Ruthenium-vinylidene intermediate

Catalytic Carbocyclization via Electrocyclization of Ruthenium-Vinylidene Intermediates 195... 

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). 

Movassaghi et al. [21[ reported the synthesis of substituted pyridine derivatives via ruthenium-catalyzed cycloisomerization of 3-azadienynes. To avoid the isolation of the chemically active alkynyl imines, trimethysilyl alkynyl amines served as initial substrates, as shown in Scheme 6.19. The formation of ruthenium vinylidene intermediates is accompanied by a 1,2-silyl migration according to controlled... 

Catalytic Carbocyclization via Cycloaddition of Ruthenium Vinylidene Intermediates... 

Catalytic Carbocyclization via Cycloadclition of Ruthenium Vinylidene Intermediates 209... 

Ruthenium-catalyzed 1,1-difunctionalization of alkynes can be achieved through ruthenium vinylidene intermediates. In this context, Lee s group reported the... 

Scheme 10.8 Two hydration processes involving ruthenium vinylidene intermediates.

The isomerization of terminal epoxyalkynes into furans catalyzed by RuCl(Tp)(PPh3) (MeCN) inthe presence of Et3N as abase at 80 °C in 1,2-dichloroethaneis explained by a related intramolecular nucleophilic addition of the oxygen atom of the epoxide to the a-carbon atom of a ruthenium vinylidene intermediate, as shovm by deuteration in the 3-position of the furan (Scheme 10.10) [45]. This reaction is specific for terminal alkynes and tolerates a variety of functional groups (ether, ester, acetal, tosylamide, nitrile). 

Scheme 10.14 Mechanistic proposal for cyclic enol ether and lactone formation based on a common ruthenium vinylidene intermediate.

Finally, it can be noted that some cross-dimerization of terminal alkynes with internal alkynes, where ruthenium vinylidene intermediates are postulated, have also been reported [74, 75]. 

Scheme 6.21 shows the proposed mechanism for this class of transformations, which involve (i) the formation of a ruthenium-vinylidene intermediate through... 

Scheme 1. General mechanism of nucleophilic addition to terminal alkynes via ruthenium vinylidene intermediates.

Ruthenium vinylidene intermediates have also been proposed in the mechanism of the coupling of unactivated alkenes with terminal alkynes to afford 1,3-dienes as a mixture of two isomers, linear and branched derivatives. The linear one was favored [56] (Eq. 42). The same system has allowed the ruthenium-catalyzed alkenylation of pyridine [57]. 

Several ruthenium complexes are able to promote the classical Markovnikov addition of O nucleophiles to alkynes via Lewis-acid-type activation of triple bonds. Starting from terminal alkynes, the anti-Markovnikov addition to form vinyl derivatives of type 1 (Scheme 1) is less common and requires selected catalysts. This regioselectivity corresponding to the addition of the nucleophile at the less substituted carbon of the C=C triple bond is expected to result from the formation of a ruthenium vinylidene intermediate featuring a highly reactive electrophilic Ca atom. 

It is postulated that the catalytic cycle accounting for this regioselectivity involves a ruthenium vinylidene intermediate and is quite similar to Scheme 4 with the carboxylate nucleophile instead of the carbamate. 

A related stoichiometric cycloaromatization of enediyne involving a ruthenium vinylidene intermediate takes place in the presence of RuCl(Cp)(PMe3)2/ NH4PF6 but a radical process has been proposed [45]. 

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]. 

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]. 

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. 

Dixneuf suggested that this reaction proceeds via the nucleophilic attack of a carbamate anion to the ruthenium vinylidene intermediate generated by the reaction of ruthenium complexes with terminal acetylene. The details of this reaction are discussed in Chapter 8. 

Non-conjugated terminal 2-substituted-l,5-enynes also react through a 6-endo-dig cyclization to afford 1,3-cyclohexadienes via ruthenium vinylidene intermediates. Nishibayashi and coworkers reported that 2-substituted-l,5-enynes in the presence of 5% methanethiolate-bridged diruthenium complex [Cp RuCl (p2 SMe)]2 and 10 mol% of NH4BF4 afforded the corresponding 1,3-cyclo-hexadienes in good yields (Scheme 30) [144],... 

Liu and coworkers reported the cycloisomerization of c -3-en-l-ynes to cyclopentadiene derivatives through 1,5-sigmatropic hydrogen shifts of catalytic ruthenium vinylidene intermediates (Scheme 44) [158]. 

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