One-Pot Unsymmetrical {[4 + 2] and [4 + 2]} Double Annulations of o/ o′-C-H Bonds of Arenes: Access to Unusual Pyranoisoquinolines

Article abstract of DOI:10.1021/acs.orglett.8b02068

With the aid of a transformable sulfoximine directing group, unprecedented one-pot unsymmetrical double annulations {[4 + 2] and [4 + 2]} of hetero(arenes) with alkynes are revealed under Ru(II) catalysis. Functionalization of both ortho-C-H bonds of (hetero)arene is reflected in the building of unusual 6,6-fused pyranoisoquinoline skeletons. Construction of four [(C-C)-(C-N) and (C-C)-(C-O)] bonds occurs in one step under single catalytic conditions. The challenging unsymmetrical double annulations of both o-C-H bonds of arenes with two distinct alkynes is effectively demonstrated. Control experiments and deuterium scrambling findings are shown.

Full text of DOI:10.1021/acs.orglett.8b02068

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
One-Pot Unsymmetrical {[4 + 2] and [4 + 2]} Double Annulations of
o/o′‑C−H Bonds of Arenes: Access to Unusual Pyranoisoquinolines
Majji Shankar, Koushik Ghosh, Kallol Mukherjee, Raja K. Rit, and Akhila K. Sahoo*
School of Chemistry, University of Hyderabad, Hyderabad 500046, India
S
* Supporting Information
ABSTRACT: With the aid of a transformable sulfoximine
directing group, unprecedented one-pot unsymmetrical
double annulations {[4 + 2] and [4 + 2]} of hetero(arenes)
with alkynes are revealed under Ru(II) catalysis. Functional-
ization of both ortho-C−H bonds of (hetero)arene is reflected
in the building of unusual 6,6-fused pyranoisoquinoline
skeletons. Construction of four [(C−C)−(C−N) and (C−
C)−(C−O)] bonds occurs in one step under single catalytic conditions. The challenging unsymmetrical double annulations of
both o-C−H bonds of arenes with two distinct alkynes is effectively demonstrated. Control experiments and deuterium
scrambling findings are shown.
he transition-metal-catalyzed directing group (DG)-
assisted oxidative annulation of ubiquitous arene C−H
T
bonds has been found remarkable in building novel polycyclic
heteroarenes that are commonly present in bioactive molecules
and natural products, exhibiting broad application in pharma-
ceuticals and advanced materials.1 Although monoannulation
strategies have largely been useful, the stepwise sequential
annulation of multiple C−H bonds has been shown to be
feasible for fabricating π-extended fused heterocycles (Scheme
1A).2−4 With these advances, creation of a one-pot sequential/
domino double annulation of arene motifs with alkynes was
recently accomplished, although with a narrow scope.5 By
contrast, double annulations of arene’s o-C−H bonds with
alkynes have yet to be widely achieved, as the molecular rigidity
and conformation strain hamper this reaction.5,6 Moreover,
uncontrolled reactivity, lack of selectivity, and the catalyst
dependent functionalization of a particular DG-aided o-C−H
bond raises concern.7 Despite these significant challenges, the
Rh(III)-catalyzed oxidative domino double annulation of C−H
bonds of benzoylacetonitrile,8 enamino ester,9 N-hydroxyben-
zamidines,10 or polyaromatic aldehyde11 with alkynes has
independently been studied (Figure 1B). Thus, double
annulation of both o-C−H bonds of arene moieties with two
alkynes was successful under Rh catalysis, primarily forming
C−C with either C−O/C−N bonds and are confined to 6,6-
bifused heteroarenes (Figure 1B).8−11
Figure 1. Multiple annulation of (hetero)arenes.
(2a) was explored under Ru catalysis (Table 1). Gratifyingly, the
expected pyranoisoquinoline 3a (with the inclusion of two
alkyne motifs on the periphery) was formed, albeit in 8% yield,
under the catalytic conditions of [RuCl2(p-cymene)]2 (5.0 mol
%) and AgSbF6 (20 mol %) in DCE at 120 °C for 24 h (entry 1).
Because bases significantly promote the Ru-mediated C−H
activation, various acetate bases were then carefully tested
(entries 2−6); NaOAc, KOAc, and Mn(OAc)2·3H2O were poor
(entries 2−4), whereas AgOAc was moderate (entry 5), and
finally, Cu(OAc)2·H2O was found to be the best, providing 50%
We herein developed a Ru-catalyzed transformable methyl-
phenyl sulfoximine (MPS)−DG-assisted unsymmetrical double
annulation of both o-C−H bonds of (hetero)arene carbox-
yamide with unactivated alkynes, which is unprecedented. This
process constructs pyranoisoquinoline {6,6-bifused structures
with O and N involving two distinct [4 + 2] annulations}
(Figure 1C).
