Ruthenium(IV) Intermediates in C?H Activation/Annulation by Weak O-Coordination

Article abstract of DOI:10.1002/chem.201804734

Ruthenium(IV) complexes were identified as key intermediates of C?H/O?H activations by weak O-coordination. Thus, the annulations of sulfoxonium ylides by benzoic acids provided expedient access to diversely-decorated isocoumarins with ample scope. Detailed experimental and computational studies provided strong support for a facile BIES-C?H activation, along with cyclometalated ruthenium(IV) intermediates within a versatile ruthenium(II/IV) catalysis regime (BIES=base-assisted internal electrophilic substitution).

Full text of DOI:10.1002/chem.201804734

A Journal of
Accepted Article
Title: Ruthenium(IV) Intermediates in C−H Activation/Annulation by
Weak O-Coordination
Authors: Yu-Feng Liang, Long Yang, Torben Rogge, and Lutz
Ackermann
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Ruthenium(IV) Intermediates in C−H Activation/Annulation by
Weak O-Coordination
Yu-Feng Liang,[+] Long Yang,[+] Torben Rogge,[+] and Lutz Ackermann*
Abstract: Ruthenium(IV) complexes were identified as key
intermediates of C–H/O–H activations by weak O-coordination. Thus,
the annulations of sulfoxonium ylides by benzoic acids provided
expedient access to diversely-decorated isocoumarins with ample
scope. Detailed experimental and computational studies provided
strong support for
a facile BIES-C–H activation, along with
cyclometalated ruthenium(IV) intermediates within
ruthenium(II/IV) catalysis regime.
a versatile
The mechanistic understanding of the fundamental aspects on
C–H activation[1] is paramount to the development of
synthetically useful C–H functionalization methods.[2] In recent
years, ruthenium(II)-catalyzed C–H activation[3,4] has emerged
as a particularly transformative tool in molecular syntheses, with
key applications to drug discovery[5] and agrochemistry,[6] among
others. Despite major advances, detailed mechanistic insights
into the relevance of ruthenium(IV) intermediates in C–H
activation processes continue to be limited,[7] which strongly
contrasts with recent findings on rhodium(V)[8] and iridium(V)[9]
C–H activation. Within our program on sustainable C–H
activation,[10] we have now identified a novel ruthenium(IV)
manifold for C–H/O–H activation/annulations by weak O-
coordination[11] with easily accessible sulfoxonium ylides[12]
(Figure 1). Salient features of our findings include 1) the
ruthenium-catalyzed weak O-coordination C–H/O–H activation
under redox-neutral conditions, 2) outstanding C–H activation
selectivities, and 3) efficient synthesis of isocoumarins devoid of
4-substituents, which are omnipresent in pharmaceuticals and
natural products,[13] but are not accessible by oxidative alkyne
annulation.[14] In stark contrast to rhodium or iridium catalysis
requiring strongly-coordinating N-containing directing groups,[15]
only the ruthenium(II/IV) manifold enabled C–H activation via
weak O-coordination, a strong testament to the unique power of
Figure 1. Ruthenium(II/IV)-catalyzed C−H activation by weak O-coordination.
We commenced our studies by probing the envisioned
C−H/O−H functionalization of weakly O-coordinating benzoic
acid 1a by sulfoxonium ylide 2a with [RuCl2(p-cymene)]2/AgSbF6
as the catalyst (see Table 1 and Table S1 in the Supporting
Information).[16] A base additive was found essential for the
desired C–H activation, with NEt3 identified as being optimal
(entries 1-7). Interestingly, different ruthenium(II)biscarboxylates
could be employed in the absence of silver salts, highlighting the
importance of carboxylate assistance (entries 8-10).[2] In contrast,
frequently used rhodium, iridium and cobalt complexes fell short
in delivering the desired product 3aa under otherwise identical
conditions (entries 11-14).
