Mercury-sensitized photoreactions

Sensitization by mercury is useful for performing the photoreaction on all those 1,4-dienes that lack chromophoric groups, e.g. alkyl and/or cycloalkyl substituted systems. These reactions appear to proceed by triplet energy transfer from the mercury to give vibrationally exited diene triplets. Thus, at variance with solution phase photoreactions, the exited states possess greater vibrational energy and substantially different products are possible. While such comparison is impossible for the parent system because no solution phase photochemistry has been reported, the di-ir-methane product (4) was not observed when irradiation of the 3,3-dimethyl-substituted diene (3) (equation 3) was carried out under acetone sensitization in solution. Hydrocarbon (4) was the major component under mercury-sensitized conditions in the vapor phase. ... 

The low lying ji -orbitals of conjugated dienes explain the stability of many diene metal carbonyl complexes e.g. butadiene iron tricarbonyl 382>. Photochemical preparation of the latter 286> is superior to the thermal procedure 272>. Mercury can be used as a sensitizer in the photoreaction of metal carbonyls with dienes I09.i7o,i76,24i)... 

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