TRICARBONYLIRON COMPLEXES - AN APPROACH TO ACYCLIC STEREOCONTROL

pi-Allyltricarbonyliron lactone complexes, eta(4)-dienetricarbonyliron complexes and their relatives offer an interesting approach to the pr oblem of acyclic stereocontrol. Functional groups appended to the orga nic ligand frequently adopt a preferred conformation. This, combined w ith the steric bulk of the Fe(CO)(3) moiety provides a means for contr olling the addition of reagents to such pendant functionality in a def ined manner. Thus addition of nucleophiles to aldehydes and ketones af fords a route to diastereoisomerically pure secondary and tertiary alc ohols while olefinic functionality in the side-chain can be utilised i n stereoselective dihydroxylations, Diels-Alder and Michael addition r eactions. Just as the formation of arene Cr(CO)(3) complexes modifies reactivity at the a-position of arene substituents, the Fe(CO)(3) grou p of eta(4)-diene and trimethylenemethane tricarbonyliron complexes ca n be used to stabilise an adjacent positive charge. Trapping of the ca rbocation resulting from ionisation of an alpha-carbinol occurs with h igh diastereoselectivity, providing an unusual and useful stereoselect ive S(N)1-type reaction, Such highly stereoselective reactions have be en put to good use in the preparation of a number of biologically inte resting natural products.