Imposing Latency in Ruthenium Sulfoxide-Chelated Benzylidenes: Expanding Opportunities for Thermal and Photoactivation in Olefin Metathesis

Gal Segalovich-Gerendash, Illya Rozenberg, Nebal Alassad, Noy B. Nechmad, Israel Goldberg,

Research Article

Imposing Latency in Ruthenium Sulfoxide-Chelated Benzylidenes:

Expanding Opportunities for Thermal and Photoactivation in Oleﬁn

Metathesis

Sebastian Kozuch, and N. Gabriel Lemco ﬀ *

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sı Supporting Information

ABSTRACT: Herein we show the design and synthesis of an

electron-rich, sulfoxide-chelated, ruthenium benzylidene. In contrast

to previously reported sulfoxide-chelated ruthenium benzylidenes,

this complex is more stable in a cis-dichloro conformation and is thus

latent in typical oleﬁn metathesis reactions. The complex was

characterized by NMR, UV−vis, and X-ray spectroscopy, alongside

density functional theory computations. The latent precatalyst could

be activated thermally and, depending on the solvent, by UV−C or

visible light. In addition, an original “thermo-chromatic” orthogonal

sequence was developed, further improved by the use of a thioether

chelated complex, where a divergent two-step synthesis can lead to a

dihydrofuran or a dihydropyran depending only on the order by

which the diﬀerent stimuli, heat or light, are applied.

KEYWORDS: oleﬁn metathesis, ruthenium, sulfoxide, photoinduced, sulfur-chelated

INTRODUCTION

temperature. These complexes are more stable in the active

trans-dichloro conﬁguration probably because the chelating

sulfoxide moiety is a weaker sigma electron donor ligand

■

Oleﬁn metathesis is a powerful synthetic tool in organic

chemistry that allows for the straightforward formation of

carbon−carbon double bonds. The importance of this reaction

has driven an enormous growth in the design and synthesis of

well-deﬁned catalysts, which in turn have given organic

chemists the opportunity for further enrichment and ﬁne-

tuning of selected reaction parameters and synthetic

8b,d

(

compared to the thioether).

It was our hypothesis that

strengthening the electron donation of the sulfoxide could lead

to a change in the geometry of the complex.

Herein, we report the synthesis, characterization, and

catalytic properties of the ﬁrst latent cis-dichloro sulfoxide

chelated ruthenium benzylidene. This was achieved by

increasing the electron density on the sulfur atom by

introducing an electron donating group on the corresponding

aromatic ring. The rich photochemistry of sulfoxide-ruthenium

strategies. Most recent eﬀorts in the ﬁeld of catalyst

diversiﬁcation are focused on the design and modiﬁcation of

the ligand shell around the metal center, resulting in the

improvement of selectivity, activity, or other properties. For

example, a judicious choice of ligands can inhibit the initiation

step, facilitating the design, synthesis, and study of latent

precatalysts that can be activated by external stimuli, such as

complexes provided a novel entry to photoactivation of the

metathesis precatalyst at wavelengths diﬀerent than those used

in the photoactivation of S-chelated complexes. These results

inspired the development of an original thermo-chromatic

orthogonal selective synthesis of ﬁve- or six-membered ring

substrates, depending on the sequence of the applied stimuli.

heat, acid, ultrasound, or light (Figure 1).

While many diﬀerent strategies have been used to achieve

latency, a convenient approach is the use of sulfur chelation.

These complexes exist mainly in a stable and inactive cis-

dichloro conﬁguration that can undergo isomerization to the

reactive trans-dichloro isomer to initiate the oleﬁn metathesis

Received: February 8, 2020

Revised: March 26, 2020

Published: March 27, 2020

reaction. A related family of sulfur bound ruthenium

alkylidene complexes uses sulfoxides as the chelating unit.

In contrast to the thioether-containing complexes, the

sulfoxide chelated complexes appear only in trans-dichloro

conﬁgurations and are consequently all active at room

XXXX American Chemical Society

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Figure 1. A few examples of latent oleﬁn metathesis catalysts.

Scheme 1. Synthesis of Complex 8

RESULTS AND DISCUSSION

an energy diﬀerence of 8.1 kcal/mol); thus, a pathway was

outlined for its synthesis.

