Heterocyclic Carbenes: A High‐Yielding Synthesis of Novel, Functionalized N‐Heterocyclic Carbenes in Liquid Ammonia

Abstract

To date the only free carbenes of the imidazoline-2-ylidene type to have been described in the literature are those bearing simple hydrocarbon or haloalkyl and -aryl substituents. We report here a novel, versatile and high-yielding method for the synthesis of N-heterocyclic carbenes in a mixture of liquid ammonia and aprotic organic solvents. Deprotonation of the imidazolium precursor salts proceeds under mild conditions within a few minutes at temperatures below -30 "C, and side reactions are thus avoided. The imidazolium salts are much more soluble in organic solvents if liquid ammonia is added. Furthermore, the acidity of the C-2 protons appears to be enhanced by hydrogen bonding. Not only are the known free ylidenes conveniently and quantitatively accessible by this procedure, but also novel functionalized derivatives that are not accessible by known procedures. Imidazoline-2-ylidenes with linear, branched, cyclic, heteroatom-substituted (0, N, P) and chiral hydrocarbon residues are accessible through the novel route. Stable carbene-metal "adducts" are conveniently obtained by treating the free carbenes with chloro-or acetato-bridged dinuclear metal complexes, or by displacement of coordinated ligands such as carbon monoxide, THF or acetonitrile by the free carbenes. The syntheses of novel imidazolium salts, N-heterocyclic carbenes and carbene adducts of Ru", Rh', Wo and sulfur are reported, and the structures of five products analysed by single-crystal X-ray diffraction. N-Heterocyclic carbenes bearing functionalized side chains are important because a number of these complexes show excellent activity in catalytic reactions. They do not show the typical reactivity of metal-carbon "double bonds" and are remarkably stable both thermally and chemically. For a number of reasons, they are best viewed as donor adducts of the highly Lewis basic imidazoline-2-ylidene ligands and the Lewis acidic organometallic fragments. The new synthetic procedure reported here makes Nheterocylic carbenes a generally accessible class of useful ligands in coordination chemistry and catalysis.

Highly Active Dinuclear Copper Catalysts for Homogeneous Azide–Alkyne Cycloadditions

“…(121) 1,3-Dicyclohexylimidazolium chloride was synthesized according to a procedure reported in literature. [169] A 500 ml three-necked flask was equipped with an interior thermometer and a dropping funnel. To a solution of freshly distilled cyclohexylamine (11.4 ml, 9.92 g, 100 mmol) in toluene (100 ml), solid paraformaldehyde was added , 11.5 ml, 5.80 g glyoxal, 100 mmol) was slowly added via another dropping funnel.…”

Topological Analysis of Experimental Charge Densities of Organometallic Carbene Complexes

“…Mössbauer spectroscopy and X-ray crystallographic studies of the compound, [ 3 ], have recently been carried out at various temperatures 1,2 . The X-ray study revealed that a temperature dependent electron transfer takes place between only two of the three Fe-atoms at temperatures higher than 35 K. Below this temperature the valence of all three Fe-atoms is trapped.…”

2-Methyl-1,4-Benzodioxan Substituted Ag(I)-N-Heterocyclic Carbene Complexes and Ru(II)-N-Heterocyclic Carbene Complexes: Synthesis, Crystal Structure and Transfer Hydrogenation of Aromatic Ketones

“…In the commonly used method in the synthesis of Ag(I)-NHC complexes, a silver base has been used as agent of deprotonation. In this context silver bases (Ag 2 O, AgOAc, and Ag 2 CO 3 ) and the solvents (1,2-dichloroethane, acetone, acetonitrile, DMF, DMSO and methanol) have been used [27][28][29][30]. Ruthenium complexes, recently playing an influential role in a variety of studies on catalytic processes have made a great progress.…”