Unexpected Stability of CO-Coordinated Palladacycle in Bidentate

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Unexpected Stability of CO-Coordinated Palladacycle in Bidentate Auxiliary Directed C(sp3)−H Bond Activation: A Combined Experimental and Computational Study Yi Jiang,†,∥ Shuo-Qing Zhang,‡,∥ Fei Cao,§ Jiao-Xia Zou,† Jing-Lu Yu,‡ Bing-Feng Shi,*,‡ Xin Hong,*,‡ and Zhen Wang*,†,§ †

School of Pharmacy, Lanzhou University, Lanzhou, Gansu 730000, China Department of Chemistry, Zhejiang University, Hangzhou 310027, China § State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000, China

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‡

S Supporting Information *

ABSTRACT: We have thoroughly studied four kinds of different palladacycles with 8-aminoquinoline as the auxiliary and carbon monoxide (CO), acetonitrile (CH3CN), pyridine, and dimethyl sulfoxide (DMSO) as the ligands, respectively, using crystallographic analysis, cyclic voltammetry (CV), stoichiometric reactions, and computational calculations. A higher oxidation potential in CV and lower chemical reactivities to react with weaker electrophilic reagents are presented for CO-coordinated palladacycles, which has been supported by DFT studies that the CO-coordinated Pd complex has the highest ΔG value for the initial ligand exchange process. Meanwhile, a rigid structure can be observed and relatively high activation energy (ΔG⧧ = 54.2 kcal/mol) is needed for further migratory insertion. All this evidence suggests that the CO-coordinated Pd complex is a kinetically and thermodynamically stable intermediate, which accounts for its reluctance for migratory insertion and the formation of the corresponding carbonylation product.

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INTRODUCTION Transition-metal-catalyzed C−H bond activation has emerged as an efficient and powerful protocol in organic synthesis for

one of the most versatile bidentate directing groups and has been widely used in the realm of C−H activation since its first introduction by Daugulis.5 Afterward, within this auxiliary, numerous elegant works on C(sp3)−H functionalization have been revealed by Chen,6 Shi,7 Nakamura,8 Kanai,9 Chatani,10 Ge,11 Maiti,12 etc.13 via Pd, Ni, Co, Fe, and Cu catalysis. In 2017, Shi and co-workers reported a Pd-catalyzed alkoxycarbonylation of a C(sp3)−H bond, employing ethyl chloroformate instead of carbon monoxide as the carbonyl source7h (Scheme 1). Actually, according to the report, COand CH3CN-coordinated palladium complexes were obtained in this transformation first, whereas no further transformation took place for the former to afford the corresponding carbonylation product under various conditions, in contrast to an alkoxycarbonylation product smoothly generated for the latter. The widespread explanation for such a phenomenon is that π-acidic CO affiliated with the metals may restrain the subsequent migratory insertion process.14 However, an explanation for the lack of reactivity remained unclear and intrigued us to investigate this system further. Herein, we report a systematic study on the effect of the CO ligand on the chemical reactivity of palladacycles on the basis of a

Scheme 1. Stoichiometric Reactions of Palladacycles in Shi’s Previous Work7h

decades,1 due to its efficiency in converting the ubiquitous inert C−H bonds to valuable carbon−carbon2 bonds as well as carbon−heteroatom bonds.3 In this context, several directing groups have been developed, which provided good regioselectivities and greatly enriched the scope of C−H activation reactions.4 Among them, 8-aminoquinoline arguably served as © XXXX American Chemical Society

Received: February 10, 2019

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DOI: 10.1021/acs.organomet.9b00087 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

and N-phthaloyl substituted at the tertiary α-carbon were examined as well, and no carbonylation product was obtained, either (2k−o). Meanwhile, no desired outcome was seen for the reaction with a substituent on the quinoline ring (2p). Finally, considering that the reductive elimination process could be boosted by a large steric hindrance group,15 an abietic acid derivative was used in the protocol; However, there was still no carbonylation product provided (2q), which suggests a difficult migratory insertion process of CO from the side face. To confirm the precise structures we obtained, compounds 2a,f,i,k,q were unambiguously determined by X-ray crystallography and the terminal CO ligands were observed by the characteristic infrared absorption (2066−2096 cm−1). Ligand Exchange. On the basis of the above results, to better understand how the CO ligand affects the subsequent conversion of the palladacyclic intermediate, three Pd(II) complexes with different ligands have been prepared, through ligand exchange. All of these ligand-exchanged products were obtained in quantitative yields at room temperature (Scheme 3, complexes 3−5). Complexes 3 and 47h have been isolated and confirmed to be the reactive intermediates in many related reports.5b,7b,i,12a,17 Complex 5 was the first Pd(II) intermediate that was prepared in C(sp3)−H bond functionalization with 8aminoquinoline as an auxiliary and DMSO as a ligand. It is worth noting that all these complexes have been characterized by X-ray crystallography. Crystallography Study. From the crystallographic data, the coordinated bond length of a Pd atom with different ligands were easily obtained and are given in Table 2. The data for complexes 2f and 3−5 match well with the known reports when the corresponding ligands were coordinated (ΔL ≤ 0.077 Å),16 and no obvious bond length differences were observed for these palladacycles. Cyclic Voltammetry Study. Subsequently, to explore the reactivity differences among four Pd complexes, we studied these cyclometalated Pd(II) complexes from an electrochemistry aspect. From the cyclic voltammograms (Figure 1), it is clearly presented that the highest oxidation wave was possessed by complex 2f at about 1.38 V, and the DMSOcoordinated complex 5 was in second place at about 1.29 V. Nearly the same oxidation potentials were shown for complexes 3 and 4. These data provided evidence that oxidation of complexes 2f and 5 could be more difficult, indicating the strong interaction of Pd−C(CO) leads to decreased electron density on the central Pd atom, which may cause low chemical activity and carbonylation barriers for complex 2f. Chemical Reactivity Study. To test the hypothesis we concluded from CV studies, several electrophilic reagents were employed to separate the chemical reactivity of these four complexes. In Daugulis’s report,5b a complex with a structure similar to that of complex 4 was shown to be able to transform into the alkylated or arylated products smoothly, with alkyl−I or aryl−I as the corresponding alkylation or arylation reagents. In such transformations, organoiodides served as both electrophile and oxidant. Taking this into consideration, we treated these cyclopalladated complexes in methyl iodide (Me−I) and iodobenzene (Ph−I) directly (Table 3), different from Daugulis’s reaction conditions.5b However, no evident difference was detected, and moderate yields were obtained for the reactions between all these palladium complexes and organoiodides to afford compound 6 and 7. Therewith, the less reactive bromobenzene (Ph−Br)

crystallographic analysis, CV, chemical transformations, and DFT computations.

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RESULTS AND DISCUSSION Reaction Optimization. To thoroughly investigate the aforementioned question, the CO-coordinated palladium Table 1. Optimization of the Conditionsa

entry

gas

1 2 3 4 5 6 7 8 9 10 11c

CO CO CO CO CO CO CO CO CO CO CO+Air

temp (°C) 110 110 110 110 110 110 room room room room room

temp temp temp temp temp

concn (mmol/mL)

sol

yieldb (%)

0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.10 0.20 0.50 0.50

TFE toluene dioxane DCE DMF HFIP HFIP HFIP HFIP HFIP HFIP

62 55 60 41 57 70 79 79 88 >95 95

a Reaction conditions: 1a (0.1 mmol, 1.0 equiv), Pd(OAc)2 (0.1 mmol, 1.0 equiv), CO (1 atm), 12 h. HFIP = hexafluoroisopropyl alcohol. room temp = room temperature (23−27 °C). bIsolated yields. cVCO:Vair = 1:1.

complex has to be prepared on a large scale. The synthetic procedure was optimized with 8-aminoquinoline-derived 1a as the model substrate (Table 1). After several commonly used solvents were screened, the desired Pd complex 2a was obtained in a yield of 70% in HFIP (entry 6). It is noteworthy that its X-ray crystal structure was first characterized by us. 2a was found to be surprisingly stable to air and moisture, and no decomposition product or starting material was detected over 1 year of examination, which was in accordance with Shi’s report.7h Unexpectedly, when the reaction was conducted at room temperature, an even higher yield was observed (entry 7, 79%). To further improve the yield, this reaction was examined with different successive different concentration gradients; a quantitative yield was obtained when the concentration of 1a was 0.5 mmol/mL (entry 10), and the efficiency did not decrease even on a gram scale. Surprisingly, air had a minor effect on this transformation (entry 11). Substrate Examination. With the optimized conditions in hand, we tried to investigate whether the subsequent CO migratory insertion into a Pd−C bond could be affected by adjusting the electronic and steric effects on the α-carbon center of the amides, to afford the desired succimide chemicals (Scheme 2). Unfortunately, no carbonylation product was detected when a sterically hindered or electron-withdrawing group was introduced on the quaternary α-carbon of the amides (2a−e). Identically, aryl substituents with either an electron-donating or electron-withdrawing group showed no promotion to the carbonylation product (2f−j); merely the C(sp3)−H activated complexes were obtained, rather than C(sp2)−H complexes. After that, several amides with alkyls B


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Organometallics Scheme 2. Substrate Scope of 8-Aminoquinoline Directed Amidesa

a

Standard conditions.

