Allenylidene reactions with ynamines

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]... 

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... 

Allenylidene complexes also undergo [2+2] cycloaddition reactions with alkynes, but in addition to the cycloadducts linear insertion products are also obtained. For example, diarylallenylidene complexes of chromium and tungsten react with MeC NEth2 or PhCMI NEt2 to give both linear adducts 36 and cycloadducts 37. The linear adducts are formed via an initial cycloaddition of the ynamine to the C =C bond in the allenylidene complex, followed by cycloreversion. The cyclobutenylidene complexes are formed by [2+2] cycloaddition of the ynamine to the C =C double bond . 

Group 6 allenylidenes also react with the carbon-carbon triple bond of ynamines to yield similar cyclobutenylidene derivatives 88 along with the corresponding alkenyl-aminoallenylidenes 89 (Scheme 32) [286]. These aminoallenylidene complexes result from a formal [2-1-2] cycloaddition between the ynamine C=C and allenylidene Cp=Cy bonds followed by cycloreversion. A stepwise cyclization initiated by the addition of the nucleophilic R C=CNEt2 carbon at the C or Cy position has been proposed in the formation of these isomeric products. As commented previously, unlike their Cr and W counterparts, the reactions of... 

Besides Selegue s methodology, several synthetic alternatives of ruthenium allenylidene complexes have been reported. The most popular involves trapping of transient butatrienylidene or pentatetraenylidene intermediates with nucleophiles [26-29]. Although alcohols, amines, or thiols have been usually employed in these reactions leading to the corresponding heteroatom-substituted allenylidenes, in some cases the use of carbon-centred nucleophiles, such as pyrroles, has been described [185, 186]. Quite recently, a systematic route to prepare sequentially polyalkenyl-allenylidene complexes has also been discovered (Scheme 11) [187— 189]. The first step consists of the insertion of the ynamine MeC=CNEt2 into the... 

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