Study On The Synthesis Of Thiophene-based Helicenes Bearing Chiral Alkyl Chains And The Chiral Resolution Of Thiophene-based Helicenes

The research work in this thesis is composed of four parts:Part I. Study on synthesis of [7]Helicene and Double Helicene Bearing Chiral Alkyl Chains1) Starting from dithieno[2,3-b:3’,2’-d]thiophene (20), the alkylation was occurred in the presence ofstrong bases, LDA (lithium diisopropylamide) and t-BuOK reacted with20and (S)-3,7-dimethyl-l-bromooctane (13) to obtain (S)-3,7-dimethyl-octanyl-dithieno[2,3-b:3’,2’-d]thiophene (1). The brominationof1was occurred in the presence of1.0equiv of NBS in CHCl3/AcOH (1:1) to generate5-bromo-2-[(S)-3,7-dimethyl-octanyl]dithieno[2,3-b:3’,2’-d] thiophene (3). The efficient bromine dance reaction wasobserved to3by using LDA in THF at-78°C to generate4-bromo-2-[(S)-3,7-dimethyl-octanyl]-dithieno[2,3-b:3’,2’-d] thiophene (4). Following the Li/Br exchange on4, the resultant aryllithium specieswere oxidized with CuCl2to generate5,5’-di[(S)-3,7-dimethyl-octanyl]-3,3’-bis-dithieno[2,3-b:3’,2’-d]thiophene (5). LDA-mediated lithiation was introduced to remove the protons on α positions of5to givedilithiated5, the reaction of dilithiated5with bis(phenylsulfonyl)sulfide gave the annelated product[7]helicene (6). On the other hand, dilithiated5could be oxidized with CuCl2to generate double helicene(7). Therefore, the two helicenes,6and7were obtained in ca.10.5%and ca.6.0%isolated yields,respectively in five steps with20as starting material. The title compounds and the intermediates werecharactered by1H NMR,13C NMR, MS, HRMS and IR.2) The title compounds6and7showed no optical rotations due to the too weak contribution fromchiral induction of the chiral chain,(S)-3,7-dimethyl-octanyl group in the formation of helicenes.Part Ⅱ. Study on the induced synthesis of dioctyl-[7]Helicene (10) by (-)-SparteineStarting from dithieno[2,3-b:3’,2’-d]thiophene (20), the alkylation was occurred in the presence ofLDA (lithium diisopropylamide) reacted with bromooctane, and5-octyldithieno[2,3-b:3’,2’-d]thiophene (21)was obtained. The bromination of21was occurred by using NBS (1equiv) in CHCl3/AcOH (1:1) to obtain5-bromo-2-octyldithieno [2,3-b:3’,2’-d] thiophene (22). After the efficient bromine dance reaction of22byusing LDA in THF at-78°C,4-bromo-2-octyldithieno[2,3-b:3’,2’-d]thiophene (23). Following the Li/Brexchange on23, the resultant aryllithium species were oxidized with CuCl2to generate5,5’-dioctyl-3,3’-bis-dithieno[2,3-b:3’,2’-d]thiophene (8). LDA/(-)-sparteine-mediated lithiation was introduced to remove the protons on α positions of8to give dilithiated8, the reaction of dilithiated8withbis(phenylsulfonyl)sulfide gave the annelated product dioctyl-[7]helicene (10). The total yield in makinghelicenes10was in ca.10.9%yields with20as starting material. The dioctyl-[7] helicene showed nooptical rotations, that means that (-)-sparteine had not shown efficient chiral induction in the formation ofdioctyl-[7] helicene.Part Ⅲ. Study on the chiralresolution of [7]helicene and double helicene by chiral preparativechromatographySeven Helicenes were analysed by HPLC using a RC-SCDP5chiral column. The chiral resolution ofnaphthalene cored double hexathia[7]helicene was achieved on preparative scale by HPLC using aRC-SCDP5chiral column(250×10mm) with n-heptane/2-propanol (98:2, v/v) mixture as the mobile phase.Thus, starting from30mg of naphthalene cored double hexathia[7]helicen, two chiral fractions wereobtained. The earlier eluting fractions was8.6mg,~80%ee; The specific rotation value was found as[α]17D=-241.8°(c=0.0061g/mL,CHCl3). Later eluting fractions was7.7mg, with high ee as>99%; Thespecific rotation value was observed as [α]D17=+487°(c=0.0039g/mL, CHCl3). The chiral resolution yieldof the two chiral fractions was in26%.Part Ⅳ.Attempt to the preparation of spirobi(bis-dithieno[2,3-b:3’,2’-d] thiophene silole)s.With two important functionalized compounds5,5’-di(trimethylsilanyl)-3,3’-dibromo-2,2’-bis-dithieno-[2,3-b:3’,2’-d]thiophene (25) and5,5’-dioctyl-2,2’-dibromo-3,3’-bis-dithieno[2,3-b:3’,2’-d] thiophene (9) assubstrates, the Li/Br exchange at-78°C and the reaction with tetrachlorosilicane were tried, respectively tosynthesize spirobi(bis-dithieno[2,3-b:3’,2’-d]thiophene silole)s, but failed. The reason might be from thestrong intramolecular tension, which causes the difficulties in the formation of four carbon-silicon bonds atthe same time. The relative research is still in progress.