Redox-Neutral [4 + 2] Annulation of N-Methoxybenzamides with Alkynes Enabled by an Osmium(II)/HOAc Catalytic System

Article abstract of DOI:10.1021/acs.orglett.9b03827

By making use of a direct C-H activation strategy, an efficient osmium(II)-catalyzed redox-neutral [4 + 2] annulation of N-methoxybenzamides with alkynes has been accomplished. Computational and experimental studies revealed that such transformation leading to the synthesis of the isoquinolone core might follow an Os(II)-Os(IV)-Os(II) catalytic pathway, in which an unusual HOAc-assisted oxidative addition of osmium(II) into the N-O bond to generate the osmium(IV) species was involved as one of the key transition states. Further exploration of divergent C-H activation reaction modes enabled by the osmium(II) catalyst has also been exemplified for one-pot assembly of other either linear or cyclic products.

Full text of DOI:10.1021/acs.orglett.9b03827

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Redox-Neutral [4 + 2] Annulation of N‑Methoxybenzamides with
Alkynes Enabled by an Osmium(II)/HOAc Catalytic System
Jian Yang,§ Liexin Wu,§ Huiying Xu, Hui Gao, Zhi Zhou,* and Wei Yi*
Guangzhou Municipal and Guangdong Provincial Key Laboratory of Protein Modification and Degradation & Molecular Target and
Clinical Pharmacology, State Key Laboratory of Respiratory Disease, School of Pharmaceutical Sciences & the Fifth Affiliated
Hospital, Guangzhou Medical University, Guangzhou, Guangdong 511436, China
S
* Supporting Information
ABSTRACT: By making use of a direct C−H activation strategy, an
efficient osmium(II)-catalyzed redox-neutral [4 + 2] annulation of N-
methoxybenzamides with alkynes has been accomplished. Computational
and experimental studies revealed that such transformation leading to the
synthesis of the isoquinolone core might follow an Os(II)−Os(IV)−Os(II)
catalytic pathway, in which an unusual HOAc-assisted oxidative addition of
osmium(II) into the N−O bond to generate the osmium(IV) species was
involved as one of the key transition states. Further exploration of divergent
C−H activation reaction modes enabled by the osmium(II) catalyst has
also been exemplified for one-pot assembly of other either linear or cyclic
products.
ver the past decades, a transition-metal-catalyzed direct
C−H activation/annulation cascade has received con-
Scheme 1. Exploration and Development of Direct C−H
O
Activation Enabled by the Osmium(II) Catalysis
siderable attention since it can enable the site-/region-selective
synthesis of a broad range of heterocyclic frameworks from
simple starting materials in a step- and atom-economical
fashion via the directing-group-assisted controllable cleavage of
the inert C−H bonds.1 Since the groundbreaking work by the
research groups of Murai,2a Lewis,2b and Fujiwara,2c
compelling progress has been made in this hot area of
research. Accordingly, most of the first- and second-row
transition metals, particularly ruthenium,3 rhodium,4 and
palladium complexes,5 have been developed as the catalysts
of choice thus far. In sharp contrast, the third-row transition
metals have been investigated much less than the first- and
second-row transition metals in this field.6,7 Especially, to the
best of our knowledge, there are no examples involving
osmium complexes as the catalysts to address the issue of
direct C−H functionalization, mainly because they are
considered less active on account of their slower ligand
exchange kinetics.8a Indeed, our preliminary investigation also
showed that the reaction of the classical 2-phenylpyridine with
either styrene or diphenylacetylene under osmium(II) catalysis
was stopped completely at the step of the cleavage of the ortho-
C−H bond to deliver the almost equivalent five-membered
osmacycle species as the dominant product (Scheme 1a).
However, in basic research, the most interesting findings are
often obtained in less explored areas. In this vein, recent
studies have demonstrated that the osmium species are a class
of versatile catalysts. For instance, several newly synthesized
osmium complexes have shown to possess relevant catalytic
performance in transfer hydrogenation (TH), hydrogenation
(HY), as well as dehydrogenation (DHY) processes with
activities comparable to or even higher than those of the
ruthenium analogues.8,9 Driven by these elegant works and the
successful history of the development of their ruthenium
counterparts in C−H activation chemistry, we envisioned that
the careful switching of the ligand, the DG, and/or related
reaction parameters might control the polarized nature of the
Os−C bond through changing the coordination environment
between the osmium metal and the substrate, which then
tuned the reactivity and the selectivity of the Os−C species to
ensure sufficient interactions of the Os−C bond with a proper
coupling reagent, thus leading eventually to facile construction
of ideal complex products via osmium-catalyzed C−H
activation reactions. As part of our ongoing efforts, herein,
we would like to disclose the first osmium(II)-catalyzed and
HOAc-assisted redox-neutral C−H activation/[4 + 2]
annulation example for the synthesis of isoquinolones (Scheme
Received: October 27, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03827
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
1b). Combined computational and experimental studies
clarified the Os(II)−Os(IV)−Os(II) catalytic reaction path-
way for the above proof-of-principle demonstration. In
addition, the applicability and versatility of the osmium(II)-
catalyzed system have been examined as exemplified by the
synthesis of 2-alkynylbenzamide, five-membered indole, six-
membered isoquinoline, and seven-membered 1H-benzo[c]-
azepine-1,3(2H)-dione skeletons through divergent C−H
activation reaction modes.
