(Triethoxysilyl)Buta-1,3-Dienes—New Building Blocks for Stereoselective Synthesis of Unsymmetrical (E,E)-1,4-Disubstituted 1,3-Dienes

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Communication 1-(Triethoxysilyl)buta-1,3-dienes—New Building Blocks for Stereoselective Synthesis of Unsymmetrical (E,E)-1,4-Disubstituted 1,3-dienes

Justyna Szudkowska-Fr ˛atczak 1,2, Mariusz Taczała 1,2 and Piotr Pawlu´c 1,2,*

Received: 8 August 2015 ; Accepted: 14 October 2015 ; Published: 28 October 2015 Academic Editor: Łukasz John

1 Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, Poznan 61-614, Poland 2 Center for Advanced Technologies, Adam Mickiewicz University, Umultowska 89c, Poznan 61-614, Poland; [email protected] (J.S.-F.); [email protected] (M.T.) * Correspondence: [email protected]; Tel.: +48-618-291-700; Fax: +48-618-291-555

Abstract: A convenient methodology for the highly stereoselective synthesis of unsymmetrical (1E,3E)-1,4-disubstituted 1,3-dienes based on palladium-catalyzed Hiyama cross-coupling reaction of 1-(triethoxysilyl)-substituted buta-1,3-dienes with aryl iodides is reported.

Keywords: buta-1,3-dienes; organosilicon dienes; C–C bond formation; Hiyama cross-coupling; palladium catalyst

1. Introduction Highly conjugated π-electron compounds such as aryl-substituted dienes and polyenes have gained a lot of attention because of the wide range of their applications in functional materials such as organic fluorescent probes, electroluminescent devices, and nonlinear optical materials [1–5]. Among the syntheses developed to access aryl-substituted buta-1,3-dienes, the transition metal-catalyzed cross-coupling reactions are of prime importance because of their high stereoselectivity [6]. A number of methodologies for the stereoselective preparation of 1,4-disubstituted buta-1,3-dienes based on the palladium-catalyzed cross-coupling of vinyl halides with alkenyl-substituted organometallic compounds of boron [7], tin [8], zinc [9], silicon [10] or zirconium [11,12] have been developed over the last three decades. The complementary synthetic routes involving 1,4-bis-metallated 1,3-butadienyl building blocks are represented by the palladium-catalyzedcross-coupling of aryl or alkenyl halides with 1,4-bis(silyl)- [13,14], 1,4-bis(stannyl)- [15,16], 1,4-bis(boryl)- [17] or 1-boryl-4-stannylbuta-1,3-dienes [18–20]. An alternative approach based on cross-coupling reactions of organometallic reagents with 1,4-diiodobuta-1,3-diene has also been reported [21,22]. The palladium-catalyzed and fluoride-promoted cross-coupling of unsaturated organosilicon compounds with aryl or alkenyl halides (Hiyama coupling) has been recently employed as a mild and efficient alternative to the well-established Stille, Negishi, and Suzuki reactions, taking intoaccount the commercial availability, high stability, and low toxicity of silicon derivatives [23–25]. In view of the above advantages, we have successfully applied various unsaturated organosilicon precursors such as (E)-silylstyrenes [26–28], 1,1-bis(silyl)alkenes [29,30], (E)-1,2-bis(silyl)alkenes [31] and vinylcyclosiloxanes [32] as versatile double-bond equivalents in the construction of π-conjugated systems. On the other hand, reports on the successful cross-coupling of silylated buta-1,3-dienes with aryl or alkenyl halides are strongly limited, mainly due to the complexity of their synthesis. Denmark has reported an efficient [Pd2(dba)3]-catalyzed coupling of 1,4-bis(silyl)buta-1,3-dienes

Materials 2015, 8, 7250–7256; doi:10.3390/ma8115378 www.mdpi.com/journal/materials Materials 2015, 8, 7250–7256

containing two distinct silyl groups (-SiMe2OH and -SiMe2Bn) with aryl iodides to construct unsymmetrical 1,4-diaryl-1,3-butadiene derivatives [13]. The synthesis was possible thanks to Materials 2015, volume, page–page the difference in reactivity between the silanol and silyl groups and the application of reactivityconditions between developed the bysilanol Denmark and silyl and groups co-workers and the for application the efﬁcient of coupling conditions of developed silanols with by Denmarkaryl halides. and Theco‐workers starting for 1,4-bis(silyl)buta-1,3-dienes the efficient coupling of silanols were obtained with aryl by halides. a sequential The starting three-step 1,4‐ bis(silyl)butaprocedure: rhodium-catalyzed‐1,3‐dienes were obtained ethynylsilane by a sequential dimerization, three deprotection‐step procedure: of the rhodium terminal‐catalyzed alkyne, ethynylsilaneand platinum-catalyzed dimerization, hydrosilylation. deprotection Thisof the method terminal has beenalkyne, successfully and platinum extended‐catalyzed to the hydrosilylation.cross-coupling with This alkenyl method iodides has been and appliedsuccessfully as a key extended step in theto the synthesis cross‐ ofcoupling immunosuppressive with alkenyl iodidesagent RK-397 and applied [33]. as a key step in the synthesis of immunosuppressive agent RK‐397 [33]. Recently, we have reportedreported aa newnew methodmethod forfor thethe synthesissynthesis ofof 1-silyl-substituted1‐silyl‐substituted buta-1,3-dienes,buta‐1,3‐dienes, based on the [RuHCl(CO)(PCy33)2]]-catalyzed‐catalyzed silylative silylative coupling coupling of terminal ( E))-1,3-dienes‐1,3‐dienes with vinylsilanes [[34].34]. The The reaction reaction provides provides a a facile facile and and straightforward straightforward access to ( E,E))-dienylsilanes,‐dienylsilanes, including alkoxy alkoxy-substituted‐substituted silanes in a highly stereoselective fashionfashion (Scheme(Scheme1 1).).

