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Sciences pures --- Zuivere wetenschappen --- 576.809.518 : 547.53 --- 547.53 : 576.809.518

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Helicenes are ortho-condensed polycyclic aromatic or heteroaromatic compounds with inherent chirality, due to the helical shape of their Pi-conjugated systems. The helical intramolecular distortion of ortho annulated polycyclic compounds arises due to steric overcrowding of the terminal rings. In helicenes and heterahelicenes, the minimum requirement for helicity to occur is the presence of five ortho annulated rings. During the last decade, helicenes have received a lot of attention not only because of their aesthetic appearance but also for their interesting symmetry and extraordinary optical and electronic properties. Helicenes have evolved to be more than an academic curiosity owing to their wide domain of potential applications in fields ranging from asymmetric catalysis to organic electronics (OLEDs, photonics etc). Within this work we have successfully investigated synthetic routes towards tetrathia[7] helicenes and their functionalized derivatives. An efficient route towards the synthesis of symmetrical and unsymmetrical benzo fused tetrathia[7]helicenes substituted with electron donor (ED) and electron acceptor (EA) groups was developed. A common, readily available precursor 1,2-bis-(2-thienyl)-benzene was used to synthesize different helicenes through a Wittig reaction, Stille coupling and modified oxidative photocyclization using visible light. Tailoring the precursors to cis-helicene-like conformation has led to an increase in the efficiency of the photocyclization reaction, thus overcoming the drawbacks associated with this process. Helicenes have a potential of being used as non-linear optical (NLO) materials owing to their intrinsic chirality. Chiral helicenes are non-centrosymmetric and therefore allow a second order NLO response to be observed due to the helical motion of the polarizable electrons. Based on the above methodology, the synthesis of [9]- and [11]thiahelicenes, as well as the preparation of lower homologues such as [5]-, [6]-, and [7]thiahelicenes was realized successfully. Efficient palladium catalyzed coupling reactions were employed. Triorganoindium derivatives were selectively mono-cross-coupled with N-methyl-3,4-dibromomaleimide followed by Stille coupling with the readily available building block naphthodithiophene. Oxidative photocyclization of the conjugated precursors using visible light was employed to synthesize a series of thia[n]helicenes. This modular synthetic methodology is independent of the length of the helical backbone and applicable to a wide variety of helical frameworks.Helicenes are complementary to the biaryl-type ligands of C2-symmetry but have advantages like higher thermal stability for preventing racemization and higher rigidity, thus competing against “traditional” ligands possessing central, axial or planar chirality. The utilization of the potential of helically chiral catalysts relies on development of efficient synthetic strategies applicable for a variety of helical structures. We have investigated different strategies based on diastereomeric separation forthe synthesis of thia[n]helicenes. We have introduced chiral auxiliaries at different positions on the helicene core and have investigated the effect of the position of the chiral group, its proximity to the helical core as well as the variation of the chiral group itself. Diastereoselective photocyclization has successfully led to formation of thia[5]helicenes in relatively good selectivity. The synthetic pathways towards diaza[5]helicenes involves Wittig reaction followed by photochemical electrocyclization. Diversely functionalized diaza[5]helicenes have been synthesized starting from 6,9-dichloro-5,10-diaza[5]helicene, which was prepared from a readily available quinoline building block. The helicene skeleton was functionalized by a variety of O-, S-, N-, and C-centered nucleophiles using nucleophilic aromatic substitution reactions and palladium-catalyzed reactions like Suzuki coupling and Buchwald-Hartwig aminations. A resolution strategy based on diastereoisomeric separation by substitution of the dichloro derivative with a chiral amine has been described. As a proof-of-concept this methodology was extended to the synthesis of diazadithia[7]helicenes. These helicenes were synthesized from the readily available building block ethyl 7-chloro-8-formylthieno[3,2-f]quinoline-2-carboxylate by a Wittig reaction–photocyclization strategy. Optical resolution via diastereoisomeric separation gave the chiral forms. Racemization studies confirmed that the enantiopure forms of these [7]helicenes are conformationally stable compared to their lower analogues. The solid-state structures of the novel diazadithia[7]helicenes were determined by single-crystal X-ray diffraction. The crystal structures show columnar stacking in antiparallel fashion. The HOMO–LUMO gaps of the new compounds were determined on the basis of electrochemical and optical measurements. Based on the HOMO–LUMO gap and the presence of an irreversible oxidation feature it may be suggested that these molecules can be better suited as an electron conductor, when used for possible applications in organic electronics.Finally, we explored some non-photochemical oxidative routes to synthesize a variety of substituted naphthodithiophene building blocks through DDQ/acid mediated oxidative cyclizations. The versatility of the Scholl reaction using the DDQ/acid system was demonstrated by the preparation of a novel substituted tetrathia[7]helicene where 3 new C-C bonds were formed in a one pot procedure. The new DDQ/acid method was compared to the known strategies such as FeCl3 oxidation and oxidative photocyclization. By protecting the 1,2-bis(2-thienyl)benzene precursors, it is possible to direct the intermediates to controlled cyclization and effectively suppressing the polymerization. The highly reactive alfa-position of the terminal thiophenes can allow for further functionalization. The efficient preparation of a variety of naphthodithiophene building blocks, the extension to a non-photochemical synthesis of [n]helicenes,and the ease of isolation of the products are arguments for the use of DDQ/acid system for this Scholl reaction.

academic collection --- 547.53 <043> --- Benzene and its homologues. Hydrocarbons containing a benzene nucleus, in general--dissertaties --- Theses

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Polycyclic aromatic hydrocarbons --- Hydrocarbures aromatiques polycycliques --- Polycyclic aromatic hydrocarbons. --- 547.53 --- Benzenoid hydrocarbons --- PAHs (Chemicals) --- Polyarenes --- Polynuclear aromatic hydrocarbons --- Hydrocarbons --- Polycyclic aromatic compounds --- Benzene and its homologues. Hydrocarbons containing a benzene nucleus, in general --- 547.53 Benzene and its homologues. Hydrocarbons containing a benzene nucleus, in general --- Hydrocarbures --- Chemistry of natural organic substances --- Aromatic hydrocarbons --- Isomerism and isomers --- Polycyclic compounds