Ruthenium complexes alkyne reactions

In order to further explore the reactivity of the homobimetallic ruthenium complexes, the reaction of 4 with terminal alkynes was investigated. Thus, when phenylacetylene or ferf-butylacetylene was added to a solution of complex 4 in CH2CI2 or benzene, the rapid and quantitative formation of the corresponding rathenium- vinylidene complexes, 8 and 9, respectively, was observed (13). The formation of 8 and 9 can be rationalised by the displacement of the ethylene ligand by the respective acetylene followed by an alkyne-to-vinylidene transformation. 

Carbonylation of alkynes is a convenient method to synthesize various carbonyl compounds. Alper et al. found that carbonylation of terminal alkynes could be carried out in aqueous media in the presence of 1 atm CO by a cobalt catalyst, affording 2-butenolide products. This reaction can also be catalyzed by a cobalt complex and a ruthenium complex to give y-keto acids (Scheme 4.8).92... 

Cationic ruthenium complexes of the type [Cp Ru(MeCN)3]PF6 have been shown to provide unique selectivities for inter- and intramolecular reactions that are difficult to reconcile with previously proposed mechanistic routes.29-31 These observations led to a computational study and a new mechanistic proposal based on concerted oxidative addition and alkyne insertion to a stable ruthenacyclopropene intermediate.32 This proposal seems to best explain the unique selectivities. A similar mechanism in the context of C-H activation has recently been proposed from a computational study of a related ruthenium(ll) catalyst.33... 

Ruthenium complexes do not have an extensive history as alkyne hydrosilylation catalysts. Oro noted that a ruthenium(n) hydride (Scheme 11, A) will perform stepwise alkyne insertion, and that the resulting vinylruthenium will undergo transmetallation upon treatment with triethylsilane to regenerate the ruthenium(n) hydride and produce the (E)-f3-vinylsilane in a stoichiometric reaction. However, when the same complex is used to catalyze the hydrosilylation reaction, exclusive formation of the (Z)-/3-vinylsilane is observed.55 In the catalytic case, the active ruthenium species is likely not the hydride A but the Ru-Si species B. This leads to a monohydride silylmetallation mechanism (see Scheme 1). More recently, small changes in catalyst structure have been shown to provide remarkable changes in stereoselectivity (Scheme ll).56... 

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-... 

After extensive screening of various aldehydes to optimize the reaction conditions, it was found that aromatic aldehydes were able to serve as a carbon monoxide source, in which the electronic nature of the aldehydes is responsible for their ability to transfer CO efficiently [24]. Consequently, aldehydes bearing electron-withdrawing substituents are more effective than those bearing electron-donating substituents, with pentafluoro-benzaldehyde providing optimal reactivity. Interestingly, for all substrates tested the reaction is void of any complications from hydroacylation of either the alkene or alkyne of the enyne. Iridium and ruthenium complexes, which are known to decarboxylate aldehydes and catalyze the PK reaction, demonstrated inferior efficiency as compared to... 

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. 

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). 

The proposed mechanism involves the formation of ruthenium vinylidene 97 from an active ruthenium complex and alkyne, which upon nucleophilic attack of acetic acid at the ruthenium vinylidene carbon affords the vinylruthenium species 98. A subsequent intramolecular aldol condensation gives acylruthenium hydride 99, which is expected to give the observed cyclopentene products through a sequential decarbonylation and reductive elimination reactions. 

Quite recently, some mononuclear ruthenium complexes such as [(p-cymene)RuX-(CO)(PR3)]OTf (X = Cl, OTf, R = Ph, Cy) have been found to work as catalysts for the propargylation of aromatic compounds such as furans, where some ruthenium complexes were isolated as catalytically active species from the stoichiometric reactions of propargylic alcohols (Scheme 7.27) [31]. The produced active species promoted the propargylation of furans vdth propargylic alcohols bearing not only a terminal alkyne moiety but also an internal alkyne moiety, indicating that this propargylation does not proceed via allenylidene complexes as key intermediates. 

A variety of alkenylalkylidene ruthenium complexes have been obtained by a quite different route the reaction of alkylidene-ruthenium complex with a functional alkyne via a. [2 + 2] cycloaddition [56]. 

Trust s group has shown that another selective reaction involving C—O bond formation followed by rearrangement and C—C bond formation occurred when Cp-containing ruthenium complexes were used as catalytic precursors. With RuCl(Cp)(PPh3)2 in the presence of NH4PF6, an additive known to facilitate chloride abstraction from the metal center, the addition of allylic alcohols to terminal alkynes afforded unsaturated ketones [46, 47]. It has been shown that the key steps are the... 

The stereoselective synthesis of 1,4-disubstituted-l,3-dienes proceeds by head-to-head oxidative coupling of two alkynes with formation of an isolable metallacyclic biscarbene ruthenium complex [23], as shown in Scheme 6. Several key experiments involving labeled reagents and stoichiometric reactions and theoretical studies support the formation of a mixed Fischer-Schrock-type biscarbene complex which undergoes protonation at one carbene carbon atom whereas the other becomes accessible to nucleophilic addition of the carboxylate anion (Scheme 6) [23]. 