The desired complex was synthesized according to Scheme

■

In a most inﬂuential ﬁnding, Grela et al. discovered that adding

a strong electron withdrawing group (EWG), i.e., a nitro

group, to the benzylidene ring in a Hoveyda−Grubbs complex

. Thiocyanation of N,N-dimethylaniline 2, followed by

can signiﬁcantly enhance its catalytic activity. Concomitantly,

reduction of intermediate 3 with LiAlH , aﬀorded thiophenol

it was also shown that an electron donating group (EDG)

4 in good overall yields. 4 was mixed with commercially

available 2-ﬂuorobenzaldehyde in dry DMF in the presence of

potassium carbonate to provide the aromatic nucleophilic

substitution product 5. Then, thioether 5 was oxidized with

hydrogen peroxide in hexaﬂuoroisopropanol (HFIP), followed

by a classical Wittig reaction that produced the desired styrene

ligand 7. Finally, a simple ligand exchange with G-III yielded

complex 8 as the major product with 90% yield.

reduced the activity of the complex.

As previously

mentioned, sulfoxide-chelated ruthenium catalysts are not

latent for typical oleﬁn metathesis reactions at ambient

conditions. Nevertheless, we hypothesized that increasing the

electron density on the sulfur atom could stabilize a cis-

dichloro conﬁguration, leading to a new type of latent complex.

To support this theory, preliminary density functional theory

(

DFT) calculations were conducted on a sulfoxide-chelated

The novel ruthenium complex was fully characterized by H-

ruthenium benzylidene complex containing a dimethylaniline

EDG, 8. Indeed, the computational studies predicted a

signiﬁcant preference for the cis-dichloro conﬁguration (with

and C NMR, HR-MS, UV−vis, and single crystal X-ray

spectroscopy (Figure 2).

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measured and compared to that of complex 10 (Figure 3 ).

Research Article

including implicit solvent with the SMD method in the

single point energies and Gibbs energy corrections with the

geometry optimization method. The computations indicate

−

that compound 8 in its cis form is 8.1 kcal mol more stable

than its trans isomer, supporting the experimental ﬁndings.

However, the DFT results for 9 are conﬂicting, as it also

suggested a cis conformation for this complex (8.2 kcal mol⁻

lower than the trans), in stark contrast to the experimental

outcome. This contradiction between theory and experiment

remains an open question that we will attempt to address in

the near future.

To complete the characterization of 8, its UV−vis

absorption spectrum in methylene chloride (DCM) was

Figure 2. Single-crystal X-ray structure of 8. Ellipsoids are shown at

the 50% probability level. Hydrogen atoms and solvent molecules

were omitted for clarity.

Several insights regarding the structure of 8 could be

attained from its synthesis and characterization. First, the trans-

dichloro complex was observed as the kinetic product, which

quickly isomerized to the cis geometry in solution, mimicking

the behavior observed for the thioether complexes (see

Supporting Information). Moreover, as predicted by compu-

tations, the heteroatom that chelates the ruthenium center is

the sulfur and not the oxygen (the latter being 10.6 kcal/mol

higher; see Supporting Information).

Table 1 compares critical bond lengths between complex 8

and relevant examples of both cis-dichloro and trans-dichloro

Figure 3. UV−vis absorption spectrum of complexes 8 (black) and 10

(red) in DCM.

7g,10b,c

S-chelated complexes from the literature.

Notably, the

Ru−S bond in complex 8 is signiﬁcantly shorter than the Ru−S

bond in cis-dichloro thioether complex 10 (2.258 Å vs 2.347

Å) (and also shorter than the Ru−S bond for a recently

The most salient diﬀerence between the spectra is a strong

absorption band for complex 8 at 303 nm. The same additional

absorption appearing on 8 but not on 10 can be seen in the

computed TD-DFT spectra (see Supporting Information ),

conﬁrming the validity of the computational results.

reported nonchelated DMSO complex), making this the

shortest Ru−S bond to date for S-chelated Ru alkylidenes. In

addition, 8 also portrays the shortest C

all the complexes shown.

