Ruthenium vinylidenes reaction with alcohols

Most efforts to explore the reactivity of ruthenium carbene complexes have employed the alkoxycarbene species so readily synthesized from the inter- or intramolecular reaction of vinylidene complexes with alcohols. These electrophilic alkoxycarbene complexes exhibit only limited reactivity at Ca, primarily with hydride reagents. For example, treatment of the 2-oxacyclopentylidene complex 97 with NaAlH2(OCH2CH2OMe)2 affords the neutral 2-tetrahydrofuranyl complex (98) [Eq. (89)] (55), as was anticipated from similar reductions of iron carbene complexes (87). 

A. G. (1980) Cyclopentadienyl-ruthenium and -osmium chemistry X. Reactions of vinylidene complexes with alcohols and water synthesis of alkoxy (alkyl) carbene, aryl and alkyl complexes. Australian Journal of Chemistry, 33, 1471-1483 (c) Bruce, M.I., Swincer, A.G., Thompson,... 

Thiolate-bridged diruthenium complexes such as Cp RuCl(p2-SR)2RuCp Cl catalyze the propargylic substitution reaction of propargylic alcohol derivatives with various carbon-centered nucleophiles [118-120]. Ketones [119] (Eq. 88), aromatic compounds [120] (Eq. 89), or alkenes thus selectively afford the corresponding propargylated products with C-C bond formation. An allenylidene intermediate is proposed in these reactions. They are detailed in the chapter Ruthenium Vinylidenes and Allenylidenes in Catalysis of this volume. 

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 vast majority of work exploring the reactivity of ruthenium viny-lidene complexes has focused on the attack of alcohols at the electrophilic a carbon of monosubstituted vinylidenes, resulting in the formation of ruthenium alkoxycarbene complexes. Bruce and co-workers have determined, for example, that the phenylvinylidene complex 80 is slowly transformed in refluxing MeOH to the methoxycarbene complex 82 in good yield (73,83). The mechanism for this reaction must involve initial attack of the alcohol at the electrophilic Ca to form a transient vinyl intermediate 81 which is rapidly protonated at the nucleophilic Cp, generating the product carbene 82 [Eq. (79)]. In contrast to monosubstituted vinylidene complexes, disubstituted vinylidene complexes are generally unreactive to nucleophiles even the relatively small dimethylvinylidene complex 83 shows no reaction with MeOH after 70 hours at reflux [Eq. (80)]. 

Contrary to the previous pathway of P-H addition to alkyne - that is, via alkyne insertion into the M-P bonds - this reaction has been shown to proceed via the nucleophilic attack of the phosphine to a ruthenium-vinylidene intermediate to yield the anti-Markovnikov product with a predominant (Z -stereoisomer (Scheme 8.36). Indeed, it has been shown that [Cp RuL2] X intermediate gives vinylidene species with propargyl alcohols. The (Z)-isomer is formed as the major product, but iso-merizes easily into the ( )-isomer upon isolation by chromatography over silica gel. 

Water-soluble ruthenium vinylidene and allenylidene complexes were also synthetized in the reaction of [ RuC12(TPPMS)2 2] and phenylacetylene or diphenylpropargyl alcohol [29], The mononuclear Ru-vinylidene complex [RuCl2 C=C(H)Ph (TPPMS)2] and the dinuclear Ru-allylidene derivative [ RuCl( x-Cl)(C=C=CPh2)(TPPMS)2 2] both catalyzed the cross-olefin metathesis of cyclopentene with methyl acrylate to give polyunsaturated esters under mild conditions (Scheme 7.10). 

The homobimetallic, ethylene-ruthenium complex 15, which contains three chloro bridges, was readily obtained from the reaction of [RuCl2(/ -cymene)]2 with 1 atm of ethylene [34]. In 2009, Demonceau and Delaude [34] showed that complex 15 could be a useful precursor to allow subsequent access to the diruthenium vinylidene complex 16, allenylidene complex 17, and indenylidene complex 18 (Scheme 14.8). Upon reaction with propargylic alcohol, complex 15 afforded vinylidene complex 16, which converted into the allenylidene complex 17 in the presence of molecular sieves [34]. As shown in the acid-promoted intramolecular rearrangement of mononuclear ruthenium allenylidene complexes [19, 20, 32], the addition of a stoichiometric amount of TsOH to complex 17 at -50 °C led to the indenylidene binuclear complex 18 [34]. Complex 18 has been well... 

Intermolecular addition of alcohols to catalytic ruthenium vinylidenes is far more difficult than the addition of water except when allylic alcohols are employed (Scheme 9) [92-96]. In this case, the reaction of an allylic alcohol with a terminal alkyne catalyzed by CpRuCl(PPh3)2 afforded a p,Y-unsaturated ketone. The initial ruthenium oxacarbene obtained by addition of the alcohol to the ruthenium vinylidene evolves through a Claisen rearrangement to a Jt-allyl ruthenium species. Reductive elimination then gives rise to the final unsaturated ketone. 

Isolable ruthenium vinylidene and carbene complexes are involved in the coupling of alkynes with allylic alcohols. Some of these transformations were previously known from model reactions. The system aUows the synthesis of a large range of enones (Scheme 32). While most coupling reactions, particularly those applied to organic synthesis, remain faithful to palladium, molybdenum carbonyl complexes, too, have found uses. 

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. 

Allenylidenes could be considered as divalent radicals derived from allenes. In a similar way to vinylidenes, allenylidenes can be stabilized by coordination with transition metals and again ruthenium is one of the most widely used metals. Metal-allenylidene complexes can be easily obtained from terminal propargylic alcohols by dehydration of the initially formed metal-hydroxyvinylidenes, in which the reactivity of these metal complexes is based on the electrophilic nature of Ca and Cy, while Cp is nucleophilic. Catalytic processes based on nucleophilic additions and pericyclic reactions involving the it system of ruthenium allenylidenes afford interesting new structures with high selectivity and atom economy. 

Thiolate-bridged dirutheniutn complexes catalyze the [3-f3] cycloaddition reaction between propargylic alcohols and cyclic 1,3-dicarbonyl compounds to afford 4,6,7,8-tetrahydrochromen-5-ones or 4//-cyclopenta[b]pyran-5-ones [193] and with 2-naphthols or phenols to afford l//-naphtho[2,l-b]pyrans and 4//-l-benzo-pyrans, respectively [194]. This cycloaddition is considered to proceed by stepwise propargylation and intramolecular cyclization (carbon and oxygen nucleophile additions) reactions, where ruthenium allenylidene and vinylidene complexes are the key intermediates (Scheme 57). Enantioselective mthenium-catalyzed [3-f3] cycloaddition of propargylic alcohols with 2-naphthols has also been described [195]. 

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