Allenylidene electrophilic additions

Scheme 14 Typical nucleophilic and electrophilic additions on metal-allenylidenes...

As already commented in the introduction of this chapter, regardless of its substitution pattern, the main trends of allenylidene reactivity are governed by the electron deficient character of the C and Cy carbon atoms of the cumulenic chain, the Cp being a nucleophilic center [9-15]. Thus, as occurs with their allcarbon substituted counterparts, electrophilic additions on 7i-donor-substituted allenylidene complexes are expected to take place selectively at Cp, while nucleophiles can add to both C and Cy atoms. However, the extensive 71-conjugation present in these molecules results in a reduced reactivity of the cumulenic chain and, in some cases, in marked differences in the regioselectivity of the nucleophilic additions when compared to the all-carbon substituted allenylidenes. In the following subsections updated reactivity studies on 7i-donor-substituted allenylidene complexes are presented by Periodic Group. 

The alkenyl(amino)allenylidene complex 41 is also prone to undergo electrophilic additions at the Cp atom of the cumulenic chain. Thus, treatment of 41 with HBF4-OEt2 led to the spectroscopically characterized dicationic vinylidene complex 65 (Fig. 10) [52, 53]. Related Cp-protonations of complexes 35 (Fig. 6) have also been described [49]. 

The reactivity of Group 6 allenylidenes [M(=C=C=CR R )(CO)5] (M = Cr, W R and R = alkyl, aryl or H) towards nucleophiles is clearly dominated by the additions at the electrophilic a-carbon. In this sense, the most common reaction of these complexes (usually generated in sim) is the addition of alcohols R OH across the C =Cp bond to afford Fischer-type a,p-unsaturated alkoxycarbene... 

Although the chemistry of pentatetraenylidene complexes [M]=C(=C)3=CR R has not received as much attention as that of aUenylidenes, there is ample experimental evidence to confirm the electrophilic character of the C, Cy and carbons of the cumulenic chain [26-29, 31]. Thus, treatment of tra s-[RuCl(=C=C=C=C=CPh2) (dppe)2][PFg] (132) with alcohols or secondary amines resulted in addition of the nucleophilic solvent across the Cy=Cs double bond to give alkenyl-allenylidenes 138 (Scheme 48) [358]. In chloroform, electrophilic cyclization with one of the Ph groups occurred to give 139. This transformation is actually the parent of the later observed allenylidene to indenylidene intramolecular rearrangement (Scheme 15). 

The use of methanol or ethanol as solvent (or sometimes the molecule of water resulting from the spontaneous dehydration) often leads to the isolation of a Fischer-type alkoxy- or hydroxy-carbene [M]=C(OR)CH=CR R instead of the desired allenylidene. Addition of nucleophiles to allenylidenes dominates the reactivity of these electrophilic groups (see below). Nevertheless, in some cases, the use of silver (I) salts Ag[X] (X = PFg, TfO, BF4 ) results in a more practical and flexible synthetic method since the use of nucleophilic polar solvents can be avoided. 

Highly reactive organic vinylidene and allenylidene species can be stabilized upon coordination to a metal center [1]. In 1979, Bruce et al. [2] reported the first ruthenium vinylidene complex from phenylacetylene and [RuCpCl(PPh3)2] in the presence of NH4PF6. Following this report, various mthenium vinylidene complexes have been isolated and their physical and chemical properties have been extensively elucidated [3]. As the a-carbon of ruthenium vinylidenes and the a and y-carbon of ruthenium allenylidenes are electrophilic in nature [4], the direct formation of ruthenium vinylidene and ruthenium allenylidene species, respectively, from terminal alkynes and propargylic alcohols provides easy access to numerous catalytic reactions since nucleophilic addition at these carbons is a viable route for new catalysis (Scheme 6.1). 

Here, we shall focus on ruthenium-catalyzed nucleophilic additions to alkynes. These additions have the potential to give a direct access to unsaturated functional molecules - the key intermediates for fine chemicals and also the monomers for polymer synthesis and molecular multifunctional materials. Ruthenium-catalyzed nucleophilic additions to alkynes are possible via three different basic activation pathways (Scheme 8.1). For some time, Lewis acid activation type (i), leading to Mar-kovnikov addition, was the main possible addition until the first anfi-Markovnikov catalytic addition was pointed out for the first time in 1986 [6, 7]. This regioselectiv-ity was then explained by the formation of a ruthenium vinylidene species with an electron-deficient Ru=C carbon site (ii). Although currently this methodology is the most often employed, nucleophilic additions involving ruthenium allenylidene species also take place (iii). These complexes allow multiple synthetic possibilities as their cumulenic backbone offers two electrophilic sites (hi). 

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

Although complex 43 does not react with phenylacetylene and methane, the products resulting from the formal addition of H-C bonds of these molecules to the C -Cp and Cp-C. bonds of the allenylidene ligand of 43 can be easily obtained [23]. Because in the allenylidene ligand of 43, the C and atoms are electrophilic centers, and the Cp atom is nucleophilic, the synthetic strategy involves the initial nucleophilic attack of the corresponding carbanion at the C or atoms and the subsequent protonation of the resulting allenyl or alkynyl derivatives. 

Because the EHT-MO calculations on the model cation [Ru(ti5-C5H5) (C=C=CH2 (C0)(PH3)]+ indicate that the C and Cp atoms of the allenylidene unit are electrophilic and nucleophilic centers, respectively, and the H-O hydrogen atoms of water and alcohols are electrophilic, it has been proposed that the transition states for the above mentioned additions require heteroatom-C interactions, which labilize the O-H bonds, favouring the migration of the H-O hydrogen atoms to the Cp atom of the allenylidene. Thus, the lower nucleophilicity of the H-C(sp)carbon atom of phenylacetylene and H-C(sp ) carbon atoms of methane and acetone could explain why the additions of the latter substrates to the allenylidene ligand are kinetically disfavored processes [23]. 

Only a few examples have been obtained through the classical methodologies followed in group 6 metal chemistry. Most rf -Cs Fischer-type ruthenium and osmium carbenes arise from the nucleophilic additions of alcohol and amino groups at the electrophilic carbenic Ca-atom of both allenylidene and vinylidene complexes. The fate of the reaction depends on the electrophilicity as well as the steric hindrance around the Ca-atom, which can control its accessibility, especially for bulky nucleophiles. These features have been thoroughly discussed in a recent review. ... 

Fischer-type osmium alkoxycarbenes are scarce when compared to those of ruthenium. The addition of alcohols to the electrophilic Go -atom of vinylidene or allenylidene groups has also proved to be, as for the ruthenium complexes. 

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