Ethylene complexes, photochemical synthesis

The study of species in which ethylene is coordinated to transition metal centres holds great interest in areas of catalytic and polymerization chemistry (7). The bonding of the ethylene ligand to the metal centre in such species has been compared to that of the dihydrogen complexes described above (14,15,22). Photolysis of chromium hexacarbonyl, Cr(CO)6, in conventional solvents in the presence of dissolved ethylene gas is known to lead initially to a highly labile species in which one CO ligand is replaced by ethylene. Further photolysis leads to a more stable compound which contains two ethylene ligands trans to each other across the metal centre (25), equation 3. The conventional synthesis is experimentally difficult the two photochemical... 

Very recently the synthesis of a covalently bound chlorophyll-a-dimer analogue has been reported703). It was shown that bis-(chlorophyllide-a)ethylene glycol diester with porphyrin rings held together via nucleophilic hydrogen bridging closely resembles in its spectral properties, photochemical activity and redox potential to Chl-aj and therefore can serve as an in vitro model for the native Chl-ai-dimer-protein complex. 

The initial idea in the preparation of complex 25 was to use tamoxifen as a vector for the potentially cytotoxic Cp2TiCl2 2 (section 2, this book). Preparation of complex 25, in which the aromatic p ring of tamoxifen is substituted by a Cp2TiCl2 entity, required development of a novel synthesis (Scheme 3.10) [72]. This synthesis starts with a McMurry coupling reaction between propionyl cyclopentadienyl manganese tricarbonyl 21 and the ketone 22, permitting preparation of the complex diphenyl-ethylene 23. This is converted to a cyclopentadienyl 24 via photochemical decomplexation, and immediately recomplexed in the presence of CpTiClj to give 25, which is isolated as the chloride. 

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