Scheme 1. Synthesis of 1 1-(triethoxysilyl)buta-1,3-dienes.‐(triethoxysilyl)buta‐1,3‐dienes.

Since the starting 1‐(triethoxysilyl)buta‐1,3‐dienes can be easily prepared with good yield in a Since the starting 1-(triethoxysilyl)buta-1,3-dienes can be easily prepared with good yield in one‐step process from inexpensive and commercially available substrates, we have envisaged that they a one-step process from inexpensive and commercially available substrates, we have envisaged could be used as coupling partners for the stereoselective synthesis of 1,4‐disubstituted buta‐1,3‐dienes that they could be used as coupling partners for the stereoselective synthesis of 1,4-disubstituted via Hiyama coupling with aryl or alkenyl iodides. Therefore, herein we report our results on the use buta-1,3-dienes via Hiyama coupling with aryl or alkenyl iodides. Therefore, herein we report our of 1‐(triethoxysilyl)buta‐1,3‐dienes as new platforms for the installation of aryl groups onto the C=C core results on the use of 1-(triethoxysilyl)buta-1,3-dienes as new platforms for the installation of aryl which leads to unsymmetrically (E,E)‐1‐aryl‐ or (E,E)‐1,4‐diaryl‐substituted buta‐1,3‐diene derivatives. groups onto the C=C core which leads to unsymmetrically (E,E)-1-aryl- or (E,E)-1,4-diaryl-substituted buta-1,3-diene derivatives. 2. Results and Discussion 2. ResultsHaving and established Discussion an efficient protocol for the highly selective synthesis of 1‐silylHaving‐buta‐1,3‐ establisheddienes, we subsequently an efficient investigated protocol fortheir thereactivity highly towards selective selected synthesis aryl and of alkenyl1-silyl-buta-1,3-dienes, iodides under Hiyama we subsequently cross‐coupling investigated conditions. their Initial reactivity studies towards were selectedcarried out aryl using and 1alkenyl‐phenyl iodides‐4‐(triethoxysilyl)buta under Hiyama‐1,3 cross-coupling‐diene (mixture conditions. of isomers: Initial (E studies,E)/(E,Z)/( wereZ,Z) carried = 83:15:2) out using and iodobenzene1-phenyl-4-(triethoxysilyl)buta-1,3-diene in the presence of [Pd2(dba)3 (mixture] catalyst (4 of mol isomers: % Pd) and (E,E )/(tetrabutylammoniumE,Z)/(Z,Z) = 83:15:2) fluoride and (TBAF) (2 equiv.) as an activator. After several attempts, we found that its reaction with 1.2 equiv. of iodobenzene in the presence of [Pd2(dba)3] catalyst (4 mol % Pd) and tetrabutylammonium fluoride aryl(TBAF) iodide (2 equiv.) conducted as an in activator. tetrahydrofuran After several (THF) attempts, at 65 °C wefor found24 h exclusively that its reaction afforded with the 1.2 coupling equiv. productof aryl iodide(E,E)‐1,4 conducted‐diphenylbuta in‐ tetrahydrofuran1,3‐diene 1, as a (THF)single stereoisomer at 65 ˝C for in 24 89% h exclusively yield (Table afforded 1, entry the1). Similarcoupling reactivity product of (E ,1E‐)-1,4-diphenylbuta-1,3-dienephenyl‐4‐(triethoxysilyl)buta 1,‐1,3 as‐diene a single was stereoisomer observed with in 89% other yield aryl (Table iodides1, containingentry 1). Similarelectron reactivity‐withdrawing of 1-phenyl-4-(triethoxysilyl)buta-1,3-diene or electron‐donating groups (Table was 1, observed entry with2–4). otherThe stereoselectivityaryl iodides containing of the Hiyama electron-withdrawing coupling was high. or electron-donating Although the starting groups silyldiene (Table1 ,consisted entry 2–4). of a mixtureThe stereoselectivity of geometrical of isomers, the Hiyama in all coupling cases the was (E, high.E) double Although‐bond thegeometry starting was silyldiene strongly consisted favored 1 (99%)of a mixture as measured of geometrical by H NMR isomers, (Nuclear in all Magnetic cases the Resonance (E,E) double-bond Spectroscopy) geometry and GC was‐MS. strongly (Gas chromatography–massfavored (99%) as measured spectrometry). by 1H NMR The (Nuclear (E,E)‐1,4 Magnetic‐diarylbuta Resonance‐1,3‐dienes Spectroscopy) 1–4 were isolated and GC-MS. and characterized(Gas chromatography–mass spectroscopically spectrometry). (see Supporting The ( E,Information;E)-1,4-diarylbuta-1,3-dienes Figures S1–S8). 1–4 Although were isolated the 1 stereochemistryand characterized of dienes spectroscopically 1–4 cannot be (see directly Supporting derived Information; from the H NMR Figures spectra S1–S8). on the Although basis of the protonsstereochemistry of the diene of dienesmoiety, 1–4 the cannotanalysis be of directlyspin systems derived by means from of the MestReC1H NMR NMR spectra software on the (Mestrelab basis of Research,the protons Santiago of the de diene Compostela, moiety, the Spain) analysis [21] as of well spin as systems comparison by means with literature of MestReC data [35–38] NMR software allowed us(Mestrelab to confirm Research, the diene Santiagostructure. de Compostela, Spain) [21] as well as comparison with literature dataThe [35– 38palladium] allowed us‐catalyzed to confirm Hiyama the diene structure.coupling proceeded efficiently also for other 1‐(triethoxysilyl)The palladium-catalyzed‐substituted dienes Hiyama such as 1 coupling‐methoxy‐4 proceeded‐(triethoxysilyl)buta efficiently‐1,3‐diene also (mixture for other of isomers:1-(triethoxysilyl)-substituted (E,E)/(E,Z)/(Z,Z) = 68:20:12) dienes (Table such 1, as entry 1-methoxy-4-(triethoxysilyl)buta-1,3-diene 5–6) or 1‐(triethoxysilyl)penta‐1,3‐diene (mixture of isomers: (E,E)/(E,Z)/(Z,Z) = 71:16:13) (Table 1, entry 7). The noteworthy feature of these processes is that the formation of 1‐substituted buta‐1,3‐diene (via protodesilylation) was suppressed. The formation of biaryls (by competitive homo‐coupling7251 of aryl iodides) was observed under given conditions in 5%–10% yield. It is worth noting that the Hiyama coupling processes proceeded in a highly stereoselective manner to yield products containing (E,E)‐dienes as predominant compounds;

