AngewandteChemie

German Edition: DOI: 10.1002/ange.201913060Strained Ring Systems Hot PaperInternational Edition: DOI: 10.1002/anie.201913060

Controllable Si@C Bond Activation Enables Stereocon-trol in the Palladium-Catalyzed [4++2] Annulation ofCyclopropenes with BenzosilacyclobutanesXing-Ben Wang+, Zhan-Jiang Zheng+, Jia-Le Xie, Xing-Wei Gu, Qiu-Chao Mu,Guan-Wu Yin, Fei Ye, Zheng Xu, and Li-Wen Xu*

Dedicated to the 100th Anniversary of Nankai University

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Abstract: A novel and unusual palladium-catalyzed [4++2]annulation of cyclopropenes with benzosilacyclobutanes isreported. This reaction occurred through chemoselective Si@C(sp2) bond activation in synergy with ring expansion/insertion of cyclopropenes to form new C(sp2)@C(sp3) andSi@C(sp3) bonds. An array of previously elusive bicyclicskeleton with high strain, silabicyclo[4.1.0]heptanes, wereformed in good yields with excellent diastereoselectivity undermild conditions. An asymmetric version of the reaction witha chiral phosphoramidite ligand furnished a variety of chiralbicyclic silaheterocycle derivatives with good enantioselectivity(up to 95.5:4.5 er). Owing to the mild reaction conditions, thegood stereoselectivity profile, and the ready availability of thefunctionalized precursors, this process constitutes a useful andstraightforward strategy for the synthesis of densely function-alized silacycles.

Introduction

Silacycles are widely used in developing organosiliconchemistry and corresponding organosilicon materials withunique optical and electronic properties.[1] In this regard, four-membered silacyclobutanes (SCBs) among the most impor-tant sila synthons and have attracted considerable interestowing to their high ring strain (150 kJmol@1) and promisingLewis acidity.[2] The strain energy inherent to small-ringsystems often brings forth unique properties and reactivitiesthat give rise to an array of extremely important andcharacteristic transformations.[3] Accordingly, the cleavageof silicon–carbon bonds as well as their pyrolytic and photo-lytic degradation offer many exciting possibilities for newtransformations of silacyclobutanes and their application insynthesis.[4] However, the development of catalytic reactionsof silacyclobutanes (SCBs) with general synthons, such asalkenes, alkynes, and carbonyl compounds, is not an easy taskbecause difficulties are often encountered in controlling theregio- and stereoselectivity of these reactions (Scheme 1).[5]

In this context, Hirano et al. reported the first nickel-catalyzed ring-opening reaction of silacyclobutanes withterminal alkenes to give linear vinyl silanes in good yieldsbut no cyclization or enantioselective process (Scheme 1 a).[6]

Whereas ordinary silacyclobutanes generally failed to under-go addition across the carbon–carbon double bond of alkenesto give silacyclohexanes,[7] the reaction of alkynes withsilacyclobutanes proceeded through ring expansion when

treated with a palladium or rhodium catalyst (Scheme 1b).[8]

The intermolecular cycloaddition of silacyclobutanes withalkynes ranks as the most attractive route to the constructionof silacyclohexene six-membered-ring systems. Since Sakuraiand Imai reported the first palladium-catalyzed cycloadditionreaction of silacyclobutanes with alkynes in 1975, the researchgroups of Oshima, Shintani, Hayashi, Song, and others havemade important breakthroughs in developing regio- andstereoselective versions of the approach based on the Si@Cbond cleavage of silacyclobutanes.[5–11] However, the stereo-control of ring expansion of silacyclobutanes (SCBs) withalkynes is still not easy; for example, in a recent study by Songet al. an asymmetric rhodium-catalyzed reaction of SCBs withalkynes only gave 86:14 to 91:9 er.[8i]

