Ruthenium(II) Catalyzed Regiospecific C-H/O-H Annulations of Directing Arenes via Weak Coordination

Article abstract of DOI:10.1021/acs.orglett.5b02967

Ruthenium(II) catalyzed oxidative C-H/O-H annulations have been demonstrated using two different directing arenes viz. 2-arylquinolinone and 2-arylbenzoxazinone with internal alkynes. Regiospecific annulations have been observed for both directing arenes via the assistance of weaker carbonyl oxygen in the presence of a stronger nitrogen-directing site. In this substrate-controlled convergent protocol the weaker directing group dictates the annulation path.

Full text of DOI:10.1021/acs.orglett.5b02967

Letter
pubs.acs.org/OrgLett
Ruthenium(II) Catalyzed Regiospecific C−H/O−H Annulations of
Directing Arenes via Weak Coordination
Arghya Banerjee, Sourav Kumar Santra, Prakash Ranjan Mohanta, and Bhisma K. Patel*
Department of Chemistry, Indian Institute of Technology Guwahati, North Guwahati 781039, India
S
* Supporting Information
ABSTRACT: Ruthenium(II) catalyzed oxidative C−H/O−H annulations have been demonstrated using two different directing
arenes viz. 2-arylquinolinone and 2-arylbenzoxazinone with internal alkynes. Regiospecific annulations have been observed for
both directing arenes via the assistance of weaker carbonyl oxygen in the presence of a stronger nitrogen-directing site. In this
substrate-controlled convergent protocol the weaker directing group dictates the annulation path.
Scheme 1. Catalyst vs Substrate Controlled Annulations
ontrolling site selective C−H activation in substrates
possessing more than one type of C−H bond is an
C
inherent and long-standing challenge in organic chemistry. One
possible solution to this problem is either via catalyst or
substrate control. Despite fast-paced progress in catalytic C−H
bond functionalizations, strategies for preferential site selectiv-
ities are limited.1 The ability to induce site-selectivity in
substrates having multiple C−H’s has tremendous importance
in the field of diversity oriented synthesis of complex
molecules.2 Oxidative C−H annulation is one such powerful
atom and step economical strategy, applied for the synthesis of
bioactive complex polycyclic molecules.3 In this context site
selective annulation of substrates enable the construction of
diverse molecular scaffolds from common starting material.
Majority of oxidative annulations have been achieved using Rh
or Pd catalysts, whereas the relatively inexpensive Ru-catalysts
have been recently introduced as a suitable alternative.4
Synthesis of polycyclic π-conjugated aromatics and hetero-
aromatics have received considerable attention in recent times
due to their potential utility as organic electronic materials.5
Their synthesis using C−H annulations is an obvious and
attractive alternative to multistep routes.
Inspired by nature’s approach in site selective C−H bond
functionalizations, efforts have focused on designing catalysts,
substrates or manipulation of ligands to provide switchable site-
selective C−H bond functionalizations.2f,6 The development of
strategies capable of divergent functionalizations at distinct C−
H sites through catalyst control is limited (Scheme 1).7
However, substrate-controlled selectivity in C−H bond
functionalizations through the cleavage of similar C−H bonds
in analogous directing arenes is unfamiliar so far. In
multidirecting systems, generally the stronger directing group
controls the C−H bond functionalization. However, after the
seminal work by Yu et al. where a weak directing group
overrides the stronger one and controls the C−H activation,8
there is no other report of this kind. Herein, we report first
oxygen-directed C−H annulation in the presence of a strong
nitrogen-directing group in 2-arylquinolinones and 2-arylben-
zoxazinones (Scheme 1).