2 Materials 2015, 8, 7250–7256 of isomers: (E,E)/(E,Z)/(Z,Z) = 68:20:12) (Table1, entry 5–6) or 1-(triethoxysilyl)penta-1,3-diene (mixture of isomers: (E,E)/(E,Z)/(Z,Z) = 71:16:13) (Table1, entry 7). The noteworthy feature of these processes is that the formation of 1-substituted buta-1,3-diene (via protodesilylation) was suppressed. The formation of biaryls (by competitive homo-coupling of aryl iodides) was observed under given conditions in 5%–10% yield. It is worth noting that the Hiyama coupling Materials 2015, volume, page–page MaterialsMaterials 2015 2015, volume, volume, page–page, page–page processesMaterialsMaterials 20152015 proceeded,, volumevolume,, page–pagepage in a–page highly stereoselective manner to yield products containing (E,E)-dienes Materials 2015, volume, page–page Materials 2015, volume, page–page ashowever,Materials predominant 2015 trace, volume amounts compounds;, page–page of the however, respective trace (E,Z amounts) isomers of (1% the–5%) respective were also (E, Zdetected) isomers using (1%–5%) the GC were-MS however,however,however, trace trace trace amounts amounts amounts of of theof the the respective respective respective (E ( ,E(ZE,Z), Zisomers) )isomers isomers (1%–5%) (1% (1%–5%)–5%) were were were also also also detected detected detected using using using the the the GC GC GC‐MS-MS‐MS alsomethodhowever,however, detected (Scheme trace trace using 2).amounts amounts theThe GC-MS (E ,of Eof) the- the1- methodaryl respective respective-4-methoxybuta (Scheme ( E(E,Z,Z) )2isomers ).isomers- The1,3-dienes ((1% (1%–5%)E,E–)-1-aryl-4-methoxybuta-1,3-dienes5%) 5–6 were andwere (also Ealso,E) -detected 1detected-arylpenta using using-1,3 the -thediene GC GC 5–6- 7MS‐MS methodhowever,methodmethod (Scheme (Scheme (Schemetrace amounts 2). 2). 2).The The The ( E of( ,E(E Ethe,)E,‐E)1- )‐1respective‐aryl-1aryl‐aryl‐4-‐4‐methoxybuta-4methoxybuta‐methoxybuta (E,Z) isomers‐1,3-1,3‐1,3‐ (1%–5%)dienes-dienes‐dienes 5–6 5 were –5–6 6and and and also (E ( ,E(E E,detected)E,‐E)1-)‐1‐arylpenta-1arylpenta‐arylpenta using‐1,3 -the1,3‐1,3‐diene -GCdiene‐diene‐MS 7 7 7 andwerehowever,methodmethod (E isolated,E)-1-arylpenta-1,3-diene (Scheme (Schemetrace and amounts characterized 2). 2). The The of ( E( Ethe,E,E) 7 - )respectivespectroscopically1‐1 were-aryl‐aryl- isolated4‐4-methoxybuta‐methoxybuta (E,Z) and isomers (see characterized -Supporting1,3‐1,3 (1%–5%)-dienes‐dienes 5were spectroscopically5–6Information;–6 and and also ( (E Edetected,E,E))- 1‐Figures1-‐arylpentaarylpenta (see using S9 Supporting– the-S14).1,3‐1,3 GC-‐diene diene‐MS 7 7 weremethodwerewere isolated isolated isolated (Scheme and and and characterized2). characterized characterizedThe (E,E)‐1 spectroscopically‐aryl spectroscopically spectroscopically‐4‐methoxybuta (see (see (see‐1,3 Supporting Supporting ‐Supportingdienes 5–6 Information; Information;and Information; (E,E)‐1 ‐Figuresarylpenta Figures Figures S9–S14). S9‐ S9–S14).1,3–S14).‐diene 7 Information;methodwerewere isolated isolated (Scheme Figures and and 2). S9–S14).characterized characterizedThe (E,E)‐1‐aryl spectroscopically spectroscopically‐4‐methoxybuta (see (see‐1,3 Supporting ‐Supportingdienes 5–6 andInformation; Information; (E,E)‐1‐arylpenta Figures Figures S9‐ S9–S14).1,3–S14).