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Title: Synthetic studies and applications of helicenes
Author: Judge, Dilraj
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2013
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Helicenes are ortho-fused aromatic molecules rendered chiral due their unique helical topology. These fascinating molecules with their extended electronic conjugation have vast potential in a variety of applications. Indeed, their applicability has been highlighted in the fields of catalysis1 and molecular recognition2 amongst others. Current methods employed to synthesise helicenes offer little means of stereocontrol. Our group aims to develop a unified, general strategy to enable the asymmetric synthesis of unfunctionalised carbo- and hetero- helicenes. Two chiral relay protocols to control the sense of helical chirality have been investigated: (i) axial chirality to helical chirality and (ii) planar chirality to helical chirality (Figure A1). The use of tricarbonyl chromium moieties as the planar chiral source and probing the conformation of hindered axially chiral compounds is described. In the synthetic route towards [6]helicene, the 1,8-diarylnaphthalene motif represents a key intermediate. Such motifs were prepared by a double Suzuki-Miyaura cross coupling reactions using two different substrates: employing diboronic acid i and bis-iodide iii. The latter substrate afforded dimethoxy v in excellent yield. Subsequent treatment with Cr(CO)6 provided an efficient route to access compound vi which exhibits both axial and planar chirality. These 1,8-diarylnaphthalene motifs exist as atropisomers: syn (meso)- and anti (chiral)- isomers (FigureA2). Further dynamic onformational studies, employing NMR techniques, on these compounds successfully determined the barriers to isomerisation (Table A1), in addition to investigating the solvent dependency on the anti:syn ratio. In the synthetic route towards aza[6]helicene, the development of a C-H activation/arylation reaction of tricarbonyl chromium complexed 2-phenylpyridines has been accomplished. Optimisation of the conditions for the two-step process involving C-H activation by the formation of a isolable palladacycle ix followed by an arylation using various aryl boronic acids furnished triaryl products xii and xiii in moderate to excellent yields (Scheme A3). This methodology was developed into a one-pot activation/arylation process resulting in key intermediate arylated products for the synthesis of aza[6]helicene. The ability of [7]helicene to function as a chiral molecular tweezer of certain metallic cations is described. Formation of a 1:1 Ag:[7]helicene complex was monitored by NMR spectroscopy and its structure was determined by X-ray crystallography (Figure A3).
Supervisor: Fuchter, Matthew Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available