Tricarbonyliron complexes: an approach to acyclic stereocontrol

pi-Allyltricarbonyliron lactone complexes, eta(4)-dienetricarbonyliron complexes and their relatives offer an interesting approach to the problem of acyclic stereocontrol. Functional groups appended to the organic ligand frequently adopt a preferred conformation. This, combined with the steric bulk of the Fe(CO)(3) moiety provides a means for controlling the addition of reagents to such pendant functionality in a defined manner. Thus addition of nucleophiles to aldehydes and ketones affords a route to diastereoisomerically pure secondary and tertiary alcohols while olefinic functionality in the side-chain can be utilised in stereoselective dihydroxylations, Diels-Alder and Michael addition reactions. Just as the formation of arene Cr(CO)(3) complexes modifies reactivity at the a-position of arene substituents, the Fe(CO)(3) group of eta(4)-diene and trimethylenemethane tricarbonyliron complexes can be used to stabilise an adjacent positive charge. Trapping of the carbocation resulting from ionisation of an alpha-carbinol occurs with high diastereoselectivity, providing an unusual and useful stereoselective S(N)1-type reaction, Such highly stereoselective reactions have been put to good use in the preparation of a number of biologically interesting natural products.