An Orchestrated Unsymmetrical Annulation Episode of C(sp2)-H Bonds with Alkynes and Quinones: Access to Spiro-isoquinolones

Article abstract of DOI:10.1021/acs.orglett.8b00468

A nontrivial Ru-catalyzed one-pot sequential oxidative coupling of a (hetero)arene/vinylic/chromene system with alkyne and quinone is presented; the methyl phenyl sulfoximine (MPS) directing group is vital. This cyclization forms four (two C-C and two C-N) bonds in a single operation and produces unusual spiro-fused-isoquinolones with a broad scope. The release of phenyl methyl sulfoxide makes the MPS group transformable. A deuterium scrambling study sheds light on the reaction path.

Full text of DOI:10.1021/acs.orglett.8b00468

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
An Orchestrated Unsymmetrical Annulation Episode of C(sp2)−H
Bonds with Alkynes and Quinones: Access to Spiro-isoquinolones
Kallol Mukherjee, Majji Shankar, Koushik Ghosh, and Akhila K. Sahoo*
School of Chemistry, University of Hyderabad, Hyderabad-500046, India
S
* Supporting Information
ABSTRACT: A nontrivial Ru-catalyzed one-pot sequential
oxidative coupling of a (hetero)arene/vinylic/chromene sys-
tem with alkyne and quinone is presented; the methyl phenyl
sulfoximine (MPS) directing group is vital. This cyclization
forms four (two C−C and two C−N) bonds in a single oper-
ation and produces unusual spiro-fused-isoquinolones with a
broad scope. The release of phenyl methyl sulfoxide makes the
MPS group transformable. A deuterium scrambling study sheds light on the reaction path.
he synthetically validated annulation tool has had a pro-
found impact in chemistry, because this method reliably
T
transforms the readily accessible less-functionalized compounds
to structurally complex molecules.1 In this regard, the transition-
metal (TM)-catalyzed and directing group (DG)-supported
activation, functionalization, and annulation of inert C−H bonds
are incomparable.2−5 Obviously, the functionalization of
environmentally different C−H bonds with distinct coupling
partners results in structurally diverse molecular scaffolds.6
A viable synthetic example of this is the directed double func-
tionalization of proximal C−H bonds with identical functional
groups.7 By contrast, the chelation-assisted unsymmetrical func-
tionalization of C−H bonds is often nonselective, uncontrolled,
and unproductive;8 however, a sequential two-step synthetic
process under divergent DGs and/or different catalytic con-
ditions is implemented for the construction of novel molecular
scaffolds.9 A one-pot, single DG-enabled unsymmetrical
tandem o-C−H difunctionalization of arenes has recently
been presented via the intramolecular o-C−H hydroarylation
and intermolecular o′-C−C/C−N bond formations,10a and
also, the o-C−H alkylation and o′-C−H amidation of
N-phenoxyacetamide.10b
Figure 1. Unsymmetrical annulation of C(sp2)−H bonds.
Usually, two synthetic steps are essential to realize a cascade
2-fold annulation of arenes with different coupling partners;
hence, the stitching of distinct functional groups (FG1/FG2)
requires different catalytic conditions (Figure 1A).9b,c We have
recently demonstrated the direct double annulation of
transformable methylphenyl sulfoximine (MPS)-DG-aided
C(sp2)−H bonds with different alkynes.11 Thus, the one-pot,
2-fold unsymmetrical C−H diannulation of (hetero)arenes with
distinct coupling partners (alkyne and olefin) in the presence of
a single DG under single catalytic conditions is a worthwhile
endeavor, which has, so far, not been reported. We propose that
alkyne over olefin can undergo multiple annulation with
C(sp2)−H bonds and generate AB (monoannulation) and ABB
(diannulation) products,11a which eventually obstructs the
formation of the projected unsymmetrical diannulation com-
pound ABC′/ABC (Figure 1B). Moreover, the quinone moiety
is an effective Michael acceptor and is susceptible to Heck cou-
pling; thus, α,α′-difunctionalization of the quinone motif to
install a spiro-skeleton is possible, although challenging.12 How-
ever, the Ir-catalyzed oxidative coupling of isoquinolone with
benzoquinone is feasible for constructing novel spiro-fused
heteroarenes.13 Thus far, there have been no reports on the respec-
tive one-pot unsymmetrical multiple annulations of heteroarene/
vinylic system from a simple carboxylate precursor.
Based on our preliminary understanding of the MPS-DG
assisted annulation of hetero(arene)/vinylic systems, we herein
invent a tunable one-step synthetic technique for the divergent
Received: February 8, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00468
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
annulation of C(sp2)−H bonds with alkyne and quinone
(Figure 1C). The strategy involves monoannulation of proximal
C−H bond of MPS-DG bearing amide and alkyne under cost-
effective, air-stable Ru catalyst in the presence of acid source.14
The in situ formation of isoquinolone/pyridone through proto-
demetalation restrict further diannulation with alkyne and the
oxidizable MPS-DG helps regeneration of the active catalyst.
Next, the base-promoted annulation with quinone delivers the
synthetically promising unusual spiro-fused-isoquinolones of
pharmaceutical and material importance.15 The isolation of
methylphenyl sulfoxide, which is the sole precursor of MPS,
endorses MPS-DG as transformable.16
The feasibility of the one-pot three component cascade
annulations (envisaged in Figure 1C) is probed by subjecting
MPS-coupled 5-methyl-thiophene-2-carboxylate (1a) with
diphenylacetylene (2a) and benzoquinone (3a) in the presence
of Ru-catalysts (see Table 1). Pleasingly, the predicted product
pleasingly, 4aaa was isolated in 35% yield (entry 2 in Table 1).
This information encouraged us to survey the reaction via
turnwise addition of 2a and 3a. Thus, the entire transformation
was performed accordingly [the reaction, at first, was performed
at 60 °C in the presence of 1.2 equiv of 2a for 10 h; subse-
quently, 3a was introduced and the mixture was then heated at
120 °C for 10 h]; this process resulted 45% 4aaa, along with
the diannulation product 4aa′ (18%) (entry 3 in Table 1).
Interestingly, annulation between 1a and 2a in the presence of
2 equiv AcOH restricted the formation of 4aa′ to 11%; mean-
while, the delivery of 4aaa was affected (entry 4 in Table 1),
which is a consequence of ineffective binding of Ru to the NH
moiety of monoannulation product in an acid medium. Inter-
estingly, the addition of NaHCO3 base, along with 3a, helped
the production of 4aaa (62%), endorsing the requirement of
base after first cyclization (entry 5 in Table 1).
Encouraged by the observation, various bases (Na2CO3,
K2CO3, K3PO4, and KH2PO4) were screened (see entries 6−9 in
Table 1); KH2PO4 was found to produce the optimum yield of
4aaa: 66% (see entry 9 in Table 1). Although the overall
reaction in 1,2-DCE was better over 1,4-dioxane (entries 9 and
10 in Table 1), the use of a mixture of solvents in this 2-fold
cyclization strategy [1,2-DCE for the first annulation and
subsequent addition of 1,4-dioxane in the second annulation]
led to 72% 4aaa (entry 11 in Table 1). This transformation, in
the absence of Cu(OAc)2·H2O, provided 4aaa in only 35%
yield, along with 40% of the monoannulation compound 4aa
(entry 12 in Table 1), which suggests that the oxidant is indis-
pensable, since it helps the regeneration of active catalyst after
second annulation. Moreover, the reaction in the presence of
oxidant O2 or CuBr2 was not effective (see entries 13 and 14 in
Table 1).9j Thus, the one-pot synthetic avenue for the unsym-
metrical annulation of 1a with 2a and 3a under the optimized
catalytic conditions (entry 11 in Table 1) smoothly provided
unusual spiro-fused isoquinolone 4aaa.
The synthetic generality of this unprecedented two suc-
cessive unsymmetrical annulation is validated by examining
MPS-assisted cyclization of C(sp2)−H bonds of challeng-
ing heteroarene/vinylic/chromene systems with alkynes
and quinones (Scheme 1−3). Thus, the MPS-coupled
thiophene-2-/benzothiophene-2-carboxylate (1a−1d) were
reacted with 2a and 3a to afford the desired spiro-products
4aaa−4daa (72%−82%); the modifiable Br-group survived to
provide 4caa. The thiophene-3-/benzothiophene-3-carboxy-
lates also participated, delivering 4eaa (62%) and 4faa (51%).
Similarly, the spiro-product from oxygen-bearing heteroarene
4gaa (72%) was readily isolated from the cyclization cascade of
MPS-bearing furan-3-carboxylate (1g) with 2a and 3a; identical
reaction in 1.0 mmol scale resulted in a 66% yield of 4gaa.
The naphthaquinone (3b) also participated in the cascade cycli-
zation episode with 1b/1g/1h and 2a to deliver 4bab (70%),
4gab (74%), and 4hab (57%), respectively (see Scheme 1).