Scheme 2. Scope of N-Methoxybenzamides for the Synthesis
of Isoquinolones
a
Preliminarily, in a search for appropriate DGs that enable
osmium(II)-catalyzed diversified C−H functionalization, we
turned our attention to N−OR-substituted benzamides, which
represent a type of versatile ODG for the construction of
various heterocycles via transition-metal-catalyzed transforma-
tions.4d,10−12 To our delight, the treatment of the simple N-
methoxybenzamide 3a with diphenylacetylene 4a in the
presence of [Os(p-cymene)Cl2]2 (5 mol %) and NaOAc (0.5
equiv) in methanol led to the formation of isoquinolone
product 5aa in 21% isolated yield along with the disappearance
of most of the starting materials, which confirmed our
aforementioned inference. Encouraged by this result, we next
screened the experimental parameters systematically to figure
out the optimal conditions for the catalytic C−H functional-
ization (see Table S1 in the Supporting Information for a
detailed optimization). After a number of trials, we were
pleased to identify the following optimized conditions: 3a (1.2
equiv), 4a (1.0 equiv), [Os(p-cymene)Cl2]2 (5 mol %),
NaOAc (0.5 equiv), and HOAc (1.0 equiv) in TFE at 60 °C
for 24 h under an atmosphere of air, delivering the desired
isoquinolone product in 85% yield.
Having the optimized conditions in hand, the exploration of
the generality and substrate scope with respect to N-
methoxybenzamides was first conducted. As shown in Scheme
2, a variety of substituted N-methoxybenzamides were
employed to react with 4a, and the corresponding
isoquinolones were constructed effectively. Various commonly
encountered functional groups including alkyl (5ba−da),
methoxyl (5ea), halogens (5fa−ha), phenyl (5ia), cyano
(5ja), and nitro group (5ka) were fully tolerated in this
transformation, furnishing the desired products in moderate to
good yields. In addition, the reaction efficiency was not
influenced by the position of the substituent; N-methox-
ybenzamides with substituents at either para-, ortho-, or meta-
position were all good reactants to afford the desired
isoquinolones in good yields. Of note, when meta-methyl-
substituted N-methoxybenzamide was subjected to the stand-
ard conditions, the C−H bond cleavage occurred at the less
hindered site with exclusive regioselectivity (5na), whereas the
meta-chloro- and bromo-substituted substrates resulted in the
generation of a mixture of regioisomers (5oa−pa and 5oa′−
pa′), revealing that the nature of the substituent had a clear
effect on determining the reaction outcome. Having disclosed
the good functional group tolerance for various functionalized
phenyl amides, we were next intrigued to further extend the
scope to heterocyclic scaffolds, which represent a class of
challenging substrates for the application in the field of
transition-metal-catalyzed C−H functionalization. Delightfully,
several O- or N-containing skeletons including coumarin,
pyridine, and piperazine derivatives readily participated in this
reaction to yield the corresponding isoquinolones (5qa−ta),
which further enhanced the versatility of the developed
osmium(II)-catalyzed protocol.
a
Reaction conditions: 3 (0.24 mmol), 4a (0.2 mmol), [Os(p-
cymene)Cl2]2 (5 mol %), NaOAc (0.5 equiv), and HOAc (1 equiv) in
TFE (0.2 M) at 60 °C for 24 h without exclusion of air or moisture;
isolated yields were reported. Performed on a 5.0 mmol scale with
b
2.5 mol % of [Os(p-cymene)Cl2]2.
