Allenylidene-chromium

Scheme 2 Synthesis of amino-allenylidene chromium complexes 5...

Related reactions have also been performed starting directly from M(CO)6 precursors, via decar bony lation (UV irradation) of the corresponding intermediate [M =C(0Li)C=CCR20Li] and subsequent treatment with COCI2 [43, 90, 93]. However, these reactions are not always straightforward and, in some cases, different types of products derived from subsequent cyclization or addition reactions have been obtained. As an example, reaction of the intermediate chromium complex obtained from Cr(CO)6 and [C=CCMe20] with MeCOCl led to the bicyclic dinuclear allenylidene-carbene complex 3 (see Fig. 3) [94]. 

Scheme 17 Some reactions of chromium allenylidenes with phosphines...

Otherwise, while the reactions of chromium mono(amino)allenylidene complexes 5 with dimethylamine afforded the aIkenyl(amino)carbene derivatives 49 by means of the expected N-H addition across the C (=Cp double bond (Scheme 13)... 

Scheme 12 S5mthesis of tetracarbonyl-chromium allenylidene complexes 48...

Scheme 13 Reactivity of chromium allenylidene complexes 5 towards amines...

Otherwise, treatment of the chromium complex 10 with an excess of [W (C0)5(THF)] afforded the tungsten allenylidene 61 by transmetallation of the cumulenic ligand and further addition of W(CO)s to the A-atom of the heterocyclic substituent (Scheme 18) [30]. The related chromium complexes [Cr... 

Scheme 18 Transmetallation of an allenylidene ligand from chromium to tungsten...

Unlike the chromium and tungsten allenylidenes 46, the coupling of the indenyl-ruthenium(II) allenylidene SO with ynamine MeC=CNEt2 takes place regioselectively... 

Scheme 2.20 Coupling between chromium and tungsten allenylidene complexes and ynamines.

The stoichiometric transfer of allenylidene ligands from one metal fragment to another metal center has been scarcely documented, the only examples known involving the allenylidene transfer from chromium compounds [Cr(=C=C= CR R )(C0)5] (R R = aryl, amino or alkoxy groups) to [W(C0)5(THF)] [9d]. DFT calculations indicate that the reaction proceeds by an associative pathway, the initial reaction step involving the coordination of W(CO)5 to the Cc(=Cp bond of the allenylidene ligand in the chromium precursor. 

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