All DFT computations were carried out with Gaussian at

the MN15/def2-TZVP(DCM)//MN15/def2-SVP level,

−Ru bond for

benzylidene

15,16

Table 1. Bond Lengths of Complexes 8−11

In parentheses, the computed values.

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Table 2. RCM Reactions Catalyzed by 8 under Diﬀerent Activation Stimuli

Conditions: 2 mol % catalyst loading, 0.1 M substrate in 0.5 mL solvent, irradiation was performed in a Rayonet photoreactor, temperature in the

reactor while running reactions was 30 °C. Conversions were monitored and determined by H NMR (see Supporting Information) (in

parentheses, dark control experiments).

RELATIVE REACTIVITY STUDIES

is possible, it is not an eﬃcient way to activate 8. We then

turned our attention to activation with light. Notably, after 24

h irradiation at 350 nm in DCM, only 10% conversion to the

RCM product was observed by NMR, as well as signiﬁcant

decomposition of the complex. Shorter wavelengths, i.e., 300

and 254 nm did not induce RCM in DCM at all due to

decomposition of the complex. Thus, irradiation with a less

energetic light source, 419 nm, was attempted with the

expectation that this would reduce catalyst decomposition. To

our great satisfaction, 74% RCM conversion could be observed

■

Having structurally characterized complex 8, its activity in the

benchmark ring closing metathesis reaction (RCM) of diethyl

diallylmalonate (DEDAM) was studied. To our satisfaction, 8

was indeed found to be latent for RCM after 24 h at room

temperature both in DCM and toluene (Table 2, entries 1 and

). The complex was then stimulated by two methods: heating

thermal) and UV irradiation (photochemical). Heating the

toluene solution to 80 °C aﬀorded about 30% conversion after

day, highlighting the fact that even though thermal activation

(

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in this case (Table 1, entry 10), making it the ﬁrst ruthenium−

sulfur chelated complex that can be eﬃciently photoactivated

with visible light.

Irradiation with 419 nm light proved to be much less

eﬃcient in other solvents compared to DCM (Table 1, entries

clearly shows that colored impurities in the samples may

hinder catalyst activation within this protocol.

Furthermore, the catalytic activity of precatalyst 8 for the

diﬃcult cross metathesis (CM) with acrylates was also studied.

Under the visible light protocol, CM of 1-octene with methyl

acrylate (1:3) in DCM aﬀorded just 34% conversion. Even

though conversions in this case are relatively low, to the best of

our knowledge this is the ﬁrst example where visible light

photoinduced CM with an acrylate is successful.

1−13). However, the photochemical behavior of 8 in toluene

proved to be quite surprising. The best wavelength for

activation in toluene was found to be 350 nm, where about

0% conversion was achieved after 23 h of irradiation.

One of the most widely used oleﬁn metathesis reactions,

especially when these are photoactivated, is ring-opening

metathesis polymerization (ROMP). Thus, photoinduced

ROMP (PROMP) of dimethyl 5-norbornene-2,3-dicarboxylate

was initiated in toluene by irradiation with 254 nm light and in

methylene chloride with 419 nm light (Table 3). Both these

Nonetheless, the most surprising result was that irradiation

at 254 nm gave fair RCM conversions (Table 1, entry 5),

making this the ﬁrst example of an S-chelated complex that can

be activated for metathesis by irradiation with UV-C. Because

no signiﬁcant diﬀerence could be observed in the computed

absorption spectra of 8 in DCM and toluene, we carried out an

experiment to determine whether aggregation induced

emission in toluene, derived from the lower solubility of the

complex in this solvent, could be the cause for the activity.

Indeed, an additional emission in the UV-A region was

observed when a toluene solution of 8 was irradiated with UV-

C. This phenomenon was not observed when the same

experiment was repeated in DCM (see Supporting Informa-

tion). To further study this interesting phenomenon, the

photoinduced RCM of 1,7-octadiene (OD) with UV-C in

toluene and with visible light in DCM was also studied.

Indeed, the results in toluene were similar for both RCM

reactions. On the other hand, visible light induced RCM of

OD in DCM was even more eﬃcient than the RCM of

DEDAM. Nearly full conversion to cyclohexene was observed

after prolonged irradiation times, disclosing that the precatalyst

can be activated even after days of continuous visible light

irradiation.

