Ruthenium(II)-Catalyzed C?H Activation/Annulation of Aromatic Hydroxamic Acid Esters with Enamides Leading to Aminal Motifs

Article abstract of DOI:10.1002/ejoc.202001632

Hydroxamic acid ester directed C(sp²)?H activation/annulation strategy has been reported employing electron-rich enamides under Ru(II)-catalysis to access aminal frameworks. Both N-vinyl acetamide and N-vinyl formamide delivered aminals bearing

Full text of DOI:10.1002/ejoc.202001632

Communications
doi.org/10.1002/ejoc.202001632
Very Important Paper
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Ruthenium(II)-Catalyzed CÀ H Activation/Annulation of
Aromatic Hydroxamic Acid Esters with Enamides Leading
to Aminal Motifs
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Suman Dana,[a] Popuri Sureshbabu,[a] Chandan Kumar Giri,[a] and Mahiuddin Baidya*[a]
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Hydroxamic acid ester directed C(sp2)À H activation/annulation
strategy has been reported employing electron-rich enamides
under Ru(II)-catalysis to access aminal frameworks. Both N-vinyl
acetamide and N-vinyl formamide delivered aminals bearing
dihydroisoquinolinone motif in good yields. Under modified
reaction conditions, vinyl acetate bestowed the selective
formation of 3,4-unsubstituted isoquinolones. The protocol
operates under mild conditions, where the CÀ H ruthenation
step is reversible and apparently not the rate-determining step.
In the past two decades, the advancement in the transition-
metal catalyzed CÀ H activation/annulation strategies have
fetched dramatic influence in the field of organic synthesis,
offering a wide range of synthetic transformations that could
readily generate important carbocyclic and heterocyclic archi-
tectures from relatively simplified progenitors.[1] Particularly, the
use of cheap and bench-stable Ru(II)-catalysts have exhibited
tremendous potential, wherein various weakly coordinating
common functional groups have been harnessed as directing
groups to govern the reaction efficacy along with the
regioselectivity.[2,3] In this regard, owing to the unique reactivity
profile of NHÀ OMe functionality, CÀ H bond activation of
aromatic hydroxamic acid esters have been extensively
leveraged.[4,7] While the available NÀ O bond efficiently serves as
Scheme 1. Hydroxamic acid ester directed CÀ H activation reactions under
an internal oxidant, obviating the need for stoichiometric
Ru(II)-catalysis.
metallic and nonmetallic oxidant additives, the increased
nucleophilicity of the nitrogen-center could be judiciously
engaged for annulation reactions under mild conditions. In
2012, Li and Wang employed aromatic hydroxamic acid esters
for ortho-olefination with activated olefins under Ru(II)-catalysis
(Scheme 1a).[5] The treatment of relatively unactivated olefins
such as styrene and norbornadiene as well as internal alkynes
bestowed annulated products with ruthenium catalyst
(Scheme 1a).[5–6] Recently, we have also reported a spirocycliza-
tion reaction with quinones to access diverse spiroisoindoli-
nones in a straightforward manner (Scheme 1b).[7] Despite these
advancements, CÀ H activation/functionalization reaction of
arene hydroxamic acid esters with electron-rich olefins under
ruthenium catalysis remained elusive.
Aminals embedded heterocycles are privileged frameworks
present in numerous natural products and bioactive molecules
(Scheme 1c).[8] They are also widely used as surrogates of imines
in diverse organic reactions.[9] Consequently, developing suc-
cinct synthetic routes to access functionalized aminals is highly
desirable. We envisioned that an annulative coupling of
aromatic hydroxamic acid esters with electron-rich olefins such
as enamides would be a straightforward method (Scheme 1c).
Herein, we report the successful enactment of this approach
under ruthenium catalysis leading to aminal frameworks
embracing dihydroisoquinolinone motif.
It is worth noting that enamides were previously employed
as coupling partners in CÀ H activation/annulation processes
under Rh-catalysis.[10] With benzoic acids, Fu and Chen reported
an annulation where N-vinyl formamide delivered isoquinolones
and N-vinyl acetamide produced 3-acetaminoisochromanone
[a] S. Dana, P. Sureshbabu, C. K. Giri, Dr. M. Baidya
Department of Chemistry, Indian Institute of Technology Madras
Chennai 600036, Tamil Nadu, India
E-mail: mbaidya@iitm.ac.in
http://chem.iitm.ac.in/faculty/baidya/
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejoc.202001632
Part of the “YourJOC Talents” Special Collection.
