Amino-allenylidenes ruthenium

Scheme 9 Synthesis of amino-allenylidene ruthenium(II) complex 38...

Scheme 10 Synthesis of alkenyl(amino)allenylidene ruthenium(II) complexes 41...

As commented previously, alkenyl(amino)allenylidene ruthenium(II) complexes 41 are easily accessible through the reaction of indenyl-Ru(ll) precursors with ynamines (Scheme 10) [52-54]. Based on this reactivity, an original synthetic route to polyunsaturated allenylidene species could be developed (Scheme 19) [52, 53]. Thus, after the first ynamine insertion, complex 41 could be transformed into the secondary derivative 62 by treatment with LiBHEts and subsequent purification on silica-gel column. Complex 62 is able to insert a second ynamine molecule, via the cyclization/cycloreversion pathway discussed above, to generate the corresponding dienyl(amino)allenylidene species. Further transformations of this intermediate in the presence of LiBHEts and Si02... 

Otherwise, the reactions of indenyl-ruthenium(II) allenylidenes [RuCty -CgHy) =C=C=C(R )Ph (PPh3)2][PF6] (R = H, Ph) with ynamines R C CNEtj (R = Me, SiMea) have been reported to yield the alkenyl(amino)allenylidene complexes 41 via insertion of the ynamine into the Cp=Cy allenylidene bond (Scheme 10) [52, 53], This insertion process involves an initial nucleophilic addition of the ynamine at Cy atom of the cumulene, which leads to the cationic alkynyl intermediate complexes 39. Further ring closing, involving the Cp atom, generates the [2+2]... 

The allenylidene-ruthenium system also appeared as effieient catalyst precursors for enyne metathesis and they were applied to the preparation of fluorinated cyclic amino esters with a 1,3-diene structure allowing Diels-Alder reactions (Scheme 20) [43],... 

Scheme 20. Amino esters by eonsecutive enyne metathesis/Diels—Alder reaction. 4.2 Allenylidene-ruthenium and polymerization...

Reductively induced alterations from cumulenic to alkynyl resonance structures have been observed for mononuclear and dinuclear ruthenium allenylidene complexes. The half-wave potentials for the one-electron reduction of allenylidene complexes [ Ru = C = C = C(ER )(R )] ( Ru = trun.s-Cl(L2)2Ru L2 = chelating diphosphine ER = NR2, SR, SeR, aryl, alkyl R = aryl, alkyl) strongly depends on the nature of the ER substituent. Amino- and aryl-substituted congeners with reduction potentials of ca. -2.2 V and -1.0 V, respectively, constitute the two extremes within this series. These sizable potential... 

Only a few examples have been obtained through the classical methodologies followed in group 6 metal chemistry. Most rf -Cs Fischer-type ruthenium and osmium carbenes arise from the nucleophilic additions of alcohol and amino groups at the electrophilic carbenic Ca-atom of both allenylidene and vinylidene complexes. The fate of the reaction depends on the electrophilicity as well as the steric hindrance around the Ca-atom, which can control its accessibility, especially for bulky nucleophiles. These features have been thoroughly discussed in a recent review. ... 

In contrast to many studies on cycloaromatization via transition metal-vinylidene complexes as key reactive intermediates, only one example of such a reaction via transition metal-allenylidene complexes has been reported to date. In 2008, Yada et al. reported the formation of substituted fiirans 78 from 3-butyne-l,2-diols 77 in the presence of a catalytic amount of thiolate-bridged diruthenium complex (Scheme 21.33) [45]. This methodology was also applied to the formation of a substituted pyrrole 80 from l-amino-2-butyn-2-ol 79. It is noteworthy that thiolate-bridged diruthenium complexes worked as effective catalysts toward cyclization involving both ruthenium-allenylidene and ruthenium-vinylidene complexes as key reactive intermediates. 

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