Intermolecular s-bond cross-exchange reactions betweentwo small-ring systems are very interesting processes thatenable the facile rearrangement of four chemical bonds toform larger ring systems with 100 % atom economy (Sche-me 1c–e). In this regard, Murakami and co-workers reporteda pioneering example proceeding through nonpolar s-bondcross-exchange between a cyclobutanone C@C bond anda silacyclobutane Si@C bond to give the correspondingsilabicyclo[5.2.1]decanes with good diastereoselectivity(Scheme 1c).[9] Similarly, a transition-metal-catalyzed reac-tion between silacyclobutanes and three-membered-ringsystems also provided a possible s-bond cross-exchangeapproach to give larger silacycles. In 2008, Saito et al.reported a nickel-catalyzed ring-expansion reaction of ben-zosilacyclobutanes with three-membered ethylcyclopropyli-deneacetate to give benzosilacycloheptenes containing a sev-en-membered ring (Scheme 1d).[10] However, the E/Z ratiofor the corresponding product is not good. Recently, Zhaoand co-workers reported a novel s-bond cross-exchange

Scheme 1. Transition-metal-catalyzed reactions of silacyclobutanes andbenzosilacyclobutanes with alkenes, alkynes, and small rings with highring strain: Divergent strategies from alkene-assisted ring opening toring expansion based on Si@C bond activation and C@C bondactivation.

[*] X.-B. Wang,[+] Dr. Z.-J. Zheng,[+] J.-L. Xie, X.-W. Gu, Q.-C. Mu,Dr. G.-W. Yin, Dr. F. Ye, Dr. Z. Xu, Prof. Dr. L.-W. XuKey Laboratory of Organosilicon Chemistry and Material Technologyof the Ministry of Education, Hangzhou Normal UniversityNo. 2318, Yuhangtang Road, Hangzhou 311121 (P. R. China)E-mail: liwenxu@hznu.edu.cn

Prof. Dr. L.-W. XuSuzhou Research Institute and State Key Laboratory for OxoSynthesis and Selective Oxidation, Lanzhou Institute of ChemicalPhysics, Chinese Academy of Sciences (P. R. China)

[++] These authors contributed equally to this work.

Supporting information and the ORCID identification number(s) forthe author(s) of this article can be found under:https://doi.org/10.1002/anie.201913060.

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reaction between cyclopropenones and (benzo)silacyclobu-tanes in the presence of a Pd or Ni catalyst (Scheme 1 e).[11]

All these approaches constitute interesting and uniqueroutes to structurally diverse silacycles from six-to-eight-membered-ring systems. These great contributions suggestthat the challenge in developing an asymmetric version of thenickel- or palladium-catalyzed ring expansion of (benzo)sila-cyclobutanes is in trapping of the intermediates resulting fromSi@C bond or C@C bond cleavage. The above ring-expansionprocesses have two limitations: 1) Except for the ringexpansion of SCB with alkynes, no enantioselective versionof these ring expansions based on Si@C bond activation hasbeen reported. 2) The [4++2] annulation reaction of (benzo)-silacyclobutanes with three-membered-ring compounds toform the corresponding silabicyclo[4.1.0]heptanes with highring strain has not been reported. The similarity in ring strainof three- and four-membered-ring compounds, such as(benzo)silacyclobutanes and cyclopropenes, resulted in facilecompetitive Si@C bond cleavage and C@C bond cleavage(Figure 1a). In analogy with the transformations reported bySaito and Zhao,[10,11] the reaction of (benzo)silacyclobutanesand cyclopropenes would possibly lead to the formation ofnovel silacycles a-1 or a-2. Thus, we investigated theconstruction of novel silacycles by the transition-metal-catalyzed functionalization of cyclopropenes as based on thetrapping of intermediates formed during Si@C bond cleavage.

Cyclopropenes are unique three-membered compoundsand extremely important building blocks in organic synthesis

owing to their inherent ring strain and unusual electronicproperties similar to those of (benzo)silacyclobutanes andcyclopropenones. In the past years, numerous transformationsbased on the direct functionalization of prochiral or achiralcyclopropenes has consistently been a hot research topic insynthetic chemistry and organometallic catalysis.[12] However,there are few reports on the functionalization or cycloaddi-tion of cyclopropenes by a Si@C bond-cleavage-initiatedtandem reaction process.[12x] Until now, the transition-metal-catalyzed cycloaddition of silabutanes with alkenes, includingthree-membered cyclopropenes, has not been explored.

