Cation Radicals in Solution

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The Organic Synthesis Program supports Professor Nathan L. Bauld's investigations of the chemistry of cation radicals in solution. Professor Bauld, of the Department of Chemistry of the University of Texas at Austin, is developing a convenient mechanistic criterion for distinguishing electrophilic additions to alkene pi bonds from additions proceeding via initial electron transfer reactions. Formation of homobenzylic cations through addition of electrophiles to aryl vinyl ethers and sulfides, wherein the alkene is insulated from the aromatic ring by virtue of the heteroatom, correlates with the Hammett sigma value, while one-electron oxidation of such substrates correlates with the Hammett-Brown sigma-plus value. In contrast, when the alkene is directly attached to an aryl ring, both protonation and ionization correlate preferentially with sigma-plus. Professor Bauld is developing a unique set of substituent parameters which promise significantly better correlations for cation radical formation, permitting the distinction between electrophilic and electron transfer reactions of styrene derivatives. A new addition polymerization strategy based upon sequential cation radical reactions offers an attractive route to Diels-Alder and other cycloaddition polymers and will be explored mechanistically and as a method for the synthesis of new materials. Professor Nathan L. Bauld, of the Department of Chemistry of the University of Texas at Austin, with support from the Organic Synthesis Program, carries out studies of the formation and reaction chemistry of two classes of reactive molecules, one (`cations`) bearing a net positive charge, the other (`cation radicals`) having a positive charge and also containing an unpaired electron in addition to those electrons involved in forming the chemical bonds in the molecule. He is developing a set of parameters to describe the reactions which can lead to the formation of either of these classes of molecules, determining which types of molecules lead to simple cations and which to cation radicals. The importance of these unusual species is highlighted by Professor Bauld's investigations of a new method for the formation of polymeric materials based upon the reactions of cation radicals. This chemistry promises to afford both new materials and milder routes to known materials displaying a range of important properties.