Ruthenium alkylidene catalyst

Transition Metal-Carbene Complexes in Olefin Metathesis 

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Initial reports of cross-metathesis reactions using well-defined catalysts were limited to simple isolated examples the metathesis of ethyl or methyl oleate with dec-5-ene catalysed by tungsten alkylidenes [13,14] and the cross-metathesis of unsaturated ethers catalysed by a chromium carbene complex [15]. With the discovery of the well-defined molybdenum and ruthenium alkylidene catalysts 3 and 4,by Schrock [16] and Grubbs [17],respectively, the development of alkene metathesis as a tool for organic synthesis began in earnest. 

Metathesis of 1-octene leads cleanly to ethene and 7-tetradecene, but as the reaction proceeds also 2-octene is formed and metathesis products derived from the isomerisation reaction. It was found that after prolonged reaction times decomposition of the ruthenium alkylidene catalyst occurs. At least eight different products were formed and several of them have been identified [37], Figure 16.22 shows the identified compounds derived from Grubbs 1st generation catalyst (the 2nd generation gives basically the same result [38]). 

The in situ preparation of a ruthenium-alkylidene catalyst for olefin metathesis is the first step for extending this high-throughput approach toward other catalytic transformations and opens up the way to the screening of azolium salt libraries for olefin metathesis reactions. ... 

A Grubbs-type ruthenium complex and a Hoveyda ruthenium complex were compared under similar conditions for recycled activity. Both the reference catalysts showed a large drop in metathesis activity in the subsequent tests. For example, a Grubbs-type ruthenium alkylidene catalyst showed a drop of nearly 50% conversion in the second run. 

Research Focus Identification of high-activity ruthenium alkylidene catalysts for ringopening and ring-closing metathesis reactions. 

The nucleophilic ruthenium alkylidene complex Cl2(PPh3)2Ru=CHCH= CPh2 also appeared to catalyse the polymerisation of norbornene the catalyst has been shown to be living with the norbornene monomer [63], Another highly strained monomer, bicyclo[3.2.0]hept-6-ene, has been polymerised in a living system with this ruthenium alkylidene catalyst [92] ... 

To a solution of 5 (30 mg, 0.114 mmol) in dry degassed CH2C12 (25 mL) was added, via syringe, a solution of ruthenium alkylidene catalyst G1 (5 mg, 0.006 mmol) predissolved in CH2C12 (6mL). The resulting purple solution was heated to 45 °C for 21 h and then concentrated under reduced pressure to afford an oily brown residue. Purification by MPLC [pre-packed silica gel column 2 x 50 cm, eluent ethyl... 

A cyclobutene was recently obtained by a related reaction of dimethyl acetylenedicarboxylate with ethylene in the presence of a cationic ruthenium alkylidene catalyst precursor [43] (Eq. 35). 

The range of ruthenium alkylidene catalysts active in radical chemistry was further enlarged to the readily accessible vinylidene complexes 4 and 5 [33]. Catalyst precursors 4 and 5 were tested for the ATRA of polyhalogenated alkanes with various olefins. Substitution of one phosphine in 4 by an NHC improves its catalytic efficiency. This is a surprising result given that 3 is more ac-... 

Nugent s catalyst, however, failed to effect the cyclization of the aminodiene 106, that was cyclized to the pyrroline 107 either by Grubbs ruthenium alkylidene catalyst, PhCH=Ru(PCy3)2Cl2, in 77% yield or by Schrock s catalyst, PhC(Me)2CH=Mo=N[2,6-(iPr)2C5H3] [OCMe(CF3)2]2 in 60% yield (Equation (123)).149... 

A similar strategy has been used to build robust conformationally restricted cyclic dinucleotides. Thus, dinucleotide with two allyl chains reacts in the presence of ruthenium alkylidene catalyst and leads to the unsaturated cyclic dinucleotide. On addition of hydrogen (1000 psi H2) the saturated related dinucleotide was obtained, which appears to be more stable toward nucleophiles than the parent unsaturated one [85] (Scheme 39). 

A typical ruthenium alkylidene catalyst (Figure 5) can be anchored through the neutral ligand (phosphine or carbene), the anionic ligand (where X is usually a phenolate), or even the alkylidene (where R = phenyl), and Barrett and Hoveyda have reported strategies. However such work is in its infancy and further development is required before these techniques become commercially viable. 

Grubbs examined the effect that a C2 symmetric imidazolin-ium ligand would have on dichloro-, dibromo-, and diiodo-ruthenium alkylidene catalysts (112a-f) (Scheme 18). The new complexes had activities and stabilities similar to the parent complex (4a) and were thus tested for asymmetric induction in enantioselective desymmetrization reactions. The A rei using these new complexes were low, leading to very low... 

The cycle outlined in Fig. 4.32 indicates that the overall metathesis activity of the catalysts is determined by the relative magnitudes of several rate constants (i) the rate constant for phosphine dissociahon ( i) dictates the rate at which the 16-electron pre-catalyst complex enters the catalytic cycle, (ii) the ratio of k- to k2 controls the rate of catalyst deachvation (by re-coordination of phosphine) versus catalyhc turnover (by coordination of olehnic substrate and subsequent steps), and (iii) the rate constant for metallacycle formahon k ) determines the rate of the carbon-carbon bond formation. High olefin metathesis activity is expected when a ruthenium alkylidene catalyst exhibits fast initiation (a large value of k ), high selectivity for binding olefins relative to phosphines (a small value of k-x/k ), and fast metallacyclobutane formation (a large value of k ). 

Another important mechanistic issue is the thermal decomposition of ruthenium alkylidene catalysts. To understand the decomposition pathways available in these systems, the thermolysis of two ruthenium alkylidene complexes, the propylidene (PCy3)2(Cl)2Ru=CHEt (3) and the methyhdene (PCy3)2(Cl)2Ru=CH2 (4), was examined in detail [93]. These two compounds were chosen because a variety of alkylidenes [as modeled by the propylidene (3)] and the methyhdene (4) are key intermediates in a range of olefin metathesis reachons with terminal alkenes. The studies revealed that the thermal decomposihon of the propylidene... 

Equally important as recycling, problems involved with the use of certain reagents can often be completely avoided if they are attached to a resin. For instance, the ruthenium alkylidene catalysts typically employed to initiate metathesis reactions often decompose into darkly colored, metal-containing by-products, unwanted materials that sometimes require special protocols to remove and which are... 

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