (IBS)
were obtained. The structure of the Z-isomer 3b was unambiguously
confirmed by X-ray crystallographic analysis.6 This approach
and Department of Chemistry, Korea Advanced Institute of Science and Technology
(KAIST), Daejeon, 305-701, Korea. E-mail: hongorg@kaist.ac.kr;
provides an attractive solution to the current limitations on
Fax: +82 42-350-2810; Tel: +82 42-350-2811
the E-stereoselective coupling in the hydroarylation reactions.
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cc03602f
However, this method was not suitable for the hydroarylation of
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Table 1 Ruthenium-catalyzed hydroarylation of chromonesa
Table 1 (continued)
Entry
1
Condition A
Condition B
Entry
11
Condition A
Condition B
12
2
13
3
4
5
a
Reactions were carried out under the following reaction conditions:
chromone (1.0 equiv.), alkyne (1.5 equiv.), [Ru(p-cymene)Cl2]2 (5 mol%),
AgSbF6 (16 mol% for condition A, 16 + 20 mol% for condition B),
Cu(OAc)2 (10 mol%), and AcOH (2 equiv.) in DCE at 100 1C for 2–6 h.
b
[Ru(p-cymene)Cl2]2 (8 mol%), AgSbF6 (28 mol% for conditions A, 28 +
20 mol% for conditions B). Isolated yields. DCE = 1,2-dichloroethane.
aliphatic alkynes, affording a mixture of the E- and Z-products
(Scheme S1, ESI†).6
It is conceivable that the Z-alkene products arose from the
isomerization of the syn addition products, and AgSbF6 played
a pivotal role in facilitating the isomerization process under the
catalytic conditions employed here. To test this hypothesis, we
investigated the isomerization reaction using the isomerically
pure E-isomer 2b as a substrate (eqn (1)). Among the Ag species
screened, AgSbF6 was the most effective catalyst for promoting
isomerization.6 The use of DCE as a solvent was necessary to
achieve a high selectivity and reaction efficiency. No obvious
effects on the isomerization were observed upon the addition of
2,6-di-tert-butyl-4-methyl-phenol (BHT), suggesting that a radical
mechanism was unlikely to be operative.
6b
7
8
(1)
We proposed a plausible catalytic mechanism for the hydro-
arylation of chromones (Scheme 2). Cationic Ru(II) species was
prepared in situ upon treatment of the [Ru(p-cymene)Cl2]2
precursor with the AgSbF6 additive, and AgCl precipitated as
a byproduct. A seven-membered ruthenacycle intermediate II
was formed through coordinative insertion of the alkyne into
the resulting aryl–Ru bond of the intermediate I. Protonolysis
of the Ru–C bond of intermediate II afforded the E-alkene 2a
9
10
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Table 2 Scope of the Ag-catalyzed isomerization reactiona
Scheme 2 Plausible reaction pathway.
and regenerated Ru(II). In the presence of an active AgSbF6
species, the isomerization process would be expected to initiate
through the formation of the alkyl cation III and bond rotation
to drive the transformation of E-alkenyl chromone 2a into the
thermodynamically more stable Z-isomer 3a. Both proton shifts
(Ha and Hb) and 13C NMR shift (CQO) revealed a good
correlation with the p-electron densities of intermediate III.6,7
Recently, Glorius’s group accomplished a phosphoryl-related
directing rhodium catalyzed hydroarylation.4f It was reported that
the use of phosphonate esters as the substrates yielded a mixture of
olefinic E–Z isomers (Scheme 3a). Because the AgSbF6-catalyzed
alkene isomerization in DCE was observed in our study, it
was reasoned that the E- and Z-mixture might have arisen from
isomerization of the E-alkene by the action of the catalytically active
AgSbF6 species. This prediction was tested by conducting the
hydroarylation reaction in the presence of additional 0.5 mol%
[RhCp*Cl2]2 under otherwise previously reported conditions. Indeed,
the hydroarylation of the phosphonate ester substrates delivered the
E-selective products. Furthermore, we were delighted to observe that
the Rh-catalyzed hydroarylation of the phosphonate esters with
alkynes allowed for the straightforward synthesis of the Z-selective
products in the presence of additional 20 mol% AgSbF6.
a
Reactions were conducted with substrate (1.0 equiv.), AgSbF6 (20 mol%),
and AcOH (2 equiv.) in DCE at 100 1C. b AgSbF6 (10 mol%) was used.
system was amenable to a variety of directing groups and permitted
the construction of a series of Z-alkene-substituted arenes. In the
majority of cases, high selectivity (490%) was observed.
In summary, we developed a new protocol to effect alkyne
hydroarylation in a stereodivergent manner. The remarkable
switching of the product E–Z-stereochemistry was facilitated by
an active AgSbF6 species, which catalyzed the isomerization of
the generated E-alkene.
Next, the unusual effects of AgSbF6 prompted us to investigate
the generality of the phenomenon. As shown in Table 2, we explored
the substrate scope of the transformation of the isomerically pure
E-alkenyl arenes into the Z products. To our delight, this catalytic
This research was supported by National Research Founda-
tion of Korea (NRF-2011-0016436) and the Institute for Basic
Science (IBS) in Korea. M. Min is the recipient of a Global PhD
Fellowship (NRF-2011-0007511).
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