Table 3. ROMP of Dimethyl 5-Norbornene-2,3-

dicarboxylate

activation

method

time conversion (

(g/mol)

entry solvent

(h)

PDI

Tol-d₈

UV 254 nm

Vis 419 nm

399,200

580,900

1.4

2.0

DCM-

Conditions: 1 mol % catalyst loading, 0.5 M substrate in solvent,

irradiation was performed in a Rayonet photoreactor. Conversions

were determined by HNMR integration of the oleﬁ n signals.

Determined by triple detector GPC analysis (see Supporting

Information ).

To summarize the photochemical activation of RCM, it was

shown that there is a strong solvent dependency which dictates

which solvent should be used depending on the color of light

used. If UV-C (254 nm) is needed, then toluene should be

used as a solvent, but if irradiation is in the visible range (419

nm), then DCM was shown to be the best solvent for this

reaction.

wavelengths had not been previously used for PROMP with S-

chelated catalysts. Happily, the polymerization in toluene-d₈

proceeded as expected (Table 3 and Supporting Information).

Unfortunately, with DCM-d as the solvent and 419 nm light,

the reaction was much slower and aﬀorded only 35%

conversion after 3 days of irradiation. Notwithstanding the

relatively low conversions, in both cases the photochemical

reactions produced well-deﬁned metathesis polymers that

could be isolated and characterized, expanding the possibilities

of using diﬀerent light sources for PROMP with S-chelated

photolatent catalysts.

A wider scope of photoinduced oleﬁn metathesis reactions

was then probed. The self-metathesis reaction of methyl oleate

was ﬁrst studied. Neat methyl oleate was heated at 100 °C for

4 h in the presence of 1 mol % of 8. GC-MS analysis showed

the expected self-metathesis products with some isomerization

byproducts typically obtained when the reaction is run at high

temperatures. In contrast, when the reaction was carried out

in the visible light protocol with DCM as the solvent, self-

metathesis proceeded without any double bond migration

byproducts. In addition, the useful self-metathesis reaction of

jojoba oil was also considered. Under thermal activation, neat

jojoba oil was consumed, and the expected volatile C18

component (together with the isomerized homologues) could

be readily observed by GC-MS analysis. However, contrary to

the observation with the methyl oleate, the reaction did not

proceed at all when carried out in DCM with visible light

irradiation. The extracted jojoba oil usually possesses a

yellowish tint derived from impurities present in the jojoba

seeds. To our satisfaction, when the jojoba oil was puriﬁed

with activated charcoal to remove the colored impurities, the

visible light protocol with DCM as the solvent eﬃciently

promoted the self-metathesis of this natural product (see

Supporting Information). This example is quite revealing and

REGIOSELECTIVE THERMO-CHROMATIC

ORTHOGONALITY

The fact that 8 could be activated by two diﬀerent wavelengths

■

inspired us to develop a novel chromatic orthogonal

18,19

system.

Speciﬁcally, given that visible light could be used

in a much more eﬀective manner for catalyst activation, we

decided to probe the use of the ubiquitous o-nitrobenzyl

protecting group to help guide the sequence of the

photoinduced reaction.

Thus, the novel substrate 1-((((3R,4R)-4-(allyloxy)hexa-1,5-

dien-3-yl)oxy)methyl)-2-nitrobenzene (14) was prepared from

(3R,4R)-3,4-dihydroxy-1,5-hexadiene (12) (Scheme 2).

First, the conditions needed for the cleavage of the

photolabile protecting group (PPG) in compound 14 were

studied. Indeed, facile photocleavage was conveniently

achieved both in DCM and toluene with 300 nm light.

Unfortunately, irradiation of 14 at 419 nm in the presence of 8

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https://pubs.acs.org/doi/10.1021/acscatal.0c00676.

