molecules

Communication 0 Synthesis of 1,1 -Bis(1-Methyl/Chloro-2,3,4,5- Tetraphenyl-1-Silacyclopentadienyl) [Ph4C4Si(Me/Cl)-(Me/Cl)SiC4Ph4] from Silole Anion − + + [MeSiC4Ph4] •[Li or Na ] and Silole Dianion 2− + [SiC4Ph4] •2[Li ]; Oxidative Coupling of Silole − + + Anion [MeSiC4Ph4] •[Li or Na ] by Ferrous Chloride (FeCl2) and Oxidative Coupling and 2− + Chlorination of Silole Dianion [SiC4Ph4] •2[Li ] by Cupric Chloride (CuCl2)

Jang-Hwan Hong

Department of Nanopolymer Material Engineering, Pai Chai University, 155-40 Baejae-ro (Doma-Dong), Seo-Gu, Daejon 302-735, Korea; [email protected]; Tel.: +82-42-520-5755

 Academic Editor: Arnaud Gautier  Received: 20 September 2019; Accepted: 15 October 2019; Published: 19 October 2019

Abstract: A reaction of silole anion {[MeSiC Ph ] [Li+ or Na+](1) with anhydrous ferrous chloride 4 4 −• (FeCl2) in THF (tetrahydrofuran) gives 1,10-bis(1-methyl-2,3,4,5-tetraphenyl-1-silacyclopentadienyl) [Ph4C4Si(Me)-(Me)SiC4Ph4](2) with precipitation of iron metal in high yield. Silole dianion {[SiC Ph ]2 2[Li+](3) is added to anhydrous cupric chloride (CuCl ) in THF at 78 C, then the 4 4 −• 2 − ◦ dark red solution changes into a greenish solution. From the solution, a green solid is isolated, and stirring it in toluene at room temperature provides quantitatively 1,10-bis(1-chloro-2,3,4,5- tetraphenyl-1-silacyclopentadienyl) [Ph4C4Si(Cl)-(Cl)SiC4Ph4](4) with precipitation of copper metal in 2 toluene. The green solid is suggested to be 1,10-bissilolyl bisradical [Ph4C4Si-SiC4Ph4] • (8), and lithium cuprous chloride salts {[Li CuICl ]+ [CuICl ] }. Both reactions are initiated by single-electron transfer 2 2 • 2 − (SET) from the electron-rich anionic silole substrates (1 and 3) to iron(II) and copper(II).

Keywords: silole; anion; dianion; oxidation; oxidative-chlorination; SET (single-electron transfer); cupric chloride; ferrous chloride

1. Introduction Recent decades have witnessed tremendous progress in the field of group 14 metallole dianions and the related metallole anions [1–6] since the first silole dianion and germole dianion were reported [7,8]. 1 In particular, the syntheses and characterizations of the silole anion [1-tert-butyl-SiC4Ph4] −, the silole 2 2 2 dianion [SiC4Ph4] −, the bissilole dianion [Ph4C4Si-SiC4Ph4] −, and the germole dianion [GeC4Ph4] − as aromatic compounds [8–10], previously considered merely as intangible intermediates, have been the starting point for exploring the synthetic, theoretical, and materials chemistry of group 14 metalloles and their anions [11–27], with increasing ASE (Aromatic Stabilization Energy) of the η5-lithium salts 2 + [C4H4E] − 2[Li ][16,17]. • 2 2 Very recently, conspicuous silole dianions {[SiC2(SiMe3)2C2Ph2] −, SiC2(SiEt3)2C2Ph2] −}, silole anion 1 2 {[1-mesityl-SiC2(SiMe3)2C2Ph2] −}, and bissilole dianion {[Ph2C2(Me3Si)2C2Si-SiC2(SiMe3)2C2Ph2] −}

Molecules 2019, 24, 3772; doi:10.3390/molecules24203772 www.mdpi.com/journal/molecules MoleculesMolecules 2019 2019, ,24 24, ,x x 22 ofof 1010 Molecules 2019, 24, 3772 2 of 10

2 3 2 2 2 2− 2 3 2 2 2 2− VeryVery recently,recently, conspicuousconspicuous silolesilole dianionsdianions {[SiC{[SiC2(SiMe(SiMe3))2CC2PhPh2]2]−, , SiCSiC2(SiEt(SiEt3)2)CC2PhPh2]]2−},}, silolesilole 2 3 2 2 2 1− 2 2 3 2 2 anionanion {[1-mesityl-SiC{[1-mesityl-SiC2(SiMe(SiMe3))2CC2PhPh2]1]−},}, andand bissilolebissilole dianiondianion {[Ph{[Ph2CC2(Me(Me3Si)Si)2CC2Si-Si- were reported as having2− enhanced aromatic electronic structures, due to their trialkylsilyl groups on two SiC2(SiMe3)2C2Ph2]2− } were reported as having enhanced aromatic electronic structures, due to their SiCα-carbons2(SiMe3 of)2C the2Ph silole2] } were rings reported [28–31]. as having enhanced aromatic electronic structures, due to their trialkylsilyl groups on two α-carbons of the silole rings [28–31]. trialkylsilylDuring groups the past on three two α decades,-carbons allof the of thesilole group rings 14 [28–31]. metallole and bismetallole dianions, from During the past three decades, all of the group 14 metallole and bismetallole dianions, from siliconDuring to , the havepast three been exploreddecades, all as of readily the group available 14 metallole highly aromatic and bismetallole systems [dianions,5,6], since from the to lead, have been explored as readily available highly aromatic systems [5,6], since the siliconfollowing to mesomericlead, have structuresbeen explored have hadas readily been suggested available tohighly denote aromatic their electronic systems characteristics [5,6], since the in following mesomeric structures have had been suggested to denote their electronic characteristics in followingthe pioneering mesomeric works structures (Figure1)[ have8–10]. had been suggested to denote their electronic characteristics in thethe pioneering pioneering works works (Figure (Figure 1) 1) [8–10]. [8–10].

2-2- 2-2- 2-2- 2-2- 2-2- 2-2-

EE EE EE EE EE EE a b c d e f a b c d e f FigureFigure 1. 1. Mesomeric MesomericMesomeric structures structuresstructures of of group group 14 14 metalloles metalloles dianions. dianions.

