This review provides applications of organoiron compounds in organic synthesis, focusing on [(η 5 -C 5 H 5 )(CO) 2 Fe]-substituted complexes. Covering recent work the preparation of representative compounds, their reactivity and some details of selected reaction mechanisms are discussed.

Methods for the synthesis of cyclic [h 5 -C 5 H 5 (CO) 2 Fe]-substituted (Z)-enals from cyclic ketones are described. With a single transformation five-membered lactams as well as lactones are accessible from these organometallic reagents.Many naturally occurring sesquiterpenes and diterpenes have an a,b-butenolide or g-lactone moiety in common as part of the molecule. 1,2 In connection with the total synthesis of terpene lactones, the regioselective annulation of butenolides starting from ketones is of interest. 3,4 This holds true for the construction of structural analogs, e.g. lactams. Syntheses of annulated butenolides from ketones have been accomplished in moderate yield for instance by Pummerer reaction of g-(phenylsulfinyl)-a,b-unsaturated aldehydes, available from a-hydroxy-methylene ketones, furnishing (phenylthio)furans and by subsequent hydrolysis butenolides. 4 Palladium-catalyzed methodologies have been shown to be extremely useful, too. 5 Recently, we reported on the synthesis and reactivity of cyclopentadienyl(dicarbonyl)iron-substituted (Z)-alkenals, iron complexes that previously have not been investigated. 6-8 These compounds were found to undergo novel intramolecular cyclocarbonylations leading to dihydropyrrolones from electron-rich primary amines, 6 and, upon treatment with sodium borohydride or K-Selectride, a,b-butenolides or g-lactones. 7 The participation of p-alkene hydridoiron intermediates has been proposed for the reduction steps within the reaction cascades yielding dihydropyrrolones and g-lactones. Lately, we successfully accomplished the synthesis of 5-substituted a,b-butenolides from cyclic b-iron-substituted alkenals and organolithium reagents. 9 Although the mechanisms of these complex reaction cascades require further clarification and yields need to be improved, one clear advantage of these iron compounds as stoichiometric metal organic reagents is the ease of access to either lactones or lactams, each from the same iron complex by one reaction step. As part of our studies on the synthesis and application of iron-substituted alkenals we here disclose our results concerning the access to different cyclic [Cp(CO) 2 Fe]-substituted a,b-unsaturated carbonyl compounds and their reactivity.In previous studies we focused on the synthesis of ironsubstituted enals from the corresponding b-halo-substituted alkenals and sodium ferrate by an addition/elimination process. 6-8 Due to the lability of b-halovinyl aldehydes, 10 especially with an aliphatic substitution pattern, we decided to investigate the synthesis of iron compounds from suitable vinyl triflates in more detail. Therefore, the synthesis of ethoxycarbonyl-substituted iron complexes 3a,b from the triflates 2a,b and sodium ferrate was examined (Schemes 1 and 2). Transformation of these iron complexes to the desired aldehydes 6a,b (Scheme 2) by a two-step reduction/oxidation reaction sequence seemed to be feasible.C-acylation of 4-phenyl-cyclohexanone using diethyl carbonate and sodium hydride as base yielded 1a analytically pur...

Reactions of organolithium and Grignard reagents with cyclic [h 5 -C 5 H 5 (CO) 2 Fe]-substituted enals provide a,b-butenolides by a reaction cascade involving an intramolecular cyclocarbonylation step.

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