Inspired by our previous DFT study on the mechanism ofthe palladium-catalyzed cycloaddition of silacyclobutaneswith dimethyl acetylenedicarboxylate,[13] we wonderedwhether the reactivity of cyclopropenes and (benzo)silacy-clobutanes could be controlled by the selective cleavage of anC@C or Si@C bond as enabled by treatment with anappropriate transition-metal catalyst. A cycloaddition reac-tion involving the Si@C bond cleavage of (benzo)silacyclobu-tanes would generate novel silacycles that are not availablefrom nature or by other approaches at present. Herein, wereport the reaction of benzosilacyclobutanes with cyclopro-penes as a novel ring-expansion method for the constructionof unexpected silabicyclo[4.1.0]heptanes but not seven-mem-bered silacycles (a-1 or a-2) under mild reaction conditions(Figure 1b). The good regio-, diastereo-, and stereoselectivityin the Si@C bond-cleavage-initiated functionalization/cyclo-addition between three- and four-membered-ring systemswere enabled by an electron-deficient phosphorus ligand anda chiral phosphoramidite ligand, respectively. Bicyclo-[4.1.0]heptane structural units are prevalent in numerousbioactive natural products.[14] Thus, the reaction of (benzo)-silacyclobutanes and cyclopropenes to give structurally di-verse silacycles (silabicyclo[4.1.0]heptanes) significantly ex-pands the potential of the silasubstitution strategy for thesynthesis of bioactive molecules and natural products (Fig-ure 1c).[15]

Results and Discussion

We initiated our studies by evaluating the catalytic activityof PdCp(h3-C3H5) in the reaction of benzosilacyclobutane 1aand cyclopropene 2a in dichloromethane (DCM) at 30 88C inthe presence of various phosphine ligands (Table 1). In mostcases, the reaction afforded the desired benzosilabicyclo-[4.1.0]heptane 3a with excellent diastereoselectivity (> 99:1dr), albeit in low to moderate yield (20–62%), with differentligands. It was found that the 3,5-(CF3)2-substituted triarylphosphine ligand L7 guaranteed the catalytic efficiency ofPdCp(h3-C3H5) to afford 3a in 62 % yield with > 99:1 dr asdetected by 1H NMR spectroscopy. This result implies thatthe electron-deficient phosphine ligand is crucial to thepalladium-controlled Si@C bond cleavage and subsequentfunctionalization/cycloaddition of the cyclopropene. WithPdCp(h3-C3H5) and L7 as the key catalyst system, wecontinued to investigate the effect of ligand amount, temper-ature, and solvents (see Tables S1–S4 in the SupportingInformation). However, despite great efforts toward improve-

Figure 1. Competitive reaction pathways for the s-bond cross-exchangereaction and [4++2] annulation of (benzo)silacyclobutanes and cyclo-propenes on the basis of Si@C and C@C bond activation.

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ment of the functionalization/cycloaddition reaction, our bestresult was 74% yield in EtOAc in the presence of PdCp(h3-C3H5) (5 mol%) and L7 (10 mol%). We then investigatedvarious palladium sources under the model reaction con-ditions. Subsequent optimization (see Tables S5–S8) con-firmed that Pd(dba)2 was a suitable catalyst in the presence ofa slightly larger amount of L7 (12.5 mol%) at 30 88C (Table 1).Under these conditions, 3a was formed in 82% yield (isolatedin 78 % yield with > 99:1 dr. The simplest silacyclobutane(SCB) is not suitable for this ring expansion/cycloadditionreaction [Eq. (2) in Table 1], which guarantees the good

chemoselectivity of the Si@C(sp2) bond cleavage but not theSi@C(sp3) bond cleavage of 1a.

Next, the reaction scope was examined under the optimalreaction conditions with various benzosilacyclobutanes 1 andcyclopropenes 2 (Scheme 2). Regardless of the type of

substituent on the aromatic ring of the cyclopropene, includ-ing electron-neutral, electron-donating (Me, OMe, etc.), andelectron-withdrawing groups (F, CF3, Cl, Br), the correspond-ing silabicyclo[4.1.0]heptanes 3a–cc and 3 ii were obtained ingood yields (50–90 %) with > 99:1 dr. These experimentalresults showed that different substituents and substitutionpatterns on the phenyl ring of the cyclopropene and differentester groups were tolerated. Moreover, the methyl substitu-ents of benzosilacyclobutane 1 a could be replaced with otheralkyl or aromatic groups. For example, substituted benzosi-lacyclobutanes with diethyl and diphenyl substituents on thesilicon atom exhibited good activity in palladium-promotedSi@C bond cleavage without any negative effect on thereaction outcome (products 3dd, 3 ff, and 3jj), includinga benzosilacyclobutane with a fluorine substituent on thearomatic ring (product 3jj). However, increasing the steric

