Ruthenium complex allenylidene

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. 

Although the ruthenium allenylidene complexes 2 have not yet been as comprehensively studied as their carbene counterparts, they also seem to exhibit a closely related application profile [6]. So far, they have proven to tolerate ethers, esters, amides, sulfonamides, ketones, acetals, glycosides and free secondary hydroxyl groups in the substrates (Table 1). 

The central Co,=Cp double bond of an allenylidene backbone can also react with a variety of dipolar organic substrates to yield cyclic adducts. Most of the cychza-tion processes reported occur in a stepwise manner via an initial nucleophilic attack at the Coi atom and further rearrangement of the molecule involving a coupling with the Cp carbon. Representative examples are the reactions of the electron-poor ruthenium-allenylidene complex 46 with ethyl diazoacetate and 1,1-diethylpropar-gylamine to yield the five- and six-membered heterocyclic compounds 82 and 83, respectively (Scheme 29) [260, 284]. 

The observation by Selegue in 1982 [13] that 16 electron ruthenium(II) intermediates could activate terminal propargyl alcohols into ruthenium-allenylidene complexes, via 3-hydroxyvinylidene-metal intermediates, showed not only thatthese allenylidene complexes were stable toward the action of the released water but, especially, that it could be an excellent way to generate allenylidene-metal complexes from easily accessible sources, the propargyl alcohols (Equation 8.1). 

Scheme 8.10 Ruthenium allenylidene complexes with chelating NHC ligand.

Scheme 8.11 Formation of indenylidene ruthenium complex accelerated bythe protonation of ruthenium allenylidene complex.

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... 

Treatment of vinyl ether 165 with the water-soluble ruthenium allenylidene complex [(RuCl(/r-Cl)-(C=C=CPh2)(TPPMS)2)2]Na4 in CHCl3/water and CDC13/D20, respectively, afforded dioxepanes 166 and 167 (Scheme 45) <2003EJI1614> however, this method is of little synthetic interest. 

From ruthenium allenylidene complexes, nucleophilic addition at the less hindered Cy represents the most classical initial step leading to catalytic transformations. 

Recently, complex Q [103] and other neutral and cationic ruthenium allenylidene complexes (U, V) [104] have been reported as efficient catalyst precursors for the ROMP of cyclic olefins such as cyclooctene and norbor-nene. 

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). 

Protons on the 8 carbon of a ruthenium allenylidene complex are acidic, and deprotonation at this position often occurs to give ene-yne derivatives. Reaction of I with cyclic propargyl alcohols of type 110, for example, did not yield the allenylidene complexes, but rather the ruthenium ene-yne products (111) were isolated [Eq. (97)] (78). Reaction of the cor-... 

The formation of allenylidene derivatives from ethynyl-hexanol and alkenyl-vinylidene mononuclear complexes (9), the formation of mononuclear ruthenium allenyl complexes from terminal alkynes (10), the intermediacy of ruthenium-allenylidene complexes in forming propargylic alcohols (II), and in the cyclization of propargyl alcohols (12), and the use of mononuclear ruthenium compounds in allylic alkylation catalysis (13) have also been reported. 

Besides Selegue s methodology, several synthetic alternatives of ruthenium allenylidene complexes have been reported. The most popular involves trapping of transient butatrienylidene or pentatetraenylidene intermediates with nucleophiles [26-29]. Although alcohols, amines, or thiols have been usually employed in these reactions leading to the corresponding heteroatom-substituted allenylidenes, in some cases the use of carbon-centred nucleophiles, such as pyrroles, has been described [185, 186]. Quite recently, a systematic route to prepare sequentially polyalkenyl-allenylidene complexes has also been discovered (Scheme 11) [187— 189]. The first step consists of the insertion of the ynamine MeC=CNEt2 into the... 

Reductively induced alterations from cumulenic to alkynyl resonance structures have been observed for mononuclear and dinuclear ruthenium allenylidene complexes. The half-wave potentials for the one-electron reduction of allenylidene complexes [ Ru = C = C = C(ER )(R )] ( Ru = trun.s-Cl(L2)2Ru L2 = chelating diphosphine ER = NR2, SR, SeR, aryl, alkyl R = aryl, alkyl) strongly depends on the nature of the ER substituent. Amino- and aryl-substituted congeners with reduction potentials of ca. -2.2 V and -1.0 V, respectively, constitute the two extremes within this series. These sizable potential... 

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