Process for preparing heterocyclic carbenes

Details Process for preparing heterocyclic carbenes Process for preparing heterocyclic carbenes

1998-11-02

2000-02-15

Higel, Floyd D.

Organic compounds -- part of the class 532-570 series

Organic compounds

Heterocyclic carbon compounds containing a hetero ring...

5482642, 5482646, 5483001, 5483471, C07D23310, C07D23312, C07D23314, C07D23316, C07D24908

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060254963

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BRIEF SUMMARY
Heterocyclic carbenes have in recent times been found to be useful as complexing ligands for a wide variety of metals, with the corresponding metal complexes having a high thermal and chemical stability and very good catalyst properties in the homogeneous catalysis of various reactions.
Metal complexes of metals of metals of the 8th, 9th and 10th groups of the Period Table containing heterocyclic monocarbenes or dicarbenes as ligands are described, for example, in the European Patent Application No. 0 719 753 as suitable catalysts for reactions leading to the formation of carbon-carbon, carbon-hydrogen and carbon-silicon bonds. Furthermore, in the German Patent Application number P 44 47 067.3, cobalt or rhodium complexes having heterocyclic monocarbene or dicarbene ligands are used as catalysts for the hydroformylation of olefinically unsaturated compounds to give aldehydes.
According to the European Patent Application 0 719 758, it is also possible to prepare aromatic olefins from haloaromatics and olefins via a Heck reaction in the presence, as catalysts, of palladium complexes containing heterocyclic carbenes as ligands.
Furthermore, the German Patent Application number P 44 47 070.3 discloses the use of complexes of the lanthanides having heterocyclic carbenes as complexing ligands as catalysts for reactions which are catalyzed by Lewis acids, e.g. for preparing polylactides, and for various CH, CC, CSi and NC linkage reactions.
Metal complexes of heterocyclic carbenes thus have a wide range of catalytic applications; the synthesis of these compounds is therefore of great importance. On this subject, one is frequently directed to the free heterocyclic carbenes whose preparation has, however, hitherto been tied to very specific reaction conditions which greatly restrict the variety of classes of materials which can be used as starting material. Thus, according to the known synthetic methods, only a comparatively small selection of heterocyclic carbenes has hitherto been obtainable, in particular 1,3-dimethylimidazolin-2-ylidene and 1,3-bis(adamantyl)imidazolin-2-ylidene.
The process for preparing free heterocyclic carbenes of the imidazole type described in J. Am. Chem. Soc. 1991, 113, pp. 361-63 comprises reacting an imidazolium salt with a deprotonation reagent in a polar aprotic solvent at relatively high temperatures.
The deprotonation reagent used here is sodium hydride in the presence of catalytic amounts of dimethyl sulfoxide (DMSO) or potassium tert-butoxide; the polar aprotic solvent used is tetrahydrofuran (THF).
The work-up of a free carbene prepared in this way is usually carried out by filtering off the precipitated salts, removing the solvent under reduced pressure and distilling or subliming the residue containing the carbene in a high vacuum at relatively high temperatures. This procedure has the disadvantage that the frequently temperature-sensitive free carbenes are subjected during the purification to a thermal stress which leads to the formation of downstream products and thus to losses in yield. In addition, for reasons of solubility and/or volatility, only a very small selection of carbenes is obtainable in good yield and in pure form, in particular in the case of oily imidazolium salts and their carbene products. A further disadvantage is that in the known procedure the deprotonation rate using the customary reagents and solvents is low, in particular at the relatively low temperatures which are desirable for the stability of the carbenes formed. If the higher temperatures actually required for the deprotonation are employed, the carbenes formed decompose completely or partially even at room temperature. This is compounded by the fact that most polar aprotic solvents such as DMSO or acetonitrile can only be obtained in anhydrous form with a considerable outlay in terms of apparatus and money. In addition, restrictions are placed on the solvents used in terms of their acidity; thus, nitromethane is unsuitable as solvent because of its relatively high acidity, although it has good

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