Moreover, the N-bearing heterocycles pyrrole and indole skel-
eton were stitched with 2a and 3b accessing the poly fused spiro-
skeletons 4iab and 4jab, albeit in moderate yield. The product
4gac was isolated from the annulations of 1g with 2a and
2-chloro-benzoquinone (3c); X-ray analysis confirms the
structure of 4gac.17 Annulations of challenging o-substituted
arene motif with 2a and 3b delivered 4kab.
a
Table 1. Optimization of Reaction Conditions
additive
(2.0 equiv)
base
(2.5 equiv)
temperature
yield of 4aaa
entry
(°C)
(%)
bc
,
1
2
120
20 (35)
35 (21)
45 (18)
26 (11)
62 (07)
40 (10)
39 (10)
61 (06)
66 (05)
50 (15)
72 (04)
bc
,
60/120
60/120
90/120
90/120
90/120
90/120
90/120
90/120
90/120
90/120
90/120
90/120
90/120
3
4
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
5
NaHCO3
Na2CO3
K2CO3
6
7
8
K3PO4
9
KH2PO4
KH2PO4
KH2PO4
KH2PO4
KH2PO4
KH2PO4
d
10
11
12
13
14
e
fg
35 (40) ,
h
43 (16)
i
39 (0)
a
Conditions: 1a (0.18 mmol, 1.0 equiv), 2a (1.2 equiv), Ru-catalyst
(10 mol %), AgSbF6 (40 mol %), Cu(OAc)2·H2O (1.0 equiv), additive
(2.0 equiv), DCE (1.5 mL) at 90 °C for 10 h; then BQ (2.0 equiv),
base (2.5 equiv) and 1,4-dioxane (1.5 mL), at 120 °C for 10 h; yield of
b
4aa′ (diannulation with alkyne) is shown in parentheses. All of the
c
d
reactants in one-pot synthesis. 2a (1.0 equiv). 1,4-dioxane instead of
e
f
DCE. Addition of 1,4-dioxane (1.5 mL) after first cyclization. In the
g
absence of Cu(OAc)2·H2O. Yield of 4aa (monoannulation with
h
i
alkyne) is shown in parentheses. O2 used as oxidant. CuBr2 used as
oxidant.
4aaa (20%) and diannulation compound 4aa′ (35%; from 1a
and 2a) formed when the reaction was executed under the cata-
lytic conditions [Ru-catalyst (10 mol %), AgSbF6 (40 mol %),
and Cu(OAc)2·H2O (1.0 equiv) in 1,2-dichloroethane (DCE)
at 120 °C for 10 h] (entry 1 in Table 1). This result is partic-
ularly noteworthy, because MPS-bearing heteroarenes and alkynes
are amenable to annulation, which occurs even at 60 °C.11a
To realize better turnout of 4aaa, the annulation episode
among 1a, 2a, and 3a was conducted at 60 °C (where alkyne
undergoes cyclization) and 120 °C (where quinone commences
annulation) in a single pot under one catalytic condition;
We next probed the alkyne scope in this one-pot two-
successive annulation of MPS-bearing heteroarene and qui-
none (Scheme 2). Pleasingly, 1,2-diarylacetylenes {having
para-substituents on the arene moiety, electron-donating
B
DOI: 10.1021/acs.orglett.8b00468
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
products 4gea (68%), 4gca (66%), and 4gfa (67%) were
undeniably accessed when 1g coupled with 2e (p-Me−)/2c/
electron-withdrawing (p-COMe) group containing alkyne (2f)
and 3a, respectively.
Scheme 1. Cascade Annulation of Heteroaryls 1 with
Diphenylacetylene 2a and Quinone 3
Inspired from the unsymmetrical diannulation of heteroaryls
with alkynes and quinones (see Schemes 1 and 2), the identical
reaction in the challenging vinylic system was surveyed next
(Scheme 3), because vinylic systems have a tendency to
Scheme 3. Cascade Annulation of Vinyl/Chromenes 5/6
with Alkynes (2) and Naphthaquinone 3b
polymerize under the oxidative conditions and also are effective
Michael acceptors.18 Gratifyingly, the reaction of N-(meth-
acryloyl)-MPS (5a) with 2a and 3b under the optimized
catalytic conditions in entry 11 in Table 1 delivered 7aab in
62% yield. To further authenticate the synthetic viability of this
2-fold annulations of acrylamides, the reaction between 5a, the
electron-rich (Me/tBu/OMe) group containing para-substi-
tuted 1,2-diarylacetylenes, and 3b delivered the desired spiro-
fused novel heterocycle manifolds 7aeb (61%), 7abb (70%),
and 7acb (62%).