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derivatives.[10a] They also observed a self-dimerization reaction
Table 1. Optimization of reaction conditions.[a]
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in the case of N-vinyl benzamides in presence of Ag2O oxidant
and LiOtBu base.[10b] However, to the best of our knowledge, the
reactivity of enamides has never been studied with Ru(II)-
catalyzed CÀ H bond activation reactions.
We began our investigation utilizing p-toluic acid-derived
hydroxamic acid ester 1a and commercially available N-vinyl
acetamide 2a as model substrates (Table 1). Delightfully, when
they were exposed to 5 mol% of [Ru(p-cymene)Cl2]2 in the
Entry
Deviation from final conditions
Yield [%][b]
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4
none
83%
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trifluoroethanol instead of HFIP
methanol or iso-propanol instead of HFIP
DCE instead of HFIP
NP[c]
NP[c]
trace
79
°
presence of NaOAc (0.5 equiv.) in HFIP solvent at 60 C, the
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desired annulated product 3a was obtained in 83% isolated
yield after 4 h (Table 1, entry 1). Further experimentation
revealed that the solvent plays a crucial role in product
formation.[11] While trifluoroethanol (TFE) delivered 16% yield of
3a (entry 2), methanol and iso-propanol were found ineffective
for this transformation (entry 3). Other common organic
solvents such as 1,2-dichloroethane (DCE) and tetrahydrofuran
(THF) were also incompetent to produce 3a even after
prolonged reaction time (entries 4 and 5). Next, the role of the
base was examined. While KOAc displayed comparable reac-
tivity (entry 6), other inorganic bases such as K3PO4 and K2CO3
gave inferior results (entry 7 and 8). Increasing the loading of
NaOAc could not improve the yield (entry 9), whereas the
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THF instead of HFIP
KOAc instead of NaOAc
K3PO4 instead of NaOAc
K2CO3 instead of NaOAc
1.0 equiv. NaOAc
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trace
NR[d]
NP[c]
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without NaOAc
without Ru(II)-catalyst
Pd(OAc)2 instead of Ru(II)-catalyst
[a] Reaction conditions: 1a (0.15 mmol), 2a (1.5 equiv.), [Ru(p-cymene)Cl2]2
(5.0 mol%), NaOAc (0.5 equiv.), HFIP (0.5 mL), 60 C, 4 h (reaction was set
up under air atmosphere). [b] Isolated yields. [c] NP=no product formation
with the recovery of 1a. [d] NR=no reaction with the recovery of 1a.
°
Suman Dana was born in 1992, in West
Bengal, India. After finishing his bachelor’s
degree in Chemistry (Honours) from Bankura
Christian College, India, he moved to School
of Chemistry, University of Hyderabad, where
he received his M.Sc. degree (Gold Medallist)
in 2014. Since July 2016, he is a Ph.D. student
in the research group of Dr. Mahiuddin Baidya
at the Indian Institute of Technology Madras.
His research interest revolves around transi-
tion-metal catalyzed C-H activation reactions
and their application in the synthesis of
heterocycles. He is also interested in radical
chemistry and currently exploring synthetic
strategies harnessing photo-redox catalysis.
Chandan Kumar Giri was born in Odisha, India,
in 1995. He received his bachelor’s degree in
Chemistry (Honours) from U. N. College, Soro,
Odisha (Gold Medallist, F. M. University). In
2017, he has completed M.Sc. in Chemistry
from Sambalpur University, Odisha, India.
Since January 2019, he is a research scholar at
the Indian Institute of Technology Madras,
under the supervision of Dr. Mahiuddin
Baidya. His research focuses on the transition-
metal catalyzed C–H activation reactions and
their applications in total synthesis of natural
products.
Mahiuddin Baidya was born in West Bengal,
India, in 1983 and studied Chemistry at the
Indian Institute of Technology Kanpur, India.
