Ruthenium-Catalyzed Hydroarylation and One-Pot Twofold Unsymmetrical C?H Functionalization of Arenes

Article abstract of DOI:10.1002/anie.201600649

A methyl phenyl sulfoximine (MPS) is used as a directing group in the ruthenium-catalyzed intramolecular hydroarylation of alkene-tethered benzoic acid derivatives to afford dihydrobenzofurans and indolines in good to excellent yields. A one-pot, unsymmetrical, twofold C?H functionalization involving intramolecular C?C and intermolecular C?C/C?N bond formations is successfully demonstrated by using a single set of catalytic reaction conditions, which is unprecedented thus far. A novel isoquinolone-bearing dihydrobenzofuran is constructed through an unsymmetrical twofold C?H functionalization.

Full text of DOI:10.1002/anie.201600649

Angewandte
Communications
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International Edition: DOI: 10.1002/anie.201600649
German Edition: DOI: 10.1002/ange.201600649
Annulations
Ruthenium-Catalyzed Hydroarylation and One-Pot Twofold
À
Unsymmetrical C H Functionalization of Arenes
Koushik Ghosh, Raja K. Rit, E. Ramesh, and Akhila K. Sahoo*
Abstract: A methyl phenyl sulfoximine (MPS) is used as
a directing group in the ruthenium-catalyzed intramolecular
hydroarylation of alkene-tethered benzoic acid derivatives to
afford dihydrobenzofurans and indolines in good to excellent
À
yields. A one-pot, unsymmetrical, twofold C H functionaliza-
À
À
tion involving intramolecular C C and intermolecular C C/
À
C N bond formations is successfully demonstrated by using
a single set of catalytic reaction conditions, which is unprece-
dented thus far. A novel isoquinolone-bearing dihydrobenzo-
À
furan is constructed through an unsymmetrical twofold C H
functionalization.
T
ransition metal (TM) catalyzed directing group (DG)-aided
À
functionalization of inert C H bonds is a promising method
for the step- and atom-efficient construction of versatile
molecular structures from readily accessible starting materi-
als.[1–3] Despite the growth and success of C H activation,
synthetically useful DG-promoted multiple functionalizations
À
À
Scheme 1. Unsymmetrical twofold ortho-C H functionalization.
À
of arene C H bonds are less explored, yet exceedingly
attractive. In general, the steric/electronic/coordinating effect
of the ortho-monofunctionalized product, obtained from the
DG-coupled arene, hampers the efficient introduction of
ideal (Scheme 1b). To the best of our knowledge, no such
transformations are known.
Dihydrobenzofuran skeletons are widely found in many
biologically active compounds and potential drug candi-
dates.[10] The groups of Ellman and Bergman, Rovis, Cramer,
and Yoshikai have independently demonstrated the rhodium-
and cobalt-catalyzed intramolecular hydroarylation of O-
bearing olefin-tethered arenes for the synthesis of dihydro-
benzofurans (Scheme 2a).[11–13] However, the use of air-stable
and low-cost ruthenium catalysts for intramolecular hydro-
arylation with unactivated alkenes remains underdevel-
oped.[14]
À
other functional groups to the second ortho C H bond on the
À
arene. Consequently, effective twofold C H bond function-
alization of arenes remains challenging.[4] Furthermore, the
DG and catalytic conditions are highly selective for the
respective bond formations, and as a consequence, unsym-
À
metrical functionalization of C H bonds in the presence of
one catalyst is mostly unsuccessful. Moreover, the formation
of numerous by-products and an incomplete reaction profile
involved in each step of the transformation makes the overall
process cumbersome and unproductive.[5] Notably, single DG-
We therefore devised a one-pot, unsymmetrical, twofold
À
À
enabled, symmetrical double ortho-C H functionalization of
C H functionalization using a rationally designed olefin-
tethered substrate (Scheme 1c). The reaction involves an
arenes has been thoroughly investigated (Scheme 1a).[6,7]
Recently, the groups of Yu and Gevorgyan have independ-
À
intramolecular ortho-C H hydroarylation and intermolecular
functionalization of a second ortho-C H bond. Herein, we
À
À
ently disclosed elegant two-step unsymmetrical ortho-C H
functionalizations of arenes. The reactions proceeded well in
the presence of two different catalytic systems and one DG
(Scheme 1b).[5,8,9] The realization of a one-pot, unsymmet-
discuss the methyl phenyl sulfoximine (MPS; 1) directed[15,16]
ruthenium-catalyzed intramolecular hydroarylation of an O-
tethered olefin bearing benzoic acid derivatives for the
synthesis of a wide array of dihydrobenzofurans at room
temperature (Scheme 2b). One-pot, symmetrical and unsym-
À
rical, twofold C H functionalization of arenes in the presence
of a single DG and single catalyst would essentially lead to
highly functionalized arene derivatives in a step-efficient and
byproduct-free fashion, and would therefore be considered
À
metrical, twofold C H functionalizations in the presence of
a single catalyst were successfully investigated (Scheme 2b).