‐diene 7 were isolated and characterized spectroscopically (see Supporting Information; Figures S9–S14). were isolated and characterized spectroscopically (see[Pd2 (Supportingdba)3] Information; Figures S9–S14). [Pd2(dba)3] [Pd[Pd2(dba)2(dba)3]a 3] I I T[PBdA2F(db )3] Si OEt + I I TBAF, T[PdBTBAFoA2F(dba), 3] S(i OE)t3 + I T H[PdF ,6 25(dba)TBCo A,2F3 4] h R Si(OEt)Si(OEt)( 3)3 I R1 TH[PdF 6o5(dba)Co 24] h R R1 R Si(OEt)+33 ++ RR1 THF, THF, 65,2 65C,TBAFo 24hC,3, 24h R R1 R R Si(OEt) I 1 RR1 THTBAFF 65 Co 24hR R R1 R R Si(OEt) 3 +I R1 THF,TBAF 65 C, 24h R - R11 R Si(OEt)3 + 1THF, 65oC, 24h R 54 8-9 R1 R = , e , e 3 + R1 o R 54 8 9%% R1 RRR =PPhh,MMeO ,MMe R1 THF, 65 C, 24h R 54-89%, 54-89%> - R R=Ph,MeO,Me R=Ph,MeO,Me= = , -, O, , - , - (EEE,E)54>98595%% 1 R1 R=H P,3hN-MOe2O,4 M- eO ,2 N- p (E(,E)>95%,)54-89% 9 % R1 H 3 N-O2 4 M-eO 2 N-p (EE,EE)>95%> 95% R1 =H,3-NORR=Ph,MeO,Me1 =H,3-NO= , 2, 4-MeO,2,, 4-MeO,e 2-Np, 2-Np (54-89%) R=Ph,MeO,MeR1 H 3 NO2 4 M O 2 Np (54-89%E,E)>95% R=Ph,MeO,MeR1 =H,3-NO2, 4-MeO, 2-Np (E,E)>95% R1 =H,3-NOScheme2, 4-MeO, 2. Synthesis 2-Np of (E,E)-1,4-disubstituted buta-1,3-dienes(E,E)>95%. R1 =H,3-NOScheme2, 4-MeO, 2. Synthesis 2-Np of (E,E)-1,4-disubstituted buta-1,3-dienes. SchemeScheme 2. Synthesis 2. Synthesis of ( ofE, E(E))-1,4-disubstituted‐,1,4E)‐‐1,4disubstituted‐disubstituted buta buta-1,3-dienes. buta‐1,3‐‐1,3dienes.‐dienes. SchemeScheme 2. 2. Synthesis Synthesis of of ( E(E,E,E)-)1,4‐1,4-disubstituted‐disubstituted buta buta-1,3‐1,3-dienes‐dienes.. Scheme 2. Synthesis of (E,E)‐1,4‐disubstituted buta‐1,3‐dienes. Table 1. HiyamaScheme cross 2.-coupling Synthesis of of 1 -((triethoxysilyl)butaE,E)‐1,4‐disubstituted-1,3 buta-dienes‐1,3 with‐dienes. aryl iodides. TableTableTable 1. 1. Hiyama 1. Hiyama Hiyama cross cross-coupling cross cross‐coupling-coupling‐coupling of of 1-(triethoxysilyl)buta-1,3-dienes1of ‐1(triethoxysilyl)buta -1(triethoxysilyl)buta‐(triethoxysilyl)buta‐1,3-1,3‐‐1,3dienes-dienes‐dienes with with with aryl aryl aryl iodides. iodides iodides. . TableTable 1. 1. Hiyama Hiyama cross cross-coupling‐coupling of of 1 1-(triethoxysilyl)buta‐(triethoxysilyl)buta-1,3‐1,3-dienes‐dienes with with aryl aryl iodides iodides.. Table 1. Hiyama cross‐coupling of 1‐(triethoxysilyl)buta‐1,3‐dienes with aryl iodides. RTable 1. Hiyama cross‐coupling of 1‐(triethoxysilyl)buta‐1,3Isolated‐dienes Yield with aryl iodides.Selectivity EntryEntry R (Diene)RR R Aryl Aryl Iodide Iodide Product IsolatedIsolatedIsolatedIsolated Yield Yield [%]Yield Yield SelectivitySelectivitySelectivitySelectivity EE/EZ EntryEntryEntry (Diene)RR ArylArylAryl Iodide Iodide Iodide ProductProductProduct IsolatedIsolated[%] Yield Yield EE/EZSelectivitySelectivity EntryEntry(Diene) (Diene)(Diene)R ArylAryl Iodide Iodide ProductProduct Isolated[%][%][%] Yield SelectivityEE/EZEE/EZEE/EZ Entry (Diene)(Diene)R Aryl Iodide Product Isolated[%][%] Yield SelectivityEE/EZEE/EZ Entry (Diene) Aryl Iodide Product [%] EE/EZ 11 (Diene)Ph Ph PhI PhI 8989[%] 99:199:1EE/EZ 1 1 1 PhPh Ph PhIPhIPhI 8989 89 99:199:199:1 11 PhPh PhIPhI 8989 99:199:1 1 Ph PhI 89 99:1 1 Ph PhI 89 99:1 2 6 4 NO 2 Ph 3-NO C2 H6 I4 N 2 86 >99 2 2 2 Ph Ph Ph 3‐3NO-3 3-NONO‐NO2CC62HCCH46I6H HI4 I4 I O2 868686 86 >99>99>99>99 2 Ph 3-NO2C6H4I NO 86 >99 2 Ph 3‐NO2C6H4I 2 86 >99 2 Ph 3‐NO2C6H4I OMe 86 >99 2 Ph 3‐NO2C6H4I OMe 86 >99 OMe 6 4 3 Ph 4-MeOC H6 I4 79 >99 3 3 3 PhPh Ph 4‐4MeOC-4MeOC‐MeOC6HH46IH I4 I 7979 79 >99>99>99 33 PhPh 4 4-MeOC-MeOC6HH44II 7979 >99 >99 3 Ph 4‐MeOC6H4I 79 >99 3 Ph 4‐MeOC6H4I 79 >99 3 Ph 4‐MeOC6H4I 79 >99

10 7 4 Ph 2-C 10H I7 55 99:1 4 4 4 PhPh Ph 2‐2C-2C10‐CH10H7IH I7 I 5555 55 99:199:199:1 44 PhPh 2-C2-C10HH7II 5555 99:199:1 4 Ph 2‐C1010H77I 55 99:1 4 Ph 2‐C10H7I e 55 99:1 4 Ph 2‐C10H7I OOMMe 55 99:1 Me 6 4 O 5 MeO 4-MeOC H6 I4 70 95:5 5 5 5 MeOMeOMeO 4‐4MeOC-4MeOC‐MeOC6HH46IH I4 I 7070 70 95:595:595:5 5 MeO 4-MeOC6H4I MeeO 70 95:5 55 MeOMeO 4-MeOC4‐MeOC66HH44II M O 7070 95:5 95:5 5 MeO 4‐MeOC6H4I MeO 70 95:5 5 MeO 4‐MeOC6H4I 70 95:5 66 MeOMeO PhIPhI 5454 98:298:2 6 6 MeOMeO PhIPhI MeO 54 54 98:298:2 66 MeOMeO PhIPhI MeO 5454 98:298:2 6 MeO PhI MeO OMe 54 98:2 66 MeOMeO PhI PhI OMe 5454 98:298:2 6 4 OMe 7 Me 4-MeOC H6 I4 62 99:1 7 7 MeMe 4‐4MeOC-MeOC6HH46I I4 6262 99:199:1 77 MeMe 44‐-MeOCMeOC6HH4II 6262 99:199:1 7 Me 4‐MeOC6H4I 62 99:1 7 Me 4‐MeOC6H4I 62 99:1 7 Me 4-MeOC6 H4 I 2 3 62 99:1 Reaction7 conditions:Me [diene]:[aryl4‐MeOC iodide]:[TBAF]:[PdH6 I4 (dba)2 ]3 =1:1.2:2:0.02; THF, 65 °C,62 24 h. 99:1 ReactionReaction conditions: conditions: [diene]:[aryl [diene]:[aryl iodide]:[TBAF]:[Pd iodide]:[TBAF]:[Pd2(dba)(dba)2 3] =1:1.2:2:0.02;]3 =1:1.2:2:0.02; THF, THF, 65 65 °C, °C, 24 24 h. h. ReactionReaction conditions:conditions: [diene]:[aryl[diene]:[aryl iodide]:[TBAF]:[Pdiodide]:[TBAF]:[Pd2(dba)(dba)3]] =1:1.2:2:0.02;=1:1.2:2:0.02; THF,THF, 6565 °C,°C, 2424 h.h. Reaction conditions: [diene]:[aryl iodide]:[TBAF]:[Pd2(dba)3] =1:1.2:2:0.02; THF, 65 °C, 24 h. Reaction conditions: [diene]:[aryl iodide]:[TBAF]:[Pd2(dba)3] =1:1.2:2:0.02; THF, 65 °C, 24 h.˝ ReactionAA successful, conditions:successful,Reaction [diene]:[aryl conditions: simplesimple, [diene]:[aryl, iodide]:[TBAF]:[Pdandand selective selective iodide]:[TBAF]:[Pd 2 method(dba) method3] 2=1:1.2:2:0.02;(dba) for for3] =1:1.2:2:0.02; the the THF, synthesis synthesis 65 THF, °C, 6524 h.C, of of 24 h.unsymmetricalunsymmetrical A AAsuccessful, successful,successful, simple, simple,simple ,and andand selective selective selective method method method for for for the the the synthesis synthesis synthesis of of unsymmetrical unsymmetricalunsymmetrical (E(E,E,E)-)1,4-1,4A-disubstituted- disubstitutedsuccessful, simple,butabuta-1,3- 1,3-anddieneshas-dieneshas selective prompted promptedmethod forusus the toto synthesis testtest ofthethe unsymmetrical selectedselected (E,(E(EE),‐A,E1,4EA))A‐- 1,4‐1,4 successful,disubstitutedsuccessful,‐successful,-disubstituteddisubstituted simple,simple,simple, butabutabuta ‐ 1,3 andand‐and-1,3‐1,3dieneshas ‐- dieneshasdieneshas selectiveselectiveselective methodpromptedmethodmethodpromptedprompted for forfor us thethe theusus synthesissynthesistosynthesis toto testtest test ofof of the unsymmetricalunsymmetricalunsymmetricalthethe selectedselectedselected 11-(triethoxysilyl)penta-(triethoxysilyl)penta(E,E)‐1,4‐disubstituted-1,3-1,3 -diene-dienebutainin ‐the 1,3the ‐synthesis dieneshassynthesis of of a a stereodefined promptedstereodefined ( usE(E,E ,E,E,E)-)to1,3,5-1,3,5 -hexatriene-hexatrienetest the derivative derivative selected 1(E‐((triethoxysilyl)pentaE,(1E1,‐E-)-1,4-disubstituted,(triethoxysilyl)penta(triethoxysilyl)pentaE)‐)1,4‐1,4‐disubstituted‐disubstituted‐1,3 ‐-1,3‐1,3dienein buta-1,3-dieneshasbuta‐-butadieneindiene‐1,3‐ 1,3thein‐ dieneshas ‐thethe dieneshassynthesis synthesissynthesis of prompted aofpromptedof promptedstereodefined aa stereodefinedstereodefined usus us(E , (E(EE, toE,,EtoE)to,‐,E 1,3,5E ))‐-1,3,51,3,5 testtest‐hexatrienetest‐- hexatrienehexatriene thethethe derivative selectedderivativederivativeselectedselected byby 1 its‐ its(triethoxysilyl)penta palladium palladium-catalyzed-catalyzed‐1,3 ‐ dienein Hiyama Hiyama the cross cross synthesis-coupling-coupling of a with stereodefinedwith (E(E)-)4-4-chlorostyryl-chlorostyryl (E,E,E)‐1,3,5 iodid iodid‐hexatrienee.e. TheThe optimal derivativeoptimal by1-(triethoxysilyl)penta-1,3-dienein11‐ byby(triethoxysilyl)penta‐its(triethoxysilyl)penta its palladium palladium ‐catalyzed‐-catalyzedcatalyzed‐1,3‐1,3‐dienein‐ dieneinHiyama Hiyama Hiyama thethe the synthesis crosssynthesis synthesis cross cross‐coupling‐-couplingcoupling of of of a a stereodeﬁneda stereodefined stereodefinedwith withwith (E )((‐EE4)‐)‐chlorostyryl- (4 4E(‐ -E(,chlorostyrylchlorostyrylE,E,E,E)-1,3,5-hexatriene,E)‐)1,3,5‐1,3,5 iodide.