Nowadays,Nowadays, even even group group 13 13 dilithio dilithio metallolesmetalloles dianion dianion and and dilithiodilithio transitiontransition metalloles metalloles are are preparedprepared and and characterized characterized as as aromatic aromatic dianiondianion dianion metallolesmetalloles metalloles [[32]. 32[32].]. InIn general,general, reactions reactions of of silole silole anionsanions andand silolesilole dianionsdianions withwith electrophileselectrophiles areare nucleophilicnucleophilic additionsadditions to toto E E atoms atoms atoms of of of E–X E–X E–X bonds bonds bonds in in inthe the the electrophile electrophile electrophiles,s,s, to to make make to make new new new Si–E Si–E Si–Ebonds bonds bonds (E=C, (E=C, (E Si, Si,= C,Ge, Ge, Si, Sn, Sn, Ge, X=Cl, X=Cl, Sn, XBr,Br,= Cl,I) I) [1–5]. Br,[1–5]. I) [In 1In– 5the the]. Incase case the of caseof the the of silole silole the siloledianion, dianion, dianion, the the respective respective the respective reaction reaction reaction pathway pathway pathway depends depends depends on on what what on what the the electrophiletheelectrophile electrophile is—for is—for is—for example, example, example, dichlorocyclopropa dichlorocyclopropa dichlorocyclopropane,ne,ne, diphenylcyclopropeno diphenylcyclopropeno diphenylcyclopropenone,ne,ne, 2-adamantanone, 2-adamantanone, 2-adamantanone, 1,3- 1,3- 1,3-butadienes,butadienes,butadienes, and and andN N,N,NN’-di-’-di-,N0tert-di-tert-butylethylenediimine—due-butylethylenediimine—duetert-butylethylenediimine—due to to the the to presence presence the presence of of two two of anionic anionic two anionic electron electron electron pairs pairs [33–37].pairs[33–37]. [33 Nevertheless, –Nevertheless,37]. Nevertheless, all all of of them allthem of themhave have been havebeen initiated beeninitiated initiated by by formal formal by formal two-electron two-electron two-electron transfer. transfer. transfer. − ++ ++ Here,Here, wewe reportreport twotwo oxidation oxidation reactions reactions ofof silole silole anion anion [MeSiC [MeSiC4Ph4Ph4]4]− •[Li[Li+ or NaNa+ ] withwith ferrousferrous Here, we report two oxidation reactions of silole anion [MeSiC4Ph4] •[Li• or Na ] with ferrous 22− ++ chloridechloride (FeCl (FeCl2)2) andand silole silole dianion dianion [SiC[SiC44PhPh44]]2−−•2[Li2[Li+ ] ]((11)) with with cupric cupric chloride (CuCl(CuCl22). To date,date, therethere chloride (FeCl2) and silole dianion [SiC4Ph4] •2[Li• ] (1) with cupric chloride (CuCl2). To date, there areare nono reports reports onon on thethe the single-electronsingle-electron single-electron oxidationoxidation oxidation and and/or and/or/or chlorination chlorination chlorination of of of them, them, them, as as as far far far as as as we we we know. know. know. This This This is the first report for oxidative chlorination of silole dianion with cupric chloride (CuCl )2 through a isis the the first first report report for for oxidative oxidative chlorination chlorination of of silole silole dianion dianion with with cupric cupric chloride chloride (CuCl (CuCl22)) through through a a radicalradical silole silole moiety, moiety, viavia via twotwo two single-electronsingle-electron single-electron transfers.transfers. transfers. 2. Results 2.2. Results Results + + The addition of silole anion salt [MeSiC4 44Ph− 4]− +[Li or+ Na ](1) to pale brown anhydrous TheThe addition addition of of silole silole anion anion salt salt [MeSiC [MeSiC4PhPh4]]−•[Li•[Li+• or or Na Na+]] ( (11)) to to pale pale brown brown anhydrous anhydrous ferrous ferrous ferrous chloride2 (FeCl2) in THF with stirring, immediately changed a dark violet solution into chloridechloride (FeCl (FeCl2)) in in THF THF with with stirring, stirring, immediately immediately changed changed a a dark dark violet violet solution solution into into a a black black solution, solution, a black solution, with precipitation of black particles at room temperature. From the reaction withwith precipitationprecipitation ofof blackblack particlesparticles atat roomroom temperature.temperature. FromFrom thethe reactionreaction mixture,mixture, 1,11,1′-bis(1-′-bis(1- mixture, 1,10-bis(1-methyl-2,3,4,5-tetraphenyl-1-silacyclopentadienyl)4 4 [Ph44C44Si(Me)-(Me)SiC4Ph4] methyl-2,3,4,5-tetraphenyl-1-silacyclopentadienyl)methyl-2,3,4,5-tetraphenyl-1-silacyclopentadienyl) [Ph[Ph4CC4Si(Me)-(Me)SiCSi(Me)-(Me)SiC4PhPh4] ] (2(2)) waswas obtainedobtained (2) was obtained quantitatively, and the black particle was identified as iron metal by its strong quantitatively,quantitatively, and and thethe blackblack particleparticle was was identifiedidentified as as ironiron metalmetal byby itsits strongstrong para-magnetismpara-magnetism in in para-magnetism in Scheme1. SchemeScheme 1. 1. Ph Ph44 Ph Ph Ph44 Ph44 MeMe FeCl /THF at RT FeCl22/THF at RT 1/2 SiSi SiSi 0 - 1/2 SiSi -Fe-Fe0, ,-Cl -Cl- MeMe MeMe 1 2 1 2 − + + Scheme 1. Reaction of silole anion [MeSiC4Ph4−] •[Li+ or Na+ ] (1) with FeCl2. SchemeScheme 1. 1.Reaction Reaction of of silole silole anion anion [MeSiC [MeSiCPh4Ph4]] •[Li[Li+ oror Na Na+] ]((11) )with with FeCl FeCl2. . 4 4 −• 2 2− + In another reaction, silole dianion {[SiC4Ph42]− •2[Li++] (3)} was reacted with brownish anhydrous In another reaction,reaction, silolesilole dianiondianion {[SiC{[SiC44PhPh44] −•2[Li2[Li ] ]((33)})} was was reacted reacted with with brownish brownish anhydrous anhydrous 2 • cupriccupric chloride chloride (CuCl(CuCl (CuCl2))) inin in THFTHF THF (tetrahydrofuran)(tetrahydrofuran) (tetrahydrofuran) at at at − 78−7878 C,°C, °C, then then then immediately immediately immediately the the the dark dark dark red red red solution solution solution of 2 − ◦ ofof the the silole silole dianion dianion ( 3(3)) changed changed to to a a greenish greenish solution. solution. From From the the solution, solution, a a green green solid solid was was isolated isolated

Molecules 2019, 24, 3772 3 of 10

Molecules 2019, 24, x 3 of 10 Moleculesthe silole 2019 dianion, 24, x (3) changed to a greenish solution. From the solution, a green solid was isolated3 of 10 quantitatively. 1H, 13C, or 29Si NMR spectroscopy was not applicable to the solid, due to its strong quantitatively.quantitatively. 1 H,1H, 13 13C,C, or or 29 29SiSi NMR NMR spectroscopy spectroscopy was was not not applicable applicable to to the the solid, solid, due due to to its its strong strong para-magnetism. The green solid was put into toluene, and stirring the solution at room temperature para-magnetism.para-magnetism. The The green green solid solid was was put put into into toluene, toluene, and and stirring stirring the the solution solution at at room room temperature temperature yielded 1,10-bis(1-chloro-2,3,4,5-tetraphenyl-1-silacyclopentadienyl) [Ph4C4Si(Cl)-(Cl)SiC4Ph4](4) yieldedyielded 1,11,1′-bis(1-chloro-2,3,4,5-tetraphenyl-1-silacyclopentadienyl)′-bis(1-chloro-2,3,4,5-tetraphenyl-1-silacyclopentadienyl) [Ph[Ph4C4C4Si(Cl)-(Cl)SiC4Si(Cl)-(Cl)SiC4Ph4Ph4]4 ] (4()4 ) quantitatively, with precipitation of copper metal on the surface of glassware (Scheme2). quantitatively,quantitatively, with with precipitation precipitation of of copper copper metal metal on on the the surface surface of of glassware glassware (Scheme (Scheme 2). 2).

