A Novel Method for the Demetalation of Tricarbonyliron–Diene Complexes by a Photolytically Induced Ligand Exchange Reaction with Acetonitrile

Abstract

,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 ...

Isolation, Structural Characterization, and Synthetic Application of Oxycyclopentadienyl Dianions

“…All these results reveal that 8 is a Ni 0 complex, in which the cyclopentadienone unit acts as a neutral η 4 ‐diene ligand 7. 9 The formation of 8 can be explained by an oxidation–reduction process of intermediate 7 with the elimination of two equivalents of LiCl. This method represents a novel reaction route of OCp dianions, which is also a promising synthetic method for cyclopentadienone–metal complexes.…”

Synthesis of [Bis(hexamethylene)cyclopentadienone]iron Tricarbonyl and its Application to the Catalytic Reduction of C=O Bonds

“…Several variations were introduced into the structure of the (cyclopentadienone)iron complexes 1 , both before the discovery of their catalytic applications and afterwards, with the goal to improve their catalytic activity and to expand the application scope. To this end, a first approach (Figure A) consisted of the replacement of one of the three CO moieties with other types of ligands such as nitriles, pyridines, N ‐heterocyclic carbenes, phosphines, and chiral phosphoramidites . In a second approach (Figure B) the structural elements of the cyclopentadienone ligand, such as the substituents at the 2,5‐positions,,, and/or the 3,4‐positions of the cyclopentadienone were modified ,,–.…”

A Comparative Study of Structurally Related Homogeneous Ruthenium and Iron Catalysts for the Hydrogenation of Levulinic Acid to γ‐Valerolactone

“…Fortunately, inspiration for an alternative approach was derived from literature concerning a useful transformation of Fe‐1 into the corresponding dicarbonyl mono‐acetonitrile analogue ( Fe‐3 ). While the first synthesis of Fe‐3 was achieved by Knölker et al via photochemical ligand exchange, Moulin et al devised a more facile treatment of Fe‐1 with Me 3 NO in acetonitrile at room temperature for 3 hours (Scheme , route C) . Compounds like Fe‐3 have been used successfully in (transfer) hydrogenation, and Oppenauer oxidation[43a] without the aid of an activating agent (e.g.…”