A sequential hydroarylation and annulation with alkynes,
À
through two C H functionalizations, is also reported.
[*] K. Ghosh, R. K. Rit, E. Ramesh, Prof. Dr. A. K. Sahoo
School of Chemistry, University of Hyderabad
Gachibowli, Hyderabad, Telangana 500046 (India)
E-mail: akhilchemistry12@gmail.com
To realize the feasibility of the envisioned process
sketched in Scheme 2b, the ruthenium-catalyzed intramolec-
ular hydroarylation was explored with [3-(2-methylallyloxy)-
benzoyl]methylphenyl sulfoximine (2a). Screening of the
catalysts, additives, and bases led to the optimized reaction
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201600649.
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À
Scheme 2. Unsymmetrical twofold C H functionalization.
Ts =4-toluenesulfonyl.
conditions, [{RuCl2(p-cymene)}2] (2.0 mol%), AgSbF6
(20 mol%), and Cu(OAc)2·H2O (1.0 equiv) in ClCH2CH2Cl
(DCE) at room temperature for 12 h, to access 3a (97%)
from 2a [Eq. (1)].[17]
Scheme 3. Scope of the hydroarylation reaction. Reaction conditions: 2
(0.5 mmol), Ru catalyst (2.0 mol%), AgSbF6 (20 mol%), and Cu-
(OAc)2·H2O (0.5 mmol) in DCE (2.0 mL) at RT for 8–16 h.
[a] 0.5 mol% the Ru catalyst (at 358C). [b] At 508C. [c] Based on NMR/
HPLC analysis. [d] 4.0 mol% Ru catalyst (at 658C). Boc=tert-butoxy-
carbonyl.
The generality of this novel ruthenium-catalyzed DG-
assisted intramolecular hydroarylation is showcased in
Scheme 3. The reaction of 2a (1.3 g) with 0.5 mol% of the
ruthenium catalyst at 358C for 6 hours furnished 3a (1.24 g),
thus the reaction is scalable. The 6-methyl- (2b) and 6-bromo
(2c) bearing sterically encumbered substrates smoothly
underwent hydroarylation to produce 3b and 3c, respectively,
in excellent yields. The electron-donating and electron-with-
drawing (NO2, ester) functional groups at the 5-position of
aryl moiety did not affect the reaction efficiency, thus yielding
3d–g in excellent yields. X-ray crystallographic studies
established the structure of 3e.[18] Excellent yield of the
products 3h–k were obtained from the 4-substituted benzoic
acid derivatives 2h–k, and electronically biased (OMe/NO2)
groups did not affect the yields. Construction of the highly
substituted dihydrobenzofurans 3l–n were readily obtained.