He received Ph.D. degree from Ludwig Max-
imilians Universität München, Germany in
2009 (supervisor: Prof. Dr. Herbert Mayr). After
post-doctoral studies with Prof. Hisashi Yama-
moto at the University of Chicago, USA and
Molecular Catalyst Research Center, Chubu
University, Japan (JSPS postdoctoral fellow-
ship), he joined the Department of Chemistry,
Indian Institute of Technology Madras in
2014 as an Assistant Professor. In 2018, he was
appointed as an Associate Professor in the
same department. His current research inter-
ests focus on the development of novel
concepts for transition-metal catalysis, orga-
nocatalysis, photo-redox catalysis, and syn-
thesis of bioactive molecules. He serves as an
Early Career Advisory Board (ECAB) member of
the ACS Catalysis journal.
Popuri Sureshbabu was born in the village of
Turlapadu in Andhra Pradesh, India, in 1987.
He obtained his bachelor’s degree from Kakani
Venkataratnam College, Nandigama, Andhra
Pradesh and master’s degree in Chemical
Science from Pondicherry University, Pudu-
cherry. He then started his Ph.D. on Bio-
Inorganic Chemistry under the supervision of
Dr. S. Sabiah, Pondicherry University and
received Ph.D. degree in 2018. Then he joined
the group of Dr. Jeyakumar Kandasamy at IIT-
BHU as a postdoctoral researcher, where he
worked on applications of amides and N-
nitroso chemistry in organic syntheses. Re-
cently, he moved to Dr. Baidya’s group at
Indian Institute of Technology Madras as a
postdoctoral researcher, where he is working
on transition-metal-catalyzed directed C–H
bond functionalization reactions and reactions
of nitroso compounds for the synthesis of
complex molecular frameworks.
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absence of the acetate base completely inhibited the reactivity
Unsubstituted benzamide and benzamides consisting of alkyl
substituents in various positions of the arene ring delivered
corresponding aminals in moderate to good yields (3b–3g).
The protocol also accommodated biphenyl derived amides to
generate the aminal products 3h and 3i in 63% and 67%
yields, respectively. Arenes consisting of electron-donating
substituents were also effective substrates, offering aminal
products in high yields (3j–3k). Synthetically useful halogen
functionalities such as bromo (3l) and iodo (3m) were also
sustained under the catalytic conditions, whereas electroneg-
ative fluoro-substituent (3n) resulted in reduced yield. Irrespec-
tive of the position of the amide functionality, polyaromatic
naphthoic acid-derived amides offered aminals 3o and 3p in
good yields. Notably, a biologically relevant carbazole derivative
containing a benzamide moiety produced the corresponding
aminal 3q in 50% yield.
After successfully exploring the scope of the reaction with
N-vinyl acetamide, other electron-rich olefins were examined
under the standard reaction conditions. Similar to N-vinyl
acetamide, N-vinyl formamide was also a fitting coupling
partner with diverse aromatic hydroxamic acid esters, forging
aminals 3r–3v in good to excellent yields (Scheme 2). However,
N-vinyl pyrrolidone failed to construct the desired aminal
product 3w and unreacted 1a was recovered. Next, we
wondered whether vinyl acetate can also be employed in this
transformation. Accordingly, when we assessed vinyl acetate as
a coupling partner under the developed catalytic conditions,
we observed the formation of isoquinolone 4a in poor yield (<
20%, Scheme 2). Reaction yield could be improved up to 75%
after slight modification of reaction conditions (employing
K3PO4 base) and further prolongation of reaction time. Under
the modified reaction conditions, 3,4-unsubstituted isoquino-
lones 4b and 4c were also prepared in 68% and 72% yields,
respectively (Scheme 2).
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(entry 10). The reaction was unproductive in the absence of
Ru(II)-catalyst (entry 11). Altering the catalyst to Pd(OAc)2 also
could not deliver the desired product (entry 12).
Having acquired the standard reaction conditions, we next
sought to unravel the substrate compatibility of this method
(Scheme 2). The reaction was quite general for a variety of
electronically and sterically distinct hydroxamic acid esters.
