Metal allenylidene complexes

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,... 

Fig. 13 Simplified 7i-orbital diagram for metal-allenylidene complexes...

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... 

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]. 

Ready addition of nucleophiles (Nu ) to metal-allenylidene complexes affords alkynyl derivatives. Subsequent protonation or alkylation, as described in Section 1.2.3 above, then gives the corresponding vinylidene complexes (Equation 1.8) ... 

The reactions are those of functionalized 1-alkynes, the first of which were described for alkynes bearing substituted hydroxymethyl groups, such as substituted propargyl alcohols, HC=CCRR (OH), and often proceed further to form metal allenylidene complexes, by spontaneous dehydration of a (usually unobserved) hydroxy-vinylidene complex (Equation 1.22) ... 

Preparation and Stoichiometric Reactivity of Metal Allenylidene Complexes... 

Similarly, a simplified 7t-orbital interaction for metal allenylidene complexes of the Fisher-type can be constructed, as shown in Figure 4.2. The LUMO is mainly localized at Q and C.. As a result, nucleophilic attack favors the Q and Cy positions [59-61]. It has been found that the contributions of Q and Cy to the LUMO are similar. Therefore, a dear preference for a nucleophilic attack at either Q or Cy could not be deduced [59]. On the basis of the orbital characters in the HOMO, we can again deduce that electrophilic attack occurs at the P-carbon and/or the metal center. 

Figure 4.2 Simplified n-orbital interaction diagram for metal allenylidene complexes of the Fisher-type.

In this chapter, we summarized the theoretical studies carried out on metal vinylidene complexes. Special emphasis was placed on aspects of their electronic structures, reactivities and their roles in organic reactions. Theoretical studies on the related metal allenylidene complexes have been quite limited. More theoretical studies on various aspects of these complexes, particularly on their metathesis reactivities, are clearly necessary. 

Metal allenylidene complexes (M=C=C=CR2) are organometallic species having a double bond betv een a metal and a carbon, such as metal carbenes (M=CR2), metal vinylidenes (M=C=CR2), and other metal cumulenylidenes like M=C=C= C=CR2 [1]. These metal-carbon double bonds are reactive enough to be employed for many organic transformations, both catalytically and stoichiometrically [1, 2]. Especially, the metathesis of alkenes via metal carbenes may be one ofthe most useful reactions in the field of recent organic synthesis [3], vhile metal vinylidenes are also revealed to be the important species in many organic syntheses such as alkyne polymerization and cycloaromatization [4, 5]. 

In parallel, since the first preparation of allenylidene-metal complexes in 1976, the formation of these carbon-rich complexes developed rapidly after the discovery, in 1982, that allenylidene-metal intermediates could be easily formed directly from terminal propargylic alcohols via vinylidene-metal intermediates. This decisive step has led to regioselective catalytic transformations of propargylic derivatives via carbon(l)-atom bond formation or alternately to propargylation. Due to their rearrangement into indenylidene complexes, metal-allenylidene complexes were also found to be catalyst precursors for olefin and enyne metathesis. 

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... 

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 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. 

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. 

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