Research Article

Scheme 2. Synthesis of Substrate 14

Figure 4. Thermo-chromatic orthogonal system. (a) Reaction sequence. (b) GC-MS trace of the isolated product mixture (top, heating followed by

irradiation; bottom, irradiation followed by heating; see Supporting Information for experimental details).

could not induce the desired metathesis reaction. As previously

observed with the jojoba self-metathesis reaction, it appears

that the yellowish color of the solution of compound 14

prevented the eﬃcient photoactivation of precatalyst 8 with

visible light. Therefore, a diﬀerent approach was envisioned,

dubbed the “thermo-chromatic orthogonal system”. In this, the

guiding protecting group can be removed by using light, while

the metathesis reaction can be induced by heat in an

orthogonal fashion. However, when complex 8 was thermally

activated under the reaction conditions, the eﬃciency was

poor, and only about 30% RCM conversion could be obtained.

Fortunately, using complex 10 resulted in a much more

eﬃcient system. Thus, either compound 18 or 19 could be

selectively produced as the major product with good to

excellent selectivity by just applying the diﬀerent stimuli in

opposite order. As shown in Figure 4, when the PPG was

cleaved ﬁrst by irradiation at 254 nm, followed by heat induced

metathesis, a 9:1 selectivity in favor of the dihydropyran was

obtained. This selectivity is due to the lessened steric

hindrance for the formation of the six-membered ring after

the PPG is removed. Notably, when the order was reversed

and heat preceded the light stimulus, the dihydrofuran

compound was obtained as the major product with 4:1

selectivity. Both pathways aﬀorded conversions of about 85%.

The GC-MS traces in the ﬁgure nicely highlight the divergent

selectivity that could be achieved by this simple methodology.

several experimental techniques, such as NMR, UV−vis, and

single crystal X-ray spectroscopy, and computational DFT

analyses. Its oleﬁn metathesis thermal and photochemical

induced activity was studied for several RCM, CM, and ROMP

reactions; showing that 8 can induce oleﬁn metathesis in DCM

when irradiated with visible light and also in toluene when UV-

C is used. Moreover, a novel orthogonal “thermo-chromatic”

system was developed, where either a ﬁve-membered ring or a

six-membered ring can be selectively synthesized depending on

the order of the stimuli applied. The novel interplay of light

activation and thermal activation can lead to important

advances in the ﬁeld of photochemistry, and we are currently

further developing this original methodology.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge at

■

sı

Synthetic procedures and characterization ( H-, ¹³C

NMR, and HRMS), crystallographic data summary for 8,

DFT computational details, NMR spectra of thermal

and photoactivated metathesis and GC-MS analyses

(PDF )

(CIF )

(TXT1 , TXT2 , TXT3, TXT4, TXT5 , TXT6 )

CONCLUSIONS

AUTHOR INFORMATION

Corresponding Author

■

A profound understanding of the factors that are required to

produce latent cis-dichloro ruthenium benzylidenes in S-

chelated systems led to the synthesis of the ﬁrst oleﬁn

metathesis latent cis-dichloro sulfoxide chelated ruthenium

benzylidene, 8. The new complex was fully characterized by

N. Gabriel Lemcoﬀ − Chemistry Department, Ben-Gurion

University of the Negev, Beer-Sheva 8410501, Israel; Ilse Katz

Institute for Nanoscale Science and Technology, Beer-Sheva

8410501, Israel; orcid.org/0000-0003-1254-1149 ;

832

ACS Catal. 2020, 10, 4827−4834

ACS Catalysis

Dabrowski, P.; Skowerski, K. Bis(Cyclic Alkyl Amino Carbene)

Research Article

Phone: (+972) 8646-1196; Email: lemco ﬀ @bgu.ac.il;

Active Catalysts for Ethenolysis. Angew. Chem., Int. Ed. 2015, 54,

919−1923. (b) Gawin, R.; Kozakiewicz, A.; Gunka, P. A.;

Fax: (+972) 8647-1740

Authors

Gal Segalovich-Gerendash − Chemistry Department, Ben-

Gurion University of the Negev, Beer-Sheva 8410501, Israel

Illya Rozenberg − Chemistry Department, Ben-Gurion

University of the Negev, Beer-Sheva 8410501, Israel

Nebal Alassad − Chemistry Department, Ben-Gurion University

of the Negev, Beer-Sheva 8410501, Israel

Ruthenium Complexes: A Versatile, Highly Efficient Tool for Olefin

Metathesis. Angew. Chem., Int. Ed. 2017, 56, 981 −986.

