New Synthetic Methodology for the Preparation of Alkenes, Alkynes and Allylated compounds via 2-Mercaptobenzothiazole

Feng-ling Wu (2011). New Synthetic Methodology for the Preparation of Alkenes, Alkynes and Allylated compounds via 2-Mercaptobenzothiazole PhD Thesis, School of Chemistry and Molecular Biosciences, The University of Queensland.

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Author Feng-ling Wu
Thesis Title New Synthetic Methodology for the Preparation of Alkenes, Alkynes and Allylated compounds via 2-Mercaptobenzothiazole
School, Centre or Institute School of Chemistry and Molecular Biosciences
Institution The University of Queensland
Publication date 2011-05
Thesis type PhD Thesis
Supervisor Dr Ross P. McGeary
Dr. Benjamin P. Ross
Total pages 166
Total colour pages 32
Total black and white pages 134
Subjects 03 Chemical Sciences
Abstract/Summary This thesis describes the use of 2-mercaptobenzothiazole (MBT) as an auxiliary in organic chemistry for the preparation of alkenes, alkynes and allylated compounds. The transformations are based on the nucleophilic substitution of MBT to various electron-deficient substrates and then oxidation to generate the key intermediates, benzothiazol-2-yl sulfones (BT-sulfones). Depending on the employed reaction conditions, BT-sulfones underwent Smiles rearrangements through different processes paralleling the Julia-Kocienski reaction and led to the regeneration of olefins from epoxides, alkynylation and allylation of nucleophiles. A stereospecific transformation of a number of epoxides to olefins via BT-sulfones was developed, with yields ranging from 62-83%. This process involves (1) the ring-opening of the epoxides with MBT, (2) oxidation of the derived ß-hydroxy thioethers to the key sulfone moieties, and (3) thermal or base-promoted fragmentation to the desired alkenes (Scheme 1). In view of its simplicity and mild reaction conditions, this methodology may form the basis of a new protecting group strategy for alkenes. Scheme 1. Proposed reaction for the conversion of epoxides to alkenes. We then continued to examine a transition metal-free alkyne forming reaction. This approach of alkynylation proceeds via an enolate-type Smiles rearrangement, initiated by α-deprotonation of the weakly acidic ß-carbonyl sulfone moieties. However, despite the carefully examined reaction conditions, alkynylation was found to compete with desulfonylation (Scheme 2). Based on the investigations under basic and strictly anhydrous conditions, possible mechanisms for the desulfonylation reactions were proposed. Scheme 2. Alkynylation and desulfonylation via ß-keto sulfones. To further expand the usage of MBT as an auxiliary in synthesis, a novel route to allylated adducts was also examined (Scheme 3). Key to this transformation was the in situ generation of the required ß-alkoxide sulfone moieties from different nucleophiles to BT-sulfones. The obtained alkoxides would then lead to immediate Smiles rearrangement and give rise to allylated compounds. The preliminary results indicate a promising outcome for this methodology, with the obtained allylated compound in 83% yield. Moreover, in the course of this endeavour, it was found that the treatment of BT-sulfide epoxide and BT-sulfone epoxide with silica gel led to rearrangement and gave rise to thiirane and allyloxybenzothiazole, respectively. Scheme 3. Proposed routes to allylated adducts via BT-sulfones.
Keyword Benzothiazol-2-yl sulfones (BT-sulfones), Julia-Kocienski olefination, Smiles-rearrangement, alkenes, protecting groups, alkynes, allylated compounds.
Additional Notes Coloured pages: 21, 22, 23, 25, 36, 38, 39, 47, 49, 55, 56, 63, 67, 103, 110, 111, 127, 128, 130, 132, 133, 135, 137, 147, 149, 150, 151, 152, 153, 164, 165, 166 Landscape pages: 149, 150, 151, 152, 153

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Created: Fri, 16 Sep 2011, 16:31:34 EST by Miss Feng-ling Wu on behalf of Library - Information Access Service