Gray, VJ; (2014) New Applications for Sulfur-Based Leaving Groups in Synthesis. Doctoral thesis , UCL (University College London).

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Abstract

This thesis describes the discovery and development of new routes towards substituted indoles and ynol ethers based on a sulfonate and sulfonamide leaving group, respectively. Both syntheses involve the use of these electron-withdrawing moieties as α-radical stabilising groups that can be utilised in a range of mild and facile bond-forming reactions to yield useful organic compounds. The first approach describes the synthesis of indoles utilising a water soluble, phosphorus-based chain carrier that generates a carbon-centred radical from an sp3 bromo sulfonate ester. This species can consequently undergo an intramolecular cyclisation with an aromatic ring, followed by loss of the sulfonate ester to yield a range of indoles in a chemoselective fashion. The second part of this thesis describes the synthesis of a range of ynol ethers via reaction of an aliphatic potassium alkoxide with an aromatic alkynyl sulfonamide. The mechanism of this process has been explored via a combination of synthetic chemistry and electron paramagnetic resonance spectroscopy (EPR) and the findings of these experiments will be discussed. The synthesis of tert-butyl ynol ethers in particular, allows for retro-ene decomposition to yield a ketene. This reactive intermediate can consequently undergo [2+2] cycloaddition with a ynol ether to yield tri-substituted cyclobutenones in excellent yield. Based on these findings, a new mode of reactivity for potassium alkoxides is suggested that involves these compounds ionising at room temperature under certain conditions to form either stabilised alkoxyl or trioxyl radical complexes that are observable via EPR, where the latter complex is known to decompose to alkoxyl radicals and molecular oxygen. The work presented in this thesis may have implications for other areas of science such as atmospheric chemistry and transition-metal free cross-coupling chemistry.

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