Subsequently, a diverse array of alkynes were examined to
further probe the robustness of this protocol for the efficient
synthesis of isoquinolones. As shown in Scheme 3, the
enumerated alkynes including symmetrical diaryl acetylenes
bearing various substituents were all compatible with the
standard conditions, delivering the corresponding isoquino-
lones (5ab−ah) in moderate to good yields. It was noteworthy
that the unsymmetrically substituted aryl/alkyl, alkyl/ester, or
alkyl/amido alkynes, including but-1-yn-1-yl benzene, but-2-
ynoic acid ester, as well as 1-morpholinobut-2-yn-1-one, all
resulted in the formation of the corresponding products (5ai−
al) with exclusive regioselectivity, thus affording a good
complement to previously reported transition-metal-catalyzed
examples11,13 for specific construction of isoquinolones bearing
different substituents. To better define the substrate scope, we
further explored the other alkynes such as those bearing the
bulky dialkyl group as the substituent as well as terminal and
liner alkynes (e.g., 1-hexyne, ethyl propiolate, and 4-octyne).
However, no expected annulation products were observed,
indicating that the size, type, and nature of the alkyne substrate
played a vital role in determining the outcome of the reaction.
Having established the osmium(II)-catalyzed [4 + 2]
annulation of N-methoxybenzamides with alkynes for the
construction of diverse isoquinolones, we next made an effort
to disclose other reaction modes for further evaluating the
B
DOI: 10.1021/acs.orglett.9b03827
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Letter
a
First, the N−H and C−H activations occurred via a concerted
metalation−deprotonation (CMD) mechanism,14 through
transition states TS-1 and TS-2, respectively, to yield the
five-membered osmacycle complex INT-4 with an overall
energy barrier of 22.0 kcal/mol (from Cat. to TS-2) (see
Figure S1 in the Supporting Information for detailed DFT
calculations). It was worth emphasizing that the subsequent
alkyne coordination and migratory insertion into the C−Os
rather than the N−Os bond were involved (ΔG⧧ = 26.0 kcal/
mol for TS-3 vs ΔG⧧ = 33.1 kcal for TS-3′) to generate INT-6.
From then on, a relatively high energy barrier of 24.5 kcal/mol
(from INT-6 to TS-4′) was involved for the following C−N
bond reductive elimination process, which had been proved as
the key step in precedent transition-metal-catalyzed C−H/N−
H annulation of N-methoxybenzamides.10−12,15 Alternatively,
an oxidative addition of osmium into the N−O bond
proceeded via TS-4 (ΔG⧧ = 14.0 kcal/mol) with the assistance
of HOAc,16 producing the osmium(IV) species INT-7 with a
free energy of −7.6 kcal/mol. Thereafter, the facile reductive
elimination occurred via the transition states TS-5 and TS-6
with the free energy of −2.7 kcal/mol and −3.3 kcal/mol,
respectively, followed by the protonlysis and the ligand
exchange in the presence of HOAc to deliver the final
isoquinolone product along with extrusion of the active
osmium(II) catalyst. Of note, our DFT calculations also
showed that the absence of HOAc resulted in a high energy
barrier of 16.8 kcal/mol (from INT-6 to TS-4″) for the
oxidative addition process, implying HOAc was crucial for this
transformation, which was consistent with our experimental
observation that the use of HOAc as an additive obviously
improved the reaction efficiency. Besides, following INT-7, a
sequential HOAc-assisted coordination (via TS-5′) and
reductive elimination (via TS-6′) were also ruled out since
the relatively high energy profiles (−1.6 kcal/mol for TS-5′ vs
−2.7 kcal/mol for TS-5 and 8.2 kcal/mol for TS-6′ vs −3.3
kcal/mol for TS-6) were involved in this reaction pathway for
the generation of the isoquinolone product.
Scheme 3. Substrate Scope of Alkynes
a
Reaction conditions: 3a (0.24 mmol), 4 (0.2 mmol), [Os(p-
cymene)Cl2]2 (5 mol %), NaOAc (0.5 equiv), and HOAc (1 equiv) in
TFE (0.2 M) at 60 °C for 24 h without exclusion of air or moisture;
b
isolated yields were reported. Performed on a 4.0 mmol scale with
2.5 mol % of [Os(p-cymene)Cl2]2.
application potentials of the developed osmium(II) catalytic
system (Scheme 4). As predicted, N-methyl-N-phenylnitrous
Scheme 4. Divergent C−H Functionalizations under
Osmium(II) Catalysis
Moreover, the alkyne migratory insertion process via TS-3
was detected in the whole favorable catalytic cycle, suggesting
that it should act as the turnover limiting step for this
osmium(II)-catalyzed [4 + 2] annulation.
To further gain more insight into the reaction mechanism
and validate computational results, a set of experimental
investigations were then carried out (Scheme 5). Deuterium-
labeling experiments in the presence of CD3OD resulted in no
obvious deuterium incorporation in both recovered N-
methoxybenzamide 3a and the desired isoquinolone product
5aa, revealing an irreversible C−H cleavage process for the
developed osmium(II)-catalyzed annulation (Scheme 5a).
