Transition-Metal-Stabilized Vinylidenes

Until 2008, the alkyne/vinylidene tautomerization had only been observed for metal-coordinated terminal alkynes, with the corresponding monosubstituted 

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The reaction consists formally of a 1,2 hydrogen shift. Ab initio calculations have been carried out for free HCCH. The transition state resembles the vinylidene and lies 45 kcal.mol 1 above HCCH. A transition metal fragment could favor this path by stabilizing the vinylidene species and all structures relatively close to this structure on the potential energy surface. Alternatively, the transition metal fragment can give entry to a multistep reaction pathway which is no more a 1,2 hydrogen shift. Two paths have been considered. 

Attempts to produce vinylidene in the free state result in rapid reversion to ethyne, with a lifetime of 10 ° s [1]. As with many reactive organic intermediates, however, vinylidene can be stabilized by complexation to a metal center, using the lone pair for coordination and thus preventing the reversion to ethyne. Most 1-aIkynes can be converted into the analogous vinylidene complexes by simple reactions with appropriate transition metal substrates (Equation 1.2) ... 

Conversion of the unsubstituted acetylene to vinylidene has been widely investigated both experimentally [66-68] and theoretically [69-71]. These studies showed that when metals were not involved, the formation of vinylidene from free acetylene (Scheme 4.4) was very endothermic (44—47 kcal mol ). Since most transition metal fragments can stabilize a vinylidene ligand, the tautomerization of r -acetylene to vinylidene on transition metal centers becomes feasible. Recently, Clot and Eisen-stein have thoroughly reviewed theoretical studies on various tautomerization pathways [44]. For the completeness of this chapter, we here briefly summarize the relevant theoretical flndings. 

Since, in transition metal complexes, the carbene (vinylidene) form of acetylene is often stabilized, it is quite possible that in the presence of alkali metal cation this form also becomes more stable and makes a certain contribution to activation of the triple bond. 

Iron porphyrin carbenes and vinylidenes are photoactive and possess a unique photochemistry since the mechanism of the photochemical reaction suggests the Hberation of free carbene species in solution [ 110,111 ]. These free carbenes can react with olefins to form cyclopropanes (Eq. 15). The photochemical generation of the free carbene fragment from a transition metal carbene complex has not been previously observed [112,113]. Although the photochemistry of both Fischer and Schrock-type carbene has been investigated, no examples of homolytic carbene dissociation have yet been foimd. In the case of the metalloporphyrin carbene complexes, the lack of other co-ordinatively labile species and the stability of the resulting fragment both contribute to the reactivity of the iron-carbon double bond. Thus, this photochemical behavior is quite different to that previously observed with other classes of carbene complexes [113,114]. 

Vinylidenes (M=C=CR R ) are tautomers of 1-alkynes and reformed by a formal 1,2-shift of the alkyne hydrogen from Cl to C2 (H-C=C-H C=CH2). The lifetime of the vinylidene in the free state, H2C=C=CH2 is extremely short (>10 s) therefore, unsaturated cumulenylidenes are extremely reactive and are considered to be important intermediates. The presence of a lone-pair (or two unpaired electrons) on the terminal carbon atom enables vinylidenes to be stabilized by coordination to a transition metal center. This is because some degree of backbonding from the metal to the carbon ligand may further strengthen the... 

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. 

Free vinylidene is a high-energy tautomer of an alkyne that can be effectively stabilized by coordination with transition metals [1-5]. Since the first formation and stabilization of vinylidenes at a transition metal center reported in 1966 [6, 7], a... 

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