Merging two functions in a single Rh catalyst system: Bimodular conjugate for light-induced oxidative coupling

Jinwoo Kim, Dongwook Kim, and Sukbok Chang*

Communication

Merging Two Functions in a Single Rh Catalyst System: Bimodular

Conjugate for Light-Induced Oxidative Coupling

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sı Supporting Information

III

ABSTRACT: A single molecular rhodium catalyst system (PC2-Cp Rh ) bearing two functional domains for both

photosensitization and C−H carbometalation was designed to enable an intramolecular redox process. The hypothesized charge-

•

−

transfer species (PC2 -Cp Rh ) was characterized by spectroscopic and electrochemical analyses. This photoinduced internal

oxidation allows a facile access to the triplet state of the key post-transmetalation intermediate that readily undergoes C−C bond-

forming reductive elimination with a lower activation barrier than in its singlet state, thus enabling catalytic C−H arylation and

methylation processes.

ooperative ligand participation in transition metal catalysis

has signiﬁcantly expanded the scope of accessible chemical

Scheme 1. Facilitation of Transition Metal Catalysis with

Redox-Active Ligand System

transformations either by modulating electron conﬁguration of

the metal center or via ligand−substrate interactions. In the

former case, redox noninnocent ligands, serving as an electron

reservoir, are known to confer versatile catalytic reactivity by

tuning the metal oxidation state. This internal redox process

often enables key elementary steps to operate readily, such as

oxidative addition (OA), reductive elimination (RE), or

hydrogen atom transfer, which would be otherwise challenging

(Scheme 1A). Oxidative inducement of the reductive elimi-

nation process, in particular, has been of great interest as it

constitutes a product-releasing step, which is often rate-limiting

in a catalytic cycle. This mechanistic consideration has been

more pronounced for the ﬁrst row metal complexes where the

frontier orbitals of certain ligands are in similar levels to those of

metals.

In this context, light-induced metal-to-ligand charge transfer

MLCT) has emerged as a powerful tool to access higher

(

oxidation states of the metal center in catalytic reactions

Scheme 1B, up). Recent studies demonstrated that, in addition

to the more conventional outer-sphere electron transfer in

(

photoredox catalysis, oxidation of the metallic core via MLCT

enables a targeted elementary step such as reductive

elimination or ligand transfer. An alternative way for

intramolecular modulation of the metal oxidation state was

envisioned to covalently ligate a redox-active photosensitizer

domain to the catalyst core (Scheme 1B, down). In this

system, upon irradiation, the excited photosensitizer moiety

induces an internal redox process through the intramolecular

quenching. While these charge-transfer (CT) catalyst systems

have been elegantly utilized in proton/hydride transfer and CO₂

reduction processes, their application toward organic synthesis

is still underdeveloped. To our best knowledge, for example,

catalytic C−H functionalization through CT-induced oxidative

coupling has not been reported.

Received: September 18, 2020

In this study, we disclose for the ﬁrst time a single molecular,

bimodular rhodium photocatalyst system for C−H arylation and

XXXX American Chemical Society

Journal of the American Chemical Society

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

electron oxidation of the rhodium metal center, being consistent

Communication

methylation via photoinduced oxidative coupling (Scheme 1C).

The working mode of the synthesized dual-functioning

conjugate catalysts was veriﬁed by spectroscopic and electro-

chemical analysis. The formation of high-valent charge-transfer

species was found to allow for the CT-induced C−C bond-

forming reductive elimination from the key post-transmetalation

intermediate by reducing the RE energy barrier.

On the basis of our previous result that C−C bond-forming

reductive elimination from a transmetalated intermediate can be

16a,7d,e,16b

permitted by the external chemical oxidants,

we ﬁrst

questioned whether the same strategy would also be operative

under photoredox conditions (Figure 1A). In the presence of

N,N′-bis(2,6-diisopropylphenyl)perylene diimide as a photo-

catalyst (PC1), compact ﬂuorescent lamp (CFL)-irradiation on

each solution of isolated rhodacycle species Cp*RhBQ-Ar and

Cp*RhBQ-Me aﬀorded reductive elimination products 1 and

in 95% and 87% yield, respectively (Figure 1, eq 1). In stark

contrast, no product was formed under thermal (80 °C) or

irradiation conditions in the absence of PC1, suggesting that the

process is mediated by the excited photocatalyst species. We

were pleased to see that this PC1 photoredox system was also

operative in the Cp*Rh-catalyzed sp C−H arylation and

methylation (Figure 1, eq 2). For instance, benzo[h]quinoline 3

was readily coupled with aryl and methyl boronic ester providing

C-10 arylation and methylation products 1 and 2, respectively,

using Cu transmetalation agent and (tBuO) as a mild terminal

oxidant.

