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Investigation of New Routes to Benzothiazole or 2-Aminothiophenol as Feedstocks for the Synthesis of Pepton

NICHOLLS, ALEXANDER,JAMES (2021) Investigation of New Routes to Benzothiazole or 2-Aminothiophenol as Feedstocks for the Synthesis of Pepton. Doctoral thesis, Durham University.

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Author-imposed embargo until 13 January 2024.

Abstract

2,2'-Dibenzoylaminodiphenyl disulfide is a valuable peptiser utilised in the rubber vulcanisation process. This product is generated efficiently from benzothiazole or 2-aminothiophenol, however routes to these valued intermediates are limited. In this thesis several new routes to these desired entities have been explored. While most of these routes are not expected to be cost-effective, other routes may have economic potential with further exploration.
Many substituted benzothiazole structures can be made by an array of discoveries published in recent literature. Such methods were applied to the target synthesis of the unsubstituted benzothiazole, but there were generally issues relating to this particular derivative. Legacy methods of synthesising 2-aminothiophenols from aniline, notably the Herz reaction, were analysed, however the yields and atom economy could not compete with more recent pathways.
The conversion of 2-substituted benzothiazole compounds to either 2-aminothiophenol or unsubstituted benzothiazole was explored. It was found 2-aminobenzothiazole can be converted to 2-aminothiophenol by hydrolysis or benzothiazole by hydrodeazotisation, however neither routes are economically viable based upon the current prices of these products. Furthermore, 2-mercaptobenzothiazole can be converted to benzothiazole, but is accompanied by the generation of an undesired sulfur-containing waste product. 2-Alkyl/aryl substituted benzothiazole, while easy to synthesise, were unsuitable for subsequent conversion to the target derivatives; with the exception of 2-methylbenzothiazole, which can be converted to benzothiazole by oxidation-decarboxylation.
Finally, methods for the conversion of 2-nitrochlorobenzene to 2-aminothiophenol were explored. This starting material is desirable as its price has been consistently low, in line with it being the less-desired isomer of the simple and well-established nitration of chlorobenzene process. The Zinin reaction describes this transformation using only a single reagent however, the utilisation of sulfur as a nitro-group reducing agent is inefficient and leads to a poor atom economy. Applying knowledge from this scheme allowed a new addition-elimination procedure to be developed, using 2-mercaptobenzothiazole as a nucleophile and ultimately producing both 2-aminothiophenol and benzothiazole as cleavage products. However, the complexity of the scheme and problems with the reactivity of 2-aminothiophenol meant that a suitable catalyst could not be found within the timescale of this project. Finally, these ideas were combined to develop a new pathway from the nucleophilic substitution of the chloride with sulfur disulfide, followed by a catalytic hydrogenation to give the desired dimer form of 2-aminothiophenol.

Item Type:Thesis (Doctoral)
Award:Doctor of Philosophy
Keywords:Benzothiazole, heteroaromatic chemistry.
Faculty and Department:Faculty of Science > Chemistry, Department of
Thesis Date:2021
Copyright:Copyright of this thesis is held by the author
Deposited On:14 Jan 2021 11:32

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