Hydrogen atoms in synthesis via mercury photosensitization: Hydrodimerizations and dehydrodimerizations of organic compounds

The present thesis deals with the investigation of new applications for the mercury-photosensitized reactions of organic substrates and hydrogen. A new methodology has been devised for the generation of hydrogen atoms for use in the synthesis of dimer molecules on a preparative scale. The homolysis of H{dollar}sb2{dollar} by excited mercury atoms (Hg* = Hg ({dollar}sp3{dollar}P{dollar}sb1{dollar})) is utilized to generate H atoms in the vapor phase where selective dimerization of the organic substrate occurs, even at high conversion.; In hydrodimerization reactions, H atoms add to CH{dollar}sb2{dollar}=CH-CH{dollar}sb2{dollar}X to give the intermediate radical CH{dollar}sb3{dollar}-({dollar}cdot{dollar}CH)-CH{dollar}sb2{dollar}X which dimerizes to give CH{dollar}sb3{dollar}CH(CH{dollar}sb2{dollar}X)-CH(CH{dollar}sb2{dollar}X)CH{dollar}sb3{dollar} (disproportionation products are also formed). In dehydrodimerizations, saturated substrates of the type CH{dollar}sb3{dollar}CH{dollar}sb2{dollar}CH{dollar}sb2{dollar}X react with H atoms via C-H homolysis to give the intermediate radicals CH{dollar}sb3{dollar}-CH{dollar}sb2{dollar}-({dollar}cdot{dollar}CH)X and final products CH{dollar}sb3{dollar}CH{dollar}sb2{dollar}CH(X)-CH(X)CH{dollar}sb2{dollar}CH{dollar}sb3{dollar}. These reactions show a tolerance for different functional groups and X may be an alkyl or fluoroalkyl chain or contain epoxy, ester, carboxylic acid, ketone, aldehyde, hydroxyl or silyl groups.; The H atom methods have the great advantage that H{dollar}cdot{dollar} adds selectively to the C=C bond of the organic substrate, or attacks the weakest C-H bonds of functionalized organic molecules with no C=C bonds. This constitutes an advantage in comparison to previous work on direct mercury-photosensitization: Hg* often fails to attack C-H bonds in the substrate to give dimers, but rather attacks the functional groups directly to give other products. H atoms are not sensitive to steric or polar effects. Radical fragmentation is avoided by using "high" pressures (1 atm). Intramolecular radical additions to C=C bonds and 1,2-shifts of methyl groups were also seen in some cases. Exceptional product ratios are observed for cross-reactions involving hydroxyalkyl radicals where H-bonding favors the recombination to homodimers.