It should be noted that the ﬂuorescence emission of PC1 did

not overlap with the absorption bands of the rhodacycle

Cp*RhBQ-Ar and Cp*RhBQ-Me (Figure 1B), thereby ruling

out a dipole−dipole energy transfer mechanism for the above

RE. The triplet energy of PC1 (1.11 eV) was calculated to be

signiﬁcantly lower than that of Cp*RhBQ-Ar and Cp*RhBQ-

Me (1.42 and 1.43 eV, respectively), suggesting that the RE

process is induced by a photocatalytic oxidation, rather than the

triplet sensitization of the intermediates (Figure 1C). Indeed,

the RE energy barrier from Cp*RhBQ-Ar and Cp*RhBQ-Me

was calculated to decrease substantially from 26.7 and 28.5 kcal/

mol to 6.8 and 8.6 kcal/mol, respectively, upon the single

Figure 1. (A) Photocatalytic C−C bond forming RE from rhodacycle

intermediates and its application to the catalytic C −H functionaliza-

tions. For reaction conditions, see the Supporting Information. (B)

Absorption spectrum of Cp*RhBQ-Ar and Cp*RhBQ-Me, depicted

with the emission spectrum of PC1. (C) Comparison of triplet energy

of PC1 and the rhodacycle species. S, singlet state; T, triplet state.

Figure 2. (A) Preparation of a series of Rh metallophotocatalyst bearing a mesityl acridinium (PC2) photosensitizer domain. (a) Mesityl lithium (2.0

equiv), TMEDA, Et O, −30 to 25 °C, 26 h, then nBuLi (1.2 equiv), −30 to 25 °C, 3 h, then 2,3,4,5-tetramethyl-2-cyclopentenone (4.0 equiv), 25 °C,

2 h; (b) Na CO , H O, EtOAc then H O, HClO , CH Cl , 25 °C, 1 h, 60% in two steps; (c) RhCl ·xH O (1.0 equiv Rh), iPrOH, 90 °C, 14 h, 89%;

2 3 2 2 4 2 2 3 2

(

d) AgOTf (6.0 equiv), CH Cl , 25 °C, 14 h, 71%; (e) CH C H , CH NO , AgSbF (6.0 equiv), 25 °C, 14 h, 54%; (f) 3, AgO CCF , Li CO , CH Cl ,

2 2 3 6 5 3 2 6 2 3 2 3 2 2

5 °C, 21 h then 40 °C, 4 h, 65%. (B) Single crystal X-ray diﬀraction structure of [PC2-Cp RhCl ] and PC2-Cp RhTol. 10% thermal ellipsoids for

2 2

both structures. Hydrogen atoms were omitted for clarity.

Journal of the American Chemical Society

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Scheme 2. Light-Induced RE Process from PC2-Cp Rh

Catalyst System and Its Application to the Catalytic C−H

Functionalizations

Reaction conditions: 4 (2.0 equiv), tBuOK (10 mol %), PC2-

Cp RhTol (5 mol %), (CuOTf) (C H ) (5 mol %), and (tBuO)

(

2.0 equiv) under CFL (20 W) irradiation. Reaction conditions: 5

2.0 equiv), tBuOK (10 mol %), PC2-Cp RhOTf (5 mol %), CuOAc

10 mol %), and (tBuO)₂(2.0 equiv) under 440 nm (20 W)

irradiation.

dienyl ring, where the phenylene linker was expected to bridge

the two conjugate systems, starting from a commercially

available acridone derivative 6 (Figure 2A). Complexation of

this ligand to a Rh precursor aﬀorded a dimeric rhodium species

[

PC2-Cp RhCl ] (Cp = −C Me ) in high yield, and its

structure was conﬁrmed by a single crystal X-ray diﬀraction

analysis to show that the phenyl group is staggered (61°) with

respect to the cyclopentadienyl ring (Figure 2B). The dimeric

precursor [PC2-Cp RhCl ] was then treated with silver salts to

obtain monomeric rhodium complexes PC2-Cp RhOTf and

PC2-Cp RhTol, and the X-ray crystallographic structure of the

latter was determined, displaying η -coordination mode of the

toluene ligand on the rhodium center.

A rhodacycle complex PC2-Cp RhBQ was also prepared by a

facile carbometalation of benzo[h]quinoline 3 to [PC2-

Figure 3. (A) UV−vis absorption (left) and ﬂuorescence emission

(

right) spectrum of PC2-Cp RhBQ, represented with that of control

species. 440 nm wavelength was used for the excitation for the emission

spectra. (B) Cyclic voltammogram of PC2-Cp RhBQ in THF-d at 50

Cp RhCl ] , and the ipso carbon was characterized by observing

mV/s. (C) Experimental (black) X-band EPR spectrum of PC2-

a doublet signal in C NMR spectrum (δ 175.0 ppm, J

Cp RhBQ and PC2-Cp RhOTf in CH Cl at 100 K and simulated

Rh−C

4.1 Hz in CD Cl ). Additional spectroscopic and electro-

(

blue) EPR spectrum for each case.

2 2

chemical analyses on PC2-Cp RhBQ were performed in

comparison with the photosensitizer segment (PC2-H) as

well as the parent rhodacycle Cp*RhBQ (Figure 3A). An

absorbance at 362 nm, which corresponds to the phenyl (π) to

acridinium (π*) transition in PC2-H, was decreased in the PC2-

with the mild reaction conditions (see the Supporting

Information for details).

7d,e

Encouraged by the above initial results that the product-

delivering oxidatively induced reductive elimination is enabled

by the external photoredox catalyst, we were challenged to

design a bimodular rhodium photocatalyst system bearing two

functional domains in a single molecule, thus capable of both

photosensitization and C−H functionalization. In this context,

we envisioned to covalently link a photosensitizer to a

synthetically tunable cyclopentadienyl ligand to induce an

internal oxidation of the metal center upon irradiation, thus

eventually permitting the oxidatively induced RE process. To

this end, we readily synthesized a ligand 8 bearing a 10-mesityl-

Cp RhBQ conjugate, while that at 429 nm remained

unchanged, which was ascribed to the mesityl (π) to acridinium

(π*) transition. In addition, phosphorescence at 509 nm of

PC2-H was almost completely suppressed in PC2-Cp RhBQ,

implying that an intramolecular quenching takes place

dominantly for the relaxation.

14a

On the other hand, the cyclic voltammogram of PC2-

Cp RhBQ showed one unexpected irreversible oxidation peak

at E_p_a= 0.010 V (50 mV/s, vs Fc/Fc ) in addition to the

anticipated signals assignable to photocatalyst redox pair

([PC2 /PC2]) and Rh /Rh oxidation, even without

•

−

III

-phenylacridinium (PC2) moiety linked to the cyclopenta-

Journal of the American Chemical Society

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

https://pubs.acs.org/doi/10.1021/jacs.0c09982.

Communication

Figure 4. DFT calculation studies on the hypothetical PC2-Cp RhBQ-Ar species. ε = 7.4257 for tetrahydrofuran. (A) Schematic diagram for the light-

induced internal charge transfer process and Kohn−Sham diagram of the frontier orbitals for PC2-Cp RhBQ-Ar in singlet and triplet ground state.

Isovalue = 0.1. (B) The activation barriers for reductive elimination from singlet (blue) and triplet (red) states of PC2-Cp RhBQ-Ar, depicted with the

transition state geometry. For the methyl counterpart, see the Supporting Information .