Our initial investigation started with the evaluation of
reaction conditions for oxidative annulation of 2-phenyl-
quinolinone (1) with diphenyl acetylene (a). 2-Arylquinolinone
was chosen as the model substrate as this core represents a
Received: October 13, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b02967
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Letter
highly privileged, biologically important molecular scaffold9
which has several possible C−H’s (via C−H/N−H, C−H/C−
H and C−H/O−H bonds) for oxidative annulations with an
alkyne. In an initial reaction 2-phenylquinolinone (1) was
treated with diphenylacetylene (a) in the presence of a well
explored [RuCl2(p-cymene)]2 (2 mol %) catalyst,4 Cu(OAc)2
(1.0 equiv) as the terminal oxidant and Cs2CO3 (1 equiv) as
the base in toluene (2 mL). A new product was isolated by
column chromatography, spectroscopic (1H and 13C NMR)
analysis of the product (1a) revealed the presence of two extra
phenyl moieties and the absence of singlets for C3−H and
−NH protons. This confirms the aromatization of 2-
arylquinolinone to 2-aryl-4-hydroxyquinoline along with the
addition of a diphenyl acetylene moiety via C−H/O−H
annulation. However, this unprecedented annulated product
was obtained in a low yield of 37% (Table S1, entry 1,
Supporting Information, SI). Since the tautomerization of
quinone to its corresponding phenol is favorable, 2-
arylquinolinone acts as a “masked” phenolic substrate to trigger
the C−H/O−H annulation over other possible (viz. C−H/N−
H and C−H/C−H) annulation paths (Scheme in Table S1,
SI). Switching the solvent from toluene to chlorobenzene
(PhCl) under identical reaction conditions provided the
annulated product (1a) in an improved yield of 60% (Table
S1, entry 2, SI). Among other solvents such as DMF, NMP,
DMSO and DCE tested (Table S1, entries 3−6, SI), later
provided the best yield (70%) (Table S1, entry 6, SI). Further,
by varying the oxidant from Cu(OAc)2 to Cu(OAc)2·H2O,
AgOAc, AgCO3 and AgSbF6 under identical conditions to that
of entry 6, did not improve the yield of product (1a) (Table S1,
entries 7−10, SI). Later, it was found that the use of Cs2CO3
was redundant as the yield remained unaltered (68%) (Table
S1, entry 11, SI) even when the reaction was performed in its
absence. Reaction when carried out without co-oxidant
[Cu(OAc)2] under otherwise identical conditions (to that of
entry 11) provided the annulated product (1a) in low yield
(25%) (Table S1, entry 12, SI). No annulated product was
formed in the absence of Ru(II) catalyst (Table S1, entry 13,
SI).
After establishing the optimized reaction conditions for
annulation (Table S1, entry 11, SI), we started to explore the
coupling between various 2-arylquinolinone (1−9) with
symmetrical internal alkynes (a−e). 2-Aryl ring of quinolinone
bearing electron-donating as well as electron-withdrawing
substituents were all compatible providing their desired
annulated products in moderate yields (Scheme 2). 2-
Arylquinolinone containing electron-donating substituents
such as p-Me (2), p-Et (3) in the 2-aryl ring provided their
expected annulated products (2a) and (3a) in moderate yields
(Scheme 2). Electron-withdrawing substituents such as p-F (4),
p-Cl (5), m-F (6) and o-Cl (7) present in the 2-aryl ring of 2-
arylquinolinone also underwent annulations giving their
corresponding products (4a), (5a), (6a) and (7a) respectively
in decent yields (Scheme 2). The structure of the annulated
product (5a) has been further confirmed by X-ray crystallo-
graphic analysis (Figure S1, SI). For directed C−H bond
functionalization, C−H metalation generally occurs at the less
sterically hindered C−H site of phenyl rings having the meta-
substituent. However, in 6,8 dibromo substituted quinolinone
(8) the annulation took place smoothly even at the sterically
hindered site of quinolinone ring, giving 66% yield of (8a)
along with retention of both the bromo functionality. This
observation suggests the preferential C−H/O−H annulation
Scheme 2. Annulation of 2-Arylquinolinone with Internal
Alkynes
over other C−H/N−H and C−H/C−H annulation paths. This
protocol is equally compatible with 2-cyclohexyl substituted
quinolinone (9) providing (9a) in a relatively low yield (46%).
To evaluate the scope and generality of this annulation
strategy, other symmetrical alkynes (b−e) were investigated
using 2-phenylquinoloinone (1) as the coupling partner
(Scheme 2). Symmetrical 1,2-diarylacetylene possessing elec-
tron-donating p-Me substituent (b) when coupled with (1)
provided the product (1b) in good yield (70%) (Scheme 2).
Symmetrical 1,2-diarylacetylene having electron-withdrawing p-
F substituents (c) when reacted with (1) provided an improved
yield (77%) of the annulated product (1c). Aliphatic internal
alkynes viz. 4-octyne (d) and 1,4-bis((4-methylbenzyl)oxy)but-
2-yne (e) when treated with (1) afforded their annulated
products (1d) and (1e) in 59% and 51% yields, respectively.
After the successful C−H/O−H annulation of 2-arylquino-
linone (1) directed via its weaker carbonyl oxygen in the
presence of stronger nitrogen (N) directing group, we looked
for analogous systems. With this in mind, 2-phenylbenzox-
azinone (1′) was selected as it is structurally analogous to 2-
phenylquinolinone (1) except its C3 carbon is replaced with an
oxygen atom. 2-Phenylbenzoxazinone (1′) having two different
directing sites viz. nitrogen (N1) and carbonyl oxygen atoms
both of which can potentially direct the oxidative annulations.