PhPh4 4 PhPh4 4 PhPh4 4 ClCl II II 2Cu2CuClCl2 2 RT/TolueneRT/Toluene 1/21/2 SiSi SiSi GreenGreen Solid Solid 0 0 -78-78oC/THFoC/THF -Cu-Cu SiSi ClCl LiLi++ LiLi++

33 44

22−2− +++ SchemeSchemeScheme 2. 2. 2.Reaction Reaction Reactionof of of silole silole silole dianion dianion dianion {[SiC {[SiC {[SiC4PhPh4Ph4]]4]•2[Li•2[Li2[Li ] ]((]3 (3)3 )with) withwith cupric cupriccupric chloride chloridechloride (CuCl (CuCl(CuCl2).2). ). 4 4 −• 2 3.3. Discussion Discussion Discussion + + Oxidation of the silole anion [MeSiC4Ph4]− − [Li +or Na +](1) by ferrous chloride (FeCl2) at room OxidationOxidation of of the the silole silole anion anion [MeSiC [MeSiC4Ph4Ph4]4−]•[Li••[Li+ or or Na Na+] ]( 1()1 )by by ferrous ferrous chloride chloride (FeCl (FeCl2)2 )at at room room temperature might generate the silolyl radical [MeSiC4Ph•4•]• (5) via single-electron transfer (SET), temperaturetemperature might might generate generate the the silolyl silolyl radical radical [MeSiC [MeSiC4Ph4Ph4]4] ( 5()5 )via via single-electron single-electron transfer transfer (SET), (SET), as as inasin inSchemeScheme Scheme 3. 3. 3.Then, Then, Then, the the the coupling coupling coupling of of of the the silolyl silolylsilolyl radical radicalradical ( (5()55 ))immediately immediately gives gives the thethe bissilole bissilolebissilole ( (22().2).). Alternatively,Alternatively, thethe the silolesilole silole radicalradical radical (55 ())5 is) is able able to to be be generated generated and and dimerized dimerized via via an an an unstable unstable unstable iron iron iron complex complex complex of [Ph4C4Si(Me)-Fe-(Me)SiC4Ph4], or a reductive elimination of the bissiole (2) from the iron complex, ofof [Ph [Ph4C4C4Si(Me)-Fe-(Me)SiC4Si(Me)-Fe-(Me)SiC4Ph4Ph4],4], or or a areductive reductive elimin eliminationation of of the the bissiole bissiole ( 2()2 )from from the the iron iron complex, complex, withwith an an elimination elimination of of iron iron (0). (0).

PhPh44 PhPh44 PhPh44 MeMe

FeClFeCl22 1/2 SiSi SiSi 0 - 1/2 SiSi -Fe-Fe0, ,-2Cl -2Cl- MeMe MeMe 11 22

SETSET PhPh44 x2x2

SiSi

MeMe

55

• SchemeScheme 3. 3. Generation Generation of of silolyl silolyl radical radical [MeSiC [MeSiC4Ph4Ph4]4•] ( 5()5 )via via single-electron single-electron transfer transfer (SET) (SET) and and its its Scheme 3. Generation of silolyl radical [MeSiC4Ph4]• (5) via single-electron transfer (SET) and dimerization.itsdimerization. dimerization.

AlthoughAlthough similar similar coupling coupling coupling reac reac reactionstionstions to to Wurtz Wurtz to Wurtz coupling coupling coupling are are known known are forknown for reactions reactions for of reactions of 1-choro-1- 1-choro-1- of 1-choro-1-PhPh/Me-2,3,4,5-tetraphenyl-1-silacyclopentadienesPh/Me-2,3,4,5-tetraphenyl-1-silacyclopentadienes/Me-2,3,4,5-tetraphenyl-1-silacyclopentadienes {[Cl,Ph-SiC {[Cl,Ph-SiC {[Cl,Ph-SiC4Ph4Ph4]4 ]and and 4[Cl,Me-SiC Ph[Cl,Me-SiC4] and [Cl,Me-SiC4Ph4Ph4]},4]}, with with4Ph one 4one]}, withequivalentequivalent one equivalent of of lithium lithium of lithiumor or sodium, sodium, or sodium, to to give give to the givethe respective respective the respective bissiloles bissiloles bissiloles {[Ph {[Ph {[Ph4C4C4Si(Ph)-(Ph)SiC44Si(Ph)-(Ph)SiCC4Si(Ph)-(Ph)SiC4Ph4Ph4]44 ]Phand and4] [Phand[Ph4C [Ph4C4Si(Me)-(Me)SiC44Si(Me)-(Me)SiCC4Si(Me)-(Me)SiC4Ph4Ph4]}44Ph]} [25,38], [25,38],4]} [25 ,it38 it is], is ituncertain uncertain is uncertain whether whether whether th those thoseose reactions reactions reactions proceed proceed via viavia two-electron two-electrontwo-electron transfertransfer oror one-electronone-electron transfer.transfer. NevertNevertheless,heless, thethe formationformation ofof thethe bissilolebissilole [Ph[Ph4C4C4Si(Me)-4Si(Me)- (Me)SiC(Me)SiC4Ph4Ph4]4 ] (2()2 ) inin SchemeScheme 3 3 providesprovides evidenceevidence forfor thethe dimerizationdimerization ofof thethe silolylsilolyl radicalradical • [MeSiC[MeSiC4Ph4Ph4]4•] ( 5(),5), or or for for the the reductive reductive elimination elimination of of the the bissiole bissiole ( 2()2 )with with iron iron metal metal from from an an iron iron