(4) (a) Liu, R.; Ge, H.; Chen, K.; Xue, H. Selectivity in Olefin-

Intervened Macrocyclic Ring-Closing Metathesis. ACS Catal. 2018, 8,

574−5580. (b) Butilkov, D.; Frenklah, A.; Rozenberg, I.; Kozuch, S.;

Lemcoff, N. G. Highly Selective Olefin Metathesis with CAAC-

Containing Ruthenium Benzylidenes. ACS Catal. 2017, 7, 7634−

7637. (c) Sytniczuk, A.; Dabrowski, A.; Banach, L.; Urban, M.;

Noy B. Nechmad − Chemistry Department, Ben-Gurion

University of the Negev, Beer-Sheva 8410501, Israel

Israel Goldberg − School of Chemistry, Tel-Aviv University, Tel-

Aviv 69978, Israel; orcid.org/0000-0002-3117-0534

Sebastian Kozuch − Chemistry Department, Ben-Gurion

University of the Negev, Beer-Sheva 8410501, Israel;

orcid.org/0000-0003-3070-8141

Czarnocka-Sniadala, S.; Milewski, M.; Kajetanowicz, A.; Grela, K. At

Long Last: Olefin Metathesis Macrocyclization at High Concen-

tration. J. Am. Chem. Soc. 2018, 140, 8895−8901. (d) Higman, C. S.;

Nascimento, D. L.; Ireland, B. J.; Audorsch, S.; Bailey, G. A.;

McDonald, R.; Fogg, D. E. Chelate-Assisted Ring-Closing Metathesis:

A Strategy for Accelerating Macrocyclization at Ambient Temper-

atures. J. Am. Chem. Soc. 2018 , 140 , 1604 −1607. (e) Ward, T. R.;

Sabatino, V. Aqueous olefin metathesis: recent developments and

applications. Beilstein J. Org. Chem. 2019, 15, 445−468. (f) Coperet,

C.; Allouche, F.; Chan, K. W.; Conley, M. P.; Delley, M. F.; Fedorov,

A.; Moroz, I. B.; Mougel, V.; Pucino, M.; Searles, K.; Yamamoto, K.;

Zhizhko, P. A. Bridging the Gap between Industrial and Well-Defined

Supported Catalysts. Angew. Chem., Int. Ed. 2018, 57, 6398−6440.

Complete contact information is available at:

https://pubs.acs.org/10.1021/acscatal.0c00676

Funding

The Israel Science Foundation is gratefully acknowledged for

ﬁnancial support.

(5) (a) Slugovc, C.; Perner, B.; Stelzer, F.; Mereiter, K. Second

Generation ” Ruthenium Carbene Complexes with a cis -Dichloro

Arrangement. Organometallics 2004, 23, 3622−3626. (b) Bantreil, X.;

Schmid, T. E.; Randall, R. A. M.; Slawin, A. M. Z.; Cazin, C. S. J.

Mixed N-heterocyclic Carbene/Phosphite Ruthenium Complexes:

Towards a New Generation of Olefin Metathesis Catalysts. Chem.

Commun. 2010 , 46 , 7115 −7117. (c) Piermattei, A.; Karthikeyan, S.;

Sijbesma, P. Activating Catalysts with Mechanical Force. Nat. Chem.

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

Dr. Ravindra Phatake is gratefully acknowledged for technical

assistance in the orthogonal thermo-chromatic and ﬂuores-

cence experiments.

2009 , 1 , 133 −137. (d) Theunissen, C.; Ashley, M. A.; Rovis, T.

Visible-Light-Controlled Ruthenium-Catalyzed Olefin Metathesis. J.

Am. Chem. Soc. 2019, 141, 6791−6796. (e) Pietraszuk, C.; Rogalski,

S.; Powała, B.; Mietkiewski, M.ło.; Kubicki, M.; Spolnik, G.;

Danikiewicz, W.; Wozniak, K.; Pazio, A.; Szadkowska, A.;

Koz ł owska, A.; Grela, K. Ruthenium-Amido Complexes: Synthesis,

Structure, and Catalytic Activity in Olefin Metathesis. Chem. - Eur. J.

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