Solvent-Controlled Switch of Selectivity between sp2 and sp3 Carbon-Hydrogen Bond Activation by Platinum (II)

Cyclometalation reactions have been studied intensely for the past few decades, especially those containing palladium. The factors that control the process of the C-H bond activation, however, are not yet completely understood. C-H bonds are ever-present in organic molecules, but the vast majority of them cannot be exploited for chemical reactions due to their inert and stable nature. Early attempts to activate these bonds led to very complicated mixtures of products, and therefore not an acceptable means of C-H activation due to poor selectivity. Controlling the selectivity of a reaction is one of the most important issues surrounding synthetic chemistry. It is generally recognized that aromatic C-H bonds are more likely to undergo activation by platinum complexes. However, recently it has been illustrated that there is a delicate balance between sp2 and sp3 C-H bond activation in a platinum (II) complex system.;In this study, the solvent-controlled switch of selectivity between sp2 and sp3 C-H bond activation in platinum (II) complex systems will be discussed. Ligands L1 through L3 were designed and synthesized to test the selectivity of cycloplatination of a reaction with potassium tetrachloroplatinate (II) in two different solvents, acetonitrile and glacial acetic acid. It was found that in each of the solvents used, a different isomer was produced from the complexation reaction. Reactions of L1 through L3 with potassium tetrachloroplatinate (II) in acetonitrile produced the sp2 substituted isomer (1B-3B), while the same reaction performed in glacial acetic acid formed the sp3 substituted isomer (1A-3A). It was determined through mechanistic studies that the sp 2 substituted isomer is a kinetically controlled product, while the sp3 substituted isomer is a thermodynamically controlled product. Also, it was found that the ratio of products depends on time, where as more time goes by the thermodynamically stable product begins to predominate.;Other issues examined in this study were the side reactions that occurred during the complexation of ligands L2 and L3. These side products were due to C-C bond cleavage in L2 and C-N bond cleavage in L3. These side products were characterized and studied in their own right.