Ruthenium catalysis substitution

It is now usual to promote these cycloadditions by catalysts for example, reaction with A -tosyl-ynamides, using ruthenium or copper catalysts, giving 1-substituted 5- and 4-amino triazoles, respectively the formation of the 1,4-substitution pattern with copper catalysis and 1,5-pattem with ruthenium catalysis seems to be general. The latter metal will also promote addition to internal alkynes. ... 

As carboxylic acid additives increased the efficiency of palladium catalysts in direct arylations through a cooperative deprotonation/metallation mechanism (see Chapter 11) [45], their application to ruthenium catalysis was tested. Thus, it was found that a ruthenium complex modified with carboxylic acid MesC02H (96) displayed a broad scope and allowed for the efficient directed arylation of triazoles, pyridines, pyrazoles or oxazolines [44, 46). With respect to the electrophile, aryl bromides, chlorides and tosylates, including ortho-substituted derivatives, were found to be viable substrates. It should be noted here that these direct arylations could be performed at a lower reaction temperatures of 80 °C (Scheme 9.34). 

A nitrogen atom in heteroarenes such as azoles and pyridine also acts as good anchor in directed alkenylation. The authors reported the alkenylations of phenylpyrazoles, -imidazole, and -pyridine [36]. Depending on the ratio of substrates employed, mono- and diaUcenylated products can be obtained selectively. Using the procedure, one-pot synthesis of unsymmetrically substituted 1,3-dialkenylbenzene derivatives was achieved (Scheme 18.36). In contrast to the cases using a Rh catalyst, monoalkenylated phenylpyrazoles are formed predominantly under ruthenium catalysis, even in the presence of excess alkenes [37]. 

Trimethylsilylation of Vinylboronates. Vinylboronate esters react with vinyltrimethylsilane under ruthenium catalysis to give the I trimethylsilyl-l-boronylethylene (eq 34). Such a species could be used in Suzuki-type cross coupling reactions to prepare various a-substituted vinyltrimethylsilanes. ... 

The same reaction can be carried out under catalysis of the ruthenium complex 53. The reaction mechanism is identical with the one depicted in Scheme 49. The advantage of ruthenium catalysis is that enynes with various degree of the substitution of the double bond can be used for the construction of both five- and six-membered rings, and strikingly mild reaction conditions (in many cases the reaction proceeds at room temperature). Also, a number of functional groups are tolerated. Some typical examples are given in Table 22 [63]. 

Stephenson and coworkers applied reductive photoredox catalysis to trigger radical 6-exo cyclizations of co-pyrrole or co-indole-substituted a-bromocarbonyl compounds 124 [186] as well as radical 5-exo cyclizations of 2-bromo-2-(4-pentenyl)malonates 126 (Fig. 32) [187]. These cyclization processes provide bi- or tricyclic products 125 or cyclopentanecarboxylates 127 in moderate to excellent yields. The initial radical was formed with reduced ruthenium catalyst HOB generated similarly as above from 110 and a sacrificial amine... 

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 unique transformation of formamides to ureas was reported by Watanabe and coworkers [85]. In place of carbon monoxide, formamide derivatives are used as a carbonyl source. The reaction of formanilide with aniline was conducted in the presence of a catalytic amount of RuCl2(PPh3)3 in refluxing mesitylene, leading to N,AT-diphenylurea in 92% yield (Eq. 56) [85]. They proposed that the catalysis starts with the oxidative addition of the formyl C-H bond to the active ruthenium center. In the case of the reaction of formamide, HCONH2, with amines, two molecules of the amine react with the amide to afford the symmetrically substituted ureas in good yields. This reaction evolves one molecule of NH3 and one molecule of H2. 

We have already alluded to the diversity of oxidation states, the dominance of oxo chemistry and the cluster carbonyls. Brief mention should be made too of the tendency of osmium (shared also by ruthenium and, to some extent, rhodium and iridium) to form polymeric species, often with oxo, nitrido or carboxylato bridges. Although it does have some activity in homogeneous catalysis (e.g. of m-hydroxylation, hydroxyamination or animation of alkenes, see p. 558, and occasionally for isomerization or hydrogenation of alkenes, see p. 571), osmium complexes are perhaps too substitution-inert for homogeneous catalysis to become a major feature of the chemistry of the element. The spectroscopic properties of some of the substituted heterocyclic nitrogen-donor complexes may yet make osmium an important element for photodissociation energy research. 

Ruthenium-catalyzed allylic alkylation falls between known catalysis with respect to regioselectivity and chirality of products. Like Pd, Mo, and W-but unlike Rh regioselectivity is not highly dependent on the nature of the starting carbonate. However, unlike Pd and Mo, but similar to Rh and W, substitution of the chiral substrate occurs with high retention of configuration, (equation 75). Hence the Ru system compliments other metal-catalyzed allylic systems well. ... 

The same differential behavior can be observed with amine nucleophiles. For example, calcium triflate promotes the aminolysis of propene oxide 84 with benzylamine to give 1-(A -benzyl)amino-2-propanol 85, the result of attack at the less substituted site <03T2435>, and which is also seen in the solventless reaction of epoxides with heterocyclic amines under the catalysis of ytterbium(III) triflate <03SC2989>. Conversely, zinc chloride directs the attack of aniline on styrene oxide 34 at the more substituted carbon center <03TL6026>. A ruthenium catalyst in the presence of tin chloride also results in an SNl-type substitution behavior with aniline derivatives (e.g., 88), but further provides for subsequent cyclization of the intermediate amino alcohol, thus representing an interesting synthesis of 2-substituted indoles (e.g., 89) <03TL2975>. 

Metal enolates have played a Umited role in the metal-catalyzed isomerization of al-kenes . As illustrated in a comprehensive review by Bouwman and coworkers, ruthenium complex Ru(acac)3 (51) has been used to isomerize a wide range of substituted double bonds, including aUylic alcohols (131), to the corresponding ketones (132) (equation 38) . The isomerization of aUylic alcohols affords products that have useful applications in natural product synthesis and in bulk chemical processes. An elegant review by Fogg and dos Santos shows how these complexes can be used in tandem catalysis, where an alkene is subjected to an initial isomerization followed by a hydroformylation reaction ... 

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