Transition-metal allenylidenes

Selegue s route has been widely used during the last two decades for the preparation of transition-metal allenylidene complexes, its efficiency allowing the access to a huge number of representatives. Although other synthetic alternatives of aUenylidenes are presently known, their applications have been comparatively less developed. In the following subsections updated syntheses of allenylidene complexes are presented by Periodic Group,... 

Transition-metal allenylidenes are prone to undergo cycloaddition and related cyclization reactions involving both M=Co-, Co,=Cp, and Cp=Cy bonds of the cumulenic chain. In some cases, cyclization/cycloreversion pathways have been observed leading to the final isolation of acyclic products. 

The involvement of transition-metal allenylidene complexes in homogeneous catalysis was reported for the first time by B. M. Trost and co-workers in 1992 (Scheme 35) [293-295]. The catalytic reactions allowed the preparation of a wide variety of tetrahydropyranyl and furanyl p,y-unsaturated ketones starting from hydroxy-functionalized alkynols and allylic alcohols, the key step in the catalytic... 

Metal-catalyzed substitution reactions involving propargylic derivatives have not been studied in much detail until recently [311, 312]. In this context, the ability shown by transition-metal allenylidenes to undergo nucleophilic additions at the Cy atom of the cumulenic chain has allowed the development of efficient catalytic processes for the direct substitution of the hydroxyl group in propargylic alcohols [313]. These transformations represent an appealing alternative to the well-known and extensively investigated Nicholas reaction, in which stoichiometric amounts of [Co2(CO)g] are employed [314-317]. 

Other catalytic reactions involving a transition-metal allenylidene complex, as catalyst precursor or intermediate, include (1) the dehydrogenative dimerization of tributyltin hydride [116], (2) the controlled atom-transfer radical polymerization of vinyl monomers [144], (3) the selective transetherification of linear and cyclic vinyl ethers under non acidic conditions [353], (4) the cycloisomerization of (V2V-dia-llyltosylamide into 3-methyl-4-methylene-(V-tosylpyrrolidine [354, 355], and (5) the reduction of protons from HBF4 into dihydrogen [238]. 

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... 

Besides the classical additions of carbon-centered nucleophiles to the electrophilic sites of the cumulenic chain, transition-metal allenylidenes are able to promote... 

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... 

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. 

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. 

---