Table 1. Optimization of reaction conditions.[a]
sustainable
ruthenium(II/IV)
catalysis.
Importantly,
we
furthermore disclose key mechanistic insights into crucial
ruthenium(IV) intermediates, including experimental and
computational studies.
Entry
1
Cat.
Additive
—
Yield of 3aa [%][b]
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
Ru(OAc)2(p-cymene)
Ru(O2CMes)2(p-cymene)
—
---
2
NaOAc
Na2CO3
KOtBu
NEt3
30
3
51
4
23
5
76, 75[c]
70 (80 °C)
42 (60 °C)
33[d]
65[d,e]
63[d,e]
---
6
NEt3
7
NEt3
8
NEt3
9
NEt3
10
11
12
13
14
NEt3
NEt3
[*]
Dr. Y.-F. Liang, L. Yang, T. Rogge, Prof. Dr. L. Ackermann
Institut für Organische und Biomolekulare Chemie
Georg-August-Universität Göttingen
[Cp*RhCl2]2
NEt3
13
[Cp*IrCl2]2
NEt3
<3%
---
Tammannstraße 2, 37077 Gottingen (Germany)
E-mail: Lutz.Ackermann@chemie.uni-goettingen.de
Homepage: http://www.ackermann.chemie.uni-goettingen.de/
Cp*Co(CO)I2
NEt3
[a] Reaction conditions: 1a (0.25 mmol), 2a (0.38 mmol), cat. (2.5 mol %),
AgSbF6 (10 mol %), additive (0.50 mmol), DCE (2.0 mL), at 100 °C for 16 h.
[b] Yield of isolated products. [c] DMSO (0.25 mmol). [d] Without AgSbF6. [e]
5.0 mol % of catalyst.
[+]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
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18 kcal mol-1 (Figure 2). Thereafter, ruthenium(IV) complex C
undergoes facile migratory insertion with a barrier of only ca.
7 kcal mol-1.
With the optimized ruthenium catalyst for weak O-coordination
C−H activation in hand, we evaluated the versatility of the redox-
neutral annulation (Scheme 1). Hence, benzoic acids 1 featuring
ortho-substituents smoothly reacted with sulfoxonium ylides 2.
For meta- and para-substituted arenes 1, mono-C−H
functionalization products 3 were selectively obtained, when
employing Zn(OAc)2 as the Lewis-acid additive.
Scheme 2. Double C–H functionlization with sulfoxonium ylides 2. [a] Ratio of
isomers.
Scheme 1. Ruthenium-catalyzed C–H/O–H annulation with sulfoxonium ylides
2. [a] Zn(OAc)2 (50 mol %).
Interestingly, the double C−H functionalization products 4 were
selectivity generated from meta- and para-substituted benzoic
acids 1 by the judicious choice of the stoichiometry. Thereby, the
versatile ruthenium catalysis manifold proved amenable to both
electron-rich as well as electron-deficient arenes 1 (Scheme 2).
Valuable electrophilic functional groups were fully tolerated,
including bromo, chloro, and ester substituents. Likewise, aryl-
substituted sulfoxonium ylides 2a-e bearing electron-donating or
-withdrawing groups delivered the desired products. Alkyl
sulfoxonium ylides 2g-h proved to be viable substrates likewise.
The connectivity of the products
4 were unambiguously
established by single-crystal X-ray diffraction analysis.[17]
Figure 2. Calculated Gibbs free energy profile for the formation of the key
ruthenium(IV) intermediate at the PBE0-D3(BJ)/def2-QZVP+SMD(DCE) level
of theory.
Given the unique reactivity of the ruthenium(II) catalyst, we
became attracted to delineating its mode of action. To this end,
first density functional theory (DFT) calculations were performed.