To probe the feasibility of the hypothesis (Figure 1C), the
domino [4 + 2] and [4 + 2] double annulation of N-[4-
methylbenzoyl]methylphenyl sulfoximine (1a) with 4-octyne
Received: July 2, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02068
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Letter
a
benzamides failed to undergo these domino [4 + 2] double
annulations with 2a. However, simple amide (IV) or amides
with an internal oxidizable moiety (V and VI) reacted with 2a to
yield a moderate amount of 3a. Thus, MPS−DG surfaces are
vital, as this moiety ensures the occurrence of synthetically
challenging one-pot [4 + 2] double annulations of arenes with
alkynes to directly construct 6,6-fused pyranoisoquinoline.
The reaction generality of this newly conceived Ru-catalyzed
domino [4 + 2] oxidative double annulations of N-aroyl
sulfoximines (1) with unactivated alkyne was probed under the
optimized catalytic conditions shown in entry 14, Table 1. The
results are detailed in Scheme 1. The reaction of 1 having either
Table 1. Optimization of Reaction Conditions
additive 1
(20 mol %)
additive 2
(1.0 equiv)
yield of 3a
b
entry
solvent
(%)
1
2
3
4
5
6
7
8
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
NaPF6
KPF6
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
toluene
08
trace
13
trace
36
50
39
42
38
27
NaOAc
KOAc
Mn(OAc)2·3H2O
AgOAc
t
electron-donating (e.g., Me, Bu, OMe, OPh), electron-with-
drawing (Ph, CO2Me), or labile halo groups (e.g., F, Cl, Br) in
the para position of arene with 4-octyne (2a) produced the
respective pyrano[4,3,2-ij]isoquinolines (3a−j) in moderate to
good yields. X-ray crystallographic studies elucidate the
structure of 3a (Scheme 1).13 The gram-scale synthesis of 3a
(1.47 g) proves the robustness of the annulation; isolation of
methylphenyl sulfoxide (precursor for MPS) makes MPS
transformable.5e−g The modifiable functional groups (Cl, Br,
F, and CO2Me) were tolerated. Diannulation of 2-naphthalene
(1k) and carbazole-3 (1l) carboxylic acid derivatives with 2a
independently delivered 3k/3k′ (60%) and 3l (70%),
respectively; the first annulation selectively occurs at the
sterically less hindered C−H site. Carbazole and anthracene
structural entities largely contribute to the tuning of the
photophysical properties of the molecules; the respective
carbazole and anthracene-molded pyranoisoquinoline scaffolds
(3m, n) were accessed in decent yields.14 The m-Me/m,p-
disubstituted amides 1o/1p successfully underwent annulation
with 2a, affording 3o/3p, respectively, albeit in moderate yield;
importantly, the steric bulkiness did not obstruct the second
annulation. Other internal alkynes, 3-hexyne (2b) and 5-decyne
(2c), were efficient, producing 3q (94%) and 3r (88%).
Obviously, ortho-substitution on arene hampered the domino
annulation; the reaction was thus ended with monoannulation
(4q). An important synthetic deliberation of this strategy is
showcased through the fabrication of linear 5,5′-bipyrano[4,3,2-
ij]isoquinoline (3s) and “V-shaped” 5,5′-oxidipyrano[4,3,2-
ij]isoquinoline (3t) complex skeletons from 1r and 1s,
respectively. The functionalization of four o-C−H bonds of
arenes with four alkynes and the formation of eight bonds (4 C−
C, 2 C−N, and 2 C−O) in a single operation is significant.
Inspired by the successful unsymmetrical double annulation
of (hetero)arenes with two molecules of single alkyne (Scheme
1), we next executed an identical challenging exploration with
different alkynes. Formation of several byproducts from
incomplete/nonselective annulation in each step is a potential
challenge.6,7 To address these issues, a two-step synthetic
strategy involving (i) MPS-promoted o-C−H monoannulation
with dialkyl alkyne followed by (ii) C(8)−H activation and
annulation of isoquinolone, obtained from Step-I, with
structurally diverse alkynes, was planned (Scheme 2). Gratify-
ingly, the corresponding isoquinolone scaffolds 4a (57%), 4b
(58%), and 4j (58%) were constructed from the respective
monoannulation of 1a, 1b, and 1j with 2a when the reaction was
conducted under Ru catalysis in the presence of AcOH in DCE
(Conditions A, Scheme 2).3e Presumably, acetic acid helps
protodemetalation and selectively delivers isoquinolone. Next,
independent annulation among isoquinolones 4a/4b/4j and
1,2-diaryl/alkyl alkynes led to the respective peripheral
decorated 6,6-fused pyranoisoquinolines (Scheme 2). The 1,2-
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
9
AgBF4
10
11
12
13
14
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
MeCN
trace
55
71
1,4-dioxane
1,4-dioxane
1,4-dioxane
c
d
83
a
Conditions: 1a (0.3 mmol), 2a (0.9 mmol), [RuCl2(p-cymene)]2
(5.0 mol %), additive 1 (20 mol %), additive 2 (0.3 mmol), solvent
(2.0 mL) at 120 °C. Isolated yield. [RuCl2(p-cymene)]2 (10 mol
%), AgSbF6 (40 mol %), and Cu(OAc)2·H2O (1.0 equiv). [RuCl2(p-
cymene)]2 (10 mol %), AgSbF6 (40 mol %), Cu(OAc)2·H2O (1.5
equiv), and 2a (1.2 mmol) were used.