The objective was to see whether the stronger nitrogen directs
the anuulation or here as well, the weaker oxygen dictates the
annulation as was observed in 2-phenylquinolinone (1). To
verify this, coupling of 2-phenylbenzoxazinone (1′) and
diphenylacetylene (a) was carried out under identical
conditions to that of coupling between (1) and (a) (Table
S2, entry 1, SI). A product was isolated by column
chromatography, spectroscopic (1H and 13C NMR) analysis
revealed its structure to be N-(1-oxo-3,4-diphenyl-1H-isochro-
men-8-yl)benzamide (1′a). The amidic group in (1′a) might
have originated by the hydrolytic cleavage of one of the C−O
bonds in 2-phenylbenzoxazinone (1′). Therefore, hydrated
Cu(OAc)2·H2O was used in lieu of anhydrous Cu(OAc)2
which gave a slight improvement in the yield (17%) (Table
S2, entry 2, SI). Among various other solvents such as PhCl
t
(38%), DMSO (00%), DMF (00%), NMP (00%), BuOH
(14%) and AcOH (50%) tested (Table S2, entries 3−8, SI), the
B
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Letter
later was found to be superior (Table S2, entry 8). Co-oxidant
AgOAc was found to be ideal giving 78% yield compared to
other co-oxidants such as Cu(OAc)2 (44%), AgSbF6 (62%),
AgCO3 (56%) and Ag2O (41%) screened (Table S2, entries 9−
13, SI)
After establishing the optimized conditions for yet another
C−H/O−H annulation directed by weaker carbonyl oxygen,
we then implemented this strategy for various 2-arylbenzox-
azinones (1′−16′) and symmetrical internal alkynes (a−e). 2-
Arylbenzoxazinone containing electron-withdrawing as well as
electron-donating substituents present in the 2-aryl ring all
coupled efficiently with diphenylacetylene (a) providing their
respective annulated products in moderate yields (Scheme 3).
yielded annulated products (15′a) and (16′a) in 64% and 48%
yields, respectively.
The scopes of other symmetrical alkynes were then tested
using 2-phenylbenzoxazinone (1′) as the coupling partner
(Scheme 3). Symmetrical 1,2-diarylalkynes having electron-
donating, Me (b), and electron-withdrawing, F (c), substituents
present at their para position when reacted with (1′) provided
their annulated products (1′b) and (1′c) in good yields
(Scheme 3). Aliphatic internal alkynes such as 4-octyne (d) and
1,4-bis((4-methylbenzyl)oxy)but-2-yne (e) when treated with
(1′) afforded their annulated products (1′d) and (1′e)
respectively in 77% and 44% yields.
To check the regioselectivity in both of these annulations, an
unsymmetrical alkyne 1-phenyl-1-propyne (f) was reacted
separately with 2-phenylquinolinone (1) and 2-phenylbenzox-
azinone (1′) under their respective optimized reaction
conditions [(Table S1, entry 11, SI) and (Table S2, entry 10,
SI)]. Both these directing substrates (1) and (1′) provided
single regioisomer (1f) and (1′f) (Scheme 4). The structure of
compound (1′f) was further confirmed by X-ray crystallo-
graphic analysis as shown in Figure S2 (SI).
Scheme 3. Annulation of 2-Arylbenzoxazinones with Alkynes
Scheme 4. Regioselectivity Evaluation
To check whether the electron-deficient diphenylacetylene
(a) or the electron-rich 4-octyne (d) undergoes preferential
annulation, intermolecular competitive reactions were per-
formed. Directing substrates 2-phenylquinolinone (1) and 2-
phenylbenzoxazinone (1′) were reacted separately in the
presence of an equimolar mixture (1:1) of (a) and (d) under
their respective optimized conditions. In both these cases
annulated products originating from the electron-deficient
alkyne (a) were obtained in higher proportions [(1a, 44%),
(1′a, 52%)] than products [(1d, 23%), (1′d, 27%)] derived
from electron-rich (d) counterpart (Scheme S1, SI). Thus, in
both these annulations electron deficient alkynes are better
annulating partners. This has been reconfirmed in yet another
set of competitive experiments where an equimolar mixture of
electron-deficient 1,2-di(p-fluorophenyl)ethyne (c) and elec-
tron-rich 1,2-di-p-tolylethyne (b) were reacted separately with
(1) and (1′). Here again, electron-deficient alkyne (c)
annulated preferentially giving higher yields of products [(1c,
40%), (1′c, 46%)] than annulated products derived from
electron-rich alkyne (b) [(1b, 27%), (1′b, 32%)] (Scheme S1,
SI).
