Metal-hydroxyvinylidenes

Allenylidenes could be considered as divalent radicals derived from allenes. In a similar way to vinylidenes, allenylidenes can be stabilized by coordination with transition metals and again ruthenium is one of the most widely used metals. Metal-allenylidene complexes can be easily obtained from terminal propargylic alcohols by dehydration of the initially formed metal-hydroxyvinylidenes, in which the reactivity of these metal complexes is based on the electrophilic nature of Ca and Cy, while Cp is nucleophilic. Catalytic processes based on nucleophilic additions and pericyclic reactions involving the it system of ruthenium allenylidenes afford interesting new structures with high selectivity and atom economy. 

The reaction is based on the spontaneous dehydration of the intermediate hydroxyvinylidene species formed, either via A or via B (Scheme 2.1), after the coordination of 2-propyn-l-ols at the metal center. 

The observation by Selegue in 1982 [13] that 16 electron ruthenium(II) intermediates could activate terminal propargyl alcohols into ruthenium-allenylidene complexes, via 3-hydroxyvinylidene-metal intermediates, showed not only thatthese allenylidene complexes were stable toward the action of the released water but, especially, that it could be an excellent way to generate allenylidene-metal complexes from easily accessible sources, the propargyl alcohols (Equation 8.1). 

Allenylidenes ligands are divalent radicals derived from allenes and their metal derivatives can be easily obtained from terminal propargylic alcohols by dehydration of initially formed M-hydroxyvinylidenes [174]. Since the first report of the use of transition metal allenylidene complexes in catalytic reactions by Trost [94], significant progress in this field has been made [59, 64, 65, 175]. The reactivities of metal allenylidene complexes are rationalized by considering the electrophilicity of Ca and Cy and the nucleophilicity of Cp of the M=C=C=CR2 moiety. 

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