Greaney, Michael Francis; (1999) Fluorination studies using difluoroiodotoluene. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is divided into three parts. The first part is an introduction to the fluorination chemistry of difluoroiodotoluene and consists of two reviews. The first review concerns the use of iodine(III) difluorides in the formation of carbon-fluorine bonds. The second deals with the Fluoro-Pummerer reaction, introduction of fluorine into the α-position of sulfoxides or sulfides via sulfonium ion intermediates. The second part is a discussion of the results obtained with difluoroiodotoluene in the fluorination of sulfur-containing compounds. Early results indicated that the reagent could perform Fluoro-Pummerer chemistry, and this transformation forms the basis of much of the work. A range of α-fluoro sulfides could be formed cleanly and in good yield by treatment of fi-oxo sulfides with one equivalent of difluoroiodotoluene in dichloromethane solution. Difluorination of substrates was also feasible, and treatment with excess reagent was found to produce α-fluoro sulfoxides. The reaction was established as being promoted by electron-withdrawing groups in the α-position, accordingly simple dialkyl sulfides were found not to undergo the Fluoro-Pummerer reaction with difluoroiodotoluene. In cases were substrates had β-hydrogens an elimination reaction was found to operate, producing vinyl sulfides. These vinyl sulfides could then add two equivalents of fluoride via an additive-Pummerer reaction to produce α,β-difluorosulfides. In this manner a range of novel 3,4-difluoro pyrrolidinones and piperidinones were synthesised. Certain a-phenylsulfanyl acetamides were found to be resistant to fluorination, undergoing preferential oxidation to the sulfoxides. Arguments based on a coordination of the β-carbonyl oxygen to the iodine centre of the putative iodosulfonium salt, forming a stabilised chelate complex are advanced to explain this behaviour. The chemistry of hypervalent tellurium difluorides in carbon-fluorine bond formation is discussed. The synthesis of α,β-difluoroethers from fluorodesulfurisation of thione esters and dithioorthoester derivatives using difluoroiodotoluene was attempted. Such fluorination reactions were successful but the hydrolytic instability of the products renders purification and analysis difficult. Part three is an account of the experimental results and procedures employed throughout this work.

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