Scheme 3. Ligand Exchange of CO-Coordinated Palladium Complex

C


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Organometallics

accordance with the oxidation potential of complex 5 in the CV (Figure 1). Computational Studies. Generally, prior to oxidation addition for the electrophiles, a ligand exchange process is demanded.13f,19 To help understand the CO-coordinated palladacycle’s inertness when it interacts with a weak electrophile (PhBr), we performed density functional theory (DFT) calculations for their ligand exchange process. The data demonstrate that CO-coordinated Pd complex 2a possesses the highest ΔG value for the initial ligand exchange processes among the four substrates MeI, PhI, PhBr, and PhSSPh (ΔG = 16.1, 15.6, 19.9, 12.4 kcal/mol, respectively; Table 4), which suggested that this process was not thermodynamically favored and strongly supported the conclusions we got from CV and chemical conversions that strong coordination of Pd−C(CO) leads to lower electron density on the central Pd atom accompanied by relative inertness in further transformations. In addition, we also performed calculations to further probe the carbon monoxide migratory insertion energy with Yu’s report as a comparison20 (Figure 2). Key transition state structures are shown in the right part of Figure 2. Our calculations indicate that the migratory insertion energy of 8aminoquinoline-assisted complex 2a is much higher than that of p-CF3C6F4-directed int3 (ΔG⧧ = 54.2 versus 24.2 kcal/ mol); in addition, the transition state structure of the former is rigid in comparison with the flexible structure of the latter, which accounts for the higher migratory insertion energy observed for complex 2a. The computed geometries indicate that the key to have a low barrier for insertion may be to have a flexible ligand that is able to move in the molecular plane. To support this conclusion further, we added a second CO molecule to 2a (Figure 3). With CO as the ligand instead of the ring nitrogen to obtain intermediate int6, the activation energy of TS7 is greatly decreased in comparison with that of TS1 (ΔG⧧ = 38.1 versus 54.2 kcal/mol). Meanwhile, a lower energy for int8 (ΔG⧧ = −1.0 kcal/mol) versus int2 (ΔG⧧ = 13.7 kcal/mol) is also observed, which suggests that the migratory insertion process of the flexible transition state from int6 is kinetically favored, rather than rigid complex 2a. In order to investigate if the same phenomenon exists in other systems, a series of other bidentate auxiliary (AQ, PIP, etc.) coordinated palladacycles were also checked in this model (for details, please see the Supporting Information). All these samples show a similarly high barrier for migratory insertion, and the Thorpe−Ingold effect (gem-dimethyl effect) is also observed for altering the migration barriers in this scenario, when the cyclometalated Pd complexes switch from a five- to a six-membered ring, which dramatically reduces the migration barriers. All of the computed results support the CO-coordinated palladacycle’s hindrance in migration insertion and inertness in chemical transformations, consistent with the phenomenon observed in the CV.

Table 2. Comparison of the Coordinated Bonds in Pd(II) Complexesa,b,c

a

X = C, N, N, S. bOur crystallographic data. cCrystallographic data in references.

Figure 1. Cyclic voltammograms recorded on a glassy-carbon electrode at 100 mV s−1, in CH3COCH3 containing 1 mM complex and 10 mM electrolyte (n-Bu4NClO4), negative scan.

Table 3. Chemical Reaction Study of Pd Complexesa,b

ligand reagent

CO (%)

MeCN (%)

pyridine (%)

DMSO (%)

Me−I Ph−I Ph−Br PhS−SPh

80 80 NR NRb

82 78 71 40b

81 80 70 37b

84 82 65 NRb

a

Reaction conditions unless specified otherwise: Pd complex (1 equiv, 0.05 mmol), R−X (5 equiv, 0.25 mmol), no solvent, room temperature, 48 h. bPd complex (1 equiv, 0.05 mmol), disulfide (4 equiv, 0.20 mmol), DMF (1 mL), 140 °C, 48 h.

was employed. Since a C−Br bond has a higher bond energy, it has more difficulty in realizing the oxidative addition with Ph− Br for the palladacycles.17 Unexpectedly, the reactions proceeded smoothly for complexes 3−5. However, the conversion of CO-coordinated complex 2f was suppressed completely under the same circumstance, which was in accordance with the hypothesis we concluded from CV studies that a strong coordination between CO and Pd leads to a lower reactivity but greater stability of complex 2f. After that, diphenyl disulfide was also checked in this protocol, since it has exhibited strong interactions with transition metals in many reports.18 Indeed, decreased amounts of compound 8 were provided for the transformations of complexes 3 and 4; for complexes 2f and 5, there was no C(sp3)−H bond sulfenylated product detected. This observation was in