The current transformation conveniently resulted in the
phenoxymethyl- and methoxymethyl-bearing (all-carbon
quaternary stereocenter enabled) products 3o (64%, 3:1)
and 3p (81%, 10:1), respectively.[17] The O-allyl substrate 2q
successfully underwent hydroarylation, thus delivering 71%
of 3q.[17] Internal alkenes also participated and the dihydro-
benzofuran derivatives 3r–t were synthesized in appreciable
yields.[17] The identical reactions with the N- and S-tethered
substrates were facile, thus resulting the N-Tosyl/Boc-pro-
tected indolines 4a–c (54–86% yield) and dihydrobenzothio-
phene 5 (Scheme 3).
We next examined the twofold hydroarylation of the
À
ortho C H bonds with the appended olefins. Pleasingly, the
dicyclization product 7a (96%) was exclusively formed when
6a was exposed to the optimized reaction conditions in
(Scheme 4).[18] This twofold hydroarylation with a low catalyst
loading at room temperature is a distinct advantage over the
rhodium-catalyzed transformations.[11b] Likewise, 4-methoxy-
and 4-bromo-substituted 7b and 7c, respectively, were
achieved in the corresponding yields of 97% and 94%. The
dihydroarylation of 6d proceeded smoothly to exclusively
produce 84% of 7d. Collectively, these data reveal the
potential of the MPS DG for the incorporation of two
heterocycles into an arene moiety by a twofold hydroaryla-
tion.
À
Unsymmetrical functionalization of multiple C H bonds
creates densely functionalized scaffolds with the introduction
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The success of the one-pot hydroarylation amidation
À
reaction led us to explore the unsymmetrical twofold C C
bond formations at the two ortho positions (Scheme 5). To
our delight, the reaction of 2h with phenyl vinyl sulfone (10a)
under the optimized reaction conditions led directly to the 5-
alkyl-substituted dihydrobenzofuran 11a in 73% yield in
a single operation.[15b, 20] The presence of 4-fluoro and 4-nitro
groups on the arene did not hamper the reaction, thus
delivering 11b (69%) and 11c (51%), respectively. The steric
strain of the quaternary group at the 3-position of the
dihydrobenzofuran in the transition state suppresses the b-
hydride elimination over proto-demetalation, and is presum-
ably responsible for alkylation product formation.[21] The N-
tethered 2v was also effectively reacted, thus leading to the
indoline derivative 11d (61%). In contrast, the reaction of 2h
with ethyl acrylate (10b) gave the alkenylation product 11e.
To the best of our knowledge, the one-pot unsymmetrical
functionalizations of arenes [hydroarylation/amidation,
hydroarylation/olefination] under one set of catalytic con-
ditions has not been reported previously.
Scheme 4. Twofold hydroarylation reaction. Reaction conditions: 6
(0.5 mmol), Ru catalyst (2.0 mol%), AgSbF6 (20 mol%), and
Cu(OAc)2·H2O (0.5 mmol) in DCE (2.0 mL) for 10–16 h. [a] 4.0 mol%
Ru catalyst (at 608C).
Development of a synthetic manifold for the introduction
of isoquinolone motifs into (hetero)arenes is appealing, as
they are widely found in the molecules of biological
importance.[22] We thus envisaged the introduction of an
isoquinolone scaffold into the dihydrobenzofuran skeleton
(Scheme 6).[23] Gratifyingly, the reaction of 3a and diphenyl-
of diverse substituents. Thus, ruthenium-catalyzed intramo-
lecular hydroarylation followed by intermolecular amidation
with sulfonylazides under one set of catalytic conditions is
envisioned (Scheme 5).[15a,19] Gratifyingly, the compound 9a
(72%) was isolated when 2h was exposed to the ruthenium-
catalyzed conditions in the presence of TsN3 at room temper-
ature, then with heating at 1208C. Likewise, the dihydroben-
zofurans 9b–d were successfully constructed. The same
reaction sequences were also applied to construct the N-
tethered indoline derivative 9e.