irradiation (Figure 3B). The signal at E = 0.010 V was assigned

diagram for the singlet ground state represents metal-centered

occupied frontier orbitals (HOMO and HOMO−1) and

acridine (π*)-centered LUMO (Figure 4A). On the other

to the oxidation arising from the metal-to-photosensitizer

•

−

charge-transfer state, PC2 /Rh , which is assumed to present

III

in equilibrium with the ground state, PC2/Rh . Indeed, the X-

band EPR spectrum of PC2-Cp RhBQ displayed signals

hand, in the triplet ground state ([PC2-Cp RhBQ-Ar]*), while

the α-HOMO is located on the acridine π*, the β-LUMO is

mainly composed of nonbonding metal d orbitals which

dominantly comprise metal-centered HOMO−1 of the singlet

state. Furthermore, the RE energy barrier of the postulated PC2-

corresponding to organic radical (g = 2.002) and transition

metal ion (g = 2.053 and 2.30) as well as a high-spin unpaired

electron (g = 4.30, Figure 3C). The line broadening of the

organic radical signal can be ascribed to the hyperﬁne coupling

by the acridine nitrogen, where the α-HOMO of the triplet

rhodacycle species is distributed (vide infra). Furthermore, the

Cp RhBQ-Ar intermediate decreases from 27.0 to 6.0 kcal/mol

upon the charge transfer, implying that the triplet excitation in

the photosensitizer domain will result in an internal oxidation to

form a transient Rh species, from which the C−C forming RE

hyperﬁne coupling by the Rh metal center was detected in the

process readily takes place (Figure 4B). A similar tendency was

metal-centered rhombic resonance of PC2-Cp RhOTf (g =

also observed for the methyl analogue PC2-Cp RhBQ-Me,

.047), indicating the transient formation of a high-valent Rh

where the activation energy of RE was also substantially reduced

from 30.0 to 11.1 kcal/mol upon the charge transfer (see the

Supporting Information for details).

species by the photosensitizer moiety (PC2).

Having found that the photosensitizer domain can easily

access the rhodium valence electrons, we envisaged that the

In conclusion, a single molecular rhodium catalyst system

bearing two functional domains for photosensitization and C−H

carbometalation was successfully developed for the ﬁrst time. It

was shown that the excited photosensitizer moiety generates a

PC2-Cp Rh-mediated C−C bond-forming RE process could be

promoted by the action of light-excited chromophore domain

(

PC2*) through an internal oxidation of the metal center.

Indeed, when a solution of PC2-Cp RhBQ was irradiated in the

presence of aryl- or methyl borate (4 and 5) and Cu(I)

transmetalation agent, the desired reductive elimination

products (1 and 2, respectively) were obtained in good yields

transient high-valent Rh intermediate through an internal

oxidation, thus facilitating the key C−C bond-forming reductive

elimination process in the oxidative C−H arylation and

methylation reactions.

(Scheme 2A). However, reactions in dark aﬀorded products in

less than 10% yields, again suggesting that this process is

facilitated by the light-induced internal charge transfer. More-

over, catalytic C−H functionalization by using PC2-Cp Rh

catalyst system was also successfully performed under irradiation

conditions (Scheme 2B).

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

■

Finally, DFT calculations on the postulated intermediate

PC2-Cp RhBQ-Ar were carried out for better understanding of

the light-induced RE process. The frontier molecular orbital

Experimental procedures; characterization data; spectra

for all new compounds; Cartesian coordinates of all

computed structures (PDF)

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

P.; Vicic, D. A. Ligand Redox Effects in the Synthesis, Electronic

Communication

Crystallographic data for Cp*RhBQ-Ar (CIF )

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Structure, and Reactivity of an Alkyl −Alkyl Cross-Coupling Catalyst. J.

Am. Chem. Soc. 2006, 128, 13175−13183.

(

Crystallographic data for [PC2-Cp RhCl ] (CIF)

Crystallographic data for PC2-Cp RhTol (CIF)

AUTHOR INFORMATION

Corresponding Author

Sukbok Chang − Center for Catalytic Hydrocarbon

Functionalization, Institute for Basic Science (IBS), Daejeon

■

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4141, South Korea

Dongwook Kim − Center for Catalytic Hydrocarbon

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Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

(f) Roberts, C. C.; Chong, E.; Kampf, J. W.; Canty, A. J.; Ariafard,

This paper is dedicated to Professor Pierre Dixneuf for his

outstanding contributions to organometallic chemistry and

catalysis. This research was supported by the Institute for Basic

Science (IBS-R010-D1) in Republic of Korea. Single crystal X-

ray diﬀraction experiments with synchrotron radiation were

performed at the BL2D-SMC in Pohang Accelerator Labo-

ratory.

A.; Sanford, M. S. Nickel(II/IV) Manifold Enables Room-Temperature

C(sp )−H Functionalization. J. Am. Chem. Soc. 2019, 141, 19513−

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