Cycloadditions ruthenium

Eq. 14) [81]. Although this transformation does not appear to be a metathesis reaction, it is thought to proceed via the formation of ruthenium carbene species and not via classical [2+2+2]-cycloaddition pathways. A rationale for the strong preference of the meta isomer 99 was provided on the basis of a metathesis-type mechanism. 

Transition-metal-based Lewis acids such as molybdenum and tungsten nitro-syl complexes have been found to be active catalysts [49]. The ruthenium-based catalyst 50 (Figure 3.6) is very effective for cycloadditions with aldehyde- and ketone-bearing dienophiles but is ineffective for a,)S-unsaturated esters [50]. It can be handled without special precautions since it is stable in air, does not require dry solvents and does not cause polymerization of the substrates. Nitromethane was the most convenient organic solvent the reaction can also be carried out in water. 

Associated to copper(II) pre-catalysts, bis(oxazolines) also allowed the asymmetric Diels-Alder and hetero Diels-Alder transformations to be achieved in nearly quantitative yield and high diastereo- and enantioselectivities. Optically active sulfoximines, with their nitrogen-coordinating site located at close proximity to the stereogenic sulfur atom, have also proven their efficiency as copper ligands for these asymmetric cycloadditions. Other precursors for this Lewis acid-catalyzed transformation have been described (e.g., zinc salts, ruthenium derivatives, or rare earth complexes) which, when associated to bis(oxazolines), pyridine-oxazolines or pyridine-bis(oxazolines), led to efficient catalysts. 

Viton F, Bemardinelli G, Kiindig EP (2002) Iron and ruthenium Lewis acid catalyzed asymmetric 1,3-dipolar cycloaddition reactions between nitrones and enals. J Am Chem Soc 124 4968-4969... 

Within this chapter, two sections are devoted to rhodium and ruthenium. The two main procedures using rhodium are first, the formation of 1,3-dipoles from diazocompounds followed by a 1,3-dipolar cycloaddition [10] and second, hy-droformylation [11], The ruthenium-catalyzed domino reactions are mostly based on metathesis [12], with the overwhelming use of Grubbs I and Grubbs 11 catalysts. 

Disubstituted 1,2,3-triazoles are formed in 1,3-dipolar cycloaddition of alkynylmagnesium reagents to azides. This reverse regioselectivity is also achieved in ruthenium-catalyzed cycloadditions. Examples of such reactions can be found in Section 5.01.9. 

Ruthenium(ll)-catalyzed cycloadditions of diynes with bicycloalkenes illustrate the synthetic importance of ruthena-cyclopentatrienes as biscarbenoid intermediates.380 Reaction of 1,6-diyne 448 and biscyclic alkene 449 with ruthenium catalyst afforded a mixture of biscyclopropanation product 450 and cyclotrimerization product 451 (Scheme 113). 

Itoh and co-workers reported the ruthenium(n)-catalyzed [2 + 2 + 2]-cycloaddition of 1,6-diynes with isocyanates to afford the corresponding bicyclic pyridones 163 (Scheme 72).356 357 For previously reported ruthenium-catalyzed [2 + 2 + 2]-cycloaddition of 1,6-diynes see Refs 358 and 358a, and for theoretical calculations of the cyclocotrimer-ization of alkynes with isocyanates, isothiocyanates, and carbon disulfide see Refs 359 and 359a. 

Cycloadditions on a ruthenium(n) complex between 2 equiv. of phenylacetylene and various types of isonitriles were described for the first time by Singleton.367 3673 These transformations were shown to proceed through coordinatively unsaturated ruthenacycle intermediates to furnish the corresponding imino-2,5-diphenylcyclopentadiene complexes. 

Based on this work, Itoh and co-workers developed ruthenium(n)-catalyzed [2 + 2 + 2]-cyclotrimerizations of 1,6-diynes 174 and electron-deficient nitriles (Equation (34)),368>368a These partially intramolecular cycloadditions proceed through ruthenacycle intermediates as well. The importance of using electronically activated nitriles is underlined by the fact that acetonitrile and benzonitrile gave only very low yields. 

For a related example of a ruthenium(II)-catalyzed cycloaddition of 1,6-diynes with isothiocyanates and carbon disulfide, see Yamamoto, Y. Takagishi, H. Itoh, K. J. Am. Chem. Soc. 2002, 124, 28-29. 

Intermolecular enyne metathesis has recently been developed using ethylene gas as the alkene [20]. The plan is shown in Scheme 10. In this reaction,benzyli-dene carbene complex 52b, which is commercially available [16b], reacts with ethylene to give ruthenacyclobutane 73. This then converts into methylene ruthenium complex 57, which is the real catalyst in this reaction. It reacts with the alkyne intermolecularly to produce ruthenacyclobutene 74, which is converted into vinyl ruthenium carbene complex 75. It must react with ethylene, not with the alkyne, to produce ruthenacyclobutane 76 via [2+2] cycloaddition. Then it gives diene 72, and methylene ruthenium complex 57 would be regenerated. If the methylene ruthenium complex 57 reacts with ethylene, ruthenacyclobutane 77 would be formed. However, this process is a so-called non-productive process, and it returns to ethylene and 57. The reaction was carried out in CH2Cl2 un-... 