Table 1: Optimization of the reaction conditions.[a]

Entry Variation from the standard conditions Yield [%][b] dr[c]

1 none 82(78) >99:12 with L1 instead of L7 in DCM 26 >99:13 with L2 instead of L7 in DCM 20 >99:14 with L3 instead of L7 in DCM 28 >99:15 with L4 instead of L7 in DCM 36 >99:16 with L5 instead of L7 in DCM 32 >99:17 with L6 instead of L7 in DCM 48 >99:18 with L7 in DCM 62 >99:19 with L8 instead of L7 in DCM 52 >99:1

10 PdCp(h3-C3H5) instead of Pd(dba)2 66 >99:111 Pd2(dba)3 instead of Pd(dba)2 70 >99:112 [PdCl(C3H5)]2 instead of Pd(dba)2 34 >99:113 at room temperature 50 >99:114 at 60 88C 60 >99:115 with 1.5 equiv. of 1a 39 >99:116 with 0.75 equiv. of 1a 18 >99:1

[a] See also Tables S1–S7 in the Supporting Information. Standardreaction conditions, unless otherwise noted: 1a (1 mmol), 2a(0.2 mmol), solvent (1.0 mL). When EtOAc (2 mL) was used with thesame catalyst system, 3a was formed in 78% yield. For entries 2–16, thesolvent amount was 2.0 mL, and for entries 2–14, the 1a/2a ratio was3:1. [b] The yield was determined by 1H NMR analysis of the crudeproduct. The value in parenthesis is the yield of the isolated product.[c] The diastereomeric ratio was determined by 1H NMR analysis of thecrude product. Cy = cyclohexyl, dba= dibenzylideneacetone.

Scheme 2. Scope of the palladium-catalyzed functionalization of cyclo-propenes with benzosilacyclobutanes through Si@C bond activation.Bn =benzyl.

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repulsion at the Si center was not suitable for this reaction,and a dibutyl-substituted benzosilacyclobutane gave thedesired product 3ee in poor yield. Furthermore, Si@C bondcleavage of methylphenyl-substituted benzosilacyclobutaneproved not to be easy, providing the corresponding product3gg in modest yield.

Products 3 of the palladium-catalyzed functionalization ofcyclopropenes with benzosilacyclobutanes feature at leastthree carbon stereocenters as well as a silicon stereogeniccenter if the substituents at silicon are different. However, it iswell-known that the development of an asymmetric version ofa transition-metal-catalyzed ring expansion based on Si@Cbond cleavage is quite difficult at present.[9–11] Encouraged bythe success of using Pd(dba)2 with phosphine ligand L7 tosynthesize racemic silabicyclo[4.1.0]heptanes, we carried outcareful optimization experiments with a wide range of chiralphosphine ligands (see Tables S9–S15 for comprehensiveresults) to develop a method for the enantioselective con-struction of chiral silacycles 3 based on the tandem process ofSi@C bond cleavage and the formation of new Si@C and C@Cbonds. After screening numerous solvents and palladiumcatalysts, as well as the effect of concentration on the reactionof 1a and 2a, we found that the TADDOL-derived phos-

phoramidite L30 (Scheme 3) was the most efficient ligand inDCM at room temperature, generating product 3a with a higher value (91.5:8.5 er). As anticipated, rigorous control experi-ments and screening of ligands revealed that all reactionparameters considered in this study were crucial for thecatalytic Si@C bond cleavage of benzosilacyclobutanes andsubsequent functionalization/cycloaddition of cyclopropenesfor the construction of three continuous stereocenters insilabicyclo[4.1.0]heptanes.

