Stereospecific Attack on Cations

When the reaction is irreversible, the addition of nucleophiles to tricar-bonyliron-stabilized cations such as VII normally takes place on the side of the molecule opposite the tricarbonyliron group [for example, Eqs. (34)-(42) and (114)-(120)]. 

Nucleophilic attack by borohydride, however, can result in attack on both sides. Treatment of cation XXX with NaBD gives a mixture of complexes XXVm and XXIX. 

Similar results have been obtained with methoxide addition to VII. (Hine et al., 1975). The product formed initially (kinetic control) is that arising from addition to the side opposite the tricarbonyliron group. Under the conditions of the reaction (NaOMe in MeOH) or in the presence of an acid catalyst, this product is slowly isomerized to a mixture of the products arising from addition to both sides. 

With acyclic cations, once again the kinetic product is formed when the reaction is irreversible, while reversible processes result in a more thermodynamically stable product [Eqs. (189) (A. J. Birch and A. J. Pearson, unpublished work) and (190) (Maglio and Palumbo, 1974)]. 

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