Date of Award

2016

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Chemistry and Biochemistry

Keywords

Allocolchicines, Catalysis, conjugate addition, cyclization, cycloheptynes, Nicholas reaction

Supervisor

Green, James

Rights

info:eu-repo/semantics/openAccess

Abstract

The chemistry of propargyliumdicobalt cations, known as the Nicholas reaction, has witnessed widespread use in organic synthesis mainly owing to the high stabilization of the carbocations, accompanied by their compatibility with numerous nucleophiles. However, there is a considerable shortage of studies regarding the factors affecting the stability and reactivity of the involved cations. In this project, we investigated the chemistry of a novel Nicholas carbocation, namely the benzo-homologue of the dehydrotropylium cation, known as a benzodehydrotropylium-Co2(CO)6 cation. To obtain the desired cation, preparation of the requisite alcohol precursor involved the key ring-closure of the cycloheptyne via an intramolecular Sakurai reaction. Additionally, heterocyclic-based cycloheptadienynol-Co2(CO)6 complexes were also prepared. A Lewis acid mediated ionization of the parent alcohol complex led to the in situ generation of the benzodehydrotropylium-Co2(CO)6 ion, which was trapped with several nucleophiles, where the preferred site of substitution was the site remote to the dicobalt alkyne unit. Computational studies using NICS (1) values estimated the ion's aromaticity to be one third of that of tropylium ion, which is comparable to that of the dehydrotropylium-Co2(CO)6 ion previously studied in our group.

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