Molecules 2019, 24, 3772 4 of 10

transfer or one-electron transfer. Nevertheless, the formation of the bissilole [Ph4C4Si(Me)-(Me)SiC4Ph4] (2) in Scheme3 provides evidence for the dimerization of the silolyl radical [MeSiC 4Ph4]• (5), or for the reductive elimination of the bissiole (2) with iron metal from an iron complex {[Ph4C4Si(Me)-Fe-(Me)SiC4Ph4], since the radical species of (5) might not be stabilized enough to be alone. However, from the same reaction at 78 C, the oxidation reaction, such as the reaction at room − ◦ temperature, was not observed, and it was not clear whether some kind of bond was formed between an iron atom and two silole moieties. The reaction in Scheme2 clearly shows the oxidation of the silole dianion ( 3) and chlorination of each silicon atom in a bissilole. Nevertheless, there are a couple of controversial issues surrounding the chemical bond formation of Si–Cu as an intermediate in the change of oxidation state for copper and reaction mechanism, since most of the mechanisms for copper-catalyzed organic reactions are still uncertain and controversial [39]. For more than a century, copper has been used to promote cross-coupling reactions of aryl halides with diverse nucleophiles to construct C–C and C–heteroatom bonds in organic synthesis [40]. Oxidative couplings of carboanion species, such as Grignard reagents and organo-lithium, via copper organyls with or other oxidants, are well-known reactions for organic synthesis [41,42]. Numerous successful Cu-catalyzed aerobic oxidation reactions, in which the use of Cu(II) species obviates the introduction of an additional oxidizing agent, have been developed for fine chemical, pharmaceutical, and related industries [41–45]. Although the mechanisms of those reactions, including Ullmann-type reactions, are not clearly known, in recent years those reactions are classified into three general mechanistic pathways, depending on organic substrates [39]. The first step in the most well-known mechanism among three mechanistic pathways is initiated by single-electron transfer (SET) from electron-rich substrates, such as 1,3,5-trimethoxy benzene, to Cu(II), which is reduced to Cu(I). Then, chlorination of the oxidated cationic radical of 1,3,5-trimethoxy benzene gives 2-chloro-1,3,5-trimethoxy benzene in high selectivity [25]. The high selectivity of the oxidative chlorination is attributed to its deactivation of the product 2-chloro-1,3,5-trimethoxy benzene to the Cu-catalyst, due to the electron-withdrawing effect of the substituted chlorine atom. Neutral arenes, such as benzene, are unreactive under these conditions [46–51]. At these points, the silole dianion {[SiC Ph ]2 2[Li+]} (3) is unquestionably a real, 4 4 −• electron-rich aromatic substrate. Therefore, the silole dianion (3) is readily oxidized to a silole radical anion cuprate [SiC Ph ] 2[Li CuICl ]+ (6) by cupric chloride (CuCl ) at 78 C, 4 4 •−• 2 2 2 − ◦ via single-electron transfer (SET), in Scheme4. Then the silole anion radical [SiC 4Ph4]•− (6) might be immediately dimerized to the bissilole dianion cuprate {[Ph C Si-SiC Ph ]2 2[Li CuICl ]+}(7). 4 4 4 4 −• 2 2 Several bissilole dianions, including the same bissilole dianion {[Ph C Si-SiC Ph ]2 2[Li]+}[10], 4 4 4 4 −• {[Me C Si-SiC Me ]2 2[K+]} [12], {[Ph C Me C Si-SiC Me C Ph ]2 2[Na+]} [52], and 4 4 4 4 −• 2 2 2 2 2 2 2 2 −• {[Ph C (Me Si) C Si-SiC (SiMe ) C Ph ]2 2[Na+](DME)} [31], have been reported and characterized 2 2 3 2 2 2 3 2 2 2 −• as stable aromatic systems. In addition, air oxidation of 1-silafluorenyl dianion has been reported to give 1,10-bis(silafluorenyl) dianion, besides analogue of the dianion [53,54]. The bissilole dianion (7) is still an electron-rich aromatic substrate to cupric chloride (CuCl2) in Scheme4. Therefore, two single-electron transfers from the bissilole dianion ( 7) to two Cu(II) readily 2 gives 1,10-bis(radical silolyl) [Ph4C4Si-SiC4Ph4] • (8) with two equivalents of cuprous dichloride anion I {[Cu Cl2]−}, since there is not enough space for four α-phenyls around two silicon atoms in (7) to form 1,10-disila-fulvalene in plane. The observed strong para-magnetism of the green solid is thought to be due to the radicals in (8). A stable free radical of the similar 2,3,4,5-tetraphenyl-silolyl species has also been reported [33]. Stirring the isolated green solid, the bisradical species (8) and lithium cuprous chloride salts of 2{[Li CuICl ]+ [CuICl ] }, in nonpolar toluene at room temperature, provides 2 2 • 2 − 1,10-bis(1-chloro-silole) [Ph4C4Si(Cl)-(Cl)SiC4Ph4](4) with strong Si–Cl bonds via chlorinations on each silicon atom of the bisradical species (8). There, a copper metal and active chlorine radical I (or chlorine molecule) are generated from the cuprous dichloride anion {[Cu Cl2]−} while the other chloride anion remains, then the active chlorine radical is coupled with the radical on each silicon Molecules 2019, 24, 3772 5 of 10 atom of the bis(radical silolyl) (8) in Scheme4. The coupling of the radical moiety ( 8) and the chlorine radical is consistent with other oxidative halogenation mechanisms of arenes, since the cationic radical intermediate of arene transforms into a neutral arene radical through its deprotonation step during halogenations [46–51]. Molecules 2019, 24, x 5 of 10

Ph4 Ph4 Ph4 Cl II 2Cu Cl2 1/2 Si Si 0 I -Cu , -[Li2Cu Cl3] Si Li+ Li+ Cl 3 4

II Cu Cl2 SET

Ph4

[Li CuICl ]+ 0 2 2 I + I - -Cu [Li2Cu Cl2] [Cu Cl2] -[Li CuICl ] Si 2 3

6

x2

Ph4 Ph4 Ph4 Ph4 II Cu Cl2 1/2 1/2 Si Si I + I - Si Si -[Li2Cu Cl2] [Cu Cl2]

I + Two SET 2[Li2Cu Cl2]

7 8

SchemeScheme 4. 4.Oxidative Oxidative chlorination chlorination mechanism mechanism of of the the silole silole dianion dianion {[SiC {[SiCPh4Ph]24]2−•2[Li2[Li+]}+]} ( 3(3)) by by cupric cupric 4 4 −• chloridechloride (CuCl (CuCl2),2), via via single-electron single-electron transfer transfer and and chlorination chlorination on on each each silicon silicon atom atom of of the the silole silole moiety. moiety.

InIn the the above above mechanism mechanism in in Scheme Scheme4 ,4, it it is is of of interest interest that that one one copper(II) copper(II) atom atom is is reduced reduced to to coppercopper metal metal by by two two single-electron single-electron transfers transfers via via Cu(I), Cu while(I), while the otherthe other copper(II) copper(II) atom atom is reduced is reduced into Cu(I)into viaCu(I) single-electron via single-electron transfer. transfer. Furthermore, Furthermore, among fouramong equivalent four equivalent chlorine atoms,chlorine one atoms, chlorine one atomchlorine is consumed atom is consumed by the chlorination by the chlorination of the silicon of the atom silicon in atom each siloylin each moiety, siloyl moiety, and the and other the three other I chlorinethree chlorine atoms remainatoms remain as chloride as chloride anions anions (2LiCl and(2LiCl Cu andCl), Cu sinceICl), there since is there no chlorine is no chlorine source source other II thanother two than equivalent two equivalent cupric cupric chloride chloride (Cu Cl (Cu2) inIICl reaction2) in reaction Scheme Scheme4. In general,4. In general, oxychlorinations oxychlorinations of arenesof arenes proceed proceed with with the copperthe copper catalyst catalyst in the in presencethe presence of a of chlorine a chlorine source source (LiX) (LiX) [48,55 [48,55–57].–57]. InIn modern modern chemistry, chemistry, triorganosilylcopper triorganosilylcopper compounds compounds are are important important reagents reagents and and the the use use of of themthem in in organic organic synthesis synthesis was was pioneered pioneered by by Fleming Fleming [57 [57].]. They They are are usually usually prepared prepared in situ in situ and and can becan usedbe used for thefor the functionalization functionalization of alkenesof alkenes and and other other reactions reactions under under mild mild conditions conditions [ 45[45].]. Generally, Generally, informationinformation on on the the structure structure of of the the silylcopper silylcopper compounds, compounds, in in general general type type Li[Cu(SiR Li[Cu(SiR3)32)],2], isis very very limitedlimited even even though though such such species species could could have have some some significance significance in in the the important important Müller–Rochow Müller–Rochow industrialindustrial process, process, andand there there is is still still plenty plenty of of room room for for further further development development of of copper-catalyzed copper-catalyzed silylationssilylations toto makemake carbon–silicon bonds bonds [58]. [58]. Although Although only only couples couples of bulky of bulky silyl silyl cuprates cuprates have been isolated and characterized, they have diverse structures such as ligand-stabilized silyl cuprate [Ph3SiCu(PMe3)3] [59], µ2-bridged hypersilyl cuprates [Li(THF)3][Cu2{Si(SiMe3)3}2Br] [60], Li[Cu2{Si(SiMe3)3}3] and Li[Cu{Si(SiMe3)3}2] [61], discrete linear structures of disilylcuprates Na[Cu{Si(t-Bu)3}2][THF]2,4 [62] and [K or Na][Cu2{Si(SiMe3)3}3] [63], and a nearly planar cyclic trimer of [Cu3{Si(SiMe3)3}3], with almost linear Si–Cu–Si in the solid state [64]. In particular, µ2-bridged di(hypersilyl)cuprates [Li(thf)4] [Cu{Si(SiMe3)3}2(CuCl)4] [65], which is synthesized from the reaction of copper(I) chloride with lithium hypersilanide at −78 °C in THF, is reported as an extremely

Molecules 2019, 24, 3772 6 of 10 have been isolated and characterized, they have diverse structures such as ligand-stabilized silyl 2 cuprate [Ph3SiCu(PMe3)3][59], µ -bridged hypersilyl cuprates [Li(THF)3][Cu2{Si(SiMe3)3}2Br] [60], Li[Cu2{Si(SiMe3)3}3] and Li[Cu{Si(SiMe3)3}2][61], discrete linear structures of disilylcuprates Na[Cu{Si(t-Bu)3}2][THF]2,4 [62] and [K or Na][Cu2{Si(SiMe3)3}3][63], and a nearly planar cyclic 2 trimer of [Cu3{Si(SiMe3)3}3], with almost linear Si–Cu–Si in the solid state [64]. In particular, µ -bridged di(hypersilyl)cuprates [Li(thf)4] [Cu{Si(SiMe3)3}2(CuCl)4][65], which is synthesized from the reaction of copper(I) chloride with lithium hypersilanide at 78 C in THF, is reported as an extremely − ◦ kinetically unstable intermediate to decomposing above 30 C, with precipitation of copper metal [65]. − ◦ The precipitation of copper metal from copper(I) coincides with the suggestion of the mechanism in Scheme4. Nevertheless, this does not completely rule out the possibility that the isolate green solid is II the bissilole dianion cuprate (7) containing unreacted cupric chloride (Cu Cl2), since the observed paramagnetism might be due to the copper(II) with d9 electronic structure. There is another possibility, that the disproportionation of two equivalents of cuprous chloride (CuICl), reduced from cupric II 0 chlorides (Cu Cl2) via single-electron-transfer, provides copper metal (Cu ) and cupric chlorides II I (Cu Cl2), which is then reduced to cuprous chloride (Cu Cl), generating a chlorine radical, which is used for the chlorination of the silicon atom in each silolyl moiety. In addition, a ligand exchange II reaction between the silole dianion (3) and chloride anion of cupric chloride (Cu Cl2) could afford a disilene of 1,10-disila-fulvalene, via a silolyl cuprates {(Silolyl)2Cu2}. Then, the disilene could react with chlorine to afford the chlorinated bissilole (4). Determining which structure is the active intermediate (7 or 8) is of interest for future work surrounding other oxidation reactions.

4. Materials and Methods All reactions were performed under an inert atmosphere, using standard Schlenk techniques. Air-sensitive reagents were transferred in a nitrogen-filled glovebox. THF and ether were distilled from sodium diphenylketyl under nitrogen. Toluene was stirred over concentrated H2SO4 and distilled from CaH2. Melting points were measured with Wagner & Müntz Co. (München, Germany), Capillary Type. To dry the metal chloride, hydrous cupric chloride with an excess of chlorotrimethylsilane in THF was refluxed, with stirring, under nitrogen atmosphere for several hours [66]. 1,10-Bis(1-Me-2,3,4,5-tetraphenyl-1-silacyclopentadienyl) [Ph4C4Si(Me)-(Me)SiC4Ph4](2): The silole anion {[MeSiC Ph ] [Li+] or [MeSiC Ph ] [Na+]} (1) was prepared according to the literature 4 4 −• 4 4 −• and used in situ [25]. Addition of silole anion salt (1.58 g, 3.96 mmol for the lithium salt, 1.61 g, 3.96 mmol for the sodium salt) to pale brown anhydrous ferrous chloride (0.29 g, 2.29 mmol, FeCl2) in THF changed the dark violet solution into a black solution, with precipitation of black particles. After completing the addition, it was stirred for 2 h. Workup of the mixture with distilled water and dichloromethane gave a pale green, unclear solution and black precipitation. Filtration of the mixture gave a green solution, from which removal of dichloromethane under reduced pressure gave a pale green solid. The solid was washed with ether for purification and identified as the bissilole (2) by the comparison of its analytical data with an authentic sample [25]. The back particle was identified as iron metal by its strong para-magnetism. Yield: 0.67 g (85%). 1,10-Bis(1-chloro-2,3,4,5-tetraphenyl-1-silacyclopentadienyl) [Ph4C4Si(Cl)-(Cl)SiC4Ph4](4): The silole dianion salt {[SiC Ph ]2 2[Li+]} (3) was prepared according to the literature used in situ [10]. 4 4 −• The silole dianion solution (1.79 mmol) in THF was added to brownish anhydrous cupric chloride (CuCl , 0.481 g, 3.58 mmol), with stirring, at 78 C, and it was stirred for 3 h. Then a greenish solution 2 − ◦ was obtained. After the greenish solution was filtered and THF removed from the resultant, a green solid was isolated. The solid was dissolved in toluene, and the toluene solution was stirred for 1 h at room temperature. Then, metallic copper was precipitated on the surface of the reaction flask. Filtration of the mixture gave toluene solution, and the solution was concentrated under reduced Molecules 2019, 24, 3772 7 of 10 pressure. The resultant solution was refrigerated for a couple of days and yellow crystals of (4) were obtained. Yield: 0.639 g (85%); mp 299 ◦C (Lit. 300–304 ◦C), Anal. Found: C 79.88, H 4.88, C56H40Cl2Si2 29 + calcd.: C 80.07, H 4.80; Si-NMR (CDCl3, ref; ext. TMS = 0.00), 0.24 (Si); MS (M , relative abundance) m/z 838, 419 (lit. [67]).