Scheme 6. Sequential hydroarylation and annulation. Reaction condi-
tions: 3 (0.5 mmol), Ru catalyst (5.0 mol%), AgSbF6 (20 mol%), and
AcOH (0.5 mmol) in 1,4-dioxane (4.0 mL) at 1208C for 24 h.
acetylene (12a) under the catalytic ruthenium conditions
delivered the dihydrofuran-fused isoquinolone 13a in 65%
yield. Likewise, annulation between 3q and 12a and 12b led
to 13b (77%) and 13c (70%), respectively. The use of the
unsymmetrical diarylacetylene 12c provided a mixture of
regioisomers, 13d and 13d’ (1:1; 64%). Whereas the reaction
of 3q with the alkyl-aryl alkyne 12d yielded 13e, for which
NOESY studies were used to establish the structure.[17]
Notably, the unsymmetrical twofold functionalization of
Scheme 5. One-pot hydroarylation-amidation/alkylation/alkenylation.
Reaction conditions: 2 (0.5 mmol), Ru catalyst (5.0 mol%), AgSbF6
(20 mol%), Cu(OAc)2·H2O (0.5 mmol), and either sulfonyl azide
(1.0 mmol) or alkene (1.0 mmol) in DCE (4.0 mL) at RT for 12–16 h
and then heated to 1208C for 24 h.
À
inert C H bonds, proceeding by hydroarylation and annula-
tion, leads to novel N- and O-bearing heterocycles.
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To gain some mechanistic insights, deuterium-labeling
studies were performed. At first, the hydroarylation of 2n in
CD3CO2D and CD3OD (1:1) explicitly provided H/D scram-
bled products [D]-2n/2n [Eq. (2)]. The incorporation of D
a
À
into the ortho C H bond (75%), which is sandwiched
b
À
between the DG and the O-allyl moiety, over the ortho C H
bond (55%) reveals that the tethered olefin moiety helps in
the activation of the ortho C H bond in 2n. Interestingly,
hydroarylation of [D]-2n/2n under the optimized reaction
conditions resulted in [D]-3n/3n [Eq. (3)]. To our surprise,
a significant decrease in the deuterium population at the
a
À
À
arene ortho C H bond (22%) and incorporation of only 25%
of D into the 3-methyl in [D]-3n/3n is observed [Eq. (3)].
À
Thus, cleavage of the ortho C H bond, which generally
Scheme 7. Possible mechanistic pathway.
involves a concerted metalation-deprotonation (CMD) path-
way, is reversible.[24] Furthermore, ruthenium-catalyzed
hydroarylation of 6a in the presence of Cu(OAc)2
(1.0 equiv) and CD3CO2D (5.0 equiv) afforded [D]-14/14
with 65% deuterium incorporation in the 3-methyl moiety
[Eq. (4)], thus indicating the involvement of protodemetala-
tion.[12a]
room temperature was demonstrated for the first time. The
reaction exhibits broad scope for synthesizing dihydrobenzo-
furan, indoline, and dihydro-benzothiophene derivatives. The
À
unprecedented one-pot, unsymmetrical, twofold C H func-
tionalization of arene derivatives through hydroarylation-
amidation and hydroarylation-alkylation/ alkenylation cre-
ates novel molecular scaffolds. The unsymmetrical twofold
À
functionalization of unactivated C H bonds involves hydro-
arylation and annulation (with alkynes) and leads to novel N-
and O-bearing heterocycles by incorporating dihydrobenzo-
furan and isoquinolone motifs in a single molecule for the first
time. The cleavage and recovery of the MPS group from the
products makes this strategy synthetically viable.[17]
Acknowledgments
This research was supported by the CSIR (02/(0155)/13/
EMR-II). K.G., R.K.R., and E.R. thank CSIR, India, for
a fellowship.
À
Keywords: annulations · C H activation · heterocycles ·
reaction mechanisms · ruthenium
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À
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A plausible reaction pathway is outlined in Scheme 7. The
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À
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Received: January 21, 2016
Published online: March 23, 2016
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