Formation of Diene Complexes

The least sterically hindred complex is favored in dienes containing alkyl groups. For example, the tricarbonyliron complexes of a-phellandrene are obtained with the Fe(CO)3 group on the same and opposite faces relative to the isopropyl group in the ratio of about 3 7, respectively (A.J. Birch, B. J. Chauncy and I. D. Jenkins, unpublished work.) Initial complexation [of Fe(CO)4 for example] to a group such as COOMe may result in the metal atom being directed to the same side of the molecule [Eq. (184) (A. J. Birch and B. J. Chauncy, unpublished work)]. 

The structure of XXVII has been confirmed by X-ray crystallography (B. Anderson, personal communication). 

A similar effect of COOMe has been reported [Eq. (185) (Whitesides el al., 1974)]. This kind of directive influence may be a rather general one with carbonyl groups and possibly even with olefins. 

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