The asymmetric synthesis of allenes via enantioselective hydrogenation of ketones with ruthenium(II) catalyst was reported by Malacria and co-workers (Scheme 4.11) [15, 16]. The ketone 46 was hydrogenated in the presence of iPrOH, KOH and 5 mol% of a chiral ruthenium catalyst, prepared from [(p-cymene) RuC12]2 and (S,S)-TsDPEN (2 equiv./Ru), to afford 47 in 75% yield with 95% ee. The alcohol 47 was converted into the corresponding chiral allene 48 (>95% ee) by the reaction of the corresponding mesylate with MeCu(CN)MgBr. A phosphine oxide derivative of the allenediyne 48 was proved to be a substrate for a cobalt-mediated [2 + 2+ 2] cycloaddition. 

An allenylaldehyde can be transformed efficiently into an a-methylene-y-butyro-lactone by a ruthenium-catalyzed carbonylative cycloaddition process (Scheme 16.34) [37]. The reaction mechanism may involve a metallacyclopentene, which undergoes insertion of CO and reductive elimination leading to the product. 

Itoh and coworkers111 carried out tandem [2 + 2 + 2]/[4 + 2] cycloadditions catalyzed by a ruthenium catalyst. The reaction of diyne 147 with excess norbomene 148 in the presence of ruthenium catalyst 153, for example, afforded 149. Adduct 150 either dissociated from the catalyst or reacted with another equivalent of norbornene. In the latter case, a ruthenium catalyzed Diels-Alder reaction occurred, affording hexacyclic adduct 152 via 151 (equation 43). Compounds 150 and 152 were obtained in yields of 78% and 10%, respectively. Both cycloaddition reactions proceeded with complete stereoselectivity. When 1,6-heptadiyne was used instead of 147, only trace amounts of a cycloadduct were obtained. Replacing norbornene by norbornadiene, which was expected to result in polymer formation, did not afford any adduct at all. 

If ROM-RCM of cycloalkene-yne 123, which has a substituent at the 2-position of cycloalkene, is carried out under ethylene gas, what compound is formed In this reaction, ruthenium carbene XIX would be formed via [2-1-2] cycloaddition of ruthenium methylidene carbene and alkyne as shown in Eq. (6.91). If XIX reacts with an olefin intramolecularly or ethylene, bicyclic compound 124 or triene 125... 

Another focus of this chapter is the alkynol cycloisomerization mediated by Group 6 metal complexes. Experimental and theoretical studies showed that both exo- and endo- cycloisomerization are feasible. The cycloisomerization involves not only alkyne-to-vinylidene tautomerization but alo proton transfer steps. Therefore, the theoretical studies demonstrated that the solvent effect played a crucial role in determining the regioselectivity of cycloisomerization products. [2 + 2] cycloaddition of the metal vinylidene C=C bond in a ruthenium complex with the C=C bond of a vinyl group, together with the implication in metathesis reactions, was discussed. In addition, [2 + 2] cycloaddition of titanocene vinylidene with different unsaturated molecules was also briefly discussed. 

Ruthenium vinylidene species can be transformed into small carbocyclic rings via carbocyclization reactions. Ruthenium vinylidene complex 2, generated from the electrophilic reaction of alkyne complex 1 with haloalkanes, was deprotonated with "BU4NOH to give the unprecedented neutral cyclopropenyl complex 3 (Scheme 6.2) [5]. Gimeno and Bassetti prepared ruthenium vinylidene species 4a and 4b bearing a pendent vinyl group when these complexes were heated in chloroform for a brief period, cyclobutylidene products 5a and Sb formed via a [2 + 2] cycloaddition between the vinylidene Ca=Cp bond and olefin (Scheme 6.3) [6]. 

Catalytic Carbocyclization via Cycloaddition of Ruthenium Vinylidene Intermediates... 

Aryl acetylenes undergo dimerization to give 1-aryl naphthalenes at 180 °C in the presence of ruthenium and rhodium porphyrin complexes. The reaction proceeds via a metal vinylidene intermediate, which undergoes [4 + 2]-cycloaddition vdth the same terminal alkyne or another internal alkyne, and then H migration and aromatization furnish naphthalene products [28] (Scheme 6.29). 

Ruthenium-Catalyzed Cycloaddition Reaction between Enyne and Alkene... 

The working mechanism involves a [2 + 2] cycloaddition between the Ru=C bond of ruthenium vinylidene and olefin to form the metallacyclobutane 92, which subsequently undergoes P-hydride elimination leading to the 7i-allyl hydride complex 93 and reductive elimination to furnish the conjugated trienes 89 (Scheme 6.31), and eventually to give the observed aromatic product 90. 

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