Alkynyl-allenylidene resonance

Based on a large number of stoichiometric studies, the main trends of allenylidene reactivity are presently well established [26 0]. They are governed by the electron deficient character of the and Cy carbon atoms in the cumulenic chain, the Cp being a nucleophilic center. This can also be rationalized by considering the mesomeric forms depicted in Fig. 14. However, it should be noted that, as commented on previously, only contribution from the alkynyl resonance form is supported by X-ray diffraction. 

Since the first discovery of transition metal allenylidene complexes (M=G=C=C<) in 1976, " these complexes have attracted a great deal of attention as a new type of organometallic intermediates. Among a variety of such complexes, cationic ruthenium allenylidene complexes Ru =C=C=GR R, readily available by dehydration of propargylic alcohols coordinated to an unsaturated metal center, can be regarded as stabilized propargylic cation equivalents because of the extensive contribution of the ruthenium-alkynyl resonance form... 

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... 

Due to the extensive contribution of the metal-alkynyl resonance form [M]-G G-G "R R, cationic transition metal-allenylidene complexes [M] "=G=G=GR R have been found to be excellent building blocks for the preparation of functionalized alkynyl derivatives through the addition of nucleophiles. Although the reactivity of cationic allenylidenes is governed by the electron deficiency of both the G - and G.y-atoms of the unsaturated chain, it is now well established that nucleophilic additions at G. regioselectively occur when electron-rich and/or bulky... 

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