Heteroditopic Imino N‐Heterocyclic Carbenes and Their Sulfur, Selenium, and Tungsten Tetracarbonyl Derivatives

Abstract

AbstractNew heterditopic imino N‐heterocyclic carbenes for use as chelating ligands in homogeneous catalysis are the focus of this contribution. The synthesis of the corresponding imid‐azolium precursors is accomplished in three steps by alkylation of imidazole with ketoalkyl bromides, imination of the carbonyl functionality with anilines, either by conventional methods or by an aluminum‐assisted condensation, and methylation using highly electrophilic methyl triflate. Deprotonation of the imidazolium triflates with potassium hydride gives a different outcome depending on the substitution pattern of the iminomethyl moiety: iminomethyl N‐heterocyclic carbenes containing a methylene group between the imino moiety and the N‐heterocycle are unstable because of tautomerization to the corresponding methylene‐deprotonated ylides. In contrast, iminoisopropyl N‐heterocyclic carbenes, where the acidic site of the methylene group is blocked by two methyl groups, are stable in solution, as proven by 1H and 13C NMR spectroscopy. First derivatives of these new [N,C]‐ligands include their sulfur and selenium oxidation products (iminoisopropylimidazolin‐2‐thione and ‐selenone, respectively) as well as a tungsten tetracarbonyl complex with a six‐membered [N,C] chelating moiety. All new products were fully characterized by multinuclear NMR (1H, 13C, 77Se, 183W) and IR spectroscopy, mass spectrometry, as well as by X‐ray single crystal structures. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)