b
c
d
of 3a (entry 6). We thus believe the redox-active acetate base
helps the C−H activation as well regeneration of the active
Ru(II) catalyst. Other additives, including NaPF6, KPF6, and
AgBF4, were moderate (entries 7−9). The reaction in toluene or
MeCN was ineffective (entries 10 and 11); this double
annulation in 1,4-dioxane produced 3a in 55% yield (entry
12). The use of 10 mol % of Ru catalyst under identical
conditions led to 71% 3a (entry 13). The annulation among 1a
and two molecules of 2a was smoothly commenced under the
optimized reaction conditions of [Ru(p-cymene)Cl2]2 (10 mol
%), AgSbF6 (40 mol %), and Cu(OAc)2·H2O (1.5 equiv) in 1,4-
dioxane at 120 °C for 24 h (entry 14). Notably, four bonds
[C−C and C−N along with C−C and C−O] are constructed
through the replacement of two o-C−H bonds of arene in a
single operation.12 The DG-assisted annulation of arenes is
routinely studied; however, the nonexistence of direct double
annulation of amide-enabled arenes to 6,6-fused pyranoisoqui-
noline warrants further investigation. Consequently, the role of
amide-DG in this one-pot double annulation of o-C−H bonds of
arene was examined under the optimized conditions (entry 14,
Table 1).2,3 Despite our repeated attempts, the NH-Me (I) or
NH-tosyl (II) amide and hydrazide (III) DG bearing
B
DOI: 10.1021/acs.orglett.8b02068
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Letter
a
Scheme 1. Synthesis of 6,6-Fused Pyranoisoquinoline
Scheme 2. Unsymmetrical Double Annulation of Arenes with
Different Alkynes
a
a
Conditions A: 1 (0.5 mmol), 2a (1.0 mmol), Ru catalyst (7.5 mol
%), AgSbF6 (30 mol %), AcOH (2.0 mmol), DCE (2.5 mL) at 120
°C for 20 h. Conditions B: 4 (0.3 mmol), 2 (0.45 mmol), Ru catalyst
(5.0 mol %), AgSbF6 (20 mol %), Cu(OAc)2·H2O (0.3 mmol), 1,4-
dioxane (2.0 mL) at 120 °C for 20 h.
the isoquinolone C8−H bond (eq 3). A set of KIE experiments
was performed to understand the rate-determining step (RDS)
of this multistep double annulation protocol. The intermolec-
ular competition experiment among 1b-H5, 1b-D5 (1:1), and 2a
under the optimized conditions exhibited kH/kD = 1.38 (eq 4).
Similarly, the relative rate of the reaction (kH/kD = 1.63) of 4b-
H4, 4b-D4, and 2a reveals a secondary KIE in the second
annulation to pyranoisoquinoline (eq 4). Finally, the kH/kD
value for the overall one-step double annulation of 1b-H5 and/
or 1b-D5 with 2a is ∼3.0 (competitive) and ∼1.41 (parallel),
clearly advocating that the C−H activations do not happen in
the RDS (eq 4).15 Based on the competition experiments, the
electron-rich arenes underwent reaction faster than that of
electron-poor arenes. For example, the product ratios 3a/3j
(8:1) and 3d/3g (6:1) were obtained from the reaction of 1a/1j
(p-Me vs p-CO2Me) and 1d/1g (p-OMe vs p-Br) with 2a,
respectively (eq 5). Likewise, the reaction of an equimolar
mixture of 4a and 4j with 2a led to 3a and 3j in a 4:1 ratio (eq 6).
