A study towards the synthesis of dithiadiazolyl functionalised calix[4]arenes

Applewhite, Malcolm (2012-03)

Thesis (MSc)--Stellenbosch University, 2012.

Thesis

ENGLISH ABSTRACT: Dithiadiazolyl heterocyclic radicals form part of an ever-growing research field in the quest for organic magnets and conducting materials due to the lone electron located within the heterocyclic ring. These compounds could potentially be used as electrical current conductors and transistors which may be developed into a molecular switch or other types of molecular devices. This thesis describes the successful synthesis of three nitrile functionalised calix[4]arenes, mono-, distal- and tetra-, as bulky scaffolds to be used as precursors in the synthesis of dithiadiazolyl functionalised calix[4]arenes. The crystal structures of these three nitriles are reported herein for the first time. Dithiadiazolyls tend to dimerise in the solid state, thus by selecting a calix[4]arene as a bulky R-group, it was hoped to inhibit dimerisation. Furthermore, synthesis of the radicals on different positions of the calix[4]arene may also inhibit dimerisation in the solid state. The typical reaction in the synthesis of dithiadiazolyls involves the reaction between a nitrile and lithium bis(trimethylsilyl)amide followed by the formation of the dithiadiazolylium chloride salt by the addition of sulfur dichloride. The salt is subsequently reduced giving the desired dithiadiazolyl. However, the addition of lithium bis(trimethylsilyl)amide to mono-nitrile calix[4]arene in the synthesis of the mono-dithiadiazolyl functionalised calix[4]arene was unsuccessful. To investigate the incompatible nature of the electrophile and nucleophile, computational and physical experiments were conducted on the mono-nitrile calix[4]arene derivative. These avenues were pursued to determine whether the unreactive nature of the nitrile in this case is due to electronic effects. Changes in the reaction conditions (i.e. temperature, solvent and the equivalents of nucleophile) were all varied, but this proved to be unsuccessful. Computationally, through charge calculations, it was determined that the electronic properties of the nitrile were similar to that of nitrile examples known to work in the literature. Therefore, it was established that steric effects of the calix[4]arene are playing a critical role in the unreactive nature of the nitrile. There are two non-degenerate LUMO orbitals for the nitrile as one is conjugated and the other is planar (LUMO + 1) to the aromatic system. It has been established that nucleophilic attack through the lowest energy LUMO would result in a high energy transition state due to the loss of conjugation and attack through the higher energy LUMO + 1 would result in a lower energy transition state. However, this was not possible due to the steric conditions surrounding its position relative to the nitrile. The results obtained from this study were, therefore, able to confirm that the normally suitable reaction procedure seems to be limited to less sterically encumbered nitriles.

AFRIKAANSE OPSOMMING: Dithiadiazolyl heterosikliese radikale vorm deel van 'n steeds groeiende navorsingsveld in die soeke na organiese magnete en material met geleidingsvermoë as gevolg van die ongepaarde elektron wat binne in die heterosikliese ring geleë is. Hierdie verbindings kan moontlik gebruik word as elektriese stroom geleiers en transistors wat tot 'n molekulêre skakelaar of ander tipe molekulêre toestelle ontwikkel kan word. Hierdie tesis beskryf die suksesvolle sintese van drie nitriel gefunksionaliseerde calix[4]arene, mono-, distale- en tetra-, as lywige steiers om as voorgangers in die sintese van dithiadiazolyl gefunksionaliseerde calix[4]arene gebruik te word. Die kristal strukture van hierdie drie nitriele is hier in vir die eerste keer gerapporteer. Dithiadiazolyls is geneig om te dimeriseer in die vaste toestand. Daarom is die redenasie agter die keuse van die calix[4]arene as 'n lywige R-groep, as dimeriseering te inhibeer. Sintese van die radikale op verskillende posisies van die calix[4]arene kan dimeriseering in die vaste toestand inhibeer. Die tipiese reaksie in die sintese van dithiadiazolyls behels die reaksie tussen 'n nitriel en litium bis(trimetielsiliel)amied gevolg deur die vorming van die dithiadiazolylium chloried sout deur die byvoeging van swaeldichloried. Die sout is dan gereduseer om die gewenste dithiadiazolyl te vorm. Die toevoeging van litium bis(trimethielsiliel)amied mono-nitriel calix[4]arene in die sintese van die mono-dithiadiazolyl calix[4]arene was egter onsuksesvol. Om die oënskynlik onverenigbare aard van die electrofiel en nukleofiel te ondersoek, is rekenaar-berekeninge en fisiese eksperimente uitgevoer op die mono-nitriel calix[4]arene afgeleide. Hierdie twee maniere is gevolg om te bepaal of die onreaktiewe aard van die nitriel in hierdie geval as gevolg van elektroniese effekte is. Wysigings in die reaksie omstandighede (d.w.s. temperatuur, oplosmiddel en die ekwivalente van nukleofiel) is gemaak, maar dit was onsuksesvol. Deur middle van ladings-berekeninge, is dit bepaal dat die elektroniese eienskappe van die nitriel soortgelyk is aan dié van nitriel voorbeelde wat in die literatuur geraporteer is. Daarom is dit vasgestel dat die steriese effekte van die calix[4]arene 'n kritieke rol in die onreaktiewe aard van die nitrile speel. Daar is nie-ontaarde LUMO orbitale vir die nitriel soos 'n mens is vervoeg en die ander is planêr (LUMO + 1) na die aromatiese stelsel. Daar is vasgestel dat nukleofieliese aanval deur die laagste energie LUMO sal lei tot 'n hoë energie oorgangstoestand as gevolg van die verlies van konjugasie. Aanval deur die hoër energie LUMO + 1 sou lei tot 'n laer energie oorgangstoestand. Dit was egter nie moontlik nie, as gevolg van die steries toestande rondom die orbital se posisie met betrekking tot die nitriel. Die resultate van hierdie studie bevestig, dus, dat die gewone reaksie proses beperk is tot minder steries verhinderde nitriele.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/20349
This item appears in the following collections: