Tetrasubstituted Selenophenes from the Stepwise Assembly of Molecular Fragments on a Diiron Frame and Final Cleavage of a Bridging Alkylidene.

Giacomo Provinciali,Fabio Marchetti,Stefano Zacchini,Tiziana Funaioli,Guido Pampaloni,Marco Bortoluzzi,Beatrice Campanella

doi:10.1021/acs.inorgchem.0c02748

Abstract

A series of 2,3-dicarboxylato-5-acetyl-4-aminoselenophenes, 5a–j, was obtained via the uncommon assembly of building blocks on a diiron platform, starting from commercial [Fe2Cp2(CO)4] through the stepwise formation of diiron complexes [2a–d]CF3SO3, 3a–d, and 4a–j. The selenophene-substituted bridging alkylidene ligand in 4a–j is removed from coordination upon treatment with water in air under mild conditions (ambient temperature in most cases), affording 5a–j in good to excellent yields. This process is highly selective and is accompanied by the disruption of the organometallic scaffold: cyclopentadiene (CpH) and lepidocrocite (γ-FeO(OH)) were identified by NMR and Raman analyses at the end of one representative reaction. The straightforward cleavage of the linkage between a bridging Fischer alkylidene and two (or more) metal centers, as observed here, is an unprecedented reaction in organometallic chemistry: in the present case, the carbene function is converted to a ketone which is incorporated into the organic product. DFT calculations and electrochemical experiments were carried out to give insight into the release of the selenophene-alkylidene ligand. Compounds 5a–j were fully characterized by elemental analysis, mass spectrometry, IR, and multinuclear NMR spectroscopy and by X-ray diffraction and cyclic voltammetry in one case.

Highlights

Stoichiometric organometallic reagents based on transition metals display advantageous features compared to related main group compounds, and have been successfully employed for the development of valuable synthesis strategies and the construction of structural motifs not accessible from typical organic reactions, either metal-catalyzed or not.[1]
A striking example is given by monochromium complexes with a Fischer alkylidene ligand of general formula [Cr(CO)5{ C(OR)R′}], which have emerged as versatile stoichiometric reagents for multicomponent cyclization reactions, providing access to highly functionalized organic molecules incorporating the alkylidene moiety.1a,b,2 This approach has been largely exploited for the preparation of a variety of fine chemicals.2a,3 The unique combination of the favorable properties of iron renders available iron compounds ideal candidates for the development of metal-directed processes.[4,5]
Selenophenes were obtained through a multi-step synthesis starting from [Fe2Cp2(CO)4] (Scheme 1)

Summary

Introduction

Stoichiometric organometallic reagents based on transition metals display advantageous features compared to related main group compounds, and have been successfully employed for the development of valuable synthesis strategies and the construction of structural motifs not accessible from typical organic reactions, either metal-catalyzed or not.[1]. An in situ IR spectroelectrochemical experiment was conducted on a solution of 4a in anhydrous 1,2-dme/ [NnBu4]PF6; a slow potential scan (1 mV/s) from −0.3 and +0.6 V (vs Ag) generated a sequence of IR spectra showing a shift of the terminal and bridging CO bands to higher wavenumber values (from 1931 and 1752 cm−1 to 1994 and 1837 cm−1, respectively), in agreement with the formation of [4a]+ (Figure S10a).

Results

Conclusion