Ruthenium complexes, reactions mechanistic studies

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... 

A recent study showed that 152 behaves mechanistically different from other catalysts in addition reactions of more activated halides 140, such as trichloroacetate to styrene [222]. After initial reduction to Ru(II), chlorine abstraction from substrates 140 is in contrast to all other ruthenium complexes not the rate limiting step (cf. Fig. 36). ESR spectroscopic investigations support this fact. The subsequent addition to styrene becomes rate limiting, while the final ligand transfer step is fast and concentration-independent. For less activated substrates 140, however, chlorine abstraction becomes rate-determining again. Moreover, the Ru(III) complex itself can enter an, albeit considerably slower Ru(III)-Ru(IV) Kharasch addition cycle, when the reaction was performed in the absence of magnesium. This cycle operates, however, for only the most easily reducible halides, such as trichloroacetate. 

Very recently, Grubbs and coworkers completed an analysis based on insight from mechanistic work on the relative rates of phosphine dissociation and olefin coordination (vide infra) in ruthenium alkylidene catalyzed olefin metathesis reactions. The study was based on numerous analogues of (4a), having different phosphine groups, for example, (4e), (4f), and (4g). Rates for ROMP of cyclooctadiene with the most potent of these new complexes were 340-fold greater than with (4a) (Scheme 1) ... 

Epoxidation of styrene or stilbene with the ruthenium [RuCl2(cod)L] complex of bis[(45)-(l-methylethyl)oxazolin-2-yl]methane afforded only racemic epoxide, suggesting that the reaction is not metal centered. In fact, mechanistic studies of this reaction indicate that the metal acts as a promoter for the production of i-PrOsH and that it is this species that carries out the epoxidation, either directly or by the formation of an intermediate oxo-ruthenium species. 

Murahashi and Naota studied the reaction mechanism of cyclopentadienyl ruthenium enolate complex-catalyzed aldol and Michael addition reactions [80-82]. This mechanistic study revealed that the cone angle of the tertiary phosphine ligands largely affects the stability of C- and N-bound complexes [80, 82], Thus, ligation of bulky phosphine ligand would favor the N -bound complexes [80]... 

Mechanistic studies on the reaction involving mthenium-nitrene complexes [100] or ruthenium-nitroso complexes [95] have also been reported. A stoichiometric reaction of Ru(dppe)(CO)3 (18) with ArNO gives Ru(dppe)(CO)2[CON(Ar)0] (19) (Eq. 11.51). In the first step of the catalytic reaction, nitroarene is reduced to nitrosoar-ene, while in the second step the complex 19 reacts with methanol and CO to give a... 

A mechanistic study of the reaction of ruthenium ammine complexes with NO was reported almost four decades earlier. Although [Ru(NH3)6] is very inert, an acidic solution of this complex ion was observed to undergo a rapid reaction in the presence of NO to form [Ru(NH3)s(NO)]. [164] An electrophilic substitution process was invoked, with NO considered to function as an electrophile and the product proposed to be Ru -NO". The conclusion from a subsequent study was that a bond making mechanism operates for this... 

Hydroxycarbonylation of olefins (Scheme 5.11) in fully aqueous solution was studied using a ruthenium-carbonyl catalyst with no phosphine ligands [35]. In a fine mechanistic study it was shown, that (the WGS) reaction of /ac-[Ru(C0)3(H20)3]2+ and water provided /ac-[RuH(CO)2(H20)3]+. At 70 °C and in the presence of CF3S03H the latter compound reacted with ethene (10 bar) giving a a-alkylruthenium complex, solutions of which absorbed CO and yielded the corresponding acyl-derivative ... 

The monomer is made by the Diels-Alder reaction of dicyclopentadiene with ethylene. The catalyst for ROMP is ruthenium chloride in butanol. The process is relatively simple as the two liquids (ruthenium chloride in butanol and norbornene) are directly mixed in the extruder, in air. The norbornene to ruthenium ratio is very high (c.a. 25000) and the conversion reaches 50%. As the process operates in air, a small amount of norbornene is oxidized into epoxynorbornane (the epoxide to ruthenium ratio is c.a. 5) which can accelerate the polymerization. Indeed, mechanistic studies have shown that the catalytic reaction passes through a ruthenium hydride (formed by substitution of chlorine by butoxy ligands and further p-H abstraction) or through a ruthenium oxametallacyclobutane (formed by reaction of the ruthenium complex with epoxynorbornane) [33]. 

This chapter will focus almost exclusively on alkene metathesis catalyzed by well-defined homogeneous ruthenium-catalyst systems and is divided into three sections (1) a discussion of how precatalyst structure affects the rate and mechanism of initiation in two key series of metathesis precatalysts (2) a discussion of how substrate structure, both close to and remote from the alkene termini, affects the rate and selectivity of alkene metathesis in synthetic chemistry and (3) the tools that have been used by experimental and theoretical chemists to study alkene metathesis reactions. In each case, the discussion will be focused on the specific topics interested readers are referred to a recent article which covers a wider range of the mechanistic aspects of alkene metathesis with ruthenium complexes, albeit in less depth. 

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