Novel Intramolecular Cyclocarbonylations Involving π-Alkene-Hydridoiron Intermediates: From [η5-C5H5(CO)2Fe]-Substituted (Z)-Enals toα,β-Butenolides andγ-Butyrolactones

Abstract

In order to broaden the application of [h 5 -C 5 H 5 (CO) 2 Fe]-substituted enals, reaction cascades were developed for the construction of five-membered lactone skeletons, initiated by the regioselective reduction of the aldehyde functionality with sodium borohydride or K-Selectride. Depending on the iron compound and the reagent employed, a,b-butenolides or g-lactones were obtained. The key steps in the reaction cascade for the formation of a,b-butenolides involve carbonylation and reductive elimination. Labeling experiments, which were carried out to provide mechanistic details of the subsequent g-lactone formation, are in agreement with a reduction step which involves a p-alkene hydridoiron intermediate. Proposed reaction pathways are given.

Recent chemistry of the η5-cyclopentadienyl dicarbonyl iron anion

“…Novel intramolecular cyclocarbonylations were achieved by Rück-Braun and Möller using Fp-substituted cyclic vinyl aldehydes (Scheme 34) [101]. These Fp complexes were subsequently transformed into ␣,␤-butenolides and ␥-butyrolactones.…”

Preparation and Application of Iron-substituted (Z)-Enals: Synthesis of 5-Substituted α,β-Butenolides

“…In an alternative procedure (Table 1, entry 7), the reaction of 5b with MeLi in THF was hydrolyzed with D 2 O to furnish 6c in 20% yield in addition to the deuterated γ-lactone 8c (30%). Obviously, the γ-lactone derivative is formed during hydrolysis of reaction intermediates, presumably under participation of an ironhydrido species [9,10]. The reaction of 5b with MeLi was studied under various conditions to improve the yield of 6c.…”

“…Interestingly, for reactions of compound 5d conducted in THF by prolongation of the reaction times at -78 °C and room temperature the α,β-butenolide 6b was solely obtained and isolated in 45% yield ( Suitable crystals for X-ray crystal structure analysis were obtained of compound 5b. In the solid state a distorted 1,2-diplanar conformation of the cyclohexene ring and a s-trans conformation of the alkenal moiety is observed, similar to compound 5a studied before [10,12,20]. With the iron complex 5b and 5c at hand and the model compounds 5d and 5e described previously [9], reactions with organolithium compounds RLi (R = Me, nBu) were examined.…”

Methods for Preparing Cyclic β-Cyclopentadienyl(dicarbonyl)iron-substituted α,β-Unsaturated Carbonyl Compounds: Precursors to Annulated α,β-Butenolides and γ-Lactams

“…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][7][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.…”

“…[6][7][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.…”