Mercury-photosensitized reactions

Hg Photosensitization reactions have selectivity for volatile molecules which allows partial oxidation of alkanes and selective dehydrodimerization of a wide variety of organic molecules. Hg photosensitization via the 3P1 state (Hg') allows the dehydrodimerization and functionlization of a variety of volatile organic molecules on a 1 to 50g scale in standard laboratory photochemical equipment at room temperature and pressure. (1-6) The system was initially developed for the functionalization of alkanes on a preparative scale, but it has now been extended to a wide variety of substrates. The substrate is placed in a quartz reactor with a drop of Hg and irradiated with a 254 nm low-pressure Hg lamp. The liquid phase is unreactive so when the product condenses it is protected from further reaction. This product protection by vapor pressure selectivity is critical in allowing partial oxidation, normally especially difficult in the case of alkanes where the partial oxidation products are more reactive than the starting alkanes. The origin of the effect seems, at least in part, to be the precise energy matching between the atomic Hg in the emitting lamp and the atomic Hg in the gas phase of the reactor. The spectrum of the Hg in the liquid phase shows the ca. 254 nm line is strongly broadened and shifted, so liquid phase Hg is not an efficient absorber of radiation. Non Hg' photochemistry is also largely suppressed because most of the radiation is absorbed by Hg. Hg Photosensitized reactions were very extensively studied (7) in the period 1945-73, but the focus was almost exclusively physicochemical. The use of low pressures meant vapor pressure selectivity effects were not recognized and the studies were largely restricted to simple alkanes which are of relatively limited interest in synthetic organic chemistry. A few synthetic applications of Hg photosensitization have been reported: both triplet sensitization (8) and applications to inorganic synthesis of silanes and germanes (9) were briefly explored. HE' Conditions Under our standard conditions, with reflux under N2 (Hg' conditions), Hg' is the reactive species which attacks the substrate. C-H bond scission produces C-centered radicals and H atoms. The H atoms do not recombine but tend to abstract H from substrate, fortunately with a selectivity (3" >2" > 1 ") very similar to that of Hg' itself, to give further C-centered radicals. These recombine and disproportionate. The dehydrodimer condenses and is protected from further reaction