Having identified optimal conditions for this asymmetricpalladium functionalization reaction of cyclopropenes withbenzosilacyclobutanes, we examined the enantioselectivitywith respect to representative reaction partners(Scheme 4).[16] The desired products 3 were obtained in

Scheme 3. Optimization of reaction conditions with chiral ligands forthe asymmetric palladium-catalyzed functionalization of cyclopropeneswith benzosilacyclobutanes. TMS= trimethylsilyl.

Scheme 4. Scope of the asymmetric palladium-catalyzed functionaliza-tion of cyclopropenes with benzosilacyclobutanes for the constructionof chiral silabicyclo[4.1.0]heptanes containing three continuous stereo-centers.

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moderate yields with good enantioselectivity (up to 95.5:4.5er). Similarly to the non-enantioselective version of thecycloaddition, the sequential transformation gave a singlediastereoisomer (> 99:1 dr) in all cases studied, possiblyowing to a directing effect of the carbonyl group on the esterby interacting with cyclopropene and benzosilacyclobutane.Notably, we also checked the possibility of the enantioselec-tive construction of a silicon stereogenic center, which hasbeen recognized as a challenge during the past decade.[17] Thecorresponding product 3 hh containing three continuous Cstereocenters and a Si stereocenter was formed in promisingyield with good enantioselectivity (Scheme 4), although thediastereoselectivity was low because of the inherent difficultyin the kinetic resolution of the racemic starting material,Me(Et)Si-containing benzosilacyclobutane.

To illustrate the utility of our method in downstreamtransformations and further demonstrate the value of theprepared silacycles, we synthesized the alcohol-containingsilabicyclo[4.1.0]heptane 4 a (Scheme 5a). The OH-function-

alized silacycle proved to be quite stable under the reactionconditions. Moreover, fluoride-anion-induced Si@C bondcleavage resulted in alternative access to multisubstitutedcyclopropane derivatives with excellent diastereoselectivity;such compounds are difficult to synthesize with high diaste-reoselectivity by cyclopropanation reactions (Scheme 5b–d).[18]

On the basis of our experimental results (see Scheme S3and Figure S1 in the Supporting Information) and literatureprecedent[5–9] as well as our DFT study on the palladium-catalyzed desymmetrization of silacyclobutanes with al-kynes,[13] two plausible reaction pathways are proposed forthis ring-expansion/cycloaddition reaction of cyclopropeneswith benzosilacyclobutanes (Figure 2). To better understandthe reaction mechanism, we performed simplified DFTcalculations with PMe3 as a model ligand (see Table S3; thepotential-energy-surface diagram and the related energyparameters identified for two possible reaction pathwaysare summarized in the Supporting Information). As shown in

Figure 1, path b is more reasonable than path a because of theweak activation/coordination of Pd with the ester group onthe cyclopropene (there is an energy difference of 1.7 kcalmol@1 between substrates with or without the ester moiety). Infact, our experimental results (see Scheme S3) support theimportance of the ester moiety for the activation of the C=Cbond in this reaction.

31P NMR analysis further supported the existence ofa palladium–carbonyl interaction between the catalyst andsubstrate. The addition of 2a to a mixture of Pd(dba)2 andphosphine ligand L7 resulted in the appearance of a new31P NMR peak at d = 33.62 ppm (Figure 3C), which support-Scheme 5. Application of silabicyclo[4.1.0]heptanes. a) Preparation of

alcohol-containing silacycle 4a. b–d) Preparation of chiral cyclopropanederivatives 5 through fluoride-anion-induced Si@C bond cleavage.DIBAL-H= diisobutylaluminum hydride, TBAF= tetrabutylammoniumfluoride.

Figure 2. Proposed mechanism for the palladium-catalyzed functional-ization of cyclopropenes with benzosilacyclobutanes through Si@Cbond activation.

Figure 3. Comparison of 31P NMR spectra of the ligand and Pd/L7complex in the cycloaddition of cyclopropane 2a with benzosilacyclo-butane 1a : A) only L7; B) L7 and Pd(dba)2 (2:1); C) L7, Pd(dba)2, and2a (2:1:10); D) L7, Pd(dba)2, and 1a (2:1:50); E) L7, Pd(dba)2, and 1a(2:1:10); F) L7, Pd(dba)2, 1a, and 2a (2:1:10:10); G) L7, Pd(dba)2, 1a,and 2a (2:1:50:10); the mixture was stirred for 15 min; H) L7,Pd(dba)2, 1a, and 2a (2:1:50:10); the mixture was stirred for 12 h.