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To showcase the synthetic utility, we examined the
scalability of this process with N-vinyl acetamide as well as N-
vinyl formamide (Scheme 3). To our delight, the efficacy of
small-scale reactions was retained upon scale-up, producing 3a
and 3s in 76% and 92% yields, respectively.
To gain insights into the mode of action of the catalytic
system, we next performed a set of control experiments
(Scheme 4). The addition of D2O in the reaction mixture led to
significant incorporation (~41%) of deuterium in the product,
implicating a reversible CÀ H metalation process (Scheme 4a).
The kinetic isotope effect (KIE) studies resulted in a value of
~1.46, which in combination with the earlier findings suggested
that the CÀ H bond cleavage is probably not the rate-limiting
Scheme 2. Scope of substrates. Yields of Isolated products are given.
Reaction conditions: 1a (0.15 mmol), 2 (1.5 equiv.), [Ru(p-cymene)Cl2]2
°
(5.0 mol%), NaOAc (0.5 equiv.), HFIP (0.5 mL), 60 C, 4 h (reaction was set up
under air atmosphere). [a] NR=no reaction. [b] Reaction conditions: 1a
(0.3 mmol), vinyl acetate (2d) (1.5 equiv.), [Ru(p-cymene)Cl2]2 (5.0 mol%),
°
K3PO4 (0.5 equiv.), HFIP (1.5 mL), 60 C, 36 h.
Scheme 3. Scaled-up reaction.
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electron-donating substituent is a poor leaving group such as
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acetamido or formamido functionality, the formation of 3 was
observed as an exclusive product after the reductive elimina-
tion. If the olefin contains a good leaving group such as acetate,
then compound 3 experiences a facile elimination of the
acetate functionality leading to the formation of intermediate
D, which upon tautomerization forms the 3,4-unsubstituted
isoquinolone 4.
In conclusion, we have demonstrated the annulative
coupling of hydroxamic acid esters and enamides, constructing
dihydroisoquinolinone derived aminal scaffolds in good yields.
The reaction is operationally simple, air- and moisture-tolerant,
and compatible with diverse common organic functionalities.
Under modified conditions, the Ru-catalyzed strategy was also
well-suited to utilize vinyl acetate as a coupling partner, where
3,4-unsubstituted isoquinolones were formed in high yields.
Preliminary mechanistic studies suggested a reversible CÀ H
metalation process to be involved in this reaction, where CÀ H
bond cleavage is perhaps not the kinetically relevant step.
Further exploration of hydroxamic acid ester directed CÀ H
bond activation/annulation strategies are ongoing in our
laboratory.
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Scheme 4. Control experiments for mechanistic insights.
step (Scheme 4b).[7] The presence of superstoichiometric Acknowledgements
amounts of radical scavengers did not inhibit the reactivity,
justifying the involvement of an organometallic pathway in the
process without having any single electron transfer event
(Scheme 4c).
We gratefully acknowledge DST-SERB India (CRG/2019/001003) for
financial support. S.D. thanks IIT Madras for HTRA fellowship. P.S.
acknowledges CSIR for fellowship. C.K.G. thanks UGC for JRF. M.B.
thanks IIT Madras for the support through Institute Research
Development Award (IRDA). We also thank the Department of
Chemistry, IIT Madras for instrumental facilities.
Following the literature precedents[12] and above mechanis-
tic experiments, a plausible reaction mechanism of this trans-
formation is depicted in Scheme 5. First, amide 1 undergoes
carboxylate-assisted CÀ H bond activation with Ru(II)-catalyst to
form metallacycle A. It then coordinates with the olefin 2 and
realizes a selective migratory insertion to generate metallacycle Conflict of Interest
C, which upon reductive elimination and a facile NÀ O bond
cleavage produces product 3 and regenerates the active Ru(II)-
catalyst. At this juncture, we observed two distinct selectivity
profiles with N-vinyl amides and vinyl acetate. When the
The authors declare no conflict of interest.
Keywords: Aminals
Homogeneous catalysis · Ruthenium
·
Annulation
·
CÀ H Activation
·
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Scheme 5. Plausible mechanism.
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Manuscript received: December 16, 2020
Revised manuscript received: January 27, 2021
Accepted manuscript online: January 29, 2021
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