Stilbene-attached complexes

The observation that addition of imidazoles and carboxylic acids significantly improved the epoxidation reaction resulted in the development of Mn-porphyrin complexes containing these groups covalently linked to the porphyrin platform as attached pendant arms (11) [63]. When these catalysts were employed in the epoxidation of simple olefins with hydrogen peroxide, enhanced oxidation rates were obtained in combination with perfect product selectivity (Table 6.6, Entry 3). In contrast with epoxidations catalyzed by other metals, the Mn-porphyrin system yields products with scrambled stereochemistry the epoxidation of cis-stilbene with Mn(TPP)Cl (TPP = tetraphenylporphyrin) and iodosylbenzene, for example, generated cis- and trans-stilbene oxide in a ratio of 35 65. The low stereospecificity was improved by use of heterocyclic additives such as pyridines or imidazoles. The epoxidation system, with hydrogen peroxide as terminal oxidant, was reported to be stereospecific for ris-olefins, whereas trans-olefins are poor substrates with these catalysts. 

Substances of certain other classes resemble stilbene in possessing high reactivity towards the benzoyloxy radical they include 1,4-diarylbuta-1,3-dienes [35, 36]. New principles are not involved in these studies but one point merits attention. There is great complexity in the NMR signals originating from the benzoate groups attached to units derived from one of these additives because the units may correspond to 1,2-, trans-1,4- or cis-1,4-placements. Similar efTects are found when 1,2,3-triphenylcyclopropene is used as an additive [37]. 

However, attachment of a Cr(CO)3 moiety to one of the benzene rings in cis-stilbene [86] has shown that the complex [cw-(i/ -stilbene)Cr(CO)3] efficiently undergoes photoisomerisation to the trans isomer following irradiation with light longer than 400 nm (Fig. 3.16) [7]. 

Since then, several phanes have been prepared by photoirradiation as a key step for CC bond formation (Table 19.1)." " Until the mid-1980s, stilbene, vinyl ether, and cinnamic acid moieties were used for the [2 + 2] cycloaddition, avoiding the use of labile vinylarenes. Thus, the phanes formed had some additional substituents at the tethers. This sometimes made their characterization complex. Other techniques were developed for the photochemical transformation of the simplest starting materials with vinyl groups directly attached to arene nuclei. 

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