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CONCLUSION In conclusion, we have conducted a systematic study on the factors which impeded the carbon monoxide migratory insertion of a CO-coordinated Pd complex. In our studies, an unprecedented DMSO-coordinated Pd(II) complex was identified. All of the results obtained from substrate expansion, CV, and chemical conversions give solid evidence that the strong interaction between CO and a Pd atom decreased the electron density on the coordinated Pd center, which induced D


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Table 4. Gibbs Free Energy Changes for the Ligand Exchange of Different Palladacyles with Four Kinds of Electrophiles

ligand

reactant

ΔG (kcal/mol)

CO (complex 2f) pyridine (complex 3) MeCN (complex 4) DMSO (complex 5)

MeI (complex S1)

16.1a 9.8a 7.0a 7.9a


PhI (complex S2)

15.6b 9.3b 6.6b 6.6b


PhBr (complex S3)

19.9c 13.7c 10.2c 10.7c


PhSSPh (complex S4)

12.4d 9.2d 3.3d 2.0d

a

M06/6-311+G(d,p)-SDD-PCM(MeI)//B3LYP/6-31G(d)-LANL2DZ. bM06/6-311+G(d,p)-SDD-PCM(PhI)//B3LYP/6-31G(d)-LANL2DZ. M06/6-311+G(d,p)-SDD-PCM(PhBr)//B3LYP/6-31G(d)-LANL2DZ. d M06/6-311+G(d,p)-SDD-PCM(DMF)//B3LYP/6-31G(d)LANL2DZ.

c

Figure 2. Reaction energy profile of CO migratory insertion in our protocol and Yu’s work.19 The upper Gibbs energies were obtained at the M06/ 6-311+G(d,p)-SDD-PCM(HFIP)//B3LYP/6-31G(d)-LANL2DZ level. The lower Gibbs energies in brackets were obtained at the M06/6311+G(d,p)-SDD-PCM(n-hexane)//B3LYP/6-31G(d)-LANL2DZ level.

a higher oxidation potential and lower chemical reactivities in the subsequent transformations. This conclusion was strongly supported by DFT studies as well; it was demonstrated that the CO-coordinated palladacycle 2f has a much higher ΔG value for the ligand exchange process when it interacts with

four kinds of investigated electrophiles. Furthermore, calculations also revealed that a rigid square-planar structure of a CO-coordinated Pd complex leads to a much higher activation energy being required for the migratory insertion process. All of these results indicate that the tridentate ligand makes COE


Organometallics

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ACKNOWLEDGMENTS This paper is dedicated to Lanzhou University on the occasion of its 110th birthday. Financial support was provided by the Recruitment Program of Global Experts (1000 Talents Plan). Zhejiang University is gratefully acknowledged, and calculations were performed on the high-performance computing system at the Department of Chemistry, Zhejiang University. We thank Prof. Xingang Zhang and Prof. Quanyi Zhao for helpful discussions, Prof. Pinxian Xi and Prof. Fangdi Hu for equipment for the CV study, and Dr. Yongliang Shao for X-ray analysis.

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Figure 3. Reaction energy profile of CO migratory insertion in an alternative pathway. The upper Gibbs energies were obtained at the M06/6-311+G(d,p)-SDD-PCM(HFIP)//B3LYP/6-31G(d)LANL2DZ level. The lower Gibbs energies in brackets were obtained at the M06/6-311+G(d,p)-SDD-PCM(n-hexane)//B3LYP/631G(d)-LANL2DZ level.

coordinated palladacycles unreactive and thus reluctant to undergo further transformations.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00087. General information, preparation of substrates, optimization of the reaction conditions, cyclic voltammetry study, chemical reactivity study of four palladium complexes, computational study, analytical data, X-ray crystallography details, and NMR spectra (PDF) Cartesian coordinates for the calculated structures (XYZ) Accession Codes

CCDC 1853125−1853132 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail for B.-F.S.: [email protected]. *E-mail for X.H.: [email protected]. *E-mail for Z.W.: [email protected]. ORCID

Shuo-Qing Zhang: 0000-0002-7617-3042 Bing-Feng Shi: 0000-0003-0375-955X Xin Hong: 0000-0003-4717-2814 Zhen Wang: 0000-0003-4134-1779 Author Contributions ∥

Y.J. and S.-Q.Z. contributed equally.

Notes

The authors declare no competing financial interest. F


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Organometallics

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