Ruthenium allenylidene species

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). 

The activation of propargylic ethers also provides the generation of ruthenium allenylidene species with elimination of alcohols (Eq. 13). This reaction has been used in the catalytic transformation of benzyl propargyl ethers into 1,3-dienes via dealkoxylation, addition of benzyl alcohol to the a-carbon atom of the allenylidene intermediate and hydrogen-transfer reactions according to Scheme 21 [89]. 

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). 

As an extension of this reaction, the selective intramolecular nucleophilic addition of a hydroxy group at Cy of a ruthenium allenylidene species generated by activation of propargylic alcohol by RuCl(Cp)(PPh3)2/NH4PF6 provides a ruthenium-vinylidene intermediate. The latter compound reacts with allylic alcohol via a second nucleophilic addition (Scheme 8.13) [27]. This unprecedented tandem reaction makes possible the construction of tetrahydrofuran derivatives in good yields, and has been used in the multistep synthesis of (-)calyculin A [28]. 

Nishibayashi and Sakata recently described the Ru-catalyzed [3+2] cycloaddition of ethynylcyclopropanes bearing two carboxy groups at the homopropargyUc position with aldehydes and aldimines to afford 2-ethynyltetrahydrofurans and pyrrolidines (Scheme 52) [179]. The proposed mechanism requires the formation of the ruthenium allenylidene species II by isomerization of the initially formed vinylidene I. Nucleophilic attack of species II to the aldehyde or aldimine, which are activated by BF3-OEt2, would afford allenylidene III. Final nucleophilic attack on the Cy by the oxygen or nitrogen followed by tautomerization of the vinylidene... 

Since the vinylcarbenes la-c and the aryl substituted carbene (pre)catalyst Id, in the first turn of the catalytic cycle, both afford methylidene complex 3 as the propagating species in solution, their application profiles are essentially identical. Differences in the rate of initiation are relevant in polymerization reactions, but are of minor importance for RCM to which this chapter is confined. Moreover, the close relationship between 1 and the ruthenium allenylidene complexes 2 mentioned above suggests that the scope and limitations of these latter catalysts will also be quite similar. Although this aspect merits further investigations, the data compiled in Table 1 clearly support this view. 

As commented previously, alkenyl(amino)allenylidene ruthenium(II) complexes 41 are easily accessible through the reaction of indenyl-Ru(ll) precursors with ynamines (Scheme 10) [52-54]. Based on this reactivity, an original synthetic route to polyunsaturated allenylidene species could be developed (Scheme 19) [52, 53]. Thus, after the first ynamine insertion, complex 41 could be transformed into the secondary derivative 62 by treatment with LiBHEts and subsequent purification on silica-gel column. Complex 62 is able to insert a second ynamine molecule, via the cyclization/cycloreversion pathway discussed above, to generate the corresponding dienyl(amino)allenylidene species. Further transformations of this intermediate in the presence of LiBHEts and Si02... 

Stabilization of organic vinylidene and allenylidene species via coordination to a ruthenium centre is now well established, and the stoichiometric reactivity of these highly unsaturated ligands is still under intense investigation [ 1-4], and theoretical studies are being carried out [5,6]. Most of the chemical properties of cumulenylidene structures arise from the alternate electronic distribution along the carbon chain (Fig. 1). 

The ability of the binuclear complex [Cp RuCl(p2-SR)2RuCl(Cp )] to generate cationic allenylidene complexes by activation of terminal prop-2-ynols in the presence of NH4BF4 as a chloride abstractor opens the way to a variety of catalytic transformations of propargylic alcohols involving nucleophilic addition at the Cy atom of the ruthenium allenylidene intermediate (Scheme 19). This leads to the formation of a functional ruthenium vinylidene species which tau-tomerizes into an -coordinated alkyne that is removed from the ruthenium centre in the presence of the substrate. 

It is noteworthy that the addition of thiols to form propargylic sulfides is not catalysed by the neutral complex [Cp RuCl(p2-SR)2RuCl(Cp )], but requires the utilization of a cationic precursor such as [Cp RuCl(p2-SMe)2Ru(Cp )(H20)]0Tf [85]. With this catalytic system,propargylic alcohols bearing an internal triple bond are also transformed into propargylic sulfides, which indicates that in this special case, the reaction does not involve a ruthenium allenylidene as an active species. 

The ruthenium allenylidene complexes W are excellent precursors for the catalytic dimerization of tributyltin hydride under mild conditions [ 109] (Eq. 15). In the presence of Bu3SnH, the hydride addition at Cy provides a catalytically active alkynyl ruthenium-tin species (Scheme 22). 

A class of cationic ruthenium allenylidenes, recently reported by Fiirstner and Dixneuf [259, 260], possesses similar high activity at least in RCM and presumably in ROMP. These cationic species are accessible by reaction of (p-cymene) RuCl2(PR3) with a l-prop-2-ynol, preferably with l,l-diphenylprop-2-ynol (Scheme 5.19). 

In contrast to ruthenium and osmium, the reactivity of iron allenylidenes remains almost unexplored. Only the behavior of the cationic diphenylallenylidene-Fe(II) derivative frans-[FeBr(=C=C=CPh2)(depe)2]" has been studied in detail. Thus, it has been found that this complex reacts exclusively at Cy with both neutral (amines, phosphines) and anionic (H , MeO , CN ) nucleophiles [105-107]. This behavior contrasts with that of the neutral Fe(0) derivative [Fe =C=C=C(f-Bu)2 (CO)5] which undergoes PPhs-attack at Co- to afford the zwitterionic phosphonio-allenyl species [Fe C(PPh3)=C=C(f-Bu)2 (CO)5] [104]. 

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