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ed the existence of intermediate I in this case (Figure 2).Furthermore, an excess amount of 1a was beneficial to theformation of possible intermediate IV with a 31P NMR signalat d = 20.37 ppm (Figure 3D), and the resulting reactionmixture also gave the same information (Figure 3G,H).Notably, when the reaction was carried out for 12 h and theconsumption of 2a was completed, intermediate I disap-peared, and intermediate IV was transformed into a dimericproduct formed with two molecules 1a. These experimentalresults reveal that the proposed pathway b is more favorablethan path a.

To elucidate the directing function of the ester group incyclopropenes 2, we prepared analogues 6–8 bearing methyl,alcohol, and acid groups instead of the ester and carried outthe corresponding palladium-catalyzed reaction of benzosi-lacyclobutane 1a with these cyclopropenes. None of thesereactions proceeded significantly, and only a trace amount ofthe product was detected when 6 or 7 was used (Scheme 6).These results further support the importance of the directingester group on the cyclopropene for the activation of carbon–carbon double and subsequent Si@C bond activation shown inFigure 2.

Conclusion

In summary, we have developed a palladium-catalyzeddual functionalization of cyclopropenes with benzosilacyclo-butanes through Si@C(sp2) bond activation synergized witha C=C bond-activation-initiated [4++2] annulation sequencewith the formation of new C(sp2)@C(sp3) and Si@C(sp3)bonds. In this novel cross-coupling transformation, an array ofelusive bicyclic skeletons with high strain, silabicyclo-[4.1.0]heptanes, were formed under mild conditions. In thisway, small strained cyclopropane skeletons attached tosaturated benzosilacyclohexenes were successfully construct-

ed with up to 95.5:4.5 er with the aid of a directing ester group.Owing to the mild reaction conditions, the good stereoselec-tivity profile, and the ready availability of the functionalizedprecursors, this process constitutes an unprecedented andstraightforward strategy for the synthesis of densely function-alized silacycles. Further investigations into the mechanisticintricacies of the reaction sequence and related processes arecurrently ongoing in our laboratory.

Experimental Section

General procedure : Tris[3,5-bis(trifluoromethyl)phenyl]phos-phane (L7; 16.8 mg, 0.025 mmol) or L30 (18.3 mg, 0.025 mmol) andPd(dba)2 (5.7 mg, 0.01 mmol) were dissolved in dry solvent (1 mL)under a N2 atmosphere and stirred at room temperature for about15 min. Then cyclopropene 2 (0.2 mmol) and benzosilacyclobutane1 (1.0 mmol) were added sequentially, and the mixture was stirred at30 88C for about 12 h until the starting material was consumed(monitored by TLC). The mixture was then filtered through celite,and the filtrate was concentrated to dryness. A portion of the residuewas analyzed by 1H NMR spectroscopy to determine the diastereo-meric ratio and recovered. The crude product was purified by columnchromatography to give the product 3.

Acknowledgements

This research was supported by the grants of National NaturalScience Foundation of China (NSFC 21773051, 21801056, and21703051) and Zhejiang Provincial Natural Science Founda-tion of China (LZ18B020001, LY17E030003, LY17B030005,LQ 19B040001, and LY18B020013). We thank L. Li, K. Z.Jiang, and X. Q. Xiao for their assistance with MS and X-rayanalysis, the technicians of our group, and the members of ourNMR, MS, and HPLC departments for their excellent service.

Conflict of interest

The authors declare no conflict of interest.

Keywords: palladium · ring expansion · Si@C bond activation ·silacycles · strained molecules

How to cite: Angew. Chem. Int. Ed. 2020, 59, 790–797Angew. Chem. 2020, 132, 2–807

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Scheme 6. Indirect evidence for the importance of the ester moiety oncyclopropenes 2 : Palladium-catalyzed cycloaddition of cyclopropenessubstituted with various groups under the optimized reaction condi-tions.

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Manuscript received: October 12, 2019Accepted manuscript online: November 5, 2019Version of record online: December 12, 2019

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