Reduction of [Mo(CO)(Bu C=CH)2Cp] + BF4 with KBHBu3(s) at — 78°C in an atmosphere of carbon monoxide yields a complex of a vinyl substituted y-lactone linked tj3 t]2 (220). The allylidene ruthenium complex 64, obtained by photochemical addition of one alkyne molecule to a /x-carbene derivative, is transformed into pentadienylidene complexes 65 and 66 on photolysis with more alkyne substrate. These reactions show clearly the stepwise growth of chains in alkyne oligomerizations at dimetal centers [Eq. (31)] (221). Similar reactions are also known for dinuclear iron (222), molybdenum (223), and tungsten (224) complexes. 

Recently, cyclopropane derivatives were produced by a ruthenium-catalyzed cyclopropanation of alkenes using propargylic carboxylates as precursors of vinylcarbenoids [51] (Eq. 38). The key intermediate of this reaction is a vinylcarbene complex generated by nucleophilic attack of the carboxylate to an internal carbon of alkyne activated by the ruthenium complex. Then, a [2+1] cycloaddition between alkenes and carbenoid species affords vinylcyclo-propanes. 

The precatalyst Cp RuCl(COD) allowed the head-to-head oxidative dimerization of terminal alkynes and the concomitant 1,4-addition of carboxylic acid to stereoselectively afford 1-acyloxy-l,3-dienes in one step under mild conditions [89] (Eqs. 67,68). The first step of the reaction consists in the oxidative head-to-head alkyne coupling via the formation of a ruthenacycle intermediate that behaves as a mixed Fischer-Schrock-type biscarbene ruthenium complex, allowing protonation and nucleophilic addition of the carboxylate. 

Research on intermolecular hydroacylation has also attracted considerable attention. The transition-metal-catalyzed addition of a formyl C-H bond to C-C multiple bonds gives the corresponding unsymmetrically substituted ketones. For the intermolecular hydroacylation of C-C multiple bonds, ruthenium complexes, as well as rhodium complexes, are effective [76-84]. In this section, intermolecular hydroacylation reactions of alkenes and alkynes using ruthenium catalysts are described. 

Metal complexes enable one to employ molecules that are thermally unreactive toward cycloadditions by taking advantage of their ability to be activated through complexation. Most of the molecules activated by transition-metal complexes involve C-C unsaturated bonds such as alkynes, alkenes, 1,3-dienes, allenes, and cyclopropanes. In contrast, carbonyl functionalities such as aldehydes, ketones, esters, and imines seldom participate in transition-metal-catalyzed carbonylative cycloaddition reactions. Recently, such a transformation was reported via the use of ruthenium complexes. 

Mitsudo et al. [32] found that hydroquinones can be obtained by the reaction of internal alkynes with norbornene and CO using N-methylpiperidine as a solvent in the presence of Ru3(CO)12 (Eq. 16). The reaction is proposed to proceed via the maleoyl ruthenium complex 15, which is generated from an alkyne, two molecules of CO, and ruthenium. Norbornene is inserted into this complex to give the quinone, which undergoes reduction to the final product under the re-... 

Alkenylsilanes, mainly vinyl silanes and allyl silanes or related compounds, being widely used intermediates for organic synthesis can be efficiently prepared by several reactions catalyzed by transition-metal complexes, such as dehy-drogenative silylation of alkenes, hydrosilylation of alkynes, alkene metathesis, silylative coupling of alkenes with vinylsilanes, and coupling of alkynes with vinylsilanes [1-7]. Ruthenium complexes have been used for chemoselective, regioselective and stereoselective syntheses of unsaturated products. 

Ruthenium complexes are known to be generally less reactive in hydrosilylation reactions when compared with platinum and rhodium ones. However, very selective ruthenium-based catalytic systems have been recently developed. The hydrosilylation of terminal alkynes generally tends to proceed through cis addition, resulting in trans adducts as the main products. 

While platinum and rhodium are predominantly used as efficient catalysts in the hydrosilylation and cobalt group complexes are used in the reactions of silicon compounds with carbon monooxide, in the last couple of years the chemistry of ruthenium complexes has progressed significantly and plays a crucial role in catalysis of these types of processes (e.g., dehydrogenative silylation, hydrosilylation and silylformylation of alkynes, carbonylation and carbocyclisation of silicon substrates). 

The interaction of an alkyne with (tj5-C5H5)(PR3)2RuX can result in the formation of a wide variety of ruthenium complexes. The nature of the products formed depends on the conditions used and the type of alkyne reacted. Reactions between I and terminal alkynes in the presence of ammonium hexafluorophosphate lead to the formation of cationic monosubstituted ruthenium vinylidene complexes in high yield, as shown for phenylacetylene in Eq. (61) (4,67,68). 

Complex 3c, a catalytic precursor for addition reactions to alkynes (65), reacts at room temperature with a variety of terminal alkynes in alcohols to produce stable alkoxyl alkyl carbene ruthenium(II) derivatives 109 in good yields (Scheme 7). Reaction of 3c (L = PMe3), with trimethylsilyacetylene in methanol gives the carbene ruthenium complex 110, by protonolysis of the C—Si bond, whereas with 4-hydroxy-l-butyne in methanol the cyclic carbene complex 111 is obtained (65,66). 

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