Moreover, kinetic isotope studies from a competitive experi-
ment and two parallel, side-by-side reactions probed primary
KIE values of 1.3 and 1.6, respectively, indicating that the C−
H bond cleavage might not be involved in the turnover limiting
step,17 which was in good agreement with the DFT calculation
results involving a relatively low energy barrier for the C−H
cleavage step compared with the alkyne migratory insertion
process (Scheme 5b). When N-methyl-substituted 3a was
subjected to the osmium(II)-catalyzed C−H annulation with
4a, no reaction occurred (Scheme 5c, left), revealing that the
N−H bond of NH−OMe was indispensable for this trans-
formation. Meanwhile, the treatment of N-acetoxybenzamide
with 4a under the standard conditions resulted in the
formation of benzamide rather than the desired isoquinolone
amide and oxime readily participated in the coupling with
diphenylacetylene 4a under the osmium(II)-catalyzed con-
ditions, delivering the corresponding indole derivative 6 and
isoquinoline product 7. Besides, the osmium(II)-catalyzed C−
H alkynylation or [4 + 3] annulation of N-methoxybenzamide
3a with different coupling partners were also achieved to give
the corresponding products 8 and 9 in synthetically useful
yields. Taken together, these results further illustrated the
remarkable robustness and versatility of the developed
osmium(II)-catalyzed system in mediating divergent C−H
functionalization reactions.
Given the unique features of the novel osmium(II) catalyst
in mediating the classic C−H activation/[4 + 2] annulation,
we became intrigued to elucidate the reaction mechanism. As
shown in Figure 1, DFT calculations were performed by
selecting Os(p-cymene)(OAc)2 (Cat.) as the starting point.
C
DOI: 10.1021/acs.orglett.9b03827
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Organic Letters
Letter
Figure 1. Computed Gibbs free energy changes of the reaction pathways for alkyne insertion, oxidative addition, reductive elimination, and
protonlysis.
Scheme 5. Experimental Mechanistic Studies
N-methoxybenzamides with alkynes for one-pot assembly of
isoquinolones. DFT calculations revealed that an active
osmium(IV) intermediate could be facilely formed through
HOAc-assisted oxidative addition of osmium(II) into the N−
O bond, suggesting an Os(II)−Os(IV)−Os(II) catalytic
pathway for such transformation. Further exploration on
different C−H activation reaction modes mediated by the
osmium(II) catalyst has also been successfully exemplified to
deliver either linear or cyclic frameworks including 2-
alkynylbenzamide, five-membered indole, six-membered iso-
quinoline, and seven-membered 1H-benzo[c]azepine-1,3(2H)-
dione skeletons, thereby illustrating the remarkable robustness
and versatility of the disclosed osmium(II)-catalyzed system.
Further studies on the scope, mechanism, and application of
this catalytic system to explore the new C−H activation mode
are in progress.
ASSOCIATED CONTENT
* Supporting Information
■
S
product (Scheme 5c, right), which not only proved that such a
transformation should be via an oxidative addition process of
osmium into the N−O bond to result in the cleavage of N−O
bond but also revealed that the alkyne coordination and
migratory insertion should occur before the oxidative addition
of osmium into the N−O bond.
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b03827.
Experimental procedures, characterization of products,
1
computational details, and copies of H and 13C NMR
spectra (PDF)
On the basis of both DFT calculations and experimental
mechanistic studies, a tandem C−H activation, alkyne
migratory insertion, oxidative addition, reductive elimination,
and protonolysis process involving the redox-neutral HOAc-
assisted Os(II)−Os(IV)−Os(II) pathway is deduced (see
Scheme S1 in the Supporting Information for the detailed
mechanistic proposal). However, more studies are still needed
to unambiguously verify the proposed mechanism.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: yiwei@gzhmu.edu.cn.
*E-mail: zhouzhi@gzhmu.edu.cn.
ORCID
Huiying Xu: 0000-0001-9025-4183
Hui Gao: 0000-0002-8736-4485
In summary, we have developed the first osmium(II)-
catalyzed redox-neutral C−H activation/[4 + 2] annulation of
D
DOI: 10.1021/acs.orglett.9b03827
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Organic Letters
Letter
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Notes
The authors declare no competing financial interest.
(9) For examples of Os-containing complexes in catalysis since 2016,
́
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■
We thank NSFC (21877020, 81502909, and 21603279) and
Guangdong Natural Science Funds for Distinguished Young
Scholar (2017A030306031) for financial support of this study.
̌
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DOI: 10.1021/acs.orglett.9b03827
Org. Lett. XXXX, XXX, XXX−XXX