‐hexatriene‐ hexatrieneiodide. iodid e.The derivativeTheThe derivative derivativeoptimal optimaloptimal conditionsconditionsby its palladiumestablished established‐catalyzed for for the the reactions reactions Hiyama of of crosssilyl silyl dienes‐ dienescoupling with with with aryl aryl iodides( iodidesE)‐4‐chlorostyryl were were applied applied iodide. to to the the The( E(E)-) styryl-styryloptimal conditionsbybybyconditionsconditions its itsits palladium-catalyzed palladiumpalladium established establishedestablished‐catalyzed‐catalyzed for forfor the the Hiyamathe reactions HiyamaHiyama reactionsreactions cross-coupling of crosscross silylof silyl‐silylcoupling‐ couplingdienes dienesdienes with with withwith aryl ( E aryl(aryl)-4-chlorostyrylE( iodidesE)‐)4 ‐ iodidesiodides4‐chlorostyryl‐chlorostyryl were werewere applied iodide. appliedapplied iodide.iodide. to theto Theto The Thethethe (E optimal ) (optimal‐(EstyrylEoptimal))‐-styrylstyryl iodideiodideconditions, ,providing providing established moderate moderate for yield theyield reactions (58%) (58%) of of ofthe the silyl desired desired dienes ( E( Ewith,E,E,E,E) -)aryl1-1-(4-(4 -iodideschlorophenyl)hepta-chlorophenyl)hepta were applied- 1,3,5-to1,3,5 the-triene- triene(E)‐styryl 8 8 iodide,conditionsconditionsconditionsiodide,iodide providing, providingproviding established established established moderate moderatemoderate for for for the the theyield reactions reactionsyieldyield reactions (58%) (58%)(58%) of of of silyl silyloftheof silyl thethe dienesdesired dienes dienes desireddesired with with( Ewith , (E(EE aryl, E,aryl,E Earyl),‐,E1E iodides)‐) ‐(4iodides- 11iodides‐-(4chlorophenyl)hepta(4‐-chlorophenyl)heptachlorophenyl)hepta were were were applied applied applied to to ‐to the1,3,5 the ‐the-1,3,5 ( E‐( trieneE)-styryl(E)‐-‐)trienetrienestyryl‐styryl 8 88 (Scheme(Schemeiodide, 3). 3). providing The The Hiyama Hiyama moderate cross cross-coupl- couplyielding ing(58%) process process of the proceeded proceeded desired ( inE in, Ea a, highlyE highly)‐1‐(4 ‐stereoselective chlorophenyl)heptastereoselective manner manner‐1,3,5 to to ‐yield trieneyield 8 (Schemeiodide,iodide,iodide,(Scheme(Scheme providing providing3). providing 3).3).The TheThe Hiyama moderateHiyama Hiyamamoderate moderate cross cross yield yield‐ couplingyield‐-couplingcoupl (58%) (58%) (58%)ing process of of processprocessof the the the desired proceededdesired desired proceededproceeded ( E( E(,E, inE,,E, E )-1-(4-chlorophenyl)hepta-1,3,5-triene a,ininE) highly‐) 1a‐a1‐ highly(4highly‐(4‐chlorophenyl)hepta‐ chlorophenyl)heptastereoselective stereoselectivestereoselective manner manner‐1,3,5‐1,3,5 to‐triene‐ triene yieldtoto yieldyield 8 8 8 productproduct(Scheme containingcontaining 3). The Hiyama (E(E,E,E,E,E) -)crosstriene-triene‐coupling asas thethe predominant predominantprocess proceeded compoundcompound in a highly; ; however,however, stereoselective tracetrace amountsamounts manner ofof to the theyield product(Scheme(Scheme(Schemeproductproduct 3containing ).3). 3).containingcontaining TheThe The Hiyama Hiyama (E ,(E(EE,E ,,crossEE cross-coupling )cross,‐,EEtriene))‐-triene‐trienecoupling‐coupling as asasthe thetheprocess processprocesspredominant predominantpredominant proceeded proceeded compound; compound;compound in in in a a highly a highly highly however,; stereoselectivehowever,however, stereoselective stereoselective trace tracetrace amounts manner manneramountsamounts manner toof to yieldof ofthe toyield thethe respectiverespectiveproduct ( E(containingE,E,E,Z,Z) ) andand ( (E(E,,EZ,Z,,EZ,Z))‐ )triene isomers isomers as (<5%)the (<5%) predominant were were also also detectedcompound; detected using using however, the the GC GC trace-MS-MS method.amounts method. The Theof the respectiveproductproductrespectiverespective containing containing(E ,(E(E,Z,,EE), ,ZZand)) ( andE( E,(EE,E , ,E(Z(,E),‐,)Z,trieneZ‐Z)triene,, Zisomers)) isomers isomers as as the the(<5%) predominant (<5%) (<5%)predominant were were were also also alsocompound; compound;detected detected using however, however, using the the the GC trace trace GC‐MS‐ -MSamounts amountsmethod. method. of ofThe the The the strucstrucrespectivetureture of of synthesized synthesized(E,E,Z) and triene triene(E,Z, Z was was) isomers confirmed confirmed (<5%) by by were GC GC- MS-alsoMS and anddetected NMR NMR spectroscopy usingspectroscopy the GC(see(see‐MS Supporting Supportingmethod. The structurerespectiverespectivestructurestructure of ( ofsynthesizedE(E, Esynthesized, synthesizedE,Z,Z) )and and ( trieneE(E, Ztriene, trieneZ,Z,Z ) was)isomers isomers was was confirmed confirmed confirmed (<5%) (<5%) bywere were by byGC GC GCalso ‐alsoMS‐-MS MS detected anddetected andand NMR NMRNMR using using spectroscopy(see spectroscopy(seespectroscopy the the GC GC‐MS‐MS(see method. Supportingmethod. Supporting Supporting The The Information;Information;structure ofFigures Figures synthesized S15 S15––S16)S16) triene. . was confirmed7252 by GC‐MS and NMR spectroscopy(see Supporting Information;structurestructureInformation;Information; of of Figuressynthesized synthesized FiguresFigures S15–S16). S15–S16).S15 triene –trieneS16) . was was confirmed confirmed by by GC GC‐MS‐MS and and NMR NMR spectroscopy(see spectroscopy(see Supporting Supporting Information; Figures S15–S16). CCl l Information; Figures S15–S16). CCl l Information; Figures S15–S16). [Pd2(dba)3] Cl + Cl [Pd2(dba)3] Si(OEt)3 + e Si OEt T[PBdA2F(dba)3] Me M e ( )3 + , TBoAF, Me M Si(OEt)3 I T HF ,6 5TCBo A,2F 4h Me I T HF 65 C 24 h Me 8 I THF, 65oC, 24h 8 58%8 , , 58 > % (E E, E, ) 5>89%5% (E E,E,) 95 % (E E E) > 95%