5. Conclusions Silole anion [MeSiC Ph ] [Li+ or Na+](1) was oxidized by anhydrous ferrous chloride (FeCl ) 4 4 −• 2 at room temperature; 1,10-bis(1-methyl-2,3,4,5-tetraphenyl-1-silacyclopentadienyl) (2) was obtained quantitatively with reduced iron metal. From the reaction of the silole dianion [SiC Ph ]2 2[Li+] 4 4 −• (3) with cupric chloride (CuCl ) at 78 C in THF, a green solid was isolated. The green solid in toluene 2 − ◦ was transformed at room temperature into 1,10-bis(1-chloro-2,3,4,5-tetraphenyl-1-silacyclopentadienyl) (4) quantitatively, with precipitation of copper metal on the surface of the reaction flask.

Funding: This work was supported by the research grant of Pai Chai University in 2019. (Publication). Conflicts of Interest: The author declares no conflict of interest.

References

1. Lee, V.Y.; Sekiguchi, A.; Ichinohe, M.; Fukaya, N. Stable aromatic compounds containing heavier group 14 elements. J. Organomet. Chem. 2000, 611, 228–235. [CrossRef] 2. Hissler, M.; Dyer, P.W.; Réau, R. Linear organic π-conjugated systems featuring the heavy group 14 and 15 elements. Co-ord. Chem. Rev. 2003, 244, 1–44. [CrossRef] 3. Grützmacher, H. Five-membered aromatic and antianromatic rings with , germanium, and centers: A short march through the periodic table. Angew. Chem. Int. Ed. 1995, 34, 295–298. [CrossRef] 4. Lee, V.Y.; Sekiguchi, A. Aromaticity of group 14 organometallics: Experimental aspects. Angew. Chem. Int. Ed. 2007, 38, 6596–6620. [CrossRef] 5. Saito, M.; Yoshioka, M. The anions and dianions of group 14 metalloles. Co-ord. Chem. Rev. 2005, 249, 765–780. [CrossRef] 6. Saito, M. Challenge to expand the concept of aromaticity to - and lead-containing carbocyclic compounds: Synthesis, structures and reactions of dilithiostannoles and dilithioplumbole. Co-ord. Chem. Rev. 2012, 256, 627–636. [CrossRef] 7. Joo, W.-C.; Hong, J.-H.; Choi, S.-B.; Son, H.-E.; Kim, C.H. Synthesis and reactivity of 1,1-disodio-2,3,4,5-tetraphenyl-1-silacyclopentadiene. J. Organomet. Chem. 1990, 391, 27–36. [CrossRef] 8. Hong, J.-H.; Boudjouk, P. Synthesis and characterization of a delocalized germanium-containing dianion: Dilithio-2,3,4,5-tetraphenyl-germole. Bull. Soc. Chim. Fr. 1995, 132, 495–498. [CrossRef] 9. Hong, J.H.; Boudjouk, P. A stable aromatic species containing silicon. Synthesis and characterization of the 1-tert-butyl-2,3,4,5-tetraphenyl-1-silacyclopentadienide anion. J. Am. Chem. Soc. 1993, 115, 5883–5884. [CrossRef] 10. Hong, J.-H.; Boudjouk, P.; Castellino, S. Synthesis and characterization of two aromatic silicon-containing

dianions: The 2,3,4,5-tetraphenylsilole dianion and the 1,10-disila-2,20,3,30,4,40,5,50-octaphenylfulvalene dianion. Organometallics 1994, 13, 3387–3389. [CrossRef] 11. West, R.; Sohn, H.; Bankwitz, U.; Calabrese, J.; Apeloig, T.; Müller, T. Dilithium derivative of tetraphenylsilole: An η1-η5 dilithium structure. J. Am. Chem. Soc. 1995, 117, 11608–11609. [CrossRef] 12. Freeman, W.P.; Tilley, T.D.; Yap, G.P.A.; Rheingold, A.L. Siloyl anions and silole dianions: Structure of + 2 [K(18-crown-6) ]2[C4Me4Si −]. Angew. Chem. Int. Ed. 1996, 35, 882–884. [CrossRef] 13. West, R.; Sohn, H.; Powell, D.R.; Müller, T.; Apeloig, Y. The dianion of tetraphenylgermole is aromatic. Angew. Chem. Int. Ed. 1996, 35, 1002–1004. [CrossRef] 14. Choi, S.-B.; Boudjouk, P.; Hong, J.-H. Unique bis-η5/η1-bonding in a dianionic germole. Synthesis and structural characterization of the dilithium salt of the 2,3,4,5-tetraethyl germole dianion. Organometallics 1999, 18, 2919–2921. [CrossRef] Molecules 2019, 24, 3772 8 of 10

15. Freeman, W.P.; Tilley, T.D.; Liable-Sands, L.M.; Rheingold, A.L. Synthesis and study of cyclic π-systems containing silicon and germanium. The question of aromaticity in cyclopentaidenyl analogues. J. Am. Chem. Soc. 1996, 118, 10457–10468. [CrossRef] 16. Goldfuss, B.; Schleyer, P.V.R.; Hampel, F. Aromaticity in silole dianions: Structural, energetic, and magnetic aspects. Organometallics 1996, 15, 1755–1757. [CrossRef] 17. Goldfuss, B.; Schleyer, P.V.R. Aromaticity in group 14 metalloles: Structural, energetic, and magnetic criteria. Organometallics 1997, 16, 1543–1552. [CrossRef] 18. Aubouy, L.; Huby, N.; Hirsch, L.; Van Der Lee, A.; Gerbier, P. Molecular engineering to improve the charge carrier balance in single-layer silole-based OLEDs. New J. Chem. 2009, 33, 1290. [CrossRef] 19. Bankwitz, U.; Sohn, H.; Powell, D.R.; West, R. Synthesis, soild-state structure, and reduction of 1,1-dichloro-2,3,4,5-tetramethylsilole. J. Organomet. Chem. 1995, 499, C7–C9. [CrossRef] 20. Hong, J.-H.; Pan, Y.; Boudjouk, P. A novel lithocenophane derivative of a trisgermole dianion:

[Li(thf)(tmeda)][2,3,4,5-Et4-Ge,Ge-{Li(2,3,4,5-Et4C4Ge)2}C4Ge]. Angew. Chem. Int. Ed. 1996, 35, 186–188. [CrossRef]

21. Sohn, H.; Powell, D.R.; West, R.; Hong, J.-H.; Joo, W.-C. Dimerization of the silole anion [C4Ph4SiMe]− to a tricyclic diallylic dianion. Organometallics 1997, 16, 2770–2772. [CrossRef] 22. Pan, Y.; Hong, J.-H.; Choi, S.-B.; Boudjouk, P.Surprising reaction of non-nucleophilic bases with 1-hydrosiloles: addition and not deprotonation1. Organometallics 1997, 16, 1445–1451. [CrossRef] 23. Hong, J.-H.; Boudjouk, P. Synthesis and characterization of a novel pentavalent silane: 1-Methyl-1,1- dihydrido-2,3,4,5-tetraphenyl-1-silacyclopentadiene silicate, [Ph C SiMeH ] [K+]. Organometallics 1995, 4 4 2− • 14, 574–576. [CrossRef] 24. Hong, J.-H. Synthesis and NMR-study of the 2,3,4,5-tetraethylsilole dianion [SiC Et ]2 2[Li]+. Molecules 4 4 −• 2011, 16, 8033–8040. [CrossRef] 2 25. Hong, J.-H. Dissociation of the disilatricyclic diallylic dianion [(C4Ph4SiMe)2]− to the silole anion [MeSiC4Ph4]− by halide ion coordination or halide ion nucleophilic substitution at the silicon atom. Molecules 2011, 16, 8451–8462. [CrossRef] 26. Zhan, X.; Risko, C.; Amy, F.; Chan, C.; Zhao, W.; Barlow, S.; Kahn, A.; Brédas, J.-L.; Marder, S.R. Electron affinities of 1,1-diaryl-2,3,4,5-tetraphenylsiloles: Direct measurements and comparison with experimental and theoretical estimates. J. Am. Chem. Soc. 2005, 127, 9021–9029. [CrossRef] 27. Tandura, S.N.; Kolesnikov, S.P.; Nosov, K.S.; Lee, V.Y.; Egorov, M.P.; Nefedov, O.M. Electron density distributions in substituted 2,3,4,5-tetraphenyl-1-germacyclopenta-2,4-dienes studied by NMR spectroscopy. Russ. Chem. Bull. 1997, 46, 1859–1861. [CrossRef] 28. Fekete, C.; Nyulászi, L.; Kovács, I. Synthesis and NMR characterization of 2,5-bis(Trimethylsilyl)- 3,4-diphenyl-1-silacyclopentadienyl dianion. Sulfur Silicon Relat. Elem. 2014, 189, 1076–1083. [CrossRef] 29. Dong, Z.; Reinhold, C.R.W.; Schmidtmann, M.; Müller, T. Trialkylsilyl-substituted silole and germole dianions. Organometallics 2018, 37, 4736–4743. [CrossRef] 30. Fekete, C.; Kovács, I.; Nyulászi, L.; Holczbauer, T. Planar lithium silolide: aromaticity, with significant contribution of non-classical resonance structures. Chem. Commun. 2017, 53, 11064–11067. [CrossRef] 31. Han, Z.; Xie, K.; Ma, X.; Lu, X. Synthesis and characterization of a 2, 5-bis(trimethylsilyl)-substituted bis(1,

10-silolide) dianion. Phosphorus, Sulfur, Silicon Relat. Elements 2018, 193, 1–15. [CrossRef] 32. Wei, J.; Zhang, W.-X.; Xi, Z. The aromatic dianion metalloles. Chem. Sci. 2018, 9, 560–568. [CrossRef] [PubMed] 33. Toulokhonova, I.S.; Stringfellow, T.C.; Ivanov, S.A.; Masunov, A.; West, R. A disilapentalene and a stable diradical from the reaction of a dilithiosilole with a dichlorocyclopropene. J. Am. Chem. Soc. 2003, 125, 5767–5773. [CrossRef][PubMed] 34. Sohn, H.; Merritt, J.; Powell, D.R.; West, R. A new spirocyclic system: Synthesis of a silaspirotropylidene. Organometallics 1997, 16, 5133–5134. [CrossRef] 35. Toulokhonova, I.S.; Guzei, I.A.; West, R. Synthesis of a silene from 1,1-dilithiosilole and 2-adamantanone. J. Am. Chem. Soc. 2004, 126, 5336–5337. [CrossRef] 36. Toulokhonova, I.S.; Friedrichsen, D.R.; Hill, N.J.; Müller, T.; West, R. Unusual reaction of 1,1-dilithio-2,3,4,5-tetraphenylsilole with 1,3-dienes yielding spirosilanes and elemental lithium. Angew. Chem. Int. Ed. 2006, 45, 2578–2581. [CrossRef] Molecules 2019, 24, 3772 9 of 10

37. Toulokhonova, I.S.; Timokhin, V.I.; Bunck, D.N.; Guzei, I.; West, R.; Müller, T. Silylene and germylene

intermediates in the reactions of silole and germole dianions with N,N0-di-tert-butylethylenediimine. Eur. J. Inorg. Chem. 2008, 2008, 2344–2349. [CrossRef]

