Ruthenium catalysts vinylidene

Bruneau C (2004) Ruthenium Vinylidenes and Allenylidenes in Catalysis. 11 125-153 Bruneau C, D4rien S, Dixneuf PH (2006) Cascade and Sequential Catalytic Transformations Initiated by Ruthenium Catalysts. 19 295-326 Brutchey RL, see Fujdala KL (2005) 16 69-115... 

Scheme 6.20. This ruthenium catalyst (10 mol%) was active for the cydization of ds-1 -ethynyl-2-vinyloxiranes to afford various 2,6-disubstituted phenols in reasonable yields. Under similar conditions, 1,1,2,2,-tetrasubstituted oxiranes gave the 2,3,6-trisubstituted phenols with a skeleton reorganization [22]. The 1,2-deuterium shift of the alkynyl deuterium of d-Sle was indicative of mthenium vinylidene intermediates (Scheme 6.20).

Most of these catalytic systems are able to dimerize either aromatic alkynes, such as phenylacetylene derivatives, or aliphatic alkynes, such as trimethylsilylacetylene, tert-butylacetylene and benzylacetylene. The stereochemistry of the resulting enynes depends strongly on both the alkyne and the catalyst precursor. It is noteworthy that the vinylidene ruthenium complex RuCl(Cp )(PPh3)(=C=CHPh) catalyzes the dimerization of phenylacetylene and methylpropiolate with high stereoselectivity towards the ( )-enyne [65, 66], and that head-to-tail dimerization is scarcely favored with this catalyst. It was also shovm that the metathesis catalyst RuCl2(P-Cy3)2(=CHPh) reacted in refiuxing toluene with phenylacetylene to produce a... 

For ruthenium catalysts a detailed study of [(PP3)RuH2] proposes a bis(alkynyl) complex as the real catalyst. The catalytic key step involves the protonation of an alkynyl ligand by external PhC=CH, allowing subsequent C-C bond formation between cis vinylidene and alkynyl groups [9]. 

C. Bruneau, Ruthenium vinylidenes and allenylidenes in catalysis in Ruthenium catalysts and fine chemistry, Topics in Organomet. Chem., C. Bruneau,... 

Fig. 3 Metathesis-active vinylidene, allenylidene, and indenylidene ruthenium catalysts...

Recently, new types of ruthenium catalyst precursors that perform the Markovnikov addition of carboxylic acids to terminal alkynes have been developed. The most representative examples are [RuCl2(p-cymene)]2/P(furyl)3/base [50], Ru-vinylidene complexes such as RuCl2(PCy3)2(=C=CHt-Bu), RuCl2(PCy3)(bis(mesityl)imidazolyli-dene)(=C=CHf-Bu), [RuCl(L)2(=C=CHt-Bu)]BF4 [51], and the ruthenium complexes shown in Figure 8.1 [52-54]. 

The first use of vinylidene-metal species in C-C bond formation with a ruthenium catalyst was reported by Trost in 1990. Since then, the use of vinylidene-Rh species in organic synthesis has been extended to the formation of heterocycles through cycloisomerization of alkynols and alkynyl anilines. For example, the cycloisomerization of alkynol 455, leading to the formation of pyran 456 was used for the synthesis of amino sugar 457 (Scheme 2-51). ... 

Once generated, one way to trap a vinylidene complex could be in an electrocyclic reaction. Indeed, treatment of an alkyne 8.143, having a dienyl unit attached, with a ruthenium catalyst yields a tricyclic product 8.144 (Scheme 8.40). Deuterium-labeling experiments are consistent with the alkyne-vinylidene isomerization. A tungsten catalyst, W(CO)5.THF, may also be used. ... 

Another common class of metal-carbene complexes is that of the vinylidene complexes (whose structure compares to that of allenes), isomers of metal-alkyne complexes. A well-known example is the vinylidene complex [Ru(PPh3)2Cl2( =C=CHPh)], the first unimolecular ruthenium catalyst of olefin metathesis discovered by Grubbs in 1992. - This class is extended to the allenylidenes and cumulenylidenes. ... 

Interestingly, modulation of the regioselectivity of the reaction from anti-Markovnikov (formation of the vinylidene) to Markovnikov (either by electrophilic activation of the alkynes or by oxidative addition of the acid to the ruthenium complex and subsequent migratory insertion and reductive elimination) was observed on using [(p-cymene)RuCl2]2 as the ruthenium catalyst and different phosphine ligands and bases (Table 1) [58, 61, 77]. 

Intramolecular anti-Markovnikov addition of carboxylic acids to alkynes was also achieved using ruthenium catalysts. a,co-Alkynoic acids afford the corresponding cycloalkene lactones by intramolecular addition of the carboxylic acid to the corresponding catalytic vinylidene species obtained by treatment of the... 