Optimization at the PBE0-D3(BJ)/def2-SVP level of theory and
subsequent single point calculations at the PBE0-D3(BJ)/def2-
QZVP+SMD(DCE) level of theory[18] revealed the formation of
the key ruthenium(IV) intermediate C to proceed via C–S bond
cleavage with an activation energy barrier of less than
The electronic nature of the crucial ruthenium(IV) intermediate C
was further studied by means of bond order analysis. Thus, our
analysis was suggestive of a Ru–C double bond character,
strongly supporting a ruthenium(IV) intermediate (Figure 3).[16]
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This observation can be rationalized by a base-assisted internal
electrophilic substitution (BIES),[19] rather than a concerted
metallation-deprotonation (CMD) process.[20] In good agreement
with this hypothesis, we observed significant H/D scrambling
(Scheme 3c) as well as a negligible kinetic isotope effect
(Scheme 3d). Kinetic analysis by in-operando infrared-
spectroscopy did not reveal a significant induction period, but
rather a sigmoidal curve (Scheme 3e).
Further, the mono-C−H activation product 3ga failed to deliver
double C–H activation product 4ga under otherwise identical
conditions, highlighting the importance of carboxylate assistance
(Scheme 4a). The independently prepared substituted benzoic
acid 5 was converted to product 3fa, while the cyclization was
facilitated by Zn(OAc)2 (Scheme 4b). Subsequently, we tested
the stoichiometric reaction of cyclometalated complexes 6k[15b]
with sulfoxonium ylide 2f. Intriguingly, thereby ruthenacycle 7kf
was obtained, the structure of which was unambiguously verified
by X-ray single crystal diffraction.[17] Moreover, the protonation of
7kf occurred readily to give the product 3kf (Scheme 4c).
Figure 3. Calculated structure of ruthenium(IV) intermediate C. Non-
participating hydrogens are omitted for clarity.
Moreover, we conducted intermolecular competition experiments
between differently substituted benzoic acids 1, revealing
electron-rich derivatives to react preferentially (Scheme 3a-b).
Scheme 4. Studies on potential reaction intermediates.
On the basis of our computational and experimental mechanistic
studies, a plausible catalytic cycle was proposed in Scheme 5.
The cyclometalated intermediate
6
is coordinated by
sulfoxonium ylide 2, leading to ruthenium(II) species B. This
intermediate B is transformed into key ruthenium(IV) species C
by α-elimination of DMSO. Insertion within the coordination
sphere of ruthenium(IV) species occurs next to generate six-
membered ruthenacycle D. Finally, proto-demetalation affords
the acyclic compound 5, which undergoes Lewis acid-mediated
cyclization to product 3.
Scheme 3. Key mechanistic findings.
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Scheme 5. Proposed ruthenium(II/IV) manifold.
In summary, we have unraveled ruthenium(IV) complexes as
key intermediates in C–H/O–H activation by weak O-
coordiantion. Thus, easily-accessible sulfoxonium ylides enabled
the versatile C–H activation towards isocomarines of relevance
to bioactive natural products and drug discovery. Detailed
mechanistic studies by experiment and computation identified
key ruthenium(II) and ruthenium(IV) intermediates, thus
providing proof-of-concept for a robust C–H activation/annulation
in terms of a ruthenium(II/IV) catalysis manifold.
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Generous support by the Alexander von Humboldt Foundation
(fellowship to Y.L.), the CSC (fellowship to L.Y.), and the DFG
(Gottfried-Wilhelm-Leibniz prize and SPP1807) is gratefully
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The authors declare no conflict of interest.
Keywords: C−H activation • mechanism • ruthenium(IV) •
sulfoxonium ylide • weak coordination
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Yu-Feng Liang, Long Yang, Torben
Rogge, and Lutz Ackermann*
Page No. – Page No.
Ruthenium(IV) Intermediates in C−H
Activation/Annulation by Weak O-
Coordination
Text for Table of Contents
Ru(IV) for O: Versatile C–H/O–H functionalizations by weak O-coordination were accomplished with sulfoxonium
ylides through a ruthenium(II/IV) manifold.
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