Conjugated diene complexes decomplexation

Addition to unsaturated centres (C=0, C=N, C=C) adjacent to the diene can occur in a diastereoselective fashion, and asymmetric synthesis can be carried out if the diene complex is optically active. As Fe(CO)3 coordinates from one face of the unsymmetrically substituted conjugated dienes, the complexes are chiral and can be resolved to the optically active forms 93 and 94, which are used for asymmetric synthesis. The optically active acetyldiene complex 95, obtained by the acetylation of the optically active diene complex 94, reacts diastereoselectively with PhLi to give 96. The optically active tertiary alcohol 97 is obtained by its decomplexation. The enantiomer 100 can be synthesized by the opposite operation namely the benzoylation of 94 to give 98, and subsequent reaction of MeLi gives 99. The enantiomer 100 is obtained by decomplexation [16]. 

Diene complexes can also be decomplexed by treatment with strong Lewis acids (Scheme 10.10). CO insertion occurs, leading to formation of a cyclopentenone 10.33. This reaction is, formally, a cycloaddition of carbon monoxide with a diene, followed by migration of the resulting alkene into conjugation. 

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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