Ruthenium-vinylidene active species

From simple terminal alkynes, the catalytic system generated in this case is also thought to proceed via a ruthenium vinylidene active species and is very efficient for the formal elimination of tvater by formation of an organic adduct (Equation 10.3) [12]. 

The formation of vinylcarbamates is restricted to terminal alkynes, which is in line with the formation of a metal vinylidene intermediate, and also to secondary amines. Indeed, a catalytic reaction also takes place under similar conditions with primary aliphatic amines but it leads to the formation of symmetrical ureas (Scheme 3) [10]. The catalytic system generated in this case is also thought to proceed via a ruthenium vinylidene active species and is very efficient for the formal elimination of water by formation of an organic adduct. The proposed general catalytic cycle, which applies for the formation of vinylcarbamates and ureas, is shown in Scheme 4 [11]. 

The proposed mechanism involves the formation of ruthenium vinylidene 97 from an active ruthenium complex and alkyne, which upon nucleophilic attack of acetic acid at the ruthenium vinylidene carbon affords the vinylruthenium species 98. A subsequent intramolecular aldol condensation gives acylruthenium hydride 99, which is expected to give the observed cyclopentene products through a sequential decarbonylation and reductive elimination reactions. 

This qfdization represents a rare example in which ruthenium vinylidene is capable of activating a tethered it-alkyne toward nucleophilic addition, as shown in species lOS this species ultimately formed the observed products via intermediate 106 (Scheme 6.36). 

Terminal alkynes can undergo several types of interaction with ruthenium centers. In addition to the formation of ruthenium vinylidene species, a second type of activation provides alkynyl ruthenium complexes via oxidative addition. 

These reactions illustrate the importance of ruthenium vinylidene species, as activated forms of terminal alkynes, in catalysis, because they favor the addition of O-nudeophiles (carbamic and carboxylic acids, alcohols, water) to terminal alkynes and completely reverse the expected regioselectivity of the addition. These examples also show that the activation processes are very sensitive to the nature of the nucleophiles, and the success of the awtt-Markovnikov addition to terminal alkynes is highly dependent on both the electron richness and steric hindrance of the ancillary ligands coordinated to the active site. 

The most efficient catalyst precursors were found in the RuCl2(arene)(phos-phine) series. These complexes are known to produce ruthenium vinylidene species upon reaction with terminal alkynes under stoichiometric conditions, and thus are able to generate potential catalysts active for anti-Markovnikov addition [8]. Dienylcarbamates could also be selectively prepared from conju-... 

Terminal alkynes can undergo several types of interaction with ruthenium centres. In addition to the formation of ruthenium vinylidene species, a second type of activation provides alkynyl ruthenium complexes via oxidative addition. When these two types of coordination take place at the same metal centre, the migration of the alkynyl ligand onto the Ca atom of the vinylidene can occur to form enynyl intermediates, which upon protonation by the terminal alkyne lead to the formation of enynes corresponding to alkyne dimerization... 

The selective intramolecular nucleophilic addition of a hydroxy group at Cyof a ruthenium allenylidene generated by activation of propargylic alcohol by RuCl(Cp)(PPh3)2/NH4PF6 provides a ruthenium vinylidene species, which reacts with allylic alcohols as previously described in the section Formation of Unsaturated Ketones (Eq. 11, Scheme 18) [79]. This unprecedented tandem reaction makes possible the construction of tetrahydrofuran derivatives in good yields and has been used as a key step in the synthesis of (-)calyculin A [80]. 

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. 

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). 

The first example of anti-Markovnikov addition of 0-nucleophiles to terminal alkynes was actually the catalytic addition of ammonium carbamates generated in situ from secondary amines and carbon dioxide to give vinylcarbamates. This was also the first suggestion of a ruthenium-vinylidene intermediate as a catalytic active species for organic synthesis (Scheme 8.20) [6, 7]. 

Various situations are analyzed where the two metal centers play a role in one of the coordination modes A-E. There are many cases in which bimetallic catalysis can occur with the two metals acting cooperatively, for instance, in the dimerization of alkynes at two ruthenium metal centers, where a ruthenium-vinylidene species is formed, which is able to subsequently activate the second alkyne reactant through a C-H cleavage on the second ruthenium center. The coupling of these two moieties occurs on this dinuclear platform to provide the enyne product molecule. Examples are also presented where bimetallic catalysts cooperatively activate substituted alkynes in the catalyzed formation of heterocycles. 

The mechanism (Scheme 60) involves coordination of the terminal alkyne to the ruthenium atom followed by the formation of the vinylidene complex 135. Coordination of the allyl alcohol followed by addition of the alcohol to the ruthenium vinylidene complex leads to the ruthenium carbene complex 136. Metalla-Claisen rearrangement produces the jr-allyl-acylruthenium complex 137, which undergoes a reductive elimination to give the product 133 and regenerates the catalytically active ruthenium species. The regioselectivity of the coupling is independent of the site of ionization and the new bond formation occurs on the more substituted terminus of the double bond of the rr-allyl-ruthenium complex. 

Cyano-derivatives can be readily obtained by a ruthenium-catalyzed addition of various hydrazines to terminal alkynes [89] in which the cyano carbon atom arises from the terminal alkyne carbon atom. The tris(pyrazolyl)borate (Tp) complex RuCl(Tp)(PPh3)2 (1 mol%) was found to be the most active catalyst, and N,N-dimethylhydrazine (5 equiv.) the best nitrogen source. The proposed mechanism involves the nucleophilic attack of the nitrogen nucleophile on the a-carbon of a vinylidene intermediate (Scheme 8.27). Proton migration in the resulting a-hydrazi-nocarbene, followed by deamination, would give the nitrile derivative and regenerate the catalytic species. 

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... 

The iron species [Fe(X)2 CN(PP)CH(Me) = CH(Me)N(Pr ) ] (X = Cl, Br), containing highly donating imidazolyidene ligands, have been found to be extremely active and efficient catalysts for the atom transfer radical polymerisation of styrene and methylmethacrylate. A variety of indenyl ruthenium complexes containing either phenylacetylide (C = CPh) or vinyl (CH = CHPh) ligands have been found to catalyse the dimerisation of phenylacetylene to ( )-and (Z)-l,4-diphenyl-l-en-3-yne with the activity of the catalyst dependent upon the nature of the phosphine co-ligand bound to ruthenium. The vinylidene-ruthenium(II) complexes [Ru(Cl)(L)2(C = CHR)] (R = Bu, ferrocenyl L =... 

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