Scheme 2. Cascade Annulation of Heteroaryls 1 with
Alkynes (2) and Benzoquinone 3a
The product 7adb with the labile chloro group in the
periphery was also constructed. The meta-Me bearing
1,2-diarylalkyne participated in the cascade annulation,
accessing 7ahb. The spiro compound 7agb was isolated from
the annulation of 5a, unsymmetrical n-butyl-phenyl alkyne 2g,
and 3b; the regioselective monoannulation of 5a with 2g makes
this process feasible. The 2H-chromene-3-carboxylate deriva-
tive 6a successfully underwent annulations with 2a and 3b to
deliver the conjugated spiro species 8aab in 38% yield.
To understand the possible reaction pathway, a few control
experiments have been performed (see Scheme 4). The reac-
tion in the absence of oxidant Cu(OAc)2 led to the mono-
annulation product (69%); the reductive cleavage of MPS
group herein helps oxidation of Ru-catalyst and keeps the
catalytic cycle active (see eq 1 in Scheme 4). However, the
reaction of 4ba with benzoquinone under the optimized
condition led to 35% of the desired product 4baa (see eq 1 in
Scheme 4). Product 4gaa-D with 33% deuterium incorporation
[−tBu (2b)/−OMe (2c)] or −Cl (2d) group} smoothly reacted
with 1b and 3a under the optimized procedure to afford spiro-
fused enlarged isoquinolones 4bba (75%), 4bca (67%), and
4bda (69%). Similarly, the furan-enabled spiro-isoquinolone
C
DOI: 10.1021/acs.orglett.8b00468
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
C(sp3)−H, generates more-stable 6-membered ruthenacycle
15.20 Reductive elimination of 15 finally leads to spiro-fused-
isoquinolone 4. The Cu(OAc)2 helps to regenerate the active
Ru-catalyst.19
Scheme 4. Control Experiments
The synthetic manipulation of peripheral olefin moiety in the
spiro-isoquinolone scaffold was further elaborated by perform-
ing [4 + 2] cycloaddition of 4gea with cyclopentadiene, result-
ing in complex molecular entity 16 (eq 4).
In summary, we have revealed, for the first time, the
Ru-catalyzed MPS-assisted 2-fold unsymmetrical cyclization of
heteroarenes/vinylic systems with different coupling partners
(first with the alkynes and then with the quinones); both these
annulations are realized via one-pot synthesis, resulting in four
bonds (two C−C and two C−N). These highly orchestrated
cyclization techniques are largely suitable for the fabrication of
unnatural spiro-isoquinolinones.
in the quinone moiety was detected when the reaction was per-
formed in CD3CO2D (eq 2 in Scheme 4), reflecting the occur-
rence of protodemetalation in the transformation.19 Hence,
formation of the desired spiro-product is possible through the
metalation of acidic C(sp3)−H bond, followed by C−N reduc-
tive elimination (see the mechanistic cycle in Scheme 5).
Scheme 5. Proposed Mechanistic Profile
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b00468.
Detailed experimental procedures and the physical prop-
erties of the compounds (NMR data, characterization,
and spectra) (PDF)
Accession Codes
CCDC 1819534 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, U.K.; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail addresses: akhilchemistry12@gmail.com, akssc@
uohyd.ac.in.
Interestingly, the reaction in the presence of 2 and 5 equiv of
BQ produced 35% and 48% of 4aaa, respectively (see eq 3 in
Scheme 4); thus, the weak oxidant BQ is incapable in carrying
out the transformation. Consequently, the oxidant Cu(OAc)2
plays a vital role in retaining the catalytic cycle via revival of
active Ru(II) species.5
Based on the precedence and the current observation, the
plausible reaction pathway is outlined in Scheme 5.3,5 The reac-
tion initiates with the coordination of MPS to the active Ru-
species, forming the ruthenacycle 9 via chelation-assisted C−H
metalation. Subsequent alkyne coordination−insertion to 9 pro-
vides seven-membered ruthenacycle 11 through 10. Next, syner-
gistic C−N reductive elimination and N−S cleavage affords 12
in situ, which simultaneously undergoes proximal C−H
metalation to deliver 13. Insertion of benzoquinone to 13
gives 7-membered ruthenacycle 14; protodemetalation of 14
(see eq 2 in Scheme 4), followed by metalation of acidic
ORCID
Akhila K. Sahoo: 0000-0001-5570-4759
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank SERB (No. EMR/2014/385) for financial support
and University of Hyderabad (UoH; UPE-CAS and PURSE-
FIST) for overall facility. K.M., M.S., and K.G. thank CSIR,
India for fellowship. Dr. K. Nagarjuna (UoH) is thanked for the
crystallographic studies.
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