Ruthenacyclobutane complexes

The reaction of the cationic ruthenium complex [(CO)2(PhMe2P)Ru(773-allyl)]+ with sodium borohydride produces the corresponding ruthenacyclobutane complex among the principal products <2003JCD2603>. 

Despite the wealth of information provided by mechanistic studies, the preferred orientation of the alkylidene/olefin and ruthenacyclobutane complexes relative to neutral ligand (L) remained unclear. As of the first edition of this book in 2003, evidence had begun to be accumulated for both the side-bound (17/18) and... 

A ruthenacyclobutane complex underwent P-carbon elimination to form a it-allyl(methyl)ruthenium complex (Scheme 1.35) [43]. Kinetic studies revealed that... 

X-ray crystallography, 3, 623 Ruscodibenzofuran synthesis, 4, 698, 709 Ruthenacyclobutane, 3-cyano-synthesis, 1, 667 Ruthenium complexes with pyridines, 2, 124 triazenido, 5, 675 Rutin... 

From the mechanistic point of view, the observed competitive reactions can be explained by considering two different pathways (Scheme 114). The intermediacy of ruthenacyclopentadiene 453 or biscarbenoid 452, formed from the reaction of a diyne and a ruthenium(ll) complex, is postulated in the proposed mechanism. Cyclopropanation of the alkene starts with the formation of ruthenacyclobutane 456, which leads to the generation of the vinylcarbene 457. Then, the second cyclopropanation occurs to afford the biscyclopropyl product 458. Insertion of the alkene 459 into the ruthenacyclopentadiene 453 affords the ruthenacycloheptadiene 454. The subsequent reductive elimination gives the cyclotrimerization product 455. The selectivity toward the bis-cyclopropyl product 458 is improved with an increasing order of haptotropic flexibility of the cyclopentadienyl-type ligand. 

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-... 

If cycloalkene-yne 65 having an o -alkynyl substituent at an olefinic position in a cycloalkene is treated with a ruthenium catalyst, what kinds of products are produced. In this reaction, ruthenium mono-substituted carbene complex XVII is anticipated to be formed from a highly strained ruthenacyclobutane intermediate. If it then reacts with ethylene, triene 67 should be formed, but if XVII reacts with an alkene part intramolecularly, bicyclic compound 66 should be formed via ruthenacyclobutane (Scheme 23). 

Reaction of the complex 24 with terminal alkene 25 generates styrene and the real catalytic species 27 via the ruthenacyclobutane 26. The complex 24 is commercially available, active without rigorous exclusion of O2 and water, and has functional group tolerance. Carbonyl alkenation is not observed with the catalysts 22 and 24. Their introduction has enormously accelerated the synthetic applications of alkene metathesis [11]. 

The cyclo addition of the alkene to the ruthenium vinylidene species leads to a ruthenacyclobutane which rearranges into an allylic ruthenium species resulting from / -elimination or deprotonation assisted by pyridine and produces the diene after reductive elimination (Scheme 16). This mechanism is supported by the stoichiometric C-C bond formation between a terminal alkyne and an olefin, leading to rf-butatrienyl and q2-butadienyl complexes via a ruthenacyclobutane resulting from [2+2] cycloaddition [62]. 

Enyne derived from ditosyl o-phenylenediamine 257 formed in the presence of benzylidene ruthenium carbene complex a nine-membered ring 258 in 5% yield (Equation 30) <20000L543, 2001S654>. Dimerization was a major by-process (22% yield) along with formation of a small amount of 259 (5% yield), which was explained by /3-hydride elimination from the intermediary ruthenacyclobutane. 

The mechanism of Ru-alkylidene-catalyzed reactions has been investigated. Note that Grubbs first- and second-generation catalysts are 16-electron species, so if the first step involves complexation of an alkene to the metal, this process could occur in an associative or dissociative manner. Evidence suggests (see Scheme 11.5) that this occurs in a dissociative manner, however, first forming a 14-electron intermediate 25 and then 26a or 26b after complexation of the alkene. Gas-phase mass spectral evidence supports the initial formation of 25. Complexes similar to 26a and b have been isolated from reaction mixtures under appropriate conditions, but ruthenacyclobutane 27 has not been directly observed until quite recently.37... 

Similarly, thermolysis of ruthenacyclobutane 61 produces Jt-allyl complex 62 [77]. The reaction involves (3-methyl transfer from the central carbon of the ligand to the metal via a formal 16-electron unsaturated intermediate. A kinetic investigation in the presence of excess phosphine revealed that the process is reversible. 

It is well known that the reaction of the Grubbs complexes with alkyl vinyl ethers readily occurs at room temperature to yield the ruthenium complex with alkoxy-substituted carbene ligand, via a ruthenacyclobutane intermediate (Scheme 11) [28]. 

Ruthenacyclobutane Intermediates Derived from Phosphonium Alkylidene Complexes... 

In 2008, Piers and coworkers [43] reported the preparation and spectroscopic characterization of ruthenacyclobutane and ruthenium alkylidene/olefin complexes associated with ring-closing metathesis (RCM) catalysis. Exploiting the reversibility associated with the RCM reaction, phosphonium alkylidene complex 28a was reacted with 1 equiv of ethylene at —78 °C, followed by the addition of an excess of RCM product 29 (2-3 equiv), to afford a 90% NMR yield of metallacycle 30 (Scheme 8.9). This use of reverse engineering was found to minimize the formation of the thermodynamically favored, unsubstituted ruthenacycle 23 to only 10% yield. Later studies found that, similar to the phenomena that had been observed with propene, the a-monosubstituted metallacycle derived from ethylene and the propagating alkylidene of 30 could also be observed if the reaction temperature was further lowered (e.g., —76 C) [41]. It was additionally reported that the use of the more bulky phosphonium alkylidene 28b afforded lower reaction yields relative to 28a (60-70% vs. 90%). 

As previously discussed, the unfavorable equilibria associated with ligand dissociation during the initiation step of an olefin metathesis reaction have traditionally hindered the direct observation of metathesis-active ruthenacyclobutane intermediates [24]. Thus far, we have seen that the use of phosphonium alkylidene complexes, such as 22, can enable facile access to metallacycle formation by providing an alternative route for catalyst initiation. However, despite the utility of these trialkylphosphonium alkylidene catalysts, their preparation requires a multi-step synthetic route that requires the use of costly reagents [28]. In addition, the vinyl trialkylphosphonium salt generated following the reaction of 22 presents a less relevant model in comparison to the styrene (34) formed from the commercially available benzylidene catalysts. 

Studies of catalyst decomposition in the presence of substrate have mostly focused on ethylene. In particular, it has been demonstrated that ethylene can induce the degradation of methylidene complex 19 to produce propylene as the main volatile organic byproduct [3, 39]. The proposed mechanism for this degradation involves the ruthenacyclobutane intermediate (20) undergoing a P-hydride elimination to form a ruthenium allyl-hydride species (21), which subsequently affords the propylene complex (22) upon reductive elimination (Scheme 11.8). 

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