We therefore presume that the electronic nature of the arenes
influences the RDS of this double annulation.
a
Conditions: 1 (0.5 mmol), 2 (2.0 mmol), Ru catalyst (10 mol %),
AgSbF6 (40 mol %), Cu(OAc)2·H2O (0.75 mmol), 1,4-dioxane (3.0
b
c
mL) at 120 °C for 24 h. Gram scale: 1a (1.37 g, 5.0 mmol). 1 (0.3
mmol), 2 (2.1 mmol), Ru catalyst (15 mol %), AgSbF6 (60 mol %),
Cu(OAc)2·H2O (0.9 mmol), 1,4-dioxane (3.0 mL) at 120 °C for 50
h.
diaryl alkynes having electron-donating (tBu, OMe) and
electron-withdrawing (F, Cl, COMe) substitutions on arene
successfully participated in this unsymmetrical annulation
(Scheme 2).
To gain mechanistic insight into the current transformation,
various control experiments were planned and assessed (Figure
2). The deuterium scrambling study of 1b in the absence (eq 1)
or presence (eq 2) of 4-octyne (2a) provided 35 and 33% D-
incorporation in the arene o-positions of precursor 1b,
respectively. Hence, the current C−H activation of arene is
reversible (eqs 1 and 2); the participation of alkyne for C−H
activation is therefore unviable. However, annulation of N-Me-
isoquinolone (4a-Me) with 2a under Ru catalysis was
unsuccessful (eq 3); the NH-isoquinolone is therefore essential
for the synthesis of pyranoisoquinoline via isoquinolinol-
directed annulation with alkyne. Exposing 4a-Me to the
optimized conditions in CD3CO2D resulted in 48% D-
incorporation of C8, reflecting the O-directed metalation of
Based on the previous observations and control experiments
in Figure 2, probable pathway of MPS-assisted one-pot double
annulation of arenes is sketched (Figure 3).
The reaction begins with the coordination of MPS to the
active Ru catalyst and activation of the o-C−H bond of arenes 1b
to form the respective species A. Next, the coordination of
alkyne to the cyclometalation complex A and insertion makes a
seven-membered Ru metallacycle. As MPS−DG is trans-
formable, the Cu(OAc)2/AcOH-promoted concomitant ex-
pulsion of methylphenyl sulfoxide delivers isoquinolone
architecture 4b′.2,3 Next, imide-assisted C(8)−H/C(peri)−H
activation forms C, and the insertion of alkyne provides seven-
membered metallacycle D. Finally, Cu-mediated reductive
C
DOI: 10.1021/acs.orglett.8b02068
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
pounds 3e, 3j, 3l, 5d, and 5n in acetone were examined (Figure
4).13 The fluorescence spectra of 3 and 5 show emission maxima
Figure 4. Normalized absorption (solid lines) and PL spectra (dotted
lines) of derivatives of 3 (3e, 3j, 3l) and 5 (5d, 5n) dispersed in acetone
medium (1 × 10−5 M).
at 429−536 nm, with broad bandwidths and high intensities.
Based on their properties, these compounds will act as a
fluorescent probe for biological labeling and organic light-
emitting diode applications.
In summary, a one-pot unsymmetrical multiple annulative
functionalization method for (hetero)arene o-C−H bonds with
alkynes is developed with the aid of modifiable MPS under Ru
catalysis, which offers a new avenue for the synthesis of
conjugated heterocycles. The current protocol gives convenient
access to 6,6-fused pyranoisoquinoline from readily available
carboxylic acid derivatives and alkynes via the construction of
four (C−C and C−N and C−C and C−O) bonds in a single
operation involving domino [4 + 2] double annulations. The
unsymmetrical double annulations of both o-C−H bonds of
arenes with two distinct alkynes are also evaluated. Gram-scale
synthesis determines the robustness of the catalytic system.
Deuterium scrambling studies and control experiments support
our understanding of the reaction mechanism. Steady-state
absorption and photoluminescence measurements are also
provided.
Figure 2. KIE and competition experiment studies.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b02068.
Detailed experimental procedures, NMR spectra, and X-
ray crystallographic data (PDF)
HRMS data (PDF)
Accession Codes
CCDC 1846554 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
Figure 3. Proposed mechanistic cycle.
elimination from Ru-embedded species delivers the desired
pyranoisoquinoline 3b with the generation of active catalyst for
the next cycle.
Most of the synthesized compounds are brightly fluorescent.
To understand the photophysical properties of the present
pyranoisoquinolines, the steady-state absorption and photo-
luminescence (PL) measurements for the respective com-
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: akhilchemistry12@gmail.com or akssc@uohyd.ac.in.
D
DOI: 10.1021/acs.orglett.8b02068
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
ORCID
Letter
Akhila K. Sahoo: 0000-0001-5570-4759
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by SERB (EMR/2014/385) and
University of Hyderabad (UoH; UPE-CAS and PURSE-FIST).
M.S., K.G., K.M., and R.K.R. thank CSIR, India, for fellowships.
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