The [221] cycloaddition of two alkynes and carbon monoxide in the presence of pentacarbonyliron represents a useful method for the construction of five-membered ring systems. [1, 2] Applications of the resulting tricarbonyl(h 4 -cyclopentadienone)iron complexes to organic synthesis are feasible by demetalation to the free cyclopentadienones. This transformation was achieved by oxidation with trimethylamine Noxide. [1,3] Recently we reported a novel method for the demetalation of tricarbonyl(diene)iron complexes by a photolytically induced exchange of the carbonyl ligands by acetonitrile. [4] Herein we describe an alternative procedure for the ligand exchange at tricarbonyl(h 4 -cyclopentadienone)iron complexes and the subsequent demetalation in the air.Tricarbonyl(h 4 -cyclopentadienone)iron complexes undergo a transformation similar to the Hieber reaction. [5] Thus, reaction of complex 1 a with aqueous NaOH in THF leads to an equilibrium of the corresponding hydrido complexes 2 a and 4 a in a ratio of about 13:1 (Scheme 1). Tricarbonyl(cyclohexa-1,3-diene)iron complexes are inert under these conditions. Addition of H 3 PO 4 affords 2 a in 94 % yield, while reaction with NaH shifts the equilibrium towards the salt 4 a Scheme 1. a) 1m NaOH/THF (1/2); b) C 5 H 11 I; c) H 3 PO 4 ; d) air, daylight, Et 2 O/THF, Na 2 S 2 O 3 , Celite, 3 h; e) NaH, Et 2 O/THF.(82 % yield). Reaction of the hydrido complex 2 a with 1-iodopentane provides the iodo complex 3 a in 98 % yield. A related transformation is reported for the hydrido complex [CpFe(CO) 2 H]. [6] The addition of 1-iodopentane after the reaction of 1 a with NaOH affords an equilibrium of the iodo complexes 3 a and 5 a that is shifted again by addition of H 3 PO 4 or NaH, respectively. Preparation of the iodo complex 3 a without isolation of the intermediate hydrido complex 2 a increases the yield (98 % based on complex 1 a).The 13 C NMR and the IR data of the hydrido complex 2 a and the iodo complex 3 a suggest an h 5 -coordinated hydroxycyclopentadienyl ligand for both compounds. [7] A characteristic structural feature of the hydrido complex 2 a is the unsymmetrical arrangement of the coligands, which is apparent from two CO signals in the 13 C NMR spectrum. This assignment was confirmed by an X-ray structure determination of complex 2 a (Figure 1), [8] which shows an h 5 -coordinated hydroxycyclopentadienyl ligand and a C1ÀO1 bond length of 1.366 . [9] A loss of C S symmetry was also found for the hydrido complex 4 a from the 13 C NMR spectrum, which exhibits the two signals for the carbonyl ligands and a peak at d 170.13 for C1. [7] Figure 1. Molecular structure of 2 a in the crystal. Selected bond lengths []: FeÀC1 2.

The photochemical exchange of all three carbonyl ligands with acetonitrile converts tricarbonyliron-diene complexes into the very labile triacetonitrile-iron-diene complexes. These easily demetalate in high yields to the corresponding free ligands on injection of air at -30°C [Eq. (1)]. The novel demetalation procedure is applied to the tricarbonyliron complexes of cyclopentadienones, cyclohexa-1,3-dienes, and buta-1,3-dienes.

,3-diene)iron complexes are a useful class of organometallic compounds with versatile applications to organic synthesis.[1] The coordination of the conjugated diene to the transition metal fragment leads to a significant alteration of its reactivity. Therefore, the tricarbonyliron fragment has been used for the stabilization of labile hydrocarbons and as a protecting group for dienes.[1] After the desired transformations at the ligand of the tricarbonyl(h 4 -1,3-diene)iron complex a demetalation is required to provide the free diene. This decomplexation of tricarbonyliron complexes is usually achieved under strong oxidizing reaction conditions, for example with ferric chloride, [2] ceric ammonium nitrate, [3] trimethylamine N-oxide (TMANO), [4] cupric chloride, [5] or hydrogen peroxide/sodium hydroxide. [6] In connection with investigations of the iron-mediated [221] cycloaddition [7,8] and our studies directed towards the application of tricarbonyliron complexes to the synthesis of alkaloids [9] we required a method for demetalation of tricarbonyliron complexes by using extremely mild reaction conditions. Herein we describe a novel procedure for the demetalation of tricarbonyliron ± diene complexes using a photolytically induced exchange of the carbonyl ligands by acetonitrile at low temperature and subsequent demetalation in the air.Although the iron-mediated [221] cycloaddition has been known for four decades, [10] only a few very limited applications were reported because of the difficulties associated with the demetalation of the resulting tricarbonyl(h 4 -cyclopentadienone)iron complexes. We recently demonstrated that selective demetalation is feasible using trimethylamine N-oxide by careful control of the reaction conditions. [7a,b] However, the yields in some cases were only moderate. Therefore, we set out to develop a novel demetalation procedure in which the bonding of the metal fragment to the diene becomes labile by exchange of the carbon monoxide ligands. Acetonitrile ligands appeared to be promising candidates for such a transformation in the coordination sphere of the metal since they are rather poor acceptors. Thus, their introduction should result in a decreased back donation of electrons from the filled iron d orbitals to the ligand and the resulting complexes should be more easily oxidized.The tricarbonyliron complex 1 a [7a] is stable at room temperature in the air. No acetonitrile complexes are observed on refluxing a solution of 1 a in acetonitrile for 29 h in the dark. However, exposure to daylight at room temperature results in a very slow formation of the monoacetonitrile complex 2 a along with the demetalated cyclopentadienone 5 a. Irradiation of a solution of complex 1 a in acetonitrile under argon atmosphere using a medium-pressure mercury lamp accelerates the ligand exchange dramatically and leads to a stepwise exchange of all three carbonyl ligands (Scheme 1, Table 1).Photolysis of 1 a in acetonitrile at À 30 8C afforded after 1 h the diacetonitrile complex 3 a in 76 % as dark ...

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Durch sequentiellen Austausch eines Carbonylliganden durch einen Hydridliganden mit NaOH und einen Iodliganden mit Iodpentan und anschließende Umsetzung mit Säure werden Tricarbonyl(η4‐cyclopentadienon)eisen‐Komplexe in Dicarbonyl(η5‐hydroxycyclopentadienyl)iodeisen‐Komplexe überführt. Diese demetallieren beim Kontakt mit Luft an Tageslicht hochselektiv zu den entsprechenden freien Liganden. Unter basischen Bedingungen reagiert der Iodkomplex zu einem anionischen Dicarbonyl(η4‐cyclopentadienon)iodeisen‐Komplex, der im Festkörper dimer vorliegt und dessen Natrium‐Gegenion nahezu quadratisch‐planar koordiniert wird.