SchemeScheme 3. 3. Synthesis Synthesis of of ( E(E,E,E,E,E)-)1-1-(4-(4-chlorophenyl)hepta-chlorophenyl)hepta-1,3,5-1,3,5-triene-triene. . Scheme 3. Synthesis of (E,E,E)-1-(4-chlorophenyl)hepta-1,3,5-triene. 3 3 3 3 33 33 Materials 2015, volume, page–page however, trace amounts of the respective (E,Z) isomers (1%–5%) were also detected using the GC‐MS method (Scheme 2). The (E,E)‐1‐aryl‐4‐methoxybuta‐1,3‐dienes 5–6 and (E,E)‐1‐arylpenta‐1,3‐diene 7 were isolated and characterized spectroscopically (see Supporting Information; Figures S9–S14).

[Pd2(dba)3] I TBAF Si(OEt) 3 + THF, 65oC, 24h R R1 R R1

54-89% R=Ph,MeO,Me (E,E)>95% R1 =H,3-NO2, 4-MeO, 2-Np Scheme 2. Synthesis of (E,E)‐1,4‐disubstituted buta‐1,3‐dienes.

Table 1. Hiyama cross‐coupling of 1‐(triethoxysilyl)buta‐1,3‐dienes with aryl iodides.

R Isolated Yield Selectivity Entry Aryl Iodide Product (Diene) [%] EE/EZ

1 Ph PhI 89 99:1

2 Ph 3‐NO2C6H4I 86 >99

3 Ph 4‐MeOC6H4I 79 >99

4 Ph 2‐C10H7I 55 99:1

5 MeO 4‐MeOC6H4I 70 95:5

6 MeO PhI 54 98:2

7 Me 4‐MeOC6H4I 62 99:1

Reaction conditions: [diene]:[aryl iodide]:[TBAF]:[Pd2(dba)3] =1:1.2:2:0.02; THF, 65 °C, 24 h.

A successful, simple, and selective method for the synthesis of unsymmetrical (E,E)‐1,4‐disubstituted buta‐1,3‐dieneshas prompted us to test the selected 1‐(triethoxysilyl)penta‐1,3‐dienein the synthesis of a stereodefined (E,E,E)‐1,3,5‐hexatriene derivative by its palladium‐catalyzed Hiyama cross‐coupling with (E)‐4‐chlorostyryl iodide. The optimal Materialsconditions2015 established, 8, 7250–7256 for the reactions of silyl dienes with aryl iodides were applied to the (E)‐styryl iodide, providing moderate yield (58%) of the desired (E,E,E)‐1‐(4‐chlorophenyl)hepta‐1,3,5‐triene 8 yield(Scheme product 3). The containing Hiyama cross (E,E‐coupling,E)-triene process as the proceeded predominant in a compound;highly stereoselective however, manner trace amounts to yield ofproduct the respective containing (E ,E(E,Z,E),E and)‐triene (E,Z ,asZ) the isomers predominant (<5%) were compound; also detected however, using trace the GC-MSamounts method. of the Therespective structure (E, ofE,Z synthesized) and (E,Z, trieneZ) isomers was conﬁrmed (<5%) were by GC-MSalso detected and NMR using spectroscopy(see the GC‐MS method. Supporting The Information;structure of synthesized Figures S15 andtriene S16). was confirmed by GC‐MS and NMR spectroscopy(see Supporting Information; Figures S15–S16).

