Group 9 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,... 

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]. 

In sharp contrast with the remaining transition metals, Group 6 allenylidenes [M (=C=C=CR R )(C0)5] (M = Cr, Mo, W) containing non-donnor substituents at Cy are in general thermally unstable [11-15]. For this reason, most of the reported examples are substituted derivatives bearing heteroatomic Ti-donor groups [9]. In this sense, the first stable Group 6 allenylidenes reported in the literature were the amino-allenylidene complexes 2, prepared by E. O. Fischer and coworkers by a Lewis-acid induced elimination of ethanol from the 3-dimethylamino-l-ethoxy-3-... 

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) ... 

The conformational orientation adopted by the allenylidene group =C=C= CR R also merits comment. Thus, in half-sandwich complexes, mainly derived from [M(77 -CxHy)L2] (M = Fe, Ru, Os) metal fragments, a marked preference of the allenylidene group to adopt a vertical orientation in which the ipso carbon atoms of the R /R substituents are contained in the molecular plane (pseudo mirror plane bisecting the half-sandwich metal fragment) is observed. Preference for this conformation arises from the dominant metal ( xy-Cp back donation of the metal-HOMO into the allenylidene-LUMO n orbital (see Fig. 13). In contrast to this general trend, an unusual horizontal orientation of the allenylidene group was... 

The regioselectivity of the nucleophilic additions on allenylidene complexes (C vs Cy) is subtly controlled by the electronic and steric properties both of the substituents on the unsaturated hydrocarbon chain and the ancillary ligands on the metal atom, as well as by the nucleophile employed. In this section we will summarize the nucleophilic reactivity of mononuclear Group 6-9 allenylidenes. 

In this context, it should also be noted that an oxidatively induced C -P coupling has been described in the oxidation reactions of complexes tra i-[RhCl (=C=C=CR2)(P/-Pr3)2] (R = Ph, o-OMeCeH4) with CI2 or PhlClj, affording phos-phonio-allenyl products [RhCl3 C(P/-Pr3)=C=CR2 (Pi-Pr3)]. They are formed by migration of one Pi-Pr3 group from the metal to the allenylidene a-carbon in the six-coordinate Rh(III) intermediates [RhCl3(=C=C=CR2)(Pi-Pr3)2] [205]. 

This synthetic strategy proved to be unsuitable for [M(CO)5] (M = Cr, W) metal fragments due to the thermal instability of the corresponding non-donor-substituted allenylidenes [M(=C=C=CR R )(C0)5] (R, R = usually alkyl or aryl groups). An alternative general synthetic procedure using deprotonated functionalized acetylenes has been successfully applied (for example tris-amino or alkoxo prop-1-ynes see Equation 2.2) [4a]. 

Figure 2.4 Example of octahedral and coordinate allenylidene complexes of Group 8 metals.

The stoichiometric transfer of allenylidene ligands from one metal fragment to another metal center has been scarcely documented, the only examples known involving the allenylidene transfer from chromium compounds [Cr(=C=C= CR R )(C0)5] (R R = aryl, amino or alkoxy groups) to [W(C0)5(THF)] [9d]. DFT calculations indicate that the reaction proceeds by an associative pathway, the initial reaction step involving the coordination of W(CO)5 to the Cc(=Cp bond of the allenylidene ligand in the chromium precursor. 

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