Variation of the electronic and steric nature of the ruthenium catalysts allow the complementary carboxylative cyclization of 1,6-diynes (Scheme 36) [150]. Lee and coworkers described how a variety of carboxylic acids condense with 1,6-terminal diynes in the presence of catalytic amounts of [Ru(p-cymene)Cl2l2. P(4-F-C6H5)3 and 4-dimethylaminopyridine to give cyclohexylidene enol carboxylates with exclusive ( )-selectivity. The proposed mechanism involves the initial formation of a ruthenium vinylidene species I followed by intramolecular cyclization induced by the nucleophilic attack of the carboxylate anion to afford a vinylmthenium species II. Final protonolysis furnished the product and turns the catalyst over. 

The excellent coordination properties of alkynes with ruthenium catalysts led to their use as partners for the coupling with a large variety of unsaturated molecules. The first examples of dimerization of terminal alkynes involved acetylide or vinylidene intermediates. By contrast, with (CsR5)Ru catalysts, a completely different stoichiometric head-to-head coupling of alkynes has been discovered affording ruthenacyclopentatrienes, which are cyclic biscarbenes produced by the oxidative coupling of two alkynes [1-3] (Scheme 1). 

Figure 2 Ruthenium catalysts with borate-, carbene-, carborane-, arena-, and vinylidene-based ligands.

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 =... 

Liu, R.-S. (2008) Catalytic transformations of terminal alkynes by cationic tris(l-pyrazolyl) borate ruthenium catalysts versatile chemistry via catalytic allenylidene, vinylidene, and Ti-alkyne intermediates. Synlett, (6), 801-812. 

The addition of carbamates to acetylene itself was also possible in the presence of ruthenium catalysts, namely RUCI3.3H2O and the polymeric [RuCl2(norbor-nadiene)] but in relatively modest yields of 10-46% [28, 29]. The formation of vinyl carbamates is restricted to terminal alkynes, which is in line with the formation of a metal vinylidene intermediate, and also to secondary amines. However, a catalytic reaction also took place under similar conditions with primary aliphatic amines but it led to the formation of symmetrical ureas [30, 31]. The catalytic system generated in this case is 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 vinyl carbamates and ureas, is shown in Scheme 7. 

Hydrophosphination of propargylic alcohols with ruthenium catalysts RuCl (PPh3)2Cp and RuCl(cod)Cp resulted in the product formation with the phosphorus atom attached to terminal position (Scheme 8.59) [143]. The reaction mechanism was proposed to involve intermediate ruthenium vinylidene species. 

In 1998, Wakatsuki et al. reported the first anti-Markonikov hydration of 1-alkynes to aldehydes by an Ru(II)/phosphine catalyst. Heating 1-alkynes in the presence of a catalytic amount of [RuCljlCgHs) (phosphine)] phosphine = PPh2(QF5) or P(3-C6H4S03Na)3 in 2-propanol at 60-100°C leads to predominantly anti-Markovnikov addition of water and yields aldehydes with only a small amount of methyl ketones (Eq. 6.47) [95]. They proposed the attack of water on an intermediate ruthenium vinylidene complex. The C-C bond cleavage or decarbonylation is expected to occur as a side reaction together with the main reaction leading to aldehyde formation. Indeed, olefins with one carbon atom less were always detected in the reaction mixtures (Scheme 6-21). 

A most significant advance in the alkyne hydration area during the past decade has been the development of Ru(n) catalyst systems that have enabled the anti-Markovnikov hydration of terminal alkynes (entries 6 and 7). These reactions involve the addition of water to the a-carbon of a ruthenium vinylidene complex, followed by reductive elimination of the resulting hydridoruthenium acyl intermediate (path C).392-395 While the use of GpRuGl(dppm) in aqueous dioxane (entry 6)393-396 and an indenylruthenium catalyst in an aqueous medium including surfactants has proved to be effective (entry 7),397 an Ru(n)/P,N-ligand system (entry 8) has recently been reported that displays enzyme-like rate acceleration (>2.4 x 1011) (dppm = bis(diphenylphosphino)methane).398... 

The proposed mechanism involves either path a in which initially formed ruthenium vinylidene undergoes nonpolar pericyclic reaction or path b in which a polar transition state was formed (Scheme 6.9). According to Merlic s mechanism, the cyclization is followed by aromatization of the ruthenium cyclohexadienylidene intermediate, and reductive elimination of phenylruthenium hydride to form the arene derivatives (path c). A direct transformation of ruthenium cyclohexadienylidene into benzene product (path d) is more likely to occnir through a 1,2-hydride shift of a ruthenium alkylidene intermediate. A similar catalytic transformation was later reported by Iwasawa using W(CO)5THF catalyst [14]. 

Various cycdization products have been observed in the cycloisomerization of 3,5-dien-l-ynes using [Ru(Tp)(PPh3)(CFl3CN)2]PF6 catalyst the cyclization chemos-eledivity is strongly dependent on the type of substrate structures, which alters the cycdization pathway according to its preferred carbocation intermediate. The reaction protocols are summarized below ruthenium vinylidene intermediates are responsible for these cyclizations (Scheme 6.10). 

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