Conjugated diene complexes synthesis/isomerization

Unconjugated dienes form the 1,3-diene complexes after isomerization to conjugated dienes. Formation of the stable conjugated diene complexe is the driving force of the isomerization. For example, the 1,4-diene in the synthetic intermediate 29 of prostaglandin A can be protected as the diene complex 30 after isomerization to the conjugated diene when it is treated with Fe2(CO)9. This method was applied to the synthesis of prostaglandin C (13). The diene complex 30 is stable for the oxidation of the lactol and introduction of the a-chain [7]. 

Even non-conjugated di-olefins may be used in these systems as precursors for the (conjugated diene)metallocene synthesis. They become isomerized under the reaction conditions at the bent metallocene system.81 Even vinylcyclopropanes yield the corresponding (l,3-diene)metallocene complexes when treated with the reagent 71. The three-membered ring is readily opened under the typical reaction conditions.82... 

Conjugated dienes can be selectively hydrated to ketones in the presence of cationic ruthenium complexes with bipyridyl ligands. The role of ruthenium is to catalyze the isomerization of allylic alcohols formed by the addition of water to diene. This method allows one to convert butadiene to methyl ethyl ketone in high yield [187]. Hydration of triple bonds is one of the oldest catalytic processes of organic chemistry. Though this reaction has no industrial value, it can serve as a tool of fine organic synthesis. The hydration can be catalyzed by rhodium salts under phase-transfer conditions [188]. The more exotic process of the hydrolysis of phenylacetylene to toluene and carbon monoxide catalyzed by ruthenium complex should also be mentioned [189] ... 

French workers have extended this reaction to develop a stereoselective synthesis of -ene-ynes (Descoins and Samain, 1976). Whereas cyclopropyl alkynyl carbinols gave mixtures of ene-ynes of variable isomeric composition under acidic conditions, the corresponding cobalt complexes yielded the E isomers almost exclusively after oxidative decomplexation. Yields for the three step sequence were good. The enhanced selectivity presumably is a result of the steric demand of the cobalt carbonyl moiety. The conjugated ene-ynes are useful intermediates in the preparation of insect pheromones, several of which contain the ,Z-diene unit. 

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