38. Jutzi, P.; Karl, A. Synthese und reaktionen einiger 1R, 1R0, 2, 3, 4, 5-tetraphenyl-1-silacyclopentadiene. J. Organomet. Chem. 1981, 214, 289–302. [CrossRef] 39. McCann, S.D.; Stahl, S.S. ChemInform abstract: Copper-catalyzed aerobic oxidations of organic molecules: Pathways for two-electron oxidation with a four-electron oxidant and a one-electron redox-active catalyst. Acc. Chem. Res. 2015, 46, 1756–1766. [CrossRef] 40. Sambiagio, C.; Marsden, S.P.; Blacker, A.J.; McGowan, P.C. Copper catalysed Ullmann type chemistry: From mechanistic aspects to modern development. Chem. Soc. Rev. 2014, 43, 3525–3550. [CrossRef] 41. Punniyamurthy, T.; Rout, L. Recent advances in copper-catalyzed oxidation of organic compounds. Co-ord. Chem. Rev. 2008, 252, 134–154. [CrossRef] 42. Allen, S.E.; Walvoord, R.R.; Padilla-Salinas, R.; Kozlowski, M.C. Aerobic copper-catalyzed organic reactions. Chem. Rev. 2013, 113, 6234–6458. [CrossRef][PubMed] 43. Wendlandt, A.E.; Suess, A.M.; Stahl, S.S. Copper-catalyzed aerobic oxidative C—H functionalizations: Trends and mechanistic insights. Angew. Chem. Int. Ed. 2011, 50, 11062–11087. [CrossRef][PubMed] 44. Shi, Z.; Zhang, C.; Tang, C.; Jiao, N. Recent advances in transition-metal catalyzed reactions using molecular oxygen as the oxidant. Chem. Soc. Rev. 2012, 41, 3381. [CrossRef][PubMed] 45. Fleming, I. Organocopper Reagents: A Practical Approach; Taylor, R.J.K., Ed.; Oxford University Press: Oxford, UK, 1994; Chapter 12; p. 257. 46. Zhang, C.; Tang, C.; Jiao, N. Recent advances in copper-catalyzed dehydrogenative functionalization via a single electron transfer (SET) process. Chem. Soc. Rev. 2012, 41, 3464. [CrossRef][PubMed] 47. Girard, S.A.; Knauber, T.; Li, C.-J. The cross-dehydrogenative coupling of C (sp3)-H bonds: a versatile strategy for CC bond formations. Angew. Chem. Int. Ed. 2014, 53, 74–100. [CrossRef]

48. Yang, L.; Lu, Z.; Stahl, S.S. Regioselective copper-catalyzed chlorination and bromination of arenes with O2 as the oxidant. Chem. Commun. 2009, 42, 6460–6462. [CrossRef] 49. Chen, X.; Goodhue, C.E.; Hao, X.-S.; Yu, J.-Q.; Hao, X.; Yu, J. Cu(II)-catalyzed functionalizations of aryl C—H

bonds using O2 as an oxidant. J. Am. Chem. Soc. 2006, 37, 6790–6791. [CrossRef] 50. Uemura, T.; Imoto, S.; Chatani, N. Amination of the ortho C–H bonds by the Cu(OAc)2-mediated reaction of 2-phenylpyridines with anilines. Chem. Lett. 2006, 35, 842–843. [CrossRef] 51. Menini, L.; Gusevskaya, E.V. Novel highly selective catalytic oxychlorination of phenols. Chem. Commun. 2006, 2, 209–211. [CrossRef] 52. Yamaguchi, S.; Jin, R.-Z.; Tamao, K.; Shiro, M. Silicon-catenated silole oligomers: Oligo(1,1-silole)s. Organometallics 1997, 16, 2486–2488. [CrossRef] 53. Liu, Y.; Stringfellow, T.C.; Ballweg, D.; Guzei, I.A.; West, R. Structure and chemistry of 1-silafluorenyl dianion, its derivatives, and an organosilicon diradical dianion. J. Am. Chem. Soc. 2002, 124, 49–57. [CrossRef] [PubMed] 54. Liu, Y.; Ballweg, D.; Müller, T.; Guzei, I.A.; Clark, R.W.; West, R. Chemistry of the aromatic 9-germafluorenyl dianion and some related silicon and carbon species. J. Am. Chem. Soc. 2002, 124, 12174–12181. [CrossRef] [PubMed] 55. Menini, L.; Gusevskaya, E.V. Aerobic oxychlorination of phenols catalyzed by copper(II) chloride. Appl. Catal. A Gen. 2006, 309, 122–128. [CrossRef] 56. Menini, L.; Santos, J.C.D.C.; Gusevskaya, E.V. Copper-catalyzed oxybromination and oxychlorination of primary aromatic amines using LiBr or LiCl and molecular oxygen. Adv. Synth. Catal. 2008, 350, 2052–2058. [CrossRef] 57. Fleming, I.; Hill, J.H.M.; Parker, D.; Waterson, D. Diastereoselectivity in the alkylation and protonation of some β-silyl enolates. Chem. Commun. 1985, 6, 318–321. [CrossRef] 58. Wilkinson, J.R.; Nuyen, C.E.; Carpenter, T.S.; Harruff, S.R.; Van Hoveln, R. Copper-catalyzed carbon–silicon bond formation. ACS Catal. 2019, 9, 8961–8979. [CrossRef] 59. Cowley, A.H.; Elkins, T.M.; Jones, R.A.; Nunn, C.M. Phosphane-stabilized CuI and AgI silanes. Angew. Chem. Int. Ed. 1988, 27, 1349–1350. [CrossRef] Molecules 2019, 24, 3772 10 of 10

60. Heine, A.; Herbst-Irmer, R.; Stalke, D. [Cu2R2BrLi(thf)3], R=Si(SiMe3)3–a complex containing five-coordinate silicon in a three-centre two-electron bond (thf= tetrahydrofuran). Chem. Commun. 1993, 23, 1729–1731. [CrossRef] 61. Klinkhammer, K.W. Synthesis and crystal structure of the two lithium hypersilylcuprates

LiCu2[Si(SiMe3)3]3[Li7 (O-tBu)6][Cu2{Si(SiMe3) 3}3]. Z. Anorg. Allg. Chem. 2000, 626, 1217–1223. [CrossRef] 62. Lerner, H.-W.; Scholz, S.; Bolte, M. The sodium cuprate (tBu3Si)2CuNa: Formation and X-ray crystal structure analysis. Organometallics 2001, 20, 575–577. [CrossRef] 63. Klinkhammer, K.W.; Klett, J.; Xiong, Y.; Yao, S. Homo- and heteroleptic hypersilylcuprates—Valuable reagents for the synthesis of molecular compounds with a Cu Si bond. Eur. J. Inorg. Chem. 2003, 2003, 3417–3424. − [CrossRef] 64. Klett, J.; Klinkhammer, K.W.; Niemeyer, M. Ligand exchange between arylcopper compounds and bis(hypersilyl)tin or bis(hypersilyl)lead: Synthesis and characterization of hypersilylcopper and a stannanediyl complex with a Cu Sn bond. Chem. Eur. J. 1999, 5, 2531–2536. [CrossRef] − 65. Heine, A.; Stalke, D. Preparation and structure of two highly reactive intermediates: [Li(thf)4][Cu5Cl4R2] and [Li(thf)4][AlCl3R], R=Si(SiMe3)3. Angew. Chem. Int. Ed. 1993, 32, 121–122. [CrossRef] 66. So, J.H.; Boudjouk, P. A convenient synthesis of solvated and unsolvated anhydrous metal chlorides via dehydration of metal chloride hydrates with trimethylchlorosilane. Inorg. Chem. 1990, 29, 1592–1593. [CrossRef] 67. Sohn, H.; Huddleston, R.R.; Powell, D.R.; West, R.; Oka, K.; Yonghua, X. An electroluminescent polysilole and some dichlorooligosiloles. J. Am. Chem. Soc. 1999, 121, 2935–2936. [CrossRef]

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