Irrespective of their positions, the presence of electron-
withdrawing substituents in the 2-aryl ring of 2-arylbenzox-
azinones such as p-F (2′), p-Cl (3′), p-CF3 (4′), m-F (5′), o-Cl
(6′), o-Br (7′) and o-I (8′) all provided their expected
annulated products (2′a), (3′a), (4′a), (5′a), (6′a), (7′a) and
(8′a) respectively in decent yields (Scheme 3). The presence of
moderately electron-donating p-Me (9′), p-Et (10′) and
strongly electron-donating p-OMe (11′), p-SMe (12′)
substituents in the 2-aryl ring of benzoxazinone all afforded
moderate yields of their annulated products (9′a), (10′a),
(11′a) and (12′a) respectively (Scheme 3). Electron-with-
drawing Br substituent presents at the C7 position of
benzoxazinone provided the annulated product (13′a) in
good yield (70%). Further, the presence of an extra aryl ring
at C7 of benzoxazinone afforded the annulated product (14′a)
in moderate yield (58%). The presence of methyl and
cyclohexyl substituents at the C2 position of benzoxazinone
On the basis of experiments performed (Scheme 4 and S1,
SI) and earlier reports, plausible mechanisms are depicted for
these annulations (Scheme S2, SI). In both these cases the
reactions are initiated via the displacement of a chloride ligand
of [RuCl2(p-cymene)]2 with an acetate anion either from
C
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AgOAc or Cu(OAc)2. Under the reaction conditions 2-
phenylquinolinone (1) aromatizes to 4-hydroxy-2-phenylquino-
line (A). In the next step, a five-membered ruthenacycle
intermediate (B) is generated from (A) (Scheme S2, SI).
Migratory insertion of alkyne into the Ru-carbon bond of
intermediate (B) afforded the intermediate (C). For unsym-
metrical alkynes the Ru-carbon bond will be favorable at the
alkyne carbon having higher electron density thus accounting
for the regioselectivity in Scheme 4. Finally, a reductive
elimination provided the expected annulated product (1a)
along with the generation of catalyst Ru(0) (Scheme S2, SI).
This Ru(0) is oxidized to an active Ru(II) catalyst in the
presence of oxidant Cu(OAc)2 or by an areal oxidation.4j
Directing substrate 2-phenylbenzoxazinone (1′) undergoes
initial hydrolytic cleavage under the reaction conditions to
give N-benzoylanthranilic acid (A′). To confirm this when
substrate (1′) was subjected to the reaction conditions but in
the absence of diphenylaetylene (a), gave N-benzoylanthranilic
acid (A′) as the exclusive product. Further, N-benzoylanthra-
nilic acid (A′) when treated with diphenylaetylene (a) under
the exact reaction condition (Table S2, entry 10, SI) provided
product (1′a) but in a mere yield of 28%. Lower yield of
product (1′a) obtained using (A′) may be due to the chelation
of cat. Ru(II) with a large excess of (A′).10 In the next step, the
in situ generated (A′) forms a five-membered ruthenacycle
intermediates (B′) via a weak oxygen directed selective
metalation (Scheme S2, SI). Migratory insertion of alkyne
into the Ru-carbon bond of intermediate (B′) generates the
intermediate (C′). Reductive elimination in the next step
provided annulated product (1′a) along with the generation of
Ru(0) (Scheme S2, SI) which is oxidized further to Ru(II).
In conclusion, weaker oxygen-directed regiospecific annula-
tions have been developed for directing arenes viz. 2-
arylquinolinone and 2-arylbenzoxazinone in the presence of a
stronger nitrogen-directing group. This is a unique demon-
stration of directing group controlled annulation in multi-
directing systems.
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ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge on the
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Corresponding Author
*E-mail: patel@iitg.ernet.in.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
B.K.P. acknowledges the Department of Science and
Technology (DST) (SB/S1/OC-53/2013), New Delhi, the
Council of Scientific and Industrial Research (CSIR)
(02(0096)/12/EMR-II) and MHRD: 5-5/2014-TS-VII for
research grants. A.B. and S.K.S. thank CSIR for fellowships.
(10) Sarangapani, M.; Ravinder, V. Indian J. Chem. 2002, 41, 2060.
D
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