Tricarbonyliron complexes: an approach to acyclic stereocontrol

Abstract

π-Allyltricarbonyliron lactone complexes, η⁴-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 α-position of arene substituents, the Fe(CO)₃ group of η⁴-diene and trimethylenemethane tricarbonyliron complexes can be used to stabilise an adjacent positive charge. Trapping of the carbocation resulting from ionisation of an α-carbinol occurs with high diastereoselectivity, providing an unusual and useful stereoselective S_N1-type reaction. Such highly stereoselective reactions have been put to good use in the preparation of a number of biologically interesting natural products.