Isopolyhalomethanes: Their formation, structures, properties and cyclopropanation reactions with olefins

An overview of recent research on isopolyhalomethanes is presented that examines their formation, structure, properties and cyclopropanation reactions with olefins. Time-resolved experimental results show isopolyhalomethanes are formed vibrationally hot within a few picoseconds following solvent induced geminate recombination of the initially produced haloalkyl radical and halogen atom fragments of the parent polyhalomethane molecule photodissociation. The carbenoid behavior of the CH₂I-I isomer toward C=C bonds can be largely attributed to the CH₂I⁺I^- radical ion pair character that activates the CH2_I moiety and the sp² bonding character of the C atom in the isopolyhalomethane species. Time-resolved resonance Raman and density functional theory studies on selected isopolyhalomethanes and their cyclopropanation reactions with olefins indicates the isopolyhalomethane lifetime and chemical reactivity relative to the intitially formed haloalkyl radical fragments helps to determine the efficiency of the isopolyhalomethane cyclopropanation reactions. The chemical reactivity of the CH₂I-I and the classic Simmons-Smith IZnCH₂I carbenoids towards olefins can be largely explained by their differing energetics for releasing the leaving group, steric demands and electrophilic character of the methylene group to be transferred.