Scheme 3. Synthesis of (E,E,E)-1-(4-chlorophenyl)hepta-1,3,5-triene. 3 3. Experimental Section

3.1. General Procedure for the Synthesis of (E,E)-1,4-disubstituted buta-1,3-dienes (1–7) A mixture composed of 1 mmol of 1-(triethoxysilyl)buta-1,3-diene with THF (10 mL) was placed under Ar atmosphere in a Schlenk bomb flask fitted with a plug valve. At room temperature, 2 mmol of TBAF (1M solution in THF) were added and the mixture was stirred for 10 minutes. After this time, 1.2 mmol of the respective aryl iodide and 0.02 mmol (18.3 mg)of [Pd2(dba)3] were added and the reaction mixture was stirred under argon for 24 h at 65 ˝C. After the reaction was completed (GC-MS analysis), the volatiles were evaporated under vacuum and the crude product was chromatographed on silica gel (eluent—hexane/ethyl acetate 8:2) to afford the analytically pure products. The structures of synthesized (E,E)-1,4-disubstituted buta-1,3-dienes were confirmed by GC-MS and NMR spectroscopy matching data reported in the literature: (E,E)-1,4-diphenylbuta-1,3-diene 1 [35], (E,E)-1-(2-naphthyl)-4-phenylbuta-1,3-diene 4 [38], (E,E)-1-(4-methoxyphenyl)penta-1,3-diene 8 [39].

3.1.1. (E,E)-1-methox-4-(4-methoxyphenyl)buta-1,3-diene (5); yellow oil; Yield: 0.13 g (70%)

1 H NMR (300 MHz, CDCl3): δ = 3.86 (s, 6H), 5.78 (d, 1H, J = 16.1 Hz), 6.02 (dd, 1H, J = 8.8, J = 16.1 Hz), 6.34 (d, 1H, J = 15.8 Hz), 6.63 (dd, 1H, J = 8.8, J = 15.8 Hz), 7.02–7.10 (m, 2H), 13 7.27–7.32 (m, 2H); C NMR (75 MHz, CDCl3): δ = 57.0, 62.0, 110.9, 116.6, 126.4, 128.2, 131.9, 139.5, 153.0, 161.1; MS (EI, 70 eV) m/z (rel. int.): 190.0 (100%), 147.0 (60), 131.0 (25), 115.0 (50), 103.0 (25), 91.0 (50), 77.0 (25), 51 (15); Anal. Calcd for C12H14O2: C, 75.76; H, 7.42. Found: C, 75.66; H, 7.49.

3.1.2. (E,E)-1-Methoxy-4-(phenyl)Buta-1,3-diene (6); yellow oil; Yield: 0.035 g (54%)

1 H NMR (300 MHz, CDCl3): δ = 3.84 (s, 3H), 6.63 (d, 1H, J = 14.8 Hz), 6.88 (d, 1H, J = 8.8 Hz), 6.89–6.95 (m, 2H), 7.34–7.39 (m, 3H), 7.47–7.49 (m, 2H); MS (EI, 70 eV) m/z (rel. int.): 160.0 (100%), 145.0 (40), 129.1 (50), 116.0 (90), 103 (50). Anal. calcd for C11H12O: C, 82.46; H, 7.55. Found: C, 82.65; H, 7.72.

3.2. Synthesis of 1-(4-chlorophenyl)hepta-1,3,5-triene (8) A mixture consisting of 1 mmol of 1-(triethoxysilyl)penta-1,3-diene with THF (10 mL) was placed under Ar atmosphere in a Schlenk bomb ﬂask ﬁtted with a plug valve. At room temperature, 2 mmol of TBAF (1 M solution in THF) was added and the mixture was stirred for 10 minutes. After this time, 1.2 mmol of (E)-β-iodo-4-chlorostyrene and 0.02 mmol (0.018 mg) of [(Pd2(dba)3] were added and the reaction mixture was stirred under argon for 24 h at 65 ˝C. After the reaction was completed (GCMS analysis) the volatiles were evaporated under vacuum and the crude product was chromatographed on silica gel (eluent—hexane/ethyl acetate 8:2) to afford the analytically pure product.

7253 Materials 2015, 8, 7250–7256

(E,E,E)-1-(4-chlorophenyl)hepta-1,3,5-triene (8); yellow oil; Yield: 0.048 g (58%)

1 H NMR (300 MHz, CDCl3): δ = 1.81–1.83 (m, 3H), 5.77–5.86 (m, 1H), 6.59 (dd, 1H, J = 9.2, J = 16.3 Hz), 6.65–6.66 (m, 1H), 6.87 (dd, 1H, J = 8.5, J = 15.9 Hz), 6.95 (d, 1H, J = 10.4 Hz), 13 7.28–7.39 (m, 5H); C NMR (75 MHz, CDCl3): δ = 19.14, 127.56, 128.74, 128.78, 128.87, 129.48, 131.74, 131.95, 133.25, 134.27, 135.71; MS (EI, 70 eV) m/z (rel. int.): 204.0 (80%), 189.0 (100), 169.1 (40), 153.1 (60), 141.0 (50), 124.9 (70). Anal. Calcd for C13H13Cl: C, 76.28; H, 6.40. Found: C, 76.40; H, 6.52.

4. Conclusions In conclusion, 1-(triethoxysilyl)-substituted buta-1,3-dienes have been applied as new building blocks for palladium-catalyzed Hiyama coupling to yield 1,4-disubstituted (E,E)-1,3-dienes containing aryl or methoxy groups. Although the starting silyldienes consisted of a mixture of geometrical isomers, the Hiyama coupling proceeded in a highly stereoselective manner to yield products containing (E,E)-dienes as predominant products. Preliminary results on the application of 1-(triethoxysilyl)-substituted buta-1,3-dienes in the synthesis of (E,E,E)-triene skeleton have also been reported.

Supplementary Materials: The following are available online at www.mdpi.com/1996-1944/8/11/5378/s1. Acknowledgments: Financial support from the National Science Centre (Poland), Grant No. Opus 2011/03/B/ ST5/01034 is gratefully acknowledged. Author Contributions: Justyna Szudkowska-Fr ˛atczakand Mariusz Taczała performed the experiments and analyzed the data. Piotr Pawlućdiscussed the experiment and wrote the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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